Danish Development Center for District Energy

Transcription

1 Idea Competition for students on district heating and district cooling CO 2 based district energy system Christian Nørr Jacobsen, Chrnjacobsen@gmail.com Christian Vang Madsen, ChristianVMadsen@gmail.com MSc Eng students, Sustainable Energy, DTU In collaboration with DTU Mechanical Engineering and Rambøll A/S 1 Abstract The energy system in Denmark is one of the most efficient in the world, largely due to the co production of electricity and heat. In recent years however, the large developments in various renewable energy sources, producing only a single energy form, are pointing to some problematic system integration issues. The proposed idea is a new highly flexible district energy system based on CO 2, which addresses these problems and hopefully increases the systems overall exergetic efficiency. The system consists of a CO 2 network with both a liquid and vapor line, pressurized to ensure a saturation temperature close to the ground s. With decentralized equipment the CO 2 can then be utilized to provide all temperatures from approximately 70 C to 150 C, meaning that all standard temperature needs can be satisfied, in addition to ensuring easy integration of low temperature energy sources. Due to the composition of the network a synergy effect is enabled, meaning that customers utilizing refrigeration will produce heating for the rest of the network, just as the production of heat will in effect produce cooling for the network, decreasing the need for primary energy inputs.

2 2 Introduction The proposed idea for this competition comes from a master thesis currently carried out in collaboration with DTU and Rambøll. The thesis was started in mid February and the final hand in is scheduled to be mid august, meaning that results of comparison between the existing district energy system and the proposed system has not been obtained at the final hand in day of this competition. The master thesis and the idea for this competition is inspired by an article [1], which proposed the main idea behind the system with compressed CO 2 as an energy carrier instead of water. This project has modified the system both in terms of the components at the customers and at the plant side to simulate the Danish district energy system. The Danish energy system is one of the most efficient in the world, partly because of the extensive use of low temperature reject heat from power production in district heating networks. This ensures a very high energy utilization, and thereby exergetic efficiency. [2] This system is constructed around the pretense that electricity is produced by means of burning fuel to drive generators, ensuring continuous availability of low temperature reject heat. In recent years the extensive development of the renewable energy sector, primarily wind power, which does not supply reject heat, has lead to situations where the needed electricity production has not delivered the required amount of heat for the district heating networks, thereby forcing the utilities to run boilers solely for heat production. This results in periods with bad exergetic system performance, which should be avoided. These situations are likely to occur far more often in the future, as the renewable energy sector is further developed, and a variety of solutions are presently being discussed, including large central solar heat implementation, the use of large electric heating elements in the district heating plants or the implementation of large heat pumps to efficiently turn surplus electricity into heat. This assignment investigates another possibility, which requires far more extensive modifications to be made in the existing district heating network, but on the other hands also address a variety of other less than perfect points in the existing network. 3 Current system and drawbacks The district energy system in Denmark is one of the best systems in the world when it comes to having high energy efficiency and with good reason as Denmark since the oil crisis has been aware of optimizing the energy sector [3]. This has for instance led to an advanced and very extensive district heating network, from which most of the population receives domestic hot water and space heating. Due to the extent of the network, decentralized heat production from heating plants and other heat sources such as solar or geothermal heat can be connected to the network and supply many customers in a much more energy efficient way than if every customer had a private furnace. There is however a number of drawbacks to the current system, which will briefly be explained in the following section, mainly: 1. Energy loss from pipes 2. High temperature requirements 3. Only heat supplied 4. Large space requirements in underground city infrastructure Christian Nørr Jacobsen s Rambøll A/S and DTU MEK p. 2/10

