Next Generation of Seasonal Gravel-Water Thermal Energy Store Design and Operating Results from Eggenstein-Leopoldshafen, Germany

Transcription

1 Next Generation of Seasonal Gravel-Water Thermal Energy Store Design and Operating Results from Eggenstein-Leopoldshafen, Germany Roman Marx, Dan Bauer, Harald Drueck University of Stuttgart, Institute of Thermodynamics and Thermal Engineering (ITW), Research and Testing Centre for Thermal Solar Systems (TZS), Pfaffenwaldring 6, Stuttgart, Germany, Phone: , Fax: , 1. Introduction Seasonal thermal energy storage is an integral component for central solar heating plants to achieve high solar fractions. For economical and ecological reasons the systems must be designed very carefully. One focus of the design is the dimensioning of the seasonal thermal energy store as well its construction. In Eggenstein-Leopoldshafen located in the south-west of Germany the latest generation of a gravel-water thermal energy store (GWTES) was built in 2007 and is in operation since 2009 [1], [2] and [3]. The store is integrated into Germany s first central solar heating plant with seasonal thermal energy storage (CSHPSS) realised with existing buildings (Figure 1). After a major refurbishment of the buildings and the heating network m² flat plate collectors (FC) have been installed. The store has a volume of m³. Discharging of the seasonal GWTES down to temperatures of around 10 C is facilitated by the use of a 60 kw th heat pump [4]. Thus, the usable thermal capacity of the GWTES can be increased by more than 40 % compared to discharging from 80 C to the network return temperature of 40 C. For backup heating two 600 kw gas boilers and a 30 m³ hot water buffer store are available. The system is designed to achieve a solar fraction up to 40 % of the total heat demand. According to calculations, primary energy savings of 65 % may be achieved due to the refurbishment of the buildings and the integration of the CSHPSS. This corresponds to an annual reduction of CO 2 emissions of 390 t. refurbished school refurbished public pool 600 m² FC refurbished gym fire station 1000 m² FC new gym gravel water thermal energy store Figure 1. CSHPSS with m² flat plate collectors and seasonal GWTES in Eggenstein-Leopoldshafen (Germany)

2 2. The Gravel-Water Energy Store in Eggenstein-Leopoldshafen The GWTES closes the gap caused by the seasonal mismatch between high solar thermal supply in summer and high heat demand in the winter period. With regard to the construction of the seasonal thermal energy store in Eggenstein- Leopoldshafen several boundary conditions had to be considered. As the ground water level is only 7.5 m below the ground surface level, the store had to be constructed in such a way that the thermal insulation is protected from penetration of ground water. Unrestricted accessibility and trafficability was required by the customer due to the location within the school yard. Therefore a totally safe construction had to be selected even in the case of a total failure of the store liner. Hence, the concept with a GWTES was favoured over a hot water thermal energy store. The geometry of the store consists of two truncated cones, see Figure 2. Two thirds of the volume of the store are located below ground surface level. It is filled with washed mm gravel to a height of 2.5 m. In the remaining volume, up to the ground surface level, the excavated gravel/sand is refilled in order to reduce construction costs. The upper third of the store is formed as a truncated cone with washed mm gravel. Charging and discharging of the GWTES is realised directly by two vertical wells. The deep well is embedded into the bottom gravel layer, the shallow well into the top gravel layer. Special attention was given on the thermal insulation type, thickness and protection. The full surface of the store even the bottom is thermally insulated. Expanded glass granules at the bottom truncated cone and foam glass gravel at the top truncated cone of the store were employed as insulation materials. Foam glass gravel was favoured over expanded glass granules for the upper part as it is shapeable due to its relatively high friction angle. The insulation thickness is 0.5 m at the bottom of the store and increases up to 0.8 m at the top. To protect the thermal insulation from getting wet from any direction it is filled into 35 chambers of HDPE liners. Furthermore the internal liner is equipped by a water vapour barrier to protect the thermal insulation from getting wet from the inside of the store by water vapour diffusion. After their construction all chambers were evacuated. The evacuation system serves additionally as leak detection system. Due to the HDPE liner the maximum operating temperature of the store is limited to 80 C. Figure 2: Scheme of the gravel water thermal energy store (GWTES) in Eggenstein-Leopoldshafen More detailed information about the GWTES in Eggenstein-Leopoldshafen and about seasonal thermal energy storage in general can be found in [5], [6] and [7]. The latest innovations applied in the GWTES in Eggenstein-Leopoldshafen in comparison to older GWTES such as the one in Chemnitz [8] and Steinfurt [9] are summarised as follows: - for the first time the excavation of the pit was partly refilled into the store to reduce the construction costs - the construction of the thermal insulation of the GTWES was further improved and kept more simple by using chambers that are filled with the insulation material; the chambers have been evacuated as leak detection system

