Projektinfo 01/2013

Storing solar energy in the groundGermany’s largest solar thermal plant has a collector surface area of around 7,400 m2

Solar thermal energy is intended to cover 50 per cent of the heating requirements of a residential area being developed on the site of a former army barracks in Crailsheim-Hirtenwiesen. To ensure that this is possible throughout the year, a seasonal borehole thermal energy storage system has been installed with a volume of 10,000 m3 of water equivalent. This is the most cost-effective version to be produced so far. However, before drilling could commence for the storage system, a specialist team had to search the area for unexploded ordnance.

To enable the American “McKee Barracks” to become a residential area, the Crailsheimer Bau- und Entwicklungsgesellschaft transformed five buildings in the former barracks into refurbished freehold apartments. The Stadtwerke Crailsheim municipal utility company has installed solar collectors on their roofs. In addition, a new housing estate has been created across a 32-hectare site, which includes not just a sports hall and secondary school with solar roofs but also detached, semi-detached and terraced houses and apartment buildings. The remaining two thirds of the collectors are situ-ated on a noise protection barrier that separates the commercial area from the residential district. These feed heat into a 480-m3 hot water buffer storage tank, while the roof collectors feed heat into a 100-m3 buffer storage tank. The challenge is to store the energy from the summer sun for the winter heating period with as little loss as possible. This is where the seasonal borehole thermal energy storage (BTES) system comes into use. If the solar thermal energy in the two buffer storage tanks is no longer sufficient, the seasonal thermal storage unit transfers heat into the system. This functions in combination with the second and larger buffer storage tank, which is situated between the collector array and the borehole thermal energy storage system. The buffer storage tank can feed heating energy around the clock into the long-term

Detailed information on energy research

This research project is funded by the

Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)

heating depot. This therefore enables a certain time lag in charging the BTES unit. The advantage of this is that the maximum charging capacity of the borehole thermal energy storage system can be maintained considerably under the maximum heat output from the collector array. This enables the size of the storage system to be reduced by such an extent that, despite the need for the additional buffer storage tank, the economic efficiency of the overall system is improved. A further reason for the low invest-ment costs of around 50 euros per cubic metre of water is that the project team deployed shanks on the borehole heat exchangers with already installed horizontal piping. “Previously, equally long boreholes have always been drilled into the ground that are connected together with tubes. The whole thing has been welded together on site, which is susceptible to faults, can cause contamination and is correspondingly expensive,” explained the head of the Solites Research Institute, Dirk Mangold, who has been advising the project. “In Crailsheim we have for the first time deployed borehole heat exchangers in different lengths and also ensured the necessary hydraulic com-pensation. This has enabled us to dispense with the previously used balancing valves,” which has saved ma-terial costs and reduced the workload. In addition, cross-linked polyethylene was used as the material for the borehole heat exchangers in contrast to the first storage generation that deployed expensive polybutene. Heat pump reduces storage lossesThe task of the electrically driven compression heat pump (coefficient of performance: 4.8) is to discharge the bore-hole thermal energy storage unit to a temperature that is as low as possible. “A special feature of the heat pump used by us is that it can cope extremely well with large differences in the source temperature,” explains Sebas-tian Kurz, Head of Planning at the Crailsheim municipal utility company. “Although the temperatures in the stor-age system can range between 20 and 50 °C, the heat pump must still provide a constant supply temperature of 60 °C.” The use of the heat pump enables the storage system to be discharged to lower temperatures. This therefore increases its efficiency: the heat losses are reduced while the usable temperature difference and the associated volume-related storage capacity increase. Particularly in spring, the rate of utilisation of the collector arrays on the noise protection barriers also increases as a result of the lower collector return temperatures. If the heating requirements of the residential area can no longer be met using solar thermal energy, the local heating network provides backup heat to achieve the necessary supply temperature. This is based on two gas boilers and a natural gas-fired CHP plant.