3 3.1 Energy loss from pipes The district heating system today is based on water as an energy carrier, and this have its benefits but also its drawbacks. The drawbacks are highlighted here as these are the ones sought addressed by the proposed system. Using water as an energy carrier means that the temperature needs to be between C in the district heating network [4], depending on the outside temperature. This results in a high temperature difference between the water and the surrounding soil, and high heat losses are unavoidable even with thick insulation. The lower the ground temperature, the higher the heat loss from both pipes and buildings, which is why 120 C is periodically needed as outlet temperature from the plants during a cold winter to ensure adequate heat supply to consumers. As a result the CHP plants must run in full backpressure mode and produce less electricity at the cost of producing heat, in addition to boilers being utilized to meet demands. This leads to lower exergetic efficiency since heat at these temperatures represent relatively low exergetic values. 3.2 High temperature requirements Another drawback is that low temperature heat sources such as solar heating, geothermal or waste heat from the industry, often have a too low temperatures to be directly implemented in the district heating supply line. Solar heating has a very high potential, and is able to supply high enough temperatures during the summer, but have too low temperature during most of the spring and fall and almost never during winter [5]. Geothermal is a stable heat source, but has the drawback that the boreholes have to be several kilometers deep in order to reach feasible temperatures to be used directly for district heating. The temperature of waste heat from the industry can vary a lot, and each case must be considered individually in relation to available temperature, amount of heat available, and if it is economically feasible in relation to the energy tax laws. Presently these sources are only sparingly implemented, and often the temperature is boosted using heat pumps to allow for appropriate output temperatures to the network. The exception is solar heat, which has seen large expansion in recent years. These systems can deliver adequate temperatures during the summer, but is rarely in operation during the spring and autumn and not at all during the winter, leading to very low utilization hours which impact the economy adversely. 3.3 Only heat supplied When considering cooling, the standard in Denmark is that each consumer has their own electrically driven heat pumps installed, which removes heat from the desired areas and releases it outside. In recent years district cooling has become an interesting alternative to some consumers, depending on their location and demands. District cooling is usually limited to specific locations where low temperature sources are easily available such as near a sea or lake, since free cooling is currently the most efficient way of producing cold water. If the cold water is to be produced from absorption cooling, a stable heat supply is required near the plant. If neither a cold nor hot source is available, only compressor driven cooling can produce the cold water and then the benefits of having the cooling produced centrally is very small, especially when considering some of the drawbacks. For example the small temperature difference between supply and return pipe leads to a need for high mass flow rates if large energy amounts are to be moved, resulting in large pipe dimensions which can be very hard to fit in the closely packed infrastructure of a city. Another drawback is the fact that presently only temperatures above 0⁰C is possible because water is used as an energy carrier, which means that only air conditioning purposes can be satisfied, leaving many consumers with a demand for refrigeration equipment even with district cooling installed. Christian Nørr Jacobsen s Rambøll A/S and DTU MEK p. 3/10

4 4 System description of CO2 district energy system In the proposed system, which is based on article [1], pressurized CO 2 is pumped to the consumers instead of water. This is done in two separate pipes, one containing liquid CO 2 and another containing vapor at close to the same pressure. The pressure level is chosen based on the ground temperature to minimize heat losses from the pipes to the surrounding soil. The overall system idea being that the consumers can obtain any desired temperature level by either evaporating or condensing CO 2 from one pipe to the other. One of the largest differences between the existing and proposed system, is the fact that what is presently seen as energy consumers will, to some degree, become energy producers of either cooling or heating. I.e. when a consumer evaporates CO 2 liquid for air conditioning purposes, vapor is returned to the network in effect producing heating for another consumer. Due to this energy exchange between consumers, the network could theoretically balance itself out, meaning that no energy is supplied centrally, except for the electricity for pumps and compressors. The superstructure of the system is illustrated in Figure 1 below. Figure 1: Superstructure of proposed CO 2 energy system. The direct result of this change of systems is expected to be a decreased need for heat output from the plant and increased need for electricity to drive compressors, both at the plant and at the consumers. The merits of this change will be discussed further in section 5. From the functional point of view, the proposed system requires a variety of different changes to be made, both plant side and consumer side. In the following, the most important changes will be briefly explained. Christian Nørr Jacobsen s Rambøll A/S and DTU MEK p. 4/10

