Is there a limit to wind power?

[Skeptik22]

How would you be aware of that, in any case? How many MW of solar power are landing on your garden?
But what would be the efficiency of energy conversion with your two trees? As a transducer from moving air to heated tree, they could be very low in efficiency. Have you tried measuring air temperatures? Even if you have, could you actually relate them to power transfer?

Not sure how I would be aware .. I think that was the point.. UK insolation according to Wiki averages at 120 watts per sq metre.. I can see plants absorbing this, and temperature rises in the ground / brickwork etc ..

The figures quoted for windmills appear to be somewhat larger, and someone suggested that a windmill was the equivalent of a couple of trees ... it just intrigued me as to where this energy manifested itself.. I am fairly sure I do not have hot trees..

The idea of efficient energy transfer in trees is also intriguing .. where has the energy gone that has been efficiently converted... as opposed to that which has been inefficiently discarded ?

If this has been returned to the wind, then does a 1 windmill = x trees relation exist?

 

[CWatters]

I recall reading an article some years ago about a plan to fix electricity "generators" to trees. The idea being to generate electricity as the tree bends and flexes. I forget the details.

 

[OmCheeto]

...
The idea of efficient energy transfer in trees is also intriguing .. where has the energy gone that has been efficiently converted... as opposed to that which has been inefficiently discarded ?

Heat and torque.
If trees don't experience torque, then why do they fall over in wind storms?

I recall reading an article some years ago about a plan to fix electricity "generators" to trees. The idea being to generate electricity as the tree bends and flexes. I forget the details.

I'm always suspicious when architects and artists get involved in engineering problems:

Click on the image for the article at gizmag​

Very pretty, but probably very spendy for their output. But then again, until "those people" get over how "ugly" conventional wind turbines are, it might be a good idea.

 

[Tatersoup]

Probably off point, but you could do it until all birds that flew were nearing extinction from getting caught in the blades, causing the environmentalists who decided massive wind farms were green to implode on themselves.

 

[msumm21]

Regarding what a wind turbine does to the air locally, without getting too complicated it can be estimated using simple approximations (e.g. Betz as mentioned earlier) to get
1. the air slows down as it approaches the rotor
2. the pressure rises at it approaches the rotor
3. the pressure drops passing through the rotor
4. the pressure rises again after the rotor

Some numbers that might be useful are (precise numbers depend on a lot of things)
1. wind slows to about 2/3 it's normal speed at the rotor (this is optimal via a momentum theory calculation)
2. wind slows to about 1/3 it's speed downstream (in reality turbulence changes this a lot)
3. more than 1/2 the wind energy (kinetic) contained in the "cylinder of wind" that passed through the rotor is taken

Regarding what it does more globally, I don't know what kind of studies have been done. I assume they'd be very uncertain. The implication in a link from an earlier post that a 1C temperature change was caused by wind farms seems motivated by a political agenda, unless they have some really amazing physics to show that wind farms were responsible for the change.

Yeah those artistic wind machines are usually very inefficient in terms of energy extracted per unit area and cost, compared to a "normal" wind turbine.

 

[psycho rich]

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

I have noticed an increase in wind speed at our windsurfing beach as the tide rises.
We have also have had a large area of desert get covered with grass . That has had a big effect
The Big Island (world's largest mountain) cast a shadow nine miles out to sea.
I don't think human intervention can compare to natural changes.
A town I lived in didn't have flies (too windy) now has flies and fruit trees now bear fruit.
I heard a wind mill engineer say that most wind mills never amortize . Perhaps design improvements will change that

 

[gary350]

I did a lot of research on wind power about 10 years ago. For best results you need a different wind mill for each wind speed. The large 3 blade wind mills you see poping up all over the country side are for moderate wind speed about 15 mph.

The old farm wind mills that had about 12 to 14 blades in a circle will operate in 2 mph wind but above 15 mph the automatic brake shuts them down.

As I recall ever time the blade diameter doubles the power tripples.

For some reason people like to say, the 3 blade wind mills are sub sonic. NO S#%T. My bicycle is sub sonic. My SUV is sub sonic. My RV is sub sonic.

