Off-design behavior investigation of the combined supercritical CO₂ and organic Rankine cycle

To provide a higher energy conversion efficiency, an organic Rankine cycle (ORC) is coupled to a supercritical carbon dioxide cycle (sCO₂) for energy cascade utilization. However, power cycles often operate during a wide range of loads and the heat sink temperature varies over time. Therefore, the off-design behavior analysis is vital to better define cycle performance and build control systems. In this paper, detailed design and off-design models of the cycle components are developed to evaluate the cycle performance at different operating conditions. The results show that the cycle thermal efficiency can reach up to 44.39% when the inlet pressures of the sCO₂ and ORC turbine equal to 20.77 MPa and 754.04 kPa, respectively. The variable-speed control strategy is used to regulate the maximum pressure, the mass flow rates and the component behaviors to achieve operational goals. Based on this method, the combined sCO₂-ORC cycle can be matched well with the nuclear reactor during a wide range of power loads (20–110% normalized power load) and heat sink temperatures (10–40 °C). The cycle performance decreases with plant loads, and an opposite trend is observed when the heat sink temperature changes. Compared with the stand-alone recompression sCO₂ cycle, the combined sCO₂-ORC cycle can gain a thermal efficiency improvement of 1.09–3.36%. But the cycle performance deteriorates significantly when the heat sink temperature is high due to the increase in the compression power consumption. From an economical point of view, the recompression sCO₂ cycle can gain a decrement by 2.32% for the levelized cost of electricity by integrating an ORC, and further cost optimization of reactors and turbines can make the cycle more commercially competitive.