It is visible for miles around, yet few people know anything about it.

A quick vox pop around Bradford centre brought similar responses: "I've seen it in the distance." "Somewhere near Thornton, I think." "I've driven past it." "I don't know what it powers."

A mile or so across the moor as the crow flies from the top of Thornton Road, and you arrive at Ovenden Moor wind farm on Hollin Hill - an example of green' technology that meets the average annual domestic needs of 6,700 households and saves around 11,700 tonnes of carbon dioxide being emitted into the atmosphere.

On this exposed spot 23 turbines, each standing 32 metres high with three 16 metre fibreglass blades, are set out in two lines forming a V' shape, to make best use of the prevailing south-westerly wind.

It is a blustery stretch of land, 1,300ft above sea level, with a high average wind speed. "It is rare that there is no wind up here," says Tim Morgan, asset leader for E.ON UK Renewables, who monitor and operate the farm from their Coventry base.

The turbines, which rotate at up to 35 revolutions per minute, start operating at a wind speed of four metres per second (one metre per second is the equivalent of 2.2mph). "This produces a small amount of electricity," says Mr Morgan. "As the wind increases the amount increases and at 13 metres per second the turbines are generating their peak output."

In very high winds the turbines, which start revolving by wind alone, are switched off to safeguard the mechanism. This usually takes place at 25 metres per second.

"They could also get damaged by a lightning strike," says Mr Morgan. "This has happened once since the wind farm opened 13 years ago. A blade was damaged and we had it repaired."

A computerised system enables the turbines to be shut down from anywhere in the country, even through a telephone line, but this does not often happen. "They are all working 95 per cent of the time," says Mr Morgan, tapping into the small console, which gives details of the wind speed at that moment, as well as operating hours on any particular day of any month. Standing among the grasses and heather, with the twittering of birds all around, the tall structures, their blades swishing through the air, appear eerie yet elegant. Occasionally, clanks can be heard, echoing throughout the hollow structure, as the mechanism holding the blades turns slightly into the wind.

"It is a fascinating place," says Mr Morgan. "We get a lot people taking photographs from the road. Most people find them rather attractive, and some people have described them as beautiful'."

That is not always the case, and plans to erect wind farms often encounter fierce opposition, with many believing them to be a blot on the landscape.

The turbines generate power by driving a generator through a gearbox, then through a transformer at the base of the turbine. A cable runs from the wind farm, underground across the moors to a sub-station in Denholme, where it is fed into the local distribution network, or local grid'. "It supplements the local supply, so we know the power is going to homes in this area," adds Mr Morgan. "We are very keen for people to find out more about the wind farm and think it is important for people to know why it is here and what it is for."

E.ON is looking to involve local people, and those from the wider area, as much as possible, through public events aimed at raising awareness of how the wind farm operates and what happens to the end product.

On July 8 anyone wanting to find out more about it can attend an open day at the site. The day presents a rare opportunity to take a closer look at the turbines and ask questions about them. There will be activities for children including face painting and kite making.

Visits by representatives of the company have already held a schools' open day.

In addition to the open day, people are also being invited to draw on their creativity to produce artwork on the theme of wind power and the Ovenden Moor wind farm for a competition.

They are looking for artwork from schools, classes or individuals of all ages. Entries will be displayed in shop windows throughout Thornton during October, followed by a people's vote which will allow members of the public to choose their favourite piece of artwork. There will be prizes for the winners.

Thornton resident Roland Arnison thought up the idea. "There is a thriving art community in and around the Thornton area and the wind farm is a well-known local landmark."

The wind farm is owned by Yorkshire Windpower, a joint venture between E.ON and Bristol-based Energy Power Resources. Under this partnership, Ovenden Moor and a similar wind farm at Royd Moor, Penistone, South Yorkshire, are managed.

It is only in the past five years that this clean' form of energy has really taken off. E.ON - which sells electricity as Powergen - has interests in 20 operational windfarms from Cornwall to Scotland, the oldest being built 14 years ago in Norfolk.

Ovenden Moor was completed with financial help from the European Union under the Thermie' programme, which aims to support development of green' technologies.

The Government has set targets for the UK to generate ten per cent of electricity from renewable sources by 2010, increasing to 15 per cent by 2015, with a further 20 per cent to be achieved by 2020.

Currently renewables represent around three per cent of electricity generation so the targets will be very challenging to meet. Says Mr Morgan: "We are aiming to provide customers with ten per cent by 2010, and are currently supplying between three and four per cent."

The power generated at Ovenden Moor is mixed' with the electricity supply from various sources including the coal-fired power stations at Drax and Ferrybridge, both of which are visible from the wind farm. "When it is not generating, power is taken from other resources," says Mr Morgan.

l Ovenden Moor wind farm will be open to the public on Saturday, July 8, from 10am to 4pm. Competition entries for the artwork must be delivered to South Square Art Gallery, Thornton Road, Thornton, by September 23. More information on the competition is available from info@thorntonarttrail. or from Thornton Art Trail, South Square Centre, Thornton, Bradford BD13 3LD.