Rooftop revolutions

Remember when everything was looking bright for micro-wind power? Rooftop turbines were the talk of the town. A flurry of innovative devices started to appear. The Tory leader decided to have one installed at his London home. The DIY chain B&Q started selling them at under £1,500 a pop.

Well, that was last year. Now, after the initial rush of enthusiasm (shared, let’s be honest, by Green Futures), domestic-scale wind is facing up to a harsh dose of realism. Ask the tough question ‘do rooftop turbines work?’ and the answers sound like fudge. Even when they do, they only provide something like a fifth of the electricity the average household needs - and lag well behind some other green technologies on the energy and carbon savings you get for your money [see ‘Snapshots’ below]. Were they brought too soon to market with too many problems unresolved? Maybe the hope was that the ‘early adopters’ would all be well-disposed greenies, who’d tolerate some initial shortcomings that could hopefully be ironed out later...

Snapshots Flash picture: Nothing like a turbine on your roof for telling your neighbours you’re green. But will it say you’re smart?

Financial picture: Home roof turbines suddenly became more ‘affordable’ - but few of us are likely to find them cost-effective as yet. A well-sited tower is a better prospect. The DTI cites an example where a grid-linked turbine with 6kW capacity (sufficient for all of the electricity requirements of two or three typical households) costs £20,000 to install. This, it says, means a payback time of 29 years at current electricity prices - about half as long as for solar PV. But you’ll recoup the cost of unglamorous loft insulation ten times faster.

Big picture: The UK government’s 2006 microgeneration strategy estimates that the various technologies could provide 30-40% of electricity needs by 2050, with micro-wind making up 4%. But home microgen enthusiasts would do better to look into the benefits of a domestic central heating and power unit - or, in the right circumstances, a ground source heat pump.

“It’s precisely these small discreet models that are likely to have least impact on supplies of renewable energy.”

The engineering equations, it turns out, are pretty intractable [see box below]. Wind energy works best when it’s big and brash - whereas the new devices are as small and discreet as possible. The aim was to make them acceptable in towns and cities, where 90% of people in Britain live. But, in so doing, the designers ran straight into the problems of turbine size and actual usable wind speed.

Wind Essentials When it comes to generating electricity from the wind, size matters. The output of a wind turbine is related to the square of the diameter of the blades. So a two-metre diameter turbine generates four times as much power as the one-metre version.

Wind speed is even more important. The turbine output is related to the cube of the wind speed. The simple maths says that a turbine generating a worthwhile 1kW at 12.5 metres/second (about 28mph) would generate only 125W at half this speed.

Turbulence is troublesome. Steady wind is best, with no gusts or sudden changes in direction. Crowded city buildings disrupt the flow. Most turbines do turn to track the wind, but the delay as they do so causes a drop in their output.

Town dwellers may be well advised to wait for clearer guidance later this year, when a Carbon Trust research project should help with forecasting urban wind speeds and assessing the potential of small-scale turbines.

Wind speed is the first thing you need to check when picking a site for a turbine. Helpfully, the DTI publishes year-round averages, area by area down to one square kilometre. So any small wind enthusiast who’s fully exposed to the prevailing wind can look up their local figure. If that’s good enough, it’s game on.

All well and good - for the owners of country houses. In town, though, all those buildings get in the way. Wind speeds can vary from chimneypot to chimneypot. It’s hard to get clear exposure without putting up a nice high tower - the very proposition over which neighbours and planners are most likely to take a dim view. And turbine makers tend to be reticent about their practical performance. “It bothers me that companies still won’t say what they’re getting from their wind turbines,” says Anne Wheldon, technical director of the Ashden Awards for Sustainable Energy. It would be really useful to know.”

As Georgina Wong of the British Wind Energy Association candidly admits, “there’s a lot we don’t know about small wind turbines. There are some places where they perform brilliantly and others where they are not so good.”

