International Journal of Energy Research, Vol.45, No.2, 1818-1835, 2021
Investigation of off-design characteristics of an improved recompression supercritical carbon dioxide cycle for concentrated solar power application
The off-design characteristics of an improved recompression supercritical carbon dioxide cycle integrated with a two-stage intercooled main compressor are investigated with a focus on the concentrated solar power application. An off-design model is established for each crucial component of the cycle system of 100-megawatt scale. Four cycle control schemes with different main compressor configurations or/and cycle maximum pressure modes are evaluated and compared. A sensitivity analysis is performed on the parameters related to the cycle thermal input and ambient condition to predict the off-design characteristics due to the plant dispatch and ambient condition change in a solar power plant. The off-design results regarding the cycle thermodynamic performance and operational issue prevention are presented. The effect of the design-point value of the main compressor inlet temperature on the off-design characteristics is evaluated with the comparison among the results at three design points. The results reveal that the compressor surge may occur to the main compressor with basic configuration as the main compressor inlet temperature decreases to a certain value beneath the corresponding design point. By contrast, the surge risk can be prevented with the modified main compressor configuration by activating the recirculation system and the cycle can thus operate normally in the entire off-design range of main compressor inlet temperature. The off-design change in thermal input has overall limited effects on the cycle system control. No operational compressor issues occur for the main compressor with either basic or modified configuration as the thermal input deviates from the design points and varies in the studied ranges. The cycle maximum pressure mode has slight effects on the cycle thermodynamic performance as the thermal input deviates from the design point. The flexible cycle maximum pressure mode has slightly lower sensitivity to the thermal input variation in net output power due to the counteraction of mass flowrate variation. The selection of the design-point value of the main compressor inlet temperature has significant effects on the off-design characteristics of the cycle. A low design-point value of the main compressor inlet temperature leads to less demanding control action for operational issue prevention whereas a high design-point main compressor inlet temperature results in overall more stable thermodynamic performance under off-design conditions. Among other schemes, the fixed maximum pressure mode with the modified main compressor configuration is found to be the most satisfactory one due to the consequent superior efficiency, steady net output power, and free of hazardous operating issues despite the relatively demanding task of compressor surge prevention. The developed control scheme can be further improved by implementing parametric optimization during the off-design operation.