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Storing solar energy in the ground
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 situated 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.
© Stadtwerke Crailsheim GmbH
© ITW Stuttgart
© ITW Stuttgart
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 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 investment 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 compensation. This has enabled us to dispense with the previously used balancing valves,” which has saved material costs and reduced the workload. In addition, crosslinked 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 losses
The task of the electrically driven compression heat pump (coefficient of performance: 4.8) is to discharge the borehole 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 Sebastian Kurz, Head of Planning at the Crailsheim municipal utility company. “Although the temperatures in the storage 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.