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Reassessing a wind farm’s potential


The ever-growing database of offshore wind farm production data provides the opportunity to improve the understanding of wind energy potential at existing and future sites significantly. In principle, the wind turbines provide a dense measurement system of the wind conditions. These need to be distinguished from all other factors that drive production at the site.

The ‘post-construction analysis’ of the wind farm production data can provide the tools for this exercise. The industry-wide goal should be to make full use of the potential of the production data in order to gain a better understanding of offshore wind farm performance in the past and in the future.

The results of a post-construction analysis of production data can have major implications not only for the shareholders of a particular wind farm, but also for the industry as a whole.

For wind farm operators, insights into wind farm performance can have a decisive influence on detailed follow-up investigations or negotiations with manufacturers.

Wind farm shareholders, on the other hand, can refer to the results in connection with refinancing and the selling of shares. For operators of multiple wind farms, the question also presents itself of what can be learned from the performance of the farms and the portfolio to date for future projects.

The industry-wide implications became more than evident when a press release from one of the largest offshore shareholders, Ørsted, created quite a stir in 2019, claiming that revenues of around two percent lower should be assumed when planning offshore wind farms. Internal data analyses had revealed that the wind farm wakes, and the large-scale blockage of the flow had been significantly underestimated in the yield assessments up to that point. In an industry where yield projections are generally calculated using similar models, the report created unease about the validity of the applied methods. The question of how credible the projections were and whether fundamental investigations of all methods are necessary has been a topic of lively discussion ever since.

The issue for the wind farm operators is that, especially in the early years of a wind farm’s operation, the pre-construction site assessment is the only reference for the measured production. This assessment usually relies on a short-term wind measurement campaign at usually just one or few locations across the site’s area. The projection to the whole site area and to the operational period as well as the estimation of the wind farm downtimes and wind turbine component failures all have to rely on models or educated guesses. However, with the right reference data, an analysis of the first months of operation can already reduce the uncertainty about the farm’s long-term potential.

So, which data needs to be manipulated for the wind farm to reveal its potential?

First, the performance assessment of wind farms only makes sense in relation to the prevailing wind conditions. In 2017, Fraunhofer IWES addressed the lack of offshore wind indexes with highly resolved numerical weather simulations aggregated in the Fraunhofer Offshore Wind Index (FROENIX). Offshore, these optimized meteorological simulations can provide a precise estimate of the wind resource. The indexing of the wind farm production already prevents many unnecessary discussions about the state of the wind farm in months or years with a low wind resource.

Second, corrections for technical downtime and enforced power restrictions by the electric grid operator need to be applied to unmask the production potential at the site. This can be done by calculating the amount of underproduction during these periods. An onshore guideline for this procedure exists in the German Technical Guideline 10 (TR10). However, for application offshore, the different characteristics of offshore sites and wind farms need to be considered.

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