The (Lost) Art of Wind Turbine Technology Selection

Cost, Brand Aren’t the Only Factors to Consider

By Aaron Anderson, PE

Wind energy development is complex. It requires careful evaluation of numerous factors, including a site's wind resource, permitting requirements, financing structure, balance-of-plant design and more. Before ground is ever broken, a typical owner invests many years and countless dollars into consideration of these basic elements of wind farm development. However, perhaps no factor influences the long-term viability of a project more than selecting the optimal wind turbine technology. And, unfortunately, most wind farm owners are applying less and less rigor to this critical step.

The primary difficulty with selecting the right technology is that not all wind turbines are created equal. They vary in size, performance, cost, reliability and even appearance. However, as more players continue to enter the growing wind industry, a greater number of inexperienced developers are entering into multimillion-dollar agreements for wind turbines based solely on name recognition or other reasons that are equally unrelated to long-term project viability.

The contributing factors to a failed turbine selection are plentiful, although several have become prevalent. The following is a sample of five common mistakes during wind turbine technology selection, as well as strategies that can make your choice of technology a successful one.

Overvaluing Capacity Factor

Of any metric considered as part of a technology selection effort, perhaps none gets more attention than capacity factor. And, while this efficiency indicator has its place in any wind turbine evaluation, it should not be the only factor in one's decision.

The key issue with overreliance on capacity factor is deception. Consider that in a given layout, it is not uncommon to observe a capacity factor differential of up to 10 percent between competing turbine models. However, many project owners erroneously equate this differential with viability, assuming that the less efficient machine is also less attractive for their site.

On the contrary, if that less efficient machine is also double the capacity (e.g., 3.0 MW versus 1.5 MW), it may produce nearly twice the annual energy despite the 10-point disparity in capacity factor. Recognizing that balance-of-plant construction costs are generally not linear (i.e., it doesn't cost twice as much to build the project with the 3.0-MW turbine), the needle often tends to move toward the bigger machine, particularly at larger wind farms, projects with attractive power purchase agreement (PPA) rates and projects that are not capacity-constrained.

Capacity factor can be a misleading statistic. This metric clearly deserves consideration, but as part of any successful turbine selection strategy, capacity factor should be limited to a financial modeling input. Consideration of this metric in any greater light may result in a highly efficient, yet underperforming, project.

Site Suitability

While capacity factor may be the most mistakenly-relied-upon metric during turbine selection, perhaps none is more commonly misused than site suitability. When selecting an optimal turbine for a particular project site, one must always consider the operating and design characteristics of the machine against that site's terrain and wind resource. Many owners limit this evaluation to selecting a machine based solely on its Internation Electrotechnical Commission (IEC) classification (e.g., IA, IIB, etc.). However, while the IEC guidelines may be useful as a preliminary screening technique, the suitability of a turbine for a particular site extends well beyond these codes.

There are a variety of considerations that go into determining site suitability. While these generally vary by site, there are a few critical questions that should always be asked when evaluating the operating and design characteristics of a turbine.

What is the project terrain like?

A project site with complex terrain may result in areas of significant upflow and elevated turbulence levels, potentially affecting the longevity and long-term energy production of the machine. Similarly, if conditions are sufficiently severe, it may be necessary to use a higher-classification machine than would otherwise be called for by the IEC guidelines.

What is the wind shear across the swept area of the turbine?

Evaluation of wind shear is common, although most assessments end at hub height. However, it is important to recognize that shear can decrease (or even become negative) across the top half of the swept area. Take care in crafting a realistic estimate of shear that not only extrapolates from the top of the met mast to hub height, but also from hub height to the upper tip of the blade. An unrealistic estimation of shear may cause energy production to be overstated and turbine loading conditions to be incorrectly modeled.

What is the shape of the project site's wind distribution?

Understanding the average wind speed at a site provides only an initial gauge of how well a turbine will perform. More critical is the distribution of the wind speeds that produce that average, as a turbine may perform starkly different if a site has a high occurrence of upper-level wind speeds (i.e., low Weibull k value) compared to a site with a high frequency of mid-range wind speeds and few near the upper range of the power curve (i.e., high Weibull k value).

