How Much Wind Does It Take To Turn A Wind Generator?

Wind power is one of the fastest growing renewable energy sources in the world. Wind turbines convert the kinetic energy in wind into mechanical power that can generate electricity. According to research, global installed wind power capacity reached 743 GW in 2020 and is expected to grow rapidly in the coming years.

Wind turbines work by using blades that capture the wind’s kinetic energy as they spin around a rotor. This rotational motion turns a shaft connected to a generator to produce electricity. The amount of power generated depends on the turbine’s size and the wind’s speed.

This article provides an overview of how much wind speed is required to turn a wind turbine and generate electricity. Key factors like cut-in speed, rated speed, capacity factor, and real-world examples will be covered.

Wind Speed Measurements

Wind speed can be measured using several different units, including:

• Miles per hour (mph): A common unit used in day-to-day weather reports in the United States.
• Kilometers per hour (km/h): The metric unit for measuring wind speed, commonly used internationally.
• Meters per second (m/s): The SI unit for measuring wind speed in science and engineering.
• Knots: A nautical unit often used in aviation and marine applications. 1 knot = 1 nautical mile per hour.
• Beaufort scale: An empirical 12-point scale (0-12) that relates wind speed to observed conditions on land and sea. It provides qualitative descriptions of wind effects at different speeds.

Wind turbines are designed with cut-in, rated, and cut-out speeds defined for optimal power generation. These specifications are often provided in meters per second (m/s), as it allows for precise wind speed measurements important in engineering. However, equivalent speeds in mph or km/h may be given for reference.

Converting between units is possible using simple formulas. For example, to convert 10 mph to m/s, multiply by 0.44704 (10 mph x 0.44704 = 4.470 m/s). Resources like conversion charts and calculators can also be helpful for translating between wind speed units.

Factors That Affect Wind Turbine Operation

Several key factors determine how much wind is needed for a wind turbine to operate and generate electricity. These include wind speed, air density, and blade size/design.

Wind speed is the most obvious factor. Wind turbines need a minimum wind speed, known as the cut-in speed, to start generating power. The cut-in speed is typically around 7-11 mph for most modern wind turbines. As wind speed increases above the cut-in speed, the turbine generates more power, up to its maximum rated output. Most turbines reach their rated power around 25-35 mph wind speeds.

Air density also affects how much power a wind turbine can extract from the wind. Denser air means more mass is passing through the turbine blades, allowing them to generate more power from the same wind speed. Air density depends on factors like altitude and temperature. Colder, lower altitude locations tend to have denser air.

The design of the turbine blades, including their size, shape and aerodynamic properties, determines how efficiently they can convert wind energy into rotational energy. Larger blades sweep a greater area and harness more wind energy. Advanced blade shapes and airfoil designs also maximize lift while minimizing drag.

By optimizing these factors of wind speed, air density, and blade design, modern wind turbines can effectively generate electricity at wind speeds as low as 7 mph.

Cut-in Wind Speed

The cut-in wind speed is the minimum wind speed at which a wind turbine starts generating power. Most commercial wind turbines have a cut-in speed between 3-4 meters per second (m/s), which is around 7-9 miles per hour (mph) [1]. When the wind speed is below the cut-in speed, there is not enough force for the turbine blades to overcome friction and inertia to start rotating. Therefore, no electricity is generated at very low wind speeds.

The exact cut-in speed varies across wind turbine models and depends on the design of the turbine. Larger rotor blades require less wind speed to start spinning, so modern utility-scale wind turbines often have lower cut-in speeds. Additionally, the tower height affects wind speeds – higher towers harness stronger winds, allowing turbines to generate power at lower wind speeds. Advanced control systems can allow turbines to optimize their performance right at the cut-in wind speed threshold.

Rated Wind Speed

The rated wind speed is defined as the minimum wind speed where the wind turbine reaches its maximum power output. This is the optimal wind speed where the turbine generates its rated power capacity, which is typically between 11-15 m/s for modern commercial-scale wind turbines (Simpson 2022). At the rated wind speed, the turbine operates at its rated power regardless if the wind speed increases. This is because the rotational speed and power output of the generator are limited by design.

Most wind turbine manufacturers indicate the rated wind speed in the turbine’s specifications. Finding this optimal rated wind speed is an important part of wind turbine design. The goal is to maximize energy production by reaching rated power at lower wind speeds. However, the rated wind speed cannot be too low otherwise the turbine components will experience excessive loading. Engineers must find the right balance through analysis and simulations (Anemoiservices 2021).

At wind speeds above the rated level, the turbine utilizes control systems to limit power output to the rated capacity. This prevents damage from excessive rotational speeds and forces. Common control methods include blade pitch regulation, generator torque control, and yaw adjustment. The optimal rated wind speed maximizes the number of hours the turbine can operate at its peak capacity.

