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STATUS OF SEASONAL THERMAL ENERGY STORAGE IN GERMANY by ∗Volkmar Lottner1 and Dirk Mangold2 BEO, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany, Fax:+49-2461-613131, [email protected] 2

Institut für Thermodynamik und Wärmetechnik, Universität Stuttgart, Pfaffenwaldring 6, D-70550 Stuttgart, Germany, Fax:+49-711-685-3242, [email protected]

KEY-WORDS Seasonal storage, solar heating, large scale storage systems, thermochemical energy storage

Abstract The paper presents a summary and review of the present status of R&D of seasonal thermal energy storage activities in Germany. Two different strategies are in investigation: small scale decentralized solar assisted heating systems of single family houses as well as large scale district heating with central seasonal stores. Sensible and thermochemical energy storage technologies show different technical and economic prospects in both heating schemes. R&D efforts on large scale storage technologies are included in the programme Solarthermie-2000 of the Federal Ministry of Economics and Technology (BMWi). The review includes a comprehensive technical-economic evaluation of first pilot and demonstration plants with different seasonal thermal energy storage technologies.

Introduction Heating of buildings offers a great potential of saving fossil fuels. At present in Germany, one third of the total energy demand falls into this end use energy sector. During the last 25 years the specific heating demand of new buildings has been reduced considerably. Today various energy saving measures are standard, e. g. improved heat insulation of walls, roofs and low-e windows, highly energy efficient gas- and oil condensing burners, heat pumps and solar thermal domestic hot water systems. Present building codes prescribe a low energy heating standard for the construction of buildings. At present, the specific space heating demand of buildings is limited to maximal 50100 kWh/m²a, but it will be reduced by another 30 % in the new code which will be probably operative in the year 2001. Legislation and implementation of energy saving measures have a very high priority in the energy policy of the Federal Republic of Germany. The government has confirmed the commitment of a reduction of CO2-emissions into the atmosphere by 25 % for the year 2005, compared to the year 1990. R&D on seasonal thermal energy storage is funded in the governmental programme on energy research and technology of the Federal Ministry of Economics and Technology (BMWi). Various thermal energy storage technologies have been investigated since 1974. In the first period, the programmes focused on basic research including model calculations, laboratory experiments and the construction of small scale pilot plants. The technical and economic feasibility of the storage concepts had to be proven. As a result of these investigations high priority has been given to R&D on large scale thermal energy storage in district heating systems with the programme Solarthermie-2000. In the following paragraphs the present status of R&D for both decentralized and centralized seasonal thermal energy storage concepts is summarized. 1

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STATUS OF SEASONAL THERMAL ENERGY STORAGE IN GERMANY TERRASTOCK 2000, Stuttgart, Germany, August 28 until September 1, 2000

Decentralized Solar Assisted Heating of Low Energy Buildings Recently the interest on decentralized solar assisted heating plants with seasonal storage has been renewed after the implementation of the building codes for low energy buildings. Hot water stores and thermochemical storage systems are both technically feasible seasonal storage concepts. Seasonal Hot Water Stores

excessive thermal losses. Due to the high specific storage costs of small scale stores, seasonal storage of solar energy is not cost-effective in this field. The residual heating demand of low energy buildings can be covered by storage volumes in the order of 10 to 50 m³. In the project zero-heating energy house in Berlin a vertical, well insulated 20 m³ hot water tank has been installed to store solar heat from the 54 m² solar collector roof. Monitoring, however, revealed that the heating demand could not be covered completely by solar energy. Due to the behaviour of the inhabitants the heating demand of the house was considerably higher than calculated by dynamic simulation programmes. The incorrect control of the solar heating system caused operational problems and destroyed the thermal stratification of the hot water store. In a consecutive project the building and heating concept has been redeveloped by a building company. A long term monitoring programme is being carried out on a pilot building. Further optimization of the concept is being continued to start the market introduction. Thermochemical Energy Store Thermochemical storage systems offer qualitative advantages compared with hot water stores: smaller storage volumes due to higher energy storage density and in principle no thermal energy losses even for long storage periods. Thermochemical stores constitute the central component of heat transformers and chemical heat pumps. Low temperature ambient or solar heat can be used for a low temperature heating system. The economics of this concept is still uncertain, but there is a potential that the performance can be improved and material and equipment costs can be reduced in mass production. R&D efforts are focused on the further development and optimization of storage materials and the storage equipment (storage design, heat exchanger). New porous solid adsorbent materials (zeolithe-water, silicagel-water) with higher energy storage density, which can be produced at acceptable costs, are being investigated and developed. A main goal is to increase the energy storage density and to reduce the desorption temperature of water from the adsorbent. The dynamical characteristics of the components and of the whole storage system are investigated and optimized. Some first pilot installations of stores in technical scale are monitored.

