Thermodynamic design and analysis of closed-loop CO2 power cycle for fusion power plant

UK Industrial Fusion Solutions Ltd is paving the pathway for a commercial magnetically-confined fusion power plant, known as the Spherical Tokamak for Energy Production (STEP), with the ambition of building a STEP Prototypic Powerplant (SPP) by 2040. The design of such a power cycle poses the following key challenges: 1) high power cycle conversion efficiency to overcome large plant parasitic loads, 2) integration of different heat sources at multiple temperature levels from the tokamak in-vessel components, including low temperature heat sources, 3) high power cycle operational flexibility, and 4) high reliability and plant life for the intermittent pulse mode of operation during the initial phases of the SPP. Closed-loop CO2 power cycles show promise in realising high efficiency (>550 °C); efficient integration of low-grade heat by capitalizing on the enthalpy gap due to the real gas effect of CO2; and high power density CO2 turbines & compact heat exchangers signifying the potential of realising operational flexibility. This paper compares the thermodynamic performance of three novel CO2 cycle configurations, namely 1) transcritical CO2 cycle variant; 2) supercritical CO2 cycle variant; and 3) transcritical CO2 blend based power cycle variant (using SO2 as the dopant), to efficiently integrate four different heat sources at different temperature levels from the fusion machine, demonstrating the feasibility of using such a power cycle design for SPP.