


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 backup 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
Back to top
