Thermo-economic analysis of a solar district heating plant with an air-to-water heat pump

Solar district heating systems are essential in reducing carbon emissions and alleviating the energy crisis. By integrating with air-to-water heat pumps, it is possible to enhance the system’s efficiency and flexibility. However, the complementary mechanisms and coupling effects between the heat pump and the other components of the system are not clear. To address these issues, it is necessary to investigate the thermo-economic performance of the system, taking into account energy price fluctuations to achieve a more efficient and economically viable operational model. This study focuses on a solar district heating system in the Danish city of Ørum and investigates the impact of integrating a large-scale air-to-water heat pump. Based on energy and economic analyses, the impact of the heat pump on the system efficiency and the levelized cost of heat are investigated for 2020 and 2022. The results show that the heat pump can convert low-cost electricity into heat which will be stored in the tank, increasing the tank’s heat storage content by 1.2 times. The medium to low-temperature water in the tank can preheat the heat pump’s compressor, raising the COP of the heat pump from 3.3 to 3.5 and increasing the annual utilization cycle of the tank by 1.4 times by reducing the return water temperature to the bottom of the tank. Additionally, the system performance has been improved due to the installation of the heat pump, which increased the system COP from 1.22 in 2020 to 2.62 in 2022. Subsequently, the CO 2 emission has been decreased from 192 kg/MWh to 74 kg/MWh with a larger share of sustainable energy. Due to the heat pump operation, the system-levelized cost of heat could be reduced to 68.6 EUR/MWh, compared to 94 EUR/MWh if the heat pump is removed. The investigation also revealed that the heat pump improved the flexibility of the heating system in response to energy prices, especially during the 2022 energy crisis in Europe. The findings of the paper provide a good reference for large solar heating applications. • Three-year monitoring of a solar district heating plant without and with a heat pump. • With the economizer of the heat pump, heat storage capacity increased by 20%. • With the HP, the system COP increased from 1.22 to 2.62. • Installing HP, the system CO 2 emission factor decreased from 192 kg/MWh to 74 kg/MWh. • The heat pump increased the system’s economic flexibility.