Wind farms interfere with the weather A recent study with the KNMI (Dutch Meteorological Institute) weather model shows that wind farms influence the weather. The wind near wind farms decreases on average and the temperature and humidity also change. It is not the case that wind energy causes climate change because the effect of wind farms on the weather is local. Nevertheless, the effect is still noticeable in certain weather situations at a great distance from the wind farm. Wind farms are atmosphere mixers Rotating rotor blades of a wind turbine convert kinetic energy from the wind into electricity. This reduces the wind behind the wind turbine. In addition, the rotor blades mix the layers of air and create eddies (turbulence) so that moisture and heat in the air mix better. That can cause clouds to dissolve or form. In short: the weather changes due to wind turbines, but the question is how and how much. Weather model including the effect of wind farms More and larger wind farms are being built. That is why the KNMI has been using a version of the HARMONIE-AROME weather model since last summer to make weather forecasts and warnings, including the effect of those wind farms on the atmosphere. With this version, the wind calculated by the model comes in the vicinity of the wind farms Wake effect greatest in stable weather conditions The effect that a wind farm has on the area in the wind shadow of the wind farm (i.e. downstream) is called the wake effect. This effect is greatest when the atmosphere is stable. Stable means that the sea or the surface of the earth is colder than the air above it. Above sea, this mainly happens in spring and early summer. Because cold air is heavier than warm air, there will be less vertical mixing in a stable atmosphere than in an unstable atmosphere (where the warm, light air sits at the bottom and wants to rise). Less mixing with the “undisturbed” air layers above the wind farm means that wake effects are less quickly nullified and are still present at a greater distance from the wind farm. In stable situations, we sometimes see a decrease in the wind at 50-150 km from the wind farm. The effects on temperature and moisture are also greatest in stable situations. Above the sea, the air in the lowest layer of the atmosphere is not only colder in stable weather, but also more humid. The wind turbines transport this colder and moister air to layers of air above the axis height of the turbines (figure 2). This may mean that the chance of fog decreases and the chance of low clouds increases, but that has not yet been investigated. Not just wake effects It matters whether you have an isolated row of wind turbines or a row that is part of a wind farm as the front row. If they are both fully exposed to the wind (i.e. not in the wake of other wind turbines), the insulated row generates more energy. The wind farm is an obstacle that the wind wants to get around and over. This ensures that the wind is slowed downstream of the wind farm. This effect is also implicit in the weather model, but how well the model reproduces this effect has not yet been investigated. What is the effect if the number of wind farms is expanded significantly? In 2020, the entire North Sea had 19 gigawatts of installed capacity. In 2050 that is expected to be about ten times as much! The installed capacity is the amount of energy that wind turbines can produce when they are running optimally. In reality, the yield will be lower due to, for example, wake effects or wind turbines standing still for maintenance. What will happen to the weather if we fill the North Sea with wind turbines? How much greater are the wake effects if so many wind farms are added? Is there a maximum amount of wind energy we can produce in the North Sea? The KNMI has investigated this together with the Whiffle company and TU Delft in the WINS50 project. It appears that in 2050 wind farms will be in each other's wind shadow much more often (figure 3). To be able to make a good estimate of the electricity production of wind farms in the North Sea, it is therefore even more important that we understand how wind farms influence the weather. KNMI climate report by Ine Wijnant, Natalie Theeuwes, Andrew Stepek (KNMI) and Peter Baas (Whiffle)