Multi-Objective Technoeconomic Optimisation of Closed Loop CO2 Power Cycle for Spherical Tokamak Fusion Reactor

The UK Industrial Fusion Solutions Ltd is paving pathway for a commercial magnetically confined fusion power plant, namely Spherical Tokamak for Energy Production (STEP), with the ambition of building a STEP Prototypic Powerplant (SPP) by 2040. The fusion power cycle has conflicting requirements, thus pushing the boundary of conventional cycles. For example, large plant parasitic load necessitates high power conversion efficiency, demanding high operating temperature of tokamak, whilst the power cycle has to also integrate multiple heat grades from different tokamak in-vessel components, including low temperature heat received from high surface heat flux plasma facing components. Similarly, the power cycle has to exhibit high operational flexibility whilst also achieving high reliability and plant life for intermittent pulse mode operation of SPP, during the initial phases. Closed-loop CO2 power cycle show promise in realising high efficiency, efficient integration of low-grade heat by capitalising the enthalpy gap due to real gas effect of CO2, and high-power density CO2 turbine & compact heat exchangers signifying the potential of realising operational flexibility. A multi-objective techno-economic optimisation of CO2 power cycle integrating four heat sources from the SPP is presented in this work. This work maximises the efficiency whilst minimising the power block capital cost by optimising 10 process variables. This work also presents the challenges of integrating multiple heat sources from tokamak with novel CO2 cycle configurations which are the derivatives of transcritical CO2, supercritical recompression CO2, partial cooling and precompression CO2 cycles with intercoolers, demonstrating the feasibility of using such a power cycle design for SPP.