Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: John Chapman on 25/07/2009 20:15:02
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I wonder if you clever physisisicists could clear up a question that has bothered me for years.
A wind turbine has blades which are pushed by moving air and transmit some of that energy to an electrical generator. Everyone knows that. But if the turbine is taking energy from the wind then how does this manifest itself in the air behind the turbine combined to the air in front? It cannot be moving slower or else the air would compress, which would be to actually transfer energy TO the air. Since air continues to move into the turbine at a useful speed it follows that it MUST be moving away behind it at the same useful speed. So no energy has been taken from the wind!?
Where has my logic let me down?
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It's not a full answer, but maybe get the ball rolling:
If you're extracting energy, you must be impeding the motion of the air in some way.
By impeding the motion, you're going to have a slightly higher air-pressure in front of the turbine, and a slightly lower pressure behind it.
I suppose you have lower pressure but higher velocity air behind the turbine to make the number of molecules add up. Presumably the air behind is slightly colder? (adiabatic expansion)
In an open system (ie not one where the turbine is within a long, enclosed tube) of course there will be turbulence and the air will flow around and the lower pressure won't be distinctly measureable for very far.
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The wind could still impart a force to the propeller blade even if it leaves at the same speed, if the air flow has changed direction.
(cf. scalar (http://en.wikipedia.org/wiki/Scalar) and vector (http://en.wikipedia.org/wiki/Euclidean_vector) quantities).
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Hi techmind
Adiabatic cooling? That’s a clever idea. You’re just showing off now. Although wouldn’t the pressure gradient be the other way round. If you slowed down the passage of air wouldn’t it ‘bunch up’ behind?
I suppose you could have lower pressure but higher velocity air behind the turbine to make the number of molecules add up.
Whaaay. I've got to try and get my head round that.
Hi RD
If the wind doesn't lose speed when redirected and the redirection is the result of work done isn't that the ingredients for perpetual motion?
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It is important to look at all the relevant variables in order to understand problems like this one.
The mass of air entering may be the same as the mass leaving but that's not necessarily all that counts. The same number of links of a bicycle chain are approaching the wheel sprocket as are leaving (the chain is not stretching all the time) but work is done on the wheel.
The work done on the blades is based on a force times the distance - or a pressure difference times volume of air moving. All that you need is for the pressure upstream to be greater than the pressure downstream, for energy to be transferred to the turbine.
It's the same for a water turbine in a parallel sided pipe - the pressures are different on either side and that accounts for the energy (=work) transfer. This is easier, possibly, to think about.
In the case of the air turbine, there is added complication in the airflow and the downstream air affects a wider area than just that of the turbine etc. etc. but what I have said will apply and 'allows' it to work without violating any Physical Laws.
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Hi sophiecentaur
The bicycle chain analogy is an excellent one and it tells me that you are all obviously right but I still don't understand why. So what is the difference between the bit of chain before and after the the drive sprocket? If I analysed the air before and after the turbine what different properties would each have?
Or to put it another way, if I built a second turbine closely behind the first, why wouldn't the second be as productive?
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Hi techmind
... wouldn’t the pressure gradient be the other way round. If you slowed down the passage of air wouldn't it ‘bunch up’ behind?
Doh! I must be taking my stupid pills tonight. I wish I hadn't said that!
If you slowed down the passage of air then of course it would bunch up in front. That would make the air behind a lower pressure. That would mean that the air passing through would have to travel faster to catch up with the neighbouring molecules of air going around the turbine which means I now understand what techmind meant about lower pressure and high velocity producing the same number of molecules passing through but in a different energy state before and after the blades.
It all makes more sense now.
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With regard to large wind turbines, and not the smaller portable variety used to power stuff on boats or on camp sites, the turbine blades aren't 'pushed' by the wind; the aerofoil cross-section of the blades and their angle in to the wind generates lift over the blade by creating a low-pressure region on one side of the blade when the wind blows across it, which is much more efficient than simply using air pressure force to turn the blades (which is more akin to a water wheel than a turbine). Energy is still taken out of the wind to do this, resulting in turbulence, but in typical modern wind turbines the area of the blades is only a small fraction of the area of the disk swept by the blades and most of the wind passing through the swept disk is not intercepted by the blades and is unaffected.