5 4.1 Plant side In the present water based system, the plant side is the sole energy producer, controlling temperature output, flow rate and pressure. In the proposed system this role changes to merely insuring the appropriate pressure in both pipes, depending on real time demand patterns. The pressure control is achieved using large pumps and compressors to pressurize the needed amount of CO 2 and then leading it directly into the supply pipes. To ensure the ability to rapidly supply the network in case of large sudden fluctuations in consumption, a large separator tank could be utilized from which both vapor and liquid could be drawn as needed. Another major difference will be the necessity of reversing the flow in the plant, as heating demand will exceed cooling demands in the winter whereas cooling demands might exceed heating demands in the summer, leading to situations where the plant must supply cooling by condensing CO Vapor production Due to the Danish weather conditions the plant will most often be required to deliver heating, however the low temperature in the supply pipes enables easy use of readily available low temperature heat sources to achieve the needed evaporation. An example could be the Danish power plant Avedøre, which in full condensing mode utilizes input cooling water at 10⁰C and heats it to 19.3⁰C before pumping it out again, delivering 302MJ/s of energy [6]. This temperature is high enough to evaporate CO 2 at 8⁰C, thereby achieving the full electricity output from condensing mode while still utilizing the reject heat as district heating. In cases where the heat demand exceeds what the CHP plants can deliver, other heat sources can easily be utilized by throttling the liquid pressure down to achieve a suitable evaporation temperature and compressing it back to supply pressure again after evaporation. This approach enables the use of very low temperature sources such as ground source loops, waste water or lake or seawater, while increasing the use of electricity to drive the compression Liquid production In cases where cooling demands are predominant, the reverse would be the case, the plant side would then need to condense vapor to liquid using a heat sink. Preferably a sink with a temperature below 8⁰C is available, however otherwise the vapor pressure can be throttled down to a suitable condensation temperature and then afterwards be increased again using pumps. 4.2 Consumer side From the consumer s point of view, the proposed system requires larger investments than the present one, because more machinery is needed to obtain the desired energy service. The specific machinery requirements are dependent on the needed energy service, but will always consist of a combination of heat exchangers, compressors, pumps and valves. All of these parts are currently being used in the refrigeration industry, ensuring that the technology is well developed and readily available. In the following a simple introduction to the different possibilities will be given Heating demand With the existing H 2 O based system a heat exchanger is sufficient to obtain both space heating and domestic hot water, with the proposed CO 2 based system there are two different methods to achieve the needed temperature levels. Christian Nørr Jacobsen s Rambøll A/S and DTU MEK p. 5/10

6 Open loop heat pump With an open loop heat pump vapor is drawn from the vapor supply pipe and then compressed to the needed temperature, then a counter flow gas cooler transfers the heat to a water loop. Hereafter the CO 2 gas is expanded through a turbine connected to the compressor to regain some of the energy. Afterwards the CO 2 will be a mix of vapor and liquid, which is separated in a separator tank from where gas and liquid can be led back to the supply lines, using valves to ensures the output pressure is identical to the supply system pressure. This use of decentralized separator tanks has the added advantage that they will function as buffer capacity for the system, thereby ensuring that fluctuations in demand are smoothed out. It should however be noted that the tank should be thermally insulated and have a one way valve installed between the tank and the liquid line, since it otherwise will assume a steady state at approximately 10⁰C and cool the room where it is placed Closed loop heat pump In the closed loop heat pump, the CO 2 is condensed at the system pressure around 8⁰C, and then pumped back to the liquid line, thereby functioning as a heat source for a secondary heat pump, which can then raise the temperature to the required level. The advantage of this setup is that the possibility of using another refrigerant makes it easier to adapt the system to specific temperature needs, thereby potentially raising the COP of the system. Additionally a failure in the heat pump will not lead to release of CO 2 from the transmission system, thereby possibly decreasing the need for safety valves Cooling demand The choice of cooling system depends on the temperature demand, the two most simple systems will be briefly explained Passive comfort cooling Comfort cooling can be obtained passively simply by evaporating liquid directly in an evaporator. This is possible because the system has a slightly higher pressure in the liquid line to avoid unintended evaporation in the transmission pipes. Therefore the higher pressure in the liquid line will force liquid into the evaporator without the need for pumps or compressors, leading to a passive cooling effect at 8⁰C, which is suitable for comfort cooling. Christian Nørr Jacobsen s Rambøll A/S and DTU MEK p. 6/10