If your building a wind mill build it for the wind speed and conditions you have. Some places are not suited for wind mills like middle Tennessee where the wind is very gusty 5 mph for 45 seconds then 30 mph for 60 second then dead calm for 30 seconds then 15 mph for 45 seconds all day long day after day.

Wind mills need a safety brake for when the wind becomes too high the blade rotates parallel to the wind and the brake locks the blades.

 

[A.T.]

it just intrigued me as to where this energy manifested itself.. I am fairly sure I do not have hot trees..

Obstacles to wind create turbulence which over time dissipates as heat, somewhere downwind.

 

[msumm21]

As I recall ever time the blade diameter doubles the power tripples.

should be quadruples (power proportional to the area of the rotor)

If your building a wind mill build it for the wind speed and conditions you have. Some places are not suited for wind mills like middle Tennessee where the wind is very gusty 5 mph for 45 seconds then 30 mph for 60 second then dead calm for 30 seconds then 15 mph for 45 seconds all day long day after day.

Yep, large and consistent winds make wind turbines more economical.

Wind mills need a safety brake for when the wind becomes too high the blade rotates parallel to the wind and the brake locks the blades.

Yep, like anything they are only made strong enough to handle a certain amount of loads. Once the wind speed is high enough that those loads will be surpassed, the control system puts it into another mode to reduce the forces exerted on the blades. This can include what you said among other things.

 

[psycho rich]

should be quadruples (power proportional to the area of the rotor)
Yep, large and consistent winds make wind turbines more economical.
Yep, like anything they are only made strong enough to handle a certain amount of loads. Once the wind speed is high enough that those loads will be surpassed, the control system puts it into another mode to reduce the forces exerted on the blades. This can include what you said among other things.

should be quadruples (power proportional to the area of the rotor)
Yep, large and consistent winds make wind turbines more economical.
Yep, like anything they are only made strong enough to handle a certain amount of loads. Once the wind speed is high enough that those loads will be surpassed, the control system puts it into another mode to reduce the forces exerted on the blades. This can include what you said among other things.

A turbine designer has come up with a multi bladed wind mill . It has a large containment ring , allows much higher rotational speeds. The blades are self correcting (spring loaded). He has a sailing background and developed a self trimming blade. The blade has pivot aft of the center of effort . The spring tension forces the blade into the low wind speed position , as the apparent wind speed increases , the blade flattens out.
His wind mill has a much greater efficiency than the popular three design.

 

[msumm21]

A turbine designer has come up with a multi bladed wind mill . It has a large containment ring , allows much higher rotational speeds. The blades are self correcting (spring loaded). He has a sailing background and developed a self trimming blade. The blade has pivot aft of the center of effort . The spring tension forces the blade into the low wind speed position , as the apparent wind speed increases , the blade flattens out.
His wind mill has a much greater efficiency than the popular three design.

By efficiency here I assume you mean power extracted per unit area (not cost)? There have been lots of things like this proposed, e.g. diffuser augmented wind turbines for good aerodynamic efficiency, but I don't think anyone has ever found a way to make them compete with the typical wind turbines you see in most wind farms, in terms of the overall cost of energy.

 

[psycho rich]

By efficiency here I assume you mean power extracted per unit area (not cost)? There have been lots of things like this proposed, e.g. diffuser augmented wind turbines for good aerodynamic efficiency, but I don't think anyone has ever found a way to make them compete with the typical wind turbines you see in most wind farms, in terms of the overall cost of energy.

Good question , the design I'm talking about has much more blade area at the perimeter where the torque is higher. I can't remember his estimates on efficiency but I do remember being impressed . Someone said earlier that as the area doubles the force triples

 

[CWatters]

Increasing blade area doesn't necessarily improve performance. Adding blades or blade area increases torque but reduces rpm. As I understand it turbines need rpm more than they need torque. In addition.. Even if you add a short duct the airflow is not forced to go through the disc it can diverge and go around it.

 

[A.T.]

