Wind Energy Basics

Wind is the largest source of renewable energy in the United States, providing clean electricity from land and offshore to individual homes, remote farms, small communities and large cities alike.

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Wind energy is old—so old that ancient Egyptians used this bountiful, blustery resource, according to the U.S. Energy Information Administration, to propel their boats down the Nile River. The first wind turbines (or windmills, as they were originally called) were made from abundant materials, such as wood or reeds, which were woven into tight blades and spun to pump water for farms, grind grain, and, eventually, power entire communities.

Today's wind turbines use sleek, modern materials to generate clean, renewable energy almost anywhere in the world.

What Is Wind Energy?

To answer this question, it's best to start with another: What is wind?

Wind is born when pockets of the Earth's craggy surface get different amounts of sun and cool or heat faster than others nearby. To balance those differences, like mixing hot and cold water in a bathtub, air moves around the world—gaining or losing speed as it dips through valleys and sprints over rivers. That creates—you guessed it—wind.

Wind can be powerful enough to whisk birds through the sky, move sailboats across the ocean, and even rip trees from the ground. In comparison to all that, pushing wind turbine blades is easy! It's that movement of the turbines that creates electricity.

Want to know how much wind energy is humming across your state? Check out NREL's wind maps on WINDExchange.

How Do Wind Turbines Work?

Wind turbines, like windmills, catch the wind's energy with propeller-like blades. These blades can have a horizontal axis, like a fan, or vertical one, like a merry-go-round. The most common design is a tall tower with three large blades on a horizontal axis. But some vertical-axis wind turbines look like eggbeaters, while others look like the windmills that populated farms a century ago.

Unlike fans, which use electricity to move air, wind turbines use moving air to generate electricity. When the wind blows, its force turns the blades, which runs a generator and creates clean electricity. But some turbine designs can produce more clean energy than others. For example, because winds can be more powerful and less volatile higher in the atmosphere, placing turbines on towers 100 feet (or 30 meters) tall—about the height of the Statue of Liberty—can help them generate more electricity. Wind turbine operators can also shift their machines to face directly into the wind—a technique called yawing.

Can't picture this? Watch the U.S. Department of Energy's wind energy animation to see wind turbines in action.

How Do We Get Our Wind Energy?

There are several ways to get power from wind energy. Wind turbines can be built on land, on lakes or in the ocean, in remote wilderness far from the power grid, within cities, or across vast plains.

One wind turbine can power an individual home or farm, but several built close together form a wind energy plant, or wind farm. Wind plants can be land-based or offshore, and they can be hybrid plants (meaning, they include other sources of energy, such as solar energy). Wind energy researchers are trying to learn how many wind turbines built in which arrangements can maximize energy production in wind plants.

Today, most grid-connected wind plants are at least 1 megawatt or larger. The biggest wind farm in the United States spans 100,000 acres (enough to cover half of New York City) and can power more than 250,000 homes.

What Are the Major Applications of Wind Energy?

Wind energy has three major applications: land-based, distributed, and offshore.

Land-Based Wind Energy Is the Most Common

The majority of turbines are installed on land. And land-based wind energy is one of the lowest-cost sources of electricity generation, as highlighted by the U.S. Department of Energy.

Researchers at NREL are categorizing wind resources on land and advancing wind turbines to more efficiently generate electricity at even lower cost.

Distributed Wind Energy Powers Remote and Local Communities

Distributed wind energy is a distributed energy resource, meaning it produces a smaller-scale unit of power. In this case, it comprises one or more wind turbines, which range from a kilowatt to several megawatts in capacity.

These typically land-based turbines operate locally to provide energy for individual buildings or small communities, though they can be connected to a power grid at the distribution level. Some wind turbines can even pop up as mobile, on-demand sources of clean power in disaster or defense scenarios.

Homeowners, farmers, businesses, and industries make use of clean, distributed wind energy to pump water (to use as drinking water, to irrigate farms, and more), to lower electric bills, and to reduce air pollution.

Distributed wind turbines are improving all the time, growing stronger, less costly, more resilient, and more efficient.

