How much energy is in the wind?

We're only going to do a few simple sums because the wind is very variable in the UK; accurate calculations are pretty pointless. Six months' data from a small wind turbine (which I hope to have soon) is much more valuable than pages of algebra.

A cubic metre of air weighs about a kilogram. It doesn't appear to weigh anything - because it's surrounded by air, but if you measured it in a vacuum, that's what it would weigh.

Energy is worked out using the formula E=½mv²

(m is the mass of moving air in kilograms and v is its speed in metres per second)

10 mph is 4.47 metres per second; let's call it 5 for simplicity. So 25 mph (a strong breeze) is about 12 metres per second.

We have to work out, for our imaginary turbine, how much energy we could extract.

Imagine a turbine with a diameter of 1.5 metres, sweeping out an area of about 1.8 square metres. We have chosen 1.5 m because this is the size of a wind turbine we are currently testing.

If the wind speed is 12 metres per second, all of the air in a "corridor" 12 metres long will reach our turbine in each second.

The volume of air will be 1.8 x 12, or 22 cubic metres. So m is about 22kg.

The energy per second is ½ x 22 x 12²

-which comes to about 1600 Joules per second.

You can't extract all of it. Imagine you're standing behind this rotor, using it to 'shelter' from the wind.

How much would it protect you?

You'd still be in a strong breeze; the blades would be rotating rapidly. (If they're not, there's something wrong with the rotor.)

I'd guess you can extract about a quarter of the energy, at most. It depends on the type of turbine.

So you're down to 1600/4, or 400 watts, whilst the wind is blowing at 25 mph.

A big disadvantage with a wind turbine is that the windblows at varying speeds and sometimes it does not blow at all, so you cannot guarantee an "electrical supply" at a given time.

An advantage with a small wind turbine setup, located in a suitable 'windy' position, is that if you include a large capacity battery in your system, then you can provide a limited but secure electrical supply independent of the National Grid.

This can be used to provide back-up for the following:

1.Backup for computer system.
2.Backup for gas central heating electrical system.
3.Low energy lighting.
4.Very limited microwave cooking.

N.D./ Habitat21

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