What about micro-generation? If you plop one of those mini-
turbines on your roof, what energy can you expect it to deliver?

Assuming a windspeed of 6 m/s, which, as I said before, is above the av-
erage for most parts of Britain; and assuming a diameter of 1 m, the power
delivered would be 50 W. That’s 1.3 kWh per day – not very much. And in
reality, in a typical urban location in England, a microturbine delivers just
0.2 kWh per day – see p66.

Perhaps the worst windmills in the world are a set in Tsukuba City,
Japan, which actually consume more power than they generate. Their in-
stallers were so embarrassed by the stationary turbines that they imported
power to make them spin so that they looked like they were working!

Notes and further reading

page no.

264The maximum fraction of the incoming energy that can be extracted by a
disc-like windmill...
There is a nice explanation of this on the Danish Wind
Industry Association’s website. [yekdaa].

267Usually, wind turbines are designed to start running at wind speeds around
3 to 5 m/s
. [ymfbsn].

a typical load factor for a good site is 30%. In 2005, the average load factor
of all major UK wind farms was 28% [ypvbvd]. The load factor varied
during the year, with a low of 17% in June and July. The load factor for
the best region in the country – Caithness, Orkney and the Shetlands – was
33%. The load factors of the two offshore wind farms operating in 2005 were
36% for North Hoyle (off North Wales) and 29% for Scroby Sands (off Great
Yarmouth). Average load factors in 2006 for ten regions were: Cornwall 25%;
Mid-Wales 27%; Cambridgeshire and Norfolk 25%; Cumbria 25%; Durham
16%; Southern Scotland 28%; Orkney and Shetlands 35%; Northeast Scotland
26%; Northern Ireland 31%; offshore 29%. [wbd8o]

Watson et al. (2002) say a minimum annual mean wind speed of 7.0 m/s is
currently thought to be necessary for commercial viability of wind power.
About 33% of UK land area has such speeds.

Figure B.9. An Ampair “600 W” micro-turbine. The average power generated by this micro-turbine in Leamington Spa is 0.037 kWh per day (1.5 W).
Figure B.10. A 5.5-m diameter Iskra 5 kW turbine [www.iskrawind.com] having its annual check-up. This turbine, located in Hertfordshire (not the windiest of locations in Britain), mounted at a height of 12 m, has an average output of 11 kWh per day. A wind farm of machines with this performance, one per 30 m × 30 m square, would have a power per unit area of 0.5 W/m2.