Offshore Wind Energy Is Newer but Growing

Wind turbines used in offshore wind energy can be even larger than on land, with towers one-and-a-half-times the height of the Washington Monument and blades as long as a football field (as noted in to the U.S. Department of Energy's list of 10 things you may not have known about wind energy).

These behemoths depend on strong sea breezes to spin turbines that are either anchored to the seafloor (called fixed-bottom wind turbines) or installed on platforms that float (called floating wind turbines). Offshore wind turbines can provide electricity to power ocean-based research and equipment, such as unmanned robots used for marine exploration; remote or island communities disconnected from the grid; or entire cities back on land.

The offshore wind energy industry is growing. Researchers are identifying massive amounts of potential wind energy off U.S. coastlines, and the Biden administration has set a goal to install 30 gigawatts of wind energy by . That will require us to grow the offshore wind energy workforce as well as the supply chain to build and infrastructure to deploy new offshore wind farms.

The U.S. Department of Energy indicates that, as of , nearly 80% of the nation's electricity is used to power our coastal and Great Lakes states, where most Americans live. So, even though offshore wind energy is a relatively new industry in the United States, it could soon provide clean, renewable electricity to many U.S. communities. America's first offshore wind farm, located off the coast of Rhode Island, powered up in December . Today, many more projects are in development along the U.S. East Coast that could send power back to the grid. And, with technology advancements for floating offshore wind energy, wind farms are coming to the West Coast as well.

Why Do We Need Wind Energy?

Wind energy is one of the largest sources of clean, renewable energy in the United States, making it essential to a future carbon-free energy sector. Wind turbines do not release emissions that pollute our air or water, and they can be built with minimal impact to the environment or livelihoods of nearby residents. Farmers and ranchers, for example, can lease their land to wind farms and, because the turbines take up minimal space, continue to grow crops or raise livestock while earning a steady income.

But wind energy can be even better.

The wind energy industry is working to figure out what areas of research need more attention to expand the use of wind energy. This includes understanding how wind interacts with a turbine behind (downwind from) another one, evaluating the best ways to verify new technologies, and incorporating feedback from communities living near or using wind turbines.

Researchers are studying different materials and designs that could make wind turbine blades lighter, longer, more durable, and better at creating energy. New technologies could also make wind turbines less expensive to manufacture, install, operate, and maintain, making wind energy more accessible to more people. As a bonus, some new materials and processes could make wind turbines more reusable or recyclable, which can cut down on waste, too.

Scientists are also studying ways to limit the impact wind turbines can have on wildlife. For example, sound and light could warn birds and bats to fly around wind turbines. And even though wind turbines must be installed far enough from homes that they produce noise no louder than a refrigerator's hum, researchers have discovered ways to further reduce their noise levels. Because wind turbines are a significant source of clean energy, they lower pollution to help keep the Earth (and, therefore, its birds, bats, and humans) healthy.

Additional Resources

For more information, visit NREL's Wind Energy Research site or the following resources:

Wind Energy Basics
U.S. Department of Energy's Office of Energy Efficiency and Renewable Energy

Small Wind Electric Systems
U.S. Department of Energy's Energy Savers Program

Wind Energy Educational Resources
U.S. Department of Energy's WINDExchange Initiative

If you want to learn more, please visit our website Vertical Axis Wind Turbine.

Recently published research by Oxford Brookes University suggests that H-shape Darrieus vertical-axis turbines (VAWTs) installed a certain way could outperform “traditional propeller-type” wind turbines or HAWTs in wind farms. 

When set closely interspaced in pairs, VAWTs increase each other’s performance by up to 15%, the UK-based institute said in its press release, triggering much media attention. Business publication Forbes wrote in May: “A recent discovery by engineers of Oxford Brookes University’s School of Engineering, Computing, and Mathematics could change the design of offshore wind farms forever.” 

But how realistic are predictions gained from two-dimensional modelling of theoretical VAWTs with 20-metre rotors compared with current “traditional” offshore giants? And who will develop and build them?

Computer simulation

The study, “Numerical modelling and optimisation of vertical-axis wind-turbine pairs: A scale-up approach” was originally published in Elsevier’s Renewable Energy journal in March. It is reported to be the first to comprehensively analyse many aspects of wind-turbine performance, with regards to array angle, direction of rotation, turbine spacing and number of rotors. 