The BTES system can grow in parallelThe long-term thermal storage system currently consists of 80 borehole heat exchangers, but this can be expanded. Depending on how much the residential area grows, the number of borehole heat exchangers can be increased up to twice the current amount. The project developers have ensured that the heat bunker can be expanded so that the hottest area is always situated in the centre, both in the original and expanded condition. This means that the additional boreholes are drilled concentrically to the existing storage system. The resulting shape and tem-perature distribution keep the heat losses to a minimum. Originally it was only intended to charge the storage system

with solar thermal energy. An enlargement would have entailed increasing the collector surface area. The technology has meanwhile further developed and the operators are considering using the depot as an additional storage possibility for exhaust heat from combined heat and power generation, i.e. as a multifunctional storage system. Since almost 40 building plots in Crailsheim-Hirtenwiesen have yet to be built, a final decision on expanding the system will not be made for at least another one and a half years.

It’s even sunnier on flat surfacesIn Crailsheim-Hirtenwiesen there was an insufficient surface area on the roofs to house the necessary number of collectors. For this reason the project participants installed around two thirds of the 7,410 m2 of collectors on a noise protection barrier. This was built up using demolition material from the bar-rack buildings. The surface of the barrier is correspondingly uneven. This is reflected in the irregular arrangement of the first 13 collector arrays on the east

2 BINE-Projektinfo 01/2013

Barrier c

ollecto

r arra

ys

(4,913 m

2 )

Backup heating plant

Hirtenwiesen II local heating network

Building co

llecto

r arra

ys

(2,492 m2 )

Heat pump 530 KW

Borehole thermal energy storage system 37,500 m³

100 m3

480 m3

Buffer storage tank 1 (100 m3)

Buffer storage tank 2 (480 m3)

Fig. 1 Schematic diagram of the solar local heating system. Source: Solites

ErdreichDichtbahn

Verfüllmaterial mit geringer Wärmeleitfähigkeit

Schutzverrohrung

Doppel-U-Rohr--Erdwärmesonde

Thermisch verbessertes

Verfüllmaterial

Schaumglasschotter

Schichten-wasserführende Schicht

Gips-keuper

4 m

Letten-keuper

22 m

obererMuschel-kalk

GeovliesDrainage-SchotterGeovliesWirrgelegeGeovliesDichtbahn

Schaum-glas-schotterGeovlies

Detail-Ausschnitt

Drainage

Geovlies

GOK

SoilSealing membrane

Fill material with low thermal conductivity

Protective pipe

Double U-shaped pipe borehole heat exchanger

Thermally improved fill material

Foam glass gravel

LayersWater-bearing layer

-Middle Keuper

4 m

Lower Keuper

22 m

Upper Muschel-kalk

-

Drainage

Ground level

Grass

Drainage gravel

Fibre matting

Geotextile fabric

Geotextile fabric

Foam glass gravel

Geotextile fabric

Geotextile fabric

Detail section

Sealing membrane

Fig. 2 Stratum water occurs in the upper five metres of the earth surrounding the BTES system. Therefore, in order to reduce heat losses, the boreholes here have a larger diameter and have been clad with insulating material. Source: Solites

barrier. Their substructure consists of two concrete beams on which a steel girder is installed. With the other collectors on the west barrier the project participants opted for an alternative substructure: they poured a concrete slab onto the barrier and thus created a flat, smooth surface area. Large-scale collectors with support rails and brackets have been installed on it. This has created an aesthetically pleasing overall appearance. It also provides ad-ditional benefits for the collector manufacturer: it receives a exact surface specification and can precisely produce the collector arrays.

Target value for solar savings fraction surpassedFrom March 2012 to February 2013, the solar share of fraction of the local heating system amounted to 51%. The target value of 50% was therefore even slightly exceeded. However, the value for the entire year in 2012 fell just short of the target value (Fig. 5). The reason for this is explained by Janet Nußbicker-Lux from the Stuttgart Institute for Thermodynamics and Thermal

