Servo-aeroelastic tailoring of wind turbines using new active-to-passive control systems

近年来，可再生能源的成本稳步下降。正如工业增长数据所证实的那样，这一因素，加上社会经济原因，使可再生能源越来越具有竞争力。考虑到风力涡轮机（WTs），有一些有趣的技术挑战与制造更大、更耐用的转子有关，以更便宜、更经济的方式产生更多的能量。转向更大转子的基本原理是，在目前的设计中，wt产生的功率理论上与叶片长度的平方成正比。此外，更高的wt在更高的高度运行，平均风速也更大。因此，在一般情况下，一个转子可以产生更多的能量比两个转子与一半的面积。然而，更大的叶片更重、更昂贵，而且越来越容易受到更大的空气动力和惯性力的影响。事实上，已经证明它们在长度和质量之间表现出三次关系，这意味着随着叶片尺寸的增加，材料成本、惯性和自重效应的增长速度快于能量输出。此外，更大的叶片也会对机舱部件的设计产生连锁影响。转子扫过的风场在时间和空间上都是不同的。因此，叶片的力和扭矩分布在频率为转子速度的整数倍时表现出强烈的峰值。附加峰是由轻阻尼结构模态引起的。叶片上的载荷结合起来在转子上产生不平衡载荷，这些载荷传递给轮毂、主轴承和其他传动系统部件。这些不平衡载荷是造成某些部件寿命等效疲劳载荷的主要原因，可能导致结构过早失效。随着叶片尺寸的增大，不平衡载荷增大，谱峰频率减小。因此，随着涡轮机的尺寸越来越大，它们的影响也越来越大。在这种情况下，改进叶片设计的需求是显而易见的。质量效率越来越高的涡轮机，也能够收获更多的能量，这一概念立即具有吸引力。在这个项目中，研究了一种用于水平轴WTs的新型自适应叶片概念的可行性。通过适当地调整叶片的弹性响应以适应空气动力压力，有可能提高涡轮机的年发电量，同时减轻结构负荷。这些改进是通过利用结构各向异性和几何诱导耦合提供的能力，以被动自适应的方式获得的。其中，诱导弹性扭转可以根据功率要求来改变叶片截面的迎角，即弹性扭转随风速的变化与弯曲载荷成比例。自适应行为允许叶片几何形状遵循理论上的最佳形状发电密切（作为远场风速的函数变化）。这个概念保留了以前提出的设计的负荷缓解能力，同时提高了能源生产。在结构上，自适应行为是通过合并离轴复合材料层和掠叶平台的弯曲-扭转耦合能力来实现的。有可能，仅由发电机扭矩控制的自适应叶片可以达到与当前最先进的功率标准相当的功率标准，同时通过挑战对主动俯仰控制系统的需求，大大降低了风力涡轮机的复杂性、成本和维护。

In recent years, the cost of energy produced by renewable supplies has steadily decreased. This factor, together with socio-economical reasons, has made renewable energies increasingly competitive, as confirmed by industry growth figures. Considering wind turbines (WTs), there are some interesting technical challenges associated with the drive to build larger, more durable rotors that produce more energy, in a cheaper, more cost efficient way. The rationale for moving towards larger rotors is that, with current designs, the power generated by WTs is theoretically proportional to the square of the blade length. Furthermore, taller WTs operate at higher altitudes and, on average, at greater wind speeds. Hence, in general, a single rotor can produce more energy than two rotors with half the area. However, larger blades are heavier, more expensive and increasingly prone to greater aerodynamic and inertial forces. In fact, it has been shown that they exhibit a cubic relationship between length and mass, meaning that material costs, inertial and self-weight effects grow faster than the energy output as the blade size increases. In addition, larger blades also have knock-on implications for the design of nacelle components.The wind-field through which the rotor sweeps varies both in time and space. Consequently, the force and torque distributions for the blades exhibit strong peaks at frequencies which are integer multiples of the rotor speed. Additional peaks are induced by lightly damped structural modes. The loads on the blades combine to produce unbalanced loads on the rotor which are transmitted to the hub, main bearing and other drive-train components. These unbalanced loads are a major contribution to the lifetime equivalent fatigue loads for some components which could cause premature structural failure. As the size of the blades increase, the unbalanced loads increase and the frequency of the spectral peaks decrease. Hence, they have an increasing impact as the size of the turbines become bigger.In this scenario, the demand for improvements in blade design is evident. The notion of increasingly mass efficient turbines, which are also able to harvest more energy, is immediately attractive.The viability of a novel adaptive blade concept for use with horizontal axis WTs is studied in this project. By suitably tailoring the elastic response of a blade to the aerodynamic pressure it could be possible to improve a turbine's annual energy production, whilst simultaneously alleviating structural loads. These improvements are obtained in a passive adaptive manner, by exploiting the capabilities that structural anisotropy and geometrically induced couplings provide. In particular, induced elastic twist could be used to vary the angle of attack of the blade sections according to power requirements, i.e. the elastic twist is tailored to change with wind speed proportionally to the bending load. The adaptive behaviour allows the blade geometry to follow the theoretically optimum shape for power generation closely (which varies as a function of the far field wind speed). This concept retains the load alleviation capability of previously proposed designs, whilst simultaneously enhancing energy production. Structurally, the adaptive behaviour is achieved by merging the bend-twist coupling capabilities of off-axis composite plies and of a swept blade planform. Potentially, an adaptive blade, controlled only by generator torque, could perform to power standards comparable to that of the current state-of-the-art-while greatly reducing complexity, cost and maintenance of wind turbines, by challenging the need for active pitch control systems.