One of the simulations performed by Steven and Liu, which shows that just behind the hill (grey circle) a lot of turbulence (the ‘clouds/air flows in blue/green’) is created.
Srinidhi Gadde / UT
Placing a windmill after a hill must almost result in weaker wind and therefore less energy yield. Not a handy move if you want to get everything out of a turbine. But that’s not right. This is apparent from research by Luoqin Liu and Richard Stevens of the physics of fluids group at the University of Twente. Their simulations show that a windmill behind a hill can generate more energy than a turbine in areas with few differences in height. NEMO Kennislink talks to Stevens about the discovery.
Did that outcome surprise you too?
“We really didn’t expect this. We honestly assumed that a windmill would catch less wind behind a hill. However, this was not apparent from our research. We wanted to check this out because there is a lot of hilly terrain in the world. How well do windmills work in these areas? We wanted to know that.”
Why is it still advantageous to place windmills behind a hill?
“It doesn’t apply in all cases, though. These are places where you can place the wind turbine at the right distance from the hill. In the case we investigated, it is about five hundred meters away. In addition, the wind conditions are important. You need a local rapid, a so-called low level jet, just above the hill. This causes the low pressure area that develops behind the hill to pull the strong current from above the hill down. As a result, the wind turbine generates more energy. Whether this occurs depends on the location, but these kinds of circumstances occur regularly.”
Do we now have to put windmills behind hills en masse?
“It is still too early for that. We started this as fundamental research, but we need to look even more closely at where the discovered effect occurs. It could very well be that you can place windmills behind an elevation in Limburg or the dune area. But this needs to be explored even more specifically. In the future, it would be nice if there was a simulation that predicts per location how well a windmill is doing in a certain area. Incidentally, the best option is still to place a windmill on a hill, but there is not always space there. In some cases, that spot is also not accessible, so placing a turbine behind the hill or dune is an alternative.”
Visualization of the research of Liu and Stevens. In this side view of the hill (circle) and windmill (dash) you can see how the wind current with a low wind force (blue) bends upwards behind the hill. As a result, the windmill comes into a stronger wind current (red) and can therefore supply more energy.
Does this research also help if the land is flat?
“It looks like that. By building a hedge or wall in front of a wind turbine, you could achieve a somewhat similar effect. We are also investigating that. If you build a hedge in the right way, the wind will not pass over the ground, but will be deflected towards the turbine. Simply put, you can see it as a kind of springboard.”
Luoqin Liu en Richard Stevens, ‘Effects of atmospheric stability on the performance of a wind turbine located behind a three-dimensional hill’, Renewable Energy 175 (September 2021) 926-935. DOI: 10.1016/j.renene.2021.05.035
Source: Kennislink by www.nemokennislink.nl.
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