Thermodynamic analysis and optimisation of a novel transcritical CO2 cycle

Abstract

Transcritical and supercritical CO2 cycles are simple, low-cost, environmentally friendly, and compact solutions for harnessing a wide range of thermal energy sources such as waste heat, solar-thermal, geothermal, and nuclear reactors. In a supercritical CO2 cycle, compression begins near the critical-state (30.98 °C and 7.3773 MPa), whereas CO2 condensation occurs below the critical-state in a transcritical CO2 cycle, and the required sink temperatures are difficult to attain in the hot arid climates. The current study presents and optimises a novel transcritical CO2 power cycle that can be used in hot climates and offers very compact system which is vital for engine waste heat recovery. The proposed system achieves CO2 condensation by expanding pre-cooled CO2 from some intermediate pressure. It uses phase-wise CO2 separation in the separator and two recuperators to recycle waste heat from the hot CO2 stream leaving the turbine, reducing heat input into the heater and heat rejection by the pre-cooler. To achieve the best thermal efficiency, a differential evolution optimisation is used, and the phase-wise split of CO2 is tuned to provide excellent matching of temperature profiles while achieving the required pinch-point conditions in both the recuperators, resulting in a minimal low exergy destruction. The maximum thermal efficiency of the proposed cycle is 39.82%, while the optimal supercritical recuperative-Brayton cycle offers efficiency of 37.77%, under the same initial conditions. Under optimal current cycle conditions, thermodynamic processes avoid operating close to the CO2 critical point, necessitating less demanding and simpler turbomachinery designs.