The wind is not an inexhaustible source of energy either, researchers make clear: Using the example of offshore parks in the North Sea, they show how astonishingly far the systems slow down the wind. As a result, the electricity yield can suffer considerably: The shadow effect can reduce the performance of neighboring wind farms by up to 25 percent, especially in stable weather conditions, according to the modeling. The scientists say that the results can now be used to make optimal use of the limited area potential.
Wind power has become an important component in the electricity mix of renewable energies. The so-called offshore wind farms are considered to be particularly productive: More and more countries are building sometimes huge systems at sea in order to tap energy from the mostly steady flow of air. The expansion of wind energy in the German Bight and the Baltic Sea has also led to considerable success: Today, wind turbines with an output of around 8,000 megawatts, which corresponds to around eight nuclear power plants, are rotating there. One might think that more wind turbines could simply be set up there. But unfortunately the power of the wind is not as inexhaustible as it seems: rotors brake the air flow and can thus take the drive off the following.
The shadow effect in view
Because of this known braking effect, the individual wind turbines in offshore systems are set up at certain minimum distances from one another. It is also already known that wind farms, due to their shadow effect, also have large-scale effects on air currents and can thus influence neighboring plants. Since the location options are limited and the wind farms are therefore partly built as close to one another as possible, an assessment of this effect and its impact on the profitability of the plants is important. The scientists working with Naveed Akhtar from the Helmholtz Center Hereon in Geesthacht have now dedicated themselves to this research topic.
For their study, they used a modeling system for the entire North Sea that incorporates data from weather services. This enables it to resolve the regional weather situation in detail and calculate how the weather and climate or wind conditions are behaving. Data on the positioning and dimensions of existing and planned wind farms in the North Sea were then integrated into the calculations. The researchers used the planning for the North Sea from 2015 as a basis. This also includes wind farms, some of which have not yet been built. The study thus takes a look into the future – to a certain extent, the target state of offshore expansion.
Braking effect in stable weather conditions
The results show how widespread the braking effect can be: On average, it extends 35 to 40 kilometers – in certain weather conditions even up to 100 kilometers, the researchers report. This can reduce the output of a neighboring wind farm by 20 to 25 percent, which is ultimately reflected in economic losses. However, it depends on the time of year, as the results make clear: the braking effect of the wind farms goes very far, especially in stable weather conditions – when there is only a little turbulence in the atmosphere. This is especially the case in March and April. In stormy times – especially in November and December – the atmosphere is so mixed that the shadow effects of the wind farms hardly matter, according to the model simulations. The researchers were then able to confirm that the theoretical results reflect reality based on comparisons with wind measurements from two research platforms in the North Sea from 2008 to 2017.
As you point out, what is special about the work is that a full ten-year period has now been calculated for the first time for the entire North Sea: “Conventional flow models for analyzing wind farms have a very high spatial resolution, but only look at a wind field over short periods of time,” says Akhtar. “In addition, it cannot be used to determine how a wind farm changes the air flow over a large area.” The researchers have now succeeded in doing this. The results are important for the expansion of wind power in Germany and can now be used in expansion planning or in assessing the energy losses in the case of tight arranged wind farms.
In further investigations, the scientists now want to turn their attention from the impairment of the energy yield to the ecological effects of the shadow effect of the offshore parks. Because wind and waves mix the sea and thereby change the salt and oxygen content of the water as well as its temperature or the amount of nutrients in certain water depths. “We would now like to find out how the reduced mixing affects the creatures in the sea,” says Akhtar.
Source: Helmholtz Center Hereon, specialist article, Scientific Reports, doi: 10.1038 / s41598-021-91283-3