# Heat Energy

## Main Question or Discussion Point

This is actually pretty embarrassing for me but a question just popped in my head today and in my three years of studying Engineering Physics nobody gave me the answer:

Why is it that heat effects are always considered as energy loss?

Really. I understand that's not really convenient if energy dissipates from your device to the outside environment, but isn't it possible to compensate that effect by building a second system around your device wherein you do something useful with that heat? (I don't know, like for instance create energy by letting a thermal flow drive some sort of fan, a sort of wind energy)

Or is the rub simply that this process always keeps occuring: so that in your second system there is also heat dissipation, in the system you build around that one too, etc. etc.

But even then, doesn't that make it odd that in practically every device there is no such thing as a second system? Or would such a system simply be ridiculously inefficient? If so, why? Is it because this heat dissipation is not controlled in any way, and so you can't design your second system in such a way that it functions in harmony with your dissipation effects?

## Answers and Replies

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Gokul43201
Staff Emeritus
Gold Member
Look up "cogeneration". Utilising waste heat becomes spatially and economically feasible at the large scales.

Right, but cogeneration systems still see heat as an energy loss. Basically, what they do is minimize the energy loss, by using the waste heat to produce the required heat in certain secundary systems, instead of using useful energy to produce the required heat. But still the division between useful and lost energy is made.

Maybe this is because useful energy can be converted into heat, but heat can't be converted into useful energy?

Hm. I guess I just wanted a good reason to say why it's never smart to convert heat energy into some other form. Maybe the best reason I will be able to find is Carnot efficiency. And that's a damn good reason, of course - hell I won't be so whacky to start questioning thermodynamics - I just wonder if thermodynamics isn't limiting the picture too much. In simple, orderly behaving systems, sure you'll always have to consider heat dissipation as a loss.
But suppose you get something like this:

you have your heat dissipation, which you try to store somehow, with a minimum amount of work, and get some heat building up. In the environment where you are building up this heat, you establish a chemical reaction which is pretty straight forward and natural and easy to keep in storage under normal conditions, but which is totally altered when it reaches some far-from-equilibrium state. It's pretty reasonable to suppose a far-from-equilibrium state that behaves chaotic (such things exist), and I'm not sure but I suppose that this chaotic behaviour can be used for certain purposes.

Anyway, this hypothetical scene is just an illustration, my point here is: in normal conditions, it's never sensible to do something different with heat than.. well, heat. By doing something different with it, (correct me if I'm wrong), you are already taking a detour along the efficiency road and your overall efficiency will be less.
However, suppose you need a certain temperature difference to start up something. I always imagine a mathematical landscape with local minima and maxima, some local maxima being actual plateaus, and the system being stranded somewhere at a local maximum; the system simply needs a big enough push to roll off the local maximum plateau, then move up the slope of the global maximum, and reach the most beneficial state of the system.
To reach this global maximum, a certain 'thermal push' is needed. So, in such cases, maybe it is interesting to store heat energy?

I guess that makes me feel uneasy about thermodynamics. It looks so damn... linear - when surely, it can't be as easy as that?

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ZapperZ
Staff Emeritus
But this is the very reason why it is rather easy to break a vase into many pieces, but it isn't easy to get it back into its original form.

You are missing another consideration within Thermo. It isn't just a matter of the First Law (conservation of energy), but also the Second Law (entropy). It isn't just the energy content, but also the FORM of that energy that can affect the amount of useful work that can be extracted. Heat is the "easiest" form of randomized motion, the very same way that a shattered vase is the easiest randomized form.

Zz.

Sorry to be so stubborn: but I don't see how that is any help. We're not talking about a closed system here. Naturally, the 2nd law implies that it is impossible to store heat energy without help from an external energy source. That's not the point. Let the closed system be the open system I described above, plus some sort of external energy source big enough to 'push' the system into its global maximum state. This is perfectly possible (again, correct me if I'm wrong), even considering the first and second law of thermodynamics.

The whole point of what I'm saying boils down to: are there situations conceivable where the increase of performance by the system, because it is being pushed into its optimal state, is so high that even though you had to deliver 'extra' energy from an external source, it is still worthwhile to do so?

Or: is it possible to prove that such situations can never exist?

ZapperZ
Staff Emeritus
Tsunami said:
Sorry to be so stubborn: but I don't see how that is any help. We're not talking about a closed system here. Naturally, the 2nd law implies that it is impossible to store heat energy without help from an external energy source. That's not the point. Let the closed system be the open system I described above, plus some sort of external energy source big enough to 'push' the system into its global maximum state. This is perfectly possible (again, correct me if I'm wrong), even considering the first and second law of thermodynamics.

The whole point of what I'm saying boils down to: are there situations conceivable where the increase of performance by the system, because it is being pushed into its optimal state, is so high that even though you had to deliver 'extra' energy from an external source, it is still worthwhile to do so?

