Study on design error of ground source heat pump system and its influencing factors

The ground heat exchanger (GHE) is vital to the energy conservation and economy of a ground source heat pump system (GSHPS). The ASHRAE method is the commonest way to design GHEs. However, the mathematical model of this method includes some assumptions and simplifications that may make the results inaccurate. In this paper, a dynamic simulation model of a GSHPS was established and verified. The GSHPS designed according to the ASHRAE method was investigated annually. The result shows that the ASHRAE method tends to overestimate the length of the GHEs by up to 13.90% under the reference condition. The design error of the ASHRAE method is affected by many factors. When the drilling spacing is less than 4 m, the required pipe length is longer, and the design error becomes smaller. When the drilling distance is between 4 m and 6 m, the influence of drilling spacing is small. As the thermal conductivity of the backfill material and the fluid flow rate in the tube increase, the design errors also increase. The drilling depth is less important in the design error of the ASHRAE method.