Simulated performance of a solar-assisted heat pump system including a phase-change storage tank for residential heating applications: A case study in Madrid, Spain

This paper reports the simulation results of two different solar-assisted heat pump (SAHP) systems able to cover a significant part of the space heating demand of a single-family house located in Madrid (Spain) using solar energy. Most SAHP systems reported in the literature serve large buildings located in cold climates using long-term (seasonal) storage tanks, a solution that leads to high investment, operation and maintenance costs that greatly limit their applicability to milder climates and/or small-scale buildings. To explore this potential application in more detail, this study examines the integration of a water-to-water heat pump in two solar house-heating systems equipped with different short-term (diurnal) stores: water, and latent (containing a phase-change material-PCM) tanks for the weather conditions of Madrid. The SAHP system's energy performance is assessed using the TRNSYS program, implementing in the simulation scheme an experimentally validated slab-like macro-encapsulated PCM tank model (as a TRNSYS component) for the simulations including the PCM tank. Simulation results indicated that the integration of the intermediate PCM tank in the SAHP system may perform unexpectedly, generating degradation in the system performance if only conventional thermostatic control strategies are considered, diminishing the solar energy transferred to the heat pump up to approximately a 30%, and lowering the heating availability of the system from 98.9% to 72.8%. An analysis of this thermal performance degradation led to the conclusion that more advanced control strategies based on accurate knowledge of forecasts for the following days' heating requirements, PCM temperatures and associated states of charge of the tank from the history of the fluid inlet/outlet temperatures are required.