Or: is it possible to prove that such situations can never exist?
No, I'm not talking about a closed system either. The "system" being heated, cooled, expanded, and contracted in a carnot cycle isn't a closed system. Yet, look at the net entropy change. That's the limiting, ideal case for all real cycle.

The problem remains that NOT ALL heat in a system can be used to do work. This is essentially a part of the 2nd Law. So this is why your idea of using ALL of the "heat loss" to generate useful work will run into trouble.

Zz.

Clausius2
Gold Member
Tsunami said:
This is actually pretty embarrassing for me but a question just popped in my head today and in my three years of studying Engineering Physics nobody gave me the answer:

Why is it that heat effects are always considered as energy loss?

Really. I understand that's not really convenient if energy dissipates from your device to the outside environment, but isn't it possible to compensate that effect by building a second system around your device wherein you do something useful with that heat? (I don't know, like for instance create energy by letting a thermal flow drive some sort of fan, a sort of wind energy)

Or is the rub simply that this process always keeps occuring: so that in your second system there is also heat dissipation, in the system you build around that one too, etc. etc.

But even then, doesn't that make it odd that in practically every device there is no such thing as a second system? Or would such a system simply be ridiculously inefficient? If so, why? Is it because this heat dissipation is not controlled in any way, and so you can't design your second system in such a way that it functions in harmony with your dissipation effects?
I will talk you as a quasi-engineer (damn undergraduate project!! :rofl: ).

As Gokul has pointed, there are cogeneration systems which take advantage of this heat released. When some device (power plant, engine, or any machine) dissipates some amount of heat you have to take into account two points:

i) this dissipated energy is not entirely recoverable into mechanical energy form. As Zz has said, the second principle forbids extracting all the mechanical energy available into this heat released. The conversion in mechanical energy again uses to have an efficiency approximately of 40% in the best case.

ii) the heat released has to have an appropriate "thermal level" in order to be useful for further uses. I mean, an exhaust flow of gas of some machine at 40ºC, or the heat exhausted in the condenser in a Rankine power plant at 35ºC, both of them has none useful use. Heats at 100ºC or so can be re-used to feed the generator of an absorption refrigeration machine, or for a wide range of industrial processes. Taking an advantage of this heat is wide practiced in industry.

An example of this cogeneration is your car. If your car is enough old, when you turn on the heating in winter, the engine is cogenerating the heat exhausted to the cooling circuit and gives it the utility of warming you inside the habitacle.

ZapperZ said:
No, I'm not talking about a closed system either. The "system" being heated, cooled, expanded, and contracted in a carnot cycle isn't a closed system. Yet, look at the net entropy change. That's the limiting, ideal case for all real cycle.

The problem remains that NOT ALL heat in a system can be used to do work. This is essentially a part of the 2nd Law. So this is why your idea of using ALL of the "heat loss" to generate useful work will run into trouble.

Zz.
That's already a big help... thanks for spelling it out.

russ_watters
Mentor
Tsunami said:
Right, but cogeneration systems still see heat as an energy loss. Basically, what they do is minimize the energy loss, by using the waste heat to produce the required heat in certain secundary systems, instead of using useful energy to produce the required heat. But still the division between useful and lost energy is made.
There is no way to completely eliminate energy loss because devices that use energy require a source at a high energy level, a sink for a low energy level, and the energy capture device to be somewhere in between (it can never be all the way at the bottom). Getting useful work requires the flow of energy from high to low and the amount of flow depends on the difference in energy level. The only time this doesn't apply is if heat itself is the useful energy you are looking for (ie, in an electric heating coil).

For thermodynamic systems, the energy level difference comes from difference in temperature - and the larger the difference in temperature, the easier it is to extract useful work.

But even for purely mechanical systems, the same set of rules applies. A hydroelectric dam requires a difference in height of the water to make it flow and produce energy. And the losses in the turbine and pipes are all friction: all heat.

HERE is some good info on the 2nd law of thermo as applied to hydro power, heat engines, etc.

edit: oy, big mistake...fixed now.

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FredGarvin
Tsunami,
You may want to look up 'Availability' in your thermo text.

Unfortunately, thermodynamics does limit the way we are and zillion generations have passed trying to negate the two famous laws but none succeeded in inventing the PMMs. Generally, heat is classified as low grade energy and mechanical work(in literal sense) and electricity are classified as high grade energy. It is just not possible to convert, fully, a low grade energy to a high grade energy.

However, there are some drawbacks in the present system of energy generation and usage and with the advent of better technology and controls, it may be good in future.

For example, Lord Kelvin long back proved the thermodynamic inefficiency of burning wood to heat up the rooms. The COP of burning wood is 1 and if this heat is used to run a heat engine which subsequently runs a heat pump the COP can be raised beyond 6. But you have to cough of good amount of capital cost. We alwasy try for a breakeven.

I seldom find it easy to discuss these issues without a word of philosophy. Just keep in mind that nature takes its own ways.