7 Refrigeration In case lower temperatures are needed, an expansion valve is used to lower the pressure from the liquid line to the desired temperature after which then an evaporator transfers the energy to the CO 2, before compressing the CO 2 back to the correct pressure for the vapor line. After the compressor the hot CO 2 gas is cooled to the desired temperature by evaporating CO 2 directly from the liquid line to the vapor line, thereby producing additional vapor for the network. Using this method, temperatures down to approximately 70⁰C can be obtained, which should satisfy the needs of any normal consumer. If lower temperatures should be needed, the system can be used as a heat sink for another refrigeration cycle Organic Rankine Cycle The increased electricity consumption of the proposed CO 2 system could be somewhat alleviated by using Organic Rankine Cycles in situations where relatively hot temperature sources are available. In this case, instead of transferring the energy directly to the network, an intermediate loop would extract part of the energy as electricity before evaporating the CO 2 liquid to vapor. This method could turn sources such as industrial waste heat and solar thermal production into electricity production units, while still producing heat for the network. 5 Pros and cons of the CO2 district energy system. In the following section the most important advantages and disadvantages of the proposed system will be discussed. 5.1 Increased electricity production One of the major disadvantages of the system is the increased use of electricity for the compressors. This however is alleviated somewhat by a range of different points which must be considered. Firstly, the decrease in needed output temperature from power plants will raise their electrical efficiency by approximately 6%, while still supplying the surplus thermal energy to the system. Depending on which COP can be assumed, more than 20% of the heat will still be supplied from the same amount of input fuel to the power plants. Industrial waste heat can be used to supply both electricity and heat for the network using Organic Rankine Cycles, leading to a situation where many industrial thermal sources and solar production facilities will be able to supply part of the need for electricity. In the future, an increasing amount of the Danish electricity consumption is expected to come from renewable sources such as wind. This is a problem for the current system since wind power does not produce heat, meaning that boilers are often employed during the winter to meet demand. With the proposed system this is Christian Nørr Jacobsen s Rambøll A/S and DTU MEK p. 7/10

8 not the case. Instead the production from wind power will decrease the overall emissions of the system, since the thermal sources are mostly emissions free, see section 5.3, leaving only the emissions from power production. That being said, more work should be done to consider the best way to ensure a certain degree of time correlation between renewable electricity production and consumer thermal production. Fortunately this is already an area under intensive research in regard to the large number of individually installed heat pumps in Denmark, and their potential to stabilize the fluctuating production from wind turbines. 5.2 Synergy effect One of the most interesting things regarding the proposed system is the synergy effect that arises between heating and cooling consumption in the CO 2 network. When a customer produces cooling from liquid CO 2 the gas vaporizes and goes to the vapor supply line and thereby becomes available to other customers as a heat source. The same synergy effect occurs when a customer produces heating from CO 2 where the gas condenses and goes to the liquid supply line and becomes available to other customers as a heat sink for production of cooling or refrigeration. This synergy effect creates a balance in the network to some extent, and thereby limits the energy input needed to condense or evaporate CO 2 at the plant side in order to fulfill the demands of the consumers. This could potentially contribute to high energy savings in periods where the demand for cooling and heating are similar in size. To illustrate this effect, the following Figure 2 has been created 1, which illustrates an estimation of the average consumption of different energy services pr m 2 in a city environment during a year. Figure 2: Estimated average energy consumption pr m2 in an urban environment 1 1 Based on hourly consumption patterns from Residential buildings, supermarkets, Malls and office buildings and the area distribution of Copenhagen. A range of simplifications has been made, for more information contact the authors. Christian Nørr Jacobsen s Rambøll A/S and DTU MEK p. 8/10

