The wind wheel is generally composed of blades, wheel valleys, cover plates, connecting bolt components and shrouds. The wind wheel is the most critical component of a wind turbine, which converts the kinetic energy of the air into mechanical energy. The wind wheel of most small wind turbines consists of 3 blades. The tip speed of wind turbine blades can reach 50~70m/s. A 3-blade impeller with this tip speed can usually provide the best efficiency, but a 2-blade impeller only reduces the efficiency by 2%~3%. It is even possible to use a single-blade impeller, which has a balanced weight, but its efficiency is somewhat lower, usually 6% lower than a 2-blade impeller. Fewer blades naturally reduce the cost of blades, but this comes at a price. For blades with a well-balanced shape, the speed of the impeller with fewer blades will be faster, which will cause problems such as blade tip noise and corrosion. The 3-blade impeller is more satisfying from an aesthetic point of view. The 3-blade impeller is balanced in force and the wheel teaching structure is simple. The wheel-loaded structure of the 2-blade and single-blade impeller is usually more complicated, because the speed of the blade changes when it sweeps through the wind. , In order to limit the fluctuation of the force, the wheel valley has the characteristics of a seesaw. The seesaw wheel valley allows the impeller to tilt back or forward a few degrees in the plane of rotation. Due to the oscillating motion of the blades, the load on the blades due to wind gusts and shear will be significantly reduced during the weekly rotation.
The blade materials include wood, aluminum alloy, nylon, glass fiber reinforced plastic, reinforced glass plastic (GRP), carbon fiber reinforced plastic (CFRP), etc. At present, the most widely used glass fiber reinforced plastic blade. For small wind turbines, such as the diameter of the impeller is less than 5m, the material selection is usually concerned with efficiency rather than weight, stiffness and other characteristics of the blade. The structure of wind turbine blades generally has 6 forms, as shown in Figure 1.
(1) Solid wooden blades. This blade is made of high-quality wood, carefully processed, and its surface can be covered with a layer of glass fiber or other composite materials to prevent rain and dust from eroding the wood, and at the same time, it can improve the performance of the blade. When wind turbines use wooden blades, they are glued together with many longitudinal wooden strips, as shown in Figure 1(a).
(2) The rear edge of the airfoil of some wooden blades is filled with very light foam plastic, and the surface is covered with glass fiber to form a whole, as shown in Figure 1(b). The advantages of using foam plastic can not only reduce weight, but also make the airfoil center of gravity move forward to the 14th chord length of the leading edge, which can reduce the undesirable vibration caused by the rotation of the blade.
(3) In order to reduce the weight of the blades, some blades use a metal tube as a stress beam to form a honeycomb structure, and use foamed plastic, balsa wood or other materials as the intermediate filling, and a layer of glass fiber is coated on the outside, as shown in the figure. As shown in 1(c).
(4) In order to reduce costs, some wind turbine blades use metal extrusions, or use glass fiber or epoxy resin to pump and form, as shown in Figure 1(d) (but the entire blade cannot be squeezed into a tapered shape , That is, the width, thickness, etc. cannot be changed, and it is difficult to achieve high efficiency).
(5) In order to achieve more economical effects in some small wind turbines, the blades are made of tube beams and glass fiber skins with aerodynamic shapes. The glass fiber skin is thicker and has a certain strength. At the same time, some foam material ribs can be bonded in the glass fiber skin, as shown in Figure 1(e).
(6) The blades are made of pipe beams, metal ribs and skins. The metal skin is made into an aerodynamic shape, and the skin, ribs and pipe beams are bonded together with rivets and epoxy resin, as shown in Figure 1(f).
In addition to the use of wood materials for the blades of some small wind turbines, the blades of wind turbines usually use glass fiber or high-strength composite materials, and the materials of the blades are constantly improving. A blade with a streamlined section, under certain conditions, can obtain a lift that is more than 20 times higher than the drag compared to an ordinary blade, which is an ideal blade shape.