Wind turbines are one of the most popular forms of renewable energy. Despite vocal detractors, opinion polls regularly find that more than eight out of 10 people are in favour of wind energy and less than one in 10 are against it.

The pros and cons of wind power regularly fill the pages of newspapers, yet it’s still quite difficult to pin down the facts about it. Two things there’s no dispute about are that:

• the UK is the windiest country in Europe, and
• the wind blows most in winter, which is when we use most electricity.

There’s considerable agreement that wind has huge potential for affordable, large-scale generation through onshore and off shore wind farms.

At a domestic level, things are not so clear. There isn’t a one-size-fits-all answer. Whether wind is right for you will depend on your location and circumstances. If you have a clear site for the turbine, away from obstructions, with high average wind speeds, it’s worth investigating.

Wind turbines are mounted either on a building or, preferably, on a mast.

• The wind rotates the blades of the turbine
• which turns a rotor shaft
• which generates low voltage DC (direct current) electricity.
• An inverter converts it to AC (alternating current or mains type) electricity.

What happens next depends on whether or not the system is connected to the grid.

In off-grid systems the power generated charges a bank of batteries. To get the most from the system, you can programme it to divert electricity to other uses such as water or space heating if the batteries are full.

If you are connected to the mains grid, then when you generate more electricity than you use, you can sell the excess to your supplier. When you are not generating enough to cover your needs, you can buy electricity from your supplier.

Micro turbines, mounted on the roof of your house, are designed to supplement the mains electricity supply, reducing the amount you need to buy. They can plug into a standard 13amp socket.

Turbine life is up to 22 years. They need servicing every few years to ensure they are efficient. Battery life is shorter – about six to 10 years.

The success of wind power depends totally on the suitability of your site. The two key factors are:

• An average wind speed of at least 5 or 6 m/s (metres per second).
• No obstacles nearby which might reduce the wind speed or create turbulence. These include buildings, trees or hills. An ideal site is a smooth hill top with a clear, open stretch to the prevailing wind.
  

Until now, it has been difficult to measure wind speed accurately and affordably. A wind map and a link to the DTI database of average wind speeds are available through the British Wind Energy Association (BWEA) website or Encraft has a free wind speed calculator. These are given in meters per second at 10m high. However, these should be viewed as a starting point only, as the figure is an average for the square kilometer that you live in.

Actual wind speeds for your site can vary enormously from the average. Field trials by the Energy Saving Trust and Encraft both found that average wind speeds at many sites in the trial are significantly lower than indicated by the DTI database. Both studies also found that building-mounted turbines in urban and suburban areas are unlikely to perform well.

The best way to find out how windy your site really is is to measure it using a wind gauge (or anemometer). Getting a professional in to do this tends to be prohibitively expensive for a small turbine. Better Generation has just plugged the gap in this market with the launch of its Power Predictor (it just sneaks in under £100). It measures both solar and wind energy at your site, capturing the data, which you can then plug into to your computer to get a report of the suitability, cost and payback times of various products, completely tailored to your site.The longer you use it, the more accurate the predictions.

More detailed information on good and bad sites for a small wind turbines is also available from BWEA.

Turbines can either be mounted on a free standing tower (mast) or micro turbines can be fixed to the roof of a building.

Wind speeds increase with height, so it is generally considered best to put your turbine on a mast. A small increase in the wind speed will lead to a proportionally much larger increase in the amount of electricity generated (the change is cubed – so twice the wind speed will lead to eight times as much power; half the wind speed will generate an eighth as much). A tower also helps avoid turbulence caused by trees or buildings.

Data for the performance of building-mounted turbines is thin on the ground. Evidence from the Warwick Wind Trial suggests that claims to generate 2,000-4,000kWh per year made by manufacturers are ‘over-estimating performance by a factor of around 20 for the majority of UK sites, and probably between two and five for even apparently attractive building-mounted locations’.

