Optimization of Advanced High-Temperature Brayton Cycles with Multiple-Reheat Stages

This paper presents an overview and a few point designs for multiple-reheat Brayton cycle power conversion systems (PCSs) using heat from high-temperature molten salts or liquid metals. All designs are derived from the General Atomics gas turbine-modular helium reactor (GT-MHR) power conversion unit (PCU). Analysis shows that, with relatively small engineering modifications, multiple GT-MHR PCUs can be connected together to create a PCS in the >1000 MW(electric) class. The resulting PCS is quite compact, and results in what is likely the minimum gas duct volume possible for a multiple-reheat system. To realize this, compact plate type liquid-to-gas heat exchangers (power densities from 10 to 120 MW/m3) are needed. Different fluids such as helium, nitrogen and helium mixture, and supercritical CO2 are compared for these multiple-reheat Brayton cycles. For turbine inlet temperatures of 900, 750, and 675°C, the net thermal efficiencies for helium cycles are 56, 51, and 48%, respectively, and corresponding PCU power densities are 560, 490, and 460 kW(electric)/m3, respectively. The very high PCU power densities could imply a large material saving and low construction cost, and bring down the specific PCU cost to about half that of the current GT-MHR PCS design.