Simulation-based performance evaluation of large-scale thermal energy storage coupled with heat pump in district heating systems

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Large-scale water-based thermal energy stores (TES) coupled with heat pumps (HPs) are a key element in District Heating (DH) systems to achieve an increase of the share of renewables.The implementation of a 1D equation- based TES model, coupled with a 2D or 3D finite element-based model of the surrounding ground provides a robust and reliable tool that can be used in the study of large-scale TES systems. In this work, a simplified equation-based model for the HP using the TES as evaporator heat source is implemented to investigate its impact on the TES performance and on the surrounding ground temperatures. Two TES types (buried tank and shallow pit) are analysed under the same conditions (size, envelope characteristics, ambient temperature, ground properties and system boundary conditions). The parameters used to evaluate the coupled TES-HP performance consider both the environmental aspect (ground temperature and CO2 emissions) and the technical performance (energy and exergy efficiency).The integration of the HP proves to be effective in increasing the TES efficiency by 6 % (from 87 %) compared to the reference case without an HP in case of an insulated tank TES, and by 16 % (from 64 %) in case of an insulated shallow pit. The TES exergy efficiency analysis emphasizes the better performance of the tank over the shallow pit, with the tank geometry outperforming the shallow pit in all the investigated configurations (+ 25 % in the reference case without HP, + 20 % with 6 MW HP). However, when extending the exergy analysis to the system, the difference in exergy efficiency between the two geometries is strongly reduced (+ 5 % on average for the tank with respect to the shallow pit); the impact of thermal insulation is also less noticeable. In particular, in the system analysis the configuration of shallow pit with HP shows a performance comparable with the tank without HP. In terms of CO2 emissions, the integration of the HP coupled with a TES allows for a reduction of 33 % for the tank and 29 % for the shallow pit compared to the respective reference cases, due to the better utilization of the TES capacity. The resulting ground temperature near the TES is also influenced by the use of the HP, with an average ground temperature 5 K lower than in the reference case without HP at a ground depth between of 7 to 15 m. The contribution of thermal insulation appears however necessary for both geometries, as it contributes most to the reduction of ground temperature.