Performance analysis and experimental validation of a solar-assisted heat pump fed by photovoltaic-thermal collectors

Solar energy is called to play a relevant role in meeting the energy requirements of heating and cooling in industry and households. Its widespread use depends on the development of cost-effective and highly efficient systems. The joint installation of heat pumps and photovoltaic/thermal (PVT) panels can greatly contribute to these targets, since solar energy provides a heat source that allows operating the evaporator of the heat pump cycle at a higher temperature than the air temperature in winter. The coefficient of performance (COP) of the heat pump therefore increases because of the reduction of the electricity expense on the compressor. Additionally, the solar system is allowed to work in a low temperature range, so the heat loss from the collector to the surrounding areas decreases, resulting in higher collector efficiencies. The solar assisted heat pump is conceived in this work as a support system to be installed in existing facilities with yearly more or less constant thermal demands being supplied by conventional boilers. This paper shows the simulation of such a system, performed in Trnsys (a flexible graphically based software used to simulate the behaviour of transient systems specialized in the solar energy field), and its experimental validation. From the collecting operating parameters (irradiance, temperature, pressure and mass flow measured on the facility) and the actual design of the system defined by the size of the solar field, the storages and the heat pump capacity, the final objective of the work is to analyse the most appropriate size of the solar field, storage (water tanks) and heating power of the heat pump for optimizing the efficiency of the system. Results show that the working cold temperature ranges from 10 to 20 degrees C and the seasonal COP rose to 4.62, instead of 2.96 when working between 7 and 10 degrees C. The 67.6% of the electricity demanded by the heat pump is provided by the photovoltaic production (9309 kWh/yr of the 13,771 kWh demanded by the heat pump are renewable). The economic analysis indicates a payback period of about six years, what makes the proposal greatly attractive. (C) 2018 Elsevier Ltd. All rights reserved.