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Author Topic: What's wrong with Thermodynamics?  (Read 11032 times)

Offline Geezer

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What's wrong with Thermodynamics?
« on: 15/02/2012 06:08:28 »
FLAME ON:

A certain geezer by the name of Carnot made it perfectly clear (a very long time before a lot of other brilliant physicists) that the efficiency of any heat engine, be it nukeular, thermo-mumbo-jumbo or even quanto-solaristic, is determined by the DIFFERENCE in temperature between the "hot side" and the "cold side".

Why do so many people think they might be able to "get around" this limitation? To me, it's pretty much the same as saying that  e=mc^2 is only a minor inconvenience.

FLAME OFF.


 

Offline damocles

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Re: What's wrong with Thermodynamics?
« Reply #1 on: 15/02/2012 07:32:10 »
About 5 and 6 years ago I surveyed my advanced undergraduate chemistry students, who had just audited a "Fuels and Energy" module in their Chemistry or Environmental Science subject.

The majority of them seemed to think that the second law of thermodynamics (in its Carnot cycle and similar manifestations) was a limitation to our present state of knowledge rather than an inevitable "law of nature". This may or may not be a reflection on the quality of my teaching!

A few pedantic points:
  • the Carnot equation is an inequality -- it represents a maximum efficiency that can never quite be achieved in a practical heat engine, and usually the efficiency is quite a bit less.
  • it is not universal, in that there are situations where there is no effective "temperature" because there is not a Boltzmann distribution (e.g. lasers). In these situations an equivalent and equally severe restriction applies, but it can not be so simply expressed.
  • the actual maximum efficiency is given by (Thi-Tlo)/Thi which means that the best way to achieve a high efficiency is to relocate to the Antarctic or to a crater well near the lunar pole  ;)
 

Offline Geezer

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Re: What's wrong with Thermodynamics?
« Reply #2 on: 15/02/2012 08:10:39 »
Quite right!

I should have said maximum efficiency. In practice, we would be fortunate to get anywhere near that limit.

The sad thing is that many people seem to assume that it's "just a matter of time" before we get there. The reality is that we will get there at about the same time as we can travel at the speed of light.

Basically, that's my point. No amount of technomumble can overcome the limitations imposed by thermodynamics, but that does not seen to be universally accepted.
 

Offline CliffordK

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Re: What's wrong with Thermodynamics?
« Reply #3 on: 15/02/2012 09:11:00 »
The problem is that...

Say one has a resistance electrical heater.

One puts in 1kWh, and I get out about 3,412 BTU's of heat.

However, working a little smarter, one can run that same 1kWh into a heat pump with a good low-grade heat source, and get back the equivalent of about 15,000 BTU's of heat.

The question is if we take the 3,412 BTU's of resistance heat, or the 15,000 BTU's of Heat pump heat, and tried to generate electricity with it, how much electricity would we get back?

And, I assume that is where the Carnot efficiency comes into play.  In the first case of the resistance heater, we don't do very well.  In the second case with the heat pump, assuming our heat source and cooling source is the same, we can come closer to recovering our energy investment.

So, the only way we can actually have a net electricity output is if we have independent heating and cooling sources.  I.E.  gaining energy from solar heating, geothermal energy input, or perhaps chemical energy input.

 

Offline Geezer

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Re: What's wrong with Thermodynamics?
« Reply #4 on: 15/02/2012 09:37:59 »
The problem is that...

Say one has a resistance electrical heater.

One puts in 1kWh, and I get out about 3,412 BTU's of heat.


That's your problem, right there. What you get out is 1kWh of energy (aka heat). You can measure it it in kWh, BTU, or megaJules, but it is still energy.
 

Offline imatfaal

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Re: What's wrong with Thermodynamics?
« Reply #5 on: 15/02/2012 13:02:28 »
It seems there is a similar mind block when thinking about The Uncertainty Principle - too many people, even science journo.s appear to think that this is a solely measuring problem that could/will be solved by greater technology, clever experiments, powerful statistical methods (ie that there is a clearly defined and exact position, momentum, time, energy etc and we are just too stupid to find it). 

