Like many words in the English language, ‘overbearing’ has a nautical connection. It describes a manoeuvre in which one sailing ship steers directly downwind towards another, effectively snatching away the overborne vessel’s wind to leave it powerless.
Wind turbines can overbear each other, too. As developers seek to build ever more of them — globally, installed onshore wind capacity rose to almost 500 gigawatts last year, up from just 92 GW in 2007 — some of the best blustery locations are getting crowded. That could be a problem. To work best, wind turbines need to capture a clear and uninterrupted stream of moving air. Anything that gets in the way — from mountains and buildings to a rival wind farm — reduces wind speed and the electricity generated. Such obstacles also break up the air flow and the resulting turbulence increases noise, as well as wear and tear on the turbine blades.
A study published in Nature Energy this week shows just how overbearing this effect can be (J. K. Lundquist et al. Nature Energ. https://doi.org/10.1038/s41560-018-0281-2; 2018). It analysed the change in electricity production at a wind farm in West Texas when another farm was built a few hundred metres upwind and switched on 18 months after the first farm opened. The researchers estimate that the downwind farm may have lost 5% of its potential on average, and as much as US$2 million annually in electricity production. Texas is unusual: it has the largest number of wind turbines in the United States, with more than 12,000 devices spread across 131 separate farms. Inevitably, the separate projects are clustering at the best sites, which have reliable wind and access to transmission lines. In the study, some of the turbines in the upwind farm stand just 300 metres from some of the downwind turbines.
But the study authors say the impact could stretch much further. Under the right atmospheric conditions, the decreases in downwind wind speeds can extend for 50 kilometres or more. Almost 90% of US wind farms have a neighbouring project within 40 km, and so could be affected. (Of course, not all of them would be affected all the time, because the wind changes direction. The Texas study looked only at the impact under the prevailing southwesterlies.) There is also inevitably internal disruption within a single wind project, with the upwind turbines creating a wake that reduces the output of those behind.
One solution to wind farms treading on each other’s toes is to leave the land behind and head to the vast spaces of the oceans. But offshore wind farms — typically much more expensive to build and run — also tend to compete for the best sites. In 2014, the Danish firm DONG Energy Wind Power (now Ørsted, based in Skærbæk) published data to show how the performance of its long-standing project at Nysted, close to the island of Lolland in the Baltic Sea, was being undermined by a another company’s wind farm constructed just 3 km away (N. G. Nygaard J. Phys. Conf. Ser. 524, 012162; 2014).
What can be done? Technical fixes to the design or layout of projects are difficult, especially as wind turbines grow larger and more powerful. Some engineers have proposed offshore turbines that float and can shift position to reduce wake as the wind moves, but that’s clearly impossible on land. Could rules and restrictions work? A legal analysis by the study authors found no relevant legislation in place in the United States. As a comparison, solar-power efficiency in California is protected by regulations to limit the amount of shadow from neighbouring properties that can fall on panels during peak operating hours.
Where they exist, restrictions on the construction of wind turbines often focus on more immediate risks. In a 2008 dispute between rival developers who wanted to build wind farms on adjoining properties in North Dakota, officials ruled only that each turbine must be placed further than its own height from the boundary, so that if it fell it would not land on the other side. Wind shadow wasn’t considered.
It’s crucial in a warming world to support efforts to boost wind power, and therefore important to install wind farms responsibly to ensure that we harness as much energy as possible, even if the facilities are close together. That means it’s important to craft regulations to support such development.
One country has long taken an enlightened view, and could offer a model to follow. The Netherlands is famous for its windmills, many of which still function, thanks to a law that guarantees each mill can continue to fill its sails with the necessary wind (called its molenbiotoop, or windmill biotope) by restricting development within 375 m. The law has led to some creative solutions: in 2010, a flour mill in Spijkenisse from the 1860s was cut from the ground, raised and placed on a 7 m-high concrete collar to allow houses to be built nearby. Where there’s a mill, there’s a way.
Nature 563, 599-600 (2018)