Presentations

Abstract

Accurate energy yield assessments (EYA) are critical for long-term financial viability of wind energy projects, yet significant discrepancies between pre-construction estimates and actual operational production remain a common industry challenge. Understanding the root causes of these variances is essential for reducing bias and providing greater confidence in wind farm development and investment.  This work presents comparative real-world case studies for several development and operational projects in Finland. The catalyst for this study was the identification of significant divergences between pre-construction and operational EYAs for a variety of projects from different wind farm owners and operators across Finland.  Until now pre-construction assessment methods have focussed on wind resource at hub height, and it is often assumed that a logarithmic boundary layer wind speed profile and relatively constant wind direction occurs across a turbine rotor.  With the increase in turbine size and hub heights, particularly in relatively low wind markets such as Finland, we are regularly seeing tip heights up to 250 m.a.g.l, with shear and direction assumptions requiring additional scrutiny.   Wind data from tall masts and lidars in Finland has been analysed. A relatively high frequency of unusual wind flow profiles was observed in lidar data, with periods of negative wind shear in the upper part of the rotor, despite a more normal wind shear up to hub height. Additionally, periods of wind direction veer above 30° across the measurement range have been identified. With many historical pre-construction assessments based on masts only, such flow characteristics would be missed even when utilising a best-practice hub-height mast. Whilst lidars offer the opportunity to measure above hub height, reduced lidar data availability has sometimes been observed to often correspond with such unusual flow events.  Turbine power curves are typically valid for a narrow range of flow conditions. Measurements at some Finnish sites indicate that turbines could be operating outside this range over 75 % of the time. It is considered that a proportion of the observed discrepancy between the pre-construction and operational EYAs is due to lower-than-expected wind resource across the entire rotor and reduced performance due to these unusual wind conditions.  This was investigated by reviewing SCADA data from turbines with co-located lidars. To understand the impact on operational performance and isolate specific loss drivers, factors such as rotor equivalent wind speed ratio, atmospheric stability, temperature profile, shear, veer, and turbulence are considered in order to assess potential methods for improved estimation of wind resource and turbine performance in pre-construction analysis. With the issues surrounding reduced lidar availability in such flow conditions, potential proxies from reanalysis data have been investigated to ensure that the actual frequency of unusual wind flow conditions is understood. Finally, methodologies for the prediction of the impact of unusual wind flow on wind resource and turbine performance are proposed, so that such conditions can be considered in pre-construction energy yield assessment, and greater confidence in project investment decisions is obtained.