Silent revolution

A radical design for wind turbines aimed at harnessing the winds that gust in city streets could be a common sight in London in the run-up to the 2012 Olympics. Stuart Nathan reports

Cities can be windy places, especially in the UK, but urban wind turbines are a rare sight. The architects behind the London Eye are trying to change that with an ambitious plan to install 6,600 wind turbines in London in time for the 2012 Olympics, boosting the city’s green credentials. But these turbines won’t be the familiar triple-bladed units; they’ll be unlike anything the city’s ever seen.

David Marks and Julia Barfield, the celebrated husband and wife architectural team, have come up with the Beacon, an array of five, vertical-axis wind turbines set in a Y-shaped structure, specifically designed to generate the maximum power from the awkward wind conditions that occur in cities. ‘We live in one of the windiest countries in the world,’ David Marks said. ‘There’s no shortage of it anywhere.’ However, exploiting it is not simple, he said.

Although we are used to seeing power stations on the fringes of urban centres, or on the coast, there is a logic to generating power within the city.

‘The Victorians got it right when they decided to build their power plants in the centre of cities, because that’s where the power is required,’ Marks said. ‘They only moved them out of the city because it’s a dirty technology. But wind really is clean and city centre turbines would avoid the fantastic losses in energy production and transmission that you get with remote generation.’

There are two problems with urban wind generation, one linked to the nature of the wind and the other with the nature of cities. Average windspeeds in urban environments tend to be low, with occasional gusts of very high wind; the wind is also turbulent, with rapid changes of direction. This means conventional, horizontal-axis turbines are inefficient. They can only generate power when facing into the wind and, while they are tracking to take up the optimum orientation, they are not generating.

The second problem is that the noise and vibration generated by wind turbines is not acceptable in cities; it would disturb the large numbers of people who live and work nearby and the vibration could damage buildings and infrastructure.

The Beacon design avoids these problems because of the turbines it uses. The QuietRevolution turbine, developed by engineering consultancy XCO2, is a triple-bladed, vertical-axis turbine that is claimed to be silent and vibration-free.

‘When I first saw the design of the QR turbine, it rang a bell with me,’ Marks said. ‘I could immediately see that this had advantages and potential in an urban environment that the normal wind turbines, which we were used to seeing, didn’t have.’

Marks and Barfield had been looking at other vertical-axis turbines to incorporate into the i-360 project, a needle-like tower with a doughnut-shaped observation deck, planned for Brighton; the QR design was a natural choice for the urban wind energy project that was also on their drawing boards.

Vertical-axis turbines will spin in wind from any direction, said Robert Webb, XCO2 managing director. ‘We’ve done some modelling using real wind data that predicts we’ll be about 30 per cent better in terms of energy output than an equivalent-sized propeller-type turbine,’ he said.

A single QR turbine is rated to produce 6kW of instantaneous power in a wind speed of 12.5m/sec. ‘But that’s a little misleading,’ Webb said. ‘A more relevant way of talking about it is the annual average energy use and we usually quote 10,000kWhr/yr in a windspeed of 5.9m/s, which is fairly typical of urban settings.’

The vertical configuration also produces less noise. Although the turbines spin at a higher rate of revolutions per minute than a horizontal-axis turbine, the velocity of the blades through the air is lower. For a horizontal turbine, the tips of the blades — which create vortices of turbulent air as they travel, giving rise to the noise — have to describe a large circle with each revolution, while the vertical configuration has the blades parallel to the axis of rotation, and rotating 1.5m from the axis, leading to a relatively low blade velocity.

The QR has three blades twisted into a characteristic helical S-shape which, according to Webb, eliminates vibration. ‘The ideal, aerodynamically, is a single blade but that would need a counterweight, which isn’t practical,’ he said. ‘Two blades give you a fluctuation in the resulting loads on the turbine. But three blades smooth out the load. Along with the twisted geometry of the blade, it means that the wind pulls each blade around on the windward and leeward sides of the turbine, spreading the torque evenly over the course of each revolution. You smooth out the load, so it’s constant.’

Twisting the blade geometry also has a ‘happy side-effect, not altogether unlooked-for’, Webb added. The shape of the blade becomes a curve shape known as a catenary — the type of curve taken up by a chain hanging by its ends — which is easier to make, lighter and more efficient then an untwisted shape.

The lightness of the blade is also an important consideration, Webb said, as this reduces the centrifugal force acting on the blades as they spin. ‘We wanted a very high performance and because the centrifugal force is quite a high proportion of the structural loads the blades experience, the heavier the blade, the greater the centrifugal load. So we wanted something very, very light. We’ve been able to take the latest developments in pre-preg carbon fibre, working with people who design and make light aircraft. Our blades are 7m long, and they weigh about 4kg.’

The framework in which the Beacon’s five QR turbines sit was relatively simple to design, Marks said. ‘You need to get the turbines above the roofs, and in central London most of the buildings are below 50m in height, because of 1950s planning regulations. We worked with XCO2 and the structural engineers to get the optimum arrangement.’

The Beacon’s Y-shaped support structure is 40m high and each unit can generate some 50,000kWhr/yr.

The turbine is designed to be sited anywhere in the city with an appreciable wind, such as along major roads, banks of rivers and on roundabouts, and Marks is so confident of its potential he has proposed an ambitious scheme to site 6,650 around London in time for the 2012 Olympics. This, he argues, would meet half of the London Mayor’s target to generate 665GWhr/yr of electricity from renewable sources in a single step.

The scheme would include 600 Beacons along the length of the Thames; 1,800 around the M25 and 4,170 along major roads.

‘That’s four for every square kilometre or one for every 24 hectares,’ Marks said. ‘That’s not really a dense population, when you compare it to the number of bus stops or lamp posts on Transport for London’s roads.’

Marks said it is the efficiency of the design that allows a relatively small number of turbines to generate in excess of 330GWhr/yr.

‘To get the same amount of energy, you’d need 600,000 small building-mounted horizontal axis turbines and that’s never going to happen, is it? But you can imagine 6,650 Beacons. It’s an achievable target.’

At a cost of £100,000-150,000 per turbine, the total cost would be less than 10 per cent of the cost of the Olympics.

Some argument is to be expected about aesthetics but Marks believes the Beacon is an elegant design with a great deal of appeal. ‘On the skyline, they almost disappear — they look very light and airy. You need to consider the visual impact on the built heritage and access for installation and maintenance. But most people I’ve shown it to say it looks rather good.’

No decisions have yet been taken on the London proposal but the Beacon is ready for deployment, Marks said, and the need to harness renewable power is pressing.

‘This is something that needs to be done,’ he said. ‘Everyone agrees it needs to be done, and it can be done. We can do it — let’s get on with it.’