## Wind

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The UK has the best wind resources in Europe.

Sustainable Development Commission

Wind farms will devastate the countryside pointlessly.

James Lovelock

How much wind power could we plausibly generate?

We can make an estimate of the potential of on-shore (land-based) wind in the United Kingdom by multiplying the average power per unit land-area of a wind farm by the area per person in the UK:

power per person = wind power per unit area x area per person.

Chapter B (p263) explains how to estimate the power per unit area of a wind farm in the UK. If the typical windspeed is 6m/s (13 miles per hour, or 22 km/h), the power per unit area of wind farm is about 2 W/m2.

Figure 4.1. Cambridge mean wind speed in metres per second, daily (red line), and half-hourly (blue line) during 2006. See also figure 4.6.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

### Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

This figure of 6m/s is probably an over-estimate for many locations in Britain. For example, figure 4.1 shows daily average windspeeds in Cambridge during 2006. The daily average speed reached 6m/s on only about 30 days of the year - see figure 4.6 for a histogram. But some spots do have windspeeds above 6m/s - for example, the summit of Cairngorm in Scotland (figure 4.2).

Plugging in the British population density: 250 people per square kilometre, or 4000 square metres per person, we find that wind power could

Figure 4.1. Cambridge mean wind speed in metres per second, daily (red line), and half-hourly (blue line) during 2006. See also figure 4.6.

Figure 4.2. Cairngorm mean wind speed in metres per second, during six months of 2006.

generate

2 W/m2 x 4000 m2/person = 8000 W per person, if wind turbines were packed across the whole country, and assuming 2 W/m2 is the correct power per unit area. Converting to our favourite power units, that's 200 kWh/d per person.

Let's be realistic. What fraction of the country can we really imagine covering with windmills? Maybe 10%? Then we conclude: if we covered the windiest 10% of the country with windmills (delivering 2 W/m2), we would be able to generate 20 kWh/d per person, which is half of the power used by driving an average fossil-fuel car 50 km per day.

Britain's onshore wind energy resource may be "huge," but it's evidently not as huge as our huge consumption. We'll come to offshore wind later.

I should emphasize how generous an assumption I'm making. Let's compare this estimate of British wind potential with current installed wind power worldwide. The windmills that would be required to provide the UK with 20kWh/d per person amount to 50 times the entire wind hardware of Denmark; 7 times all the wind farms of Germany; and double the entire fleet of all wind turbines in the world.

Please don't misunderstand me. Am I saying that we shouldn't bother building wind farms? Not at all. I'm simply trying to convey a helpful fact, namely that if we want wind power to truly make a difference, the wind farms must cover a very large area.

This conclusion - that the maximum contribution of onshore wind, albeit "huge," is much less than our consumption - is important, so let's check the key figure, the assumed power per unit area of wind farm (2 W/m2), against a real UK wind farm.

The Whitelee wind farm being built near Glasgow in Scotland has 140 turbines with a combined peak capacity of 322 MW in an area of 55 km2. That's 6 W/m2, peak. The average power produced is smaller because the turbines don't run at peak output all the time. The ratio of the average power to the peak power is called the "load factor" or "capacity factor," and it varies from site to site, and with the choice of hardware plopped on the site; a typical factor for a good site with modern turbines is 30%. If we assume Whitelee has a load factor of 33% then the average power production per unit land area is 2 W/m2 - exactly the same as the power density we assumed above.

Consumption Production

Wind: 20 kWh/d

Figure 4.3. Chapter 4's conclusion: the maximum plausible production from on-shore windmills in the United Kingdom is 20 kWh per day per person.

Power per unit area wind farm 2 W/ m2 (speed 6m/s)

Table 4.4. Facts worth remembering: wind farms.

Population density of Britain

250 per km2 ^ 4000 m2 per person

Table 4.5. Facts worth remembering: population density. See page 338 for more population densities.

Figure 4.6. Histogram of Cambridge average wind speed in metres per second: daily averages (left), and half-hourly averages (right).