Capacity factor and generator power per swept rotor area

Engineering System Design
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AWE Advantages
More full load hours, more constant electricity production, less intermittency, better
system integration
Higher capacity factor (60%-80% or beyond) compared to conventional wind
(30-60%) and solar (15-25%)

The expected higher capacity factor may result from more regular and constant high-altitude winds.

AWE can also benefit from a more flexible swept area by adapting to wind speed. The following methods (and others) can be explored: varying the kite’s trajectory amplitude, varying the altitude to take advantage of the wind gradient, and adding or removing units for a kite train (e.g., stacked Daisy).

If even more consistent power production is desired than is possible with high-altitude winds, the swept area can be increased based on a lower “generator power per swept rotor area”.

The expressions “generator power per swept rotor area” or “rotor loading” are used by the expert Henrik Stiesdal in his laudatory evaluation of the KiteX wind turbine which has been the subject of numerous comments and several topics on this forum.

The most compelling small-scale wind power concept I have encountered to date.

Henrik Stiesdal

Pioneer of the modern wind turbine industry

A relevant excerpt:

The TWT11 exhibits a rotor loading of under 45 W/m² (generator power per swept ro-
tor area), which is significantly lower than the 200–250 W/m² typical of modern on-
shore wind turbines. This low loading supports efficient energy capture at moderate
wind speeds. Based on preliminary assessments, I consider it plausible that under
typical onshore European conditions the TWT11 could achieve capacity factors in the
range of 30–40%, despite the relatively simple blade aerodynamics. This would
translate to an estimated annual energy yield of 13,000–17,000 kWh.

Note that the giant wind turbine below has a rotor loading slightly higher than the 250 W/m² indicated above.

SANY SI-270150 - 15,00 MW - Éolienne

Densité de puissance 1: 262.0 W/m²
Densité de puissance 2: 3.8 m²/kW

Densité de puissance (in French) is power density (translated into English) or “rotor loading” or “generator power per swept rotor area”.

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This is all getting very confusing. Right after scolding that multi-savonius “full power at 3 m/s” guy for bragging about a huge amount of swept area compared to power output, you start congratulating KiteX for the same exact thing. Meanwhile “W in Disguise” decides to “close” then “reopen” the topic so I end up scrolling through all sorts of confusing links, having no idea where I am in the scheme of things and whether this message will even be posted. When i look at the members, everyone such as you, Pierre, Roddy, me, etc. are identifyable people. Everyone except the people arbitrarily deleting, opening, closing etc. people’s messages. All very weird, but as we know, this field is full of weirdos! :slight_smile:

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KiteX is an ultra-lightweight horizontal axis wind turbine for fairly weak winds, and is working. From the website:

The TWT-11 produces ~15,000 kWh per year - enough to cover most of a Northern European farm or rural home - 13× more energy per kg of material than traditional small wind.

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This is nonsense.

In a good wind site (roughly an 11–12 mph or 5 m/s average wind speed), a Bergey Windpower Co. Excel 10 wind turbine produces between 13,600 and 18,000 kWh per year. Under optimal conditions with higher average winds, production can reach up to 25,000 kWh per year. [1, 2, 3]

