ARTIC SOLAR CITY. Master Thesis Ignacio de Lis & Lidia Sáenz 17/06/2017

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1 ARTIC SOLAR CITY Master Thesis Ignacio de Lis & Lidia Sáenz 17/06/2017 LULEÅ UNIVERSITY OF TECHNOLOGY Department of Civil, Environmental and Natural Resources Engineering

2 Abstract The importance of renewable energies will continue to grow over the coming years as a substitute of fossil fuels. In this scenario, prove the profitability of green technologies in places with adverse weather conditions is a technological challenge. This master thesis analyses the profitability of using two renewable energy sources, the sun and the ground, to cover the heating demand of a fictive city of inhabitants in the north of Sweden, where the solar radiation received is quite low. Nevertheless, these two renewable energy sources will not cover the entire heating demand of the city, since an 8% of the total heating demand will be covered by auxiliary electricity. On the one hand, a Borehole Thermal Energy Storage (BTES) is used to store the energy coming from the sun and to cover the heating demand of the city during winter time, when there is no solar gain. On the other hand, solar collectors are used not only to cover the heating demand of the city during spring, summer and autumn, but also to recharge the storage during this time. Furthermore, a low temperature district heating network has been considered to connect the energy sources to the end customers of the city. The heating system has been designed and dimensioned by using the software TRNSYS, which is a simulation program widely used in the renewable energy field to simulate the behaviour of transient systems. Moreover, the costs of the heating system have been estimated to determine the project profitability. The results obtained from the simulations performed in TRNSYS show that the heating system designed and dimensioned is capable of covering both the DHW and the space heating demand of the city. However, the economic analysis carried out shows the non-profitability of the heating system designed (e.g. the payback time is almost 40 years). This profitability would improve if any kind of subsidy would be provided by either the Government or the City Administration. Another conclusion drawn from the economic analysis is the high cost of using solar collectors to recharge the storage, since a great number of solar collectors is required to be able to recharge it sufficiently. Consequently, there could be cheaper alternative energy sources (e.g. biomass) that could be used to recharge the storage with lower costs, which would lead to an increase of the project profitability. Therefore, this master thesis shows that covering the heating demand of a city in the north of Sweden with two renewable energy sources, the sun and the ground, is technically feasible but non-profitable. Nevertheless, some improvements can be made in the heating system designed, as well as some subsidies could be received to reduce the costs and make it profitable. Keywords: renewable energies, heating system, solar collectors, borehole thermal energy storage, district heating network, heating demand, TRNSYS.

3 Acknowledgments Firstly, we most especially want to thank Kjell Skogsberg, the main supervisor of this master thesis, for his support, commitment, dedication and interest. Secondly, we also appreciate the support given by Göran Hellström, not only with the software TRSNSY but also with the dimensioning of the BTES and many more things. Thirdly, we would like to thank Damiano Varagnolo, for his help with the presentation of the master thesis, and Jenny Lindblom, for the theoretical framework provided about solar heating systems. Fourthly, we also wish to convey our sincere thanks to Daniel Eriksson, from Piteå Energi, for his support with the consumption data estimation and with the district heating network performance. Finally, we would like to thank our families and friends for their support throughout the project.

4 Index Abstract... 1 Acknowledgments Introduction Energy Situation Background Description Goals Method Limitations Theory Solar Heating systems Solar Irradiation Solar energy in the World Solar thermal in Europe Solar Collectors Thermal Energy Storage Surface and above ground technologies Underground Thermal Energy Storage (UTES) Phase Change Material Energy Storage (PCMES) Thermal Energy Storage via chemical reactions District Heating System Method Heating System District Heating Network Borehole Thermal Energy Storage Customers Heating Installations Solar Field Results and Discussion Model results Borehole Thermal Energy Storage (BTES) District Heating Network Customers Heating Installations Economic Analysis Conclusions and Future Research References... 50

5 Annexes Annex 1. TRNSYS model Annex 2. Heating demand estimation Annex 3. District heating network parameters Length Pipes Heat Exchangers Variable Speed Pump Valves References Annex 4. BTES parameters References Annex 5. Influence of BTES inputs and parameters Annex 6. Costumers heating installations parameters Single-family dwellings Apartment blocks Industries References Annex 7. Accumulators design and dimensioning References Annex 8. Solar field parameters References Annex 9. Dimensioning procedure Annex 10. Energy balance Annex 11. Budget Borehole Thermal Energy Storage District Heating Network Single-family dwellings installation Apartment blocks installation Industries installation Solar field CAPEX OPEX Economic indicators References

6 Introduction 1. Introduction 1.1. Energy Situation Nowadays, the world is undergoing a major transition from conventional energy sources to renewable energies; there is an urgent need of turning the development in society to a sustainable future. However, the vast majority of the global energy consumed is still produced from fossil fuels, as it is possible to check in the following figure. Figure 1. World consumption (million tonnes oil equivalent) [1] Therefore, much still remains to be done in order to build a more sustainable world. In particular, Sweden is intended to achieve a 100% renewable energy production by In order to achieve this goal, green energy projects must be promoted, financed and implemented. National strategies are pointing at a more efficient use of energy and transport, in order to reduce emissions from the energy and transport sector. By 2050, the total emissions in Sweden should be lower than 4.5 tons of carbon dioxide equivalents per capita per year. The project described in this master thesis represents a green energy project in which two renewable energy sources, the sun and the ground, are used for supplying the required heating for a small city in the north of Sweden. Almost all the required energy is obtained from the sun, by using solar collectors, while the ground source is used as a heat storage, by using boreholes Background This master thesis constitutes a follow-up to the project carried out by Ignacio de Lis in the course Natural Energy Resources, at Luleå Tekniska Universitet [2]. Solar and thermal energy were combined in that project to design and dimension the heating system of a fictive city of 10,000 inhabitants in the vicinity of Luleå. The design shown in the figure below was chosen for the heating system. 5

7 Introduction Figure 2. Scheme and operation modes of the heating system [3] According to this design, there are two different operation modes. On the one hand, in the left, the winter operation mode (November February), in which the solar heat is sent to the evaporator of the heat pump and to the borehole, in order to increase the efficiency of the heat pump, by increasing the heat pump inlet temperature (evaporator temperature). According to this operation mode, both water and space heating are obtained from the heat pump. Solar collectors only lead to an increase of the heat pump efficiency (COP) and to an increase of the borehole temperature. On the other hand, in the right, the summer operation mode (March October), in which the domestic hot water demand is covered by solar heat, while space heating is provided by the heat pump. If solar collectors cannot cover the domestic hot water demand, it will be covered by the heat pump or by auxiliary electric heaters located in the hot water storage tank. Once the heating system configuration was chosen, the dimensioning of the Borehole Thermal Energy Storage and the Solar Collector System was carried out using the softwares Earth Energy Designer and Polysun, respectively. Among the results obtained, it should be highlight the small roof area occupied by the solar collectors, only a 3,5% of the total roof area in the single-family dwellings and a 9% of the total surface area in the apartment blocks. This means that the solar collector system could be maximized by occupying the whole surface area of the roofs. Then, a much larger heating demand could be covered only with the solar heat. Consequently, the main goal of this master thesis is to maximize the solar collector system in order to be able to cover both hot water and space heating demand of the fictive city during the whole year only with solar heat and a borehole thermal energy storage, but without heat pump Description A prestudy of a solar heated fictive small city located in the vicinity of Luleå is carried out in this master thesis. The task mainly includes system design, dimensioning of the solar plant and the heat storage (BTES) together with a rough cost estimation. Heat pumps are not used in the borehole thermal energy storage to avoid operational issues and costs. Therefore, the heating demand of the city for the whole year must be covered almost in its entirety with solar and ground heat. Only an 8% of the heating demand will be covered by auxiliary electricity. 6

8 Introduction The city studied in this project is a fictive one with 10,000 inhabitants, half of the persons living in single-family dwellings and the remaining people in apartment blocks. Furthermore, the different industries, public buildings and commercial premises (hereinafter referred to as industries for short) of the city have been also considered. Consequently, the heating system designed must cover not only the heating demand of the houses and apartments, but also the heating demand of the industries. On the other hand, all the single-family dwellings, apartments and industries are assumed to be connected between them and with the borehole thermal energy storage through a district heating network Goals The objective with this study is to identify the conditions and possibilities of using solar heat in combination with ground-source heat in a city. The main goals of this project are described below. Design and dimension a heating system able to cover the heating demand for the whole year of a small city in the north of Sweden by using solar collectors and a borehole thermal energy storage, without heat pump. Prove the feasibility of solar technology even in one of the most northern regions of the world, where climate conditions are less suitable for solar plants than in most parts of the world. Calculate the economic profitability of the project Method The design and the dimensioning of the heating system of the arctic solar city have been done by collecting information about solar energy and thermal energy storage concerning available systems, problems and earlier studies. In order to evaluate the design of the arctic solar city heating system and study the influence of the different parameters in the whole system (e.g., the depth and temperature of the boreholes, the number of solar collectors, the thermal influence of neighbouring boreholes, etc.), the software TRNSYS have been used. TRNSYS is an extremely flexible graphically based software environment used to simulate the behaviour of transient systems. This software, which is widely used in the energy field, simulates the performance of an entire energy-system by breaking it down into individual components. By using TRNSYS tools, it is possible to build up a model for the heating system of the city. A complete description of the model designed is described later Limitations The limitations of this master thesis are listed below. Most of them are limitations of the software used (TRNSYS). TRNSYS limitations: - Non-convergences issues when dimensioning the water tanks. - Non-convergences issues when working with large storage accumulators. - Too many non-convergences when simulating 10 years since the non-convergences issues accumulate over the simulation time. - The heat exchanger used in the UFH circuit does not work properly with high temperatures (explained later). 7

9 Introduction - The type used for the water tanks has not a vent valve to vent steam when the water boils. - Modelling individually all the houses, apartments and industries is not possible, since they are a lot, so some assumptions have been considered. In particular, we have simulated one single-family dwelling, ten apartments and one industry and we have assumed that the remaining houses, apartments and industries work in the same way. This assumption leads to an oversizing of the system, since it means that the consumption peaks are going to be at the same time in all the houses, apartments and industries. - TRNSYS is not able to consider the efficiency drop of the solar collectors over the years. - Coaxial pipes are not modelled in TRNSYS. BTES limitations: - Estimate the fluid-to-ground borehole thermal resistance (a thermal response test in situ is required). Economic limitations: - It would be reasonable that either the government or the administration of the city would provide any kind of subsidy to the project. However, since there is not a project like this in Sweden, it is not possible to know if any subsidy would be provided, so we have not considered any kind of subsidy for the calculation of the project profitability. - The price of some components has been calculated with interpolations and approximations due to the difficulty in finding the prices of exactly the same components that we have designed in our heating system. 8

10 Theory. Solar Heating Systems 2. Theory 2.1. Solar Heating systems Solar Irradiation The solar irradiation from the sun falling on to our atmosphere is TWh/h. The energy radiated by the sun is transferred without losses through space in a direction normal to the surface of the sun. The radiation from the sun falling on the earth outside the atmosphere is 1367 W/m 2. We know this data as the Solar Constant (G sc), which is a mean value that varies about ± 45 W/m 2 over the year depending on the latitude, since the angle of incidence of the solar irradiance results in a lower radiation at higher latitudes [3]. There are two reasons for this: - One reason is that the distance that the solar radiation has to travel in the atmosphere is longer at higher latitudes, resulting in increased absorption and reflection before reaching the earth. - The other reason is that the higher angle of incidence results in a lower irradiance on the horizontal ground. However, this may be compensated by a tilted surface towards the sun. The horizontal irradiation all over the world is shown in Figure 3, where it is possible to check that low figures are obtained for northern latitudes, e.g. Sweden. [3] Figure 3. The annual global irradiation in the world on a horizontal surface (kwh/m 2 ) [7] Since the north of Sweden does not appear in the Figure 3, the horizontal irradiation in all Sweden is shown in the following figure. 9

11 Theory. Solar Heating Systems Figure 4. The annual global irradiation in Sweden on a horizontal surface (kwh/m 2 ) [7] The maximum annual global mean irradiation is around 1100 kwh/m 2 in the south of Sweden and about 900 kwh/m 2 in the north Solar energy in the World Solar energy has a big part to play in reducing future carbon emissions and ensuring a sustainable energy future. It can be used for heating, cooling, lighting, electrical power, transportation and even environmental clean-up. The global average solar radiation, per m 2 and per year, can produce the same amount of energy as a barrel of oil, 200 kg of coal, or 140 m 3 of natural gas. Global installed capacity for solar-powered electricity has seen an exponential growth, reaching around 227 GWe at the end of It produced 1% of all electricity used globally. Germany has led PV capacity installations over last decade and continues as a leader followed by China, Japan, Italy and the United States. [8] Figure 5. Solar installed Capacity by region. 10

12 Theory. Solar Heating Systems Figure 6. Installed Global capacity Solar thermal in Europe In Europe exists an organism, The European Solar Thermal Industry Federation (ESTIF), which is the voice of the solar thermal industry, actively promoting the use of solar thermal technology for renewable heating and cooling in Europe. [9] The European Union and their member states committed to achieve a 20% of renewable energies over the final energy consumption in Europe by Space heating will have a relevant contribution since it represents a 49% over the total energy demand in Europe. [10] Solar Collectors The two main types of solar energy technologies are photovoltaic and thermal collectors. Photovoltaic collectors convert solar radiation directly into electricity, without the use of any heat engine, and are increasingly popular in building integration purposes (such as using photovoltaic tiles as roof shingles) as well as for small- and large-scale devices, from watches to satellites. Solar thermal collectors can be used for domestic heating and hot water, but large solar collectors plants can also be used for either industrial heat purposes or electricity generation based on the same mechanisms as fossil fuels. [8] The solar energy uses have verified significant growth in recent decades and, despite the different utilization forms developed, its most common use is yet the domestic hot water systems. These systems use collectors to capture the heat from the solar radiation. That heat is then transferred to a working fluid that can go to a storage device or directly supply heat to the heating system of the user. Solar collectors absorb solar radiation, convert it into heat, and transfer useful heat to the solar system. Solar thermal systems are generally comprised of solar panels, a twin coil cylinder, pump and controller station. The panels are usually roof mounted however they can also be mounted on the building walls or on frames on the ground. The solar panels receive free energy from the Sun throughout the year even when it is a cloudy day. As it is shown in the figure 6, this is the basic and most common installation of a solar heating system. Installing a solar thermal system can drastically reduce your fuel bills. [27] 11

13 Theory. Solar Heating Systems Figure 7. Basic scheme of a solar heating system There are different types of solar collectors to heat up fluids that differ by the temperature of the application. The most common systems are analyzed below: Unglazed liquid flat-plate collectors are made of a black polymer, they do not have a frame and insulation at the back. In that way, they have a low delivery temperature and high losses to the surroundings. Figure 8. Unglazed liquid flat-plate collector Glazed liquid flat-plate collectors have a flat-plate absorber (which often has a selective coating) fixed in a frame between a single or double layer of glass and an insulation panel at the back. Much of the sunlight (solar energy) is prevented from escaping due to the glazing. These collectors are commonly used in moderate temperature applications (e.g. domestic hot water, space heating, year-round indoor pools and process heating applications). 12

14 Theory. Solar Heating Systems Figure 9. Glazed liquid flat-plate Evacuated tube solar collectors have a selective coating enclosed in a sealed glass vacuum tube. Their thermal losses (convection and conduction) to the environment are extremely low and they have an energetic efficiency really high, even in colder climates, where they can reach higher temperatures. They do not have thermal losses due to the negative pressure inside the tubes. Systems presently on the market use a sealed heat-pipe on each tube to extract heat from the absorber (a liquid is vaporized while in contact with the heated absorber, heat is recovered at the top of the tube while the vapor condenses, and condensate returns by gravity to the absorber). Evacuated collectors are good for applications requiring energy delivery at moderate to high temperatures (domestic hot water, space heating and process heating applications typically at 60 C to 80 C depending on outside temperature), particularly in cold climates. [5] The heat gain is the double than in flat plate collectors (800Wh/m 2 and year) but the price is also the double, around 500 /m 2 for vacuum. Figure 10. Evacuated tube solar collector 13

15 Theory. Thermal Energy Storage 2.2. Thermal Energy Storage Thermal Energy Storage (TES) consists basically on storing the surplus energy obtained during a certain time, for using it when there is a lack of energy that needs to be covered. The energy is stored as heat in three possible ways, either as sensible heat, when there is a temperature difference in a body without phase change, as latent heat, when a phase change occurs in the substance at constant temperature, or as thermo-chemical energy, when chemical reactions occur [11]. Two different types of thermal energy storage are considered, according to the timescale. Shortterm storage covers energy peaks for short periods of time (e.g. the surplus energy collected during the day is used to cover the energy demand in the evening of the same day), while seasonal storage covers energy needs for larger periods of time (e.g. the surplus energy collected during summer time is used to cover the energy demand in winter). There are a wide variety of different ways and locations to store the heat. The use of hot water tanks is the most common technology in short-term storage (e.g. domestic hot water in residential buildings). Regarding seasonal storage, some of the main technologies available are described below Surface and above ground technologies Storing the heat in the surface is less common than storing it underground, as it is less efficient. Nevertheless, some technologies should be emphasized Pit storage A pit heat storage consists on a large water reservoir that is used to store the heat. A shallow excavation is made in the ground, filled with water and covered by a floating insulated cover, as it is shown in Figure 11 [12]. Figure 11. Pit heat storage This type of technology is highly developed in Denmark. In fact, the largest pit storage in the world is in Vojens, a small town in Denmark, with a capacity of m 3 [13] Large water tanks Large scale water storage tanks are built above the ground and insulated on the roof and in the vertical walls. Concrete is usually used for manufacturing the tank. The first pilot storages are in operation in Hamburg and Friedrichshafen since Another water storage tank was built in Munich in 2006 [14]. 14