If wind turbines operated more like water wheels then there would be a very noticeable difference between the wind upstream and downstream of the turbine, as there is with water wheels.
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Hi sophiecentaur
The bicycle chain analogy is an excellent one and it tells me that you are all obviously right but I still don't understand why. So what is the difference between the bit of chain before and after the the drive sprocket? If I analysed the air before and after the turbine what different properties would each have?
Or to put it another way, if I built a second turbine closely behind the first, why wouldn't the second be as productive?
The difference, for the chain, is in the tension.
(Tension difference) times speed = power transferred
LeeE
most of the wind passing through the swept disk is not intercepted by the blades and is unaffected.
The aerodynamic effect of a blade extends way beyond the metal of the blade itself. If any air gets through the turbine 'unaffected' by it then the turbine has been designed wrong and should have more blades b ecause it is missing out on some available power.
I agree with what you say about the way the aerofoil of the blades works - low pressure appearing on one face of each blade, which causes the blade to turn. That is an important detail of the operation. However, the turbine acts as a whole on the flow of air through and past it and the 'mean' effect on the air has to be that the pressure upstream is higher than the pressure downstream. I realise that is a bit of a circular argument but it just has to be true because you have to account for the lost energy yet the constant volume flow.
J.C.
A turbine placed downstream of the first on would not have the same pressure difference across it. Moreover, the upstream turbine would also suffer if energy were being taken by the downstream one. It's a bit like the skeins of ducks flying along, 'slipstreaming' the lead duck. The lead duck would actually do better on its own and is helping the ones which are following it.
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LeeE
most of the wind passing through the swept disk is not intercepted by the blades and is unaffected.
The aerodynamic effect of a blade extends way beyond the metal of the blade itself. If any air gets through the turbine 'unaffected' by it then the turbine has been designed wrong and should have more blades b ecause it is missing out on some available power.
Yes, the turbulence produced by the blades do extend beyond the blades themselves, but mostly in the form of downwind tip vortices. Also, they don't want to intercept all of the available wind passing through the disk because in farms it would upset the operation of nearby turbines, and downwind of a 'farm' there would be havoc, with the farm effectively terraforming the land downwind of it, which wouldn't be very eco-friendly.
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Interesting. What would be the actual effects on the ground?
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Chapman: imagine each of the individual oxygen, nitrogen, etc. particles flowing through the windstream, say, at 20mph. The wind turbine is at rest, and the particles that make it up are vibrating quite slowly, compared to the wind. They have a much lower energy.
As the high-energy wind particles bombard the particles making up the turbine, the energy of the turbine particles increases, and this increase in energy is manifested in the rotation of the blades.
Of course, as Entropy says, the energy of the wind particles is reduced as they come into contact with the lower energy particles of the turbine blade, albeit to a relatively minimal degree.
It's like the reverse of passing an ice cube through a tub of warm water. The ice cube gets a little warmer, and the water gets a little colder.
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pshmell
I note your 'particle' explanation, which has been popular with GCSE teachers for some years. It's all very well but very few of the particles of the air actually come into contact with the blades. The air behaves in a macroscopic way and I don't see that the microscopic approach helps. If anything, it introcuces confusion - would you really want to describe the function of an aerofoil in terms of particles?
It seems to be introducing an extra layer into the problem - like introducing the behaviour of electrons when designing a simple circuit.
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pshmell
I note your 'particle' explanation, which has been popular with GCSE teachers for some years. It's all very well but very few of the particles of the air actually come into contact with the blades. The air behaves in a macroscopic way and I don't see that the microscopic approach helps. If anything, it introcuces confusion - would you really want to describe the function of an aerofoil in terms of particles?
It seems to be introducing an extra layer into the problem - like introducing the behaviour of electrons when designing a simple circuit.
Heh yeah I understand. But you never really understand a process until you understand it under different levels of magnitude, from multiple perspectives. The visualization I was trying to give is what I believe Chapman was looking for. Who knows, he hasn't replied. He seems to already understand the process for the most part, he's just having trouble visualizing a certain aspect of it.
You can say a pattern of flowing energy described by the windflow pushes the blades, and I can say that particles bombard the blade and induce movement. Neither of us is wrong, and neither of us is Absolutely right. We are both describing the same process from different perspectives: the macroscopic and the microscopic, respectively.
