Thermal and economic performance of horizontal ground source heat pump systems with different flowrate control methods

The horizontal ground heat exchanger (HGHE) integrated with the heat pump unit can be defined as the horizontal GSHP system. Most horizontal GSHP systems are always operating at a rated flowrate in the secondary loop, resulting in the electricity waste of circulation pump when the system operates in partial load. This study aimed to compare the thermal and economic performance of horizontal GSHP systems with different flowrate control approaches. Two novel variable flowrate regulation methods, namely the segmented and time-varying flowrate control (SFC and TFC) method, together with the traditional constant flowrate control (CFC) method, were employed in the secondary loop of system. 3D numerical model of the HGHE considering the isotropic porous soils and varying ambient environmental conditions was built and coupled with the model of heat pump to analyze the heating and economic efficiency of the horizontal GSHP system. Considering that the buried depth of HGHE, H, may vary in different conditions, the impact of flowrate control methods in various H was analyzed. The results reveal that compared with the CFC and SFC method, the variation range of the fluid temperature difference between the inlet and outlet of the HGHE, delta T-f, and system COP, COPsys, are smallest and the level of delta T-f and COPsys are greatest for the TFC method. The system operation costs and electricity charges proportion of water pump can decrease by up to 9.80% and 47.26% as the control method varies from CFC to TFC, but the differences between the variable flow control technologies are not significant. The discrepancy of COPsys and operation costs between the constant and variable flowrate control methods decrease by 39.13% and 48.72% with the increases of H. This study highlights the significance of using variable flowrate regulation methods in the secondary loop of the horizontal GSHP system to improve thermal and economic performance.