# Is there a limit to wind power?

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1. Mar 5, 2015

### Skeptik22

Is there a limit to the amount of energy which can be extracted from the wind?

There are a huge number of windfarms springing up around the world.. all taking energy from the wind.

The assumption seems to be that this is limitless and "free".

Clearly this is not possible.

The question is (I think) - how will the transfer of energy (from the wind) manifest itself on the climate, and could the impact be measured?

The only research I have seen to date, relates to finding the "best" location for a windmill, and some on the effect on local weather conditions.

2. Mar 5, 2015

### A.T.

So far we extract wind energy near to the ground, which is equivalent to putting up some high obstacles, like trees.

3. Mar 5, 2015

### Skeptik22

Agreed .... the question still remains .. how much energy can you extract before it has an effect ... one could argue that any energy extracted must have some effect... but that it would be so insignificant that it cannot be detected .. the question is still ... if one continued to extract more and more ..at what point could the effect be detected?

Also agree that the immediate effect would be at ground level.. but there must be some point at which perturbations at ground level have an effect on the overall system?

My analogy would be of throwing pebbles into a river .. a couple would go unnoticed .. a lorryload would have some effect..

4. Mar 5, 2015

### A.T.

On a global scale, that energy is extracted from the wind anyway by interaction with the ground / natural obstacles. On a local scale though, you change where it is extracted, and where and it gets dissipated as heat.

5. Mar 5, 2015

### Skeptik22

Sorry to be pedantic .. but the windfarm has been specifically added to the environment to extract N gigawatts which previously would have been involved in the climate system..

In the river analogy, the pebbles are added to the existing riverbed .. altering the flow..

6. Mar 5, 2015

### A.T.

...and dissipated somewhere else. All the energy that goes into creating wind, is also dissipated by interaction with the surface/obstacles. Regardless what the obstacles are. The question is just where it and when it happens.

Adding a layer of pebbles, which is only a tiny fraction of the river depth, will not have much effect on the river globally. Also, you have to consider all the wind obstacles humans have already removed and will keep removing by deforestation.

Last edited: Mar 5, 2015
7. Mar 5, 2015

### Pierce610

The energy can be absorbed from windpower until the wind exists. So the limit depends only by weather condition, firstly the global warming, since the impact of wind farms on it, also if they slow down the speed of air particles assorbing kinetic energy, is still probably too small to detect, like it happens with other masses of obstacles, or to make some previews. So I've really more interest about the risk of a global warming increase and I think the principal negative impact for a wind farm is still the rumour.

8. Mar 5, 2015

### CalcNerd

Technically, you can't beat Betz! Someone (named Betz) did the math and found that you cannot extract more than approx. 60% of the energy of wind. ie do a mass flow problem with a circular area of where the wind propeller operates and the maximum amount of energy that can be extracted is not quite 60% theoretical. In reality, it is less, usually a LOT less.

And most (if not ALL) turbines have to feather (turn off and turn sideways) so as to not overload their power output capabilities once wind speeds exceed a certain value. This is because it is no longer safe to operate the turbines due to wind shear stress on their structure or buckling of their foundations. Wind speed energy is a cubic function. At low to moderate wind, there is very little energy to be harvested. Once the wind speed is above 15 mph, it starts to become attractive (lower winds can generate energy, but most turbines are underutilized with these breeze speeds). I believe 30-40 Mph is the typical cut out (turn off) speeds for most wind turbines, but you should reference other sources before you quote that. At 30 mph most turbines are running well over their design output ratings (ie in danger of burning up or wearing out their bearings, if they were operated for extended times)

That is why there needs to be a certain distance between large wind turbines for best energy capture (and safety, nobody wants to play dominos on a large scale).

Thanks A.T. I have edited to correct.

Last edited: Mar 5, 2015
9. Mar 5, 2015

### A.T.

Betz

10. Mar 5, 2015

### SteamKing

Staff Emeritus
Yes, there is, but it's due to factors which have nothing to do with the amount of wind.

When the tax incentives and subsidies propping up the financial side of wind farms are removed, they tend to become an uneconomical means of power generation.

http://www.telegraph.co.uk/news/ear...m-plans-in-tatters-after-subsidy-rethink.html

11. Mar 5, 2015

### Skeptik22

Maybe my pebble analogy is not entirely valid .. I saw the lorryloads as piles rather than a thin uniform layer ... maybe a windmill is not such a barrier.

I have seen Betz before in another discussion and maybe I misunderstand ... If you have a wind hitting a mill what is the difference between the air prior to the mill and that downstream?

If N gigawatts have been removed .. is it slower ? denser? higher pressure?

12. Mar 5, 2015

### Skeptik22

13. Mar 5, 2015

### Pierce610

14. Mar 5, 2015

### jbriggs444

Kinetic energy to heat conversion has nothing to do with the climate effect reported in that article. With or without turbines, 100% of wind energy ends up as heat.

15. Mar 5, 2015

### Khashishi

A windfarm has tougher restrictions than the Betz limit, because wind turbines in close proximity will interfere with each other. The front ones may get efficiencies close to the Betz limit (yes, turbines are that good), but they cause turbulence in the air stream which interferes with the efficiency of the back turbines. The calculation of efficiency becomes a lot more difficult. There is a limited amount of energy you can extract from some volume of air above a land area. You can increase the available energy by building taller turbines, but it gets more expensive and structurally difficult.

16. Mar 5, 2015

### A.T.

It's comparable to a couple of high trees. You could compare the rate at which we are removing trees, to the rate we build windmills.

http://en.wikipedia.org/wiki/Deforestation_by_region

World annual deforestation is estimated as 13.7 million hectares a year, equal to the area of Greece. Only half of this area is compensated by new forests or forest growth.

17. Mar 5, 2015

### Skeptik22

[QUOTE=It's comparable to a couple of high trees.

I was beginning to wonder of this really was a physics forum... question was not about whether a windfarm was a GOOD THING, or not. Nor was it about the cost .. it was about detecting and measuring the effect.

1 windmill = 2 tall trees

sounds like a measurement... and yet... I have 2 tall trees in my garden and am not aware of any Megawatts being dissipated

18. Mar 5, 2015

### Bystander

Rough numbers regarding wind energy: global average of 2 W/m2 to maintain a global average wind speed of 10 m/s; that's also a global average energy of moving air that is ~ 500 kJ/m2; average viscous dissipation of 2 W/m2. Play with those numbers as you will, keeping in mind that they are estimates. They may have been refined somewhat over the past fifty years, but they haven't changed much. How much interference in water transport, precipitation patterns, evapotranspiration, and other atmospheric transport might result from wind farms is anybody's guess, and to my knowledge, not been examined in any detail.

19. Mar 6, 2015

### A.T.

"A couple of" doesn’t mean exactly two. If the total size of the tree crowns is comparable to the disc area of the windmill, the removed wind energy will be comparable.

20. Mar 6, 2015

### Skeptik22

Thanks for the numbers, although I am not sure what could be done with them.

This is interesting

http://www.mdpi.com/2072-4292/6/12/12234