The Bergey weighs about what, maybe 500 kg? What’s that, about twice what the kiteX turbine weighs? This would mean the more accurate statement would be 2x more energy per kg of material than traditional small wind. If they are somehow adding tower weight, the Bergey COULD be mounted on a much lower tower, but it is usually placed higher for smoother winds and better production. I do agree that a setup that does not require a crane for every service or maintenance issue is a good idea. Really, cranes are getting ridiculously expensive to hire, as in a year or two of electric bills just to hire the crane for a few hours, and so just a few crane sessions in a decade will completely ruin the economics of even bothering with owning a turbine. (How would I know this???) And of course without the current “global warming derangement syndrome” government incentives, the manufacturers would sell pretty much ZERO turbines, since these windmills could never pay for themselves without the artificial incentives. The world needs affordable small wind, and we are nowhere close. Cheap solar panels put almost all small wind companies out of business years ago. Bergey survives only because they spend as much effort lobbying congress and working the government rebate and tax incentive system, as they put into the engineering of their turbines - probably far more. The entire “distributed wind” movement is mostly just Mike Bergey “working the system”, with the rebates and incentives literally crafted around the Bergey Excel 10 so the government officials in charge of regulating “small wind” even have a job. For example, the Bergey Excel 10 uses heavy pultruded blades with way too steep of a pitch at the tips, making for an extremely noisy turbine. Yet partly because of these ultra-heavy blades, the turbines tend to survive strong wind events, and basically there has been no other choice in small wind, so the agencies either approve the Bergey and promote it, or they are out of a job. Meanwhile the Bergey is extremely noisy, under several conditions, including high wind operation, unloaded operation when the inverter “lets go” of the turbine because of excessive output, or even when one small corner of the leading edge tape comes loose, making a horrendous noise in strong winds or especially when running unloaded in high winds. Check out this video I made of my Bergey making blade noise a few years ago:(91) Bergey 10 kW Wind Turbine Leading-Edge Tape Noise - YouTube

But back to the subject: So the long-time small-wind standard Bergey 10k supposedly uses twice as much material for the same energy capture, right? Well, as flawed as the Bergey turbines are, and they are FAR from perfect, I’m pretty sure a Bergey will last WAY more than twice as long as a kiteX product in its current state. Let’s remember the news we just read: The kiteX turbine is awaiting proof that it can last even 6 months. They had better hope for no severe storms during this period. Any turbine that is efficient at harnessing the power of the wind is easily overwhelmed by that same efficiency when strong winds hit! Any new product can CLAIM it “should” last for many years of nonstop use, but anything that CAN go wrong WILL go wrong, and Mother Nature is not kind to wind turbines - especially turbines of light construction. So I hope for the best from KiteX, but only many turbines in many locations for many years will discern whether any model is even marginally reliable. :slight_smile:

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Bergey excel 10, rotor diameter 7 m, so swept area = 38.465 m².

• AWEA RATED POWER: 8.9kW @ 24.6mph (11m/s)

Rotor loading about 8900/38.465 = about 231 W/m².

KiteX:

Swept Area
TWT-11
89 m²

From Henrik Stiesdal’s assessment (initial post):

Hence:

From KiteX’s website:

I would say: Maybe, (or maybe less than) 13× more swept area per kg of material than traditional small wind. I would say about 5x by taking account of rotor loading between 234 W/m² (Bergey) and 45 W/m² (KiteX).

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It seems you may be confusing energy production per kilogram, and energy production per square meter of swept area. If the two turbines have the same energy production per year, and the Bergey weighs twice as much, the ratio is 2x. Am I missing something?

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“2x more energy per kg of material”: it is right.

Right.

It is about swept area.

The higher capacity factor is achieved with a lower rotor loading (W/m²) as wanted by KiteX, so according to the power/swept area ratio, not directly the power/mass ratio. But both power/swept area and power/mass ratios are relevant means to partially assess efficiency.

Also: this comment where KiteX is compared to another 5 kW wind turbine of comparable mass but smaller size and operating with higher winds (nominal wind speed of about 12 m/s, compared to only 7 m/s for KiteX).

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Which also affects the siting. KiteX are not needing to operate in the 11m/s average field

Their power level saturated and stayed steady at only 6.5m/s

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An excerpt:

With little fanfare, NREL released updated data showing that, with current technology, wind turbines could generate more than enough energy at 55% CF to power the entire US. However the real stunner is that near future turbine technology (140 m towers) could boost that to 65% CF. With the current national average wind CF (pg 34) at about 33%, this represents a near doubling. According to NREL using current technology and siting it in prime locations, wind power CF already can exceed that of natural gas. Using ‘near future’ technology wind power’s CF will exceed the CF of both coal (61%) and natural gas (48%) achieved nationwide in recent years.

