Low Wind Speed and Medium Wind Speed Turbines or New Large Diameter Turbines with Low Generator Ratings

By Paul Gipe

Technology Makes High Penetration of Wind Energy More Likely

Reduces Need for Storage and New Transmission Capacity

Michael Hoexter asked me what I thought of all the excited talk about new wind turbines with high capacity factors. I get such requests all the time. People I’ve never met or heard of writing a one line query asking me what I think of such-and-such a wind turbine. Hoexter is different though. He is an insightful analyst of economic and energy policy and he knew there must be more to the story than what he found from the fluff pieces circulating on the web.

My answer was short: “It’s about time.”

That brief answer says a lot and was sufficient for the time being, because a fuller answer required a lot of explanation, and not a little bit of history.

I’ve written about this general topic in the past and there was a nagging thought that I’d examined this very question in one of my books. As it happens, I wrote about this subject nearly twenty years ago in Wind Energy Comes of Age (1995), and I’d been writing about power ratings of wind turbines for much longer.

As I said, “It’s about time.”


Others have written about this topic. I recommend the technical articles by the Deutsches Windenergie Institut’s Jens-Peter Molly, Rated Power of Wind Turbines: What is Best?, and French industry analyst Bernard Chabot’s Wind Power Silent Revolution: New Wind Turbines for Light Wind Sites. Eize de Vries at Windpower Monthly has also covered this subject with his Finding the optimum low-wind design combination.

In short, manufacturers are now offering very large diameter wind turbines with relatively low power ratings for low to moderate wind speed sites. For example, a wind turbine that would have been rated 3 MW a few years ago is now being offered as a 2 MW turbine, sometimes even less. These wind turbines will deliver high capacity factors, and thus, the reason for the hype.

Is this a good thing? Yes, absolutely. Here’s Chabot’s summary of why:

  • More generation and higher penetration rates relative to installed capacity,
  • Expanded opportunities through use of lower-wind sites,
  • Less opposition to wind as less high-wind, high-value sites are now required,
  • Less demand on grid operators,
  • Less demand for new transmission capacity or capacity upgrades, and
  • Wind turbines with large rotors relative to their generator size will allow easier integration of wind energy in the grid, and allow us to put the wind turbines where the people are, that is, near our cities, towns, and villages.

Why this is so is more complicated.

What is a Wind Turbine?

In essence, a wind turbine is a rotor to capture the wind and a generator to produce electricity. Long ago I learned from physics professor Vaughan Nelson at West Texas State University that it is the area of the wind stream intercepted by a wind turbine—the swept area—that largely determines how much energy the wind turbine will capture.

Obviously, the generator is a critical component, but it is not the most critical component in what makes a wind turbine—a wind turbine. It is the rotor powered by the wind that separates a wind turbine from a steam generator, for example.

Generator Ratings

Wind turbines are designed with a specific combination of rotor and generator for a specific wind resource.

While I am in the habit of referring to wind turbines by their rotor diameter—shorthand for the turbine’s swept area—the media, utility engineers, and even some in the wind industry simply use the generator size in kilowatts (kW) or megawatts (MW).

The generator will produce it’s “rated power” at a certain wind speed. (See Wind Turbine Rating for a fuller explanation.) The wind doesn’t always blow at this speed and this is where descriptions such as this complicate our understanding of what a wind turbine will produce.

Swept Area Trumps Generator Ratings

I’ve railed against the use of generator ratings as a measure of wind turbine size or potential for decades now. See Generator Ratings & Capacity Factors: Why You Should Avoid Them, where I used the example of a V82 and a V80.

The Vestas V82 is a larger—more powerful–wind turbine than the Vestas V80 even though it is rated at 1.65 MW and the V80 is rated at 1.8 MW. How can this be?

The V82 intercepts ~5% more of the wind stream than the V80. For low and moderate wind sites, the V82 will out produce the V80. At higher wind sites where the V80’s larger generator will be used more often, the V80 will generate slightly more than the V82.


To illustrate that this isn’t a quirk, here’s another example. At one time, Nordex offered its N80 rated at 2.5 MW and its N90 at 2.3 MW. The N90 sweeps ~25% more of the wind stream than the N80 and consequently, generates considerably more electricity—even though it has a lower generator rating than the N80.

