Numerical simulation of ground source heat pump systems considering unsaturated soil properties and groundwater flow

This paper analyzes the influence of unsaturated soil properties and groundwater flow on the performance of ground source heat pump (GSHP) systems. A mathematical model of GSHP systems, which considers the influence of varying unsaturated soil thermal properties, groundwater table depth and saturated groundwater flow, is introduced and coupled to a heat pump model to consider dynamically-changing loads due to specific heat pump coefficient of performance (COP) characteristics. The model is validated against experiment results and an analytical model reported by others. Finally, numerical simulations are performed to investigate the effect of inclusion of a heat pump, moisture content variations, groundwater table fluctuations and groundwater flow rates on the performance of the GSHP system. Results show that neglecting variations in moisture content in unsaturated soil could lead to underestimating the heat transfer capacity of the soil. A rising groundwater table is beneficial to the operation of the GSHP system. The influence of groundwater table fluctuations should be considered, otherwise it may result in a error of 3-4% in the outlet fluid temperature. When calculating the influence of groundwater flow rates on the performance of the GSHP system, the effect of the position of the groundwater table should also be considered.