3 - the double truncated cone shape of the store improves the surface-volume ratio in comparison to a single truncated cone or pyramid shape and thus the heat losses will be reduced - the use of foam glass gravel as cost-effective recycled thermal insulation material - integrating a simple direct charging and discharging unit into the store facilitated by two wells Some steps of the construction of the store are shown below in Figure 3. Figure 3: Construction of the GWTES in Eggenstein-Leopoldshafen; the first chambers filled with expanded glass granules (top left); filling with gravel into the bottom of store (top right); refilling the excavated sand (bottom left); banking up the foam glass gravel onto the top of the store (bottom right) For the accompanying research of the CSHPSS and the GWTES in particular extensive measurement equipment has been installed (see Figure 4). The store is equipped by 56 Pt100 temperature sensors and 8 high sensitive heat flux sensors. There are installed 38 temperature sensors inside the store on different horizontal layers and vertical axis, 10 temperature sensors in 4 lances in the ground beside and underneath the store and 8 temperature sensors coupled with the heat flux sensors spread over the outer liner of the store s thermal insulation. Thus it is possible not only to measure the store s temperature but also the vertical and even horizontal stratification and to detect regions of higher fluid flow during charging and discharging phases by transient temperature changes. With the temperature sensors in the lances inside the ground around and underneath the store it is possible to evaluate the thermal interaction of the store with the ambience. Finally, it is possible to detect the heat fluxes at certain spots of the thermal

4 insulation by using the heat flux sensors coupled by each one temperature sensor on the internal and one on the outer liner of the store s thermal insulation. Figure 4: Location of sensors in and around the store (T temperature sensor; HFS heat flux sensor) 3. Results and discussion Up to now the entire system is still struggling with some technical problems; e. g. the discharging cycle of the GWTES is not running reliable, due to hydraulic problems. Nevertheless valuable measurement results were gained by monitoring the store s thermal behaviour in detail. In specific, almost stationary interactions between the store and the ambience could be evaluated during the standstill periods, when no charging or discharging occurred. Thus the heat losses of the store could be determined based on the decrease in temperature and the measured heat fluxes at the heat flux sensors. In Figure 5, the stored amount of thermal energy is illustrated for the first three consecutive years of operation. Figure 5: Stored amount of thermal energy (0 C equates to 0 MWh) and weekly energy difference The amount of thermal energy refers to 0 C as theoretically lowest possible discharging temperature due to water is used as brine. Hence, the initial amount of thermal energy starts at

5 50 MWh which corresponds to a mean store temperature of 15 C. The maximum mean temperature of the store of 61 C was measured in Due to hydraulic problems, the charging cycle of the GWTES didn t work properly in 2011 and so only small amounts of thermal energy were charged into the store. In addition, the weekly difference of energy is added to the graph in Figure 5. In cooling-off periods, which are only caused by heat losses, the weekly reduction of the stored amount of thermal energy is between 1.0 and 2.5 %. This corresponds to a weekly temperature decrease of K. The heat losses of the store are between 9 and 25 kw which corresponds to heat fluxes of about 8 22 W/m² uniformly spread over the entire surface of the store. In Figure 6, the temperature profiles are displayed along the centre axis (according to Figure 4) for the and the Those two profiles represent a fairly high charged store and a discharged store. For the temperature profile in August a typical stratification can be monitored inside the store. The temperature gradient differs slightly in the different layers (gravel and sand) due to marginal differences in the permeability, heat capacity, heat conductivity and the locations of the two wells, which influence the temperature distribution by the fluid flow. The temperature profile in January does not show the expected stratification. Contrariwise, the upper half of the store is up to 5 K colder than the lower half and so an inverted stratification develops. This can only result through higher heat losses at the top than at the bottom of the store. height above store's bottom h / m amb amb soil insulation gravel sand gravel insulation soil temperature / C Figure 6: Temperature profiles along the centre axis of the store at as example for a charged store and at as example for the store cooled off by heat losses To investigate the location of the heat losses more detailed the measured heat fluxes and coupled temperatures were analysed. Based on the measured data the calculated effective heat conductivity λ eff at the location of the heat flux sensors (HFS) around the envelope of the store is depicted in Figure 7 beginning from the first charging period up to the beginning of 2012.