And there lies the trouble with the world: Disputation is a proof of not seeing clearly.
In God's eyes, the wind simply blows and the blade simply moves :)
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But the notion that all the air particles that affect the blade actually hit it is wrong. The pressures on the different parts of the blade are due to the statistical behaviour of the particles in the bulk of the air and, if you want to use particles in your explanation, you also have to include the statistical behaviour. That just opens a whole new can of worms. And for what benefit? If you want to give the particle explanation for the way the air behaves then, by your argument, you would need to do the same thing to explain why the blade turns due to the photon reactions between the atoms in the solid metal. Would that make sense? Would it be appropriate?
Could you explain, using particles, in a way which would help a 'beginner', how the Bernouli Effect works and how it produces a pressure reduction. Yes, of course there is an explanation but does it help?
I believe that splitting a problem into shells is the way forward - in life as well as in Science.
On a point of accuracy, btw, the particles of the blade have almost the same kinetic energy as those of the moving air because they are at the same temperature; the average (vibrational) Kinetic Energy of all particles will be the same. The difference due to the bulk motion of the air is very small as the majority of energy in the situation is thermal.
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Interesting. What would be the actual effects on the ground?
I don't know for sure, but I'd expect it to be chaotic i.e. gusty, but with no constant overall direction and lots of small localised short-lived vortices. Birds would have a very hard time trying to fly in this region and any loose soil on the ground is likely to be alternately picked up and then thrown back down somewhere else. Any trees that managed to survive would present quite a spectacle in autumn when they shed their leaves. I'm really just guessing at the effects but I think the ground level turbulence is a pretty safe bet.
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Would that make sense? Would it be appropriate?
Could you explain, using particles, in a way which would help a 'beginner', how the Bernouli Effect works and how it produces a pressure reduction. Yes, of course there is an explanation but does it help?
I understand quite clearly that only a very small fraction of the air particles actually collide with the particles that make up the blade.
Your explanation of bulk movement is completely meaningless without understanding the microscopic perspective as well. And my microscopic explanation is meaningless without your bulk energy flow explanation. Don't take it so personally, it's science, man.
And yes, every perspective "helps" see the bigger picture. The Universe is not partial to any perspective, explanation or theory. You must understand many perspectives, or your own will become meaningless.
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Pshmell
I'm not taking it personally. I'm being pedantic, which is essential if one is to get a better understanding.
The macroscopic explanation which involves fluid dynamics and thermodynamics is complete and thorough. It yields accurate predictions by and large.
The limited particle description which you introduced does not yield correct results because it is too simplified. In fact, it is misleading and does no favours to anyone trying to understand what is going on in an air turbine. As I said before, until you can explain Bernouli in terms of particles, it is a failure.
You would say I was daft if I demanded an explanation which involved string theory. That demonstrates that there is a point beyond which it is not appropriate to venture. Without a much fuller model, I say the particle approach is not appropriate.
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The macroscopic perspective you describe works best to you because you, in your own perspective, are a macroscopic system, and thus you have been brought up to understand things on the level of magnitude with which you most identify.
There is no "true" perspective, there is only true for you. The more perspectives from which you view a system, the more you understand the dynamics of how they interact with one another.
Much like differentiation and integration of mathematical functions, you understand how things work better when you know what patterns are visible under different levels of magnitude.
Although, now we are talking philosophy, not science. Moving forward ;)
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I wouldn't disagree with you too much on that.
However, I am not aware of a particle based analysis of an aerodynamic situation / problem so, as far as I can see, it is a matter of horses for courses. The macroscopic approach solves this one. If we happened to be talking about spectral lines, then a macroscopic analysis doesn't work and we need to talk QM -I would be just as much against the 'other' approach where it did not apply.
If you, or anyone, could come up with a mathematically based particle approach to the turbine problem I would be only too happy to use that 'perspective'. But you could be sure that it would boil down to statistics - which takes you back to the Gas Laws.
I often ask the following question of people who demand that we should try to 'umderstand in depth' everything that goes on. When you turn on the light in your front room, how many times do you consider the extra shovelfull of coal that someone organises to be put into a boiler somewhere in the Midlands to enable it to happen? All of our lives, including Science, involves compartments.