SOLAR DISTRICT HEATING WITH SEASONAL STORAGE IN ATTENKIRCHEN

SOLAR DISTRICT HEATING WITH SEASONAL STORAGE IN ATTENKIRCHEN M. Reuss, W. Beuth, M. Schmidt, W. Schoelkopf Bavarian Center of Applied Energy Research, ZAE Bayern Walther-Meissner-Str. 6, D-85748 Garching, Germany Tel: +49-89-356-250-30 mailto:reuss@muc.zae-bayern.de 1. INTRODUCTION Energy conservation through energy storage combined with intensive use of renewable energies will support the effort to meet the objectives of reduction of climatic gases formulated in the Kyoto protocol. Potential studies have shown that in Germany about 20 25 % of the end-energy consumption for space ing and domestic hot water can be covered by solar energy. Solar district ing with seasonal storage can cover half of this. The first projects realized in Germany were equipped with big water storages made of concrete, insulated and sealed with stainless steel liners like in Hamburg-Bramfeld and Friedrichshafen. The subsequent one built in Hanover- Kronsberg was constructed out of a special high quality concrete without liner /1/. In Chemnitz and Steinfurt- Borghorst gravel water pits were installed. The plant in Neckarsulm has large borehole storage (BTES) with the big advantage of enlargement according to the growth of the system. Due to the local hydro-geological situation the solar district ing in Rostock uses aquifer storage (ATES). Water storages with its environmentally-friendly and everywhere available storage material have the big advantage of a high capacity and excellent transfer properties. Rock, soil and groundwater as storage material of borehole and aquifer storage have a significant lower capacity and transfer is limited. Nevertheless a detailed analysis of realized projects has shown that insulated water storages are more expensive than borehole or aquifer storage. The combination of an underground water storage thermally coupled to borehole storage promises the combination of operational advantages of both systems by reducing the economical disadvantages. This storage type was realized in the solar district ing system in Attenkirchen, a small community near Freising north of Munich. 2. DESCRIPTION OF THE SYSTEM A solar district ing basically consists of the solar collector field, the seasonal storage, a supplementary ing and the district ing network. For solar space ing the supply structure and the buildings have to meet certain requirements. A compact district ing system has to be accompanied by improved thermal insulation of the houses. Attenkirchen is developing a new area for 30 homes in single occupancy and duplex houses. This settlement represents the typical size of new housing areas in rural communities in Germany nowadays. The buildings are designed with an improved insulation standard which results in a total demand for space ing (385 MWh/a) and domestic hot water (102 MWh/a) of 487 MWh/a. For this area a solar district ing with seasonal storage was planned /2/ with the goal to substitute about 50 % of primary energy compared to conventional ing. Nevertheless the investment for insulation is in direct competition with the investment for solar components, from an optimum insulation standard on it is more economic to use solar ing. The Solar Collector Most solar district ing systems use flat plate collectors which are combined to larger modules. From an energetic point of view this is favorable because piping can be reduced and partly be integrated in the collector itself. Large collector modules allow attractive architectural solutions. The Solar Roof system realized in

Attenkirchen shows additional economic advantages as roofing functions are taken over by the collector which consists of prefabricated modules of about 22 m². Figure 1: Solar roof collector in Attenkirchen The solar collector field with a total area of 836 m² (764 m³ aperture) has a copper absorber with a selective surface (Tinox) to achieve good thermal performance. The Storage The objective of 50 % savings of primary energy by solar district ing requires seasonal storage. Based on experiences with different types of underground thermal energy storages mentioned above an underground concrete water store combined with a field of borehole exchangers was investigated in a previous research project SOLEG sponsored by the Bavarian Research Foundation. Detailed system analysis in a feasibility study showed technical and economic advantages of this storage system /3/ which therefore was realized in Attenkirchen to gain also practical experiences. Figure 2: Combined water pit / borehole storage The central water pit (Figure 2) made of prestressed concrete serves as short-term or buffer store while the surrounding borehole field represents the long-term storage. The pit measures 9.00 m in diameter and 8.50 m in depth with a total volume of 500 m³. This combination allows a simpler and cheaper construction of the water store. The concrete vessel is built without a diffusion tight stainless steel liner and without thermal insulation on the bottom and side walls. Heat transport by conduction and vapor diffusion through the concrete walls are gains in the borehole storage and thus can be recovered from there. The top area of the whole storage is insulated by a 20 cm thick layer of polystyrene (XPS). In Attenkirchen 90 borehole exchangers of 30 m deep were installed in three rings surrounding water store which gives a volume of 10,500 m³. The average volumetric capacity of the underground measure at this location is 2.7 MJ/m³/K. Thus the borehole storage volumes corresponds to 6,800 m³ water equivalent and both

