Ground-source heat pumps using phase change materials

Abstract

Employing Phase Change Materials (PCMs) is normally considered as an effective measure to store thermal energy, by means of their latent heat during phase changing. But, it could also represent a method to smooth the thermal wave generated from operations of thermal machines, such as ground-source heat pumps (GSHPs). This paper evaluates the application of PCMs through numerical modelling to solve the heat transfer in ground carried out by a horizontal and shallow ground heat exchangers (GHXs), when coupled to a GSHPs. The PCMs are assumed to be mixed directly with backfill material close to the GHXs or placed in a surrounding shell that is in direct contact with the heat exchangers. Results showed that the use of the PCMs incorporated with GHXs meets the instantaneous heating demand by the GSHPs, and reduces the sudden cooling wave on the ground interface. By calibrating the amount and the properties of the PCM in accordance with the energy requirements of the GSHP, it is possible to balance the heat extraction of the operating time to the heat recovered during the off time of the GSHP. As a consequence, the peak temperature results could be smoothed by up to 0.7 K in comparison to the case without PCMs. Thus, higher coefficients of performance (COP) are expected for GSHPs. Moreover, the underground thermal energy storage is recovered for shallow GHXs, getting over the seasonal variations due to weather change.