Friday, October 15, 2010

How Efficient are Vertical Axis Wind Turbines compared to Horizontal?

Are you looking to build a wind turbine, or to buy a windmill, and you want to know whether it is better to get yourself a vertical axis wind turbine or a conventional horizontal windmill?  It is a good idea to educate yourself on the differences between vertical and horizontal axis wind turbines.  Obviously, you want to get the one that has the best return on investment, whether you will buy a wind turbine or build it yourself.





The challenge when it comes to a wind turbine is to extract energy from the wind.  According to physics, it is impossible for a windmill to exceed 59% efficiency when extracting kinetic power from the wind and converting it into mechanical power.  The remaining 41% of the power must remain in the wind.

Lets cut to the chase!  You don't really want to read a 2000+ word article to discover which one is the best, right?  How about I give you the answer to the question now, then if you are still hungry for more information you can read the math formulas and physics involved explaining just why one is better than the other.

The best type of wind turbine is ...
Of the 59% possible efficiency available to convert the kinetic energy in the wind into mechanical energy:
  • Horizontal Wind Turbines: close to 50%
  • Vertical Axis Wind Turbines: around 10% (though an excellent design could reach near 15%)

The bottom line is that while vertical axis wind turbines can make a great do-it-yourself project, and on a small scale they can even be a worthwhile project -- in reality they are far less efficient than horizontal axis wind turbines.

There are also additional design flaws that can lead to more wear-and-tear on vertical axis wind turbines making them less financially viable from the aspect of creating a long-term supply of reliable renewable energy.

If that was all you wanted to know, thanks for visiting!  If you actually want to know the math and physics behind why horizontal is better than VAWT designs, please continue reading.

How much energy is available in the wind?

This is the formula for a conventional horizontal wind turbine:

Where R is 1/2 the diameter of the windmill blades (the radius from the center to the outer edge), and where v is equal to the wind velocity.  This math is for an efficient wind turbine (approximately 50% efficient) at a moderate speed of wind.  (Though there are a good deal of low quality wind turbines on the market that would not produce half this much).

Putting this example to the test, lets assume it is a medium sized diy type of windmill (the diameter is 3 meters -- which means the radius would be 1.5 meters).  Also assume a wind of 5 meters a second (about 11 mph).

P(power in watts) = 1.0 x 1.5 squared(radius of blade, equals around 1.22) x 5 (meters a second to the third degree which equals 125).

For this example, we have a grand total of 153 watts (1.0 x 1.5 x 1.22 x 5).

The next logical step would be to give you the math of how much power you can expect from a vertical axis wind turbine.  Unfortunately, after many hours of searching, I have been unable to discover the math.  Let me explain where I got the 10% to 15% efficiency that I mentioned above.

To the left is a Darrieus VAWT. There are two models of vertical axis wind turbines, one uses what is called a "Savonius rotor", while another uses what is called the "Darrieus" model.  Following are pictures of each type.

Where did I get my 10% to 15% efficiency remark then?  Mick Sagrillo is an expert on residential wind power.  In an article on "Mother Earth News" he was quoted with these figures (actually he stated it was 5% to 10% efficient but could perhaps be tweaked to 15%).

A lot of what we see today are Savonius rotors. They’re very crude, very low-tech, very inefficient. We’re talking about something that operates in the, say, 5 to 10 percent efficiency range. People have been able to tweak the efficiency rate — ideally, they might be as high as 15 percent.

To the left is a picture of the Savonius VAWT.  The Darrieus model is no different.  They both operate using the same principals and at the same efficiency.  Whereas a horizontal axis wind turbine continually has its blades against the wind, it will be extremely more efficient than the vertical axis which has its blades on the far-side-of and at times slightly working against the wind to get back into its original position.  This inefficiency reduces what you can expect from a VAWT to 5% to 10% whereas an efficiently designed horizontal axis wind turbine will close to 50% efficiency.  Perhaps the reason that the physics and math are not readily available for VAWT is that it is not a selling feature and thus is not a part of the promotional materials of VAWT systems.

