What is the most efficient design for a wind generator?

Last update: May 15, 2013

It seems as if every week a new wind generator innovation is unveiled, sometimes in technology blogs, sometimes in TV segments, once even on a TED Talk.  They all claim to be better than the iconic, triblade, horizontal-axis wind turbines we are most familiar with.  So what is the most efficient design for capturing wind energy?  If each design were subject to a constant wind source, and each design had the same surface area (for blades, aerofoils, or other component), which would generate the most electricity over the same time interval?

20081124-energy-ball-wind-turbine

Short Answer:

A modern horizontal-axis, triblade wind turbine would generate the most electricity. Claims of superior performance by alternate technologies accompanied by requests for investment should be viewed extremely skeptically.


Long Answer:

Maximum potential generation from a volume of wind is determined by Betz’ Law (alternately known as Betz’ Limit). Betz calculated that the maximum power that could be gained from the wind was 59.3% of its total energy. [1]

Wind generation devices include:

  1. Triblade horizontal-axis wind turbines
  2. Vertical-axis wind turbines with aerodynamic blades
  3. Cabled, flying wind generators (prototypes at present)

    [9]

  4. Horizontal-axis wind generators of various types with no aerodynamic component to the blades
  5. Vertical-axis wind generators of various types such as the Savonius  generator without aerodynamic blades
  6. Various devices that look like jet engines, or jet engines with big funnels, cones with pistons (the Saphonian [5]) or corkscrews
  7. Towers that use passive solar heating around their base to create strong winds flowing up the tower past wind turbine blades spinning inside the tower [4]

How do they stack up?

There are about 300,000 triblade horizontal-axis wind turbines generating power today at the utility, and no other form of wind generation at that scale at all.[7]  They are the winning form of generation because they are the most effective.  The reasons are easy to explain:

  1. Aerodynamic blades add a component of lift-related force to drive the blade faster.  This is a significant advantage over windmills whether horizontal- or vertical-axis. Any even adequately designed wind turbine with aerodynamic blades will always generate more electricity than the best generator without aerodynamic lift as a component of energy capture.
  2. The blades of the triblade design are always flying through clean air.  The turbulence of the previous blade’s passage has been carried downwind by the time the next blade passes the same point. Vertical-axis wind turbines, whether bladed or pure drag forms, are flying through turbulent air a significant percentage of the time.  The clean air allows the triblade HAWTs a sizeable advantage. [2]

    [8]

  3. The blades of the triblade design are always presented at the optimal angle to the oncoming wind.  Aerodynamically bladed vertical-axis wind turbines change the angle of their blades to the oncoming wind constantly as they rotate, and only a portion of even the best designs are at an optimal angle at any given time.  Aligning the blades of HAWTs to the oncoming air requires trivial amounts of energy compared to this advantage. [2] Savonius wind generators (named after a Finnish engineer who created a common variant in 1922) are even worse as they capture wind in the concavity in half of their surface area and shed wind on the convex portion with attendant drag and additional turbulence on the other half of their surface area.  As an example of the magical thinking often present in wind generation innovation, I analyzed a potential investment for a small firm in micro-generation capability and saw that the inventor had created 5 ‘innovations’ around the basic Savonius premise that took it from a cheap form of energy sufficient for minor irrigation uses to a very expensive form of generation of energy sufficient for minor irrigation uses. For context, here is a cost-effective Savonius irrigation windmill made out of an old plastic barrel and some scrap lumber.
  4. Triblades scale up well. One of the biggest advantages is that you can put a very big set of blades on a very tall tower and gather lots of wind above the point where it slows down due to contact with the ground.

    Many ‘innovative’ designs have been proposed that use some sort of venturi effect in combination with turbine rotors, but the fundamental problem is that in order to gather sufficient wind, you have to scale the outer shell up to the point where weight and material costs become prohibitive.  An outer shell has to scale up at least to the square of the diameter and likely more. A 3 MW wind turbine with 80 meter blades can catch a subset of the energy from 20,096 square meters of air. A venturi shell at that scale would have a circumference of 251.2 meters, would likely have to be at least 10 meters in breadth before noticeable effects started kicking in and would weigh an enormous amount.

