Importance of long-term ground-loop temperature variation in performance optimization of Ground Source Heat Pump system

In a recent article published in this journal, a series of design optimization was executed for a ground source heat pump system. The optimization was conducted based on a COPall index, which considers both the hydraulic loss on the buried pipe network, as well as the soil thermal balance over a one-year period. In that article, it was concluded that a borehole spacing of 4m is the optimal value. In this short communication, the comprehensive COPall index is re-evaluated with the same system setup with both the TRNSYS and OpenGeoSys-TESPy software, but over a 20-year period. The results show that the borehole heat exchanger array with a spacing of 4m will suffer severe heat accumulation over a 20-year operation. This causes the soil temperature to rise by 5.68 degrees C, along with a decrease of COPall from 4.59 to 4.18. Due to the smaller increase of ground-loop temperature and lower electricity consumption from heat pumps, a larger spacing of 6m will bring better COPall value over the long term, and thus should be recommended. The extended numerical study in this work suggests that when evaluating the performance of a ground source heat pump system, different time duration will lead to different results. Therefore, the variation of long-term ground-loop temperature needs to be quantitatively evaluated in advance, and the system optimization is recommended to be conducted over the entire life cycle of the system.