Numerical simulation of horizontal spiral-coil ground source heat pump system: Sensitivity analysis and operation characteristics

This study concerns the operation characteristics of horizontal spiral-coil ground source heat pump (GSHP) system. A numerical model is established, which takes into account the geothermal gradient, as well as the varying ambient air temperature. More importantly, a heat pump model is also included to consider dynamically-changing loads due to specific COP characteristics. The accuracy of the model is validated with experimental results in Metz's work and the model presented by Mei. Circulating fluid temperatures are compared with and without considering the effect of heat pump COP. Influence factors affecting the operation characteristics of horizontal spiral-coil GSHP system are investigated and their influence degrees are also compared. Impacts of daily variations of thermal load and operation models on the performance of horizontal spiral-coil GSHP system are also investigated. Results show that without considering the effect of heat pump, the difference between average inlet fluid temperatures in cooling and heating models can be 4.1% and 11.5%. Among factors influencing the performance of horizontal spiral-coil GSHP system, soil thermal conductivity and pipe spacing are main factors. When analyzing the annual or average operation characteristics of horizontal GSHP system, daily variations of load can be neglected. Under the condition of equal total loads, the average performance of intermittent operation is poorer than continuous operation and day-high/night-low operation. And continuous operation has the best performance.