As wind farms expand, some can accidentally "steal" each others' wind – causing worries over some countries' energy transition to net zero.

As offshore wind farms are expanding around the world in the race to meet net zero climate targets, a worrying phenomenon is attracting growing attention: in some conditions, wind farms can "steal" each other's wind.

"Wind farms produce energy, and that energy is extracted from the air. And the extraction of energy from the air comes with a reduction of the wind speed," says Peter Baas, a research scientist at Whiffle, a Dutch company specialising in renewable energy and weather forecasting. The wind is slower behind each turbine within the wind farm than in front of it, and also behind the wind farm as a whole, compared with in front of it, he explains. "This is called the wake effect."

Simply put, as the spinning turbines of a wind farm take energy from the wind, they create a wake and slow the wind beyond the wind farm. This wake can stretch more than 100km (62 miles) for very large, dense offshore wind farms, under certain weather conditions. (Though more typically, the wakes extend for tens of kilometres, according to researchers). If the wind farm is built upwind of another wind farm, it can reduce the downwind producer's energy output by as much as 10% or more, studies suggest.

Colloquially, the phenomenon is known as wind theft – though as Eirik Finserås, a Norwegian lawyer specialising in offshore wind energy, notes: "The term wind theft is a bit misleading because you can't steal something that can't be owned – and nobody owns the wind."

Still, he points out that the phenomenon can have a number of negative consequences for wind farm developers, and even, potentially, cause problems across borders (more on this later). There are in fact a number of ongoing disputes between wind farm developers over alleged wind theft, raising concerns in countries that rely on ramping up offshore wind energy to meet their net zero climate targets.

While the problem of wind theft has been long known in principle, it is growing more pressing due to the scale and speed of the offshore expansion, and the size and density of offshore wind farms, experts say.

In the North Sea, which is seeing an offshore wind boom, the impact of such wakes on offshore energy production is likely to increase in the next decades as the sea becomes more crowded with wind farms, according to simulations by Baas together with researchers from the Delft University of Technology and the Royal Netherlands Meteorological Institute. The denser and bigger the wind farm, the stronger the wake effect, Baas says.

A new research project in the UK, launched this spring, aims to provide a clearer picture of the wake effect to help governments and developers improve their planning, and avoid disputes. The project will model wakes and their impact on wind farms' output in

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