What is the most efficient design for a wind generator?

It seems as if every week, a new wind generator innovation is unveiled, sometimes in technology blogs, sometimes in TV segments, once even on TED Talks.  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 240,000 triblade horizontal-axis wind turbines generating power today.[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.  (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

The solar-wind tower with turbine blades comes closest to being an interesting technology, however maintenance is never explored. The turbines are usually envisioned as being stacked 3-7 or more horizontally up the length of the tower.  They will be operating in very hot winds, likely 45 degrees Celsius or higher at velocities of 50 kph and higher.  Effectively, the device is a convection oven that would cook a human in short order. Working inside of the tower would require refrigerated suits and breathing gear, if the wind velocity made that feasible at all.  Pulling the turbine head or blades out of the tower to service them would be an extraordinary job. Closing the wind input would require closing gates on a diameter of five kilometres.

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

9 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

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