I feel your pain
 

Offline syhprum

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Re: What's wrong with Thermodynamics?
« Reply #6 on: 15/02/2012 15:31:54 »
"The question is if we take the 3,412 BTU's of resistance heat, or the 15,000 BTU's of Heat pump heat, and tried to generate electricity with it, how much electricity would we get back?"
It would depend upon how good a cold sink you had available , if you were on the ISS with a large radiator pointing to the darkness of space I presume from your 15000 btu,s (what ever they may be) you could get more than 1Kw output.
 

Offline Geezer

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Re: What's wrong with Thermodynamics?
« Reply #7 on: 15/02/2012 19:03:47 »
The problem is that...

Say one has a resistance electrical heater.

One puts in 1kWh, and I get out about 3,412 BTU's of heat.

However, working a little smarter, one can run that same 1kWh into a heat pump with a good low-grade heat source, and get back the equivalent of about 15,000 BTU's of heat.

The question is if we take the 3,412 BTU's of resistance heat, or the 15,000 BTU's of Heat pump heat, and tried to generate electricity with it, how much electricity would we get back?

And, I assume that is where the Carnot efficiency comes into play.  In the first case of the resistance heater, we don't do very well.  In the second case with the heat pump, assuming our heat source and cooling source is the same, we can come closer to recovering our energy investment.

So, the only way we can actually have a net electricity output is if we have independent heating and cooling sources.  I.E.  gaining energy from solar heating, geothermal energy input, or perhaps chemical energy input.



Clifford,
 
What Carnot basically says is that the thermal efficiency of any heat engine cannot exceed 1 minus the ratio of the low/high absolute temperatures.
 
For example, suppose we have a heat source at 50C and an ambient heat sink at 25C, the ratio turns out to be a pretty big number (298/323 = 0.92). So, a perfect heat engine would only be able to produce 80 watts of power by processing 1000 watts of heat.
 
Even more depressing, just because the temperature difference is large, it might not help the efficiency. If the high temp is 1500K and the low 1000K, that's a pretty big gradient, but the Carnot efficiency is still only 33% because 1000/1500 is still a large part of 1. That's why running a heat engine "hotter" won't necessarily make it do any more work from the energy supplied.
 
Things get a lot better as the cold side gets closer to absolute zero, but our ambient (even at the poles) is far above zero.
 
Sadly, what all this means is that to extract work from "warm" sources, the mechanism has to process huge amounts of the warm stuff and process it with hardly any losses, otherwise no work will be extracted at all!
 

Offline Bored chemist

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Re: What's wrong with Thermodynamics?
« Reply #8 on: 15/02/2012 20:33:07 »
"The question is if we take the 3,412 BTU's of resistance heat, or the 15,000 BTU's of Heat pump heat, and tried to generate electricity with it, how much electricity would we get back?"
It would depend upon how good a cold sink you had available , if you were on the ISS with a large radiator pointing to the darkness of space I presume from your 15000 btu,s (what ever they may be) you could get more than 1Kw output.


cough dimensions! cough.
 

Offline syhprum

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Re: What's wrong with Thermodynamics?
« Reply #9 on: 15/02/2012 20:43:23 »
Apologies for dimensions muddle I don't realy know what BTU,s are of course if energy is being produced at so many BTU,s per second or minute or what have you then we could talk about KW,s or maybe horse power or CV or PF.
 

Offline Geezer

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Re: What's wrong with Thermodynamics?
« Reply #10 on: 15/02/2012 23:47:17 »
Probably best to stick with megajoules (MJ). I don't know about you, but expressing energy in terms of power in time (as in kWH) always confuses the heck out of me.
 
Power is derived from energy. Energy is not derived from power, although the the electric companies might want you to think that it is to make it harder to understand what you are actually getting for your money.
 
