Energy and exergy analysis of a biomass-CO2 transcritical brayton cycle

The biomass-COQ transcritical Brayton cycle is an emerging technology that offers high efficiency, compact system design, and environmental benefits for biomass-based power generation. In this paper, a theoretical study is carried out to evaluate the energy and exergy performance of a biomass power system with a transcritical COQ Brayton cycle under various operating parameters, including heat source temperature and mass flow rate, CO2 turbine inlet and outlet pressures, and heat sink temperature. Notably, this theoretical analysis is incorporated with CFD modelling of the system's gas heater to achieve more accurate and reliable predictions. Simulation results indicate that both thermal and exergy efficiencies increase with higher heat source temperatures, greater heat source mass flow rates, and lower heat sink temperatures. Furthermore, an optimal cycle pressure ratio exists to maximize both thermal and exergy efficiencies. Under the designed operating conditions, with a turbine inlet pressure of 12 MPa, a turbine outlet pressure of 5.0871 MPa, a heat source temperature of 1073.15 K, and a heat sink temperature of 288.15 K, the optimal thermal efficiency and exergy efficiency are 13.5 % and 18.4 %, respectively. Moreover, the exergy analysis reveals that the gas cooler, accounting for 33.4 %, and the gas heater, at 25.3 %, contribute the most to irreversibilities, underscoring the importance of their optimisation. The model development and simulation are essential for understanding system operations, and facilitating the optimisation of system structural designs and control strategies.