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Fig. 2: System diagram for the solar local heating supply system.
© ZAE Bayern
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Heat pump supplements storage concept

The ambitious goal of the research project was to meet half of the energy requirements for space and domestic water heating with solar thermal energy. The key component that has made this value at all possible comprises a seasonal thermal storage system in combination with a thermally operated heat pump. For technical and economic reasons, the project participants in Munich opted for an underground hot water-based heat storage tank. Precast concrete units with thin, prestressed walls were used here for the first time. The walls and roof are covered with thermal insulation comprising cellular glass particles in a membrane formwork; the storage tank is insulated underneath with foam glass ballast. The inside surface is clad with a stainless steel skin. The draw-off points are located in the hottest, upper layer and the coldest, lower layer. In addition, the storage tank is also equipped with a movable draw-off device that can be positioned between the upper 15 and 30% of the water volume.

The combination of a heat storage system with a lithium bromide absorption heat pump was based on the following concept: the storage tank is fed via a stratified charger that feeds in the water at the respective level in accordance with the temperature. If the solar thermal system is unable to completely meet the heating requirements in winter, the system is coupled to the district heating network belonging to Stadtwerke München. The district heating, which has a temperature between 80 and 120 °C, drives the heat pump. This in turn uses the solar storage tank as a heating source that can cool down to around 10 °C. The heat pump transfers the extracted solar thermal and drive heat to the local heating network and thus raises the supply temperature to 60 °C nominal. At the same time it enables the storage tank to cool down below the return temperature. The advantages: Heat losses are reduced, the thermal bunker is discharged at lower temperatures and can therefore take up more solar thermal energy. The collector input temperatures also drop, which increases the solar yield. The district heating deployed as additional energy is used efficiently, since for each kWh of district heating, 1.5 to 1.7 kWh of useful heat can be supplied to the local heating network.

Stratified chargers present a challenge

A challenge for the storage technology is presented by the stratified charger, which is designed to provide temperature-oriented stratification of the solar energy. This enables the proportion of directly used solar energy to be considerably increased. The loading device used until now has, however, been unable to achieve the desired stratification. This is because there is still very little practical experience with stratification chargers used for relatively large storage tanks such as in Munich. For example, it was shown in the case of the commissioned supplier that merely scaling up standard stratification chargers for the 5,700-m³ storage tank was insufficient. A solution for the technical problems that have occurred is currently being worked on.

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Accompanying scientific research/monitoring
ZAE Bayern - Würzburg

Technical and economic project support
Solites

System operator
Stadtwerke München GmbH