Sliding Pressure Inventory Control of a Supercritical CO2 Cycle for Concentrated Solar Power-Analysis and Implications

This paper presents the use of sliding pressure inventory control (SPIC) of a 10 MW supercritical carbon dioxide Brayton cycle for concentrated solar power, incorporating printed circuit heat exchangers. Load regulation using SPIC for three representative ambient conditions 45 degrees C, 30 degrees C, and 15 degrees C are considered. While a wide operating range from 10 MW to less than 1 MW part load is obtained, a notable cycle efficiency decline at part load is also seen. Irreversibility analysis reveals that deterioration in recuperator and turbomachinery performance are primarily responsible for cycle performance degradation at part load. Nevertheless, useful inferences are obtained from the 10 MW SPIC irreversibility study. With a slightly increased value of heat exchanger length, a non-condensing 1 MW subcritical CO2 cycle operating between 35 bar/53 bar is found to be as efficient as a 1 MW supercritical CO2 cycle operating between 88 bar/210 bar. The major benefit of choosing the subcritical CO2 cycle for 1 MW scale applications is the significantly reduced turbomachinery speed (similar to 26,000 rpm) in comparison with supercritical CO2 turbomachinery (similar to 67,000 rpm) for the same power scale. These advantages are found to be true for air-based ideal gas cycles operating between 35 bar/53 bar too, with the latter requiring nominally smaller heat exchangers than the subcritical CO2 cycle. The final choice of working fluid, however, for these low-pressure cycles would depend on practical considerations, such as material compatibilities at high temperatures, corrosion considerations, and cost.