Field test on energy performance of medium-depth geothermal heat pump systems (MD-GHPs)

•The practical energy performance of MD-GHPs applied for space heating is introduced.•The MD-GHPs reduce the space occupation and eliminate thermal imbalance of the ground.•The average temperature of the outlet water from the ground can reach 33.0 °C.•The COP of the heat pump reaches 5.43 and the COPhs of heat source reaches 4.58. This paper introduces the practical energy performance of medium-depth geothermal heat pump systems (MD-GHPs), which use vertical concentric tube as ground heat exchangers (GHEs) to extract heat from the medium-depth geothermal energy for space heating in buildings. Hereby the medium-depth geothermal energy refers to heat embodied in rocks nearly 2–3 km underground with temperature around 70–90 °C. The operation performance and energy efficiency of MD-GHPs is tested in five residential buildings in Xi'an, North China. According to the short-term and annual field test results, the energy performance of MD-GHPs as well as the heat transfer performance of GHEs are introduced and analyzed. Results show that the average temperature of the outlet water from the ground can reach 33.0 °C, promoting the performance of whole systems greatly, with the COP (Coefficient of Performance) of the heat pump reaching 5.43 and the COPhs of heat source reaching 4.58. Therefore, the energy saving rate (ESR) of the MD-GHPs can reach 41.0%, 29.3% and 47.5% compared with air source heat pump systems (ASHPs), conventional ground source heat pump systems (SD-GHPs) and gas boilers.