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 COP_all 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 COP_all 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°C, along with a decrease of COP_all 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 COP_all 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.