Experimental investigations of the performance of a solar-ground source heat pump system operated in heating modes

• A set of SGSHP experimental system which can meet various operation modes was built.
• The experimental studies on the performance of SGSHPS operated in different heating modes were undertaken.
• The effects of two heat source combination on performance of SGSHPS were investigated.
• A dynamic simulation program was constructed to investigate performance of the SGSHPS.

Solar-ground source heat pump system (SGSHPS) is a new type of high efficiency, energy saving and environmental protection air-conditioning technology. In this paper, experimental studies and numerical simulation on the performance of a SGSHPS operated in different heating modes were carried out. The experimental system was installed in Nanjing of China and solar collectors were coupled with ground heat exchangers (GHE) through an insulated water tank. Four operation modes including ground source heat pump (GSHP), combined operation mode, day and night alternate operation mode and solar U-tube feeding heat alternate operation mode were investigated during winter season. The heat pump performance, solar collecting performance and borehole wall temperature variations were analyzed and compared for various modes. The experimental results indicate that for the combined operation mode, the system operation efficiency during day can be improved by the assistance of solar energy, and the excess solar energy collected during day can be stored in ground by the GHE to improve the operation performance of GSHP during night. The proportions of heat source burdened by solar and geothermal energy are 43.3% and 50.2% respectively. For the alternate operation modes, the temperature resumption of ground surrounding the GHE can be well achieved due to the intermittent heat extraction of GHE or feeding solar heat into ground and thus the overall utilization efficiency of solar and geothermal energy can be improved greatly. During the whole experimental period, the average COPs are 2.37 and 2.72 for GSHP and SAHP operation mode respectively, and the corresponding parameters are 2.69, 2.65 and 2.56 for the combined operation mode, day and night alternate operation mode and solar U-tube feeding heat alternate operation mode, respectively. The average solar collecting efficiency are 43.6%, 47.3% and 38.8% for the combined operation mode, SAHP operation mode and solar U-tube feeding heat operation mode, respectively. Based on the unit modeling, a dynamic simulation program was constructed to investigate the seasonal performance of the SGSHPS operated in different heating modes, the simulation results show that the seasonal average COP are 3.67, 3.64, 3.52 and 3.48 for the combined operation, day and night alternate operation, solar U-tube feeding heat and GSHP mode, respectively. From the view of improving the overall efficiency and increasing heat source fraction of solar energy, the combined operation mode is the best.