    Other ‘innovative’ designs fly wind-capturing devices of some sort or other — blimp-shelled turbine blades, frames with turbines, kites with turbines — into wind that’s more constant and higher off of the ground.  The problem is that these are constantly running into scale limits.  The blimp-shelled wind generator starts having rigidity problems long before it gets to utility-scale generation.  The flying kites with blades start requiring massive and very long cables in order to resist the forces. Generally these prototypes are very interesting and never see the market. All of them start requiring massive ground installations with extraordinarily large winches when you want utility levels of generation. When you start thinking ship-hawser levels of strength multiplied by kilometres of cable you start realizing that the weight and expense of the cable alone becomes prohibitive at any useful level of generation.[3]

  5. Triblades just sit in one place on a big pillar when they are generating electricity.  This is very efficient, which is one of the reasons that they payback the energy used in construction faster than any other form of electrical generation. [6] One wind farm in Australia generated 302 times the electricity that was used to start them, brake them and turn them into the wind over a year. Compare this to the requirements for a flying wind turbine which has to be hauled in when the wind doesn’t blow, launched when the wind starts and has a heavy cable potentially kilometres long adjusted to maximize generation regularly.
  6. The graph below is from the 2006 book by E. Hau., Wind Turbines: Fundamentals, Technologies, Application, Economics. Springer. Germany. 2006. Even at the time, this was not new news, but merely an obvious statement to include in textbooks.fig06-2

See my related post Invest carefully; wind energy ‘innovations’ are rarely kosher for the questions to ask about any innovative piece of wind generation, especially if someone is asking you to put money into it.

References:

[1] http://en.wikipedia.org/wiki/Betz’_law
[2] Why aren’t vertical axis wind turbines more popular?
[3] Are airborne wind turbines a plausible source of cheap energy?
[4] http://www.dailymail.co.uk/sciencetech/article-2019197/Arizona-solar-power-tower-worlds-2nd-tallest-building.html
[5] http://www.energymatters.com.au/index.php?main_page=news_article&article_id=3325
[6] Wind turbines pay back total environmental ‘debt’ in under six months
[7] http://www.gwec.net/global-figures/wind-in-numbers/
[8] http://www.windpowerengineering.com/construction/simulation/seeing-the-unseeable-in-a-rotor-wake/
[9] http://www.skysails.info/english/power/power-system/skysails-power-system/
[10] Paul Gipe’s excellent material on wind generation economics

13 comments

  1. […] with all of the professional people, there is a physically optimal design that the vast majority of wind generators have converged to: the three-bladed, horizontal-axis […]

  2. I would imagine Turbine convection towers would undergo maintenance at night.
    Also, people get hardened to working in hot steel blast furnaces and around glass casting
    lines.

  3. Noel Dean Calvert · · Reply

    Pat. The temperatures they are talking about inside these convection towers would not dissipate before the passing of the night to day once again. I have lived in iraq under temperatures ranging close to those mentioned here, and the cement towers never cooled enough to expect that a metal convection tower like this would cool enough to allow anyone to enter. Also the heat produced would keep the towers operating through the night as well unless actively cooled.

  4. thA ministA of truth · · Reply

    the VAWT is the most efficient design and will operate in 0.2 mph winds. reason that big electric has horizontal triblade turbines is so they can charge more money for the power when the turbines is inoperable due to low winds. this does not happend with good VAWT designs. MONEY MONEY MONEY

    1. References and evidence are required on barnardonwind for claims like this. I agree that VAWTs work in lighter winds, but lighter winds also have a lot less energy as energy increases with the square of velocity. Physics is why very low wind operation is meaningless of utility-scale generation.