Even if you add a short duct the airflow is not forced to go through the disc it can diverge and go around it.

Can you shape the duct such that more air will go through the disc? In the same way a wing makes more air flow above it.

http://www.rtbx.nl/image/index4.jpg

 

[Andrew Mason]

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

On the other hand, if you factor in the adverse health effects of fossil fuel burning, particularly coal, the death toll and injuries from mining coal (particularly in China) - not to even mention the eventual costs of global climate change - one may be able to make a case that power generation using fossil fuel is (already) more uneconomical.

AM

 

[psycho rich]

On the other hand, if you factor in the adverse health effects of fossil fuel burning, particularly coal, the death toll and injuries from mining coal (particularly in China) - not to even mention the eventual costs of global climate change - one may be able to make a case that power generation using fossil fuel is (already) more uneconomical.

AM

Perhaps a transmission could convert torque to increased r.p.m.
I for see a day when we use wind power to force sea water to higher elevations where it's run thru a solar still . The excess water is used for hydro power. It's a giant battery with no emissions .

 

[billy_joule]

Perhaps a transmission could convert torque to increased r.p.m.
I for see a day when we use wind power to force sea water to higher elevations where it's run thru a solar still . The excess water is used for hydro power. It's a giant battery with no emissions .

All the wind turbine schematics I've seen have gearboxes.
Pumped storage is in use around the world, current capacity is over 100 GW:
http://en.m.wikipedia.org/wiki/Pumped-storage_hydroelectricity

 

[Wes Tausend]

QUOTE Skeptic22 in post #1, "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".

Actually, the last statement is only partly true. The derived power is not "limitless", but it is "free". OmCheeto is correct in assessing that the atmospheric wind is solar powered, and that is what limits it, the total small percentage of solar power that falls upon earth. On the other hand, the derived power is "free" in the sense that if we do not use it, it is wasted elsewhere in friction losses with various earthern structure. Since all the wind is eventually lost to entrophy, and therefore converted to heat anyway, the net earthern energy change to windmills from trees etc (net change in what climate would have been with no manmade obstacles) is zero. The total earthern solar power undergoes entropy change back to heat, is re-radiated away into empty space in all cases and it is equal no matter which "windkiller" it comes from.

So A.T. and jbriggs444 are absolutely correct in their assessments. The idea that the wind energy is "free" is not only possible, but clearly mandatory.

Omcheeto always seems to offer interesting proposals to catch us off guard. He suggested that we could hang a balloon with a tab of tape protruding from it and passing wind would produce a global torque. This is sort of true if we set the experiment up and blow on the hanging balloon with a nearby fan. In actuality, I think the balloon will soon bob in the breeze and the tab will scoot around to the backside, perhaps oscillating from one side to the other because the moment the tab rotates all the way around to the other side, it will be blown backwards. Which brings up an another interesting scenario.

Suppose we attach the fan to the experimental balloon too. This may even be possible with the new tiny personal battery powered fans now available. In this case the little fan will always blow directly on the tab if we strategically place it. Unfortunately only part of the airflow will strike the tab and the rest will miss. I suggest that the balloon will rotate backwards since the fan will now basically operate as a fanjet motor with a minor contrary obstacle (the tab) in it's wake. The same would happen if we were to set up a sailboat and try to blow ourselves forward by catching the breeze from a forward-facing fan set up behind the sail on deck. The boat would likely sail backwards.

Suppose we did set the boat up and it did sail backwards? If we let it continue and it struck a reef, wouldn't this reef began to apply a continuous torque to mother Earth besides wrecking the rudder? Unfortunately the fan would create a small "waste" breeze behind us (ahead of the bow), and this would circle the Earth and create a headwind (against the stern). Except the waste exhaust would actually first dissipate against Earth from all the trees. But the trees would create a nulling counter-torque in doing so. The only thing that would create a complete rotational torque on Earth would be a chemically fueled rocket engine. I once naively proposed this to my young grandson as a method to slowly rotate the moon so we did not have to look at the same old side all the time. This is before I found out about tidal locking. Whoops.