together 7,300 m³. The borehole exchangers are constructed as double-u-loops made of Polybuten pipes (20 x 2.3 mm) with spacers every 2nd meter. They are mounted in grouted boreholes of 150 mm diameter. For investigation of the long-term performance of thermal grouts two different materials were used. Half of the boreholes is grouted with a bentonite/cement/quartz-sand/water suspension while for the other half the convenience blend ThermoCem was used. The System In a systematic analysis of different system concepts with various backup systems those options with s showed technical and operational advantages as the useful temperature heave in the store is bigger. The high dependency of the system and storage performance on the return temperature of the district ing is avoided. As shown in the system scheme (Figure 3) discharging of the water store as well as of the borehole storage can be done directly or by s depending on the temperature levels. Additionally in case of demand in the houses direct supply is possible. This option is used for domestic hot water in spring, summer and autumn. Furthermore low price electricity can be used for discharging the borehole store with the during night. This is charged in the water storage for direct use later. Figure 3: Scheme of the system The solar field delivers the collected to the water store with high and to the borehole storage with low priority while the storage temperatures were used as control criterion. 2. CONSTRUCTION AND OPERATIONAL EXPERIENCES Construction of the System In summer 2000 the construction work started with the installation of the solar collector and the district ing. The collector was built in 11 m long and 2 m wide prefabricated roofing units including all structural components like the rafters. Mounting of these units proved to be easy and fast without any technical problems and extra costs. The construction of the storage and the ing central started early in 2001. Because of the narrow space available the concrete store has to be built first followed by the drilling work and mounting of the borehole exchangers

(Figure 4). The early coming winter made it necessary to postpone the connection work of the plastic pipes to spring 2002. During the whole construction period no significant technical problems occurred. As the first houses were occupied in February 2002 the system was put into operation with the water store at the end of January. During 2002 four more houses and an indoor tennis court were connected to the district ing. Actually 16 homes and an indoor tennis court are connected to the district ing which corresponds to about 2/3 of the total load. From an economic point of view this is disadvantageous as only part of the potentially deliverable can be sold while the full capital costs have to be paid. Figure 4: Construction of the concrete store (left) and drilling of the boreholes (right) Investment Costs The total investment for the system is summarized in Major mistakes in the control program are permanent operation of s at full speed which results in an enormous electricity consumption. The high ing rate in the district ing is responsible for the rather low differences between supply and return temperature and thus the higher losses. Pump operation in the storage - collector loop Table 1. The amounts given there are the real settled costs without VAT and without subsidies. The specific costs for the solar system (collector loop incl. exchanger) amount to 302.50 Euro/m² referred to the aperture area of 764 m² or 276,- Euro/m² referred to the gross collector area of 836 m². Considering that the collector replaces the roof completely these costs of the roof in the order of about 75,- Euro/m² can be credited to the collector ending up with collector system costs of 200,- Euro/m² gross area. The water storage costs 406.40 Euro/m³ and the borehole storage 46.- Euro/m borehole length or 18.30 Euro/m³ - water equivalent. The specific costs of the total storage amount to 45.- Euro/m³ - water equivalent. There is still a potential of cost reduction for the water pit by about 20-30 % if the excavation can be done in the regular way and the concrete structure is not prestressed. So far this is the cheapest seasonal storage realized in Germany. Operational Experience The water store is operated as a non-pressurized open system. As a major technical problem turned out that no submersible at the required size for hot water (up to 90 C) was available on the market at this time. Thus the s have to be installed in the ing center above the water level of the store. The installed special bleeding required several modifications in the hydraulic system. Significant problems occurred with the control program because of technical incompetence of the programmer who was not able to understand the system behavior and thus did not deliver a control program which allows full automatic operation of the system. A complete new control program from scratch is required which is actually under development. Because of the complicated legal process this work could not be started before October 2005.

