Wind turbines come in various designs, but they are generally divided into two main categories: horizontal-axis wind turbines (HAWTs) and vertical-axis wind turbines (VAWTs). In HAWTs, the rotor shaft is parallel to the direction of the wind, while in VAWTs, the rotor shaft is perpendicular to the wind direction. This fundamental difference affects how each type captures and converts wind energy.
This article focuses on vertical-axis wind turbines, which can be further categorized into resistance-type and lift-type designs. Resistance-type VAWTs rely on the drag force created by air flowing over the blades, while lift-type VAWTs use aerodynamic lift generated by the shape of the blades. As the rotational speed increases, the resistance decreases significantly, while lift increases, making lift-type turbines more efficient than their resistance-based counterparts.
Several types of resistance-based VAWTs exist, such as those with flat plates or cup-shaped blades, which operate primarily on drag. These devices have a high starting torque but suffer from low tip-speed ratios and limited power output for their size, weight, and cost. Another example is the S-shaped windmill, which combines some lift with resistance but remains largely a drag-driven system.
The Darrieus turbine, invented by French engineer G.J.M. Darrieus in the 1930s, is a classic example of a lift-type VAWT. It gained attention in the 1970s when Canadian scientists conducted extensive research on it. The Darrieus design features curved, airfoil-shaped blades that generate lift, allowing for higher tip-speed ratios and greater efficiency. However, these turbines typically have a low starting torque and require an external mechanism to initiate rotation. They come in various configurations, including Φ, Δ, Y, and H shapes, and can be designed with single, double, triple, or multiple blades.
Despite being over 180 years old, the Darrieus turbine has not seen widespread adoption due to several limitations. One major issue is its inability to self-start, which makes it less practical in variable wind conditions. Additionally, the range of wind speeds and loads it can handle is narrow, and the lack of adjustability further restricts its performance.
Fixed-blade lift-type VAWTs, such as the traditional Darrieus design with a φ-shaped blade or modern H-shaped blades, have fixed blade angles that cannot be adjusted. Figure 1 illustrates the layout of the blades on a lift-type VAWT. Under ideal conditions, airfoil blades can generate lift at angles of attack between 0° and 15°, with maximum lift occurring around 8° to 13°, where drag is minimal.
Figure 2 shows the airflow and forces acting on the blades at different positions. When the blade is facing the wind (0°), the relative wind speed W creates lift L and drag D. The angle of attack α is approximately 14°, and the blade moves at about four times the wind speed, resulting in a tip-speed ratio (TSR) of 4. The resultant force F pushes the rotor forward, generating a positive moment M that drives the turbine.
However, if the wind speed doubles and the blade speed remains the same, the TSR drops to 2, increasing the angle of attack to around 27°, causing the blade to stall. At this point, lift decreases, and drag increases significantly, leading to a negative moment that can prevent the rotor from turning.
In reality, a TSR of 4 is already near the stall threshold. Below this, lift doesn't increase much, but drag rises, potentially creating zero or negative moments. While there are regions where the blade can still produce positive torque, these areas shrink as the TSR drops below 3.5.
Figure 3 illustrates the relationship between the power coefficient (Cp) and the tip-speed ratio (TSR) for fixed-blade lift-type VAWTs. The optimal performance occurs when the TSR is between 4 and 6, under ideal airflow conditions.
However, wind conditions are rarely constant, and sudden changes in wind speed or load can cause the TSR to fall below 3.5, leading to reverse torque and unstable operation. Similarly, when the load increases and the rotor slows down, the turbine may struggle to maintain stability. This sensitivity to wind and load variations is a major limitation for fixed-blade lift-type VAWTs, along with their inability to self-start, which restricts their practical applications.
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