16 Theory. Thermal Energy Storage Figure 12. Large-scale water storage tank in Munich Underground Thermal Energy Storage (UTES) Using the ground as a storage is commonly done since large storage volumes are needed. Furthermore, the mean temperature of the ground is higher than the ambient air temperature during winter and colder than the air during summer. Consequently, the ground is suitable for store the heat seasonally, heat extraction during winter and cold extraction during summer [15]. For this reason, regarding large-scale, underground storages are more common than surface storages. The three main types of UTES are described below Aquifer Thermal Energy Storage (ATES) In this type of storage, the thermal energy is stored in the groundwater and the minerals of an aquifer. An aquifer is an underground layer with large water storage capacity and high permeability, from which groundwater can be extracted using a water well. Two different wells are used since the flow direction is reversed depending on the operating mode. Cold groundwater is extracted during summer (charging period) from the cold well, heated up by the heat source (e.g. the sun) and injected into the warm well. On the other hand, warm water is extracted during winter (discharging period) from the warm well, cooled down by the heat sink (e.g. radiators of a house) and injected into the cold well. Figure 13. Aquifer Thermal Energy Storage (ATES) For high temperatures (roughly above 50 C), large storage volumes (more than m³) are required, with a favourable surface to volume ratio, since it is not possible to insulate thermally the storage volume against the surroundings at high temperatures [16]. 15

17 Theory. Thermal Energy Storage The Netherlands is the country where this method is more highly developed, as it is possible to check in the Figure 14 [17]. Figure 14. Number of installed ATES in operation in 1990, 2000 and 2010 in The Netherlands Borehole Thermal Energy Storage (BTES) Borehole thermal energy storage is the UTES most commonly used and is the one that we have selected in this project for the heating system. For this reason, it is explained in more detail than the other storage types. A borehole is a vertical hole in the ground that contains a vertical U-pipe with a carrier fluid circulating inside it, and that is filled with grout or groundwater, according to Figure 15. Consequently, the borehole acts as a heat exchanger exchanging energy between the carrier fluid inside the borehole duct and the ground. Therefore, a vertical U-tube with a fluid circulating inside it is used in this kind of storage (BTES) to either heat the ground (the storage) or extract energy from it. Figure 15. Borehole cross section Convective heat transfer is considered from the circulating fluid to the pipe and conductive heat transfer from the pipe to the borehole wall [18]. Borehole s depth is normally large, around metres long, to reduce costs and take advantage of the ground geothermal gradient. In Luleå, for example, the geothermal gradient of the ground is K/m (1.3 /100 km) [19]. As it is shown in Figure 16, the hot fluid enters the pipe, heats up the storage and exits the pipe as cold fluid when there is an energy surplus (heat injection). This process will be reversed when 16

18 Theory. Thermal Energy Storage heat from the ground needs to be extracted (heat absorption). Cold fluid will be the input to the borehole and hot fluid will be obtained at the output. Figure 16. Borehole front view Figure 16 shows a closed system in which the heat carrier fluid is not in direct contact with the borehole. This is the typical system configuration. However, opened systems also exist, even though they are not very common. In these systems, the inserting pipe outlet is placed close to the bottom of the borehole, while the extraction pipe has its inlet close to the top of the borehole, so the heat carrier fluid is in direct contact with the borehole wall, which leads to water chemistry problems [20]. Figure 16 also shows the most common type of borehole heat exchanger, the single U-tube. However, the double U-tube heat exchanger is also used frequently due to its lower thermal resistance and head loss [21]. In addition to these two types of heat exchangers, concentric or coaxial heat exchangers are also used occasionally. Figure 17. Types of borehole heat exchangers A borehole is characterised by the following parameters: Type (e.g. single U, double U, coaxial), configuration (e.g. hexagonal, rectangular), depth (normally above 200 m, since the costs are lower), spacing between boreholes (usually between 3 8 m), borehole diameter (0,115 m is commonly used in Sweden [22]), volume flow rate (0,0006 m 3 /s per borehole is commonly used [22]), U-pipe (e.g. a 40 mm polyethylene pipe with a maximum allowed pressure of 10 bar), heat carrier fluid (water is normally used, mixed at certain times with some additives as ethanol to lower the freezing point) and thermal resistance of the borehole. The thermal resistance of the borehole is the thermal resistance between the heat carrier fluid and the borehole wall. The heat transfer is greater for lower thermal resistance values. Some of 17

19 Theory. Thermal Energy Storage the parameters that have a greater influence on the borehole thermal resistance are the thermal conductivity of the filling material, the number of pipes, the pipe position and the pipe thermal conductivity [23]. A matrix of boreholes uniformly distributed is used instead of a single borehole for large-scale storage systems to increase the storage volume and, consequently, the energy extracted from the ground. The first large scale high temperature BTES was built at Luleå University of Technology, in 1982, with a storage volume of m 3 and 120 boreholes [17]. A BTES is usually used as a seasonal storage, i.e. heat is injected into the ground in summer when there is an energy surplus, while it is extracted from the ground in winter when the energy needs are higher. For large-scale storage systems, a certain number of boreholes are frequently connected in series per parallel loop and the fluid circulation through the boreholes needs to be switched depending on the season. During summer (heat injection), the fluid will circulate from the centre to the border of the BTES, to keep the storage warmer. On the other hand, the fluid will circulate from the border to the centre during winter (heat absorption), also to keep the storage warmer (heat losses will be higher if the hot fluid circulates at the border). Figure 18. Seasonal BTES Nevertheless, in some applications the BTES is not used as a seasonal storage, but only for extracting energy from the ground (e.g. a heating system with solar collectors and a BTES in which the solar heat is used to heat up the domestic hot water in summer, as well as to increase the inlet temperature of the heat pump evaporator, while the BTES is used, with the aid of a heat pump, to cover the space heating demand throughout the year, as well as the domestic hot water demand during winter (Figure 2)). A heat pump is normally used in combination with boreholes to produce a larger amount of thermal energy. This kind of installations are commonly called Ground Source Heat Pump (GSHP), and operate at low storage temperatures (usually lower than 40 C). On the contrary, BTES installations without heat pump operate at higher storage temperatures (40 80 C, approximately) [20]. A heat pump is an electrical device that extracts energy from a heat source (e.g. the ground) and transfers it to another place, by consuming electrical energy. A compressor is used to pump the refrigerant from the evaporator to the condenser. In the evaporator, the refrigerant is evaporated at low pressure, absorbing heat from its surroundings. On the other hand, the refrigerant is condensed in the condenser at high pressure, releasing the heat absorbed previously in the evaporator. Finally, an expansion valve is used between the condenser and the evaporator to lower the pressure. 18

20 Theory. Thermal Energy Storage Figure 19. Operational cycle of a heat pump The main advantage of a heat pump is its high efficiency or Coefficient of Performance (COP), since a thermal energy around four times larger than the electric power consumed is obtained at the output. Nevertheless, using a heat pump together with a BTES has some disadvantages, as for example higher investment costs, higher operational and maintenance costs (most operational and maintenance issues are related to heat pumps) or higher electricity consumption (heat pumps are not entirely carbon neutral). In order to avoid these problems, we have designed in this project a BTES without heat pumps. Consequently, the energy at the output will be lower than in a system with heat pumps, so the BTES should be larger to be able to cover all the heating demand. In Alberta, Canada, there is a BTES of 144 boreholes in operation since 2007 that works together with a district heating net heated by solar-thermal panels on garage roofs. This installation covers, without heat pumps, almost all the heating demand of the homes of the Drake Landing Solar Community [24] Cavern Thermal Energy Storage (CTES) This technology, which is less common than the ATES and the BTES, is based on using a rock cavern below the ground to store hot water. Since a large volume of water is used, it is necessary to keep a stratified temperature profile in the cavern by injecting hot water at the top of the cavern and extracting cold water from the bottom during summer time (charging period), as well as extracting hot water from the top and returning cold water to the bottom during winter time (discharging period) [25]. Figure 20. Cavern Thermal Energy Storage (CTES) The main limitation of this type of storage are its high construction costs, which make it less economically feasible than other storage technologies. 19

21 Theory. Thermal Energy Storage Phase Change Material Energy Storage (PCMES) This technology consists on collecting the energy corresponding to the latent heat of the Phase Change Material (PCM) when it is melted due to a temperature increase. When a temperature drop occurs, the PCM will solidify and the heat will be released. During melting processes of the PCM the energy density (around 100 kwh/m 3 ) is considerably larger than the energy density reached in sensible heat processes (around 25 kwh/m 3 ). The storage capacity between sensible heat processes (without phase change) and latent heat processes (with phase change) is compared in Figure 21 for a given temperature difference [11]. Figure 21. Stored heat vs Temperature for sensible (blue) and latent (red) TES Some of the materials used as PCM are paraffin, which is expensive, and ice/snow, which is the most common PCB for large-scale storage cooling applications, due to its low melting point [16]. Seasonal Snow Storage (SSS) can be applied either in the surface or underground, according to the different configurations shown in Figure 22 [26]. Figure 22. Seasonal Snow Storage location An underground storage is more efficient since the losses are lower, but the construction costs are higher. A common configuration is placing the snow in a shallow pit on the ground with wood chips at the top of the storage as insulation. A storage of this kind is in operation in Sundsvall regional hospital since 2000, with a capacity of m 3 of snow. Another larger storage ( to m 3 ) was taken into operation in 2010 at the New Chitose Airport in Sapporo, Japan [17]. This method is recommended for locations with enough snow and with periods long and cold enough Thermal Energy Storage via chemical reactions In this kind of storage heat and cold are stored thanks to thermo-chemical reactions, such as adsorption (adhesion of atoms, ions or molecules from a substance to a surface). An example is 20

22 Theory. Thermal Energy Storage shown in Figure 23, where water molecules are desorbed from the adsorbent (zeolite) during charging and adsorbed by the adsorbent during the discharge of the Thermal Energy Storage (TES) [11]. Figure 23. Thermal Energy Storage by adsorption and desorption processes This technology, which reaches high energy densities (around 300 kwh/m 3 ), is used in applications, such as domestic hot water, space heating and air-conditioning [11]. 21

23 Theory. District Heating System 2.3. District Heating System A district heating (DH) system consists on a pipe network that sends hot water from one or more heating plants to the buildings of a neighbourhood, town or city. This hot water, usually around 70 C and 120 C, is delivered to the costumers for different applications as domestic hot water, space heating or industrial processes. A substation with a heat exchanger is required for every costumer to absorb the heat from the district heating net. All the costumers are connected to the heat source through a double pipe network. One pipe circuit (primary circuit) is used to transport the heat from the heating plant to the different customers while another pipe circuit (secondary circuit) is used once the heat delivered by the primary circuit is consumed to bring back the outgoing water of the houses to the heating source, to heat it up again and restart the cycle. A good insulation must be used in the pipes of the primary circuit to minimize as much as possible the environmental losses [28]. Figure 24. District Heating System Since DH systems usually cover large areas, several heating plants need to be used to cover the large energy demand. Combined Heat and Power (CHP) or cogeneration plants are the most common heating plants used as heat sources in a DH system. In this kind of plants, both electricity and heat are produced at the same time and the heat emitted during electricity generation is recovered as energy for heating. On the contrary, power plants discard the waste emitted during electricity generation into the environment, for example through cooling towers. Consequently, the efficiency of a CHP plant is higher than a heating and a power plant separately. Nevertheless, other types of heating plants are also used in DH systems, as for example solar plants. This kind of plants, which are very interesting from the point of view of sustainability and environmental protection, collect the heat from the sun, a renewable heating source, by using either solar collectors or photovoltaic panels. A disadvantage of using a solar plant as the heat source of a DH system is that this plant needs to be complemented by either another type of energy plant independent of the weather conditions or a thermal energy storage capable of covering the energy needs when there is no sun (e.g., at night or during winter). Figure 25 shows a DH system in which only solar heat and a borehole thermal energy storage are used to cover the heating demand of a neighbourhood in Okotoks, Alberta [29]. 22

24 Theory. District Heating System Figure 25. District heating system using solar heat and a borehole thermal energy storage On the other hand, heat sources can be connected in two different ways to the district heating net, either directly or through a heat exchanger. When a heat exchanger is used (indirect connection) the heat source and the distribution system (district heating net) work separately, having the possibility of using different fluids at different temperatures and different pressures. By contrast, direct connection has some limitations regarding the water quality and its pressure, as the same fluid is going to be used in both the heat source and the distribution system. Heat sources operate normally at higher temperatures to reduce the heat exchanger size [30]. One of the biggest problems in a DH system are the heat losses along the DH network due to its high operating temperature. However, Low Temperature District Heating (LTDH) systems have been developed in recent years. This kind of system works with lower temperatures so the losses of the network are lower. Normally, these systems operate in a range between C to C for supply temperatures and C to 40 C for return temperatures [31]. A higher flow rate is required in LTDH systems to be able to cover the heating demand of the consumers. This means higher electricity consumption in pumping and higher pressures in the pipe, which could lead to larger pipe diameters (higher costs). LTDH systems are usually applied to single-family houses and multi-dwelling buildings. Two different types of substations are recommended for single-family houses, the Instantaneous Heat Exchanger Unit (IHEU) and the low temperature District Heating Storage Unit (DHSU) [31]. Figure 26. Instantaneous Heat Exchanger Unit (IHEU) 23

25 Theory. District Heating System Figure 27. District Heating Storage Unit (DHSU) The main different between these two types of substations is the storage tank that is included in the DHSU. In a IHEU, a thermostatic bypass valve is used instead of the storage tank to control the amount of water passing by the heat exchanger. A flow controller with a temperature sensor is also needed to regulate the DHW temperature. In a DHSU, the heat coming from the DH network is stored in the tank storage and supplied from the tank to the house, passing through the heat exchanger. The temperature supplied from the DH network is assumed to be roughly constant (around C) [31]. The storage tank should be dimensioned based on the heating peaks demand of the house and considering the input energy coming from the DH network to the tank (function of the flow rate and the temperature of the DH net). Regarding the storage tank, both hot water inlet and outlet need to be placed at the top of the storage tank while both cold water inlet and outlet need to be placed at the bottom to ensure a good level of stratification within the tank (the energy storage efficiency is higher in a stratification tank than in a mixed one). Regarding the heat exchanger, a high efficiency heat exchanger with a low logarithmic temperature difference needs to be used due to the low temperature of the water supplied by the DH network. Consequently, the heat transfer area of the plate heat exchanger (type of heat exchanger commonly used for DHW applications) is going to increase by a factor 3-5, compared to the heat exchangers used in normal DH systems that work with higher temperatures [31]. On the other hand, in multi-dwelling buildings the LTDH substations used are the same than the ones described above, but there are separated DHW systems, usually one per each flat (flat stations), as shown in Figure 28. Figure 28. Flat stations in a multi-dwelling building 24

26 Method. Heating System 3. Method 3.1. Heating System A heating system has been designed and dimensioned in this project to cover the heating demand (both space heating and domestic hot water) of the houses, apartments and industries of a fictive city in the north of Sweden, by combining two renewable energy sources, the sun and the ground. The city is a fictive one with inhabitants, half of the persons living in single-family houses and the remaining people in apartment blocks. It has been assumed, for simplicity, that the 5000 inhabitants living in the single-family dwellings are distributed in 1666 houses of 3 persons each one, since the average of persons living in single-family dwellings in Sweden in 2015 was 2,7, according to the Sweden Statistical Database [4]. The following plan has been selected for the single-family dwellings, with a surface area of 120 m 2. Figure 29. Single-family dwellings plan On the other hand, 2 persons living in every apartment has been assumed, since the average of persons living in multi dwelling buildings in Sweden in 2015 was around 2, depending on the circumstances [4]. The apartments buildings are composed of five floors with 4 apartments in each floor, each one of 70 m 2, as shown in the Figure 30. Figure 30. Apartments plan Lastly, regarding the industries, since we do not have any information about their geometry and dimensions, we have assumed that there are 500 buildings with the same energy requirements. 25

