Wind energy
Martin Dörenkämper is postdoctoral researcher at the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES). After obtaining his master degree in meteorology he did a PhD on wind energy meteorology at Oldenburg University. As such, he has become a leading scientist in this field. The following summary of his presentation is written by the organizers.
A 6% error in wind speed can lead to an error of 30% in power
In his first slides, Dörenkämper explained why wind energy meteorology is so important. With a simple example he showed that a 6% error in wind speed can lead to an error of 30% in power. As the share of wind energy will only increase, every bit gained in forecast accuracy will pay off. When it comes decimal differences, small- to mesoscale effects that were perhaps deemed negligable in the past now become important. The main focus of the talk was on offshore wind energy, and it was based on measurements and simulations for the North and Baltic seas, which appeared to be very different. Four overarching research topics served as a common theme through the talk: wind and stability conditions in both seas; the influence of fetch on these conditions; the effect on the power of single wind turbines, and the influence on wind farm wakes.
Indeed, a clear coastal signature is visible in the offshore wind field
One of the differences between the North and Baltic seas is the diurnal cycle. Over the Baltic Sea, there is a pronounced diurnal cycle in e.g. potential temperature, which is shifted in time with respect to the onshore cycle. Over the North Sea the diurnal signature is much less pronounced. Also noteworthy is the predominantly stable stratification due to warm air advection, especially for the Baltic sea. Using a plot of fetch length versus wind direction, Dörenkämper illustrated that this coastal signature corresponds very well with the climatological wind shear. He further elaborated by introducing the concept of a wake rose. By comparing the power between two turbines in a wind farm, it can be used to assess the power loss for each turbine for different wind directions. Strikingly, a power excess was found for wind direction that was more or less parallel to a coastline and as such represented a strong gradient in fetch length. Simulations with the Weather Research and Forecasting mesoscale model demonstrated that the model is able to reproduce this phenomenon: indeed, a clear coastal signature is visible in the offshore wind field. The coastal effect is different for stable and unstable conditions. For unstable conditions, the wind field closely resembles the coastal structure, whereas for stable conditions elongated wind streaks are visible. This result is also visible on satellite images. The streaks seem to be related to the heterogeneous coastal land-use.
With respect to the impact of stratification on individual turbines, Dörenkämper proceeded to show that under stable conditions, the power is substantially lower than what would be expected with the standard IEC (International Electrotechnical Commission) power curve. Under unstable conditions, the power yield is higher instead. This effect can lead to a 20% difference for a single turbine! Similarly, the wake effect is dependent on atmospheric stratification if wind speeds are low. Examining the wake effects by wind direction, it appears that the difference between stable and unstable situations is strongest for turbines that are spaced far apart.
In the final part of his talk, Dörenkämper showed the results of a large-eddy simulation study. They had implemented an enhanced actuator disc model that accounts for lift and drag forces, as well as the rotation of the rotor disc. Stable, neutral and unstable inflow conditions were generated by imposing a change in surface temperature. The results indicate that stable conditions lead to stronger and longer wakes. This is because under neutral and unstable conditions we see expansion of the wakes and recovery of momentum due to vertical mixing. Another noteworthy result is that for unstable conditions, wakes start to show directional variations because of the presence of roll convection.
There is still much work to do. Two important topics that Dörenkämper will proceed to investigate are the structure and behaviour of the internal boundary layers, and the interaction between different wind farms.