VAWT Design Flaw: Altitude

Whereas a horizontal wind turbine can be reliably mounted on a pole at a high altitude, vertical axis wind turbines have a considerable amount of drag.  This makes placing them on a pole at a high altitude unstable, unless they are stabilized  fastened to the ground with guyed cables.  The cables need to be secured at wide angles for the cables to be clear of the rotating blades.  This unrealistic scenario causes many manufacturers to claim that they do not need to be put in the air, in fact you can mount them to the ground or to the top of a building.  However, the order of magnitude of wind power is exponentially greater at a distances far in the air than it is on the ground.  Wind on the ground is dampened by objects, removing much of the power available in the air.  Furthermore, there is far more turbulence on the ground.  Turbulence causes wear and tear on VAWT windmills, causing them to need maintenance all that much quicker.

VAWT Design Flaw: Centrifugal Force and Weight

When you are looking to invest into a system that will consistently provide renewable energy for an extended period of time (such as 20 or more years), one of the things you must look at is maintenance costs.  So far we have discussed that you can get three times the power out of a horizontal wind turbine than you can get out of a vertical wind turbine.  We mentioned the difficulties with mounting them up high (where the wind is).  How about longevity, which one lasts longer?

The way that a conventional horizontal wind turbine is designed, its weakest point is at the base where the blades meet the core of the turbine.  This also happens to be where the blades are the strongest, because they are the thickest at this location.





With vertical axis windmills (on the Darrieus models, or some of the Savonius designs which has open space between the blades and the base) -- the blades are located at a distance away from the base of the shaft.  The centrifugal force that happens with the bulk of the blade weight at a distance from the shaft causes structural wear over time.  In addition, where the blades meet the blades is typically mounted with thinner structural components.  There is a tradeoff -- the more that VAWT manufacturers attempt to stabilize this weak design, the more they bulk down the weight of the windmill reducing its efficiency.

Is VAWT a Bad Idea?

Thus far, this article has been pretty condescending towards the idea of getting yourself a vertical axis wind turbine.  Are all VAWT simply a bad idea to invest money into?  Let me say that I would not recommend buying a large commercial VAWT unit, some of which can cost $15,000 to $30,000.  However, there are applications where a vertical axis wind turbine can make good sense.  There are small applications where efficiency is not the main concern, instead the biggest concern is reliability (one example of such would be a waterbuoy that just needs enough power to keep themselves lit at night).  In this instance, certain Savonius designs would work wonderful for this application (and be far more stable and reliable than a horizontal axis).

There are other considerations.  If you are making a do-it-yourself "build your own" vertical axis wind turbine, VAWT can be simple to make and you can often turn items you have around your house into a cheap and easy wind turbine (such as a 55 gallon drum barrel).

Many people just think they're cool.  There was a study to this effect done, where people become mesmerized by watching windmills in action.  This effect is caused by the repetition and is called the "frequency effect", similar to what you would get with watching a fire, or waves crash on the beach.  This study happened to show that people preferred the effect of VAWT over HAWT.





So this entire website is devoted to vertical axis wind turbines.  You will see them in action, some homemade ones, videos, etc.  If you are intending to build one, you should seriously consider whether you really want to build a VAWT or whether a horizontal might be better.  If you are considering purchasing a high-end retail VAWT, at least by reading all the way through this last sentence you can at least consider yourself warned of the many downsides.

5 comments:

  1. I'm considering to invest in the wind turbine. After reading this article I would consider to put VAWT outside. Thanks.

    ReplyDelete
    Replies
    1. regardless of the type of turbine used, I would never recommend having it inside!

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  2. We're planning to build a VAWT, but we're not sure which kind, i.e., Helical or H-type (Egg Beater).
    We've heard that the helical has higher efficiency but is expensive to manufacture.
    It'd be great if you could suggest which kind will offer a better efficiency-ROI trade-off
    Thanks!

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  3. It's simply not true that the Darrieus turbine is just as inefficient as the Savonius. The turbine you picture (the QR5) for instance is about 25% efficient. (http://www.bettergeneration.co.uk/wind-turbine-reviews/qr5-wind-turbine.html). You've also got to consider how good your HAWT is at catching the wind. Small turbines in urban environments tend to spin aimlessly looking for a coherent wind pattern - not a problem with VAWTs.

    That said, I'd agree with the overall conclusion.

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  4. The point is that a vertical turbines has half of it blades turning in the wind direction, and the other half against it, so the net force acting on the verticall windmill is the difference between those forces, hence the lower efficiency. But you could use windshading, a casing around the windturbine, that shades that other half, only that casing needs to be directed in the direction of the wind.

    ReplyDelete