Wind turbines in the sea pose obstacles to water and air. What invisible changes are associated with this? In a study of the southern North Sea, scientists of the Helmholtz-Zentrum Hereon have investigated the influence of a weakening of the wind on the physical conditions of the affected marine region. The results are of importance for the planning of future offshore wind farms.
The turbines of the windmills cause a weakening of the wind speed on the downwind side of the parks. This influence has already been proven in a previous study by the scientists. The so-called wake vortices created in the lee of the wind turbines are characterized by reduced wind speed and special pressure conditions as well as increased air turbulence. These deficits in wind speed can propagate up to 70 km behind the wind farms.
The current study shows a connection between wake vortices and changes in the momentum-driven exchange between atmosphere and water. This in turn can affect the horizontal currents and stratification of the water. The effects of wake vortices are strong enough to redirect the existing currents. Which can lead to a shift in the mean temperature and salinity distribution in the areas of the wind farms.
Read the full Hereon press release:
==> Offshore wind farms reshape the North Sea
Christiansen, N., Daewel, U., Djath, B., & Schrum, C. (2022): Emergence of Large-Scale Hydrodynamic Structures Due to Atmospheric Offshore Wind Farm Wakes. Front. Mar. Sci. 9:818501, doi:10.3389/fmars.2022.818501
Abstract:
The potential impact of offshore wind farms through decreasing sea surface wind speed on the shear forcing and its consequences for the ocean dynamics are investigated. Based on the unstructured-grid model SCHISM, we present a new cross-scale hydrodynamic model setup for the southern North Sea, which enables high-resolution analysis of offshore wind farms in the marine environment. We introduce an observational-based empirical approach to parameterize the atmospheric wakes in a hydrodynamic model and simulate the seasonal cycle of the summer stratification in consideration of the recent state of wind farm development in the southern North Sea. The simulations show the emergence of large-scale attenuation in the wind forcing and associated alterations in the local hydro- and thermodynamics. The wake effects lead to unanticipated spatial variability in the mean horizontal currents and to the formation of large-scale dipoles in the sea surface elevation. Induced changes in the vertical and lateral flow are sufficiently strong to influence the residual currents and entail alterations of the temperature and salinity distribution in areas of wind farm operation. Ultimately, the dipole-related processes affect the stratification development in the southern North Sea and indicate potential impact on marine ecosystem processes. In the German Bight, in particular, we observe large-scale structural change in stratification strength, which eventually enhances the stratification during the decline of the summer stratification toward autumn.
Akhtar, N., Geyer, B., Rockel, B., Sommer, P.S., & Schrum, C. (2021): Accelerating deployment of offshore wind energy alter wind climate and reduce future power generation potentials. Sci Rep 11, 11826, doi:10.1038/s41598-021-91283-3
Abstract:
The European Union has set ambitious CO2 reduction targets, stimulating renewable energy production and accelerating deployment of offshore wind energy in northern European waters, mainly the North Sea. With increasing size and clustering, offshore wind farms (OWFs) wake effects, which alter wind conditions and decrease the power generation efficiency of wind farms downwind become more important. We use a high-resolution regional climate model with implemented wind farm parameterizations to explore offshore wind energy production limits in the North Sea. We simulate near future wind farm scenarios considering existing and planned OWFs in the North Sea and assess power generation losses and wind variations due to wind farm wake. The annual mean wind speed deficit within a wind farm can reach 2–2.5 ms−1 depending on the wind farm geometry. The mean deficit, which decreases with distance, can extend 35–40 km downwind during prevailing southwesterly winds. Wind speed deficits are highest during spring (mainly March–April) and lowest during November–December. The large-size of wind farms and their proximity affect not only the performance of its downwind turbines but also that of neighboring downwind farms, reducing the capacity factor by 20% or more, which increases energy production costs and economic losses. We conclude that wind energy can be a limited resource in the North Sea. The limits and potentials for optimization need to be considered in climate mitigation strategies and cross-national optimization of offshore energy production plans are inevitable.