Abstract
The medium-depth geothermal heat pump systems (MD-GHPs) use vertical concentric deep borehole heat exchangers (DBHEs) with depth more than 2 km to extract heat from medium-depth geothermal energy, which provides a higher-temperature heat source and improve the energy performance of heat pump obviously. This paper introduces the field test on energy performance of heat pumps in MD-GHPs. Results show that the outlet and inlet water temperature of DBHEs reach 33.0℃ and 23.7℃ respectively, thus the COP of heat pumps reaches 5.70. However, the heat pump with constant speed compressor is identified unsuitable for operation with high-temperature heat source. Besides, the high water resistance, low water temperature difference and energy efficiency lead to the poor energy performance of water pumps. Thus the high-temperature heat source hasn't been fully utilized. Then based on analysis of field test results, the design parameters of heat pump are optimized and the variable speed centrifugal compressor is applied. While the design parameters and control strategy of water pumps are optimized. Finally, the optimization effect is examined through practical application and the SPFH1, WTFu, WTFg, SPFH3, and SPFH4 of MD-GHPs reach 7.71, 57.2, 97.6, 7.15 and 6.35 separately, which are obviously improved.
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