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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Wakes: Do we need different models for onshore and offshore wind farms?' taking place on Wednesday, 12 March 2014 at 16:30 -18:00. The meet-the-authors will take place in the poster area.

Patrick Volker DTU, Denmark
Co-authors:
Patrick Volker (1) F P Jake Badger (1) Andrea Hahmann (1)
(1) DTU, Roskilde, Denmark

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Abstract

Wind farm group efficiency - a sensitivity analysis with a mesoscale model

Introduction

In the future, power production from offshore wind farms is expected to increase significantly in the North Sea. Furthermore other countries have recently announced plans of developing offshore wind farms. The study of wind farm interaction is complex, since a large range of scales are involved. Whereas near turbine wakes are dependent on local turbine characteristics, wind farm wakes become a function of even the Coriolis force. Through projects, as for example the European Energy Research Alliance-Design Tools for Offshore wind farm Cluster (EERA-DTOC), we try to obtain an understanding of the importance of the various processes


Approach

It has been shown that high resolution models, such as Reynold Averaged Numerical Simulations or Large Eddy Simulations, can simulate wind turbine wakes fairly precisely. With the increasing computational power, LES simulations over entire wind farms can be performed. However, those simulations remain expensive. Mesoscale models, on the other hand, have coarse horizontal grids, but account for large scale flow properties. The drawback is that single turbine-induced wakes remain unresolved and have to be parametrised. It is shown that despite the lack of horizontal resolution the velocity development within and in the wake of the wind farm is well described.


Main body of abstract

For this investigation two wind farm parametrisations are used. The first is implemented in the publicly available Weather Research and Forecast (WRF) model and the second is developed at DTU Wind Energy and implemented in the WRF model. The WRF mesoscale model is set up in the idealised case mode. No forcing is applied at the lateral boundaries and the surface heat flux is set to zero. In this configuration the atmosphere develops a neutral boundary layer, with a changing wind speed in height due to the Coriolis force. Under these conditions we simulate a reference atmosphere, without wind farms, and one including wind farms.

To analyse the sensitivity of the shadowing effect of the up stream wind farm, we varied the wind farm size, their spacing and the atmospheric conditions. Their sizes vary from wind farms with a nominal capacity of Horns Rev I to sizes comparable to the London array. The spacing ranges from 15 to 60 km. Additionally we consider different atmospheric conditions in order to examine their relative importance in the wind farm wake recovery process. The inversion, that caps the planetary boundary layer, together with the turbulent momentum fluxes from aloft, control the wind farm wake extension, as well as its orientation. The inversion strength determines the behaviour of the wind farm induced horizontal pressure gradients. The second process regulates the amount of horizontal wind speed, in the stream and cross stream direction, that is transported downwards.



Conclusion

Since offshore wind energy is expected to increase in the future and suitable areas for the construction of offshore wind farms are limited, the study of wind farm interaction will gain importance. Within a wind farm group, we analyse the effect of wind farm size, spacing and atmospheric conditions, on the production of individual wind farms. We find that the production of downstream wind farms varies considerably with wind farm distances and atmospheric conditions. Moreover, it has to be considered that ahead of the wind farms unfavourable pressure gradients are induced.



Learning objectives
With this study we want to achieve a better understanding of the effect of wind farm interaction on the efficiency of wind farm groups. In the analysis we account for wind farm size, spacing and for atmospheric effects from the inversion layer and the lower troposphere.