Off-design operation of grid-scale pumped thermal energy storage: A comparative analysis of control strategies

This work explores and compares control strategies to enable off-design operation of grid-scale pumped thermal energy storage (PTES) systems. Four relevant control strategies are identified to provide this kind of operational flexibility: inventory control, variable speed operation, flow control with bypass valves, and variable turbomachine geometry. These are applied to a comprehensive system model of a 100MW, 10h commercial PTES concept. The system model is characterised by a dynamic consideration of the thermal reservoirs, off-design performance models for all components and a more detailed description of electrical components. Each control strategy’s operating range and its impact on component performance, system performance, and electrical energy capacity are quantified. The findings indicate that none of the strategies cover the entire operating range. Inventory control is confirmed as the most promising strategy, covering 61.4% of the theoretical nominal operating range, without significant performance loss. Variable speed covers 38.6% of the nominal operating range and shows performance drops in off-design. Bypassing the turbine enables limited flexibility on the charging side and covers the lowest operating range (22.1%). Bypass control can only modulate the discharging power by incurring high losses. Variable compressor inlet guide vanes have a similar effect to variable speed, covering 30.7% of the operating range, but at a much lower cost. Variable speed, turbine bypass, and variable compressor inlet guide vanes significantly alter the cycle’s specific work; this, in turn, causes temperature fluctuations in the liquid storage tanks. Temperature fluctuations in one cycle can affect subsequent cycles, making those control options less attractive as stand-alone options.