The importance of surface air temperature fluctuations on long-term performance of vertical ground heat exchangers

Ground source heat pump (GSHP) systems efficiently heat and cool buildings using geothermal energy accessed via ground heat exchangers (GHEs). The thermal performance of GHEs is usually studied with equivalent groups of linear heat sinks/sources or with more advanced numerical models. In most cases, in regions with common geothermal gradients, the ground temperature is assumed to be constant with depth, a simplification that is reasonable for typical borehole GHEs which are at least 50 m deep. However, the increasing use of piles (and other geo-structures) as GHEs, which are typically shallower than borehole GHEs requires a re-assessment of the thermal effects of the surface air temperature changes on GHEs. This work introduces and addresses such issues using the newly developed 3D detailed numerical model of GHEs and highlights its significant influence on thermal performance. Numerical results show that in the cases studied herein, accounting for the surface air temperature fluctuations may result in up to about 11% shorter GHEs than if these effects are ignored.