Programme Solarthermie-2000: Seasonal Thermal Energy Storage Technologies In centralized large scale solar assisted heating plants substantial cost reductions are possible by the scale of system size. With increasing storage volume the specific construction costs as well as the relative heat losses of the store decrease. In 1993 the programme Solarthermie-2000 has been launched by the Federal Ministry of Research and Technology (BMBF) to demonstrate the technical and economic feasibility of the most promising storage and system concepts in large scale. In 1998 the programme was handed over to the Federal Ministry of Economics and Technology (BMWi). Part 3 of the programme Solarthermie-2000, entitled with „Solar Assisted District Heating Systems with Seasonal Storage“, aims on the realisation of large scale, seasonal thermal energy stores that are connected to a central heating plant of a housing area. Figure 1 gives a scheme of a typical system, where heat is delivered from the heating central to the houses via the district heating net and heat transfer substations. The seasonal store is connected to the heating

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The storage technology is conventional, however long term storage requires an excellent heat insulation to avoid

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central and is heated up by thermal solar energy. This energy is collected via large collector areas that are mounted on some of the roofs in the housing area. A gas burner secures the supply temperature in the district heating net, that is mostly between 65 and 75 °C.

heating central

heat transfer substation

Brauchhot wassertap speicher water

r la so

heat transfer substation cc

rs to lec l co

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lar so

rs to ec l l co

Brauchhot wassertap speicher water

cc fresh water

fresh water

district heating net solar net

hot water store (seasonal store)

Fig. 1: Scheme of a solar assisted heating plant with seasonal store (cc: circulation of hot tap water). Mainly in the programme Solarthermie-2000 the following different concepts for seasonal stores were realized: • Hot water store (Rottweil, Hamburg-Bramfeld, Friedrichshafen-Wiggenhausen, Hannover-Kronsberg) • Gravel/water store (Solaris-Chemnitz, Steinfurt-Borghorst) • Duct store (Neckarsulm-Amorbach) • Aquifer store (Rostock-Brinkmanshöhe, Berlin-Reichstag) Long term monitoring programmes have been carried out to yield reliable technical and cost data for the evaluation of the concepts. The results will be used to further improve the technical and economic feasibility of the concepts. A main goal of the programme is to reduce the storage costs as well as the specific system costs, which are at present about three to four times higher than they should be for an economic utilization of seasonally storing solar energy or waste heat of combined heat and power plants or industrial processes. The large scale solar plants are designed to cover in average 50 % (34 to 62 %) of the annual district heating demand (hot water and space heating) by solar energy. The size of the collector area and the volume of the store result from dynamic simulation calculations of the systems. These simulations and the monitoring of the plants has confirmed that the solar fraction of in average 50 % was not achieved in the first year of operation. This results from higher thermal losses of the store during the first year, but also from higher return temperatures of the district heating system. Therefore the thermal storage capacity of the store could not be used as presupposed. On the other hand, the 3

STATUS OF SEASONAL THERMAL ENERGY STORAGE IN GERMANY TERRASTOCK 2000, Stuttgart, Germany, August 28 until September 1, 2000 solar plants operate without serious technical failures and showed by and large the expected performance. Table 1 presents the R&D plants that are realised within part 3 of the programme Solarthermie-2000.