9 As seen, the balance in the system is fluctuating throughout the year and day, but during the summer the demand for cooling on a 24 hour average exceeds that of heating, meaning that in this period, all heating energy is produced by cooling equipment throughout the network, and some heat must actually be either stored or released to the surroundings. 5.3 Inclusion of low temperature heat sources One of the major advantages of the proposed system is the easy integration of various low energy temperature sources. Even temperatures below 0⁰C can be used to produce CO 2 vapor for the system, meaning that there will never be a shortage of thermal energy. It is however clear that energy sources above the pipeline temperature is far more desirable, since no energy would then be needed to compress the vapor back to system level. Even with this in mind, there are still a large number of different sources to choose from. Even in full condensing mode a power plant produces excess heat at approximately 20⁰C which needs to be cooled to 10⁰C, cheap solar panels are far more efficient when only low temperatures are needed, in Frederikshavn sewage between 10⁰C and 20⁰C are already being used as a heat source for the district heating system and geothermal energy is much more easily obtainable when only low temperatures are needed. The easy access to low temperature sources combined with the described synergy effects and increased insulation levels, means that it is highly likely that no additional thermal energy input would be needed for the system. 5.4 Ability to supply all thermal energy services within one system Due to the use of water as an energy carrier, the existing district energy system is only able to provide heating and air condition through district cooling. District cooling however is only feasible when located near a cold source, and have large customers in need of cold water for air condition. As water has a freezing point at 0 C refrigeration cannot be produced centrally and circulated by the existing system. Another problem also arises if the existing system is to provide district heating and cooling and that is that the supply and return pipes will take up quite some space in the underground infrastructure in a city. The proposed system is able to function as a heat source and a heat sink making it possible to utilized the CO 2 to produce energy services between 70 C 150 C depending on the decentral equipment. A major benefit of the CO 2 system is that it needs only two pipelines, which do not need to have a large diameter due to lower mass flow rate, and thereby do not obtain the same space in the underground infrastructure as the existing system. Additionally every component providing a thermal energy service is connected into one district network, making it easier to balance the system centrally and exploit the synergy effect mentioned above. 5.5 Risk assessment The proposed system does not come without potential drawbacks and issues, the main concern being to have pressurized CO 2 around bar in an urban environment. Though it rarely happens, hitting a pipeline and making a hole while digging in the city is a possibility, and it has to be investigated whether the pipe will explode following decompression or just leak CO 2 to the surroundings. If CO 2 is leaked to the surroundings either from a pipeline or inside buildings where decentral energy service equipment is placed, it is a potential threat to humans whom can suffocate if the concentration is too high. CO 2 is also without smell or color making it more difficult for humans to detect. Measures can be taken to minimize the extent of potential accidents. This could be that in the pipe lines, both vapor and liquid line, for every 100 meters shut off valves with pressure sensors are placed and if the pressure Christian Nørr Jacobsen s Rambøll A/S and DTU MEK p. 9/10

10 drops dramatically between these, they will shut off the flow. Leaks cannot be completely avoided, but measures can be taken like for instance mounting CO 2 sensors close to decentral energy service equipment which will trigger if the concentration becomes too high. Additionally, color and odor, without an effect on the system, can be added to make it easier to humans to detect. 5.6 Conclusion This proposal is a radical idea on a very early stage of development, which changes the way energy services are provided to customers, but also offers interesting possibilities. A graphical illustration of the advantages and disadvantages of the two systems are presented in Figure 3 below, for easy comparison. Existing water based system Thermal losses to the ground Electricity only needed for pumps Only heat delivered Only high temperature sources useable No synergy effect Large pipe diameter Simple technology Very safe system Known system price Figure 3: Comparison between the advantages and disadvantages of the two systems Proposed CO 2 based system No thermal losses Increased electricity consumption All thermal energy services delivered All thermal sources useable Synergy effect leading to decreased need for thermal input Small pipe diameter Known technology, but more sophisticated Potential dangers in regard to high pressure pipes and leaks Potentially more expensive When considering the comparison between the systems, there are advantages and disadvantages of both, however many of the recent and future changes in the Danish energy system will benefit the proposed system far more than the existing one. In conclusion the proposed system seems extremely interesting, while still on a very early stage of development. 6 References [1] Weber C, Favrat D, Conventional and advanced CO2 based district energy systems, Energy 2010;35: [2] Mathiesen B.V, Lund H., Comparative analyses of seven technologies to facilitate the integration of fluctuating renewable energy sources, IET Renew. Power Gener., 2009, Vol. 3, Iss. 2, pp [3] Danish Energy Agency, Heat Supply in Denmark Who What Where and Why, ISBN: Electronic version: [4] Søren Vesterby Knudsen, Rambøll, Power point presentation at DTU, Energy system Analysis, Design and Optimization, 16th of february [5] Production data, Marstal Solvarmeanlæg, Accessed May [6] Elmegaard B, Houbak N, Simulation of the Avedøreværket unit 1 cogeneration plant with DNA, Department of Mechanical Engineering, Technical University of Denmark, 2002 Christian Nørr Jacobsen s Rambøll A/S and DTU MEK p. 10/10