by Michael Edmund, Editor
Raise your sail one foot and you get ten feet of wind - Chinese proverb
In 1919, the Austrian physicist Albert Betz demonstrated that a fraction no greater than sixteen twenty-sevenths of the kinetic energy of the wind may be captured under ideal conditions. Derived from scientific principles, the 59.3% Betz limit is just as absolute as Kelvin's absolute zero.
Both proponents and opponents of wind energy can agree that the wind does not blow all the time; but this technology has certainly attracted its fair share of controversy. The debate has ranged widely: over optimal locations - offshore or onshore; over environmental issues, such as landscape degradation, noise pollution and the impact upon birdlife (and let us not forget bats. The wind does blow at night, after all); and over economic ones, including the cost of the infrastructure and its overall generating efficiency. How long before investors see a return; how much can the consumer afford to subsidise the electricity, and for how long? But perhaps the cause of greatest disagreement turns upon the very unpredictability of the wind. How much can we rely upon the turbines to generate electricity when we need it, and how much can they realistically supply?
One point to make is that it can surely be no accident that modern commercial wind turbines adopt the same configuration: two, or more usually three blades rotating around the horizontal axis. In assessing the best role for the technology, we do not need to undertake a detailed exploration of optimal wind turbine design here, but it is pertinent to ask what is an appropriate scale for our ambitions for it.
Turbine Size as a Factor
Though turbines are undoubtedly becoming more efficient, the Betz limit means that none will ever be able to convert even 60% of the energy in the wind blowing past it. One obvious, but expensive, way to extract more energy from the wind is to build more wind turbines. Another is to build bigger ones.
Big, it would seem, is good.
Anyone who has followed the development of wind turbine technology will know that these structures have indeed been growing - both in size and in generating capacity. This is perhaps most noticeable with offshore turbines, where size and space are less constrained, and probably has as much to do with the economics as with advances in the technology. Siemens has been credited with supplying the first offshore wind farm in 1991 with 30KW-rated turbines, whose rotor blades were 5m long. Twenty-one years later on, we have reported upon the French offshore facility in which 5MW-and 6MW-rated turbines are under consideration. Areva's 5MW turbine has 56m long rotor blades. Those of Alstom's 6MW alternative are 73.5m long. Meanwhile, Vestas has just announced the development of an 8MW version of an existing platform. Several of the main components are close to completion, including the moulds for its 80-metre rotor blades. So, in the quest to extract as much energy as efficiently as possibly, how far could this technology take us? Upwind, a recent EU-sponsored report tried to shed some light by exploring the design limits for very large wind turbines. It did not seek to define optimal size, but rather to investigate the limits of upscaling existing technology. Its conclusion was that 20MW turbines, with rotors 125m long, were possible; but, critically, that financial considerations would determine whether or not such gigantic machines are ever built. As if to confirm the Upwind findings, Vestas has indicated that its larger turbines offer lower cost of energy without sacrificing reliability and structural integrity; and that the cost of the necessary routine maintenance visits will be reduced.
Wind Farm Size as a Factor
Meanwhile, wind farms themselves have been growing too. Onshore, the world’s largest project is currently the Gansu Wind Farm in western Gansu province in China. One of six national wind power megaprojects approved by the Chinese government, planned capacity growth is 5,160 MW by 2010, 12,710 MW by 2015 and 20,000 MW in 2020. The estimated cost is 120 billion Chinese yuan ($17.5 billion, €13.6 billion).
The 630MW London Array is currently the world's largest offshore wind farm. 151 of the 175 turbines are now installed on a 245 sq km site in the Thames estuary. If approved, a second phase will bring the total capacity to 870MW; but there are plans for a 339-turbine, 1.5GW farm off the Scottish coast that would be capable of supplying 40% of domestic demand for the whole of Scotland. The cost is a less eye-watering £4.5 billion (€5.6 billion).
If big is good, then bigger, it would seem, is better. And more expensive.
The Wind Itself as a Factor
Of course, the stronger the wind, the greater the amount of energy available before the Betz limit is reached. Apparently, the winds that blow on Saturn are among the fastest in the solar system: data from the joint NASA-ESA Cassini–Huygens mission indicate peak easterly winds of 375 metres per second (840 miles/1344 kilometres per hour). Meanwhile on Earth, hurricanes are known to release a phenomenal amount of energy. According to the Atlantic Oceanographic and Meteorological Laboratory, the winds in an average hurricane release about 1.5 TW, or about half the total electrical generating capacity on the planet. Put another way, this energy corresponds to the output of about 20,000 coal-fired power stations. In the devastation wrought by Hurricane Sandy in New York, many saw how destructive this energy can be. But Sandy had already struck the island of Cuba, blowing across the 10MW GIbara I and II wind farms at up to 180 kilometres [110 miles] per hour. Yet both were producing electricity within days: this technology is surprisingly durable.
Height as a Factor
Of course a larger rotor diameter needs a taller tower. And the wind blows harder and more consistently higher up. Could turbines located on much taller towers produce more electricity? What new heights could this thinking reach? As any transatlantic air traveller will tell you, it takes longer flying westwards, into the prevailing headwind than it does eastwards, with the tailwind. So could even the jetstream somehow be exploited? Could taller be better still? A recent article suggests that there is "enough power in Earth's winds to be a primary source of near-zero-emission electric power as the global economy continues to grow through the twenty-first century." The study found that wind turbines placed on the Earth's surface could extract kinetic energy at a rate of at least 400TW, whereas high-altitude wind power could extract more than 1,800TW, or some three orders of magnitude more than present global primary power demand. But before we plan turbines in the sky, we should take note that the study also suggested that such a high rate of extraction of energy would have "pronounced climatic consequences", while German work has found potential for substantial effects upon climate at the surface, and concludes: "jet stream wind power does not have the potential to become a significant source of renewable energy."
Higher, it seems, may not be better.
Conclusions: a perfect storm?
Debate within Europe has recently begun, its objective to start the process of finding a successor to the highly-regarded 20.20.20 energy framework. Now, against a background of financial austerity, many countries are reviewing renewable energy policy: Spain has cut all funding for new green technology, while Germany, Italy and the UK have also reduced (or are planning to reduce) their subsidies for renewable energy production. Discussion in Doha currently features substantial economic support for renewable energy in developing nations, and it is not difficult to imagine from where that support will have to be forthcoming. Meanwhile, the shale gas revolution has led to cheaper European coal imports, rising emissions and falling prices for carbon permits. Will this combination of circumstances impose as much of a barrier as the famous Betz limit? In the wake of Hurricane Sandy, it is economic winds that are blowing cold and hard across the turbines; and they might not resist as easily.