Cameron may be lucky. And any responsible supplier will do a site visit before selling you their bit of kit. Their task should soon be a bit easier on the planning side, too. The government, keen to encourage microgen by cutting bureaucratic red tape, published new draft rules in April under which householders wouldn’t need planning permission for devices less than two metres in diameter, provided they don’t stick up more than three metres above the roof line. Sadly, it’s precisely these small discreet models that are likely to have least impact on supplies of renewable energy - and could even be counter-productive to the cause of microgeneration, if people get burned by spending money on gadgets that don’t really deliver.

Happily, there are some smart technologies that promise to squeeze out more from urban wind. US-based Green Energy Technologies couples a turbine with a wind tunnel, which chief executive Mark Cironi says can double the wind speed. That means his Wind Cube can operate in a breeze as slight as two metres/second. But the tunnel is 7.5 metres long - scarcely suitable for the average domestic rooftop.

Then there’s the Quiet Revolution, which tackles the problem of wind turbulence in towns by using a vertical-axis turbine revolving around a flagpole-like mast. Rob Webb, chief executive of the Clerkenwell-based company, says these work fine with gusts and changeable wind direction; they’re always facing the right way, so they generate 20-40% more power than conventional (horizontal-axis) turbines of the same size. They’re not designed for little spaces; indeed, Webb believes “the idea that every two-up two-down can have a wind turbine is an impossible dream”. But Quiet Revolution’s smallest-yet version, a 2.5-metre model due out next year, is potentially ideal for something like street lighting, with one turbine generating enough to light up half a dozen lights.

There’s no reason to turn against all urban wind, just because it’s a whole lot easier when there’s more space. “Imagine a series of small-scale turbines strung out along the central reservations of dual carriageways and urban motorways,” says Iain Watt of Forum for the Future. “The future of small-scale wind power is not about one-size-fits-all solutions, but matching smart technology with suitable sites.” And wind is still very much a front-runner in the microgeneration stakes for well-sited community projects and offices. As it is for schools - which may enjoy a relatively uninterrupted wind flow over playing fields, with no neighbours close enough to be too concerned about a ten-metre turbine tower. A new £110 million package to create 200 carbon-neutral schools, announced in May, could help provide just the education we need.

THE CONTENDERS 1 StealthGen, Eclectic Energy, www.eclectic-energy.co.uk; Diameter: 1.1 metres; Peak output: 660W at 12.5 metres/second; Cost: £3,000;  Plus points: Small and discreet, likely to get planning permission; Claims to generate: about 660 kilowatt-hours (kWh) a year

2 Swift Renewable Devices, www.renewabledevices.com; Diameter: 2.1 metres; Peak output: 1.5kW at 12.5 metres/second; Cost: £5,500-£6,000; Plus points: Designed to avoid ‘unwanted vibration’ to buildings; could offset the energy required to manufacture it within four years; Claims to generate: 3,000 kWh a year

3 Iskra Wind Turbines (model: AT5-1) www.iskrawind.com; Diameter: 5.4 metres; Peak output: 5kW at 12 metres/second; Cost: £20,000; Plus points: low cut-in wind speed three metres/second; Claims to generate: 6,000kWh a year; Note: Turbine mounted on a tower, not suitable for roofs

4 Windsave www.windsave.com; Diameter: 1.75 metres; Peak output: 1kW at 12.5 metres/second; Cost: £1,500; Plus points: Low cost, plugs into mains; Claims to generate: about 650kWh a year

5 Quiet Revolution (model QR2.5, available late 2008) www.quietrevolution.co.uk; Diameter: 2.5 metres; Peak output: 1.25kW; Cost: £14,000; Plus points: Vertical axis wind turbine, output not affected by gusts; Claims to generate: 3,000-4,500kWh a year

6 WindCube, Green Energy Technologies, www.getsmartenergy.com; Diameter: 4.6 metres; Peak output: 53kW; Cost: about £100,000; Plus points: minimum wind speed only two metres/second; Claims to generate: 200,000kWh a year; Note: Includes wind tunnel 7.5 metres long

Mick Hamer is a freelance environmental journalist and a consultant to New Scientist. Roger East is editor of Green Futures.

24 June 2007

Mick Hamer and Roger East