Windpower Engineering & Development

  • Home
  • Articles
    • Most recent posts
    • News
    • Featured
  • Resources
    • Digital issues
    • Podcasts
    • Suppliers
    • Webinars
    • Events
  • Videos
  • 2025 Leadership
    • 2024 Winners
    • 2023 Winners
    • 2022 Winners
  • Magazine
  • Advertise
  • Subscribe

Wind project performance assessment: How to better understand the risk

By Paul Dvorak | October 4, 2012

A wind-flow model has underestimated wind-speed variations across most of a site. On hill tops, production meets prediction, but at most other locations it does not.

North America’s wind power industry has developed an unfortunate reputation for producing energy at levels less than those predicted by pre-construction energy assessments. While not all projects underperform, many do.

Ongoing research suggests the five factors covered in this article are major contributors to over prediction in energy assessments. This article provides questions that project investors can ask to help determine whether these factors have been adequately addressed. Project developers can infer from these questions ways to reduce the risk that these factors will bias a project energy assessment.

 

Flow modeling bias

The wind flow models commonly used in the industry often do a poor job of modeling wind flow across a site, with a tendency to underestimate variability in wind speeds, even in relatively simple terrain. Consider asking:

  • Has the meteorlogical (met) measurement campaign captured the range in the wind resource and characterized the impact of terrain and vegetation variation on the resource?
  • Is the exposure of the met towers consistent with the exposure of the turbines and have the relative exposures been considered when estimating the uncertainty in the analysis?
  • Has the ability of the model to cross predict the met towers on site been examined and reflected in the uncertainty analysis, with consideration for the relative exposures of the met towers and turbines?
Uncertainty in data

Unknown factors such as measurement consistency and correlation quality may increase uncertainty when using data from a long-term reference site.

Wake model bias in some conditions

Wake models typically used in the industry produce results reasonably close to actual performance on average for some project configurations and atmospheric conditions, but can also produce large over and under-estimation under other conditions. Consider asking:

  • How were variations in atmospheric stability from the average and the relationship between atmospheric stability and the wind speed frequency distribution considered in the wake modeling?
  • Do the commercial terms make meaningful provisions for testing the actual impact of a new project on another?

Project availability

Project availability has been a significant contributor to project underperformance. There are dozens of contributors to project downtime. It is insufficient for an energy assessment to assume that turbine availability will meet the manufacturer’s warranty. So consider asking:

  • Does the assumed availability decrease over time as the turbines age?
  • Have design teams considered all sources of unavailability and the difference between time lost and energy lost?
  • What is the availability track record of the organization that will be providing long-term O&M on the project?

Atmospheric stability

Recent research in North America has shown a notable reduction in power output at a given wind speed in stable conditions, which in many North American locations is typically characterized by a steep wind-shear gradient below 60 to 80 meters. The gradient may even turn negative above 80 m. Knowing the average shear exponent and verifying it with remote sensing are frequently insufficient. Consider asking:

  • Has at least a year of remote sensing data been collected at a single point on the site?
  • Has the design team considered potential for the shear profile across the rotor to differ from the 40 to 60-meter measurement?
  • Have seasonal and diurnal variations in wind shear been considered in relation to the wind speed frequency distribution?

Long-term adjustments

Knowing the correlation between on-site measurements and a nearby long-term reference station is usually insufficient. Making a long-term correction may increase uncertainty. Consider asking:

  • Does analysis show that the uncertainty in the result is lower with the long-term correction than without the long-term correction?
  • Have the following sources of uncertainty been considered? Period of record, Climatological changes, Quality of correlation, or Known and unknown inconsistencies over time

Asking the right questions prior to commencing an assessment for wind resource and project energy can significantly reduce over prediction and reduce uncertainty, making the wind industry more attractive to investors in the long term.

Michael Drunsic

Michael Drunsic, Head of Section, Energy Analysis DNV KEMA, Energy & Sustainability

Robert Poore, DNV KEMA Vice President of Business and Service Development


Filed Under: Featured, Financing, News
Tagged With: DNV KEMA, Risk
 

About The Author

Paul Dvorak

Related Articles Read More >

US government allows Empire Wind offshore project to resume construction
Richardson Electronics to deliver pitch energy modules to TransAlta wind fleets
Equinor halts work on Empire Wind offshore project after federal government order
ARESCA wants input on offshore wind standards

Podcasts

Wind Spotlight: Looking back at a year of Thrive with ZF Wind Power
See More >

Windpower Engineering & Development Digital Edition Archive

Digital Edition

Explore the full archive of digital issues of Windpower Engineering & Development, presented in a high-quality, user-friendly format. Access current and past editions, clip, share, and download valuable content from the industry’s leading wind power engineering resource.

Windpower Engineering & Development
  • Wind Articles
  • Solar Power World
  • Subscribe to Windpower Engineering
  • About Us/Contact Us

Copyright © 2025 WTWH Media LLC. All Rights Reserved. The material on this site may not be reproduced, distributed, transmitted, cached or otherwise used, except with the prior written permission of WTWH Media
Privacy Policy | Advertising

Search Windpower Engineering & Development

  • Home
  • Articles
    • Most recent posts
    • News
    • Featured
  • Resources
    • Digital issues
    • Podcasts
    • Suppliers
    • Webinars
    • Events
  • Videos
  • 2025 Leadership
    • 2024 Winners
    • 2023 Winners
    • 2022 Winners
  • Magazine
  • Advertise
  • Subscribe