6 Additionally, the temperature difference between the mean store temperature and the ambient air temperature is displayed as driving force for the heat flow at the top of the store. Surprisingly, the effective heat conductivity λ eff fluctuates strongly at the top of the store (HFS 4, HFS 7 and HFS 8), where foam glass gravel as thermal insulation material is used. In comparison to that, the effective heat conductivity λ eff at the bottom of the store (HFS 2 and HFS 5), where expanded glass granules are used, is nearly constant at all times. Especially, in periods of a high temperature difference between the store inside and the ambient air, the effective heat conductivity λ eff increases at the top of the store. This induces significantly higher heat losses at the top of the store. 0,7. 70 effective heat conductivity λ eff / W/(m K) 0,6. 0,5. 0,4. 0,3. 0,2. 0,1. HFS 4 HFS 7 HFS 8 HFS 5 store - amb tempereture diffrence / K HFS time t / dd.mm.yyyy 0 Figure 7: Effective heat conductivity λ eff at different locations on the store s envelope and temperature difference between the mean store temperature and the ambient air temperature from To investigate the thermal behaviour of the thermal insulation more detailed a test stand on laboratory scale was established. The test stand consist of a cuboid shaped box with the dimensions 100 cm x 80 cm x 75 cm (length x width x height), see Figure 8. The box is filled with foam glass gravel with a grain diameter of mm and it has been compressed slightly. This cuboid shaped box should simulate a cut-out section of the thermal insulation construction at the top of the store. At the bottom of the box a heat plate can simulate the heated store. The temperature at the outside of the insulation (on the top) can be simulated by the air temperature in the climatic chamber, where the test stand is installed. The side faces of the box are insulated by expanded polystyrene to assure adiabatic conditions. A multitude of experiments have been carried out at the test stand varying the set temperature of the heat plate and the moisture content in the insulation bulk material. In the store in Eggenstein- Leopoldshafen foam glass gravel is used with a moisture content of 5 kg water /m³ bulk material. A totally dry insulation material is favourable as moisture increases the heat conductivity [10], but in the case of the store in Eggenstein-Leopoldshafen the foam glass gravel couldn t be delivered totally dry by the supplier. Thus the influence of the moisture content was also explored in the test stand. A thermographic image of the rear side of the test stand was taken immediately after removing the polystyrene insulation, see Figure 8 (top right). The temperature of the heat plate is set to 75 C and the moisture content is set to 1 kg water /m³ bulk material. Against the expected horizontal isothermal distribution it seems like vortexes develop by free convection. Taking a closer look on the plexiglass front side of the test stand (see Figure 8; bottom left) condensation at the top

7 half can be observed and even a dripping down of the condensed water in the bulk and at the plexiglass. Hence, beside the convection in the bulk material, additionally a mass transfer occurs. This mass transfer operates like a heat-pipe effect. So addition thermal energy is transferred through the bulk material. Thus several heat transfer mechanisms accumulate which influence the effective heat conductivity λ eff. In Figure 8 (bottom right) the temperature profile along the centre axis of the box is displayed. The diagram shows the temperature profile for a moisture content of 5 kg water /m³ bulk material. Based on the development and the continuous change of the vortexes steady states are never reached. Therefore, in the graph the minimum, maximum and mean temperatures measured during the experiment are depicted. The temperature gradient changes over the height. Only on the bottom 15 cm a temperature difference is pronounced. Above the bottom 15 cm the temperature gradient decreases significantly. This indicates an increase of the effective heat conductivity λ eff and so the higher heat losses of the store can be explained max height h / m min temperature / C Figure 8: Picture of rear side of test stand (top left), corresponding thermographic image with a moisture content of 1 kg water /m³ bulk material and a set temperature of the heat plate of 75 C (top right), picture of front-side of test stand with view on the plexiglass and thermal insulation with a moisture content of 1 kg water /m³ bulk material (bottom left) and temperature profile with a moisture content of 5 kg water /m³ bulk material At a different test stand processes to dry out the bulk material in chambers via in-situ processes have been carried out. This test stand is also equipped by a heating underneath the chamber to simulate the heated store. By decreasing the pressure inside the chamber the evaporation temperature will be decreased as well and the evaporated water can be delivered out by the vacuum pump. Monitoring the temperatures and thus the vertical temperature showed that already a low pressure of less than 0.5 bar had an influence on the heat distribution. Hence, the mass transfer and the free convection had to be decreased by the decrease of pressure. This cognition was adapted on an experiment at the store in Eggenstein-Leopoldshafen, where the pressure inside the chambers at the top of the store was decreased by a vacuum pump. The result of this test is depicted in Figure 9. It represents a cut-out of the graph in Figure 7. Additionally, more heat flux sensors are included to the graph. It is obvious that during the period, where the

