What factors determine the efficiency of a pumped storage hydroelectric system?

The overall efficiency of a pumped storage hydroelectric (PSH) system is determined by a combination of factors related to the turbine-generator equipment, hydraulic losses, and operational strategies. PSH efficiency is typically defined as the ratio of the energy generated during the discharge cycle to the energy consumed during the pumping cycle. Turbine and generator efficiency is a primary factor. The efficiency of the pump-turbine units in both generating and pumping modes directly affects the overall system efficiency. Higher turbine and generator efficiencies result in less energy loss during conversion. Hydraulic losses in the penstock, reservoirs, and other water conduits also contribute to energy losses. Friction losses, turbulence, and other hydraulic inefficiencies reduce the amount of energy that can be stored and retrieved. Pumping head, which is the vertical distance the water is pumped, impacts efficiency. Higher pumping heads generally result in lower efficiencies due to increased energy requirements. The pumping cycle efficiency is generally less than the generating efficiency because the pumps typically have a lower efficiency than the turbines. Operational strategies, such as the timing and duration of pumping and generating cycles, can also affect efficiency. Optimizing these cycles to minimize losses and maximize energy storage can improve overall system performance. For example, pumping during off-peak hours when electricity prices are low and generating during peak hours when prices are high can increase the economic value of the stored energy. The use of variable speed drives can also improve the efficiency of the pumping cycle by allowing the pumps to operate at their optimal efficiency point. Round trip efficiency, which represents the percentage of electricity recovered compared to the amount used for pumping, typically ranges from 70% to 85% for modern PSH facilities.