An evaluation of the predictive accuracy of wake effects models for offshore wind farms

Abstract

Wake losses are perceived as one of the largest uncertainties in energy production estimates (EPEs) for new offshore wind projects. In recent years, significant effort has been invested to improve the accuracy of wake models. However, it is still common for a standard wake loss uncertainty of 50% to be assumed in EPEs for new offshore wind farms. This paper presents a body of evidence to support reducing that assumed uncertainty. It benchmarks the performance of four commonly used wake models against production data from five offshore wind farms. Three levels of evidence are presented to substantiate the performance of the models:• Case studies, i.e. efficiencies of specific turbines under specific wind conditions; • Array efficiencies for the wind farm as a whole for relatively large bins of wind speed and direction; and • Validation wake loss, which corresponds to the overall wake loss within the proportion of the annual energy production where validation is possible.The most important result for predicting annual energy production is the validation wake loss. The other levels of evidence demonstrate that this result is not unduly reliant on cancellation of errors between wind speed and/or wind direction bins.All of the root-mean-squared errors in validation wake loss are substantially lower than the 50% uncertainty commonly assumed in EPEs; indeed, even the maximum errors are below 25%. It is therefore concluded that there is a good body of evidence to support reducing this assumed uncertainty substantially, to a proposed level of 25%.

A linearized numerical model of wind-farm flows

“…Based on the analytical approach by Belcher et al, 20 which describes how a 2D turbulent boundary layer adjusts when facing a canopy of roughness elements, the proposed force model for wind turbines is f i D I f jUj U i . Hereby, the force intensity, I f .x, y, z; C T /, depends on the thrust coefficient and characterizes the body force similarly to the definition of the thrust force in (2). To be consistent with the linearization, the only force term should be f x D I f U 2 0 at the leading order, but this can only be carried out in case of weak canopies.…”

“…Wake effects must be considered during the layout optimization since the power losses due to wind turbine wakes can reach up to 20% in large farms . Besides that, wake losses are perceived as one of the largest uncertainties in energy production estimates for new offshore wind projects, and it is still common to assume a standard wake‐loss uncertainty of 50%, even if recent studies state that there is a good body of evidence to reduce this assumed uncertainty to 25% or even 15%.…”

Transient LES of an offshore wind turbine

“…One of the reasons for the comparatively high costs of offshore wind energy is the scarcity of long-term atmospheric measurements at existing or planned wind farms. The resource assessment at these locations is difficult and prone to large errors (Walker et al, 2016). In addition, missing measurements during operation prohibit the thorough analysis of turbine malfunctions and unexpected underperformance.…”

3-D shear-layer model for the simulation of multiple wind turbine wakes: description and first assessment

“…In this sense, wakes are the main cause of power losses in wind farms (Walker et al, 2016). In addition, wakes hitting a downstream turbine contribute to the increase in the fatigue loads of its components.…”