For example, Figure 1 illustrates the difference between two sites with an average annual wind speed of 8 meters per second. On the left, the low Weibull k value indicates a higher occurrence of upper-level wind speeds than the chart on the right, which has a higher k value and, as a result, a tighter distribution of wind speeds. Using the same 1.5-MW turbine mentioned previously, nearly 12 percent more energy would be produced based on the curve on the right despite the same average wind speed, due solely to this distribution better matching the production characteristics of the turbine.

Bid Normalization

Following an evaluation of the performance and suitability of a turbine, a request for proposal will generally be issued to procure the machines. However, another commonly overlooked technique during turbine selection is the appropriate normalization of bids.

When turbine supply proposals are received, the first step should always be to identify the applicable differences in scope. These differences may range from equipment supply (e.g., is the turbine furnished with an internal transformer or will it require an owner-supplied pad-mount?) to service offerings (e.g., is the supplier installing tower cabling or is the owner's contractor required to do it?). Similarly, different turbine manufacturers may offer varying warranty periods; climb assists versus service lifts; differing quantities of spare parts and special tools; varying durations of on-site support during field activities; and other similar disparities.

Regardless of the differences or their subtleties, bid prices must always be adjusted at the onset of any successful turbine evaluation — the goal should be an "apples to apples" comparison wherein all bids are equivalently and uniformly evaluated. Failure to do so may lead to selection of a suboptimal machine for your site, adversely affecting the long-term financial and operational performance of the project.

Service Versus Supply

Another area where a turbine evaluation often goes awry is in the assessment of the service proposals. While the cost of these proposals — generally $35,000 to $50,000 per turbine per year — seems almost negligible when compared to the turbine price, its impact can be substantial.
Nearly every turbine supplier will require the use of their O&M services throughout the turbine warranty period. Not only should this requirement be treated as an intrinsic cost of turbine selection, but it is also important to consider all aspects of the proposal. To more fully understand these, ask questions such as:

  • Does the proposal include both planned and unplanned maintenance?
  • Who pays for the crane if required for repairs?
  • Are spare parts required to be purchased upfront or at the end of the contract?
  • Is my availability guarantee covered by the service agreement, and if so, does that lower the potential limit of liability?
  • Is "in and out" coverage included in the service fee?

Answers to these questions may influence the selection of an optimal turbine, as well as potentially affect the long-term viability and production capability of every machine at the project site. They can also save millions in unplanned expenses over the life of the contract.

Risk Versus Reward

A final oversight commonly observed during the selection of an optimal wind turbine is the appropriate consideration of technology risk. As wind turbines continue to evolve and push the boundaries of technology advancement, manufacturers are constantly tweaking — or even overhauling — design strategies in hopes of squeezing out every last kilowatt from the machine. However, many owners are wholly averse to the risk that any design change introduces.

Every modification to an established turbine design carries some inherent level of risk. For instance, a new blade design may require a change to the vendor's established manufacturing process, introducing new or additional opportunities for laminate wave defects. Similarly, a new direct-drive design may eliminate the gearbox, but it likely also necessitates a radically new generator design and potentially a new generator supplier, introducing both design and supply chain risk. Regardless of the change, the risk associated with each modification should be captured and, to the extent possible, quantified.

A simple yet effective tool for visualizing risk is a "risk matrix," similar to that shown below (Figure 2). This tool graphically depicts the inherent risk associated with any potential issue by simultaneously capturing the probability of that issue occurring along with the impact if it were to occur.
As an example of how the risk matrix would be used, again consider a change in blade design. The impact of any issue involving blades will likely be significant as energy production, and ultimately revenues, will be reduced. However, if the probability of these issues appearing is low, the overall risk would likely only be quantified as moderate (dark blue). Thus, the owner may be able to accept this risk in consideration of the potential benefits it should yield.

Critical Consideration Is Required

Wind turbine selection is difficult. The pitfalls are abundant and a wrong choice may have years of lingering consequences. Similarly, picking the right machine goes well beyond a familiar name or the lowest price. It requires recognition and consideration of the countless dynamics involved with the single most important asset at a wind farm. Anything less is risk without reward.


About the Author

Aaron Anderson, PE, is a mechanical engineer and project manager in the Renewable Energy Group at Burns & McDonnell. He specializes in financial and engineering analyses of renewable and alternative energy projects, including contractual negotiations, project development, asset due diligence assessments, technology evaluations, resource and energy assessments, wind farm design, and project economic and strategy evaluations.

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