Multi-objective optimization in air-helium Stirling engine under objectives of efficiency, power, heat loss, and cost

As an external combustion powertrain, the Stirling engine uses the temperature difference between a hot and cold source to drive the piston, generating mechanical energy, which is then converted into electricity. Based on the available literature, it was established that utilizing air as the working fluid represents a more cost-effective choice than helium. However, it is noteworthy that when air is employed as the working fluid in a Stirling engine, it tends to yield lower power output and efficiency than helium. Therefore, this study examined the possibility of using a combination of air/helium as the working fluids in the Stirling engine. This research introduces a non-ideal thermodynamic model designed for a gamma-type Stirling engine. Once the model is validated, the study employs a mixture of helium and air as a working fluid in the thermodynamic model instead of a single operating fluid. Following this step, the necessary data were extracted from the thermodynamic model using the Design of Experiments approach, and a curve was fitted to the data to establish the desired functions. Finally, multi-objective optimization based on the genetic algorithm was performed on the obtained functions in the mode-FRONTIER software to determine the mixing ratio of air and helium, in which the efficiency and the power of the Stirling engine increased, and the working fluid consumption cost and heat loss decreased. The optimization results showed that, in four-objective optimization, the most optimal input parameters for reducing the cost and heat dissipation and increasing the power and efficiency were a helium percentage of 34.6%, a speed of 614 rpm, a pressure of 4.5 bars, and a hot temperature of 597°C. Optimizing with an air and helium gas mixture increased power by 124.8% compared to 100% helium without optimization.