Comparison of control strategies of an off-grid Carnot battery combined with renewable energy resources: Priority of electrical and Carnot battery

As the national grid is intermittent in Nigeria, farms use diesel generators to cover daily electricity demands, so a pumped thermal energy storage (PTES) with renewable energy resources is a feasible option to get the farm’s daily demands satisfied and avoid fuel consumption and environmental pollution. This study focuses on the reversible HP/ORC control strategies and its techno-economic optimal result. Two controlling parameters are introduced: scroll machine low-speed protection factor (j) and state of charge (SOC) judgment factor (k) for hot storage. Respectively, j will decide whether the compressor/expander must be cut off due to low expected speed, and k will determine the heat pump switching between air-cooled mode and hot-storage-cooled mode. Finally, after the simulation results gained, the optimal design thus can be determined by techno-economic criteria, levelized cost of electricity (LCOE), which considers both investment / maintenance cost and generated energy. When the ORC or electrical battery needs to work for the power demands, there are two strategies with their different priority orders to cover demands. Strategy 2 performs best in LCOE, but its ORC operating time remains at a low level. Strategy 1 makes SOC stable and always healthy but relies on more grid / diesel generators.