One last example to illustrate the point. GE’s tried and true 1.5 MW platform has been on the market for at least a decade. It began with a 71-meter diameter rotor and evolved to the 1.5 MW SL with a 77-meter diameter rotor. Though both turbines used the same generator rating the SL used a rotor that intercepted 18% more of the wind stream. Consequently, the 77-meter turbine would generate more electricity even though it had the same generator as the earlier model.

The GE example is noteworthy because GE expanded the platform even further with its 100-meter diameter model rated at 1.6 MW, effectively doubling the turbine’s swept area relative to its earliest model.

Metrics of Productivity

There are two principal metrics used to describe the productivity of wind turbines: capacity factor and annual specific yield.

Capacity factor is a measure of how much electricity the wind turbine produces relative to how much it would have produced if the turbine had run all the time at full capacity. It’s a common measure in the utility industry but often misused when applied to variable sources of generation such as wind and solar energy.

Plant factor is the British expression for the same concept. Continental Europe uses full-load hours, a more direct expression than capacity factor. It is simply how many hours the turbine would have produced annually at full output.

The most useful measure of wind turbine performance is how many kilowatt-hours of electricity a wind turbine generates relative the area swept by the rotor in kWh/m2/yr.

For a fuller discussion of this, see Measures of Wind Turbine Productivity.

Measures of Relative Swept Area

There are two measures of relative swept area, that is, how much of the wind stream a wind turbine intercepts relative to its generator capacity: specific capacity and specific area.

Specific capacity or specific power has been traditionally used and is presented in either watts per square meter of rotor swept area, W/m2, or kilowatts per square meter of rotor swept area, kW/m2. The new large diameter wind turbines have very low specific power. GE’s 1.6 MW, 100-meter diameter turbine has a specific capacity of 204 W/m2. Lower specific power delivers greater capacity factors than turbines with higher specific power for the same wind conditions.

French renewable industry analyst Bernard Chabot principally uses specific area because it is simpler to interpret—a higher number offers better performance for the same wind resource. Specific area is in units of m2/kW. GE’s 1.6 MW, 100-meter diameter turbine has a specific area of nearly 5, almost double that of turbines marketed in the mid 2000s.

The design of the Vestas V80 in our earlier example was typical of its day: 358 W/m2, or 2.8 m2/kW. The V82, in contrast had slightly less rotor loading than the V80. The GE 1500 was much more aggressive with a specific power of more than 400 W/m2, or 2.4 m2/kW.

Historical Abuse of Power Ratings

The problem with using measures of capacity (power) in describing wind turbines arises because it is easy to abuse.

Technically, wind turbines can be designed with high power ratings relative to their swept area for very windy sites. Nevertheless, some manufacturers have played on this and marketed wind turbines with very large generator ratings relative to the turbine’s swept area.

Compare the turbines in the preceding example with those in the following table.

Why? Because unsophisticated buyers often compare wind turbines on their installed cost relative to their installed capacity. By inflating the wind turbine’s rating, the manufacturer can charge more money than a competitor and still look cheaper in $/kW of installed capacity.

The most notorious example was Fayette Manufacturing. Their unreliable turbines were the bane of the California wind industry during the 1980s. Fortunately, all but one have been removed.

Manufacturer’s of Vertical Axis Wind Turbines (VAWTs) have also been prone to overrate their turbines compared to conventional wind turbines. The two FloWind turbines in this example are Darrieus wind turbines.

See Specific Rated Capacity of Wind Turbines in the 1980s for an expanded list.

Suffice it to say that despite their high power ratings these wind turbines did not generate any more electricity than other wind turbines with comparable swept areas. Because they didn’t generate any more electricity than other turbines of similar size, they produced very low capacity factors.

Wind Turbine Design and Wind Regimes

To designate which wind regimes wind turbines were designed to withstand, the International Electrotechnical Commission (IEC) set a design standard. The standard defines what wind conditions the wind turbines must endure. There are four IEC classes.

