Experimental and theoretical investigation of biomass-CO2 transcritical Brayton cycles and heat exchanger optimizations

Advanced energy conversion technologies can convert the massive amounts of waste heat rejected by industrial operations into power and useable heat. In this thesis, a small-scale transcritical CO2 Brayton Cycles power generation test system integrated with biomass has been designed and constructed with purposely selected and manufactured system components. This thesis contributes to the knowledge and characterization of biomass-CO2 transcritical Brayton Cycles. It is based on both
experimental experience, CFD models and thermodynamic models.
Heat exchangers in power cycles highly influence the efficiency of system. Finned-tube gas cooler has been widely used in industries, and finned-tube water cooler was used in the proposed system. However, due to large amount of fins and then the difficulty of evaluating tube side temperature profile. A novel 1D-3D CFD model was carried out to investigate performance of CO2 finned-tube gas cooler and correspondingly evaluate
the feasibility of using it in biomass-CO2 transcritical Brayton Cycles. The CFD model has been validated by comparing with published literatures. The novel CFD model allows to predict the finned-tube type heat exchanger with good accuracy and also to explore possible improvements or different configurations. The lower CO2 outlet temperature makes it is an alternative gas cooler used in biomass-CO2 transcritical Brayton Cycle systems. In addition, it is found that longitudinal heat conduction along fins can lead to inverse heat transfer between adjacent tubes and thus capacity degradation of the heat exchanger. Therefore, CFD models have been purposely developed for the CO2 gas cooler with split fins to quantify the effect of the heat conduction through fins. Results show that average heating capacity can be increased by 10% when split fins are applied.
A detailed CFD simulation model of a particular designed shell-and-tube supercritical CO2 gas heater in a biomass-CO2 power generation system has been developed based on actual heat exchanger structural design and applicable operating conditions. The model has been validated with both manufacturer operational data and empirical correlations. The simulation results showed that increasing flue gas mass flow rate, flue gas temperature and CO2 mass flow rate can enhance differently the heating capacity of the heat exchanger. It is also found that by minimizing the distance between hot fluid pipe inlet and cold fluid outlet ports, as well as hot fluid pipe outlet and cold fluid inlet ports, the heating capacity of the shell-and-tube heat exchanger and the performance of its associated system can be significantly improved.
In this thesis, a theoretical study was conducted to investigate the performance of biomass-CO2 transcritical Brayton cycles. The thermodynamic model was integrated with the CFD results of CO2 gas heater to precisely evaluate the power generation,
power consumption, exergy loss, system thermal efficiency and system exergy efficiency at different operating conditions. Results showed that there exists an optimal CO2 turbine inlet pressure or CO2 turbine outlet pressure to maximize the system thermal efficiency and system exergy efficiency. Model development and simulation can contribute significantly to understand the system operations and eventually optimize the system structure designs and controls.