27 Method. Heating System Before designing and dimensioning the heating system it is necessary to determine the heating demand that needs to be met. The heating demand estimation is described in detail in the Annex 2. It is important to highlight that not all the demand is covered by renewable sources (either the sun or the ground), since auxiliary electricity is used to heat up the domestic hot water from 35 C to 50 C. However, this auxiliary electricity represents a small fraction (8%) of the total heating demand (50369 MWh/year). Therefore, the heating system has been dimensioned to meet a total energy demand of MWh/year (92% of the total heating demand) only with renewable energy sources, assuming that the remaining fraction will be covered by auxiliary heaters. The heating system designed can be divided basically into four different parts, the borehole thermal energy storage (BTES), the solar field, the district heating network and the consumers installations, as shown in the Figure 31. Figure 31. Heating system sketch Solar collectors are placed on the roof of all the single-family dwellings and apartments to cover a large fraction of their heating demand during summer. However, during winter time, the heat coming from the sun is very low, so a BTES is used to cover the energy needs in these months by extracting heat from the ground. The BTES will be recharged during summer, when there is a solar energy surplus. Therefore, by combining these two technologies (BTES and solar collectors) it is possible to meet the existing heating demand of the houses and apartments. Regarding the industries, solar collectors have not been placed on the roof of the buildings due to the lack of information about their geometry and dimensions. This is a pessimistic assumption since in reality installing solar collectors on these buildings would probably be possible, which would lead to a higher efficiency and profitability of the system. The energy required by the industries will be always supplied by the heat coming from the district heating network. In addition to the BTES and the solar collectors placed on the houses and apartments, a solar field has mainly been considered to recharge the BTES during spring, summer and autumn, but also to cover the industries heating demand during these months. In order to connect the different elements of the system, a district heating network is used. Heat extracted from the BTES is sent through this network to the consumers installations in winter, while the energy surplus achieved in summer is sent from the consumers installations to the BTES through the district heating network. The energy produced on the solar field is also sent to 26

28 Method. Heating System the BTES to recharge it. Accordingly, it is obvious that the heating system designed works seasonally. As previously mentioned, the software TRNSYS has been used for modelling the heating system and simulating its performance. Furthermore, the dimensioning of this heating system has been done by following the procedure described in the Annex 9. As it is possible to check in the Annex 1, the model designed is composed of many components, so splitting the model into different parts is recommendable to better explain how the model works. The four main parts mentioned previously of the heating system are described below in more detail, including an explanation of the modelling of the different components present in each part District Heating Network A district heating network is needed to connect the different components of the heating system. An important feature of the district heating network designed in this project is that it works with relatively low temperatures (between 45 and 90 C, roughly), which reduces the total losses of the net. However, to be able to supply a certain amount of energy, the flow rate needs to be quite high to compensate for the low temperatures. One of the main parameters of a district heating network is its length. A simplification has been made to estimate the total length of the net by splitting the district heating network in three different parts, the single-family dwellings net, the apartment blocks net and the industries net. In total, 80km are needed for the double circuit of the district heating network, distributed in three different piping circuits (primary, secondary and tertiary). An explanation of how this value has been calculated is included in the Annex 3. The district heating net modelled in TRNSYS is shown in Figure 32. The different components of this diagram are analysed below. Figure 32. Modelling of the district heating network Piping The piping of the district heating network has been modelled in TRNSYS with the type 709, which models the thermal behaviour of fluid flow in a pipe. To facilitate the modelling in TRNSYS, only two pipes ( DHN-1 and DHN-2 ) have been used for both flow and return circuits, each one of 40km length, according to the total district heating network length calculated previously. This assumption is not very realistic since the flow rate decreases as it moves forward along the net, so different pipes should be considered for the different circuits of the net. From the economical point of view, the piping costs are much lower if we consider different pipe diameters along the district heating net. For this reason, a total of 5 different pipes have been considered for the 27

29 Method. Heating System district heating network designed, as shown in Table 1 and as it is explained in the Annex 3, even though these pipes have not been included in the TRNSYS model. Table 1. Pipes used in the different circuits of the district heating network DN450 [mm] DN125 [mm] DN80 [mm] DN65 [mm] DN32 [mm] L1 [m] L2 [m] L3 [m] Pre-insulated pipes are commonly used in district heating networks since they can withstand high temperatures and pressures. The parameters chosen for the pipes modelled in TRNSYS are described and justified in the Annex Heat Exchangers Four different heat exchangers are included in the model in the district heating network, as shown in the Figure 32. They are not actually 4 heat exchangers, but 4 different groups or stations, with several heat exchangers on each one. The type 5 (counter flow heat exchanger) have been used in TRNSYS to simulate these heat exchangers, since it is more efficient than the parallel flow and the cross-flow heat exchangers. Firstly, the heat exchanger called HE-1 has been used to connect the BTES and the district heating network. A very large heat transfer area is required for this heat exchanger to be able to transfer enough heat from the BTES to cover the heating demand during winter. 4 paraflow plate heat exchangers APV Z-390, with a heat transfer area of 2535 m 2 each one, have been selected for the heat exchanger HE-1 used in the model. Secondly, the solar field and the district heating network have been connected through the heat exchanger called HE-4, which requires also a large heat transfer area due to the large number of solar collectors in the solar field and, consequently, the large amount of solar heat that needs to be transferred to the district heating net. 22 units of the flat plate heat exchanger GEA Model FP15x34AL-280-FB have been chosen to represent the heat exchanger HE-4. Finally, the heat exchangers HE-2 and HE-3 have been designed to share the heat between the single-family dwellings, apartments and industries. For example, if some single-family dwelling has an energy surplus in its installation (due to the solar gains) and one apartment has an energy deficit, the excess heat will be sent from the house to the apartment by injecting energy into the district heating net through the heat exchanger. In this way, the losses of the district heating net will be lower and the efficiency of the heating system will be higher. As it is possible to see in the Figure 32, there is only one heat exchanger between the singlefamily dwellings and the apartments ( HE-2 ) and another one between the apartments and the industries ( HE-3 ). However, this is a simplification of the model. In reality, there will be one heat exchanger per single-family dwelling and per apartment block, so the heat transfer could be done not only between one house and one apartment, but also between two different houses or two different apartments. Therefore, the heat exchanger HE-2 is composed of 1666 heat exchangers (Flat Plate Heat Exchanger Model FG5X12-20) and the heat exchanger HE-3 is composed of 125 heat exchangers (Flat Plate Heat Exchanger Model FG10X12-80). 28

30 Method. Heating System The parameters of these 4 heat exchangers are described in the Annex 3. One of the main parameters of the heat exchangers is their overall heat transfer coefficient (given as UA [W/K] in TRNSYS). A value of 1000 W/m 2 K has been assumed for all the heat exchangers, as it is a common heat transfer coefficient value for a water-to-water heat exchanger [32]. Even though the heat exchanger of the solar field ( HE-4 ) does not work with water in both sides of the heat exchanger (in one side water and in the other side a mixture of water and propylene glycol), the same heat transfer coefficient value (1000 W/m 2 K) has been assumed, since the density of the mixture is almost the same than the density of the water Variable Speed Pump Due to the low temperatures of the district heating network and the large energy that needs to be covered during winter, a high flow rate in the district heating net is required. Moreover, it needs to be variable, since the heating demand is much lower in summer than in winter. Therefore, a variable speed pump has been used to vary the flow rate in the district heating network. The type 110 has been used in TRNSYS to model the variable speed pump. A maximum flow rate of 2000 m 3 /h has been used during the coldest months (when the outdoor temperature is lower than 0 C) and a minimum flow rate of 400 m 3 /h has been used during the warmer months (when the outdoor temperature is higher than 17 C). A linear function has been assumed when the temperatures vary between 0 C and 17 C. This assumption has been made based on the performance of the district heating network in Piteå, which is shown in the following figure. Figure 33. Flow rate variation depending on the outdoor temperature in the district heating network of Piteå [33] The following equations have been used in TRNSYS to calculate the signal that needs to be sent to the variable speed pump to adjust the flow rates at every time. T GT17 = GT(Tamb, 17) (1) T LT0 = LT(Tamb, 0) (2) 29

31 Method. Heating System flow pumpdhn = ( flow min flow max Tamb + flow 17 max ) (1 T GT17 ) (1 T LT0 ) + flow max T LT0 + flow min T GT17 (3) signal pumpdhn = flow pump dhn flow max (4) These equations are included in the model in the calculator sig_pump. On the other hand, 10 pumps CMX-80/160A of 18,5 kw and 210 m 3 /h each one, have been selected to achieve the total flow rate of the district heating net. These pumps are represented in the model by one single pump ( P_dhn ), whose parameters are described in the Annex Valves There are several valves in the district heating network, whose parameters are described in the Annex 3. Firstly, the valve T1 splits the flow of the district heating net in 3 different parts. One fraction is sent to the single-family dwellings, another fraction is sent to the apartments and the last fraction is sent to the industries. These fractions have been calculated as fractions of the total energy consumed per each consumer (Table 2). Frac E,dh, houses Table 2. Monthly energy fractions Energy fractions E,heat,dh Frac E,dh,ap Frac E,dh,ind 0,37 0,26 0,37 0,37 0,26 0,38 0,37 0,26 0,37 0,38 0,28 0,34 0,40 0,31 0,29 0,45 0,39 0,16 0,46 0,40 0,14 0,44 0,37 0,20 0,40 0,30 0,30 0,38 0,28 0,34 0,37 0,26 0,37 0,37 0,26 0,37 0,40 0,30 0,30 Secondly, the valves T2, T3, T6, T7, T11 and T12 have been used to distribute the energy along the different houses, apartments and industries. The valve T2 splits the flow into the energy fraction required by one single-family dwelling and the energy fraction required by the remaining single-family dwellings. The performance of the heating installation of one singlefamily dwelling is then simulated and the outlet temperature of the single-family dwelling installation is assumed to be the same than the outlet temperature of the remaining singlefamily dwellings, i.e. we simulate only the performance of one single-family dwelling installation and we assume that the heating installations of the remaining houses work in the way. This is 30

32 Method. Heating System done by setting in T3 the same inlet temperature in its two inputs. T6, T7, T11 and T12 work in the same way for the apartments and the industries, respectively. Thirdly, the function of the valves Bypass-1, Bypass-2, Bypass-3 and Bypass-4 is to bypass the heat exchangers HE-2 and HE-3 when there is no need to inject heat from the single-family dwellings to the apartments and from the apartments to the industries, respectively. The control signals of these bypass valves are calculated in the calculators Sig_ap ( Bypass-1 and Bypass-2 ) and Sig_ind ( Bypass-3 and Bypass-4 ). The following equations are used to know the status of the consumers installations (either energy deficit or energy surplus). Signal deficitsfd = GT(Tin_he3_sfd, Tout_he3_sfd) (5) Signal deficitap = GT(Tin_he3_ap, Tout_he3_ap) (6) Signal deficitind = GT(Tin_he2_ind, Tout_he2_ind) (7) Lastly, the return water coming from the single-family dwellings, apartments and industries is mixed in the valve T10, before reaching the solar field. Another two valves ( Bypass-5 and T14 ) need to be considered to bypass the heat exchanger of the solar field when there is no solar gain Borehole Thermal Energy Storage A Borehole Thermal Energy Storage (BTES) is used to cover the energy demand during winter, when there is no solar gain, and to store the surplus energy produced by the solar collectors during summer. The main feature of the BTES designed in this project is that it works without a heat pump. In this way, the maintenance and operational issues are reduced as most of the operational issues in a Ground Source Heat Pump (GSHP) system are due to the heat pumps. Moreover, the electricity consumption is reduced when no heat pumps are used so a higher fraction of the energy demand can be covered by renewable sources (e.g. the sun and the ground). Type 557 has been used in TRNSYS to model the BTES. This component ( vertical U tube or tube in tube ground heat exchanger ), which has been obtained from Tess Libraries and was developed by Hellström G. (1989) at Lund Institute of Technology [34], models a vertical ground heat exchanger that interacts thermally with the ground [35], i.e., it calculates the heat transfer in the ground. In this model, boreholes are assumed to be placed uniformly within a cylindrical storage volume of ground. Convective and conductive heat transfer are assumed within the pipes and in the storage, respectively. Moreover, the surrounding ground temperature is obtained by the superposition of three different solutions: a global temperature (large-scale heat flows from the storage to the surrounding ground), a local radial solution (heat transfer between the heat carrier fluid and the storage that covers short term variations) and a steady-flux solution (heat injection or extraction pulses of long duration). The steady-flux solution is calculated analytically with pre-calculated g- functions, while the global and local solutions are solved by using an explicit finite difference method. G-functions are non-dimensional temperature response factors that represent the 31

33 Method. Heating System temperature response to a constant heat injection pulse for a certain time step [36], i.e. they represent the thermal behaviour of a specific borehole field configuration along the time. Lastly, a steady-state balance for the heat carrier fluid (eq. 8) is applied to calculate the outlet fluid temperature of the BTES (eq. 9) by the integration of the equation 1 along the tube, where c f is the heat capacity of the fluid, ṁ f is the mass flow of the fluid, T f(s,t) is the fluid temperature along the tube, T g is the surrounding ground temperature, α is the heat transfer coefficient between the fluid and the surrounding ground and β is the damping factor [36]. c f ṁ f T f s + α(t f T g ) = 0 (8) T f,out = βt f,in + (1 β)t g (9) β = e αl c f ṁ f (10) Two different versions of this model are available in TRNSYS: 557a and 557b. The main difference between them is that the fluid-to-ground thermal resistance is known from experimental data in the Type 557b, while it needs to be calculated in the Type 557a from the geometry and the thermal conductivity of the pipes and the grout. According to [18], the Type 557b is the most accurate of the types for groundwater filled boreholes. However, since we do not have any experimental data to calculate the thermal resistance, the Type 557a has been used in this project for simulating the BTES. The parameters chosen for defining this component are listed in the Annex 4, as well as a short justification of the entered values. Furthermore, Type 557a is also characterized by the following inputs and outputs. Table 3. BTES (Type 557a) inputs and outputs Type 557a Inputs Outputs Inlet fluid temperature Outlet temperature Inlet flowrate (total) Outlet flow rate (total) Temperature on top of storage Average storage temperature Air temperature Average heat transfer rate Circulation switch Heat loss through top of storage Heat loss through side of storage Heat loss through bottom of storage Internal energy variation Regarding the inputs to the BTES, both temperature on top of storage and air temperature are equal to the ambient temperature (given data). In order to analyse the influence of the other inputs in the performance of the BTES (output), as well as the influence of its main parameters, different simulations have been carried out considering different scenarios. The main conclusions drawn from the results of the simulations, which are described in the Annex 5, are highlighted below. An increase of the mass flow rate implies a decrease of both storage and outlet fluid temperatures during winter time (extracting energy from the BTES), while the situation 32

34 Method. Heating System is reversed in summer time (injecting energy into the BTES), increasing both storage and outlet fluid temperatures with the mass flow rate. An increase of the inlet fluid temperature means higher storage and outlet fluid temperatures throughout the year. An increase of the number of boreholes results in a higher heat transfer rate (both extracted and injected), a higher storage temperature and a higher outlet fluid temperature, both on winter and summer months. An increase of the spacing between boreholes leads to higher storage and outlet fluid temperatures during winter, due to the larger storage volume (higher heat transfer rate). For the same reason, lower storage and outlet fluid temperatures will be obtained in summer. An increase of the boreholes depth implies a higher heat transfer rate, a higher storage temperature and a higher outlet fluid temperature during winter time. On the other hand, a higher heat transfer rate, a lower storage temperature and a lower outlet fluid temperature will be reached in summer time if deeper boreholes are used. Regarding the BTES designed in this project, which is composed of a total of 2000 boreholes, almost 3 years of preheating have been considered to heat the storage up to 60 C, the required initial temperature. This initial temperature needs to be at least 60 C to be able to cover the heating demand of the whole city during winter. The preheating is assumed to be done by the solar field designed for the heating system, which is composed of a total number of solar collectors. According to the Figure 34, 2 years and 8 months are required to heat up the storage from the initial temperature (3,6 C) to 60 C. It has been assumed that, as a result of a good planning, the recharge of the storage will start shortly before the project completion, so the preheating lost time after the heating plant commissioning will be 2 years (value taken into account in the calculation of the payback time). Figure 34. Preheating of the BTES On the other hand, 125 pumps MAGNA F have been selected to pump the water along the 2000 boreholes, and special polyethylene pipes ( Rehau PE-Xa 32mm SDR-11 ) have been used in the BTES due to the high temperatures of the storage inlet fluid (a maximum of 94 C). More information about the pumps and the pipes is included in the Annex 4. 33

35 Method. Heating System Customers Heating Installations The demand of three different customers, the single-family dwellings, the apartment blocks and the industries, needs to be covered by the heating system. There is one heating installation in every house, apartment block and industry, whose main function is to cover the heating demand of the corresponding house, apartment block or industry. The consumption data described in the Annex 2 have been used to design and dimension the customers heating installations. Figure 35 identifies the customers heating installations in the TRNSYS model. The single-family dwellings installations are shown on the left, the apartment blocks installations in the middle and, finally, the industries installations on the right. These three kinds of heating installations are explained below in more detail. Figure 35. Customers heating installations in the TRNSYS model Single-family dwellings As previously told, 5000 inhabitants of the city live in 1666 single-family dwellings, each one of 120 m 2. These assumptions have been taken into account together with the consumption data described in the Annex 2 for designing and dimensioning the single-family dwellings heating installations. These installations must be able to cover all the hot water and space heating demand in the worst conditions, when the consumption peaks are highest. As a result of considering that all the heating demand in the houses is going to be the same during all the year, including the peaks of demand, our system is oversized, since in reality the peaks are not going to be at the same time in all the houses. This simplification has been also assumed for the apartment blocks and the industries, even though it means the oversizing of the heating system. There are four different circuits in the heating installation of every single-family dwelling, all of them around a water accumulator, which is the core of the heating installation. As shown in the Figure 36, the water tank has 3 internal heat exchangers that are used for the DHW circuit, the solar circuit and the connection with the district heating network. There is also one input and one output of the accumulator that are used for the space heating circuit. 34

