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Wind Turbines - vawt & hawt

   

Wind Turbines

Wind Turbines

- convert the wind's kinetic energy into electricity power through a mechanical means. Wind technology is adaptable and versatile and can be one of the most cost-competitive renewable energy technologies when produced on a large scale (wind farm). Smaller systems work very well when combined with a solar array, as cloudy days are often windy and a hot sunny day may not contain a steady breeze.

The video to your below was taken Sept 16th, 2008 at Ravenswood, Ontario, Canada. These 6 wind turbines produce 10 MW of power that feeds the nearest town, Grand Bend.

 

How much power can we get from the wind?

Warning: a little bit of math follows - not to hard, bear with us. We will give examples for 2 small turbines, one with 2 foot blades and one with 4 foot blades.

Power AVAILABLE in the wind = .5 x air density x swept area x (wind velocity cubed)

Air density = 1.23 kg per cubic meter at sea level. Swept area = pi x r squared. Our 2 foot blades = 0.609m, 4 ft = 1.219m. 10 mph = 4.4704 m/s, 20 mph = 8.9408 m/s.

How much power is in the wind: 2 ft blade, 10 mph winds = .5 x 1.23 x 3.14x0.609squared x4.4704 cubed

= .5 x 1.23 x 1.159 x 89.338 = 63.7 watts

With 4 foot blades and 10 mph winds = .5 x 1.23 x 4.666 x 89.338 = 256 watts

With 4 foot blades and 20 mph winds = .5 x 1.23 x 4.666 x 714.708 = 2051 watts

That's the MAXIMUM power in the wind. However, it's impossible to harvest ALL the power. The Betz Limit tells us that the maximum percentage of power we can harvest from the wind is 59.26%.

Thus our maximum power from these turbines would be:

2 ft blades, 10 mph wind = 37.7 watts

4 ft blades, 10 mph wind = 152 watts

4 ft blades, 20 mph wind = 1,215 watts

A very well built DIY horizontal axis wind turbine would not likely get more than 65% of the above numbers. Keep these numbers in mind when a sales person starts to tell you their small wind turbine will meet all your needs....

 

How Does a Wind Turbine Work?

Wind turbines operate on a simple principle. The energy in the wind turns two or three propeller-like blades around a rotor. The rotor is connected to the main shaft, which spins a generator to create electricity.

Wind turbines are mounted on a tower to capture the most energy. At 100 feet (30 meters) or more above ground, they can take advantage of faster and less turbulent wind.

Wind turbines can be used to produce electricity for a single home or building, or they can be connected to an electricity grid (shown here) for more widespread electricity distribution.

Types of Turbines

Modern wind turbines fall into two basic groups: the horizontal-axis variety, as shown in the photo, and the vertical-axis design, like the eggbeater-style Darrieus model, named after its French inventor.

Horizontal Axis Wind Turbines - HAWT

- typically either have two or three blades. These three-bladed wind turbines are operated "upwind", with the blades facing into the wind.

Utility-scale turbines range in size from 100 kilowatts to as large as several megawatts. Larger turbines are grouped together into wind farms, which provide bulk power to the electrical grid.

Single small turbines, from 400 watts to 100 kilowatts, are used for homes, farms, telecommunications dishes, or water pumping. Small turbines are sometimes used in connection with diesel generators, batteries, and photovoltaic systems. These systems are called hybrid wind systems and are typically used in remote, off-grid locations, where a connection to the utility grid is not available. Here are some do-it-yourself plans for a small HAWT.

Myth Buster

Myth: Horizontal axis wind turbines are extremely dangerous to birds. Fact: Domestic cats and flying into windows, kills hundreds of millions of birds each year. Wind turbines affect a small fraction of that number.

Vertical Axis Wind Turbines - VAWT

Anemometer - a Drag stype vertical axis wind turbine VAWT

VAWTs come in two flavors: lift and drag based designs. The three-cup anemometers commonly used for measuring wind speed are drag-based vertical axis wind turbines (VAWT). If the velocity of the cups is exactly the same as the wind speed, we can say that the instrument is operating with a tip speed ratio (TSR) of 1. The ends of the cups can never go faster than the wind, so the TSR is always 1, or less.

The Savonius: A Useful, Drag-Type VAWT

Savonius vertical axis wind turbine VAWT

They can be made many different ways with buckets, paddles, sails, and oil drums. The Savonius rotor is S-shaped (when viewed from above). All of these designs turn relatively slowly, but yield a high torque. They can be useful for grinding grain, pumping water, and many other tasks; but are not good for generating large amounts of electricity. RPMs above 1000 are generally best for producing electricity; however, drag-based VAWTs usually turn below 100 RPM. One might use a gearbox, but then efficiency suffers and the machine may not start at all easily.

Darrieus Lift-Type Vertical-Axis Machines

The lift based vertical axis wind turbine shaped like an "eggbeater" is theDarrieus style vertical axis wind turbine VAWTDarrieus VAWT. The long VAWT blades have many natural frequencies of vibration which must be avoided during operation. A well-designed HAWT has none of these problems.

VAWTs are very difficult to mount high on a tower to capture thehigher level winds. Because of this, they are usually forced to accept the lower, more turbulent winds and produce less in possibly more damaging winds.

Guy cables are usually used to keep the turbine erect. They also impose a large thrust loading on the main turbine bearings and bearing selection is critical. Like all types of turbines, replacing main bearings requires that the turbine be taken down.

VAWT at Corpus Christi U

Darrieus' 1927 patent also covered machines with straight vertical axis blades called Giromills.

 

VAWTs have not performed well in the commercial wind turbine market though a number of smaller, household sized units have recently hit the market. In September 2012, Corpus Christi Univ installed a number of 20kw and 4kw VAWT. Info on this installation can be found here: Corpus Christi

 

We have a couple of plans for the do-it-yourself builder on this site:

Horizontal Axis Wind Turbine Plans

Vertical Axis Wind Turbine Videos

 

 

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