Taking the wind out of each other’s blades

A few small offshore wind turbines are but a drop in the ocean in terms of the impact they have on each other and on the speed of the wind in their wake.

However, as more and much-bigger turbines are added to the designated areas, the impact of each new turbine or wind farm on other turbines – and on the wind on the other side – will increase. That means offshore wind investors should start asking questions about load models sooner rather than later.

The effect on the wind’s speed and precipitation when it makes landfall is still mostly an academic issue, but worth keeping an eye on too, as well as questions of what a changing climate will do to the seasonal distribution and speed of the wind.

All in all, forecasting the load of a wind farm a decade or two from now is anything but trivial.

The impact of turbine clustering is a more obvious concern for investors in potentially congested offshore markets such as Northern European waters. One look at the map of current and proposed developments in the North and Baltic Seas makes it clear that in a decade or two – which is well within the lifetime of assets currently being developed – the regions’ mostly westerly or southwesterly winds will likely have to pass by several wind farms before making landfall.

And while more turbines will increase the overall amount of energy generated, each added turbine in a crowded field will feel the wake, or shadow, from the turbines upwind and see fewer hours of full load, potentially denting revenue.

“There is a shadow effect,” says Eigil Kaas, a professor from the Danish Meteorological Institute. “We are harvesting kinetic energy from the air, and in certain atmospheric conditions this can cause 5-10 percent less harvestable wind energy as much as 70-100km on the lee side direction [downwind] of a wind farm”.

Just how much less energy the third, fourth or fifth down-wind wind farm will generate compared to the wind farm that’s first in line is uncertain, but “our hypothesis is that there is a significant effect”, says Kaas.

Developers such as Denmark’s Orsted, Sweden’s Vattenfall and Germany’s RWE are known to have engaged with this issue already. Laggards may want to get with the programme, as the estimated load will matter from the time of the bid and onward through every step of financing.

Ignorance of the wake-effect could produce winning bids that lead to financially challenged projects down the line with repercussions for investors and wider society.

“Increasingly, wind farms need to be thought of in terms of their environment and the neighbouring wind farms that will exist over the lifetime of the wind farm,” says Jake Badger, head of wind resource assessment modelling at DTU Wind Energy.

“The critical factor is the capacity density and size of wind farm or wind farm cluster,” says Badger, referring to an analysis that showed that the average number of full-load hours for a turbine would fall from around 4,500 for a lone turbine in the German Bight to around 3,300 if 28GW were installed in a total area limited to 2,800km2 (ie, 10 MW/km2).

“There is a need for countries to co-operate to plan placements and specification of wind farms as much as possible in advance so their impact on environment and resources can be estimated and mitigated.”

The ambitious build-out could also affect the weather as turbines impact not only the speed but also the humidity of the wind in their wake. Weather models including every Northern European offshore turbine will soon be tested, and if there is a demonstrable loss of precipitation over land downwind, this could adversely affect agriculture, for example.

Climate change is another complicating factor when looking at offshore projects.

Alex Brierley, co-head of fund management at Octopus, sees that “investors are increasingly taking note of whether El Niño is happening more regularly and whether the jet stream is trending further south. They ask whether irradiation patterns and wind patterns are going to be forecastable, and will the assets actually perform? Will the wind blow in the way you expect over 35 years?”

The answer, as so often, is blowing in the wind.