What Is The Most Efficient Model Of Wind Turbine?

Wind turbines are an increasingly important source of renewable energy around the world. They use the wind’s kinetic energy to generate electricity that feeds into the grid. Wind turbines work by using blades that spin as the wind blows past them. The spinning blades turn a shaft connected to a generator which converts the mechanical energy into electrical energy [1]. Wind power produces no greenhouse gas emissions during operation and uses little water [2]. As of 2020, wind provided about 7% of total U.S. electricity generation [3]. Understanding what makes wind turbines more efficient allows us to capture more renewable energy and reduce our dependence on fossil fuels.

Factors In Efficiency

There are several key factors that impact the efficiency of a wind turbine, including size, design, and location. Generally speaking, larger wind turbines can capture more wind energy. According to the Department of Energy, wind turbines are becoming larger, with taller towers that can access faster wind speeds at higher altitudes (https://www.energy.gov/eere/articles/wind-turbines-bigger-better). The power output of a turbine is also heavily dependent on blade length, since longer blades can harvest more energy.

The overall design and aerodynamics of the turbine blades also play a major role. The blade shape, angle of attack, and material all contribute to efficiency in harnessing the kinetic energy of wind. In addition, the location and site conditions are critical factors. Consistent, fast wind speeds are ideal. Offshore locations generally provide stronger and less turbulent winds compared to onshore sites, enabling higher capacity factors (https://css.umich.edu/factsheets/wind-energy-factsheet).

Finally, the height of the turbine tower greatly impacts efficiency. At higher heights above ground level, wind speeds are faster and more consistent. Turbines that are able to reach stronger winds by using taller towers will be more efficient at converting wind energy into electricity.

Common Turbine Models

The two main categories of wind turbines are horizontal axis and vertical axis turbines. Horizontal axis turbines are the most common design and consist of a rotor shaft and electrical generator at the top of a tower. The blades face into the wind. According to Arcadia, the most common horizontal axis turbines in use today are manufactured by Vestas, Gamesa and General Electric.

Vertical axis turbines have blades that go from top to bottom and the main rotor shaft is set vertically. This type of turbine was popular in the early development of wind power. The two main types of vertical axis turbines are Savonius and Darrieus models. Savonius models have blades that catch the wind like scoops and are used mainly for pumping water, while Darrieus models have curved blades and are not commonly used in utility scale wind farms today.¹

Horizontal Axis Turbines

Horizontal axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, with the blades facing into the wind. The “horizontal-axis” design is the most common type used today due to its lower cost and higher efficiency than vertical axis turbines.

HAWTs have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane while large turbines generally use a wind sensor coupled with a servo motor to turn the turbine into the wind.

There are two main types of HAWT based on the orientation of the rotor blades relative to the wind – upwind and downwind models:

• Upwind turbines have the rotor facing the wind in front of the tower. The wind hits the blades before hitting the tower, allowing for higher efficiency. However, they require a yaw control mechanism to turn the rotor against the wind.
• Downwind turbines have the rotor placed behind the tower, facing away from the wind. They don’t require a yaw mechanism but tend to have lower efficiency as the tower can block some of the wind from reaching the blades.

HAWTs can have any number of blades, but two and three blade designs are most common. Two-bladed turbines are simpler and cheaper to manufacture but operate at higher RPMs, causing more noise and machine stress. Three-bladed turbines operate more smoothly and quietly but cost more to manufacture.

Vertical Axis Turbines

Vertical axis wind turbines (VAWTs) have the main rotor shaft arranged vertically. The main advantage of this arrangement is that the turbine does not need to be pointed into the wind to be effective. VAWTs can utilize winds from varying directions.

The two main types of vertical axis turbines are the Savonius turbine and the Darrieus turbine. The Savonius turbine uses scoops or cups to catch the wind. It is a drag-type turbine that is powered by the difference in wind speed exerted on each side of the scoop. Savonius turbines are relatively slow rotational speed but have high torque capabilities. They are primarily used where self-starting ability in low winds is important. Some key advantages of Savonius turbines are simple design, low cost, ability to accept wind from any direction, and easy maintenance [1].

The Darrieus turbine uses airfoils or blades in a curved rotor design to generate rotation from the lift force created. The Darrieus design leads to higher rotational speeds, making it better suited for electricity generation compared to the Savonius turbine. Downsides are that the Darrieus turbine can be challenging to start turning and often requires an additional starting system. Overall, the Darrieus design is capable of greater efficiency than Savonius turbines. Leading Darrieus style VAWTs like the Quiet Revolution QR5 and the Turby can achieve high efficiency while maintaining low noise profiles [2].

