Saturday, October 19, 2013

Efficiency of Vertical Axis Wind Turbines

I started this blog a long time ago because I needed to research the difference between VAWT and HAWT windmills. I'm finding myself come back to the blog after some three years now to make an update. I'm almost surprised its still here!

It seems that there are some vertical axis evangelists out there. I'm assuming they have a product to sell. I'm going to try to introduce some reason to various arguments I've seen recently.

The first issue I've noticed is on wikipedia's VAWT page. Keep in mind that this may change because I noted the issue on their talk page and it may get fixed.




Under "Advantages of vertical axis wind turbines", one of the "benefits" was that they could be packed together more tightly in wind farms. Those horrible horizontal axis wind turbines, on the other hand, needed to be spaced out on wind farms because they had the miserable effect of slowing the wind down too much if they were closer than that.

This has got to be one of the most ridiculous things I've ever read! It is paramount to saying that if you need to mop a room, it is far more efficient to buy 10 mops than it is to buy 1 mop, because 10 mops will mop a room 10 times more efficiently!

Follow the logic here with me, if you will.

vertical axis wind turbine
On the left, you have vertical axis wind turbines. You can completely stack them right next to each other in a wind farm and they do not slow down the wind at all.

horizontal axis wind turbine

On the right, you have horizontal axis wind turbines. "Unfortunately", they slow the wind down -- so much so that you have to fit 10 blade-spans between each one or they would, in fact, completely slow the air down to where there is virtually no wind.

Maybe -- just maybe -- slowing down the air is the purpose of a windmill?  Lets re-visit for a moment the original calculation about how much power is available in the wind, as an algorithm.



Betz' law says there is power in wind, and you can only get 59% of it, period.  The most efficient windmill ever conceived will only ever produce 59% at best. To put into layman's terms, imagine a windmill that was 100% efficient at removing energy from the wind.  Wind rushes at it, and on the other side of it -- wind does not move at all.  In this case, there is nowhere for the new incoming air to move.  That is impossible, so there will always be that 41% energy you cannot extract from the wind.

Now, applying this to the absurdity found on the wikipedia page -- imagine a field with HAWT in them.  They are so efficient at removing power from the air, that they have to be spaced out 10 blade-spans from each other.  PS: This is a good thing.

Now imagine another field, where you have VAWT lined up 1 by 1, right next to each other, and the ones on the back side receive just as much air flow as ones on the front side.  What does this tell you?  It tells you that VAWT are barely extracting any power from the wind at all!

As I stated in my original article 3 years ago, efficiency of vertical axis wind turbines, VAWT design at best will extract around 15% of the energy in the air, but at most times only gets 5 to 10%.

This is why you can put them close to each other in a row and not see much of a performance degradation.  Each leaves so much power left in the wind that the next one receives enough to operate.

This isn't to say that VAWT is evil.  There isn't a fight between HAWT and VAWT to see who wins.  They really do not compare well at all, they are apples to oranges.  They have different purposes for which each has a better use.

A VAWT is better at extracting wind from an area where there is a lower amount of wind available.  VAWTs are more certain to spin, as they require less energy to get them spinning.  This makes them perfect in situations where a low amount of consistent energy is desirable.

For instance, imagine a buoy which has a light affixed to it.  It also has a windmill attached, its only purpose is to provide electricity to a battery to keep a light blinking at night to alert people to stay away.

Attaching a HAWT to the buoy would be a bad idea.  There is only a small amount of power necessary -- so the extra power provided by the HAWT is unnecessary.  Furthermore, in low wind conditions it may not spin at all -- providing no electricity to power the light.  On the other hand, a VAWT will more easily spin, therefore providing the power necessary to keep the light going.  The amount of electricity necessary to power a night beacon is not enough to necessitate a large quantity of power.

In most situations however, people would like as much power out of their windmill as they can achieve.  Therefore, in most situations, people will want a horizontal axis wind turbine to supply their electricity needs.





And in highlighting the ridiculousness of that comment on the wikipedia page, which would you think is more efficient?  To fill a field with 10 HAWT, or 40 VAWT?  Just the costs of installation should be enough to answer the question!  I say 10 HAWT vs 40 VAWT because it is not unheard of for a HAWT to get 40% power extraction from wind, whereas a VAWT may get 10% (thus a HAWT is 4 times more efficient).

If you enjoyed my article, please visit me on my blog at Guru Wizard, where I discuss wind turbines and other cool stuff.

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