Thermodynamic and technoeconomic limitations of Brayton refrigeration for air conditioning

Abstract

With global cooling demand increasing, there is a need for refrigeration cycles that use low global warming potential (GWP) refrigerants. Researchers have flirted with the idea of using the Brayton cycle for refrigeration over the years, but it has yet to prove competitive in performance or cost when compared to the widely used vapor compression cycle. The recent development of an electrochemical Brayton refrigeration cycle has renewed interest in the thermodynamics of Brayton refrigeration. This work provides a parametric study of the COP of a Brayton cycle air conditioner (either mechanical or electrochemical in nature) as a function of the characteristics of the cycle components (thermal conductances and isentropic efficiencies). When the isentropic efficiencies of the adiabatic components (i.e., the compressor and turbine in the mechanical Brayton cycle) are 90 %, the cycle COP is limited to a value of ∼1. Furthermore, a thermodynamic comparison to the vapor compression cycle reveals that the Brayton refrigeration cycle generates ∼8 × more entropy, and that the thermodynamic favorability of the vapor compression cycle is due to the presence of phase change.