Wind turbines are a modern engineering success story: in three decades they have gone from being a curiosity to being a major component in the electricity generation plans of many countries. The growth in size is spectacular. In the early 1980's 50kW machines were typical, a mid-Wales windfarm I visited in 1989 had 100 turbines of 300kW each. Today, 2MW turbines are common, the first 5MW machine to come to the UK is being installed offshore: just one of todays turbines could replace 100 of those of 25 years ago! And development hasn't stopped there: one of the latest models is 7MW .
There is plenty of exellent material available on the web about wind energy and wind turbines. See for example:
British Wind Energy Association (also has information on wave and tidal systems).
Wind turbines suffer a lot of almost vitriolic comment in the press and on internet blogs. This is curious because they are one of the most harmless and environmentally-friendly ways of generating electricity ever found. Coal requires dirty and dangerous mining, all fossil fuels produce carbon emissions, nuclear produces radioactive wastes and plutonium, and even hydro means that valleys have to be drowned to make the reservoirs. Perhaps it is simply that wind is the most immediate challenge to the vested interest in existing forms of generation. Some of the arguments raised by opponents of wind energy are ...
Total UK electricity consumption is about 370 billion kWh . To generate 20% of this with wind we would need about 28 Giga Watt of wind turbine capacity (this figure allows for intermittency of the wind, i.e. it assumes that they operate at 30% of capacity). This could consist of 14,000 2MW turbines or 5,600 5MW turbines; in practice a mix would be used so the figure would be in between - perhaps 10,000. This sounds like a lot, but consider that the motor industry builds about 2 million cars a year for the UK market. Building wind turbines is a similar type of metal-manufacture to cars and if we built ALL 10,000 turbines in a single year, we would only be building one turbine for every 200 cars built that year.
We know that this isn't a problem until wind generates at least 20% of our electricity, both from theoretical studies and the experience of other countries that already have far more wind generation than the UK (Denmark has almost 20% already and is aiming for 50%). Modern electronics also allows us to reduce electricity demand when supply is low: for example some heavy industrial and commercial users of electricity have contracts which give them cheaper electricity provided they are willing to reduce their usage for a few hours when asked. In the very worst case if we made a huge miscalculation, there could be an occasional power cut during unusually calm weather. Considering that humans lived without electricity in their homes until less than 100 years ago (and many still do), this is scarcely terrifying. The worst possible nuclear accident however could kill thousands and make a large part of the UK so radioactive that it was uninhabitable.
The 30% figure or thereabouts that is often quoted is not efficiency but what is called "Capacity Factor". The Capacity Factor' of a wind turbine (or windfarm) tells you how much a turbine generated in a year as a percentage of how much it could generate if it had that perfect wind, running the generator at maximum output for the whole year. Of course it depends on how windy the site was that year - typical values are indeed about 30%. The importance of capacity factor is that the amount of kWh of energy to be expected from a turbine annually is about 30% of its maximum rating times the number of hours in a year. It should not be thought that 30% is low, on the contary it is perfectly acceptable and to be expected from a machine collecting a widely fluctuating resource.
An analogy may be useful here: Suppose that you decided to collect water by putting out a bucket each week to catch the rain. The maximum annual 'capacity' of your system would be a full bucket every week, i.e. 52 bucketfuls. The "capacity factor" of your system is how much water you actually get in a year as a percentage of that maximum. It is easy to see that if your bucket is too shallow there will be more weeks when it overflows, so you will get less water over the year. On the other hand if you use a bucket so deep that it almost never overflows, then it may only ever be full in a week when there is an exceptional cloudburst and for all the other weeks much of the capacity of the bucket is unused: so a deep bucket catches more water but its 'capacity factor' is lower. Turbine designers have a similar choice when deciding how big a generator to fit. A wind turbine would generate its maximum possible annual output if it could be sited in a wind that blew at just the right speed for the whole year to turn the generator at its maximum rated output. Of course such a perfect wind doesn't exist so designers have to select a generator to fit to their turbine that is a sensible compromise: not so small that a lot of available energy cannot be turned into electricity, and not so big that most of its capacity never gets used.
A designers job is made more difficult by the fact that the power available from the wind is proportional to the cube of the windspeed. What that means is that if the windspeed doubles, say from 10mph to 20mph, the power available goes up by 2x2x2 = 8 times. If it doubles again from 20mph to 40mph, the power available goes up by 8 times again, so it is now 8x8 = 64 times what was available at 10mph. This explains why it is vitally important to site a turbine in an area with strong, steady winds. Given the cube law, a capacity factor of 30% is actually a very good result - it is what you would get if the wind blew constantly for the whole year at 67% of the perfect windspeed for the turbine (because 0.67 x 0.67 x 0.67 = 0.3 ). So the wind turbine is not inefficient - like a fishing net or a bucket to collect rain, it must be designed to accomodate the biggest likely catch - it is to be expected that average catch over the year will be smaller than that.
