Optimization of wind turbine loads for maximum power output and low fatigue loading

Abstract: In this thesis the aerodynamic loads for maximum power output at acceptable fatigue loads on a 1.5MW onshore wind turbine are examined. The objective mainly is to investigate pitch angles where optimal value of maximum power output at an acceptable level of fatigue loading can be achieved while studying the source of fatigue loading and the constraints of increasing the coefficient of performance of wind turbine power output. A total of thirteen hub height mean wind speed profiles, at the same turbulence level, ranging from cut-in wind speed of 3m/s to cut-out wind speed of 27m/s at 2m/s incremental are simulated. The reference wind speed is set at the hub height. For reference wind set below the hub height, the logarithm wind profile is used to determine the hub height mean wind speed, and then the power law follows to determine the mean speed at other height. The speeds are determined on a meshed grid point to examine the change of wind speed and direction in time and space or turbulence which is mainly due to the shape and hostile of the terrains. Wind profile simulation is performed by TurbSim simulation code, and the resulting profile is used as input to analyze the loads at the blade root. The loads are analyzed for the wind speed above the rated wind speed, 11m/s to 27m/s, where the blades are pitched to obtain an even power output. After performing several runs to investigate the relationship of wind speed to power output and fatigue loading, the wind speed, where the load should be analyzed, is narrowed to 21m/s which is close to the cut-out wind speed. The loads at the blade root are examined using the free simulation code, FAST, for different pitch angles ranging from 7.5 degrees to 17 degrees for each hub height mean wind speeds mentioned above. For examination of the loads at the selected locations the blade root is segmented to twelve equal points located 15 degrees away to each other. The points are located in angle between 0 and 180 degrees according to Load Rose approach. The loads at the blade root are FAST output and they are used as input for post-processor MLife to analyze the fatigue load. The fatigue loads are examined in terms of damage equivalent loads of the bending moment out of plane. It is observed that pitching a blade angle has a significant effect on the power output and fatigue load, the power output increases and with undesirable fatigue load while pitching the blade angle to capture as maximum power output as possible. On the other hand, attempting to decrease the fatigue load affects the power output as well, that indicates minimizing the fatigue load cannot be achieved without affecting the power output. Output power and fatigue load relation for different pitch angle ranging from 7.5 to 17 degrees of the selected wind speed 21m/s shows that while pitching the blade the power output increases with undesirable fatigue load. In general, it can be said that expected results are achieved at pitch angle ranging from 15 to 17 degrees. However, the fatigues loads may be not are in acceptable level, hence, it will not be appropriate to conclude that these pitch angles are the optimal angles where the maximum power output and minimum fatigue load can be achieved. Furthermore, looking at only the fatigue loads the minimum fatigue load is achieved at pitch angle of 7.5 at a sacrifice of 0.6MW of the maximum output power, 1.92MW, which is significant compared to the maximum output power that can be achieved.