Cut-out Wind Speed

The cut-out wind speed refers to the maximum wind speed at which a wind turbine will operate before shutting down. Most commercial wind turbines will automatically shut down when wind speeds exceed 25 m/s or about 55 mph (Source 1). The purpose of the cut-out speed is to protect the turbine components from damage caused by extremely high winds.

When wind speeds rise above the cut-out threshold, the turbine controller activates the yaw brake and blade pitch mechanisms to stop blade rotation and feather the rotor blades. This braking action brings the rotor to a standstill position aligned with the wind direction. The nacelle is also turned out of the wind. These actions protect the turbine from excessive structural strain and fatigue. If the turbine did not shut down at very high wind speeds, the forces could overstress the components, potentially leading to catastrophic failure.

The specific cut-out wind speed varies by turbine model and manufacturer. But 25 m/s is common for large multi-megawatt utility-scale wind turbines. Offshore turbines may have slightly higher cut-out speeds around 30 m/s due to their robust designs for marine environments (Source 2). Once wind speeds drop back below the cut-out threshold, the turbine’s control system will restart the rotor.

Power Curve

A wind turbine’s power curve shows the relationship between wind speed and the power output of the turbine. It is a graph that plots wind speed on the x-axis and power output on the y-axis. As wind speed increases, the power output also increases, until it reaches the maximum or rated power output of the turbine. After this point, the power output remains constant even as wind speed continues to rise. The turbine will shut down at very high wind speeds to prevent damage – this is known as the cut-out wind speed. Key wind speed thresholds on the power curve include:

• Cut-in wind speed – the minimum wind speed where the turbine starts to generate power.
• Rated wind speed – where the turbine reaches maximum power output.
• Cut-out wind speed – where the turbine shuts down to prevent damage at high wind speeds.

The shape of the curve depends on the turbine design, but generally rises steeply at first before leveling off. The area under the power curve determines how much total energy a turbine will produce at different sites with varying wind speeds. Understanding power curves is important for properly siting and selecting wind turbines.

Capacity Factor

The capacity factor of a wind turbine measures its actual output over a period of time compared to its theoretical maximum output if it operated at full capacity 100% of the time.[1] For example, a 5 MW wind turbine with a capacity factor of 35% will produce 1.75 MW on average over a year.

According to energynumbers.info, the capacity factor for wind turbines worldwide in 2019 was around 29-35%. This means that on average, wind turbines produce only about a third of their rated maximum power. There are several reasons for this:

• Wind speed variability – wind speeds fluctuate throughout the day and seasons.
• Maintenance downtime – turbines need to be shut down periodically for servicing and repairs.
• Unexpected failures – occasional component failures like gearbox or generator malfunctions.

While the theoretical maximum output is used for rating purposes, real-world wind conditions mean turbines operate well below capacity most of the time. Understanding capacity factor helps properly size and site turbines to maximize productivity.

Real-World Examples

In the real world, wind speeds and power output can vary greatly depending on location and turbine size. According to the Wind Energy Development Programmatic EIS, the average capacity factor of wind projects in the United States ranges from 25% to 40%.¹ This means the turbines do not produce their maximum rated power output most of the time.

For example, the Roscoe Wind Farm in Texas has 627 wind turbines with a total capacity of 781.5 MW. But its average annual net generation is around 2.2 million MWh, equal to a capacity factor of 32%.² At times of high wind speeds, the farm can reach its full capacity. But at low wind speeds, the output drops significantly.

Likewise, offshore wind farms experience a range of wind speeds. According to the Block Island Wind Farm off Rhode Island, its 5 turbines with 30 MW capacity had a net capacity factor of around 37% in 2019.³ This highlights how offshore wind speeds vary throughout the year, resulting in less than full power output overall.

So while modern utility-scale wind turbines are capable of generating maximum power at high wind speeds, real-world wind variability means the average output is less than the rated capacity.

Conclusion

Wind speed and turbulence are critical factors in determining how much electricity a wind turbine can produce. The cut-in, rated, and cut-out wind speeds dictate when a wind turbine starts generating power, reaches maximum output, and shuts down to prevent damage. Understanding these wind speed thresholds and the power curve of a turbine provides insight into its real-world energy production and capacity factor.

While the optimal wind speed range varies across turbine models, there are general principles that apply. Lower wind speeds produce less power, while higher speeds allow the turbine to generate more electricity until its maximum output is reached. However, extremely high winds carry risks and force turbines to shut down. By considering these wind patterns and turbine capabilities, we can strategically site and operate wind farms to maximize clean energy production.

Overall, knowledge of wind characteristics and their interaction with turbine technology is key to evaluating the performance and potential of wind energy projects. As the world continues transitioning to renewable power, understanding these factors will assist in the smart integration of wind energy.