Location and Conditions

When selecting a site for a wind turbine, one of the most important factors is the wind resource. The higher and more consistent the wind speeds, the more efficient the turbine will be at generating electricity. According to the EIA, good locations for wind turbines have an annual average wind speed of at least 9 mph for small turbines and 13 mph for utility-scale turbines

Siting wind turbines offshore in coastal waters or large inland bodies of water can take advantage of higher average wind speeds than most onshore locations. However, installing turbines offshore comes with higher costs for construction and underwater cabling to deliver the electricity to land (1).

While offshore turbines can harness strong and steady winds, most wind development still takes place on land due to lower costs and easier accessibility for construction and maintenance. Within the U.S., some of the best onshore wind resources are found in the Great Plains, West Texas, and mountain passes (1). Site selection depends on local wind patterns shaped by terrain, weather patterns, and obstacles like buildings or trees.

(1) https://www.eia.gov/energyexplained/wind/where-wind-power-is-harnessed.php

Size and Height

The size and height of a wind turbine determine the amount of wind energy it can capture. Larger wind turbines are able to capture more energy for several reasons:

Firstly, larger rotor diameters mean a larger swept area. The swept area is the circular area that the turbine blades rotate through. A larger swept area means that more wind is intercepted by the blades. According to the U.S. Department of Energy, doubling the rotor diameter increases energy output by a factor of four (https://www.energy.gov/eere/articles/wind-turbines-bigger-better).

Secondly, greater hub heights expose turbines to stronger and less turbulent winds. Winds increase in speed with height above ground. Taller towers allow access to these faster winds. According to Vector Renewables, hub heights today are commonly over 100 meters (https://www.vectorenewables.com/en/blog/types-of-wind-turbines-which-one-generates-the-most-energy).

Finally, tapered and tubular steel towers enhance efficiency. Their design minimizes weight and material while maintaining the strength needed to support large rotor blades.

Blade Design

The design of wind turbine blades is crucial for efficiency. Modern wind turbines use curved blades with a cross sectional shape called an airfoil, similar to an airplane wing. As the U.S. Department of Energy explains, “Airfoils, the cross sectional shape of wind turbine blades, are the foundation of turbine blade designs. Generating lift and drag when the wind blows, they enable the rotor blades to extract energy from the wind” (Source).

The amount of lift and drag produced depends on the angle of attack as the blade rotates. To optimize efficiency, the blades are designed with a twist so the ideal angle of attack is maintained across the entire length. The tips are oriented at a lower pitch angle than the roots to account for the faster moving tip speed. According to an aerodynamics paper, “So the tips of real turbine blades generate much more lift than the roots. Some large wind turbines have blade tip speeds over 322 km/h (200 mph)” (Source).

The blades are also made of lightweight, resilient materials like composites of fiberglass, carbon fiber, or wood-epoxy to withstand the forces and fatigue over decades of use.

Maintenance

Regular maintenance is critical for maximizing the efficiency and longevity of wind turbines. Preventative maintenance helps minimize downtime and avoid more costly repairs down the line. According to the U.S. Department of Energy’s report on best practices, “Keeping the wind turbine properly maintained, through preventive and/or conditions-based maintenance, will minimize overall O&M costs, improve turbine reliability, and maximize power production over the life of the turbine” (source).

Typical maintenance activities include inspecting components and replacing worn parts like blades, bearings, and gearboxes. The frequency of maintenance varies based on the turbine model, but most require servicing at least every 1-2 years. Larger multi-megawatt turbines used in wind farms often require more specialized maintenance from qualified technicians. Smaller residential or commercial scale turbines can be serviced by the owner or a local repairperson. Developing a comprehensive maintenance checklist and schedule is important for keeping turbines running at optimal efficiency.

Most Efficient Models

In recent years, wind turbine technology has rapidly advanced, enabling the development of more powerful and efficient models. According to Wikipedia’s List of most powerful wind turbines, as of July 2023, the most powerful wind turbine currently in operation is the 16 MW Mingyang MySE 16-260. Other top models include the Vestas V162-6.0 MW and the Siemens Gamesa SG 14-222 DD offshore wind turbines, both rated at around 9-10 MW.

Some of the key innovations that have enabled larger, more efficient turbines include optimized rotor and blade designs, more reliable gearboxes and generators, advanced control systems, and taller towers to access steadier winds. Larger rotor diameters and taller tower heights allow modern turbines to capture more wind energy. Advanced materials like carbon fiber and new manufacturing techniques also enable lighter, stronger blades.

According to Electrek, in August 2023 the Vestas V236-15.0 MW prototype set a new world record for the most energy generated by a single turbine in 24 hours, producing 359,000 kWh. As researchers continue to innovate, wind turbines are likely to become even larger and more efficient in the coming years.