  5. Mike, I understand and agree with your points about the greater area covered by HAWT’s as having the advantage in producing more power but I think there is something you may be overlooking. Your viewpoint seems to be the “one-size-fits-all” approach and that’s just not realistic everywhere. I contend that we must use the right tool for the job at hand and that depends largely on location and power needs. Furthermore, there are numerous locations where a HAWT simply will not fit or isn’t allowed by local zoning codes. I agree that some designs are simply flights of fancy but there are a few VAWT’s that work very well for distributed (micro) power generation needs.

    1. Sean, I completely agree and usually recommend the UGE for those rare instance, at least in the developed world. I had aspects of this discussion with an Indian national and resident who was convinced he had discovered the secret sauce to making VAWTs better than HAWTS. I commended him on selling hybrid energy systems with VAWTs, solar and inverters for homes because that was a great thing to be doing in India where the grid sucks and told him to get an engineer to help him figure out where his math was wrong otherwise.

      However, it’s a niche, it’s tiny generation and it is going to have a high LCOE. That’s fine for some places with crap grids, but I’m actually a fan of solar for those applications most of the time.

      Thanks for your comments. I’ll look at my post and see if I need to strengthen my statements to this effect.

  6. J.D. Dannar · · Reply

    I Am just learning about this technology. I intend on building a generator for my small shop this spring, and since I live on a farm, space is not an issue. I just don’t know if I need storage or not, and how to switch from DC to AC, besides buying an inverter.

    1. JD, if you are grid-connected my suggestion is that storage should be fairly far down the list. Have a look at this answer to the question on Quora.

      http://www.quora.com/Energy-Storage/What-is-the-best-way-to-store-electrical-energy-generated-by-multiple-sources-like-a-number-of-wind-turbines

      If you do need storage, I’d look at the technology that people like Urban Green Energy use. They have a solid set of different energy tools and products, so I assume the they put time into thinking about the storage question for local use that I haven’t. http://www.urbangreenenergy.com

  7. Tim · · Reply

    VAWT do offer useful niche advantages, but you have to know which type of turbine is best for the application, i.e. location, and depends how much power you need.

    Some VAWTs do not need to be shut down in high winds, and thus can keep generating electricity.

    We actually need a mix of different types and any research should be welcomed

    It is understandable that certain companies will promote their products and slant the marketing to certain issues, but you have to be careful to weed out the hype and possible “over positive messages” that any company uses to make a sale. Caveat Emptor.

  8. Brenner · · Reply

    Has anyone considered the recent research on pairs of counter rotating VAWTs?

    Right now there are claims for 6 to 9 times more energy extraction for a unit area of land for VAWT arrays over HAWT arrays.

    Here’s my reference: http://arxiv.org/ftp/arxiv/papers/1010/1010.3656.pdf

    1. Yes. Dabiri is actually finding mild increases in total Darrieus VAWT output at eight rotor rotor diameter spacing. So far so good. Originally he was using 1-2 rotor diameter spacing. However, his claims of being better than HAWTS fall apart rapidly because he insists on using total land area over which a wind farm spreads as his denominator for determining energy density.

      Here are three problems with this. First is that no one actually uses energy density as a measure of value of a form of generation; it’s an argument legacy generation advocates bring forward to attack renewables with but it is meaningless and not used.

      The second is that wind farms take up 1% to 2% of the land on which they operate with the rest of the land remaining in use for whatever was going on before, usually agriculture. By using 100% instead of 1% when he was using 1-2 rotor diameters he claimed 10’times greater energy density rather than 10 times worse energy density given the mixed use nature of wind farms.

      Third is that the best Darrieus VAWTs require twice the swept area and four times the material to generate the same electricity according to actual certified results from UGE. Dabiri is making Darrieus VAWTs perform a few percent better but not four times better. His LCOE will reflect that as capital cost is linear with materials.

      I cover most of those points here: http://www.gizmag.com/dabiri-fish-school-wind-farms/28355/

      I had an extended email conversation with Dabiri on the land use myth. I expect that eventually he will stop making this point and realize his insight and findings are interesting but not as disruptive as he and others have claimed. As I point out elsewhere the real innovation in wind generation is incremental on HAWTS.

  9. Dr. Rakesh Saxena · · Reply

    Informative analysis

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