Development and simulated evaluation of inter-seasonal power-to-heat and power-to-cool with underground thermal storage for self-consumption of surplus solar energy in buildings

The adoption of renewable energy, such as solar, to meet the energy demand in buildings has become one of the keys to achieving the global target for net-zero emissions. As a result, solar photovoltaic installations have increased tremendously, giving rise to an enormous surplus of electricity generation, which has become an issue requiring alternative ways to be addressed. Underground thermal energy storage for power-to-heat operations has gained interest in this area due to its reliability, cost-effectiveness, and carbon-free nature. This study presents a novel system configuration with an operational strategy guided by a simple control method that uses surplus photovoltaic electricity to power an inter-seasonal heating and cooling system coupled with seasonal underground thermal energy storage. Two cases were developed, modeled, and simulated in the TRNSYS 18 simulation tool. Case 1 involves an air-source water-load heat pump and 1.5 m-shallow underground thermal storage with power-to-heat and power-to-cool operations. Case 2 features an air-source water-load heat pump and vertical 150 m-deep underground thermal storage with power-to-heat and power-to-cool operations. The base case involving an air-source water-load heat pump without power-to-heat and power-to-cool operations was modeled for their evaluation. In Case 1, energy savings and power-to-heat and power-to-cool efficiency of 14 % and 39 % were obtained, respectively. Similarly, energy savings and power-to-heat and power-to-cool efficiency of 13 % and 36 % were obtained, respectively, from Case 2. Both study cases displayed a self-consumption ratio of approximately 81 % compared to the base case, which had 76 %. Similarly, the surplus energy utilization ratio of about 26 % was obtained from both cases. Furthermore, 60 % and 52 % thermal efficiencies were obtained for study cases 1 and 2, respectively, for the underground thermal storage. The results demonstrate that the configuration and operational strategy implemented can seasonally utilize the available photovoltaic power and enhance the performance of the heat pumps.