Analyses and optimization of a supercritical N2O Rankine cycle for low-grade heat conversion

Abstract

Analyses and operating pressure optimization of the supercritical Rankine cycles with and without regenerator and reheating for low-grade heat conversion have been conducted using N2O as a working fluid and compared with its counterpart CO2 based on various performance indicators. N2O is better in terms of net power output, thermal efficiency and exergetic efficiency and N2O works at much lower pressures at optimum operation; whereas, CO2 is advantageous in terms of turbine size, expansion ratio and heat transfer requirement. The choice of optimum operating conditions will differ depending on the chosen performance indicator. Hence, there is a need of trade-off between various indicators. Component wise irreversibility distribution shows the similar trends for both working fluids. With the increase in cycle temperature lift, both turbine shape parameter and heat transfer requirement decrease, leading to more compactness. Higher pump and turbine isentropic efficiencies yield higher optimum turbine inlet pressure, and lower heat transfer requirement and turbine size. Uses of internal heat exchanger and reheating in the supercritical Rankine cycle not only improve the performances, it also constitutes an excellent compromise between various performance indicators based optimizations. Present study reveals that N2O is a potential option for the supercritical Rankine cycle.