Toward improving the performance of a variable pitch vertical axis wind turbine (VP-VAWT), Part 2: Multi-objective optimization using NSGA-II with CFD in the loop

Abstract

In this study, a multi-objective optimization is performed to improve the performance of a Darrieus vertical axis wind turbine (VAWT) at low tip speed ratios considering the aerodynamic and structural objectives. In this regard, firstly, a multi-objective optimization platform is developed. Using the results of part-1 of the current research, design of experiment (DOE) analysis, more effective design variables and their limited range of variation were extracted. These quantities were selected from pitch profile and blade section geometry variables, which will form a minimal optimization problem including the essential physics with reduced computational cost. The optimization is executed using a homemade NSGA-II code, validated by a benchmark model. It is embedded within the main MATLAB code, which relates and controls the flow field solution and optimization loop. Average moment coefficient in the downwind phase, the maximum moment coefficient in the upwind phase and the bending stiffness were selected as objective functions. These parameters would reflect power, structural fatigue and strength, respectively. The Pareto front charts are provided, which depict the relationship between objectives. It will then be followed by trend analysis and provide trade-off capability. Different aerodynamic aspects of the optimized VAWT are also compared with the reference VAWT, demonstrating the effects of optimum pitch profile and blade section. The trend analysis shows a compromise between structural and aerodynamic objectives, while the two indices of maximum moment and stiffness are not opposing. Moreover, a practical method for effective angle of attack identification is employed. Based on this estimation, one may deduce that the angle of attack in the upwind phase is more influenced by pitch variation than in the downwind. Interestingly, thrust coefficient and thrust angle variation range reduction would be achieved as a result of the proposed optimization, which is beneficial for the whole system's stability. According to the results, the developed framework is capable of performing other multi-disciplinary optimization challenges.