One other thing to think about is that all the winds on Earth are due to rotating flows. Whenever the wind blows one way somewhere on earth, it is blowing the other way somewhere else. (Else the wind would pile up on one end.) The rotational flows are caused by the sun and related to Earth's natural rotation. The sun heats water and air which rises in pockets of both narrow and broad columns. When the air rises (warm air and vapor rise), other air must rush into fill the pressure void. This wind is also encouraged to rotate just as does a bathtub drain. We may say all atmospheric air movement is of this nature.

Which brings us to the reason windpower is desireable. A bit ago I said that not just air, but also water rises. Very few people realize that water is lighter than air (as vapor). So extra newly heated rising water is largely responsible for our current, more aggressive weather changes. The ocean water is being heated to an extra degree (literally) by extra greenhouse gases trapping heat near the ground. The warmer water therefore evaporates easier and rises more quickly, dragging more atmospheric air with it. Caught in the throes of global warming, we now have more overall earthern wind than we had before. The solar power delivered to Earth is pretty much the same to be sure. But less re-radiation of entrophical heat is escaping to outer space. Unfortunately the usual escape route is slightly more blocked mostly by the "thicker" blanket of greenhouse gases formed by fossil-fuel carbon dioxide and the boot-strap effect of more water vapor at any given time. We need windpower to off-set this.

It is true most wind generators won't amortize. That is because they are too small, basically toys. Modern commercial units do pay for themselves.

Wes
...

 

[OmCheeto]

...
Omcheeto always seems to offer interesting proposals to catch us off guard.
...

Wonderful post!

And, I don't mean to catch anyone off guard. I just find some questions very interesting, and don't really have an answer, so I design over-simplified models in my head.

One thing I forgot,

Heat and torque.

was acoustic energy. Trees are very noisy when the wind blows. But, as you said; "Entropy".

But these hypothetical questions are very difficult to model. You have to start out with ideal "text book" initial conditions, and then start adding in your pieces of duct tape and fans.

I've been thinking about my "torque" comment the other day, and decided it might need more research. I may be completely wrong about it.

If the Earth were a solid perfect sphere, with no features, no moon, and no skin effect drag on the atmosphere, then the atmosphere would probably get tidally locked to the sun. If you added a single tree, at the equator, would not some of the energy of the atmosphere get transferred to the Earth, via the non-perfect tree?

​

​

 

[CWatters]

Can you shape the duct such that more air will go through the disc? In the same way a wing makes more air flow above it.

Sure but instead of adding mass to make a duct it's better to add mass to the blades and tower to make them bigger (larger diameter). The swept area is proportional to the square of the blade length.

 

[psycho rich]

Km

Sure but instead of adding mass to make a duct it's better to add mass to the blades and tower to make them bigger (larger diameter). The swept area is proportional to the square of the blade length.[/QUOTE sounds like Kepler ' s theory. I think the true genius is in the self trimming blades, which take advantage of constantly changing wind speeds.

 

[CWatters]

Fine for a small turbine but modern large turbines have fancy computer controls and not just for pitch. For example in light winds close to the cut in speed they use a motor to spin up the rotor when they detect the wind is fast enough. As wind speed increases so does the power output with max power output achieved at say 12m/s. At higher wind speeds the pitch is controlled to limit power output to the maximum. At say 25m/s the turbine will probably be shut down and the blades feathered to reduce load on the blades and tower.

 

[Skeptik22]

the derived power is "free" in the sense that if we do not use it, it is wasted elsewhere

Perhaps my term 'free' would be better phrased as ..without consequence...

I'm with OmCheeto with the simplified models, and the model in my head has the Earth as a kind of energy storage unit.. which has been receiving energy from the sun since dot... some of which has been reflected, some retransmitted... and some stored ... possibly in ways we do not fully understand.. but some as plant life / animal life / and the organic compounds left after their demise (including oil and gas).

The idea that this energy must be used by Man or it will be "wasted" seems a tad arrogant.