Project

Hamburg-Bramfeld

(Solar)System area 3,000 m²

Type of store

Concrete hot water store with stainless steel liner

Volume of store/ capacity 4,500 m³

Max. temperature

In operation since

95 oC

1996

FriedrichshafenWiggenhausen

5,600 m²

Concrete hot water store with stainless steel liner

12,000 m³

95 oC

1996

Hannover-Kronsberg

1,350 m²

Concrete hot water store without liner

2,750 m³

95 oC

2000

Solaris-Chemnitz

540 m² (vt)

Gravel/water store with plastic liner

8,000 m³

85 oC

1997

Steinfurt-Borghorst

510 m²

Gravel/water store with doubled plastic liner

1,500 m³

90 oC

1999

Neckarsulm-Amorbach

2,700 m²

Duct store

20,000 m³

70 oC

1999

o

RostockBrinkmanshöhe

1,000 m²

Shallow aquifer store

20,000 m³

50 C

2000

Berlin-Reichstag

chpp

Shallow and deep aquifer store

100 m³/h

10 oC /70 oC

1999

Storage Concepts The storage concepts, that are investigated in the programme Solarthermie-2000, are described in more detail in previous publications (LOTTNER et al. 1997, LOTTNER et al. 2000). This paper is restricted to the presentation of an update in particular of the pilot and demonstration projects of the programme Solarthermie-2000. Hot water Stores The cost analysis of the two plants in Hamburg and Friedrichshafen showed that the stainless steel liner is a very expensive component of the store (LICHTENFELS et al. 2000). In a new construction concept the liner can be avoided. The wall is made of high density reinforced concrete which exhibits a negligible water diffusion rate even at hot water temperatures. Extensive preliminary investigations have been carried out to test new concrete material compositions which are suitable for the required conditions (temperature, mechanical stress). The new concrete

Fig. 2:

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Construction of the store in Hannover-Kronsberg. Left picture: excavation with base plate (picture by ITW), right picture: insulation work on storage top (picture by Pki, Stuttgart).

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Table 1: Large scale seasonal thermal energy storage projects in Germany (chpp: combined heat and power plant, vt: vacuum tubes).

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material has been applied for the first time in the 2,750 m³ hot water store of the solar city project in HannoverKronsberg (REINECK et al. 2000). Figure 2 gives two pictures taken during the construction of the store. The 20 to 30 cm thick reinforced concrete wall is insulated on the outside with granulated blown-up glass that is made of recycled material. These small particles with a diameter of 2 to 4 mm are packed on site in large bags of textile fabrics. The same type of insulation has been successfully used before in the project in Steinfurt-Borghorst

steel liner leads to a certain water vapour transfer through the concrete material. Consequently the entire construction from the concrete wall to the surrounding earth has to be open for water vapour diffusion in order to avoid water condensation in the insulation. Because of this, for example, the insulation is protected from the water that can occur in the drainage with a watertight plastic layer that is open for vapour diffusion from the insulation to the surrounding drainage. The construction of the store showed that a very careful processing of the concrete is essential. Another promising new concept is a cylindrical tank made of glass fibre reinforced plastics. The compound wall consists of outer reinforced plastic liners with integrated heat insulation. In an ongoing industrial project the construction technology is being developed to reduce the specific storage costs. The development aims at a construction system of prefabricated cylindrical segments. Accompanying investigations are carried out in a 300 m³ pilot store to examine the long term material durability and the thermal performance of the store (stratification, charging devices) during seasonal operation. Gravel/Water Store Based on the satisfactory results of the first 1,000 m³ pilot plant which was built at ITW of Stuttgart University and is in operation since 1985, the storage concept was applied for the construction of a 8,000 m³ demonstration plant in the project Solaris in Chemnitz. The store was completed in 1996, however the heating plant was not ready for operation before 540 m² of solar collectors (vacuum tubes) have been installed in 1999. A long term monitoring programme will be carried out by the Technical University of Chemnitz (URBANECK et al. 2000). Another 1,500 m³ store was constructed with a modified concept for the solar assisted district heating system of the new housing project in Steinfurt-Borghorst. The store is tightened with a doubled plastic liner. The space between the two layers is evacuated to allow a permanent control of the water-tightness during construction and operation. As heat insulation material granulated blown-up glass was used for the first time for seasonal stores. The system is designed to cover about 34 % of the annual heating demand by thermal solar energy. The first year of operation in 1999 showed that the results agree with the design data (PFEIL et al. 2000). Duct Store The solar project in Neckarsulm-Amorbach is being realized in several steps. At first the feasibility of the storage concept was proven with the installation of a 5,000 m³ prototype store at the site of the plant. The heat exchanger pipes are made of polybuthene and doubled in U-shape in every borehole, see figure 3. The design data of the model calculations have been validated by the experimental results of the Fig. 3:

Double-U-pipe with spacers and installation pipe for the store in Neckarsulm-Amorbach.

monitoring programme. In 1999, the store was

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(gravel/water store). In comparison to the first stores in Hamburg and Friedrichshafen the avoidance of the stainless

STATUS OF SEASONAL THERMAL ENERGY STORAGE IN GERMANY TERRASTOCK 2000, Stuttgart, Germany, August 28 until September 1, 2000 enlarged to a storage volume of 20,000 m³. Figure 4 gives a picture taken during the construction of the enlarged storage volume. Recent results of an extensive monitoring programme which has been carrying out since 1999 are consistent with the calculations (SEIWALD et al., 2000). Data from the monitoring programme will be used to examine a more detailed duct store model which takes into account combined moisture and heat transfer in the soil (REUSS et al. 2000).

to 63,000 m³ storage volume with a solar collector area of about 6,000 m².

Fig. 4:

Drilling of the boreholes in front of the heating central with the buffer store (100 m³) and a collector field on the sports hall.

Aquifer Store In the solar assisted district heating plant of the new housing project in Rostock-Brinckmanshöhe an aquifer is used as a low temperature seasonal store. Due to the small size of the plant, the shallow 30 m deep aquifer has to be operated in a temperature range between 10 and 50 oC. Model calculations for the design of the plant showed that a maximal fraction of the stored solar heat can be recovered by a 100 kWel heat pump. The aquifer is charged with solar heat from a 1,000 m² solar collector roof. A long term monitoring programme has been started in early 2000 (SCHMIDT et al. 2000). The district heating and cooling scheme of the renovated Reichstag building and of the connected neighbouring large office buildings of the Parliament include a shallow and a deep aquifer. The deep aquifer is charged in summer with surplus heat of 70 oC from the combined heat and power plants. These plants are operated dependent on the electricity demand of the connected buildings. According to the design calculations, about 60 % of the stored heat can be recovered during the heating period from the aquifer in the temperature range between 55 and 70 oC and can supplement the absorption heat pump system. The ground water of the shallow aquifer is used at ambient temperature for the air conditioning of the buildings. An extensive long term monitoring programme will examine 6

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This year the next phase of the solar assisted district heating project has been started: the duct store will be enlarged

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the technical and economic feasibility of the concept (KABUS et al. 2000). Accompaning investigations on hydrogeochemical aspects of the aquifers are carried out in close international cooperation within the IEA-Programme Energy Conservation Through Energy Storage, Annex 12: High Temperature Thermal Energy Storage in Aquifers (SANNER et al. 2000).

Figure 5 presents the cost data of the built pilot and demonstration plants of table 1 and of studies. The specific storage costs are related to the water equivalent storage volume. Due to the lower specific heat capacity of soil and gravel, the storage volume of gravel/water-, duct- and aquifer stores has to be scaled by a factor of about 1.3 to 5. The exact scaling factor depends on the site specific geological parameters. The volume of aquifer stores cannot be exactly specified. The relevant quantity is the maximal thermal capacity of the wells for charging and discharging. 1000

900

Rottweil hot water concrete

built study

Investment cost per water equivalent [DM/m3]

Steinfurt gravel/water

800

700

600

500

Kettmannshausen hot water glass fibre

Stuttgart gravel/water

400

Hamburg hot water - concrete Bielefeld hot water concrete

Berlin-Biesdorf aquifer

300 Chemnitz gravel/water Friedrichshafen hot water - concrete

200 Neckarsulm duct

Neckarsulm duct - next phase

100 Rostock aquifer

0 100

1000

3

Volume water equivalent [m ]

10000

Potsdam aquifer

100000

Fig. 5: Specific storage costs. Figure 5 shows the strong cost degression with an increasing storage volume. The storage costs include costs of charging devices, connecting pipes from the store to the heating central, planning costs, but no VAT. Moreover, system costs like costs for heat pumps are not considered. Additional costs can arise especially for duct and aquifer stores for site exploration. High maintenance costs have to be taken into account for water treatment in aquifer stores, if necessary. The economy depends not only on the storage costs, but also on the thermal performance of the store and the connected system. Therefore each system has to be examined separately. In this context important parameters are the maximum and minimum operation temperatures of the store and of the district heating net. Obviously heat from the store can only be used without a heat pump as long as the storage temperature is higher than the return temperature of the district heating system. To determine the economy of a store, the investment and maintenance costs of the store have to be related to its thermal performance. This quantity is equivalent to the cost of the usable stored energy. Recently a national team of experts has been established to evaluate the economy of the pilot and demonstration plants of the programme Solarthermie-2000.