8 top chambers were evacuated down to a pressure of 0.4 bar, the effective heat conductivity λ eff is also decreased. Unfortunately, the entire vacuum system is not leak-proof and therefore the pressure inside the chambers quickly increased again after finishing the test. Figure 9: Effective heat conductivity λ eff at different locations at the store s envelope and temperature difference between the mean store temperature and the ambient air from Conclusions In Eggenstein-Leopoldshafen the latest GWTES in Germany was built for seasonal storage in a central solar heating plant. Many innovations e. g. new thermal insulation materials were implemented into the pilot project. Thus the construction costs could be decreased further in comparison to former projects. Charging the store for the first time by solar heat took place in Due to hydraulic problems of the discharging cycle in particular a regular discharging of the store couldn t be realised so far. Nevertheless valuable measurement results were gained monitoring the store s thermal behaviour in detail. Thus higher heat losses than expected were detected. Investigating this issue in detail higher heat losses than expected were located at the top of the store, where foam glass gravel as thermal insulation material is used. Test stands, that have been built to explore this phenomenon more detailed on laboratory scale, were established. The results of the tests show, that if foam glass gravel is used as thermal insulation material on top of a store, multiple heat transfer mechanisms overlap. Beside the heat transfer through heat conductivity in the bulk material free convection can appear. In the presence of moisture, which is the case in the store in Eggenstein-Leopoldshafen in small amounts, even an additional mass transport can occur in form of a heat-pipe effect. So, additional thermal energy can be transferred through the thermal insulation. This affects the effective heat conductivity λ eff of the thermal insulation. The higher effective heat conductivity λ eff induces higher heat losses of the store. As monitored the cooling-off rate is 1.5 K/week in maximum. Considering that the store should be discharged within the first three months of the heating period a total decrease of about K in the mean temperature is the result of the heat losses. The evacuation test at the top chambers of the store has shown that it might be still possible to improve the thermal insulation without refurbishing the store just by adding a continuously working vacuum system.

9 As result of the accompanying research it can be concluded, that bulk materials with large grain diameters are not favourable as thermal insulation material on top of thermal energy stores especially if they are used with high thicknesses. 5. References [1] Bauer, D.; Heidemann, W.; Marx, R.; Nußbicker-Lux, J.; Ochs, F.; Panthalookaran, V.; Raab, S. Forschungsbericht zum BMU-Vorhaben, Solar unterstützte Nahwärme und Langzeit- Wärmespeicher (Juni 2005 bis Juli 2008), FKZ J, Institute for Thermodynamics and Thermal Engineering (ITW), University of Stuttgart, Germany, 2009 [2] Pfeil, M. Realisierung eines solaren Nahwärmesystems mit Langzeitwärmespeicher in einem Schul- und Sportzentrum der 1960er Jahre", 17. Symposium Thermische Solarenergie, 9th to 11th May 2007, Kloster Banz, Bad Staffelstein, Germany, 2007 [3] Bauer, D.; Marx, R.; Nußbicker-Lux, J.; Ochs, F.; Heidemann, W.; Müller-Steinhagen, H. German Central Solar Heating Plants with Seasonal Heat Storage Solar Energy 84 (2010), , 2010 [4] Riegger, M.; Mangold, D. Planungsoptimierung und Bau des solaren Nahwärmesystems mit saisonalem Kies-Wasser-Wärmespeicher in Eggenstein-Leopoldshafen, 18. Symposium Thermische Solarenergie, 23rd to 25th April 2008, Bad Staffelstein, Germany, 2008 [5] Ochs, F.; Nußbicker-Lux, J.; Marx, R.; Koch, H.; Heidemann, W.; Müller-Steinhagen, H. Solar assisted district heating system with seasonal thermal energy storage in Eggenstein- Leopoldshafen, EuroSun 2008, 1. International Conference on Solar Heating, Cooling and Buildings, 7th to 10th October, Lisbon, Portugal, 2008 [6] Ochs F. Abschlussbericht zum Vorhaben Weiterentwicklung der Erdbecken- Wärmespeichertechnologie FKZ E, BMU, Germany, 2008 [7] Marx, R.; Ochs, F.; Heidemann, W.; Müller-Steinhagen, H. Innovative Example for Central Solar Heating Plant with Seasonal Thermal Energy Storage in Germany, ISES Solar World Congress, 11th to 14th October 2009, Sandton Convention Centre, Johannesburg, South Africa, 2009 [8] Urbaneck, T.; Schirmer, U. Forschungsbericht Solarthermie2000 Teilprogramm 3 Solar unterstützte Nahwärmeversorgung Pilotanlage Solaris Chemnitz FKZ O, Fakultät für Maschinenbau, Professur Technische Thermodynamik, Chemnitz University of Technology, Germany, 2003 [9] Pfeil, M.; Koch, H. Saisonaler Kies/Wasser-Wärmespeicher der 3. Generation für die Solarsiedlung Steinfurt Borghorst, 9. Symposium Thermische Solarenergie, 5th to 9th May 1999, Kloster Banz, Bad Staffelstein, Germany, 1999 [10] Ochs, F. Modelling Large-Scale Thermal Energy Stores, Dissertation, University of Stuttgart, Germany, Shaker-Verlag, ISBN , Acknowledgements The construction of the project and the ongoing scientific work has been supported by the German Ministry for Environment, Nature Conservation and Nuclear Safety (BMU). The authors gratefully acknowledge this support and carry the full responsibility for the content of this paper.