Control of supercritical CO2 Brayton cycle for fast and efficient load variation processes

Abstract

The supercritical carbon dioxide (S-CO2) cycle is considered a promising power conversion system due to its high efficiency and flexibility. This work presents a dynamic model of a 600 MWe S-CO2 recompression Brayton cycle (RBC) and validates it at both component and system levels. To ensure the safety and stability of the S-CO2 RBC during wide-load operation, some necessary control strategies are designed, including minimum pressure and temperature control, split ratio control, turbine inlet temperature control, and anti-surge control. Moreover, an innovative valve-inventory joint control method is proposed, coupled with an improved inventory tank layout, to achieve both fast and efficient load variation processes. Compared to the control strategy before improvement, the ramp-up and ramp-down rates are significantly increased by 2.65 and 2.24 times, respectively. In addition, the values of key parameters suitable for wide-load operation of the S-CO2 RBC are determined to maximize the cycle efficiency under off-design conditions. The results show that the split ratio should be dynamically adjusted to determine the optimal value that varies with the load, which can improve the cycle efficiency by a maximum of 1.13 %. Finally, the dynamic performance of the S-CO2 RBC is studied during load ramp changes and real-time microgrid load, and the variations in heat exchange, pressure, temperature, and mass flow rate of key components are revealed. Comparative analysis with previous literature demonstrates that the proposed control method provides a wider load regulation range, an increase of 7.78 % in cycle efficiency, and a reduced maximum generation frequency deviation of 0.696 Hz. There is no overpressure or compressor surge risk, and the maximum temperature change rate is only 1.3 ℃·min−1.