Combined cycle schemes coupled with a Very High Temperature gas-cooled reactor

With gradual increase in reactor outlet temperature, the efficient power conversion technology has become one of developing trends of (very) high temperature gas-cooled reactors (HTGRs/VHTRs). In this paper, physical and mathematical models of two combined cycle schemes coupled with the VHTR, Reheated Combined Cycle (RCC) and Regenerative-Reheated Combined Cycle (RRCC), are established. Their topping cycle is a no precooling and no intercooling Brayton cycle, and their bottoming cycle is a supercritical Rankine cycle. The effects and mechanism of key parameters were analyzed. Based on parameter analysis, two schemes were compared within the temperature range of the VHTRs (ROT 950-1200 °C). The analysis shows that, for the combined cycle of the VHTR, by adding a recuperator, can further utilize the helium turbine exhaust heat, then to improve the cycle efficiency. When the ROT is 1050 °C, the cycle efficiency of the RRCC (recuperator's effectiveness 0.4）is 56.4%, which is 2.18% higher than that of the RCC. But because of the regenerative process, the reactor inlet temperature (RIT) of the RRCC is higher than 490 °C, additional reactor pressure vessel (RPV) cooling system is necessary, and it increases the system complexity and reduce the cycle's efficiency. The RIT of the RCC can be controlled below 490 °C, no RPV cooling system is required. Amongst the main parameters, the recuperator's effectiveness and compression ratio are the most important factor affecting the cycle efficiency of the VHTR. The cycle efficiency can be increased by 0.61% when the recuperator's effectiveness is increased by 0.1. The main factors that limit the increase of the recuperator's effectiveness are the RIT and the system flow resistance. When the average compression ratio increases by 0.1, the combined cycle efficiency increases by 0.33% (RCC) and 0.373% (RRCC). The main factors that limit the increase of compression ratio are the manufacturing capacity of magnetic bearings, turbines and compressors. These results provide insights on the combined cycle of the VHTRs, and reveal the effects of key parameters on performance of these cycles. It could be helpful to understand and develop a combined cycle coupled with a VHTR in the future.