Numerical analysis of a novel ground heat exchanger coupled with phase change materials

Thermal energy storage with phase change materials (PCMs) is a functional strategy to minimize the sizing of air conditioning systems and reduce their primary energy consumption. This approach is well known in ground-coupled heat pump applications (GCHP), especially with use of borehole ground heat exchangers (GHEs). However, this is seldom investigated for coupling with shallow horizontal GHEs that are usually considered to be less effective for GCHP applications, due to faster depletion of the stored thermal energy as a result of the seasonal energy balance. In order to make shallow GHEs more effective, mixing PCMs directly with backfill material for a flat-panel type GHE is presented in this study. The application has been evaluated through numerical modelling to solve transient heat transfer using effective heat capacity method. Yearly performance has been simulated by taking into account the estimated energy requirement for an assumed residential building located in Northern Italy. According to hourly time series boundary conditions and annual performance, the simulation results show that employment of PCMs is able to smooth the thermal wave in the ground, improve the coefficient of performance of the heat pump (COP) and if suitably sized, prevent thermal depletion in winter by charging the PCMs naturally in summer with a shallow GHE.