Petrol/gasoline should really be priced in megajoules too.
 

Offline YG

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Re: What's wrong with Thermodynamics?
« Reply #11 on: 16/02/2012 04:52:20 »
Well, I believe there  is nothing wrong with thermodynamics and its basic principles.
All these comments on Carnot cycle efficiency and delta T as measure of conversion efficiency are, in a way, secondary arguments. The primary one is the second law of thermodynamics stating that whatever amount of internal energy is used to produce desired work, part of it inevitably is converted into entropy. The faster the conversion the higher the entropy waste. So, the entropy waste is inevitable. Unless the targeted activity is to warm the Universe and bring it to its thermal death.
« Last Edit: 16/02/2012 04:55:43 by YG »
 

Offline YG

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Re: What's wrong with Thermodynamics?
« Reply #12 on: 16/02/2012 05:00:22 »
As far as what we are purchasing from the energy sources- it is amount of work we can produce, and the amount of energy one can spend to produce the desired work... In this sense it is not that important what units are agreed to be used in day-to-day practice (J, KJ, MJ, Wsec, or kW h, or GW sec, or any other abbreviation. All this is matter of agreement and convenience. Everybody likes to deal with numbers containing just few digits (certainly except the basic constants) within the set of circumstances he is running his activities. Say, a physicist cosmologist, while interpreting the Black Hole mass/energy absorption will use BW sec, in the same time discussing the sub-atomic transformation will use, I guess, pW/event, but will use KWh  while estimating how much his electric bill is going to be 
« Last Edit: 16/02/2012 05:16:58 by YG »
 

Offline syhprum

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Re: What's wrong with Thermodynamics?
« Reply #13 on: 16/02/2012 06:46:42 »

No one has commented on my idea to increase the power available on the ISS with the aid of a Stirling engine and a large space cooled heat sink, I expected heavy criticism to fall on me
 

Offline Geezer

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Re: What's wrong with Thermodynamics?
« Reply #14 on: 16/02/2012 07:01:23 »
Well, I believe there  is nothing wrong with thermodynamics and its basic principles.
All these comments on Carnot cycle efficiency and delta T as measure of conversion efficiency are, in a way, secondary arguments. The primary one is the second law of thermodynamics stating that whatever amount of internal energy is used to produce desired work, part of it inevitably is converted into entropy. The faster the conversion the higher the entropy waste. So, the entropy waste is inevitable. Unless the targeted activity is to warm the Universe and bring it to its thermal death.

YG,

You obviously understand what is going on, so you are not in the least surprised by the Carnot limits. However, a large number of people, some of whom are well versed in many scientific fields, do not seem to be aware of the Carnot limits. If they were, they might have a better appreciation of how difficult it can be to convert heat into work.

That was the point of this topic. I was encouraging people to appreciate that there are fundamental limits associated with thermodynamics that are not dissimilar from another fundamental limit that is very well accepted; the speed of light.
 

Offline Geezer

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Re: What's wrong with Thermodynamics?
« Reply #15 on: 16/02/2012 07:28:23 »
As far as what we are purchasing from the energy sources- it is amount of work we can produce, and the amount of energy one can spend to produce the desired work... In this sense it is not that important what units are agreed to be used in day-to-day practice (J, KJ, MJ, Wsec, or kW h, or GW sec, or any other abbreviation. All this is matter of agreement and convenience. Everybody likes to deal with numbers containing just few digits (certainly except the basic constants) within the set of circumstances he is running his activities. Say, a physicist cosmologist, while interpreting the Black Hole mass/energy absorption will use BW sec, in the same time discussing the sub-atomic transformation will use, I guess, pW/event, but will use KWh  while estimating how much his electric bill is going to be 

It's not the amount of work we can produce.

What we are buying is energy. How we choose to use that energy is up to us. If we could easily compare the prices we are  paying for different forms of energy, we might be sufficiently shocked to consider alternative sources, or alternative methods of utilizing that energy.