36 Method. Heating System Figure 36. Connections in the heating installation of a single-family dwelling Figure 37 shows how the single-family dwellings heating installations have been modelled in TRNSYS. The main components are analyzed below in more detail. Figure 37. Modelling in TRNSYS of the single-family dwellings heating installation Solar collectors As previously explained, evacuated tube solar collectors have been selected in this project. The Type 71 has been used in TRNSYS to simulate the solar collectors since it models an evacuated tube collector. A total of 8 m 2 of solar collectors have been used per every single-family dwelling. All the parameters chosen for this component are described in the Annex 6. Another important parameter is the heat carrier fluid used in the solar circuit. A mixture of 50% water and 50% propylene glycol (freezing point -34 C) has been used to avoid freezing issues. Lastly, the solar circuit works with a high flow rate to decrease the outlet temperature of the solar collectors, since the water tank does not tolerate temperatures higher than 100 C. In this way, temperatures lower than 100 C are achieved in the tank, so working with high pressure in the solar circuit is not necessary. When working with high pressure (the higher pressure the higher boiling point of the fluid) the costs are higher since more expensive pumps are required to withstand the higher temperatures Water tank There is a water tank of 1000 litres in every single-family dwelling. This accumulator has 3 internal coil heat exchangers, as previously explained in the Figure 36. The placement of the 35

37 Method. Heating System different internal heat exchangers, as well as the equations used for their dimensioning are explained in the Annex 7. The type 534 has been used in TRNSYS to model the water tank. This subroutine models a fluidfilled, constant volume storage tank with immersed heat exchangers, and divided into isothermal temperature nodes (to model the stratification in the tank). The parameters chosen for this component are described in the Annex Valves The valves T2 and T3, which were already explained in the District Heating Network section, are used to split the flow rate among the different single-family dwellings, i.e. to distribute the energy coming from the district heating network among the different houses. On the other hand, the type 13 has been used to model a pressure relief valve in the solar circuit. This pressure relief valve ( PRV-1 ) discards the vapour when the liquid begins to boil. A boiling point of 106 C has been considered for the mixture of 50% water and 50% propylene glycol (1 bar) since the solar loop is not working with high pressure (the higher pressure the higher boiling point of the fluid) Heat exchanger A heat exchanger is required in the space heating circuit of every single-family house to keep the underfloor heating system temperature at 35 C. By using this heat exchanger, the temperature at the cold side of the heat exchanger will be always around 35 C, even though the temperature at the hot side of the heat exchanger is high, since only the heat required to get 35 C will be transferred through the heat exchanger. The type 650 has been used in TRNSYS to model these heat exchangers. This kind of heat exchanger can bypass hot side fluid around the heat exchanger in order to maintain the cold side outlet temperature below a certain limit, according to the Figure 38. Figure 38. Bypass heat exchanger (Type 650) Some problems have arisen when simulating these heat exchangers in TRNSYS since, at certain times, when the temperature of the heat exchanger hot side is quite high and the hot side fluid is bypassed almost in its entirety, the temperature of the heat exchanger cold side starts rising substantially above 35 C. This does not make sense since the heat transferred from the hot side to the cold side should be zero when all the fluid is bypassed, so the temperature should not increase above 35 C. We think this simulation error is due to an internal error in the component (Type 650). We have solved this problem by increasing quite a lot the flow rate of the pump P1_sh-2. However, this high flow rate has only been considered for the simulation of the 36

38 Method. Heating System model but not for the calculation of the pumping consumption, as the flow rate will be much lower in reality. The parameters of these heat exchangers are described in more detail in the Annex Pumps Four different pumps are required for each heating installation, as shown in the Figure 37. One pump is used in the solar circuit, another one in the DHW circuit and, finally, two more pumps are required in the space heating circuit. The type 3 has been used in TRNSYS to model the different pumps described above. The parameters of these pumps are described in the Annex 6. Regarding the solar circuit, a controller has been used to switch the pump off when the solar collectors do not receive solar heat. Type 2 has been used in TRNSYS to model the controller. This component compares two different temperatures and, considering two dead band temperature differences, switches the pump off or on depending on the temperature differences Loads Since we know the heating load (both DHW and space heating) required by the single-family dwellings (calculated in the Annex 2), the type 682 has been used in TRNSYS to model the DHW and the space heating loads of every single-family dwelling. This component calculates the outlet temperature of a fluid from the inlet fluid temperature, the flow rate and the heating load. The component L1_dhw models the DHW load of one single-family dwelling while the component L1_sh models the space heating load of one single-family dwelling Apartment blocks The heating demand of 125 apartment blocks, each one with 20 apartments distributed in 5 floors, has been considered for designing and dimensioning the apartment blocks heating installation, which is very similar to the heating installation of the single-family dwellings. As shown in Figure 39, there are 4 different circuits (solar circuit, DHW circuit, space heating circuit and district heating net circuit) around the water accumulator, like in the single-family dwellings installations. The accumulator has the same configuration as the one shown before in the Figure 36. The only difference between this accumulator and the accumulator of the single-family dwellings is its volume, which needs to be larger in the apartment blocks due to greater heating demand. 37

39 Method. Heating System Figure 39. Modelling in TRNSYS of the apartment blocks heating installation The types used in TRNSYS to model the different components of the system (solar collectors, water tank, heat exchanger, etc.) are exactly the same than the types used in the single-family dwellings installations. For this reason, these components are not explained again. The only difference are the parameters of these components, which differ from the parameters of the single-family dwellings installations due to the different heating demand. The parameters chosen for the heating installations of the apartment blocks are described in the Annex 6. It is important to highlight that we have considered two different heating installations per apartment block, i.e. there are two accumulators (of 5000 litres) per apartment block (20 apartments). Nevertheless, we have only simulated in the TRNSYS model one heating installation, which covers the heating demand of 10 apartments, and we have assumed the remaining heating installations works in the same way. This has been done by splitting the flow rate in the valve T6 according to the number of heating installations (250) in all the apartment blocks (125), as previously explained in the district heating network section Industries As previously told, since we do not have any information about the geometry and the dimensions of the industries, we have assumed that there are in total 500 buildings in the city with the same energy requirements. A heating installation has been designed and dimensioned for each one of these 500 buildings. The design of this heating installation is similar to the heating installations in the single-family dwellings and in the apartment blocks, except for the fact that there is no solar loop in the heating installation of the industries, since solar collectors have not been placed on the roofs of these buildings. Consequently, there are only three circuits around the accumulator, the DHW circuit, the space heating circuit and the district heating net circuit. The configuration of the water tank is the same than the one previously shown in the Figure 36, but without the solar loop. Therefore, it only has two internal heat exchangers instead of three. More information about the design of this accumulator is included in the Annex 7. Figure 40 shows how the industries heating installations have been modelled in TRNSYS. The types used in TRNSYS to model the different components of the system (water tank, heat exchanger, pumps, etc.) are exactly the same than the types used in the single-family dwellings installations. For this reason, these components are not explained again. The only difference are 38

40 Method. Heating System the parameters of these components, which differ from the parameters of the single-family dwellings installations due to the different heating demand. The parameters selected for the heating installation of one industry are described in the Annex 6. Figure 40. Modelling in TRNSYS of the industries heating installation Solar Field A solar field has been designed and dimensioned in our heating system mainly to recharge the BTES during summer but also to cover the industries heating needs during summer, since solar collectors have not been placed on the roofs of the industries. In order to maximize the heat injected in the storage, the solar field has been placed just before the BTES, as shown in the Figure 31. In this way, the heat transferred from the solar field to the district heating network will be send directly to the BTES through a heat exchanger. The main components of the solar field designed and dimensioned in TRNSYS are described below Solar collectors A total of solar collectors (each one of 2 m 2 ) have been used in the solar field. This large number of solar collectors is required to recharge sufficiently the storage to be able to finish the year at the same storage temperature than at the beginning of the year. In this way, the BTES will work properly during the following years, i.e. it will be able to cover the winter heating demand of the city over the coming years. Evacuated tube solar collectors have been used in the solar field. These solar collectors have been modelled in TRNSYS by using the Type 71. All the parameters chosen for this component are described in the Annex 8. One important parameter is the heat carrier fluid used in the solar circuit. A mixture of 50% water and 50% propylene glycol (freezing point -34 C) has been used to avoid freezing issues. Furthermore, a high flow rate has been selected for the solar circuit to decrease the outlet temperature of the solar collectors, since very high temperatures imply high temperatures at the storage inlet, which is not beneficial as more expensive materials would be required for the pipes of the BTES to be able to withstand the high temperatures (common polyethylene pipes usually withstand up to C) Pressure relief valve The type 13 has been used in TRNSYS to model a pressure relief valve in the solar circuit. This pressure relief valve ( PRV-3 ) discards the vapour when the liquid begins to boil. We have 39

41 Method. Heating System considered that the solar field works with high pressure (6 bar), not like in the single-family dwellings and the apartment blocks (1 bar). Consequently, the boiling point of the glycol mixture (50% water and 50% propylene glycol) rises up to 171 C [37]. This has been done to reduce the losses in the pressure relief valve, due to the high temperatures achieved in the solar field as a consequence of using a large number of solar collectors Pump The type 3 has been used in TRNSYS to model the different pumps required in the solar field. In total, 55 pumps of 10 kw have been chosen to pump the heat carrier fluid around all the solar field. The parameters selected for the solar field pump modelled in TRNSYS are described in the Annex 8. Several controllers are used to switch the pumps off when the solar collectors do not receive solar heat. Type 2 has been used in TRNSYS to model the controller. This component compares two different temperatures and, considering two dead band temperature differences, switches the pump off or on depending on the temperature differences. 40

42 Results and Discussion 4. Results and Discussion This section analyses the main results obtained for the heating system designed and dimensioned in this project. As previously told, the flow diagram included in the Annex 9 has been used for dimensioning the heating system. Two different parts are analysed. Firstly, the results obtained from the simulations carried out in TRNSYS of the model are described and discussed. Secondly, an economic analysis has been done to calculate the profitability of the project Model results As it is shown in the Annex 1, several printers (Type 25) and plotters (Type 65) have been used in the TRNSYS model to measure its different parameters, e.g. temperatures, energy rates, energy losses, etc. Due to the model complexity, the results obtained in the different parts of the model are analysed below separately Borehole Thermal Energy Storage (BTES) One of the criteria followed for the dimensioning of the heating plant was to use a number of solar collectors in the solar field large enough to recharge the BTES during summer. Consequently, the storage temperature at the end of the year must be almost the same than the storage temperature at the beginning of the year. As it is possible to check in Figure 41, the average temperature of the storage (in brown) starts at 60 C and finishes the year at 60 C. There is a temperature variation of roughly 10 C in the storage during the year. The minimum storage temperature (55,8 C) is recorded in mid-march, while the maximum temperature (65,4 C) is recorded in early October. Figure 41. Average and outlet storage temperature and heat transfer rate in the storage throughout the year On the other hand, the outlet temperature of the storage is also shown in the Figure 41 (in blue), as well as the energy transfer rate in the storage (in grey). It is easy to see in this figure that energy is extracted from the BTES during winter time (October - February) while it is injected into the storage during the rest of the year (March - September). In total, the amount of energy injected in the storage throughout the year is higher than the energy extracted from it, even 41

43 Results and Discussion though the initial and final temperatures of the storage are the same, since the losses in the storage need to be considered. Another simulation for 10 years of operation has been carried out in TRNSYS to verify the performance of the BTES over the years. The results of the simulation, which are plotted in the Figure 42, show the proper functioning of the BTES over the years, as the annual variation of the storage average temperature (in brown) remains constant, as well as the annual heat transfer rate in the storage (in grey). Figure 42. Average storage temperature and heat transfer rate in the storage over 10 years Nevertheless, although the system works properly over the years in these conditions, it is necessary to consider the fact that the solar collectors efficiency decreases year by year, which cannot be considered in the TRNSYS model. Consequently, the results would worsen slightly since less energy would be injected to the BTES. On the other hand, the annual heat losses of the BTES are represented in the Figure 42 by the vertical grey line obtained at the end of every year District Heating Network The temperatures at the beginning (after the BTES) and at the end (before the solar field) of the district heating network are shown in the Figure 43 in red and green, respectively. 42

44 Results and Discussion Figure 43. District heating network temperatures throughout the year (I) During winter, the initial temperature (in red) is always higher than the final temperature (in green) since the heat is extracted from the BTES and cooled down by the consumers to cover their demand. However, the consumers demand will be met in summer with the solar collectors placed on the roof of the houses and apartments and the surplus heat coming from the houses and the apartments will be used, in addition to the solar field, to cover the industries demand and to recharge the storage, by increasing the temperature of the net. For this reason, both initial and final temperatures of the district heating net are similar in summer, as shown in the figure. The inlet temperature to the BTES is also plotted in the Figure 1 (in blue). This temperature is higher than the two previously described as it is the fluid temperature after the solar field. Due to the large solar field size, the energy transferred from the solar field to the fluid of the district heating net is quite large, especially in the warmer months. Consequently, the inlet temperature of the storage greatly increases during summer, with peak temperatures up to 94 C. Since standard polyethylene pipes do not withstand these high temperatures, special and more expensive polyethylene pipes need to be used in the BTES. In particular, Rehau PE-Xa 32 mm SDR-11 pipes (T max = 100 C) have been used in the BTES. On the other hand, the district heating network temperatures at the outlet of the single-family dwellings (in blue), apartments (in magenta) and industries (in orange) are plotted in the Figure 44. The temperatures raise during summer due to the solar heat coming from the solar collectors placed on the roof of the single-family dwellings and the apartment blocks. The temperature at the outlet of the industries is lower than the temperatures at the outlet of the single-family dwellings and the apartment blocks since no solar collectors have been placed on the industries roof. Nevertheless, this temperature is quite high during summer due to the heat injected from the solar field and the solar installations of the single-family dwellings and apartments to the district heating network. It is thus possible not only to cover the industries heating demand but also to recharge the storage. 43

45 Results and Discussion Figure 44. District heating network temperatures throughout the year (II) Figure 44 also shows that the temperatures reached at the outlet of the apartments installation are higher than the temperatures at the outlet of the single-family dwellings installation. This is due to the larger number of solar collectors installed on the apartment blocks (64 m 2 of solar collectors per apartment block and 8 m 2 of solar collectors per single-family dwelling) Customers Heating Installations Single-family dwellings The temperatures of the DHW and space heating circuits in the single-family dwellings are plotted in the following figure in orange and blue, respectively. These temperatures are always above 35 C, which is the constraint that must be verified. Figure 45. Temperatures and monthly energy demand of the single-family dwellings heating installation throughout the year Maximum peak temperatures of approximately 87 C are achieved during summer at the top of the water tank. 44

46 Results and Discussion The underfloor heating temperature, which should be kept constant at 35 C, is also shown in the Figure 45 in light blue. On the other hand, the accumulated heating demand per month (both DHW water and space heating) is represented in the Figure 45. The energy required for DHW (in dark blue) is almost constant for every month, while the space heating demand (light blue) is much larger during winter months than in summer, when the heating demand is almost zero Apartment Blocks The temperatures of the DHW and space heating circuits in the apartment blocks are plotted in the Figure 46 in magenta and blue, respectively. These temperatures are always above 35 C, which is the constraint that must be verified. Figure 46 represents the outputs of only one water tank of an apartment block, which corresponds to 10 apartments (there are two water tanks per every apartment block). The other tank of the apartment block is assumed to work in exactly the same way. Figure 46. Temperatures and monthly energy demand of the apartments heating installation (10 apartments) throughout the year Maximum peak temperatures of approximately 95 C are achieved during summer at the top of the water tank. These temperatures are higher in the apartment blocks than in the single-family dwellings due to the larger solar collector area used in the apartment blocks (64 m 2 per apartment block and 8 m 2 per single-family dwelling). On the other hand, the accumulated heating demand per month (both DHW water and space heating) is also represented in the Figure 46. The energy required for DHW (in brown) is almost constant for every month, while the space heating demand (in pink) is much larger during winter months than in summer, when the heating demand is almost zero Industries The temperatures of the DHW and space heating circuits in the industries are plotted in the Figure 47 in magenta and blue, respectively. These temperatures are always above 35 C, which is the constraint that must be verified. 45

47 Results and Discussion Figure 47. Temperatures and monthly energy demand of the industries heating installation throughout the year Maximum peak temperatures of approximately 78 C are achieved during summer at the top of the water tank. These temperatures are lower in the industries than in the apartment blocks and in the single-family dwellings since there is no solar circuit in the industries heating installation. On the other hand, the accumulated heating demand per month (both DHW water and space heating) is also represented in the Figure 47. The DHW demand (in pink), which is very low, is almost constant for every month, while the space heating demand (in light blue) is much larger during winter months than in summer, when the heating demand is almost zero. It should also be noted that the space heating demand in the industries is much larger than its DHW demand. The performance of the heating system designed has been measured not only with the figures previously shown but also by doing an annual energy balance of the overall system, which includes a monthly summary of the main components of the system. This information is contained in the Annex Economic Analysis An economic analysis has been carried out in order to estimate the profitability of the heating system designed. Firstly, the costs of the project have been estimated, according to the information included in the Annex 11. One of the main costs in this kind of projects are usually the construction and procurement costs, which are listed in the Table 4. It can be highlighted that the solar field has the highest cost, followed by the district heating network. 46