“current national average wind CF (pg 34) at about 33%”:

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Capacity Factor - an overview | ScienceDirect Topics

The capacity factor is “the actual energy output of an electricity-generating device divided by the energy output that would be produced if it operated at its rated power output (Reference Unit Power) for the entire year” [77]. A high capacity factor dramatically improves the economics of the plant.

Onshore wind farms typically have capacity factors ranging from 25% to 45%, depending on the wind resource quality of the site. Offshore wind farms benefit from stronger and more consistent winds, leading to higher capacity factors, often ranging from 40% to over 55%.

The higher capacity factor of offshore wind is a key driver for its higher energy output and overall economic viability despite the greater upfront costs.

If, by some incredible turn of events, an AWE system were to become operational, perhaps it could benefit from a capacity factor above that of onshore wind turbines, by harnessing stronger and more consistent winds, knowing that the capacity factor can be reduced by flight conditions which may be variable, and by maintenance operations.

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Regarding the capacity factor, continued. Summary based on a single document, concerning the United States.

A large excerpt:

Nuclear power plants have the highest capacity factors among all utility-scale generation sources in the United States, averaging more than 90% in recent years. Nuclear’s high capacity factor results from their ability to operate continuously for long periods, up to 18 to 24 months, before they need to be shut down for refueling and maintenance. Power plants powered by coal and natural gas have much higher maintenance needs and thus need more shutdown time.

Wind (35%) and solar photovoltaic (25%) plants have much lower capacity factors due to the intermittency of the wind and solar resources. Over a day, week, month, and year, solar and wind energy vary dramatically, and significant periods of unavailability lead to a low capacity. Similarly, hydropower has a relatively low capacity factor because the hydroelectric resource potential depends on a combination of rainwater draining directly into waterways and the level of accumulated snowpack in mountainous regions that eventually melt and become runoff.2

The low capacity factors for natural gas turbines and oil-fired internal combustion engines are due to their role as “peaking” power plants. These plants sit idle most of the time. They typically operate during the hours of the highest daily, weekly, or seasonal loads, such as a summer heat wave.

Capacity factors help explain the role that different generations of sources play in electric power grids. Nuclear, coal, and natural gas combined cycle plants historically serve what is known as the “base load”: the minimum amount of electric power delivered or required over a given period at a steady rate. Baseload power plants must be able to supply electricity at a constant rate, usually at a low cost relative to other generation facilities available to the grid.

Renewable sources such as wind and solar historically were not treated as baseload power sources due to their low capacity factors. But this view is shifting for several reasons. Better electricity transmission capacity can link geographically distributed sources of renewable energy and thereby reduce aggregate intermittency. Rapid advances in utility-scale battery storage can fill the gaps that might occur between electricity demand and the supply from intermittent energy sources. Combining storage with a combination of renewable sources, including rooftop solar PV, can provide effective caseload power. For example, solar and wind power generated 22% of Europe’s electricity in 2022, for the first time overtaking natural gas (20%), and remaining above coal power (16%).3 This suggests that the historical notion of baseload power, which refers to coal and nuclear energy, is outdated.

To complete the definition of the capacity factor, I will quote an extract from a commentary (Does nuclear power have a future or will new technologies of renewable energy be developed in the energy sector? | ResearchGate) concerning the text cited above.

Correct definition, but capacity factor is an energy-yield metric, not a firm-capacity or reliability metric. Offshore wind may match or exceed the annual capacity factor of many coal plants, yet that does not mean it provides the same dispatchable capacity during peak-demand or low-wind periods. The meaningful comparison requires capacity credit/ELCC, cost, emissions, transmission, storage or backup, and location-specific production profiles.

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Nice that after 18 years of [pretending to be at the forefront of wind energy, we’re still learning our first-grade vocabulary words.