IEC Class I is for the windiest sites, those with an average annual wind speed of 10 m/s. Class II is for less windy sites with an average wind speed of 8.5 m/s at hub height. Class III is for even lower wind sites with an average wind speed not to exceed 7.5 m/s. Class IV is for very low-wind speed sites with an average wind speed of 6 m/s.

Not all wind turbines, then, are created equal. Some turbines are designed for exceptionally windy sites, and others for low wind sites. As a result the specific capacity and specific area will differ between the different IEC classes.

Wind turbines with low specific capacity or high specific area may only be suited for Class II or Class III sites. Whereas, wind turbines designed for Class I conditions can be used at any site, but because they have comparably higher specific capacity or lower specific area, they will produce lower capacity factors at low wind sites.

For example, a Class I wind turbine will have a specific capacity of approximately 400 W/m2, or a specific area of 2.5 m2/kW.

Whereas, Class II turbines will have specific capacities of 300 W/m2 to 400 W/m2, or a specific area of 2.5 m2/kW to 3.0 m2/kW.

And Class III turbines will have specific capacities of 200 W/m2 to 300 W/m2, or a specific area of 3.5 m2/kW to 5.0 m2/kW.

Specific Capacity/Area and Capacity Factor

For similar wind conditions, a wind turbine with a low specific capacity or a high specific area will produce a higher capacity factor or more full-load hours in the European system. As noted above, this relationship doesn’t hold across all wind regimes because some wind turbines are not suitable for some wind regimes. Low wind, IEC Class III turbines are not suited for IEC Class I or Class II conditions. Nevertheless, this relationship between capacity factor and specific capacity/specific area explains why manufacturers can advertise high capacity factors and—more importantly—deliver high capacity factors in the field.[1]


Why All This is Important

What is revolutionary about the new low specific capacity-high specific area turbines is not that they exist—there have always been such turbines—it is that the manufacturers and wind developers as well have finally embraced them. In the US, for example, Lawrence Berkeley’s Ryan Wiser reports that the average specific power of newly installed wind turbines has dramatically fallen from 400 W/m2 in 1998 to 283 W/m2 in 2012.

For many years those who wanted to use wind turbines in lower wind regimes, typically near where people live, were forced to use wind turbines that were designed for high wind sites. While such turbines were adequate for the task they produced very low capacity factors. This was acceptable as long as wind energy was a small part of the generating mix and there was more than sufficient capacity on the wires and electrical infrastructure to absorb peak power on those occasions when it occurred.

Manufacturer’s, meanwhile, were selling to commercial wind developers who pick the windiest sites possible to maximize their profits. This was the traditional model of power plant development since the 1940s: power plants were installed where the resource was most abundant often quite distant to where the electricity would be used. It wasn’t always so. In the early days of electricity, power plants were built in the cities where the demand was.

All this began to change as more and more of the high-wind regime sites were developed and the bottlenecks to long-distance transmission of electricity became more problematic.

Countries such as Germany and France went so far as to implement policies—feed-in tariffs differentiated by wind resource—that would enable development at lower wind speed sites. They reasoned that it would be better for the nation if wind development was not solely concentrated on the windiest coastlines or windiest mountaintops but distributed across the breadth of the country. Not only would this simplify integration of wind energy with the transmission and distribution system, but it would also reduce social conflicts to those opposed to wind turbines in scenic, but windy locales. Additionally, these policies were intended to spread the economic opportunity from wind development to all regions.

France and Germany have been successful in this regard. Wind development is geographically dispersed in both countries. In Germany it’s not uncommon to see wind turbines near the great urban agglomerations. For example, numerous wind turbines are visible from the inland harbor of Hamburg, and even from the urban core of scenic Freiburg in southern Germany.

Like his French colleague Chabot, Quebec engineer Bernard Saulnier believes the new IEC Class III turbines are not only revolutionary because they allow deploying new wind generating capacity in lower wind speed regions, but also because—whether they realize it or not–the manufacturers have declared war on the centralized generation model and the long transmission lines that are an essential part of that model.

This is good news to many environmentalists who have objected to the long-distance transport of electricity. Many environmentalists prefer that generating capacity should be “distributed” among the users of electricity so that transmission, or at least new transmission lines, are not needed. Distributed generation implies putting wind turbines and solar panels in or near urban areas where consumption is greatest.

