Wind shadows at sea
The Dutch government wants to create a capacity of 6000 megawatts of wind energy in the North Sea, spread over a number of farms. Are our wind farms at sea positioned in the right places? “They are at the moment,” says Arno Brand of ECN Wind Energy, “but new farms are going to ‘steal’ each other’s wind if they are built closer together.”
Wind turbines derive energy from the wind. They cannot convert all the energy contained by the wind, though: the theoretical limit lies at 59 per cent. And naturally we are only talking about the column of air that falls within the span of the rotor blades. The turbines in a wind farm are widely spaced, so it seems safe to assume that the kilometres-wide air current that passes through the farm loses very little energy in its entirety. “But this is not the case,” says Brand. “For a safe distance between two wind farms – safe in the sense that the farms are minimally affected by each other’s ‘wind shadow’ – you should allow for anything from ten to more than thirty kilometres. What is considered ‘not too much’ is then defined as a decrease in wind speed of no more than half a metre per second, based on an annual average of 10 m/s at 90 metres above sea level and farms with an installed capacity of 5 megawatts per square kilometre.
Existing wind farms in the Netherlands’ North Sea and current plans for the future (Illustration: Noordzeeloket).
To the third power
A decrease of 0.5 m/s does not sound like a lot. However, the energy contained by the wind equals the wind speed to the third power. Therefore, if a turbine can only count on 9.5 instead of 10 m/s, this results in a loss of 14 per cent. Of course you are always dealing with a specific wind direction. The Egmond aan Zee wind farm is only affected by the Prinses Amalia wind farm when there is a west-south-westerly wind. This is the prevailing wind, however, which should provide a considerable proportion of the yield. Over the course of a wind farm’s lifespan, therefore, 0.5 m/s can add up to a considerable sum in terms of lost yield.
And if you look at the map showing the current applications for new wind farms in the North Sea, you can see that GWS, EP and BARD (9, 10 and 11 on the map above) are almost bordering on each other in the current plans. We can also see how Rijnveld Oost (18) is virtually surrounded by Katwijk, Rotterdam Noord-West, West Rijn and Rijnveld Noord (19, 7, 16 and 17). Why have the wind farms been planned so close to each other?
The average wind speed above the Netherlands’ Exclusive Economic Zone (NEEZ) in the 1997-2002 period. Height: above average sea level.
From 2014 onward, three extremely large wind farms are going to be built along the western boundary according to current UK plans. If these plans are carried out this will result in a considerable reduction in the average wind speed in our area (Illustration: ECN).
The profitable coastal zone
The coastal zone of the Randstad area (the urban agglomeration on the west coast) is the most profitable location for a wind farm. However, a large extent of this zone is taken up by shipping routes, as the map with current applications shows.
Brand: “As a result of the limited space, the locations of wind farms have been designated without taking the potential adverse interference between farms into account. This is understandable, as we have only recently developed a mathematical model that takes sufficient account of both convective (dynamic) forces and the effects of turbulence and the Coriolis effect; a model that we could validate thanks to measurements taken at existing offshore wind farms, by the way.”
We now know that we cannot implement all the current plans without this resulting in the wind farms adversely affecting each other. And as we move within the parameters of the safe distance as projected by the mathematical model the loss increases rapidly; directly behind a wind farm with a 5 MW/km2 capacity, the wind speed drops by as much as 40 per cent.
This problem cannot be circumvented by enlarging the individual wind farms, or building wind farms with larger turbines or a greater power density. Brand’s model shows how both cases would result in farms with longer wind shadows, so that its neighbours would again need to be positioned at a greater distance; it is a choice between that or putting up with less wind. Moreover, wind turbines in larger farms catch more wind from each other, unless the distance between them is increased as well.
In deep water
The Netherlands’ energy policy demands 6000 megawatts of installed capacity at sea. There is no doubt that turbines in deep water, far out at sea, will have to supply a large share of that capacity. Brand: “Our wind atlas indicates extensive unused areas, which have the added bonus of higher average wind speeds. They are a lot further out from the coast, however, where the sea is much deeper. This entails more expensive foundations, more expensive cables to bring the energy inland and higher maintenance costs. We can’t say much about the exact difference in the costs and yields as we have only limited experience of it.”
British wind shadows
Together with Delft University of Technology and industrial partners including RWE Offshore Wind, TenneT and Eneco, ECN is working on the FLOW project, which stands for Far and Large Offshore Wind, which should, by 2013, result in a small demonstration farm in waters that are 35 metres deep and 75 kilometres out to sea.
The UK seems to be a step ahead of us here, as they are already planning three gigantic operational wind farms along the western boundary of the Dutch part of the North Sea: the Dogger Bank, Hornsea and Norfolk zones. These are much farther out to sea, in waters with depths of up to 60 metres. The aim is for them to supply 20 gigawatts and construction could start as soon as 2014. The size of Norfolk (to the west of the Randstad area) and Dogger Bank, in particular, is going to be such that these farms are going to produce considerable wind shadows that will affect the most important Dutch zones.
It appears to be highly likely that the British plans will be implemented. It may well be time for a return to an international version of the wind rights of olden days, which protected millers from new buildings being constructed in their windmill’s surroundings that threatened to obstruct the wind.
Contact
Arno J. Brand
ECN Wind Energy
Phone +31 22 456 8238
E-mail: Arno Brand
Text: Steven Bolt
Info
Wind farm design - When other wind farms are close
Wind Power Plant North Sea - Wind Farm Interaction: The effect of wind farming on mesoscale flow
Offshore Wind Atlas of the Dutch part of the North Sea
This ECN Newsletter article may be published without permission provided reference is made to the source: www.ecn.nl/nl/nieuws/newsletter-en/
