Vertical Axis Wind Generators

Anytime you see a wind generator that has a vertical axis, such as those in the collage below, you should be thinking scam. There are a few legitimate companies that create vertical axis generators just because they want to, and coupled with a high efficiency alternator might have acceptable economics - however most are scams. Vertical axis machines are extremely inefficient. When the wind is blowing, half of a vertical axis machine will always be rotating towards the wind, generating nothing but drag. Water wheels have the same problem and used to be common, but that was before the engineering and physics were understood. Some of these turbines claim to use magnetic levitation to improve efficiency, but this too is a hoax.

A legitimate design would be similar to the design shown below, a Jacobs in this case. The blades rotate to face the wind, and the propeller blades have an airfoil shape, which generates lift. The entire assembly faces into the wind.

Horizontal axis wind turbines like the one pictured above will ALWAYS be more efficient that vertical axis turbines by a wide margin. Every wonder why you have never seen an airplane driven by an eggbeater instead of propellers?

Here is one called "THE MAGLEV: The Super-powered Magnetic Wind Turbine", supposedly from China that really takes the cake. Magnetic levitation (or Magelv) means that they have a pair of magnets at the base that act as a low friction bearing. In any case, even if they could get the MagLev working and it functioned better than a thrust bearing, an inefficient design enlarged to gargantuan proportions is sheer lunacy -- design to attract foolish investors perhaps. They claim that the "Zhongke Hengyuan Energy Technology has invested 400 million yuan ($5.8 million) in building this facility" which if true, is pathetic.

Rooftop Wind Turbines

Anyone that suggests that you can mount it on your roof is either naive or scamming you. Any generator that is large enough to generate useful amounts of power (as in being able to significantly lower your power bills) will be under a great deal of stress. The power comes from slowing down the wind as it passes past the blades, and this requires a balancing force from the tower and guy wires. Roofs are usually made from 2x4 studs and thin plywood, and are totally unsuitable for mounting a wind turbine.

Aside from mounting problems, wind is extremely turbulent and considerably slower close to ground. In order to get decent performance out of a generator, you need to mount it about 30 feet above buildings and trees.

The Betz Limit

As a thought experiment, imagine what would happen if a turbine could stop all the wind blowing at it. Instead of getting maximal energy extraction, generation would halt because no wind would be passing through the blades and rotation would stop.

Somewhere there is a happy medium, a point where you are slowing down the wind, extracting energy, but still allowing significant volumes to pass through. It has been proven that this point, the maximum theoretical efficiency, is about 59%, and is referred to as the Betz limit.

The Danish Wind Association has a proof of Betz' Law. Wikipedia also has a proof of the Betz limit.

The bottom line is that no horizontal wind turbine can extract more than 59% of the wind's energy, and any one that claims fraudulently that they can, is selling you a "Betz Beater".

Outrageous Claims

One of the more ridiculous promoters of wind generators is the Earth4Energy / HomeMadePowerPlant scam, which claim you can build a home generator for less than less than $200. Statements like "why you should always build a wind generator (it only takes few hours and can be built almost for free) instead of buying it. After you read chapter 4 I’m sure you’ll know for sure what you need to do." are patently ridiculous. There are good books out there, like Homebrew Wind Power that show you how, but it is definitely not a trivial task.

Using the equations for calculating power output from the American Wind Energy Association, we get the following:

Wind Turbine Power:

P = 0.5 x rho x A x Cp x V3 x Ng x Nb

where:
P = power in watts (746 watts = 1 hp) (1,000 watts = 1 kilowatt)
rho = air density (about 1.225 kg/m3 at sea level, less higher up)
A = rotor swept area, exposed to the wind (m2)
Cp = Coefficient of performance (.59 {Betz limit} is the maximum theoretically possible, .35 for a good design)
V = wind speed in meters/sec (20 mph = 9 m/s)
Ng = generator efficiency (50% for car alternator, 80% or possibly more for a permanent magnet generator or grid-connected induction generator)
Nb = gearbox/bearings efficiency (depends, could be as high as 95% if good)

So, lets assume we see an advertisement for a 1200W turbine, and we wonder how big it might be to make this much power at a wind speed of 15 mph or 6.75 m/s, which corresponds to the light blue in the map below.

The equation now becomes:

Area = 1200W/(0.5 x 1.225 kg/m3 x .35 x (6.75m/s)3 x .80 x .95 )
Area = 23.9 m2

That gives us a diameter of 5m or about 15 feet, which is a large device. Now how big do you think this little thing is (they claim 1200 Watts).

Since it can't be more than a meter in diameter, lets determine how fast the wind would have to be:

(wind speed m/s)3 = 1200W/(0.5 x 1.225 kg/m3 x .35 x .80 x .95 x 1m2)

Wind speed = 18m/s = 42 miles per hour. You won't be getting these kind of sustained winds in the location pictured above.

If you are serious about wind generation and have the room to erect a tower, perhaps 60 feet tall in the back yard, and have the finances, then there are many good engineering companies that can install commercial equipment for you.

If you are simply interested in knowing how to make your own, want to have a state of the art understanding of the issues, or are are mechanically inclined and want to build your own from scratch, then I recommend you get the Homebrew Wind Power book.