48 Results and Discussion Table 4. Procurement and Construction Costs Construction and Procurement Costs (M ) BTES 19,08 Single-family dwellings 15,02 Apartment blocks 9,86 Industries 6,51 Solar field 32,44 DHN 23,12 Total 106,02 Secondly, once the cost estimation has been done, three economic indicators, the Net Present Value (NPV), the Internal Rate of Return (IRR) and the payback time, have been calculated to determine the economic feasibility of the project. The figures obtained for the economic indicators in this project are summarized below. Table 5. Economic indicators of the project NPV (M ) IRR (%) Payback time (year) -76,728 1,31 39,53 The calculation of these economic indicators is explained in more detail in the Annex 11. The results shown in the Table 5 can be analysed in three different parts, the NPV, the IRR and the payback time. Firstly, a negative NPV has been achieved in this project, even though a large period of time has been considered (50 years). This means that the project is not profitable, since the NPV should be higher than zero. Secondly, a value of 1,31% has been obtained for the IRR, which is lower than the interest rate assumed (5%). This means again that the profitability of the heating system designed is not good. Finally, a payback time of almost 40 years would be needed to recover the initial investment, which is such a long period of time. Therefore, all the economic indicators calculated show the non-profitability of the heating system designed. Nevertheless, the project described in this master thesis is not only about designing an energy plant to cover the energy demand of a city, but also about using a network all around the city to connect all the elements of the city. This would improve the quality and the safety of the city heating system, as well as its efficiency, but it would require also a larger investment. Due to this fact, it would be reasonable that either the government or the administration of the city would provide any kind of subsidy to the project, since it would be beneficial for the city development. In such case, the project profitability would improve. 47

49 Conclusions and Future Research 5. Conclusions and Future Research The conclusions drawn in this project are described in this section, as well as some proposals that could be developed in future research. According to the economic results described in the previous section, the most obvious conclusion is the non-profitability of the heating system designed, due to the bad economic indicators achieved (NPV, IRR and payback time). However, as previously told, it would be reasonable that either the government or the administration of the city would provide any kind of subside to the project, since it would be beneficial for the city development. In such case, the project profitability would improve. Secondly, regarding the recharge of the BTES with solar heat, two conclusions can be highlighted. On the one hand, the fact that recharging the storage with solar heat is quite expensive. As shown in Table 4, within the procurement and construction costs of the project, the solar field has the highest costs due to the large number of solar collectors required to be able to recharge the storage during summer. Consequently, another renewable energy source as for example biomass (there is a lot of biomass production in the north of Sweden) could be used to recharge the BTES instead of solar heat in order to reduce costs. An economic study of the different alternatives for recharging the storage could be carried out in the future to economically optimize the recharge of the BTES. On the other hand, the heat transferred from the solar field to the BTES, i.e. the recharge of the storage, is larger if we send directly the heat through a heat exchanger without using a large storage accumulator in the solar field since the inlet temperatures to the BTES are higher. Nevertheless, by using a large storage accumulator in the solar field it is possible to spread the injection time and decrease the temperature peaks, which could be beneficial since the losses would be reduced and cheaper pipe materials could be used. Therefore, it would be interesting to analyse in more detail in future research the possibility of using a large storage accumulator in the solar field. The limitation on the number of solar collectors placed on the roof of the single-family dwellings and the apartment blocks is another important conclusion drawn. This means that using a lot of solar collectors on the roof of the houses and apartments is not beneficial since it would mean higher temperatures and higher losses, due to the fact that the water accumulator does not tolerate high temperatures (higher than 100 C). Therefore, a solar collectors area considerably smaller than the roof surface area available must be chosen to increase the heating system efficiency. It is also beneficial for the system using a flow rate in the solar collectors circuit as large as possible to decrease the peak temperatures at the outlet of the solar collectors. Lastly, a positive conclusion can be remarked, which is the fact that the BTES dimensioned in this project is able to cover the whole heating demand of the city during winter without using heat pumps. In this way, quite operational and maintenance issues are avoided. Regarding the DHW temperatures, a special heat exchanger (Duck Foot Heat Exchanger) designed by Luleå University of Technology has been considered to kill bacteria, such as Legionella, when the water temperatures inside the accumulators are low. However, this heat exchanger has not been analysed in detail due to lack of time. Therefore, it would be interesting to analyse in more detail in future research not only how this kind of heat exchanger works but also its consumption and cost. 48

50 Conclusions and Future Research Furthermore, another interesting idea that could be developed in future research is combining the heating system designed with a cooling system to be able to cover not only the heating demand of the city but also its cooling demand, e.g. the industries cooling demand. In this way, the efficiency of the heating system would improve. Finally, it would be also interesting to study the heating system designed in a different emplacement with more suitable climate conditions, by making some changes in the heating system dimensioning, to check how the profitability of the project would improve. 49

51 References 6. References [1] BP, June 2016, BP Statistical Review of World Energy June [2] Ignacio de Lis, October Artic Solar City Project, Luleå University of Technology. [3] Elisabeth Kjellsson, Solar Collectors Combined with Ground-Source Heat Pumps in Dwellings Analyses of System Performance, Lund University. [4] Statistics Sweden, Households' housing 2015: Households in multi-dwelling buildings often smaller, Statistical news from Statics Sweden. [5] Clean Energy Project Analysis RETScreen Engineering & Cases Textbook. [6] Jenny Lindblom. Natural energy resources. Solar thermal Application. LTU. [7] Meteotest database Meteonorm [8] World Energy Council Solar energy resources. [9] The European Solar Thermal Industry Federation (ESTIF) [10] Werner Weiss, AEE (Institute for Sustainable Technologies), Peter Biermayr (Vienna University of Technology). The European Solar Thermal Industry Federation (ESTIF). Study about the Thermal Solar energy potential in Europe. [11] IEA-ETSAP and IRENA, Thermal Energy Storage. Technology Brief. [12] Morten Vang Jensen, Seasonal pit heat storages Guidelines for materials & construction. Solar Heating & Cooling Programme, International Energy Agency. [13] Wittrup Sanne, Verdens største damvarmelager indviet i Vojens. Ingeniøren. [14] Sun & Wind Energy, Seasonal storage a German success story. [15] Bo Nordell, Mohamed Grein & Mohamad Kharseh, Large-scale Utilisation of Renewable Energy Requires Energy Storage. Luleå University of Technology, Sweden. [16] Thomas Schmidt, Solites, Solar district heating guidelines Storage. SDH Solar District Heating. [17] Bo Nordell, Underground Thermal Energy Storage (UTES). Div. Architecture and Water, Luleå University of Technology, Sweden. [18] Åsa Thorén, Practical evaluation of borehole heat exchanger models in TRNSYS. Division of Applied Thermodynamics and Refrigeration, KTH School of Industrial Engineering and Management, Sweden. [19] Jordi Jové Manonelles, Julio Large-scale Underground Thermal Energy Storage Using industrial waste heat to supply district heating. Escola Politècnica Superior, Universitat de Lleida, Spain. [20] Lavinia Gabriela Socaciu, Seasonal Sensible Thermal Energy Storage Solutions. Department of Mechanical Engineering, Technical University of Cluj-Napoca, Romania. 50

52 References [21] Kun Sang Lee, Underground Thermal Energy Storage. Department of Natural Resources and Environmental Engineering, Hanyang University, South Korea. [22] Bo Nordell & Göran Hellström, BTES for Heating and Cooling. Design Workshop, NATO Advanced Study Institute on TESSEC. [23] Georgi K. Pavlov, Bjarne W. Olesen, Seasonal solar thermal energy storage through ground heat exchangers Review of systems and applications. Department of Civil Engineering, Technical University of Denmark. [24] SAIC, June 28, Conference Drake Landing Solar Community. International District Energy Association (IDEA). [25] Bo Nordell, Large-scale Thermal Energy Storage. Div. Architecture and Water, Luleå University of Technology, Sweden. [26] Kjell Skogsberg, Natural Energy, Solar heat and Thermal Energy Storage. North Sweden Energy Agency & Luleå University of Technology. [27] Solar Utilities, renewable energy solutions. Solar Thermal (Hot Water) [28] Roger Sallent Cuadrado, Return temperature influence of a district heating network on the CHP plant production costs. Department of technology and built environment, University of Gävle, Sweden. [29] Gordon Howell, Borehole Field in the The Drake Landing Solar Community Okotoks, Alberta. American Association of Physics Teachers. [30] Peter Mildenstein. District Heating and Cooling Connection Handbook. Part II. Heating section. International Energy Agency IEA District Heating and Cooling. [31] Peter Kaarup Olsen, Christian Holm Christiansen, Morten Hofmeister, Svend Svendsen, Alessandro Dalla Rosa, Jan-Eric Thorsen, Oddgeir Gudmundsson, Marek Brand, Guidelines for Low-Temperature District Heating. Danish Energy Agency. [32] Andre G. McDonald & Hugh L. Magande, Introduction to Thermo-Fluids Systems Design. Appendix C. Heat Exchanger Design. [33] Daniel Eriksson, Pite Energi, Piteå, Sweden. [34] Hellström G., Duct Ground Heat Storage Model, Manual for Computer Code. Department of Mathematical Physics, University of Lund, Sweden. [35] TESS. TESS Component Libraries, General Descriptions. Thermal Energy System Specialists. [36] M. De Rosa, F. Ruiz-Calvo, J. M. Corberán, C. Montagud and L. A. Tagliafico, Borehole modelling: a comparison between a steady-state model and a novel Dynamic model in a real ON/OFF GSHP operation. University of Genoa & Universitat Politècnica de València. [37] Kjell Skogsberg, Solar heating systems design. North Sweden Energy Agency & Luleå University of Technology. 51

53 Annexes Annexes Annex 1. TRNSYS model Annex 2. Heating demand estimation Annex 3. District heating network parameters Annex 4. BTES parameters Annex 5. Influence of BTES inputs and parameters Annex 6. Customers heating installations parameters Annex 7. Accumulators design and dimensioning Annex 8. Solar field parameters Annex 9. Dimensioning procedure Annex 10. Energy balance Annex 11. Budget 52

54 Annex 1. TRNSYS model Annex 1. TRNSYS model Figure A1.1. Trnsys model of the heating system designed

55 Annex 2. Heating demand estimation Annex 2. Heating demand estimation We have estimated the heating demand of the houses, apartments and industries (including in this group not only industries, but also commercial centers, shops, etc.) of the fictive city studied in this project. This estimation has been validated by Pite Energi. The consumption data for each kind of building is explained in the next tables. Table A2.1. Data for single-family dwellings E,heating,per house E,dhw,per house Data for single-family dwellings kwh/year kwh/year E,dhw,per house,daily 9,3 kwh/day E,heat,total kwh/year Assumed house area 120 m2 E,specific 96,7 kwh/m2,year Inhabitants in houses persons Inhabitants per house 3 persons No of houses houses Group-factor 0,75 - P,max, dh system kw T,dhw,initial 5 o C T,dhw,final 50 o C T,dhw,increase 45 o C T,dhw,increase by dh 30 o C T,dhw,increase by electricity 15 o C BBR zon 1, houses 130 kwh/m2,year Miljöbyggnad silver, max 75% of BBR E,specific,max 97,5 kwh/m2,year Miljöbyggnad guld, max 65% of BBR E,specific,max 84,5 kwh/m2,year Table A2.2. Data for Apartments Data for apartments E,heating,per apartment kwh/year E,dhw,per apartment kwh/year E,dhw,per apartment,daily 5,5 kwh/day E,heat,total kwh/year Assumed house area 70 m2 E,specific 81,4 kwh/m2,year Inhabitants in apartments persons Inhabitants per apartments 2 persons No of apartments houses Group-factor 0,75 - P,max, dh system 4301 kw T,dhw,initial 5 o C

56 Annex 2. Heating demand estimation T,dhw,final 50 o C T,dhw,increase 45 o C T,dhw,increase by dh 30 o C T,dhw,increase by electricity 15 o C BBR zon 1, apartments 115 kwh/m2,year Miljöbyggnad silver, max 75% of BBR E,specific,max 86,25 kwh/m2,year Miljöbyggnad guld, max 65% of BBR E,specific,max 74,75 kwh/m2,year Table A2.3. Data for Indutries Data for industries etc. E,industries etc., proportion of homes 0,5 kwh/year E,industries etc.,total ,3 kwh/year Proportion dhw 0,1 E,industries etc.,dhw kwh/year E,industries etc.,heating kwh/year Group-factor 0,75 - P,max, dh system 5332,405 kw T,dhw,initial 5 o C T,dhw,final 50 o C T,dhw,increase 45 o C T,dhw,increase by dh 30 o C T,dhw,increase by electricity 15 o C The first part of the houses and apartments tables shows the energy that the respective buildings are going to consume (both space heating and hot water). After that, the dimensions of the buildings are used to calculate the specific energy per square meter. The next task alludes to the number of people who lives in each kind of building. The last part refers to the efficiency of the buildings. The industries table is similar but it does not include information about people and efficiency. As an important factor, the group factor must be defined, which is used to reduce the peak power and is related with the size of the buildings and the consumption rate (hot water and space heating). Boverket's building regulations, BBR, is the building code in Sweden and contains mandatory provisions and general recommendations. It considers three kinds of buildings and gives to each category a factor that establish the maximum limit of consumption in kwh/m 2 year. The three categories of buildings are bronze, silver and gold, depending on the sustainable character of the buildings. Once the tables are analysed, the consumption data for all the year needs to be calculated accurately with the temperature data that we have gained through Energykontor (North Sweden Energy Agency). In the tables A2.5, A2.6 and A2.7 the calculations for one hour are shown. They have been interpolated for the remaining hours of the year to obtain a reliable consumption data to cover with our heating system.

57 Annex 2. Heating demand estimation Regarding the data validated by Piteå Energy, the following distribution of the hot water consumption during the day has been considered. Table A2.4. Share dhw for houses, apartments and indutries Kl. Share dhw Kl. Share dhw Kl. Share dhw 0-1 0,0% 0-1 0,0% 0-1 1,0% 1-2 0,0% 1-2 0,0% 1-2 1,0% 2-3 0,0% 2-3 0,0% 2-3 1,0% 3-4 0,0% 3-4 0,0% 3-4 1,0% 4-5 0,8% 4-5 0,8% 4-5 1,0% 5-6 3,8% 5-6 3,8% 5-6 1,0% ,4% ,4% 6-7 1,0% 7-8 3,8% 7-8 3,8% 7-8 7,91% 8-9 1,9% 8-9 1,9% 8-9 7,91% ,9% ,9% ,91% ,0% ,0% ,91% ,5% ,5% ,91% ,8% ,8% ,91% ,0% ,0% ,91% ,0% ,0% ,91% ,8% ,8% ,91% ,7% ,7% ,91% ,1% ,1% ,91% ,2% ,2% ,0% ,7% ,7% ,0% ,8% ,8% ,0% ,8% ,8% ,0% ,0% ,0% ,0% ,0% ,0% ,0% The reference hour has been taken on March 8 th, between 8 and 9 am, as shown in the Tables A2.5, A2.6 and A2.7. The temperature of the air at that time was -0,5 C. The following parameter in the tables ( Heat-degree-days ) has been calculated by comparing the limit temperature with the air temperature, i.e. it represents the temperature difference that needs to be covered by the heating system due to the space heating requirements. Furthermore, the energy consumption of the different buildings for heating purposes has been calculated according to the following equations. Energy consumption for one building - Space heating consumption T heat degree day (1) E heating [kwh/h] = E heating per house T sum - Domestic hot water heating consumption E dhw [kwh/h] = E dhw per house daily S dhw (2)

58 Annex 2. Heating demand estimation - Total energy consumption Consumption for all the buildings - Space heating E total [kwh/h] = E heating + E dhw (3) E heating,dh [ kwh h ] = E heating N buildings - Domestic hot water heating (4) E dwh,dh [ kwh h ] = E dwh N buildings Tdwh increase by dh (5) T dwh increase - Domestic hot water heating covered by electricity E dwh,el [ kwh h ] = E dwh N buildings Tdwh increase by electricity (6) T dwh increase - Total energy consumption E heat,dh [ kwh h ] = E heating,dh + E dwh,dh (7) E heat total = E heat,dh + E dhw,el (8)

59 Annex 2. Heating demand estimation Table A2.5. Single-family dwellings consumption For one house Total T,air Heat-degree-days E,heating E,dhw E,total E,heating,dh E,dhw,dh E,dhw,el E,heat,dh E,heat,total No Date Kl. [oc] [oc,h] [kwh/h] [kwh/h] [kwh/h] [kwh/h] [kwh/h] [kwh/h] [kwh/h] [kwh/h] mar ,5 17,5 1,399 0,176 1, , ,985 97, , ,984 Table A2.6. Apartments consumption For one apartment Total T,air Heat-degree-days E,heating E,dhw E,total E,heating,dh E,dhw,dh E,dhw,el E,heat,dh E,heat,total No Date Kl. [oc] [oc,h] [kwh/h] [kwh/h] [kwh/h] [kwh/h] [kwh/h] [kwh/h] [kwh/h] [kwh/h] mar ,5 17,5 0,631 0,104 0, , ,963 86, , ,130 Table A2.7. Industries consumption Total T,air Heat-degree-days E,heating,dh E,dhw,dh E,dhw,el E,heat,dh E,heat,total No Date Kl. [oc] [oc,h] [kwh/h] [kwh/h] [kwh/h] [kwh/h] [kwh/h] mar ,5 17,5 2578, , , , ,743

