Off-design operation of the dry-cooled supercritical CO2 power cycle

• Past studies disagree on sCO2 cycle off-design performance. • Pressure operating strategy and compressor speed control improves performance. • Increasing turbine pressure maintains power output during elevated temperatures. • Compressor pressure control is effective for efficient plant turn-down. • Main compressor speed control is more beneficial than recompressor speed control. The supercritical CO 2 recompression cycle (the ‘sCO 2 cycle’) is highly efficient, scalable, simple, and compatible with dry cooling. Two scenarios where dry-cooled sCO 2 cycles may operate off design are (1) during periods of high ambient temperature, and (2) when operating at part-load output (in load-following roles). Understanding sCO 2 cycle off-design performance is essential for financial feasibility assessment, detailed design, and providing control system setpoints. Off-design performance can be maximised by exploiting the cycle’s pressure, temperature, and turbomachinery speed degrees of freedom. Several previous studies arbitrarily constrain these degrees of freedom, giving sub-optimal performance predictions. Other studies fail to consider CO 2 ’s steep and non-linear property variations, leading to inaccurate results. To address these limitations, this work presents a detailed sCO 2 cycle model, which captures property changes and component off-design behaviour. The model is used to systematically explore the off-design operating space, thus showing novel off-design performance trends and best operating strategies. Elevated ambient temperatures reduce power output more than efficiency, but this effect can be mitigated with appropriate operating strategies. At an ambient temperature of 40 °C (10 °C above design), design-point power output can be maintained with an 8.7% minimum turbine pressure increase, alongside compressor speed control. Alternatively, at this same temperature, design-point power output can be maintained with only a 4.4% (2 percentage point) efficiency reduction and a slightly higher (13.7%) turbine pressure increase. During turn-down, design-point efficiency can be maintained at 44% of design-point power output by using pressure and compressor speed control. Main compressor speed control alone is effective for above-design ambient temperatures, while dual compressor speed control provides significant benefits during turn-down. Compared to past sCO 2 cycle studies, this work reports better off-design performance, which demonstrates the importance of off-design operating strategies and highlights the flexibility and potential of the cycle.