Performance of a U-vertical Direct-expansion Ground Heat Pump

The direct-expansion ground-source heat pump (DX GSHP), which uses a buried copper piping network through which refrigerant is circulated, is one type of ground-source heat pump (GSHP). In this paper, the theoretical energy analysis of DX GSHP system was presented. Since there are no secondary solution loops and water circulating pumps, the energy efficiency of the heat pump unit is equal to the whole DX GSHP system. This is an important reason of high system performance of DX GSHP system. In addition, in the cooling (heating) mode, the condensing (evaporating) temperature of the DX GSHP system is much lower (higher) than the traditional GSHP system, indicating that the DX GSHP system conduct much better performance than traditional GSHP system. The experimental energy performance of a DX GSHP system was also investigated based on the actual operational data. This tested DX GSHP system, with a 42m vertical 12.7mm outside nominal diameter U-bend ground heat exchanger buried in a water well, was installed in Xiangtan, China. The experimental set-up and measurement system were also introduced in the paper. The average COP of the DX GSHP system in the heating mode was 4.72 during the testing period. The highest value reached to 5.95, which is relative much higher than the traditional GSHP system. The work done in this paper is helpful for people to realize the energy efficiency of the DX GSHP system, and promote the application of DX GSHP system.