Most people have not the faintest clue how to compare what they are getting from various suppliers, and the suppliers really like it that way because it preserves their revenue. The SI unit of energy is the joule. Other than the usual inertia and protection of self-interests, there is absolutely nothing to prevent all energy from being priced in joules (probably MJ).

For example, how many people would continue buying premium gasoline if they realized they would get more energy by buying regular gasoline?
 

Offline Geezer

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Re: What's wrong with Thermodynamics?
« Reply #16 on: 16/02/2012 08:06:03 »

No one has commented on my idea to increase the power available on the ISS with the aid of a Stirling engine and a large space cooled heat sink, I expected heavy criticism to fall on me

OK - so you want a beating? I'll give you a beating. The BTU is the BRITISH THERMAL UNIT! (Not to be confused with the brutish thermal unit, which is a different thing entirely.)

Actually, I think you are right on the money. Space is a great place for heat engines because the cold side can be incredibly cold, and because of that, the Carnot efficiency can be high. After that, it's only a matter minimizing any parasitic and entropic losses! 
 

Offline damocles

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Re: What's wrong with Thermodynamics?
« Reply #17 on: 16/02/2012 08:29:23 »
There is a major and fundamental problem with the concept of buying energy. It relates to the first law of thermodynamics. When we "use" energy, we do not consume it: it is all still there! Where? Well it might be spread around a bit, but perhaps we could collect it all up, and use it over and over without having to pay for any more?

That is where the second law comes in. It is stated in many ways, most of them pretty obscure and impossible to understand. What it is really saying is that energy comes in forms of different quality, and that in anything we do, we are turning, useful, high quality energy into useless, low quality energy.

The ultimately low quality energy is low grade heat. It is easy to add 1 litre of water at 100°C to 79 litre of water at 20°C to make 80 litre of water at 21°C. It is impossible to take 80 litre of water at 21°C and make 1 litre of water at 100°C for coffee all round (along with 79 litre at 20°C) without using energy, high quality energy, from elsewhere!

So our aim in energy conservation should always be to march down that usefulness hierarchy as gradually as possible, making good use of energy all the way down. Using LED or compact fluorescence lighting rather than incandescent is a good step for example -- more light output, and less heat for a given amount of energy.

It might also be a good move to use the hot water that is a low quality energy offshoot of nearly any heavy industry for neighbourhood domestic heating but that would depend on the energy cost of setting up and maintaining the necessary infrastructure, which would need to be calculated on a "dust to dust" basis.

Capital energy cost of compact fluorescent or LED manufacture and safe disposal must similarly be factored into consideration of replacing incandescent lamps.

What about premium gasoline? Well, you are not just buying the energy content of the gasoline, because the energy quality of the gasoline is an important factor. And that is not just a property of a particular blend of gasoline, but also of how it matches the design of the particular engine it is to be consumed in, on an effective efficiency basis. Any vehicle engine has manufacturing design specifications: if your car engine is designed to run on standard petrol, you are wasting your money and energy resources to fill up with premium. But if it is designed to run on premium blend, you will get much better performance if you do not switch to standard, because although the fuel energy content may be lower, the efficiency factor will be significantly higher.
 

Offline damocles

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Re: What's wrong with Thermodynamics?
« Reply #18 on: 16/02/2012 08:39:53 »
Actually, I think you are right on the money. Space is a great place for heat engines because the cold side can be incredibly cold, and because of that, the Carnot efficiency can be high. After that, it's only a matter minimizing any parasitic and entropic losses! 

I am not nearly so sure about that! Actually, you do not just need a low temperature, you also need a large heat sink or "thermal bath" to dump the waste low grade energy that you will produce. And space is incredibly empty!

Also space, at least in the inner solar system as far out as the orbit of Jupiter, is really very hot! The prevailing temperature is thousands of degrees. Basically, the molecules and ions that are there, many of them from the "solar wind", will not hit you very often, but they will hit you very hard when they do -- high temperature, low heat content.
 