60 Annex 2. Heating demand estimation The next tables (A2.8, A2.9 and A2.10) show the annual energy consumption in the single-family dwellings, apartments and industries. Table A2.8. Single-family dwellings annual consumption For one house Total for all houses T,limit T,air,mean E,heating E,dhw E,total E,heating,dh E,dhw,dh E,dhw,el E,heat,dh Month [oc] [oc] [kwh/månad] [kwh/månad] [kwh/månad] [MWh/h] [MWh/h] [MWh/h] [MWh/h] January 17-9, , , , , , , ,870 February 17-13, , , , , , , ,516 March 17-1, , , , , , , ,367 April 12 2, , , , , , , ,634 May 10 7, , , , , , , ,857 June 10 13,803 16, , ,670 27, , , ,465 July 10 15,560 2, , ,398 4, , , ,172 August 11 14,060 47, , ,022 78, , , ,530 September 12 7, , , , , , , ,941 October 13 4, , , , , , , ,451 November 17-4, , , , , , , ,424 December 17-1, , , , , , , ,728 MEAN/SUM 17 2, , , , , , , ,956

61 Annex 2. Heating demand estimation Table A2.9. Apartments annual consumption For one apartment Total for all apartments T,limit T,air,mean E,heating E,dhw E,total E,heating,dh E,dhw,dh E,dhw,el E,heat,dh Månad [oc] [kwh/månad] [kwh/månad] [kwh/månad] [MWh/h] [MWh/h] [MWh/h] [MWh/h] January 17-9, , , , , , , ,162 February 17-13, , , , , , , ,681 March 17-1, , , , , , , ,972 April 12 2, , , , , , , ,479 May 10 7,449 94, , , , , , ,735 June 10 13,803 7, , ,701 18, , , ,267 July 10 15,560 1, , ,050 2, , , ,073 August 11 14,060 21, , ,185 53, , , ,410 September 12 7, , , , , , , ,513 October 13 4, , , , , , , ,544 November 17-4, , , , , , , ,248 December 17-1, , , , , , , ,250 MEAN/SUM 2, , , , , , , ,333

62 Annex 2. Heating demand estimation Table A2.10. Industries annual consumption T,limit T,air,mean E,heating,dh E,dhw,dh E,dhw,el E,heat,dh Månad [oc] [MWh/h] [MWh/h] [MWh/h] [MWh/h] January 17-9, ,725 95,064 47, ,789 February 17-13, ,535 85,864 42, ,399 March 17-1, ,601 95,064 47, ,665 April 12 2, ,183 91,998 45, ,180 May 10 7, ,924 95,064 47, ,989 June 10 13,803 29,886 91,998 45, ,884 July 10 15,560 4,848 95,064 47,532 99,913 August 11 14,060 87,080 95,064 47, ,144 September 12 7, ,756 91,998 45, ,753 October 13 4, ,172 95,064 47, ,236 November 17-4, ,850 91,998 45, ,848 December 17-1, ,200 95,064 47, ,265 MEAN/SUM 13, , , , , ,064

63 Annex 2. Heating demand estimation Dimensioning the system in TRNSYS requires considering a huge variety of parameters as it has been explained during the report. For this reason, we have tried to simplify the procedure of modelling the system by fixing some variables as for example the flow rate for both the domestic hot water and the space heating circuits. Firstly, regarding the DHW circuits, the flow rates have been selected according to the maximum consumption on a day. m DHW ( kg h ) = E DHW ( kwh month ) T DHW increase by dh T DHW increase by dh T DHW increase by electricity ,18 T DHW increase by dh (9) m share DHW = m DHW 24 share (10) m share DHW is the value that we need for dimensioning the district heating system since the flow rate is not constant. We have a different consumption each hour, so it is necessary to know the maximum consumption on a day and dimensioning regarding on it. Secondly, regarding the space heating circuits, a first estimation of the flow rates has been calculated from the maximum space heating load per hour and assuming a temperature difference between the inlet and the outlet of the space heating element (underfloor heating system) of 7 C, approximately. ṁ = E SHmax c P (T 2 T 1 ) Nevertheless, the flow rates calculated have been modified slightly when working with TRNSYS to avoid non-convergence issues and to regulate the temperatures range of the space heating circuits. (11)

64 Annex 3. District heating network parameters Annex 3. District heating network parameters Length A simplification has been made to estimate the total length of the district heating network by splitting the network in three different parts, the single-family dwellings net, the apartment blocks net and the industries net. The distribution shown in the Figures A3.1 and A3.2 has been assumed in this project for the single-family dwellings and the apartment blocks. Figure A3.1. District heating network along 128 single-family dwellings Figure A3.2. District heating network along 32 apartment blocks Furthermore, three different circuits have been considered in the district heating network with decreasing pipe diameters, since the flow rate decreases as it moves forward along the net. The primary circuit, which has a large pipe diameter due to the high flow rate, distributes the energy around all the city. The secondary circuits, which have a smaller pipe diameter, take the energy from the primary circuit and distribute it along different districts. Finally, the tertiary circuits, which have the smallest diameter, take the flow rate from the secondary circuits and deliver it to the houses, apartments and industries. The figures A3.3 and A3.4 show the three main circuits considered in this project for the district heating network.

65 Annex 3. District heating network parameters Figure A3.3. Primary, secondary and tertiary circuits along 128 single-family dwellings Figure A3.4. Primary, secondary and tertiary circuits along 32 apartment blocks The lengths of the single-family dwellings net and the apartments net have been calculated by considering that the distribution shown in Figure A.3.1 for 128 houses is repeated 13 times (1666 houses) and that the distribution shown in the Figure A3.2 for 32 apartment blocks is repeated 4 times (125 apartment blocks). Regarding the industries, since we do not have any data about the geometry and dimensions of the different industries, a certain length has been assumed for each of the three different circuits of its district heating net. The lengths calculated and assumed for the different circuits of the district heating network (L1: primary circuit, L2: secondary circuit, L3: tertiary circuit) are written in the following table. Table A3.1. District Heating Network length DHN LENGTH SFD 1666 [Houses] Max flow rate Max Diameter* L [m] 2000 [m3/hr] 0, [m] L [m] 61, [m3/hr] 0, [m] L [m] 7, [m3/hr] 0, [m]

66 Annex 3. District heating network parameters Ap 125 [Ap] L [m] 2000 [m3/hr] 0, [m] L2 664 [m] 150 [m3/hr] 0, [m] L [m] 37,5 [m3/hr] 0, [m] Ind 500 [Ind] L [m] 2000 [m3/hr] 0, L [m] 60 [m3/hr] 0, L [m] 7,5 [m3/hr] 0, L1 additional** [m] Total L [m] Total L [m] Total L [m] TOTAL [m] * A maximum speed of 3,5m/s has been considered ** Additional piping between houses, apartments and industries, as well as piping to the solar field and to the BTES According to these figures, a DN450 pipe will be used in the primary circuit, a DN125 pipe will be used in the secondary circuits of the apartments, a DN80 pipe will be used in the secondary circuits of the single-family dwellings and the industries, a DN65 pipe will be used in the tertiary circuits of the apartments and, finally, a DN32 pipe will be used in the tertiary circuits of the single-family dwellings and the industries. However, only the DN450 pipe has been modelled in TRNSYS. Pipes Table A3.2. Parameters of DHN 1 (Type 709) DHN 1 (Type 709) PREMANT Rigid steel pre-insulated pipe DN450, Tmax 144 C, pmax 25bar [1] Inside diameter 0.45 m Due to the high flow rate of the DHN (max. 2000m 3 /h), a large pipe diameter is required to keep the fluid speed within the pipe below a certain limit (max. 3.5m/s). Outside diameter m Pipe length m Length of the DHN (primary side) Pipe thermal conductivity 16 W/m.K Steel Fluid density kg/m^3 Water Fluid specific heat kj/kg.k Water Fluid thermal conductivity kj/hr.m.k Water Fluid viscosity kg/m.hr Water Initial fluid temperature 55 C Number of nodes 1 - Insulation thickness 0.17 m Conductivity of insulation W/m.K Polyurethane foam (pentane-blown), manufactured from 3 components:

67 Annex 3. District heating network parameters Outer surface convection coefficient (input) polyol, isocyanate and cyclopentane 2 kj/hr.m^2.k [2], [3] Table A3.3. Parameters of DHN 2 (Type 709) DHN 2 (Type 709) PREMANT Rigid steel pre-insulated pipe DN450, Tmax 144 C, pmax 25bar [1] Inside diameter 0.45 m Due to the high flow rate of the DHN (max. 2000m 3 /h), a large pipe diameter is required to keep the fluid speed within the pipe below a certain limit (max. 3.5m/s). Outside diameter m Pipe length m Length of the DHN (secondary side) Pipe thermal conductivity 16 W/m.K Steel Fluid density kg/m^3 Water Fluid specific heat kj/kg.k Water Fluid thermal conductivity kj/hr.m.k Water Fluid viscosity kg/m.hr Water Initial fluid temperature 50 C Number of nodes 1 - Insulation thickness 0.17 m Conductivity of insulation W/m.K Polyurethane foam (pentane-blown), manufactured from 3 components: polyol, isocyanate and cyclopentane Outer surface convection coefficient 2 kj/hr.m^2.k [2], [3] Heat Exchangers Table A3.4. Parameters of HE-1 (Type 5) HE-1 (Type 5) Paraflow Plate Heat Exchanger BTES-DHN, Model APV Z-390, Amax 2535m 2, 4 units [4] Counter flow mode 2-2 indicates a counter flow arrangement Specific heat of hot side 4.19 kj/kg.k Water fluid Specific heat of cold side 4.19 kj/kg.k Water fluid Overall heat transfer coefficient of exchanger W/K Heat transfer area required: 10000m 2. U = 1000W/m 2 K (water-water) [5] Table A3.5. Parameters of HE-2 (Type 5) HE-2 (Type 5) Flat Plate Heat Exchanger SFD-DHN, Model FG5X12-20, 0.77m2, 20 plates, 1666 units [6] Counter flow mode 2-2 indicates a counter flow arrangement Specific heat of hot side fluid 4.19 kj/kg.k Water

68 Annex 3. District heating network parameters Specific heat of cold side fluid Overall heat transfer coefficient of exchanger 4.19 kj/kg.k Water W/K Heat transfer area required: 10000m 2. U = 1000W/m 2 K (water-water) [5] Table A3.6. Parameters of HE-3 (Type 5) HE-3 (Type 5) Flat Plate Heat Exchanger Ap-DHN, Model FG10X12-80, 10.32m2, 80 plates, 125 units [6] Counter flow mode 2-2 indicates a counter flow arrangement Specific heat of hot side 4.19 kj/kg.k Water fluid Specific heat of cold side fluid 4.19 kj/kg.k Water Overall heat transfer coefficient of exchanger W/K Heat transfer area required: 10000m 2. U = 1000W/m 2 K (water-water) [5] Table A3.7. Parameters of HE-4 (Type 5) HE-4 (Type 5) Heat Exchanger SF-DHN GEA Model FP15x34AL-280-FB, 280 plates, 92m 2, 22 units [6] Counter flow mode 2-2 indicates a counter flow arrangement Specific heat of hot side 3.56 kj/kg.k Water Propylene glycol (50-50) fluid Specific heat of cold side 4.19 kj/kg.k Water fluid Overall heat transfer coefficient of exchanger W/K Heat transfer area required: 2000m 2. U = 1000W/m 2 K [5] Variable Speed Pump Table A3.8. Parameters of P_dhn (Type 110) P_dhn (Type 110) Pump CMX-80/160A 18,5kW, 210m 3 /h 10 units [7] Rated flow rate kg/hr Maximum Flow rate of the DHN Fluid specific heat 4.19 kj/kg.k Water Rated power 185 kw 10 pumps of 18,5kW Motor heat loss fraction Default value Number of power 1 - Default value coefficients Power coefficient 1.0 kj/hr Default value Total pump efficiency Default value Motor efficiency Default value Valves Table A3.9. Parameters of the different valves in the DHN T1 (Type 647) Diverter valve

69 Annex 3. District heating network parameters Number of outlet ports 3 - Fraction of flow to outlet Energy fraction to the industries -1 Fraction of flow to outlet Energy fraction to the apartments -2 Fraction of flow to outlet Energy fraction to the single-family dwellings T2 (Type 647) Diverter valve Number of outlet ports 2 - Fraction of flow to outlet Energy fraction for 1665 single-family -1 Fraction of flow to outlet -2 dwellings (1665/1666) Energy fraction for 1 single-family dwellings (1/1666) T3 (Type 649) Mixer valve Number of inlets 2 - m out = m 1 + m 2 T out = (m 1T 1 + m 2T 2)/m out Bypass-1 (Type 11) Diverter valve Control signal 0/1 - Either 0 or 1, depending on the energy surplus coming from the houses and the energy deficit of the apartments Bypass-2 (Type 11) Diverter valve Control signal 0/1 - Either 0 or 1, depending on the energy surplus coming from the houses and the energy deficit of the apartments T4 (Type 649) Mixer valve Number of inlets 2 - m out = m 1 + m 2 T out = (m 1T 1 + m 2T 2)/m out T5 (Type 649) Mixer valve Number of inlets 2 - m out = m 1 + m 2 T out = (m 1T 1 + m 2T 2)/m out T6 (Type 647) Diverter valve Number of outlet ports 2 - Fraction of flow to outlet Energy fraction for 10 apartments -1 Fraction of flow to outlet -2 (half apartment block) (249/250) Energy fraction for 10 apartments (half apartment block) (1/250) T7 (Type 649) Mixer valve Number of inlets 2 - m out = m 1 + m 2 T out = (m 1T 1 + m 2T 2)/m out Bypass-3 (Type 11)

70 Annex 3. District heating network parameters Diverter valve Control signal 0/1 - Either 0 or 1, depending on the energy surplus coming from the apartments and the energy deficit of the industries Bypass-4 (Type 11) Diverter valve Control signal 0/1 - Either 0 or 1, depending on the energy surplus coming from the apartments and the energy deficit of the industries T8 (Type 649) Mixer valve Number of inlets 2 - m out = m 1 + m 2 T out = (m 1T 1 + m 2T 2)/m out T9 (Type 649) Mixer valve Number of inlets 2 - m out = m 1 + m 2 T out = (m 1T 1 + m 2T 2)/m out T10 (Type 649) Mixer valve Number of inlets 3 - m out = m 1 + m 2 + m 3 T out = (m 1T 1 + m 2T 2 + m 3T 3))/m out T11 (Type 647) Diverter valve Number of outlet ports 2 - Fraction of flow to outlet Energy fraction for 499 industries -1 Fraction of flow to outlet -2 (499/500) Energy fraction for 1 industry (1/500) T12 (Type 649) Mixer valve Number of inlets 2 - m out = m 1 + m 2 T out = (m 1T 1 + m 2T 2)/m out Bypass-5 (Type 11) Diverter valve Control signal 0/1 - Either 0 or 1, depending on the energy gain coming from the solar field T14 (Type 649) Mixer valve Number of inlets 2 - m out = m 1 + m 2 T out = (m 1T 1 + m 2T 2)/m out References [1] Premant, Premant Technical Catalogue [2] Eindhoven University of Technology. The thermal insulated pipe problem. [3] BRECSU & ETSU, 1996.The economic thickness of insulation for hot pipes. Government s Energy Efficiency Best Practice Programme, Department of Energy, Great Britain.

