How to improve the energy performance of mid-deep geothermal heat pump systems: Optimization of heat pump, system configuration and control strategy

Mid-deep borehole heat exchangers (MDBHEs) extract heat from geothermal energy with depth of 2-3 km, and provide high-temperature heat source for heat pump heating system. To improve the energy performance, this paper conducted optimization analysis on heat pump, system configuration and control strategy. Firstly, the energy efficiency and regulation characteristic of magnetic bearing variable-speed centrifugal heat pump (MBCHP) was examined through field tests and simulation analysis. Results showed that MBC-HP performed efficiently with wide-range variation of heating load and compression ratio. And average COP of MBC-HP reached 6.21 during heating season. Besides, as evaporator-side water temperature difference (dTe) reached about 10 degrees C, twostage MBC-HPs connected in series was raised to fit the large dTe, and the average COP of heat pump was improved to 6.97. Furthermore, the evaporator-side water flow rate (Ge) affected the energy efficiency of heat pump system and heat transfer performance of MDBHEs significantly. Thus the water pumps were recommended to be connected in parallel and then connected with heat pumps. Then the numbers and frequency of water pumps should be regulated to maintain dTe about 9.0 degrees C. Compared with control strategy of constant Ge, the COPs of heat pump system increased to 5.28, showing 18.7 % improvement.