A 30% capacity factor does not mean that turbines only operate for 30% of the time. A properly sited wind turbine is typically producing electricity for 70-85% of the time, but the quantity will vary depending on the wind speed. With many wind turbines in different parts of the country connected to the grid, there is an averaging effect.
Finally, the 'efficiency' of a wind turbine is a measure of how much of the available energy in the wind is converted to electricity. It varies with windspeed and manufacturers have graphs that show how their turbines perform. In practice, since the wind is free, it is more important how much the turbine costs in terms of money and energy per unit of energy generated.
If we include all the space between the turbines, a 1GW windfarm extends over about 100 sq km, so to generate 20% of UK electricity would require 2,800 sq km of windfarms. UK land area is 241,590 sq km, so that represents about 1.2% of UK surface area, or about twice London's surface area of 1,590 sq km (and in practice a large part of this might be offshore).
However lamposts are spread out over the whole of London but would not usually be said to occupy the whole of London. On a windfarm, only 1% of the land is actually used for the access roads and turbines. Normally hikers and grazing animals can walk right up to the base of the turbines, so the only area that is no longer accessible is that occupied by the towers themselves. A 3.6GW windfarm would require about 1800 turbines of 2MW each, and the 4.3m diameter towers would therefore occupy 2.61 hectares. Given the lower capacity factor of wind, this would be roughly equivalent to a 1.2GW coal or nuclear plant, and according to the UK governments nuclear consultation, the land necessary to build a 1.2GW nuclear power station is estimated at 25-75 hectares. Thus the actual inaccesible land is from 10 to 30 times LESS for wind than for an equivalent nuclear power station, and that doesn't even include the large areas of land worldwide that have been contaminated by nuclear plants.
Thus if say half of the windfarms needed to generate 20% of UK electricity were on land, they would actually use up 14 sq km for access roads and turbines, and the bases of the towers would occupy 0.1 sq km - about the same as used by 2 miles of motorway.
This is an emotive claim since people who want renewable energy also tend to care about birds and wildlife in general. The evidence so far is that a few birds have been killed by colliding with wind turbines, but this is insignificantly small compared with deaths caused by collisions with tall buildings and cars, and attacks by domestic cats (Cats are estimated to kill 27 million birds every year in Britain). The RSBP in the UK and ABC Birds in the USA are broadly in favour of wind energy.
Curiously wind turbines are relatively popular compared to most things that are built in the countryside such as electricity pylons, motorways, etc., even attracting visitors. However it is perfectly understandable that many people would prefer not to have them on visual grounds. The good news is that if and when we invent another way to generate electricity, the whole lot can be bulldozed and carted away. A few years later there will be little sign that they were ever there - though no doubt there will be a campaign for the National Trust will preserve one or two. Few other generation technologies are so benign.
There are many more claims and arguments made against wind energy, e.g. that it takes more energy to make a wind turbine than you get out (not true). Doubtless such arguments will keep coming - the latest I've heard are that offshore turbines make the UK vulnerable to attack because they will confuse air defence radar, and even that all windmills have an "unatural menace" being why Don Quixote saw them as "monstrous giants".
UK residents can buy their electricity from a company that concentrates on renewable energy. Two such companies are:
Ecotricity - concentrates on installing new wind generation. They are also developing biogas and sometimes offer offer bonds to allow individuals to invest in their company.
Switching can be done on-line and prices are similar to those of the major suppliers.
 7 Megawatt wind turbine. New Record: World’s Largest Wind Turbine.
 UK electricity consumption for 2005 CIA World Factbook.
 Capacity factor and electricity data for wind in Scotland "http://www.esru.strath . ac . uk/EandE/Web_sites/01-02/RE_info/interesting.htm" Interesting Figures on Renewables in Scotland (site "www.esru.strath . ac . uk /EandE/Web_sites/01-02/RE_info/index.htm).
 Birds: RSPB's policy on wind farms, American Bird Conservancy's Wind Energy Policy, American Wind Energy Association article. Birds deaths due to cats: How man’s best friend becomes birds’ worst enemy ... from The Times, September 5, 2007.
 The U.S. Department of Energy's Wind Powering America programme.
Paul Gipes website is a mine of information on wind: Wind-Works.