71 Annex 3. District heating network parameters [4] APV, An SPX Brand, APV Paraflow Plate Heat Exchangers For Chemical Process and Industrial Applications. [5] Andre G. McDonald & Hugh L. Magande, Introduction to Thermo-Fluids Systems Design. Appendix C. Heat Exchanger Design. [6] GEA, Hydronic List Prices. Flat Plate Heat Exchangers. Pennsylvania, USA. [7] AGP Bombas, Catálogo General. Lugo, Spain

72 Annex 4. BTES parameters Annex 4. BTES parameters Table A4.1. BTES Parameters BTES (Type 557a) Borehole Thermal Energy Storage 2000 boreholes, 200m depth, 4m spacing Storage volume m^3 Pi*Number of Boreholes*Borehole Depth*(0.525*Borehole Spacing)^2 Borehole spacing = 4m (typical value) Borehole depth 200 m Typical value Header depth 1.0 m The depth below the surface of the top of the u-tube ground heat exchangers. 1m (default value). Number of boreholes Borehole radius m A drilling diameter of mm is commonly used in Sweden [1] No. of boreholes in series 2 - No more boreholes in series since the larger pipe length (2x2x200m) the larger head losses of the pump Number of radial regions 1 - Simplification (default value) Number of vertical regions 1 - Simplification (default value) Storage thermal conductivity 3.4 W/m.K Ground properties in Luleå (taken from EED software) Storage heat capacity 2400 kj/m^3/k Negative of u-tubes/bore -2 - Double U-tube per borehole Outer radius of u-tube pipe m Pipe Rehau PE-Xa 32mm SDR-11, Topmax 93 C, Tmax 100 C Inner radius of u-tube pipe m Outer radius - thickness Center-to-center half m Common value distance Fill thermal conductivity 3.6 W/m.K Groundwater in Luleå [2] Pipe thermal conductivity 0.42 W/m.K PE-Xa 32mm SDR-11 Gap thermal conductivity 1.4 W/m.K Simplification (no gap between U-pipes Gap thickness 0.0 m and the fill material). Reference borehole flow rate 2160 kg/hr m 3 /s per borehole is commonly used [1] Reference temperature 60 C The reference fluid temperature for the calculation of the fluid to ground thermal resistance. Pipe to pipe heat transfer = Account for the heat transfer between the U-tube pipes Fluid specific heat 4.19 kj/kg.k Water Fluid density kg/m^3 Insulation indicator 2 - Insulation height fraction A layer of 1 m of soil is used as natural Insulation thickness 1 m insulator. Insulation thermal 1.4 kj/hr.m.k conductivity Number of simulation 25 - Typical value for BTES simulations years Maximum storage temperature C Assumption (default value)

73 Annex 4. BTES parameters Initial surface temperature 3.6 C Equal to the ground surface average temperature in Luleå (3.6) Initial thermal gradient C Geothermal gradient in Luleå [3] Number of preheating years 3 - Time required to heat the storage up to 60C with the energy coming from the solar collectors 70.0 C Assumption Maximum preheat temperature Minimum preheat 54.0 C Assumption temperature Preheat phase delay 192 Day 192 = (90 maximum storage temperature day (September 15 th (assumption))) Average air temperature 3 C Luleå average air temperature Amplitude of air 15 deltac Peak amplitude temperature Air temperature phase delay 295 day 295 = (90 hottest day (June 4th)) Number of ground layers 1 - Simplification (only 1 layer) Thermal conductivity of layer 3.4 W/m.K Ground properties in Luleå (taken from EED software) Heat capacity of layer 2400 any Thickness of layer 300 m Higher than borehole s depth (only 1 layer is considered) Circulation switch -1/1 - -1: the fluid circulates from the border to the center (heat abstraction) 1: the fluid circulates from the center to the border (heat injection) This value is calculated on the calculator Sig_btes by comparing the inlet and outlet temperatures of the storage Pbtes (Type 3) Pump Grundfos MAGNA F, DN32, 230V, 336W max, 10bar max, 125 units [4] Maximum flow rate kg/hr 2160kg/h per borehole Fluid specific heat 4.19 kj/kg.k Water Maximum power 50 kw Grundfos MAGNA F 336W Conversion coefficient Default value Power coefficient Default value Control signal 1 - The pump is always in operation References [1] Bo Nordell & Göran Hellström, BTES for Heating and Cooling. Design Workshop, NATO Advanced Study Institute on TESSEC. [2] Signhild Gehlin, Thermal Response Test In Situ Measurements of Thermal Properties in Hard Rock. Department of Environmental Engineering, Luleå University of Technology, Sweden.

74 Annex 4. BTES parameters [3] Jordi Jové Manonelles, Julio Large-scale Underground Thermal Energy Storage Using industrial waste heat to supply district heating. Escola Politècnica Superior, Universitat de Lleida, Spain. [4] Grundfos, Price List South-East Europe.

75 Annex 5. Influence of BTES inputs and parameters Annex 5. Influence of BTES inputs and parameters In order to analyse the influence of the inputs and the parameters of the BTES in its performance, different simulations have been carried out considering different scenarios. Firstly, the following scenario has been studied: BTES of 20 boreholes, 5 m spacing, 200 m depth, 0,0001 m 3 /s per borehole (360 kg/h * 20 boreholes (in parallel) = 7200 kg/h). Heat absorption (winter time): Tstorage = 50 C, Tfluid_in = 20 C. Figure A5.1. Scenario 1. Outlet fluid temperature (red), average storage temperature (blue) and heat transfer rate (orange) along 3 winter months (January, February and March) Logically, the average storage temperature decreases with time during winter, as heat is extracted from the storage to the fluid. The fluid temperature also decreases slightly with time. In this scenario, an increase of the mass flow in the boreholes would mean lower temperatures for both the fluid and the storage, as a larger amount of mass requires more energy from the storage to heat up the fluid. On the other hand, the higher inlet fluid temperature the lower heat that needs to be extracted from the ground and, consequently, the higher will be the storage and outlet fluid temperatures. Heat injection (summer time): Tstorage = 30 C, Tfluid_in = 55 C. Figure A5.2. Scenario 1. Outlet fluid temperature (red), average storage temperature (blue) and heat transfer rate (orange) along 3 summer months (June, July and August) During heat injection, the average storage temperature increases with time. The higher the mass flow the higher will be the temperature increase in the storage, as well as the fluid temperature.

76 Annex 5. Influence of BTES inputs and parameters According to the simulations carried out, the circulation switch input, which is going to be either 1 or -1 depending on the operational mode (either injection or absorption), has not a significant impact on the output of the storage. This input only has a greater influence for larger installations, where the storage volume is much larger and some of the boreholes are connected in series. On the other hand, some of the main parameters of the BTES have been modified to analyse its influence on the model. A second scenario, characterised by an increase of the number of boreholes, has been considered: BTES of 200 boreholes, 5 m spacing, 200 m depth, 0,0001 m 3 /s per borehole. Heat absorption (winter time): Tstorage = 51 C, Tfluid_in = 20 C. Figure A5.3. Scenario 2. Outlet fluid temperature (red), average storage temperature (blue) and heat transfer rate (orange) along 3 winter months (January, February and March) By comparing Figure A5.1 and Figure A5.3, it is possible to check that a slightly higher heat transfer rate is extracted when the number of boreholes is larger. Moreover, the decrease of the storage temperature will be lower with a larger number of boreholes, since the storage volume is bigger. Heat injection (summer time): Tstorage = 31 C, Tfluid_in = 55 C. Figure A5.4. Scenario 2. Outlet fluid temperature (red), average storage temperature (blue) and heat transfer rate (orange) along 3 summer months (June, July and August)

77 Annex 5. Influence of BTES inputs and parameters Regarding the injection period (summer time), a higher heat transfer rate is injected to the ground with a larger number of boreholes and, consequently, a higher increase in both the storage temperature and the outlet fluid temperature is achieved. Thirdly, a new scenario with a different spacing between boreholes has been analysed: BTES of 20 boreholes, 3 m spacing, 200 m depth, 0,0001 m 3 /s per borehole. As a result, a higher decrease during winter and a higher increase during summer are obtained for the storage temperature, since the lower storage volume (lower spacing) the easier it will be to heat up all this volume and the more difficult it will be to get energy from it. Heat absorption (winter time): Tstorage = 48 C, Tfluid_in = 20 C. Figure A5.5. Scenario 3. Outlet fluid temperature (red), average storage temperature (blue) and heat transfer rate (orange) along 3 winter months (January, February and March) Heat injection (summer time): Tstorage = 31 C, Tfluid_in = 55 C. Figure A5.6. Scenario 3. Outlet fluid temperature (red), average storage temperature (blue) and heat transfer rate (orange) along 3 summer months (June, July and August) Finally, a last scenario has been carried out to check that deeper boreholes imply a higher storage and outlet fluid temperatures and a slightly higher energy transfer rate from the boreholes during winter time, due to the larger storage volume, but also lower storage and outlet fluid temperatures and a higher energy transfer rate injected to the borehole during

78 Annex 5. Influence of BTES inputs and parameters summer time. Scenario 4: BTES of 20 boreholes, 3 m spacing, 250 m depth, 0,0001 m 3 /s per borehole. Heat absorption (winter time): Tstorage = 50 C, Tfluid_in = 20 C. Figure A5.7. Scenario 4. Outlet fluid temperature (red), average storage temperature (blue) and heat transfer rate (orange) along 3 winter months (January, February and March) Heat injection (summer time): Tstorage = 30 C, Tfluid_in = 55 C. Figure A5.8. Scenario 4. Outlet fluid temperature (red), average storage temperature (blue) and heat transfer rate (orange) along 3 summer months (June, July and August)

79 Annex 6. Customers heating installations parameters Annex 6. Costumers heating installations parameters Single-family dwellings Firstly, the parameters of the accumulator of the single-family dwellings are described. These parameters are introduced in TRNSYS as an external file (.txt) of the type used to model the accumulator (Type 534). It is possible to fill all the tank data with a plugin supplied by the program however it is not recommended since it does not work properly. Table A6.1. Parameters of the single-family dwellings heating installations Nodes/outputs/Internal HE/Miscellanous heat flow Tank_sfd (Type 534) 1000l triple coil accumulator temperature levels (nodes) 1 output 3 internal HE in the tank 0 losses by miscellanous heat flow Volume 1 m^3 Volume required to cover the heating demand Height 1.25 m Typical value (assumption) Tank Fluid 1-1: pure water Edge loss coefficient Node 1 Edge loss coefficient 3 3 kj/h-m2-k kj/h-m2-k Thermal losses to the environment through the sides of the storage tank. A normal value that it is commonly used Node 2 Edge loss coefficient 3 kj/h-m2-k in this kind of accumulators has been selected [1] Node 3 Edge loss coefficient 3 kj/h-m2-k Node 4 Edge loss coefficient 3 kj/h-m2-k Node 5 Edge loss coefficient 3 kj/h-m2-k Node 6 Top loss coefficient 3 kj/h-m2-k Bottom loss coefficient 3 kj/h-m2-k Additional thermal conductivity 0 - The nodes do not interact between them Inlet flow mode 1 - Location of intlet and outlet provided Entry node Exit node Overall flue heat loss coefficient node 1 0 kj/h-k Overall heat loss coefficient from node j to the gas flue [2] Overall flue heat loss 0 kj/h-k coefficient node 2 Overall flue heat loss 0 kj/h-k coefficient node 3 Overall flue heat loss 0 kj/h-k coefficient node 4 Overall flue heat loss coefficient node 5 0 kj/h-k

80 Annex 6. Customers heating installations parameters Overall flue heat loss 0 kj/h-k coefficient node 6 Heat exchanger type 3 - Coiled tube Number of heat exchanger nodes 2 - Number of nodes of the heat exchanger Heat exchanger fluid 2 Propylene Glycol and Water Volume of additive 50 % Percentage of Propylene glycol in the carried fluid Multiplier for natural convection correlation Exponent for Rayleigh number 1 - The heat exchanger and storage tank interact thermally through natural convection heat transfer from the heat exchanger outer surface to the tank fluid. It depends on the type of HE. To calculate it [2]: dttank/dt=(qin,tankqout,tank)/ctank Based on fluid properties at (Tsurf + T )/2 This requires an iterative solution as Tsurf depends on the heat transfer. [2] Geometry factor 1 - Assumption Geometry factor 0 - Assumption exponent Tube inner diameter m Assumption Tube outer diameter 0.02 m Assumption Wall conductivity 57 kj/m h k It depends on the material of the heat exchanger tube. In our case steel is used Tube length 2.5 m Annex 7 Number of tubes 8 Number of tubes in the Heat Exchanger Header Volume m^3 Annex 7 Cross sectional area m^2 Annex 7 Coil diameter 0.8 m Annex 7 Coil pitch 0.03 m Annex 7 Tank node for HX node 5 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 6 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Heat exchanger type 3 - Coiled tube (solar collector circuit) Number of heat 5 - Number of nodes of the heat exchanger exchanger nodes Heat exchanger fluid 1 - Water Multiplier for natural convection correlation 1 - The heat exchanger and storage tank interact thermally through natural convection heat transfer from the heat exchanger outer surface to the tank

81 Annex 6. Customers heating installations parameters Exponent for Rayleigh number fluid. It depends on the type of HE. To calculate it [2]: dttank/dt=(qin,tankqout,tank)/ctank Based on fluid properties at (Tsurf + T )/2 This requires an iterative solution as Tsurf depends on the heat transfer. [2] Geometry factor 1 - Assumption Geometry factor 0 - Assumption exponent Tube inner diameter m Assumption Tube outer diameter 0.04 m Assumption Wall conductivity 57 kj/m h k It depends on the material of the heat exchanger tube. In our case steel is used Tube length 1.26 m Annex 7 Number of tubes 16 Number of tubes in the Heat Exchanger Annex 7 Header Volume m^3 Annex 7 Cross sectional area m^2 Annex 7 Coil diameter 0.4 m Annex 7 Coil pitch 0.06 m Annex 7 Tank node for HX node 5 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 4 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 3 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 2 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 1 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Heat exchanger type 3 - Coiled tube (hot water circuit) Number of heat 5 - Number of nodes of the heat exchanger exchanger nodes Heat exchanger fluid 1 - Water Multiplier for natural convection correlation 1 - The heat exchanger and storage tank interact thermally through natural convection heat transfer from the heat exchanger outer surface to the tank

82 Annex 6. Customers heating installations parameters Exponent for Rayleigh number fluid. It depends on the type of HE. To calculate it [2]: dttank/dt=(qin,tankqout,tank)/ctank Based on fluid properties at (Tsurf + T )/2 This requires an iterative solution as Tsurf depends on the heat transfer. [2] Geometry factor 1 - Assumption Geometry factor 0 - Assumption exponent Tube inner diameter m Assumption Tube outer diameter 0.04 m Assumption Wall conductivity 57 kj/m h k It depends on the material of the heat exchanger tube. In our case steel is used Tube length 1.57 m Annex 7 Number of tubes 16 - Number of tubes in the Heat Exchanger Annex 7 Header Volume 0.02 m^3 Annex 7 Cross sectional area m^2 Annex 7 Coil diameter 0.5 m Annex 7 Coil pitch 0.06 m Annex 7 Tank node for HX node 1 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 2 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 3 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 4 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 5 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Solar circuit P1_sc (Type 3) Solar Pump UPS (0,08kW) Temperature rate 2-110ºC Operating point: 600l/h 5,4m Maximum flow rate 600 kg/hr 300kg/h per parallel string (maximum flow rate allowed by the solar collector) Fluid specific heat 3.56 kj/kg.k 50% water 50% propylene glycol

83 Annex 6. Customers heating installations parameters Maximum power 0.37 kw Pump UPS (0,08kW) Conversion coefficient Default value Power coefficient 1 - Default value Control signal 1 - The pump is switched off when there is no solar heat gain. Driven by the controller SC_sfd SC _sfd (Type 2) Temperature difference controller No. of oscillations 3 - Number of control oscillations allowed in one timestep before the controller is "Stuck" so that the calculations can be solved High limit cut-out 180 C The pump is switched off above this limit Input control function 1 - The output control signal from this component is hooked up to this input Upper dead band dt 3 Temp. Assumption Difference Lower dead band dt 0.5 Temp. Difference Assumption PRV-1 (Type 13) Pressure relief valve Boiling point of fluid 106 C Mixture of 50% water 50% propylene Specific heat of fluid 3.56 kj/kg.k glycol, working at 1bar Domestic Hot Water circuit P1_dhw (Type 3) Pump HW PM45 (0,37 kw) 300l/h H35m 230V Maximum flow rate 40 kg/hr Flow rate necessary to cover the demand Fluid specific heat 4.19 kj/kg.k Water Maximum power 0.37 kw Pump HW PM45 (0,37 kw) Conversion coefficient Default value Power coefficient Default value Control signal 1 - The pump is always in operation (simplification) L1 _dhw (Type 682) DHW heating load Fluid specific heat Water Load DHW load 1 house.txt kj/hr External file introduced by using the type 9 and calculated as explained in the Annex 2 Space heating circuit P1_sh (Type 3) Pump SH CPM130 (0,37kW) 600 l/h H 20m 230/400V Maximum flow rate 500 kg/hr Flow rate necessary to cover the demand Fluid specific heat 4.19 kj/kg.k Water Maximum power 0.37 kw Pump SH CPM130 (0,37kW) Conversion coefficient Default value Power coefficient Default value

84 Annex 6. Customers heating installations parameters Control signal 1 - The pump is always in operation (simplification) L1 _sh (Type 682) Space heating load Fluid specific heat Water Load Space heating load 1 house.txt kj/hr External file introduced by using the type 9 and calculated as explained in the Annex 2 P1_sh-2 (Type 3) Pump UHS CPM130 (0,37 kw) 600 l/h H 20m 230/400V Maximum flow rate 500 kg/hr Flow rate necessary to cover the demand Fluid specific heat 4.19 kj/kg.k Water Maximum power 0.37 kw Pump UHS CPM130 (0,37 kw) Conversion coefficient Default value Power coefficient Default value Control signal 1 - The pump is always in operation (simplification) HE_sh1 (Type 650) Heat exchanger SH loop Effectiveness of heat exchanger Specific heat of hot-side fluid Specific heat of hot-side fluid Number of posible spteps Cold-side set temperature Assumption 4.19 kj/kg.k Water 4.19 kj/kg.k Water Default value 35 C Underfloor heating temperature Apartment blocks Table A6.2. Parameters of the apartment blocks heating installations Nodes/outputs/Interna l HE/Miscellanous heat flow Tank_ap (Type 534) 5000l triple coil accumulator temperature levels (nodes) 1 output 3 internal HE in the tank 0 losses by miscellanous heat flow Volume 5 m^3 Volume required to cover the heating demand Height 1.25 m Typical value (assumption) Tank Fluid 1-1: pure water Edge loss coefficient Node 1 Edge loss coefficient Node 2 3 kj/h-m2-k Thermal losses to the environment through the sides of the storage tank. 3 kj/h-m2-k