The university’s research team, led by Iakovos Tzanakis, a professor in technology, design and environment, used extensive computer simulation for the in-depth study “This study evidences that the future of wind farms should be vertical,” Tzanakis said. “Vertical-axis wind farm turbines can be designed to be much closer together, increasing their efficiency and ultimately lowering the prices of electricity. In the long run, VAWTs can help accelerate the green transition of our energy systems, so that more clean and sustainable energy comes from renewable sources.”

The researchers argue that VAWT’s in wind-farm array do not suffer from HAWT-related turbulent wake issues created by the first row, which decrease the output of the rows of turbines behind by up to 40%. Using vertical- rather than horizontal-axis machines would not only eliminate this problem, they suggest, but the VAWTs would actually enhance each other’s performance. 

Two similar-size rotors were used for the research, with the second rotor placed the length of three rotor diameters downstream. The maximum increase in power output — compared with two units each operating in isolation — was achieved with the second rotor placed at a 60-degree array angle (ß) to the prevailing wind direction. 

Another variable the team introduced was letting the two rotors either co-rotate in the same direction or counter-rotate, whereby the counter-rotating pair performed better only at smaller array-angles of around -30 to +30 degrees. Minimum power output is logically achieved when the wind blows directly over the two rotors positioned in line relative to the prevailing wind direction (0 degrees). 

Finally, a two-dimensional space was an integral part of the computational fluid dynamics (CFD) modelling properties.

Performance augmentation

Lead author Joachim Toftegaard Hansen explained why the team chose an H-type Darrieus rotor shape with a 20-metre rotor diameter for the analysis: “Other studies have verified similar performance augmentations for other Reynolds numbers [to predict flow patterns] too. This is something we are possibly interested in investigating in our future studies, analysing the effects of turbine size.”

A key question is whether such envisaged VAWT-based offshore wind farms could indeed propel them at par with the latest HAWT-technology — which is now at the 11-15MW scale and has 30 years of cumulative marine experience under its belt.

Aerodynamics expert Jens Nørkær Sørensen is a professor in the wind-energy department at the Technical University of Denmark (DTU) who worked with VAWTs in the past. He questions the scientific design basis of the Oxford Brookes research effort, the design set-up and the outcomes on which the conclusions are based.

“My objections concentrate on three main points. The first is about the use of a two-dimensional shape for the CFD flow model. Wind flow in a wind-farm array should, in my view, always be studied from a three-dimensional perspective because the mixing of fresh high-energy wind from the upper, outer layers with ‘depleted-energy’ wind flow inside the wind-farm boundaries is of key importance to continuously regenerate energy for generation in downstream turbine rows.” 

This essential mixing of wind flow inside wind farms with HAWTs takes place automatically, because the spinning blades “sweep” fresh air from upper wind layers into the array during each individual rotor revolution. Tip heights of the latest and largest offshore turbines now reach up to 250 metres, and will soon hit 300 metres and more. At these altitudes wind speeds and thus energy in the wind are high. 

No wind mixing

In contrast, with VAWTs the rotor rotating-axis is vertical, so the rotor thus always spins in the same plane, Sørensen explains. As a result, there is no mixing of fresh and already “depleted-energy” wind flow, and this “used” air remains inside the wind-farm array, with a substantial negative impact on potential power production in all further rows downstream.

His second criticism is linked to the research set-up involving only two rotors. These are, according to the report, “positioned in a rotating region within a larger rectangular domain with stationary walls top and bottom, a velocity inlet and a zero-gauge pressure outlet.” This selected set-up with two turbines, plus the flow boundary conditions is depicted in the chart below from the report. 

Fig. 3. Boundary conditions. R1 was always rotating in a counter-clockwise direction, but the direction of rotation of R2 varied depending on whether the pair was co- or counter-rotating. The “dist” was the turbine spacing, and β was the array angle (Source: Oxford Brookes University)

This overall research concept contains several other fundamental design flaws, Sørensen believes. 

