Energy Efficient Integration of Heat Pumps into Solar District Heating Systems with Seasonal Thermal Energy Storage

Solar district heating (SDH) with seasonal thermal energy storage (STES) is a technology to provide heat for space heating and domestic hot water preparation with a high fraction of renewable energy. In order to improve the efficiency of such systems heat pumps can be integrated. By preliminary studies it was discovered, that the integration of a heat pump does not always lead to improvements from an overall energy perspective, although the operation of the heat pump increases the efficiency of other components of the system e.g. the STES or the solar collectors. Thus the integration of heat pumps in SDH systems was investigated in detail. Usually, the heat pumps are integrated in such a way, that the STES is used as low temperature heat source. No other heat sources from the ambience are used and only that amount of energy consumed by the heat pump is additionally fed into the system. In the case of an electric driven heat pump, this is highly questionable concerning economic and CO2-emission aspects. Despite that fact the operation of the heat pump influences positively the performance of other components in the system e.g. the STES and makes them more efficient. If the primary energy consumption of the heat pump is lower than the energetic benefits of all other components, the integration makes sense from an energetic point of view. A detailed assessment has been carried out to evaluate the most promising system configurations for the integration of a heat pump. Based on this approach a system concept was developed in which the integration of the heat pump is energetically further improved compared to realised systems. By means of transient system simulations this concept was optimised with regard to the primary energy consumption. A parameter study of this new concept has been performed to identify the most sensitive parameters of the system. The main result and conclusion are that higher solar fractions and also higher primary energy savings can be achieved by SDH systems using heat pumps compared systems without heat pumps. (C) 2014 The Authors. Published by Elsevier Ltd.