New Math Trick Helps Solar Farms Waste Less Stored Power

Solar and other renewable plants are increasingly expected to provide ancillary services to the grid, but the variability of sources like solar photovoltaics means they need additional operational flexibility. Hybrid energy storage systems offer an appropriate solution, because different technologies with complementary characteristics can be applied and leveraged to share the power demand and improve overall performance. The catch: most models in the literature assume constant efficiencies, and most formulations rely on optimisation problems too computationally demanding to be solved in real time. This paper proposes a controller that minimises the losses of a hybrid energy storage system while considering detailed power-dependent efficiency curves for each storage technology. The power sharing method is based on analytical verification of the Karush-Kuhn-Tucker conditions, which the authors say makes it computationally efficient and suitable for real-world deployment. It was applied to a test case of a 10 MW PV power plant with a hybrid energy storage system based on a lithium-ion battery and a redox-flow battery, each with 2.5 MW power and 5 MWh capacity. The main contributions were verified via numerical simulations performed in MATLAB/Simulink, while a real-time implementation deployed in OPAL-RT demonstrated the viability of the algorithm for real-time applications.