Inverter Heat Pumps as a Variable Load for Off-Grid Solar-Powered Systems

The capacity of electric air conditioning and heating systems is growing rapidly, as is the nameplate capacity of PV power plants. While the demand for cooling has a positive correlation with solar irradiance, the demand for heating has an opposite relation. In this study, different approaches to design (aggregation; thermal, battery, and implicit storage) and control (frequency conversion; variable and adaptive load) and their effects on the efficiency of an off-grid active thermal stabilisation system based on a solar-powered heat pump are analysed. The case considered is a permafrost thermal stabilisation system in Norway. It is shown that proper layout and control of the system with an adaptive load can reduce capital expenditures and the total cost of ownership by 30-40%. Increases in the capacity factor and cooling stability of the systems with aggregated and variable loads are studied. The downside is that there is an increase in the compressor's operation time by 50% with a variable load and by 25% per unit with aggregation, which means more frequent replacement in terms of motor hours. The approaches considered are applicable in a wide range of solar-powered facilities with a positive correlation between solar irradiation and energy demand, but the results are quite case-sensitive. The prospects of using excess refrigerant and soil for thermal energy storage instead of traditional electrochemical batteries are considered.