### Standard windmill properties

The typical windmill of today has a rotor diameter of around 54 metres

centred at a height of 80 metres; such a machine has a “capacity” of 1 MW.

The “capacity” or “peak power” is the *maximum* power the windmill can

generate in optimal conditions. Usually, wind turbines are designed to

start running at wind speeds somewhere around 3 to 5 m/s and to stop if

the wind speed reaches gale speeds of 25 m/s. The actual average power

delivered is the “capacity” multiplied by a factor that describes the fraction

of the time that wind conditions are near optimal. This factor, sometimes

called the “load factor” or “capacity factor,” depends on the site; a typical

load factor for a good site in the UK is 30%. In the Netherlands, the typical

load factor is 22%; in Germany, it is 19%.

### Other people’s estimates of wind farm power per unit area

In the government’s study [www.world-nuclear.org/policy/DTI-PIU.pdf]

the UK onshore wind resource is estimated using an assumed wind farm

power per unit area of at most 9 W/m^{2} (capacity, not average production).

If the capacity factor is 33% then the average power production would be

3 W/m^{2}.

The London Array is an offshore wind farm planned for the outer

Thames Estuary. With its 1 GW capacity, it is expected to become the

world’s largest offshore wind farm. The completed wind farm will consist

of 271 wind turbines in 245 km^{2} [6o86ec] and will deliver an average power

of 3100 GWh per year (350 MW). (Cost £1.5 bn.) That’s a power per unit

area of 350 MW/245 km^{2} = 1.4 W/m^{2}. This is lower than other offshore

farms because, I guess, the site includes a big channel (Knock Deep) that’s

too deep (about 20 m) for economical planting of turbines.

I’m more worried about what these plans [for the proposed London Array wind farm] will do to this landscape and our way of life than I ever was about a Nazi invasion on the beach.

Bill Boggia of Graveney, where the undersea cables

of the wind farm will come ashore.