85 Annex 6. Customers heating installations parameters Edge loss coefficient Node 3 Edge loss coefficient 3 3 kj/h-m2-k kj/h-m2-k A normal value that it is commonly used in this kind of accumulators has been selected [1] Node 4 Edge loss coefficient 3 kj/h-m2-k Node 5 Edge loss coefficient 3 kj/h-m2-k Node 6 Top loss coefficient 3 kj/h-m2-k Bottom loss coefficient 3 kj/h-m2-k Additional thermal conductivity 0 - The nodes do not interact between them Inlet flow mode 1 - Location of intlet and outlet provided Entry node Exit node Overall flue heat loss coefficient node 1 0 kj/h-k Overall heat loss coefficient from node j to the gas flue [2] Overall flue heat loss 0 kj/h-k coefficient node 2 Overall flue heat loss 0 kj/h-k coefficient node 3 Overall flue heat loss 0 kj/h-k coefficient node 4 Overall flue heat loss 0 kj/h-k coefficient node 5 Overall flue heat loss 0 kj/h-k coefficient node 6 Heat exchanger type 3 - Coiled tube Number of heat exchanger nodes 2 - Number of nodes of the heat exchanger Heat exchanger fluid 2 Propylene Glycol and Water Volume of additive 50 % Percentage of Propylene glycol in the carried fluid Multiplier for natural convection correlation Exponent for Rayleigh number 1 - The heat exchanger and storage tank interact thermally through natural convection heat transfer from the heat exchanger outer surface to the tank fluid. It depends on the type of HE. To calculate it [2]: dttank/dt=(qin,tankqout,tank)/ctan k Based on fluid properties at (Tsurf + T )/2 This requires an iterative solution as Tsurf depends on the heat transfer. [2] Geometry factor 1 - Assumption

86 Annex 6. Customers heating installations parameters Geometry factor 0 - Assumption exponent Tube inner diameter m Assumption Tube outer diameter 0.02 m Assumption Wall conductivity 57 kj/m h k It depends on the material of the heat exchanger tube. In our case steel is used Tube length 6.3 m Annex 7 Number of tubes 8 Number of tubes in the Heat Exchanger Header Volume m^3 Annex 7 Cross sectional area m^2 Annex 7 Coil diameter 2 m Annex 7 Coil pitch 0.03 m Annex 7 Tank node for HX node 5 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 6 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Heat exchanger type 3 - Coiled tube (solar collector circuit) Number of heat 5 - Number of nodes of the heat exchanger exchanger nodes Heat exchanger fluid 1 - Water Multiplier for natural 1 - The heat exchanger and storage tank convection correlation interact thermally through natural convection heat transfer from the heat exchanger outer surface to the tank fluid. It depends on the type of HE. To calculate it [2]: Exponent for Rayleigh number dttank/dt=(qin,tankqout,tank)/ctan k Based on fluid properties at (Tsurf + T )/2 This requires an iterative solution as Tsurf depends on the heat transfer. [2] Geometry factor 1 - Assumption Geometry factor 0 - Assumption exponent Tube inner diameter m Assumption Tube outer diameter 0.04 m Assumption Wall conductivity 57 kj/m h k It depends on the material of the heat exchanger tube. In our case steel is used Tube length 3.77 m Annex 7 Number of tubes 16 m^3 Number of tubes in the Heat Exchanger Annex 7 Header Volume m^2 Annex 7 Cross sectional area m Annex 7

87 Annex 6. Customers heating installations parameters Coil diameter 1.2 m Annex 7 Coil pitch 0.06 m Annex 7 Tank node for HX node 5 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 4 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 3 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 2 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 1 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Heat exchanger type 3 - Coiled tube (hot water circuit) Number of heat exchanger nodes 5 - Number of nodes of the heat exchanger Heat exchanger fluid 1 - Water Multiplier for natural convection correlation Exponent for Rayleigh number 1 - The heat exchanger and storage tank interact thermally through natural convection heat transfer from the heat exchanger outer surface to the tank fluid. It depends on the type of HE. To calculate it [2]: dttank/dt=(qin,tankqout,tank)/ctan k Based on fluid properties at (Tsurf + T )/2 This requires an iterative solution as Tsurf depends on the heat transfer. [2] Geometry factor 1 - Assumption Geometry factor 0 - Assumption exponent Tube inner diameter m Assumption Tube outer diameter 0.04 m Assumption Wall conductivity 57 kj/m h k It depends on the material of the heat exchanger tube. In our case steel is used Tube length 3.14 m Annex 7 Number of tubes 16 - Number of tubes in the Heat Exchanger Annex 7 Header Volume 0.04 m^3 Annex 7

88 Annex 6. Customers heating installations parameters Cross sectional area m^2 Annex 7 Coil diameter 1 m Annex 7 Coil pitch 0.06 m Annex 7 Tank node for HX node 1 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 2 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 3 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 4 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 5 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Solar circuit P2_sc (Type 3) Solar Pump CMX-32/125A (0,75 kw) 6000 l/h H 21m 230/400V Tmax 110ºC Maximum flow rate 2400 kg/hr 300kg/h per parallel string (maximum flow rate allowed by the solar collector) Fluid specific heat 3.56 kj/kg.k 50% water 50% propylene glycol Maximum power 0.74 kw Solar Pump CMX-32/125A (0,75 kw) Conversion coefficient Default value Power coefficient Default value Control signal 1 - The pump is switched off when there is no solar heat gain. Driven by the controller SC field SC _ap (Type 2) Temperature difference controller No. of oscillations 3 - Number of control oscillations allowed in one timestep before the controller is "Stuck" so that the calculations can be solved High limit cut-out 180 C The pump is switched off above this limit Input control function 1 - The output control signal from this component is hooked up to this input Upper dead band dt 3 Temp. Differenc e Assumption

89 Annex 6. Customers heating installations parameters Lower dead band dt 0.5 Temp. Differenc e Assumption PRV-2 (Type 13) Pressure relief valve Boiling point of fluid 106 C Mixture of 50% water 50% propylene Specific heat of fluid 3.56 kj/kg.k glycol, working at 6bar Hot water circuit P2_dhw (Type 3) Pump HW PM45 (0,37 kw) 300l/h H35m 230V Maximum flow rate 225 kg/hr Flow rate necessary to cover the demand Fluid specific heat 4.19 kj/kg.k Water Maximum power 0.37 kw Pump HW PM45 (0,37 kw) Conversion coefficient Default value Power coefficient Default value Control signal 1 - The pump is always in operation (simplification) L2 _dhw (Type 682) Temperature difference controller Fluid specific heat Water Load DHW_10 apartment s.txt kj/hr External file introduced by using the type 9 and calculated as explained in the Annex 2 Space heating circuit P2_sh (Type 3) Pump SH CPM130 (0,37kW) 600 l/h H 20m 230/400V Maximum flow rate 500 kg/hr Flow rate necessary to cover the demand Fluid specific heat 4.19 kj/kg.k Water Maximum power 0.55 kw Pump SH CPM130 (0,37kW) Conversion coefficient Default value Power coefficient Default value Control signal 1 - The pump is always in operation (simplification) L2 _sh (Type 682) Temperature difference controller Fluid specific heat Water Load SH_10 apartment s.txt kj/hr External file introduced by using the type 9 and calculated as explained in the Annex 2 P2_sh-2 (Type 3) Pump UHS CPM130 (0,37kW) 600 l/h H 20m 230/400V Maximum flow rate 500 kg/hr Flow rate necessary to cover the demand Fluid specific heat 4.19 kj/kg.k Water Maximum power 0.37 kw Pump UHS CPM130 (0,37kW) Conversion coefficient Default value Power coefficient Default value

90 Annex 6. Customers heating installations parameters Control signal 1 - The pump is always in operation (simplification) HE_sh2 (Type 650) Heat exchanger SH loop Effectiveness of heat Assumption exchanger Specific heat of hotside 4.19 kj/kg.k Water fluid Specific heat of hotside 4.19 kj/kg.k Water fluid Number of posible Default value spteps Cold-side set temperature 35 C Underfloor heating temperature Industries Table A6.3. Parameters of the industries heating installations Nodes/outputs/Internal HE/Miscellanous heat flow Tank_ind (Type 534) 5000l double coil accumulator temperature levels (nodes) 1 output 3 internal HE in the tank 0 losses by miscellanous heat flow Volume 5 m^3 Volume required to cover the heating demand Height 1.25 m Typical value (assumption) Tank Fluid 1-1: pure water Edge loss coefficient Node 1 Edge loss coefficient 3 3 kj/h-m2-k kj/h-m2-k Thermal losses to the environment through the sides of the storage tank. A normal value that it is commonly used Node 2 Edge loss coefficient 3 kj/h-m2-k in this kind of accumulators has been selected [1] Node 3 Edge loss coefficient 3 kj/h-m2-k Node 4 Edge loss coefficient 3 kj/h-m2-k Node 5 Edge loss coefficient 3 kj/h-m2-k Node 6 Top loss coefficient 3 kj/h-m2-k Bottom loss coefficient 3 kj/h-m2-k Additional thermal conductivity 0 - The nodes do not interact between them. Inlet flow mode 1 - Location of intlet and outlet provided Entry node Exit node

91 Annex 6. Customers heating installations parameters Overall flue heat loss coefficient node 1 0 kj/h-k Overall heat loss coefficient from node j to the gas flue [2] Overall flue heat loss 0 kj/h-k coefficient node 2 Overall flue heat loss 0 kj/h-k coefficient node 3 Overall flue heat loss 0 kj/h-k coefficient node 4 Overall flue heat loss 0 kj/h-k coefficient node 5 Overall flue heat loss 0 kj/h-k coefficient node 6 Heat exchanger type 3 - Coiled tube (solar collector circuit) Number of heat 5 - Number of nodes of the heat exchanger exchanger nodes Heat exchanger fluid 1 - Water Multiplier for natural 1 - The heat exchanger and storage tank convection correlation interact thermally through natural convection heat transfer from the heat exchanger outer surface to the tank fluid. It depends on the type of HE. To calculate it [2]: Exponent for Rayleigh number dttank/dt=(qin,tankqout,tank)/ctank Based on fluid properties at (Tsurf + T )/2 This requires an iterative solution as Tsurf depends on the heat transfer. [2] Geometry factor 1 - Assumption Geometry factor 0 - Assumption exponent Tube inner diameter m Assumption Tube outer diameter 0.04 m Assumption Wall conductivity 57 kj/m h k It depends on the material of the heat exchanger tube. In our case steel is used Tube length 1.88 m Annex 7 Number of tubes 16 m^3 Number of tubes in the Heat Exchanger Annex 7 Header Volume m^2 Annex 7 Cross sectional area m Annex 7 Coil diameter 0.6 m Annex 7 Coil pitch 0.06 m Annex 7 Tank node for HX node 5 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 4 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node

92 Annex 6. Customers heating installations parameters Tank node for HX node 3 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 2 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 1 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Heat exchanger type 3 - Coiled tube (hot water circuit) Number of heat exchanger nodes 5 - Number of nodes of the heat exchanger Heat exchanger fluid 1 - Water Multiplier for natural convection correlation Exponent for Rayleigh number 1 - The heat exchanger and storage tank interact thermally through natural convection heat transfer from the heat exchanger outer surface to the tank fluid. It depends on the type of HE. To calculate it [2]: dttank/dt=(qin,tankqout,tank)/ctank Based on fluid properties at (Tsurf + T )/2 This requires an iterative solution as Tsurf depends on the heat transfer. [2] Geometry factor 1 - Assumption Geometry factor 0 - Assumption exponent Tube inner diameter m Assumption Tube outer diameter 0.04 m Assumption Wall conductivity 57 kj/m h k It depends on the material of the heat exchanger tube. In our case steel is used Tube length 4.7 m Annex 7 Number of tubes 16 - Number of tubes in the Heat Exchanger Annex 7 Header Volume 0.06 m^3 Annex 7 Cross sectional area m^2 Annex 7 Coil diameter 1.5 m Annex 7 Coil pitch 0.06 m Annex 7 Tank node for HX node 1 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 2 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node

93 Annex 6. Customers heating installations parameters Tank node for HX node 3 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 4 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Tank node for HX node 5 - Tank node in which this heat exchanger node is completely immersed Fraction of HX node Fraction of heat exchanger length assigned to this heat exchanger node Hot water circuit P3_dhw (Type 3) Pump HW PM45 (0,37 kw) 300l/h H35m 230V Maximum flow rate 25 kg/hr Flow rate necessary to cover the demand Fluid specific heat 4.19 kj/kg.k Water Maximum power 0.37 kw Pump HW PM45 (0,37 kw) Conversion coefficient Default value Power coefficient Default value Control signal 1 - The pump is always in operation (simplification) L3 _dhw (Type 682) Temperature difference controller Fluid specific heat Water Load DHW_1 industry.txt kj/hr External file introduced by using the type 9 and calculated as explained in the Annex 2 Space heating circuit P3_sh (Type 3) Pump SH CPM130 (0,37kW) 600 l/h H 20m 230/400V Maximum flow rate 2000 kg/hr Flow rate necessary to cover the demand Fluid specific heat 4.19 kj/kg.k Water Maximum power 0.55 kw Pump SH CPM130 (0,37kW) Conversion coefficient Default value Power coefficient Default value Control signal 1 - The pump is always in operation (simplification) L2 _sh (Type 682) Temperature difference controller Fluid specific heat Water Load SH_1 industry.txt kj/hr External file introduced by using the type 9 and calculated as explained in the Annex 2 P2_sh-2 (Type 3) Pump SH CPM130 (0,37kW) 600 l/h H 20m 230/400V Maximum flow rate 2000 kg/hr Flow rate necessary to cover the demand

94 Annex 6. Customers heating installations parameters Fluid specific heat 4.19 kj/kg.k Water Maximum power 0.37 kw Pump SH CPM130 (0,37kW) Conversion coefficient Default value Power coefficient Default value Control signal 1 - The pump is always in operation (simplification) HE_sh1 (Type 650) Heat exchanger SH loop Effectiveness of heat Assumption exchanger Specific heat of hot-side 4.19 kj/kg.k Water fluid Specific heat of hot-side 4.19 kj/kg.k Water fluid Number of posible spteps Default value Cold-side set temperature 35 C Underfloor heating temperature References [1] S. Sathiyamoorthy, B. Elizabeth Caroline and J. Gnana Jayanthi, Emerging Trends in Science, Engineering and Technology. Proceedings of International Conference, INCOSET. [2] Tess models. Storage tank library. Type 534. Trnsys documentation.

95 Annex 7. Accumulators design and dimensioning Annex 7. Accumulators design and dimensioning The hot water accumulators have been dimensioned considering that there is one hot water accumulator for every single-family dwelling, two hot water accumulators for every apartment block and one hot water accumulator for every industry. 6 different temperature levels (nodes) of the same size have been considered in the design of all the accumulators to model the stratification within the storage tank. Moreover, accumulators with 3 internal coil heat exchangers and 1 input/output have been used in both the single-family dwellings and the apartment blocks, according to the design shown in the Figure A7.1. Figure A7.1. Water accumulator design for the single-family dwellings and the apartment blocks The input and output of the accumulator, as well as the inputs and outputs of the different internal heat exchangers, are distributed in the following nodes. Solar Collectors circuit: - Input: Node 5 - Output: Node 6 (bottom node) Domestic Hot Water circuit: - Input: Node 5 - Output: Node 1 (top node) District Heating Network circuit: - Input: Node 1 (top node) - Output: Node 5 Floor Heating System circuit: - Input: Node 5 - Output: Node 1 (top node) Firstly, the internal heat exchanger of the solar circuit has been placed on the bottom of the tank to heat up all the tank, since the heat rises. Secondly, the internal heat exchanger of the DHW circuit has its output at the top of the tank to extract the water as hot as possible. Moreover, the size of this internal heat exchanger is quite large to be able to heat up the water from 5 C to 35 C. The internal heat exchanger of the district heating net is also quite large to be able to inject enough energy during winter (energy coming from the BTES) to cover the heating demand. Moreover, the inlet of this heat exchanger is placed

96 Annex 7. Accumulators design and dimensioning at the top of the tank to achieve a space heating temperature large enough during winter (the space heating output is placed at the top of the tank). Finally, the output of the space heating circuit is placed at the top of the accumulator to be able to extract water at 35 C during winter. Despite the fact that the software (TRNSYS) does not allow to change the output position along the year, in reality it would be interesting to lower the position of the space heating output during summer since high temperatures would be achieved, much higher than the temperatures required for the underfloor heating system. On the other hand, regarding the industries, the accumulator design is similar to the one previously shown in the Figure A7.1 but without the internal heat exchanger of the solar loop, since there are not solar collectors on the industries roof. Figure A7.2. Water accumulator design for the industries The type 534 has been used in TRNSYS to model these accumulators. The main parameters of the hot water tanks designed for all the single-family dwellings, apartment blocks and industries are described in the Annex 6. Regarding the heat exchangers design, internal coil heat exchangers of stainless steel 304 have been used in the hot water accumulator, similar to the one shown in Figure A7.3. Figure A7.3. Helical-coil heat exchanger The following equations have been used for dimensioning the three coil heat exchangers [1].