Thermal accumulation as an alternative thermal storage for air-conditioning plants equipped with solar-assisted heat pump

Energy efficiency in buildings is becoming a key goal in order to ensure sustainable development and address the challenges of climate change. The building sector, in fact, is responsible in the EU for 40% of the global energy consumption for final uses and 36% of the CO2 emissions. Heat pumps are among the most interesting technologies to replace conventional systems based on fossil fuels, not only for the direct use of renewable sources available in the external environment, but also for the possibility of combining active solar systems. In order to rationalize the use of renewable energy, in this paper the performance of an innovative air conditioning system consisting of an air-to-water heat pump, photovoltaic plant, solar thermal collectors, and electrical and thermal storage systems are introduced. Regarding a reference residential building, a parametric analysis carried out in TRNSYS 18 by varying PV size, tank volume and capacity of electrical storage, has made it possible the identification of the optimal system configuration. Moreover, in order to manage rationally the energy flows, two main control strategies were implemented. The first activates the heat pump according to the temperature difference between its outlet and the thermal storage, to ensure the tank set-point temperature (reference case). In the second control strategy (smart control) the tank temperature level can exceed the set point because the heat pump is activated even when the renewable electricity exceeds the demand. So, the tank is employed as sui-generis electric storage of the PV surpluses in terms of hot/cold water produced through the heat pump. The results showed that the smart control strategy allows for almost always satisfying 50% of the thermal load using renewable energy sources and a potential decrease in CO2 emissions and an increase in the share of self-consumed electric energy.