Performance analysis and multi-objective optimization based on a modified irreversible Stirling cycle

The efficiency of the actual Stirling engine is much lower than the ideal Carnot cycle efficiency. To obtain more precise efficiency for Stirling engines, this paper proposes a modified Stirling cycle with a more accurate heat transfer process in Stirling engines based on a thermodynamic function called available potential. The finite-time thermodynamic method is used to analyze the model performance under constant heat source temperature, finite temperature difference heat transfer, and incomplete regenerative processes. A new polytropic process is introduced to model the heat transfer between the working fluid and external heat sources in which only heat above ambient temperature is converted into technical work. The regenerator is divided into numerous smaller heat reservoirs with individual temperature to analyze the incomplete regenerative processes. The expressions of the output power and thermal efficiency are obtained based on the modified irreversible Stirling cycle, and the effects of irreversible losses are analyzed to evaluate the performance of the proposed model. Results indicate that the efficiency of the modified cycle is much lower than that of the ideal Stirling cycle with an isothermal process. With the increase of the average heat transfer temperature difference, there exists an optimum value for the power of the irreversible cycle. The optimum power of the model was obtained for varying thermodynamic parameters by optimizing the average heat transfer temperature difference between the hot and cold sides. To optimize the irreversible model, the multi-objective optimization analysis is carried out based on NSGA-Ⅱ, which results in an optimized output power of 40.87 kW and an optimized thermal efficiency of 44%.