Wind Energy

Wind energy uses the wind to turn turbines that produce electricity through motion. It relies on consistent wind at higher altitudes and is typically employed through commercial wind farms, although there are some residential applications.

Key Terms

Blades: Lifts and rotates when wind is blown over them, causing the rotor to spin.


Rotor: The blades and hub of the turbine together form the rotor.


Low-speed Shaft: Turns the rod at about 30-60 rotations per minute (that is not very fast).

Gear Box: Connects the low-speed shaft to the high-speed shaft and increases the rotational speeds from about 30-60 rotations per minute (rpm) to about 1,000-1,800 rpm; this is the rotational speed required by most generators to produce electricity. The gearbox is a costly (and heavy) part of the wind turbine and engineers are exploring "direct-drive" generators that operate at lower rotational speeds and don't need gearboxes.

High-speed Shaft: Drives the generator.

Generator: Produces a 60-cycle alternating current electricity (unlike solar panels). This generator is usually not available to the general public.

Anemometer: Measure the wind speed and transmits data to the controller.

Controller: Starts up the machine at wind speeds of about 8 to 16 miles per hour (mph) and shuts off the machine at about 55 mph. Turbines do not operate at wind speeds above about 55 mph because they may be damaged by the high winds. It is also used to help prevent wildlife from being harmed.

Pitch System: Turns (or pitches) blades out of the wind to control the rotor speed, and to keep the rotor from turning in winds that are too high or too low to produce electricity.

Brake: Stops the rotor mechanically, electrically, or hydraulically, in emergencies.

Wind Vane: Measures wind direction and communicates with the yaw drive to orient the turbine properly with respect to the wind.

Yaw Drive: Orients upwind turbines to keep them facing the wind when the direction changes. Downwind turbines don't require a yaw drive because the wind manually blows the rotor away from it.


Yaw Motor: Powers the yaw drive.

Tower: Made from tubular steel, concrete, or steel lattice. Supports the structure of the turbine. Because wind speed increases with height in the air, taller towers enable turbines to capture more energy and generate more electricity. This is the rod you see that supports the turbine.


Nacelle: Sits atop the tower and contains the gearbox, low- and high-speed shafts, generator, controller, and brake. Some nacelles are large enough for a helicopter to land on.

How it Works

These many parts come together to allow the wind turbine to generate electricity. Traditionally the blades are angled slightly to catch the wind so that they can spin with the wind.

How it's Renewable Energy

The wind is actually created by the sun, through the uneven heating of the earth's surface. Therefore, just like solar energy, wind turbines will always be able to produce electricity. Furthermore, the spinning of the turbine does not produce greenhouse gas emissions.

Different Types of Wind Turbines

There are two major types of wind turbines. The first type is a horizontal axis wind turbine (HAWT), which is a normal wind turbine (as described above). However, there is a second type of wind turbine: a vertical axis wind turbine (VAWT). In VAWT, the axis sticks straight up in the sky. 


Within VAWTs, there are two main groups. There is the Savonius VAWT and the Darrieus VAWT. The Savonius VAWT has two to four "scoop" blades that use drag to take energy from the wind. The blades are oriented so that there is one cup facing the wind at all times. The back side is closed off and rounded so that there is minimal drag on them. As a result, most of the wind force is exerted on the open cup, pushing it back, which turns the axis. There is sometimes a gearbox, but sometimes not, meaning that sometimes the blades/cups are mounted on to a shaft, which goes to a gearbox, which goes to another shaft that goes to the generator, but other times they are mounted on is the same axis which is connected to the generator shaft. Furthermore, as previously noted, Savonius VAWTs operate mainly on drag. Drag occurs when there is a difference in velocity between the solid object and the fluid. In this case, that would be the rotor and air. The turbine spins because the drag of the open (concave) face of the cylinder is greater than the closed (convex) face. Consequently, because Savonius VAWTs are drag-based, it is impossible for them to move any faster than the wind, resulting in a very low efficiency; the maximum efficiency is 15%, compared to the HAWT’s 59%.


Darrieus VAWTs use aerofoils to operate a lift-based rotation system. Because Darrieus VAWTs use lift, they are able to rotate faster than the speed of the wind, unlike Savonius VAWTs. This makes Darrieus VAWTs more comparable to the horizontal axis (traditional) wind turbines, at 30 to 40% efficiency ratings.


When comparing VAWTs to HAWTs, a few factors must be considered. Firstly, the higher efficiency of HAWTs (59%) makes it obviously higher performing when the wind faces the turbine head-on. However, their efficiency, and therefore output, decreases as the wind changes angle and hits the turbine at a non-optimal angle. Comparatively, a VAWT does not change its efficiency or output as the wind angle changes because of its layout. Kids Fight Climate Change Executive Director Ajani Stella conducted a study comparing the difference in power output between Savonius VAWTs and HAWTs and found that Savonius VAWTs outperform HAWTs when the wind conditions are variable, but when the wind is steady, a HAWT is significantly better than a Savonius VAWT (to see the full results and the experiment, click here). However, Darrieus turbines use lift force instead of drag, so as they are further developed, they could rival HAWTs even at head-on wind angles.

Advantages and Disadvantages

Advantages

Due to recent government subsidies, wind energy is the cheapest form of electricity in the United States at 1-2¢ per kilowatt-hour. And wind turbine prices are fixed over twenty years, meaning that developers can build wind turbines with price certainty, helping the wind industry grow. Furthermore, just like the solar PV industry, the wind industry creates a lot of jobs. The wind industry currently employs over 100,000 Americans and is one of the fastest-growing job markets. By 2050, estimates show that the wind industry can create up to 600,000 jobs. In addition, developers can build wind turbines on existing land that has fallen into disuse to mitigate land-use concerns.


Negative Impacts

Many environmental concerns of wind turbines have already been mitigated, such as the impact on birds or bats. Birds or bats have been killed frequently by turbine blades. However, recent engineers have mostly fixed this problem by properly siting wind farms to avoid birds and bats. Besides, ongoing research is working to improve wind turbines so that they spin slower with just as much efficiency. The other main environmental concern is land use, just as with CSP and solar PV. However, the National Renewable Energy Laboratory concludes that because wind turbines must be spaced out, they occupy only 1% of the land they sit on. Thus, the rest of the land can be occupied by farms, among other uses. Besides, wind farms can sit on unused land, as aforementioned, thereby negating the concern. In addition, contrary to former President Trump's claims, wind turbines do not cause cancer.

Conclusion: Our Take

Just like solar PV, wind energy will play an important role in the emerging clean economy. Due to its low prices and wide economic opportunities, wind energy will grow rapidly. Meanwhile, the negative environmental concerns are actively being mitigated by engineers, leading to wind energy becoming one of the safest renewable energy types.

Sources

Image: Wind Turbines against a Blue Sky. September 27, 2014. Wonderful Engineering. https://wonderfulengineering.com/wp-content/uploads/2014/09/wind-turbine-15.jpg.


“How Does a Wind Turbine Work?” U.S. Department of Energy. Accessed November 2019. https://www.energy.gov/maps/how-does-wind-turbine-work.


Zemamou, M., M. Aggour, and A. Toumi. “Review of Savonius Wind Turbine Design and Performance.” Energy Procedia 141 (2017): 383–88. https://doi.org/10.1016/j.egypro.2017.11.047.

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