Research on the operation features and optimization methods of heat pumps coupled with mid-deep borehole heat exchangers: On-site measurements and comparative study

This paper conducted field tests and comparative studies on the operation performance of different kinds of heat pumps integrated with mid-deep borehole heat exchangers. Results show that the mid-deep borehole heat exchangers could provide heat source with temperature higher than 25 degrees C. But the constant-speed screw heat pumps failed to match the wide-range variation of heating capacity and compression ratio, especially the small values of compression ratio. Thus the internal efficiency of constant-speed screw compressor was lower than 0.50 during the whole heating season, and the energy efficiency of heat pumps was lower than 5.0. In comparison, the variable-speed centrifugal heat pumps preferentially adjusted compressor speed to match the variation of heating capacity and compression ratio, thus the internal efficiency of compressor could reach 0.62 during the heating season and the energy efficiency of heat pumps could reach 7.0. Then the control strategies of heat pumps were compared and optimized, where the user-side supply water temperature of heat pumps should be chosen as control value, which could response in time to the adjustment of heating capacity, and had significant influence on the energy efficiency of heat pumps. Furthermore, the control strategies of water distribution systems were analyzed, so as to improve the energy performance of the whole system. With the above optimization, the operational energy cost of mid-deep geothermal heat pumps could be reduced by 61.4 %, 48.7 %, and 69.8 % respectively compared with air source heat pumps, shallow-deep geothermal heat pumps and Gas Boiler. Also the CO2 emissions could be reduced by 61.4 %, 48.7 %, and 60.5 % respectively.