“Power conversion” is the conversion of heat-to-electricity, mediated by water becoming steam and propelling turbines. Unlike all other fusion devices, which are pulsed, the stellarator is inherently steady-state, and thus requires a different downstream power conversion. In this paper we conceive and optimize such a system for a stellarator plant equipped with plasma-facing liquid metal walls in the 700–900 °C temperature range. For maximum efficiency and safety, we rely on a “supercritical CO2 Brayton–Rankine Combined Cycle”. Despite the lofty name, this means that heat is transferred twice: first from the liquid metal to carbon dioxide (yes, the one we are trying to remove from the atmosphere), but in a special state called supercritical, halfway between liquid and gas, and secondly to water/steam. Brayton and Rankine are two important folks, with two thermodynamic cycles named after them. We obtain heat-to-electricity efficiencies of 51% (significantly higher than ~34% in tokamak reactors) and a net electrical efficiency of the complete plant, including auxiliary systems, of 34%. Finally, we discuss the technical feasibility and optimization of the two most critical components: turbo-machinery and heat exchangers. We obtain compact machines of, respectively, 1.2 m diameter and 40 m³ volume.

Even 1% of power-plant efficiency increase is worth tens of millions, and here we are talking factors, not percents... Let’s say we want to put 1 GW of electricity on the grid,. We could build a tokamak producing 6 GW heat with 34% conversion efficiency and 50% duty cycle. Alternatively, we could build a stellarator capable of 2 GW heat production with 51% conversion efficiency and near-100% duty cycle. The latter is roughly a 3x smaller investment, for the same result ! Continuous operation reduces costs even further (no thermo-mechanical transients, no intermediate energy storage, etc.). This is partly made possible by the steady state nature of stellarators, and partly by two new contributions of this paper. First, we envision a combined Brayton-Rankine cycle, more efficient than the traditional Rankine cycle of nuclear power plants. The second benefit stems from the high temperature of our first working fluid -the liquid metal-, resulting in high thermodynamic efficiency, and made possible by clever materials subject to lower evaporation than pure lithium. Finally, combining two cycles led to adopting a third, intermediate working fluid. This prevents liquid metal from entering in contact with water, with major benefits for safety.