Offline CliffordK

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Re: What's wrong with Thermodynamics?
« Reply #19 on: 16/02/2012 08:48:08 »
The ultimately low quality energy is low grade heat. It is easy to add 1 litre of water at 100°C to 79 litre of water at 20°C to make 80 litre of water at 21°C. It is impossible to take 80 litre of water at 21°C and make 1 litre of water at 100°C for coffee all round (along with 79 litre at 20°C) without using energy, high quality energy, from elsewhere!

Which is the purpose of the Heat Pump, but yes, it does require some additional energy.

Sorry to those purists when I suggested using BRITISH UNITS on a BRITISH WEBSITE to indicate the conversion from electrical energy to thermal energy.
 

Offline CliffordK

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Re: What's wrong with Thermodynamics?
« Reply #20 on: 16/02/2012 09:11:07 »
Also space, at least in the inner solar system as far out as the orbit of Jupiter, is really very hot! The prevailing temperature is thousands of degrees. Basically, the molecules and ions that are there, many of them from the "solar wind", will not hit you very often, but they will hit you very hard when they do -- high temperature, low heat content.
It is really hot in the sunshine, but really cold in the shade.

The polar craters on the moon, in mid-winter have been measured at a balmy 25K, at least on the surface, which would be excellent for a cold sink.  I think there was a discussion earlier about heat sinks to radiate heat which could be a problem on a free floating object in space though.

Looking for references I bumped into this.  Could we generate electricity directly from the polar craters?
 

Offline syhprum

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Re: What's wrong with Thermodynamics?
« Reply #21 on: 16/02/2012 09:20:47 »
Is this a British web site ?, are we to exclude non Brits from publishing ?, perhaps we true Brits could use blue text while lesser types could be made to use red.
 

Offline damocles

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Re: What's wrong with Thermodynamics?
« Reply #22 on: 16/02/2012 09:21:49 »

Sorry to those purists when I suggested using BRITISH UNITS on a BRITISH WEBSITE to indicate the conversion from electrical energy to thermal energy.

Clifford, you may or may not be aware that BRITISH UNITS like BTUs along with calories, imperial measures, avoirdupois weights have long been abandoned by scientists in every country of the world except the US, as well as by engineers in the old British Empire (but not sure about Britain itself). The United States of America is the last stand out bastion of British weights and measures (and manages to have the odd space program disaster as a consequence of unit mismanagement). There is a certain irony in that!
 

Offline imatfaal

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Re: What's wrong with Thermodynamics?
« Reply #23 on: 16/02/2012 10:00:28 »
Sorry to those purists when I suggested using BRITISH UNITS on a BRITISH WEBSITE to indicate the conversion from electrical energy to thermal energy.

further to Damocles' point - you are not even allowed to sell fruit and veg here in the UK without metric masses displayed.  Everyone from a few years older than me learnt only the metric system at school.  I know there are 12 inches in a foot, 3 feet in a yard, but would struggle to remember how many yards in a mile.  The only imperial measurement that I can think of that is still in common and official use is the mile - the foot is still common but not I think officially used by the powers that be
 

Offline CliffordK

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Re: What's wrong with Thermodynamics?
« Reply #24 on: 16/02/2012 11:03:50 »
but would struggle to remember how many yards in a mile.
It is easy enough.
440 yards to the quarter mile (which used to be the track standard until someone changed it to a metric equivalent to the mile, 1600 meters), out of which you can derive the rest.

The only imperial measurement that I can think of that is still in common and official use is the mile

Don't you still use the gallon over there.
Which the British conveniently changed AFTER the US Adopted the British Queen Anne Gallon.

Do you use Calories for nutrition?

 

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Re: What's wrong with Thermodynamics?
« Reply #24 on: 16/02/2012 11:03:50 »

 

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