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

近年来，由可再生能源供应产生的能源成本稳步下降。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，工业增长数字证实了这一点。考虑到风力涡轮机（WT），有一些有趣的技术挑战与驱动器相关，以更便宜，更经济的方式制造更大，更耐用的转子，产生更多的能量。转向更大转子的基本原理是，在当前设计中，风力发电机产生的功率理论上与叶片长度的平方成比例。此外，更高的WT在更高的海拔处运行，并且平均而言，在更大的风速下运行。因此，一般来说，单个转子可以比具有一半面积的两个转子产生更多的能量。然而，较大的叶片更重、更昂贵并且越来越倾向于更大的空气动力和惯性力。事实上，已经表明，它们在长度和质量之间表现出立方关系，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，更大的叶片也会对发动机舱部件的设计产生影响。转子扫过的风场在时间和空间上都是变化的。因此，叶片的力和扭矩分布在转子速度的整数倍的频率处表现出强峰值。轻微阻尼的结构模态会引起额外的峰值。叶片联合收割机上的载荷结合起来，在转子上产生不平衡载荷，这些不平衡载荷被传递到轮毂、主轴承和其他传动系部件。这些不平衡载荷是某些部件寿命等效疲劳载荷的主要贡献，可能导致过早的结构失效。随着叶片尺寸的增加，不平衡载荷增加，谱峰频率降低。因此，随着涡轮机尺寸的增大，它们的影响越来越大。在这种情况下，对叶片设计的改进需求是显而易见的。越来越多的质量效率的涡轮机的概念，这也能够收获更多的能量，是立即吸引人的。一个新的自适应叶片的概念，用于水平轴风力发电机的可行性进行了研究，在这个项目中。通过适当地调整叶片对气动压力的弹性响应，可以提高涡轮机的年能量生产，同时减轻结构载荷。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方式获得的。特别地，所引起的弹性扭转可用于根据功率要求改变叶片区段的攻角，即，弹性扭转被定制成与弯曲载荷成比例地随风速变化。自适应性能允许叶片几何形状紧密地遵循理论上最佳的发电形状（其作为远场风速的函数而变化）。这一概念保留了先前提出的设计的负载缓解能力，同时提高了能量生产。在结构上，自适应行为是通过合并离轴复合材料层片和后掠叶片平面形状的弯扭耦合能力来实现的。潜在地，仅由发电机扭矩控制的自适应叶片可以执行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变桨控制系统的需求来大大降低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.