German wind engineer Jens-Peter Molly also points out that low specific power-high specific area turbines use the existing network so much more effectively that it drastically reduces the need for storage of a variable resource like wind energy. Thus, argues Molly, we can rethink how best to integrate the high penetration of wind energy into the grid. Incorporating these new wind turbines in the transmission and distribution system would be much more cost-effective than adding expensive storage facilities or expanding transmission capacity with thicker cables on existing lines, or installing controversial new power lines.

Molly explains it this way.

If a wind turbine of 100m rotor diameter were equipped with a generator of only 1 kW size, it should be clear to everyone that this wind turbine could run throughout the whole year at rated power without requiring expensive storage facilities or over dimensioned grid connections because the capacity factor or the guaranteed power capacity would be almost 100%. The remuneration for the kilowatt hour generated in this way, however, would have to be very high, because with only 8,760 kWh generated at best, the expenditure for the large rotor, bearings, tower, foundations etc. could not be paid otherwise.

On the other hand the same rotor diameter could be coupled with a 10 MW generator. In this case, the wind turbine would generate the rated power only for a few hours a year, in other words, enormous costs for the mechanical and structural components of the turbine which are out of all proportion to the increased yield of the wind turbine. Even the cross-sections of the transmission lines would have to be dimensioned so as to be able to transmit the rated power generated only during a few hours per year and would be under-utilized and therefore much too expensive.

Between these two extremes there must be an optimum, says Molly. And that is why there is now a range of wind turbine designs for different wind resources.


IEC Class III turbines are not robust enough for high wind sites, says Molly, but they are well suited for large areas of countries, such as Germany, where the majority of people live and work—where the load is. He argues that it is much cheaper to pay a little extra for the generation from such a wind turbine than to either pay for storage or increased transmission capacity. Fortunately for Germany, the country’s differentiated wind tariffs are easily adapted to this requirement.

Low specific capacity-high specific area turbines increase the average power that can be delivered and it can be delivered for a longer period of time. This improves both their predictability of wind generation to grid operators while also providing the ability to deliver reserve generating capacity for emergencies, such as when a nuclear plant trips offline. And because the difference between average power and rated power is smaller, such wind turbines reduce the need for greater transmission capacity.

French engineer Chabot makes a similar observation, wind turbines with low specific power-high specific area “represent a strategic advantage for the large-scale integration of wind energy in the electricity system, as a much greater amount of TWh would be delivered with a lower total installed capacity and it could be placed nearer the centers of consumption than otherwise, reducing the cost of transmission and distribution. This is a huge advantage, says Chabot, for adapting existing infrastructure to the high penetration of renewables to come.

Long awaited, low specific capacity-high specific area turbines are the kind of technology needed to make wind energy an essential low-cost component of moving society toward 100% renewable energy.

As I said, “it’s about time.”

Bernard Chabot,  Wind Power Silent Revolution: New Wind Turbines for Light Wind Sites, Renewables International, 6 May 2013.

Jens-Peter Molly, Rated Power of Wind Turbines: What is Best?, DEWI Magazin No. 38, February 2011.

Jens-Peter Molly, Design of Wind Turbines and Storage: A Question of System Optimisation, DEWI Magazin No. 40, February 2012.

Eize de Vries, Finding the optimum low-wind design combination, Windpower Monthly, 1 July 2013.

Catherine Early, Low wind focus opens up new markets, Windpower Monthly, 1 July 013.

Geoff Henderson, Technical Comparators: Swept Area preferred over Rated Power, Windflow Technology Newsletter Nº 31, June 2011.

Paul Gipe, Generator Ratings & Capacity Factors: Why You Should Avoid Them, January 23, 2006.

Paul Gipe, Measures of Wind Turbine Productivity, August 2, 2013.

Paul Gipe, Wind Turbine Rating, August 2, 2013.

Paul Gipe, Specific Rated Capacity of Wind Turbines in the 1980s, August 2, 2013.

Lyn Harrison, Power system reserve – No need to build wind back-up, Windpower Monthly, August 1, 2013.

[1] Capacity factor is equal to the annual specific yield divided by 8,760 hours per year times the specific capacity.