Hybrid pumped hydro and battery storage for renewable energy based power supply system

It is very challenging for single energy storage to make an off-grid renewable energy (RE) system that is fully capable and reliable, unless there are an oversized generator and storage capacities which eventually lead to high dump load, due to high variability and intermittency of RE resources. In this study, a hybrid pumped and battery storage (HPBS) system is proposed to make the off-grid RE system more reliable and sustainable. Firstly sizing of RE generators and energy storages is accomplished, then a novel operating strategy of the HPBS based RE system is developed, considering the operating range of reversible pump-turbine machine, to extract maximum stored energy by operating HPBS at optimum efficiency. In the proposed model, the battery is only used in order to meet very low energy shortfalls considering the net power deficiency and state of charge, while pumped hydro storage works as the main storage for high energy demand. To make sure the functionality of each energy storage, HPBS operating strategy is developed based on the operating range, both in discharging/charging, of reversible pump-turbine machine which is defined after several simulation cases. Some indicators including storage overall performance (SOP), energy utilization ratio (EUR) and storage usage factor (SUF) are taken into account for HPBS performance analysis. It is observed, that by employing the hydro turbine with operation in range from 20% to 100%, a high SUF for pumped storage can be attained while the SOP of battery is high due to a small regular energy flow from it. After one-year simulation, the overall SOP and SUF of HPBS is calculated as 66.4% and 7.3% respectively, while EUR of the whole system is 16.5%. Finally, the energy balance of the proposed system is analyzed to reveal the detailed operational performance of HPBS.