3BINE-Projektinfo 01/2013

Engineering (ITW Stuttgart), which is conducting the ac-companying research and monitoring: “The heat pump has only been in operation since February 2012. The borehole thermal energy storage system had therefore only slightly discharged by April 2012. This meant that in the annual calculation it wasn’t possible to take full ac-count of the January and February months with very high heat consumption. Furthermore, in summer the Hirten-wiesen I local heating network was also supplied with heat in addition to Crailsheim-Hirtenwiesen. This was not intended in our original planning.” The target value for the system’s return temperature is 35 °C, which has not yet been achieved. This is simply because the hous-ing estate that is being supplied has not yet been com-pleted. To ensure that the already occupied buildings receive sufficient heat requires that the corresponding heating circuit is completely operated and that there is a minimum flow through it. Because, however, there are still not enough recipient buildings to sufficiently cool the supply water, the return temperature is still unable to reach the desired level. A further reason is due to the buildings that are still under construction. The only heat recipients here are building heating systems for which there is no coordinated control system. This means that the solar local heating system in Crailsheim-Hirtenwiesen cannot be conclusively as-sessed until the construction site has been completed and further operating experience has been gained in around two years time. Converting the long-term bore-hole thermal energy storage system, which was origi-nally planned for purely solar thermal heat, into a multi-functional storage system offers prospects for the future. Another research project will be concerned with optimising the solar share of fraction and monitoring the overall system for the multifunction storage system.

Barrier c

ollecto

r arra

ys

(4,913 m

2 )

Backup heating plant

Hirtenwiesen II local heating network

Building co

llecto

r arra

ys

(2,492 m2 )

Heat pump 530 KW

Borehole thermal energy storage system 37,500 m³

100 m3

480 m3

Buffer storage tank 1 (100 m3)

Buffer storage tank 2 (480 m3)

Fig. 3 Roof-integrated collector arrays. Source: ITW Stuttgart

Fig. 4 Backfilling the upper metres of the BTES system with insulating material. The remaining borehole heat exchanger tubes are used for horizontally connecting the piping systems. Source: Solites

Fig. 5 Solar local heating supply system in Crailsheim. Source: ITW Stuttgart

Key data

2008 2009 2010 2011 2012Collector surface area at end of year m² 1,559 5,714 5,714 7,410 7,410Heat supplied by collectors MWh 570 1,735 1,785 2,337 2,740Usable solar heat MWh 484 674 864 1,342 2,307Usable solar heat in HW I MWh 0.3 0.4 0 0 254Usable solar heat in HW II MWh 483 673 864 1,342 2,053Total heat volume in Network II MWh 2,990 3,497 4,068 3,750 4,700Heat supplied by district heating MWh 2,530 2,832 3,197 2,407 2,580Charging heat volume BTES MWh - 849 779 781 707Discharge heat volume BTES MWh - - - - 382Heat supply condenser HP MWh - - - - 1,129Heat absorption evaporator HP MWh - - - - 917Electricity consumption heat pump MWh - - - - 212Solar savings fraction % 16.2 19.3 21.2 35.8 49

Combating unexploded bombs

Drilling into the ground on the former barracks site is a hazardous undertaking. Unexploded ordnance and other munitions could still be in the ground and detonate. Therefore an ordnance disposal unit dealt with the problem on behalf of the Crailsheim municipal utility company. First of all the specialist company analysed old aerial reconnaissance photos. These photos show specific areas where munitions were dropped. Areas where bomb craters can be seen are considered to be particularly dangerous. The experts in Crailsheim scanned hazardous sections with special equipment and located larger metal parts. However, these turned out to be harmless and were removed with a digger. The advantage of such excavations is that potential unexploded bombs have sufficient space to shift out of the way onto the shovel or into the earth. This is not possible with the punctual pressure caused by drill-ings. Here there is a much greater risk of detonation. For this reason there were particularly strict safety re-quirements for the site where the borehole thermal energy storage system was installed. In areas where excavation work was carried out, the digger operators and site managers merely had to attend a training course.The costs for the entire measures were paid for by the Crailsheim municipal utility company.