 

[Skeptik22]

Regarding what a wind turbine does to the air locally, without getting too complicated it can be estimated using simple approximations (e.g. Betz as mentioned earlier) to get
1. the air slows down as it approaches the rotor
2. the pressure rises at it approaches the rotor
3. the pressure drops passing through the rotor
4. the pressure rises again after the rotor

Some numbers that might be useful are (precise numbers depend on a lot of things)
1. wind slows to about 2/3 it's normal speed at the rotor (this is optimal via a momentum theory calculation)
2. wind slows to about 1/3 it's speed downstream (in reality turbulence changes this a lot)
3. more than 1/2 the wind energy (kinetic) contained in the "cylinder of wind" that passed through the rotor is taken

Regarding what it does more globally, I don't know what kind of studies have been done. I assume they'd be very uncertain. The implication in a link from an earlier post that a 1C temperature change was caused by wind farms seems motivated by a political agenda, unless they have some really amazing physics to show that wind farms were responsible for the change.
.

This has helped my mind's model a lot.

So I have a high pressure area at point A.. a low at point B .. and a set of air pockets departing A and arriving B until such time as the pressures are equal .. those pockets encountering a mill (or a tree of comparable size) will be slowed ... this will result in the same equalization in pressures between A and B but over a longer time period ..

So weather must be affected .. just a question of how much?

 

[A.T.]

...the model in my head has the Earth as a kind of energy storage unit.. which has been receiving energy from the sun since dot... some of which has been reflected, some retransmitted... and some stored ... possibly in ways we do not fully understand.. but some as plant life / animal life / and the organic compounds left after their demise (including oil and gas).

The idea that this energy must be used by Man or it will be "wasted" seems a tad arrogant.

You are confusing wind energy with fossil fuels.

 

[OmCheeto]

Perhaps my term 'free' would be better phrased as ..without consequence...

I'm with OmCheeto with the simplified models, and the model in my head has the Earth as a kind of energy storage unit.. which has been receiving energy from the sun since dot... some of which has been reflected, some retransmitted... and some stored ... possibly in ways we do not fully understand.. but some as plant life / animal life / and the organic compounds left after their demise (including oil and gas).

The idea that this energy must be used by Man or it will be "wasted" seems a tad arrogant.

Arrogant?

On the other hand, the derived power is "free" in the sense that if we do not use it, it is wasted elsewhere in friction losses with various earthern structure.

"wasted" is just shorthand for "going to be lost to entropy anyways".

 

[Andrew Mason]

Arrogant?
"wasted" is just shorthand for "going to be lost to entropy anyways".

I agree. Mankind is not short of "energy". There is just a shortage of energy in low entropy forms that we can easily use. All we are doing by generating electricity from wind is making use of solar energy it as it degrades from low entropy energy from the sun to high entropy, low grade thermal energy on the earth. So I expect that the only significant environmental effect would be possible minor changes in wind patterns and weather locally.

AM

 

[Skeptik22]

I expect that the only significant environmental effect would be possible minor changes in wind patterns and weather locally.

AM[/QUOTE]

Interesting... as one of the reports quoted earlier suggested that hurricanes could be tamed by suitably located windfarms ..

 

[Buckleymanor]

I agree. Mankind is not short of "energy". There is just a shortage of energy in low entropy forms that we can easily use. All we are doing by generating electricity from wind is making use of solar energy it as it degrades from low entropy energy from the sun to high entropy, low grade thermal energy on the earth. So I expect that the only significant environmental effect would be possible minor changes in wind patterns and weather locally.

AM

Is all wind caused by solar energy . If the Sun was sudenly replaced by a structure of similar mass but did not radiate heat .Would there be no wind on Earth ?

 

[jbriggs444]

Is all wind caused by solar energy . If the Sun was sudenly replaced by a structure of similar mass but did not radiate heat .Would there be no wind on Earth ?

Energy conservation still applies. If there were no energy inputs, the wind would die down. You could get some minor convection as heat energy from radioactive decay in the Earth's core bleeds outward. There would be some disturbance due to the lunar tides. But by and large, yes, there would be no wind.