The Centre for Alternative Technology (CAT) does not recommend mounting a turbine on a roof; in its field trials the Energy Savings Trust found that no building-mounted turbine performed as well as expected and attributed this to inappropriate locations - all those in the test were found to have an annual measured wind speed of less than 4m/s. 

Warwick researchers suggest that micro wind is a niche technology suited to a limited number of sites in the UK and where micro turbines are viable, people would be better off going for larger systems.

The BWEA has a small wind performance and safety standard which gives two measures: a power rating and the energy output for the turbine. The latter, which is measured in kWh (kilowatt hours, the unit that electricity is measured in), is the most significant. It’s no good having a high power rating if the turbine needs hurricane strength winds to turn it and stands idle a good deal of the time.

You can find a list of the performance data provided by manufacturers on a variety of micro and small turbines at BWEA. Reviews of micro and small turbines are available at Better Generation. In its field trials, the Energy Saving Trust pointed out that a number of manufacturers' performance claims are not yet standardised or comparable. It advises you to treat power curves with caution until new standards are in place. These have been developed by the Microgeneration Certification Scheme (MCS) and are expected to be in place by the end of 2009.

While these figures can indicate comparative performance of turbines, the actual performance will depend entirely on the conditions at your site. The Warwick Wind Trials found that variation in the performance of the same wind turbine can be as much as 28 times when comparing two sites less than a mile apart.

The site is not the only factor influencing the system’s performance. As with all renewable energies, it also depends on how the user interacts with the technology and the quality of the installation.

Micro turbines, which are generally in the range of 500 watts to 2.5 kW, will cost from £1,500 upwards installed according to the Energy Savings Trust.

Small turbines, ranging in size from 2.5kW to 8kW, are a much heftier investment. Prices are likely to be anything from £11,000 up to £25,000.

Researchers at Oxford University's Environmental Change Institute analysed information from all installations that got a LCBP grant between 2006 and 2008. They calculated the following averages which may be useful as a benchmark:
The mast-based 5kWp Iskra AT5-1 cost an average £18,425 (£3,650 per kWp).
The mast-based 6kWp Proven Engineering WT6000 cost an average £19,170 (£3,200 per kWp).

The size you choose will depend on your site, the amount of power you want to generate and how much you want to spend. Connecting to the grid is expensive, and probably won’t be worth it for less than a 2kW turbine. A 5kW system is likely to generate around 15,000kWh of electricity in a year – which means it would cover its cost in 10 years.

From April 2010, the Clean Energy Cashback (CEC - the government's financial incentive scheme) will guarantee an income for electricity generated. Rates are 34.5p for installations with a capacity of less than 1.5kW; 26.7p for 1.5-15kW; 24.1p for 15-100kW; 18.8p for 100-500kW; 9.4p for 500kW-1.5MW; 4.5p for 1.5-5MW, plus 3p per kWh for each unit exported to the grid.

The CEC payments will be index-linked, and will continue for a 20 year period. To qualify you will need to use an MCS accredited installer and product.

The cheapest way to install a wind turbine is to make your own. The Centre for Alternative Technology publishes a book Windpower Workshop which describes how to build your own from second hand and scrap materials. Better Generation runs weekend turbine building courses from £200.

You will need planning permission. It’s also worth talking about your plans with your neighbours, before you apply.Up-to-date advice is available on the government’s planning portal.

New standards and safeguards are to be established to deal with noise and  vibration from wind turbines to ensure neighbours are not disturbed by the development. More information can be found on the Greener Homes planning portal. 

From the blog:

Will shadow flicker affect a wind planning application?

Calculators shed light on renewable choices

Measure the wind speed before you install a turbine

Selling the electricity you generate

Support for microgenerators increased this month (April 09)

Urban microwind doesn't work, say new trials

Wind power: accessible yet frustrating

Wind power without the guesswork - how to measure your wind speed

Who makes wind turbines in the UK

Sources:

British Wind Energy Association

Centre for Alternative Technology

Devon Association for Renewable Energy

Energy Savings Trust

Microwind – a catalyst for change in UK energy Culture A report on the Warwick Wind Trials published by Encraft