TECHNOLOGY QUARTERLY
Turning wind power on its side
From The Economist print edition
Energy technology: Wind turbines that rotate about a vertical axis, rather than the usual horizontal one, could have a number of benefits
WIND turbines are springing up in all sorts of places around the world, from China to California, but most of them have the same basic design: the blades rotate about a horizontal axis, as in an old-fashioned windmill. Such turbines can generate electricity at a cost not much higher than non-renewable, fossil-fuel sources—provided the wind is blowing, that is. But if proponents of a rival design are to be believed, electricity can be generated from wind even more cheaply, using turbines that rotate about a vertical axis, like a playground roundabout.
TMA, a company based in Cheyenne, Wyoming, announced in November that its first vertical-axis wind turbine (VAWT) would soon be ready for commercial production. The TMA system has two sets of vertical blades. The two inner blades, each shaped like a half-cylinder, catch the wind and rotate about a central axis, while the three outer blades, shaped like aircraft wings, are fixed. The interaction between the two sets of blades causes a drop in pressure in front of the rotating blades' leading edges, which further increases the rate of rotation. TMA claims that its system harvests 43-45% of the wind's available energy; conventional propeller-style turbines, in contrast, have efficiencies of 25-40%.
In winds of more than 80kph (50mph), furthermore, the blades and gearboxes of conventional turbines cannot cope with the strain, and they have to be shut down. TMA says its vertical-axis design can still work even at wind speeds as high as 110kph, however. The ability to harvest high-speed winds is particularly valuable, since each doubling of wind speed results in an eightfold increase in available energy. TMA also claims that its design is quieter and less visually obtrusive than conventional turbines.
A British consortium, Eurowind Developments, which includes VT Group, a shipbuilding and engineering company, and Mott Macdonald, a consultancy, believes VAWTs could be the best design for giant offshore turbines. Such a turbine, with a capacity of ten megawatts, would be able to power around 10,000 homes. Today's largest horizontal-axis turbines produce around five megawatts, and are proving difficult to scale up. Each blade has to be more than 60 metres long, and the bigger the blade, the greater the stress it experiences as it turns: the blade's own weight compresses it at the top of the cycle and stretches it at the bottom. As a result, blades must be made and transported in one piece, which is expensive. Reinforcing the blade to enable it to withstand these forces further increases cost and reduces efficiency.
The blades of a VAWT, in contrast, do not have to undergo this repeated stretching and compression. Nor does their cross-section vary from top to bottom, which makes them cheaper to manufacture than windmill blades, the shape of which must be painstakingly engineered. VAWT blades can also be made in pieces and joined together on site. So vertical-axis designs should enable wind turbines to be scaled up more easily, resulting in cheaper electricity, even for VAWT designs of similar efficiency to conventional turbines. “If we can build a ten megawatt turbine for only slightly more than other companies build five megawatt turbines, then the efficiency question goes out of the window,” says Steven Peace of Eurowind.
Neither TMA nor Eurowind has yet proved the technology in commercial deployments, however, and the mainstream wind industry remains sceptical about the benefits of VAWTs, in large part because the idea is not new. Simple VAWTs, with a couple of sails pushed around by the wind, have been around for centuries, and were being used in Persia thousands of years ago. In 1922 a Finnish engineer, S. J. Savonius, improved on this primitive design, and devised a turbine based on two half-cylinder blades, as TMA uses. In 1931 a Frenchman, Georges Darrieus, patented a wind turbine that operates on an entirely different principle with two thin, curved blades fixed to a central axis, in a design often compared to an egg-beater.
Turbines based on the Savonius design are already used for small-scale generation in remote locations. Even large-scale VAWTs have been tried before. In the early 1990s the British government funded a trial in Carmarthen Bay in Wales, which culminated in the construction of a 500 kilowatt, 35-metre turbine. But it failed after six months because of a manufacturing fault, and the trial was wound up shortly afterwards. The project's final report concluded that VAWTs had no applications on land, but they should be reconsidered “if offshore wind energy becomes more attractive”.
That day has now come, so it might be time to give the technology another look. Nigel Crowe, director of the British Wind Energy Association, says the use of horizontal-axis turbines has as much to do with historical factors as technological merit. “Why do we use horizontal axis turbines? Why do we use VHS, not Betamax?” he asks. “They are the ones that got accepted first, and got established in the marketplace. The industry now is going through some major changes. Maybe the goalposts have moved a bit and maybe it is the right time to look again.” With plans afoot to build wind farms off the coast of Britain and elsewhere, the fortunes of the VAWT may be about to take a turn for the better.
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