Stabilizing Supercritical CO2 Cycles Using a Propane-Based Thermal Adapter for High-Temperature Nuclear Application

• A propane “Adapter” bottom cycle is modelled to stabilize supercritical CO 2 cycles. • The adapter isolates the power system from ambient temperature fluctuations. • The specific power of the sCO 2 cycle is improved by 26 – 47% • The proposed cycle exhibits stable operating points across ambient temperatures. • At rejection above the critical temperature, the proposed cycle is more efficient. Supercritical carbon dioxide (sCO 2 ) Brayton cycles mark the next generation of thermal energy conversion owing to their greater efficiency, simplicity, lower footprint, and compatibility with high temperature energy sources such as Gen IV nuclear systems. These cycles performance is reliant on heat rejection and compressor operation near the critical point of CO 2 , wherein small temperature changes have drastic effects in thermophysical properties. The proximity of this heat sink temperature to ambient conditions renders the performance and operation of the sCO 2 cycle and power source susceptible to unwelcome transients associated with ambient temperature fluctuations. This study proposes pairing the sCO 2 cycle with a stabilizing propane refrigeration bottom cycle – termed a “Thermal Adapter” – which absorbs ambient temperature fluctuations, providing stable and favorable operating conditions to a condensing transcritical sCO 2 cycle, maintaining constant interface conditions with the nuclear heat source. Thermodynamic analyses comparing the proposed cycle to a standard recompression Brayton cycle at varying temperatures show a 26% − 47% increase in specific power for the Thermal Adapter with minimal efficiency impacts. At high ambient temperatures (> 35 ° C) the thermal adapter cycle employs greater isothermal heat transfer resulting in lower exergetic losses than the standard cycle. Under transient ambient conditions, the Thermal Adapter is more resilient and is expected to be easier to control, as the refrigerant operates further from its critical point, thereby in a more ideal regime compared to CO 2 . Further economic analysis and rigorous dynamic analysis are suggested to bolster the case for the inclusion of an isolating bottoming cycle.