A comparison between numerical methods for evaluating ground coupled heat pump systems performance

Ground coupled heat pumps are increasingly used for HVAC systems. The difficulty in sizing and predicting their behaviour and performance is well known. A suitable simulation is often advisable to help in the design choices. The code EnergyPlus is widely used in the field of building simulation and, since it includes a routine dealing with borehole heat exchangers, based on the wellknown concept of g-functions, it can be profitably used for the considered purpose. On the other hand a numerical tool, namely CaRM, based on a detailed finite difference model of both the ground and borehole heat exchangers has been developed. A comparison between the use and the results of the EnergyPlus g-functions approach and CaRM in ground subsystem modelling was carried out with particular reference to an office building with quite a critical unbalance between heat extracted from and heat injected into the ground.