How to Calculate Wind Turbine Power Output?

How to calculate wind turbine power output? It’s a simple calculation that’ll highlight the great potential of these white-spinning machines.

Before we dive into the math, let’s get a quick grasp of how these wind turbines work.

How do wind turbines work?

When the wind whooshes past a wind turbine, the blades go for a spin. These blades capture the wind’s kinetic energy, transforming it into mechanical or rotational kinetic energy.

Now, inside the wind turbine, the rotating blades turn a shaft connected to a gearbox. This action spins the generator’s rotor, which ultimately generates electricity.

To help you visualize, here’s a schematic showing the power conversion process step by step:

And, here’s what goes on inside a wind turbine:

Important Note: The power generated by wind turbines depends on several factors:  

• Mass of air (density)
• Speed of air (velocity)
• Amount of air (volume)
• The swept area of the wind turbine blades
• Blade design

Wind turbine power output calculation equations and variables

Here are the variables you need to know:

m: mass (kg)
v: wind speed (meters/second)
A: rotor swept area ()
r: radius (meters)
KE: kinetic energy
P: power
: density ()
: mass flow rate (kg/second)

With our variables defined, let’s move on to the equations.

Kinetic energy is defined as

Power, on the other hand, is kinetic energy per unit of time:

Fluid mechanics gives us the mass flow rate:

And the change in mass over a unit of time is illustrated in the cylinder schematic below, where the mass flux is the wind flowing through the turbine blades.

By plugging our mass flow rate equation into our power equation, we get

Finally, let’s talk about the Betz limit variable.

Betz Limit in wind power extraction

Way back in 1919, a German physicist named Albert Betz discovered that wind turbines couldn’t convert more than 59.3% of the wind’s kinetic energy into mechanical energy, no matter how perfectly designed. This limitation arises from generator inefficiencies, drive train friction, blade design, and wind slowing down the blades as the turbine extracts energy. As a result, the wind leaving the turbine flows slower than the wind entering it.

For a wind turbine to work, some wind must flow out from the back. If the turbine captures 100% of the wind power, the blades won’t spin because there’s no wind left to capture energy from.

Imagine the wind blockage at the turbine like a traffic jam on the highway. As cars slow down in front of you, eventually, you’ll have to slow down too, no matter how far back you are from the jam.

That’s why there’s a limit on the amount of air that can pass through a wind turbine for max power output. This limit is known as the Betz Limit.

Power Coefficient, Cp

Now that we’ve got a grip on the Betz limit, let’s check out the Power Coefficient (Cp). This nifty little number represents the ratio of power extracted by the wind turbine to the total available power in the wind source.

, where .

Remember, the Betz Limit is the highest possible value of , which is 16/27 or 0.59.

Now, we can update our power generation equation to:

Important Note: Wind turbines can’t operate at this maximum , as design requirements for reliability and durability reduce it. Plus, they’d need absolutely perfect wind conditions to max out their power output. In reality, the value usually falls between 0.25 and 0.45.

Let’s use the Vestas V164-8.0 MW as an example. This offshore wind turbine is one of the world’s largest! While I’ve never stood beside these giants, I’ve done a bunch of design work for the V90-3.0 MW. Just check out this photo I took of a V90 unit at a project site – look how tiny that guy looks next to the turbine base! Keep in mind, the V164 rotor diameter is 164 meters, while the V90’s is 90 meters.

We’ll grab our calculation data from the Journal of Physics table below, which focuses on the LEANWIND 8 MW (similar to the V164 unit):

From the table, we’ll use a wind speed of 14 meters/second for max power output. Here’s our input data:

• V164 blade length: 80 meters
• Wind speed: 14 meters/second
• Air density: 1.23
• Power coefficient: 0.23

First up, let’s calculate the swept area of the turbine blades.

With the V164 blade length as the radius variable in our equation:

Now, let’s crunch the numbers to find the power generated by the wind turning those massive turbine blades.

The rated capacity, or max power output, for the V164 is 8 MW – that’s the amount of power the turbine can produce when the wind’s blowing just right.

Important Note: Our calculation is just an estimate of the power output. For an even more accurate number, we’d need to factor in variables like:

• Wind direction
• Water vapor intrusion
• Thermal expansion
• Equipment corrosion, and wear and tear

Why is it so important to understand how to calculate wind turbine power output?

Well, it’s all about mastering the complex connections between the numerous power generation variables. Once you’ve got a grip on that, you can predict wind turbine power output more accurately.

This knowledge, in turn, allows for a smoother integration of these majestic machines into power grids. After all, power grids crave dependable energy sources, and let’s be honest, wind patterns are as fickle as they come.

And here’s the kicker – if you’re in the energy biz, you’re actually selling energy before it’s even generated! So, the better we can calculate wind turbine power output, the more successful we’ll be.

“How to calculate wind turbine power output?” wrap up

Those colossal white giants might look all innocent and straightforward as they tower in the distance. But, trust me, their design and integration into power grids are anything but child’s play. That’s why it’s crucial to learn the art of calculating wind turbine power output. Once you’ve got that down, you’ll be well on your way to truly understanding and appreciating these engineering wonders.

What do you think about wind turbine power output? Are you a believer in the ever-growing demand for wind power?

Engineer Calcs

Koosha started Engineer Calcs in 2020 to help people better understand the engineering and construction industry, and to discuss various science and engineering-related topics to make people think. He has been working in the engineering and tech industry in California for over 15 years now and is a licensed professional electrical engineer, and also has various entrepreneurial pursuits.

Koosha has an extensive background in the design and specification of electrical systems with areas of expertise including power generation, transmission, distribution, instrumentation and controls, and water distribution and pumping as well as alternative energy (wind, solar, geothermal, and storage).

Koosha is most interested in engineering innovations, the cosmos, our history and future, sports, and fitness.