Major mistakes in the control program are permanent operation of s at full speed which results in an enormous electricity consumption. The high ing rate in the district ing is responsible for the rather low differences between supply and return temperature and thus the higher losses. Pump operation in the storage - collector loop Table 1: Investment of the solar district ing Attenkirchen and specific costs awarding of contract positions sum district ing 160.600,00 pipe work 134.100,00 underground work 26.500,00 solarthermal 231.100,00 collectors 189.600,00 system costs: s, valves, etc. 41.500,00 ing central (building) 25.800,00 equipment of ing central 232.300,00 s (BTES) 25.300,00 s (HWS) 25.800,00 pipes, s, fittings, insulation 181.200,00 hot water storage 203.200,00 underground work 82.700,00 concrete work 98.800,00 hydraulic connections 10.000,00 pro rata planning 11.700,00 borehole exchangers 124.100,00 drilling 49.700,00 piping materials 41.900,00 connection of pipes 25.400,00 pro rata planning 7.100,00 control / electric connections 119.300,00 grid connection 10.800,00 planning and site management 41.700,00 connection for 6 houses 32.500,00 sum 1.181.400,00 solar 231,100 gross area aperture water pit 203,200 volume BHE s 124,100 total length ground volume water equivalent total storage 327,300 water equivalent 836 m² 764 m² 500 m³-h 2 O 2700 m 10,500 m³ 6800 m³-h 2 O 7300 m³-h 2 O 276 /m² 302 /m² 406 /m³ -H 2 O 46 /m 12 /m³ 18 /m³ -H 2 O 45 /m³ -H 2 O during night and in winter transferred solar from the store to the solar collector losses. Evaporation temperatures of s are depending on design and the used refrigerant limited to avoid overpressure failure. The hydraulic system in Attenkirchen provides therefore a temperature controlled mixing circuit to the evaporator which was not operating in the first two ing periods due to a wrong control algorithm. Thus an improved control program is required urgently and actually under development. As an additional feature and system improvement this control program will have an option of variable supply temperatures of the district ing. The low design temperature of the ing systems in the houses of about 35 C, the available domestic hot water stores and the control network are favorable for a demand related supply temperature. The background is to separate the time period of these two operation modes. Domestic hot water boilers will be charged before typical peak hours in the morning and in the afternoon to a temperature of ~50 C by increasing the net temperature. Between these charging periods the district ing supply temperature is determined by the demand of the space ing systems which is correlated to ambient temperature. This can reduce the losses in the district ing and improve the performance of the solar system without loosing comfort for the users. 3. PERFORMANCE MONITORING The system is equipped with an extended monitoring system which will allow detailed performance measurements for analysis of components, operation and optimization of control strategies. Major research topics are detailed analysis of all energy flows in the system to investigate system behavior and thermal performance as well as verification of the predicted yield of useful solar energy and solar fraction of the feasibility study. Operational features like a storage management (parallel or serial charging of water and borehole storage) or a cost optimized

operation using cheap night electricity to discharge borehole using the water store as buffer should be tested under real conditions. The first season of the monitoring was characterized by several failures of the monitoring system and long-term data losses due to severe damage of the data acquisition by a thunderstorm. Since spring 2004 the data availability increased significantly and was almost 100 % in 2005. As in the first season only a small part of the buildings were connected to the district ing a representative measured energy balance was not possible. collector water store 500 m³ change of content +1 MWh Heat exch. Heat exch. borehole storage 9350 m³ Figure 5: Energy balance of the period April 2004 March 2005 In this relevant monitoring period the system was not yet working properly because of the poor control program. The results were used for detailed analysis of the mistakes and evaluation of solutions. Nevertheless with manual control based on the results of the monitoring the system was providing to the consumer all year round without affecting their comfort. A detailed scheme of the energy fluxes in the system (2004/2005) is shown in Figure 5. In this period the water store was kept permanently on a temperature level above 50 C and the was directly transferred to the district ing, the was used only for experimentation. The specific solar gain delivered to the district ing was 373 kwh/m² (aperture) which exceeds the requirement of the funding program solarthermie2000 by 20 %. Based on these experiences first modifications were included in the control program in summer 2005. In autumn two new houses were connected to the district ing. collector water store 500 m³ change of content -17 MWh Heat exch. Heat exch. borehole storage 9350 m³ Figure 6: Energy balance of the period April 2005 February 2006 In this 2 nd evaluation period which was only 11 months long one of the two s connected to the BTES supplies now the directly to the district ing while the other one is still feeding the water store. The energy balance of the system in Figure 6 shows already the improvements in the installation and the first modifications of

