Thermodynamic study of supercritical CO2 Brayton cycle using an isothermal compressor

Abstract

In this paper, a thermodynamic study of newly suggested supercritical carbon dioxide (s-CO2) cycle layouts using an isothermal compressor is presented. The isothermal compressor is conceptually defined using the 'in-finitesimal approach' in attempt to resolve the ambiguity in its performance framework. As part of preliminary investigation, the isothermal compressor is demonstrated thermodynamically, and the calculations highlight that it reduces the compression work significantly under s-CO2 power cycle operating conditions over other representative working fluids. The in-house code is modified to allow the analysis of three s-CO2 cycle layouts, simple recuperated Brayton cycle, recompression Brayton cycle, and partial heating Brayton cycle, adopting the isothermal compressor. The cycle performance is evaluated through a sensitivity analysis of cycle design parameters, pressure ratio and flow split ratio. When the machinery is applied, the cycle net efficiency of the simple recuperated cycle and the recompression cycle is improved by 0.5% point and 1-3% points, respectively. Moreover, the partial heating cycle layout, known for its outstanding performance in waste heat recovery applications as a bottoming cycle, produces 15-18% more net work when using an isothermal compressor, compared to the reference cycle. Overall, the use of the isothermal compressor not only improves the general cycle performance, but also provides another degree of freedom for cycle design optimization of s-CO2 cycles.