Energy and exergy-based performance analyses of a combined recompression supercritical carbon dioxide-organic Rankine cycle for waste-heat recovery

The development of innovative systems for the effective recovery of useful energy from low–medium temperature waste-heat sources, has emerged as a sustainable step towards the reduction of fossil fuel consumption and environmental pollution. The supercritical carbon dioxide Brayton cycle is considered a promising technology for this conversion to electricity. In this study, energy and exergy-based performance evaluation of a combined recompression supercritical carbon dioxide Brayton cycle and organic Rankine cycle as the bottoming cycle is presented. The main aim of the system is to improve the thermodynamic performance and maximize low grade heat to electricity conversion. A parametric investigation is perform to predict the effect on the system's net power output, thermal and exergy efficiencies while varying input parameters such as compressor outlet pressure, carbon dioxide turbine inlet temperature, carbon dioxide mass flow rate, flow split fraction, low-temperature and high-temperature recuperator effectiveness. The R1234ze(Z), R1234yf and R245fa organic working fluids for the bottoming organic Rankine cycle were considered. Results obtained revealed that the R1234ze(Z)-based combined system demonstrated the highest thermal and exergy efficiency of 55.87% and 73.13% respectively. Alternatively, the R245fa-based combined system have the lowest thermal and exergy efficiencies with marginal differences. The exergetic analysis reveals that the gas-heater, recompression compressor, sCO2 turbine and main compressor have the highest exergy destruction rates of 27.6%, 20.35%, 17.32% and 16.08%, respectively, hence to decrease the exergy destruction of the system the implementation of improve design processes of these components is necessary. The study concludes that the R1234ze(Z)-based combined system showed the highest power output of 425.1kW and a fuel depletion ratio of 0.009693.