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Project participants >> Scientific project support: Solites Steinbeis Research Institute for Solar and Sustainable Thermal

Energy Systems, Stuttgart, Germany, Dirk Mangold, mangold@solites.de>> Project management: Stadtwerke Crailsheim GmbH, Crailsheim, Germany, Sebastian Kurz, sebas-

tian.kurz@stw-crailsheim.de>> Scientific and technical monitoring: Institute for Thermodynamics and Thermal Engineering (ITW),

Stuttgart, Germany, Dr.-Ing. Janet Nussbicker-Lux, nussbicker@itw.uni-stuttgart.de

Links and literature (in German)>> wwww.saisonalspeicher.de/Projekte/AktuelleProjekte | www.solar-district-heating.eu>> Bollin, E.; Huber, K.; Mangold, D.: Solare Wärme für große Gebäude und Wohnsiedlungen.

Stuttgart: Fraunhofer IRB-Verl., 2012. 160 S., ISBN 978-3-8167-8752-5, 29,80 Euro (Print), 23,80 Euro (E-Book), BINE-Fachbuch

>> Bauer, D.; Drück, H.; Heidemann, W. u. a.: Solarthermie2000plus: Wissenschaftlich technische Begleitung des Förderprogramms Solarthermie2000plus zu solar unterstützte Nahwärme und Langzeit-Wärmespeicherung. Forschungsbericht. Universität Stuttgart. Institut für Thermodynamik und Wärmetechnik (Hrsg.). Sept. 2012. 36 S., FKZ 0329607P

More from BINE Information Service>> This Projektinfo brochure is available as an online document at www.bine.info

under Publications/Projektinfos. >> BINE Information Service reports on energy research projects in its brochure series and newsletter.

You can subscribe to these free of charge at www.bine.info/abo.

Solar heat on an international scale With its “energy turnaround”, the German government is intent on providing the vast majority of Germany’s energy supplies with renewable energy by 2050. For example, with its “Offshore Wind Energy” funding programme it is spending 5 billion euros to help realise the first 10 offshore wind farms. In addition, the German Federal Ministry for the Environment is supporting the German initiative within the European Union’s “Solar District Heating in Europe” project. Here new approaches and instruments are being developed for introducing local and district solar heating systems to the European market. The aim is to further develop the existing solar thermal potential for grid-based heat supplies in residential and industrial areas. The transfer of international knowledge and technology plays an important role here. Interesting in this context is Denmark, whose favourable climatic conditions enable it to have a relatively high proportion of renewable electricity. With good wind conditions on summer days, at least one hundred per cent of the current electricity consumption is produced renewably. This is also impacting on the local and district solar heating provision in the country. Because it is also statutory regulated in Denmark that renewable electricity has priority, most of the gas-fired CHP plants are being removed from the grid. The district heating network therefore lacks a heat source. It is not financially worthwhile providing backup heating with gas-fired peak load boilers because in this case a gas tax of around 4 euro cents per kWh must be paid. In many projects, summer-based heat generation from solar thermal systems therefore provides the most economically feasible option for replacing waste heat from combined heat and power plants. In Denmark, large-scale solar thermal plants predominately supply small district heating systems. All plants consist of ground- mounted collectors, whereby it is planned to increase the current capacity from 192 MW to 255 MW in 2015. Nevertheless, combined heat and power generation should be used to provide heating during periods with low wind. The CHP plants would then secure the electricity provision and the short-term generation of peak-load electricity enables considerable revenue to be achieved. Large multifunctional thermal energy storage systems provide a possibility for storing the created waste heat. Here it is even feasible that the seasonal thermal storage of summer-based solar and surplus heat could last into the winter.

Project organisationFederal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU)11055 Berlin Germany

Project Management Organisation Jülich Research Centre Jülich Dr.-Ing. Peter Donat Zimmerstr. 26 – 27 10969 Berlin Germany

Project number 00329607H, 0329607N, 0325998A

ImprintISSN0937 - 8367

Publisher FIZ Karlsruhe · Leibniz Institute for Information InfrastructureHermann-von-Helmholtz-Platz 1 76344 Eggenstein-Leopoldshafen Germany

AuthorBirgit Schneider

Cover imageThe solar local heating system in Crailsheim-Hirtenwiesen. Stadtwerke Crailsheim GmbH

CopyrightText and illustrations from this publication can only be used if permission has been granted by the BINE editorial team. We would be delighted to hear from you.

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