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Specific Storage Costs and Economics

STATUS OF SEASONAL THERMAL ENERGY STORAGE IN GERMANY TERRASTOCK 2000, Stuttgart, Germany, August 28 until September 1, 2000

References BENNER, M.; MAHLER, B.; MANGOLD, D.; SCHMIDT, T.; SCHULZ, M.; SEIWALD, H. 1999: Solar unterstützte Nahwärmeversorgung mit und ohne Langzeit-Wärmespeicher (September 1994 bis Oktober 1998) (Solar assisted district heating with and without seasonal store, research report, in German), Forschungsbericht zum BMBF-Vorhaben 0329606 C, ITW, Uni Stuttgart, 1999, ISBN 3-9805274-0-9

the practice, in German), BINE-Informationspaket, TÜV-Verlag, Köln, 1998, ISBN 3-8249-0470-5 KABUS, F.; SEIBT, P.; POPPEI, J,: Aquifer thermal energy stores in Germany, Terrastock 2000: 8th International Conference on Thermal Energy Storage, August 28-September 1, 2000, Stuttgart, Germany, Proceedings LICHTENFELS, A.; REINECK, K.-H.: The design and construction of the concrete hot water tank in Friedrichshafen for the seasonal storage of solar energy, Terrastock 2000: 8th International Conference on Thermal Energy Storage, August 28-September 1, 2000, Stuttgart, Germany, Proceedings LOTTNER, V.; HAHNE, E. 1997: Status of seasonal thermal energy storage in Germany, Megastock `97, 7th International Conference on Thermal Energy Storage, June 18-21, 1997 Sapporo, Japan, Proceedings, Vol.2, p. 931-936 LOTTNER, V.; SCHULZ, M. 2000: Solar assisted district heating plants – Status of the German programme Solarthermie-2000, to be published in Solar Energy, 2000, Special Issue: „Large Scale Solar Heating“ PFEIL, M.; KOCH, H.; BENNER, M. 2000: The third generation of long-term gravel-water storage - SteinfurtBorghorst, Terrastock 2000: 8th International Conference on Thermal Energy Storage, August 28-September 1, 2000, Stuttgart, Germany, Proceedings REINECK, K.-H.; LICHTENFELS, A. 2000: High performance concrete hot-water tanks for the seasonal storage of solar energy, Terrastock 2000: 8th International Conference on Thermal Energy Storage, August 28-September 1, 2000, Stuttgart, Germany, Proceedings REUSS, M.; MUELLER, J.-P. 2000: Investigation of heat and moisture transport in high temperature duct storage, Terrastock 2000: 8th International Conference on Thermal Energy Storage, August 28-September 1, 2000, Stuttgart, Germany, Proceedings SANNER; B.; KNOBLICH, K. 2000: IEA ECES Annex 12 – High temperature underground thermal energy storage, Terrastock 2000: 8th International Conference on Thermal Energy Storage, August 28-September 1, 2000, Stuttgart, Germany, Proceedings SCHMIDT, T.; KABUS, F.; MÜLLER-STEINHAGEN, H. 2000: The central solar heating plant with aquifer thermal energy store in Rostock, Germany, Terrastock 2000: 8th International Conference on Thermal Energy Storage, August 28-September 1, 2000, Stuttgart, Germany, Proceedings SEIWALD, H.; HAHNE, E. 2000: Underground seasonal heat storage for a solar heating system in Neckarsulm, Germany, Terrastock 2000: 8th International Conference on Thermal Energy Storage, August 28-September 1, 2000, Stuttgart, Germany, Proceedings URBANECK, T.; SCHIRMER, U. 2000: Central solar heating plant with gravel water storage in Chemnitz (Germany), Terrastock 2000: 8th International Conference on Thermal Energy Storage, August 28-September 1, 2000, Stuttgart, Germany, Proceedings

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HAHNE, E. et. al. 1998: Solare Nahwärme - Ein Leitfaden für die Praxis (Solar assisted district heating - a guide for