“My third point is that it ignores real-life conditions inside a wind farm, whereby such ideal uniform wind flow conditions with unilateral wind direction do not exist. An inter-related research-design error is that it does not consider the complex, continuously varying wind-flow interaction effects with other turbine rows downstream,” he says. 

“Taking all these flaws and errors into account, the claimed benefit of up to 15% power augmentation for closely interspaced VAWTs in wind-farm arrays looks highly unlikely,” Sørensen says. 

“Finally, HAWTs can be located much closer to each other in wind farms because of their superior aerodynamic efficiency, therefore they offer a significantly higher yield per area (see Vertical- versus horizontal-axis turbines). Producing electricity at the lowest lifecycle-based LCoE remains key”, he concludes.

Huge instant leap

The pathway to commercialisation of VAWT technology up to state-of-the-art HAWT levels was not covered in the study but will be far from easy with many hurdles (see and uncertainties.

Two (UK) companies, Vertax Wind and Wind Power’s Aerogenerator X (below), introduced offshore-dedicated 10MW VAWT-concepts in -, but neither reached the prototype stage. Both designs come with very high specific power ratings: 649W/m2 for the Vertax 10MW design (140-metre rotor diameter and 110-metre blade length), and roughly 1,050W/m2 for the Aerogenerator X.

In comparison, the HAWT-type Siemens Gamesa SG 11-200 DD comes in at 350W/m2, with the SG 222 DD (in 14MW mode) at 362W/m2.

To develop vertical-axis turbines into full-scale competitive commercial concepts matching the latest and largest 12-15MW HAWT flagship category, technology companies are basically starting with a track record gained largely with kilowatt-class turbines and generally only limited onshore experiences. 

Such comprehensive product development processes will require a huge instant wind technology scaling leap and a massive industrialisation effort. It also requires large, strong parties with long-term commitment and deep pockets to take it to completion and then fast-track it to commercial ramp-up, leading to bankability and beyond.

Vertical- versus horizontal-axis turbines

A major benefit of vertical-axis wind turbines (VAWTs) compared with their (upwind) horizontal counterparts (HAWTs) is that they can draw wind from all directions while not needing a yaw system. 

A main disadvantage compared with modern, large-scale HAWTs is much lower aerodynamic efficiency. The Oxford Brookes University report states a maximum pressure coefficient (Cp) in the 35-40% range, compared with nearly 50% for HAWT, both for turbines operating in isolation. Vertax (pictured below) states a Cp of 38-39% design objective. 

A major consequence is that the specific power of VAWTs must be substantially lower to achieve similar energy production to compensate for their reduced aerodynamic efficiency. This means there is an inherent need to fit a bigger rotor per megawatt within the same wind classes they are designed for and the required annual full load hours.

Some VAWT designs lack self-starting capability as well, requiring an external power source (with associated energy loss) during each starting-up action, either a dual-mode generator-motor electric machine or two separate devices. 

Historic non-self-starting small-scale VAWT rotors were sometimes fitted with a small Savonius drag-type rotor inside, but this would always be at the expense of total system aerodynamic efficiency.

Another VAWT-challenge is power output control:

Want more information on horizontal axis wind turbine? Feel free to contact us.

• For stall-type variable-speed designs with a fixed blade angle, an output control option is rotor-speed control whereby a dual-mode generator-motor maximises and retains the rated output level. The main disadvantage — and safety concern in some experts’ views — is the lack of an aerodynamic braking system found in modern pitch-controlled HAWTs (three independent fail-safe aerodynamic brakes). An interlinked challenge is that continuous switching of the load direction during turbine operation in the rated power range could accelerate materials fatigue and trigger premature failures.
• With pitchable blades as an alternative solution, each blade is attached to support arms and bearings so that it can rotate. This requires at least one or more bearing supports and pitch systems for long blades, adding complexity, Capex and, likely Opex as well, compared with HAWTs. This is because the bearings and pitch systems are located in difficult-to-access exposed locations. They could be distanced around 110 metres from the rotor centre for a fictive 11MW H-type Darrieus turbine with a 220-metre diameter, 175-metre long blades and a 286W/m2 power rating.