Simulation and optimization of a hybrid unglazed solar photovoltaic-thermal collector and heat pump system with two storage tanks

Solar thermal energy systems combined with heat pumps are becoming popular solutions for covering space heating and domestic hot water demand in European Union households. There is a great potential for developing high performing systems by combining components in systems in new ways. This study focuses on the performance of a system consisting of heat pump, photovoltaic-thermal solar collector and two storage tanks. This system is proposed to potentially have better performance compared to air-to-liquid heat pump system while being less costly compared to a liquid-to-liquid heat pump system with ground sourced heat exchanger. The system was set up at the Technical University of Denmark in 2017, for providing domestic hot water. A simulation model of the system was created in the transient simulation software TRNSYS and the data from the experiment were used for its validation. The modulating heat pump was modelled via a function applied to the scale factor and the thermal mass of the heat pump was taken into account by including additional pipes around the heat pump. The performance of the system was assessed using selected key performance indicators. A parametric analysis was performed identifying component sizes that improved the system’s performance. The improved system produced 55% more electricity, had 23% lower electric consumption and wasted 11% less heat by minimizing heat losses compared to the demonstration system. The improved system had a solar thermal fraction of 0.58, a solar electrical fraction of 1.51 and a renewable energy fraction of 0.75. The system was however oversized compared to the load applied to the system as it was built to cover only domestic hot water demand. For the system to be economically attractive it must cover a larger load.