the control program. Heat losses of the water store were reduced by lowering the temperature in winter below 15 C. An annual overview of the flux in both periods is shown in Error! Reference source not found. and Error! Reference source not found.. In 2004 more was charged into the BTES but also discharged in winter. Despite less houses and a the much milder winter in 04/05 much more was discharged from the store to the district ing than in 05/06 which is due to the improvements in the control program. In spring 2006 both storages are ed up in parallel which is according to TRNSYS simulations more efficient than keeping the water store on a higher level. These improvements realized have reduced the losses and the electricity consumption significantly but there is still potential for further optimization. As the system is the only source of the district ing, the modifications have to be done step by step to avoid any fault in the supply and reduced comfort for the users. The COP of the s from the BTES was 3.2 in the 1 st period and 3.8 in the 2 nd one, while the COP of the from the water pit to the district ing reached 4.4 in 2005/2006; it was out of operation in the 1 st period. The solar fraction SF is defined as the ratio of useful solar energy delivered the district ing and the required backup energy. Related to electricity which is required for operating the s the solar fraction was 73 % in the 1 st period and 74 % in the 2 nd one. 4. CONCLUSIONS The solar district ing with seasonal storage in Attenkirchen is one of the smallest of such systems in Germany. It is designed to supply a new settlement of 30 homes and an indoor tennis court with for domestic hot water and space ing. The objective of this project was to demonstrate the feasibility of such small size plants. Additionally the new storage type should be tested in real operational conditions. A detailed analysis of the construction process, costs and system performance is carried out to gain better input data for future design and to work out the cost reduction potential compared to existing plants. The actual results of the cost analysis are very promising. The plant was built in 2000/2001 and is in operation since spring 2002. Despite the fact that the developing area is growing slowly and not yet fully occupied the experiences during planning and construction phase are mainly positive. Construction costs regarding the innovative part of the solar collectors and the storage construction came out very positive. Control has turned out to be an important issue which resulted in significant operational problems. Fortunately the monitoring has shown that a new control program can solve all major problems detected in the system. Several features and operation modes intended could not yet been tested, but available operational experiences have shown that the system concept is technically and economically promising despite several problems especially in the control. There is still a significant potential of optimization in construction costs and performance. An important lesson learned is that for the first projects with such new technical concepts an intensive coaching of designers, construction companies and installers is required. ACKNOWLEDGEMENT The construction of the solar district ing system in Attenkirchen was subsidized by the Bavarian Ministry of Economy, Work and Technology. The R&D work was carried out within projects sponsored by the German Federal Ministry for Environment, Nature Conservation and Nuclear Safety (project no. 0329607D). The authors appreciate this support very much. REFERENCES Reuss M.: Solare Nahwärmeversorgung Attenkirchen; 7. Internationales Symposium für Sonnenenergienutzung SOLAR 2004 (8.-12.09.2004) in Gleisdorf, Oesterreich, Gleisdorf 2004.

Reuss M., Mueller, J. P.: Solare Nahwaerme mit saisonaler Waermespeicherung in einem kombinierten Erdbecken/Erdwaermesonden-Speicher. 12. Internationales Sonnenforum, Freiburg, 05. 07.07.2000. Hrsg.: Deutsche Gesellschaft fuer Sonnenenergie e.v. DGS Muenchen, 2000. Mueller, J. P.: Bewertung eines Hybridspeichers zur saisonalen Waermespeicherung. VDI Fortschrittsbericht, Reihe 19 Waermetechnik/Kaeltetechnik, Nr. 127. ISBN 3-18-312719-9. Duesseldorf: VDI Verlag 2001.