Aerogenerator turbine sets sail for a greener future

It may resemble a giant rotary washing line, but it might just help Britain meet its hugely ambitious new wind energy targets. At least that's the claim of the company developing a novel "vertical axis" wind turbine dubbed the Aerogenerator. The 144-metre high V-shaped structure would be mounted offshore and capable of generating up to 9 megawatts of electricity, roughly three times as much power as a conventional turbine of equivalent size. Switching to such a design could ensure that thousands fewer turbines would be needed in order to meet the government's new wind power target, says Theo Bird, founder of Windpower, the Blyth-based firm behind the new turbine.As unique as it may sound, the Aerogenerator is in fact just the latest addition to a family of wind turbines that generate power through a rotating vertical shaft as opposed to the horizontal shafts of the more familiar windmill design. Out of reach

One of the problems with horizontal axis wind turbines (HAWT) is that the generators and gearboxes tend to be in an inaccessible position at the top of the structure's tower. This makes them difficult and costly to maintain, says Bird. Such designs also require gearing mechanisms to ensure they are always facing into the wind.

By contrast, vertical axis wind turbines (VAWT) can harness the wind from any direction, allowing them to run more efficiently without the need for such mechanisms.The idea for VAWTs has been blowing around for decades, but despite many advantages the technology has so far attracted little interest. That is about to change, according to Bird. Invented by aeronautical engineer David Sharpe, the Aerogenerator is an adaptation of the egg-whisk-shaped Darrieus wind turbine that solves a number of problems originally posed by the technology.Not least is the ability to build giant turbines, says Neven Sidor of architectural firm Grimshaws - Windpower's collaborator and the company behind the Eden Project.

A Darrieus turbine becomes unstable above a certain height. The biggest HAWTs are capable of producing 6MW of power and stand just short of 200m tall, but if you try to make them any bigger they start to become less efficient.

Enormous stressOne reason is that the weight of the turbine blades becomes prohibitive. As they turn, this places the blades under enormous stress because gravity compresses them as they rise and stretches them as they fall. The larger you make these structures, the more robust they must be in order to withstand these forces.

Add to that the cost and difficulty of building the increasingly large towers needed to keep this top-heavy structure stable and you have a major engineering challenge on your hands.

The Aerogenerator team claims its invention gets round these problems. "The centre of gravity is at the bottom," says Sidor, making the structure much more stable.

Jutting armsInstead of being mounted on a tower with "egg whisk" blades that bow outwards and meet at the top - like a typical Darrieus - Sharpe's variation has two arms jutting out from its base to form a V-shape, with rigid "sails" mounted along their length at intervals. As the wind passes over these they act like aerofoils, generating lift which turns the structure as a whole at roughly three revolutions per minute, says Bird.

Windpower recently carried out wind tunnel tests on a 6kW scaled version of the Aerogenerator at the New and Renewable Energy Centre in Blyth. These suggested that the predicted power outputs could be achieved.The beauty of this design is that no matter how high the two main structures are made it is relatively simple to make them bottom heavy. Because of this the technology lends itself to large engineering projects, which is precisely what is needed with wind power, says Bird.

Earlier this month the UK government's business secretary, John Hutton, made a commitment to create enough offshore wind turbines to supply 25 million homes with electricity by 2020. This would require roughly 7,000 large turbines dotting our coastline. With the Aerogenerator this could be achieved by building bigger and fewer, says Bird.

"This is an opportunity for this country to develop its own technology," he adds.Domestic models

But VAWTs can also be deployed on a smaller scale. A Wyoming-based firm, TMA, has spent the past 12 years developing a cylindrical version and plans to build generators in sizes from 2kW domestic models up to 525kW for wind farms.One of the advantages of VAWTs is they can be built lower, so they are less visible and can withstand much harsher environments, says TMA's John Nunley. Unlike conventional turbines his company's design doesn't need to be shut down when wind speeds exceed 64mph, he claims, and even then the structure can withstand speeds of up to 110mph.Windpower is also interested in small-scale versions and says it should have a 30m high "community scale" 50kW version available within about two years.

But it will be at least 2013 before we see Aerogenerators as powerful as 9MW - and that's assuming Bird manages to convince the right people. But if he pulls it off, he may well turn the wind power industry quite literally on its side.