VASIMR

[Giorgio]

As far as I can tell, Edwards does not actually claim that thermal energy can be converted at higher than Carnot efficiency.

I wonder WHO ever affirmed that this is possible.

His wording is strange in spots (for instance, he appears to classify internal combustion engines as separate from heat engines *cough*), but he does seem to understand what he's talking about.

Seems so..... but probably is just due to the fact that mechanical Engineers like him and me do understand about this stuff.....
Also it's classification of engines follows a logic that you are not grasping, probably becouse you are rejecting the concept of Exergy.

The stuff about the Carnot limitation being an exception seems to mean not that Carnot isn't a limit on a thermal engine, but rather that it is not what's holding us back; other things are in the way and need to be considered too.

He is actually stating something much more different and profound, but if you didn't get it I can't possibly explain it better than the way he did in those pdf.

This is wrong. Exceeding Carnot with thermal energy breaks the Second Law - period.

Again (for maybe the 30th time...), WHO ever affirmed that this is possible?

And quit talking about "exergy" as if it's some brand-new concept that invalidates conventional thermodynamics. Rejected heat at the minimum available temperature (as in a Carnot cycle) is not exergy.

Rejected heat is only useful if you are able to access a state at a lower temperature than that of the rejected heat. Regenerative cooling is a good example of this - the fuel or intake air is at a lower temperature than the exhaust, so some of the thermal energy in the exhaust can still be made useful. A bottoming cycle in a combined-cycle power plant is another good example - using the exhausted heat as the heat source for another heat engine. But this does not break Carnot - it just gets you closer.

You can dodge Carnot if you use a "combined-cycle" power plant that involves non-thermal conversion, such as a solid-oxide fuel cell operated in combination with a gas turbine (this combination is actually a nice one for a couple of reasons). But from a conversion pathway perspective this is actually a parallel implementation in which some of the available chemical energy does not pass through a thermal state at all, and thus Carnot does not apply to the whole system.

These passages makes clear to me that you do did not get anything of what I have been talking about until now.... Funniest part is that you think that Exergy should in some way invalidate thermodynamic laws, I am really not following your logic.

Anyhow, I find quite useless to try to explain again.
Keep your convinction, I will wait until technology and material advancements will allow Prof. Edwards to build an engine according 1st thermodynamic law concept, so that people like you will finally understand that Carnot is just a part of a much bigger picture.

[ladajo]

amen.
But, to be fair, I am just a patent clerk, so what do I know...

[MSimon]

How about a concrete example:

A high temp fuel cell feeds its waste heat to a thermal engine. The fuel cell is not governed by Carnot. The thermal engine is.

Or you run a gas turbine at 1300 K (roughly) and take its exhaust heat at 600 K and feed it into a steam cycle. Carnot all the way.

[D Tibbets]

My feeble attempt at boosting efficiency. A secondary or even tertiary steam turbine might marginally approach the total aviable Carnot efficiency. But never exceed it. Passing the waste heat through a series of thermocouples might retrieve a small additional amount of energy (still within the Carnot limit, but getting closer).

A cheat, if you use loose definitions. A breeder type process could be used-
Burn coal, and use steam conversion to get as much power as possible within Carnot limits. The residual waste heat is fed into an algae farm, where the increased temperature results in an assumed 3 fold increased growth rate. The algae is then dried and fed into the incinerator. This would increase efficiency in terms of the net power out vs the coal in. If every stage is very efficient, you might even exceed the output predicted by Carnot limits on the original coal. Of course to be honest you would have to include the solar energy used for photosynthesis and the CO2 from the atmosphere, the cost of drying....

Dan Tibbets

[Giorgio]

Guys,

as long as you keep thinking in terms of burning the fuel you have available to just get the heat and apply o simple Carnot cycle (or more than one) you are never going to improve the general efficiency of the system....

Let's try to get out of this "heat" issue like if is the heat the first block of our process, becouse it is not. It's the fuel the starting block.
Unless we all share the same common starting point we are talking about different things, and we will never understand each other.

[MSimon]

Guys,

as long as you keep thinking in terms of burning the fuel you have available to just get the heat and apply o simple Carnot cycle (or more than one) you are never going to improve the general efficiency of the system....

Let's try to get out of this "heat" issue like if is the heat the first block of our process, becouse it is not. It's the fuel the starting block.
Unless we all share the same common starting point we are talking about different things, and we will never understand each other.

I've noticed that.

[Giorgio]

Good to know that at least on something we agree...

[93143]

As far as I can tell, Edwards does not actually claim that thermal energy can be converted at higher than Carnot efficiency.

I wonder WHO ever affirmed that this is possible.

You did:

3) Rejected heat (which is residual Exergy of the starting system) is OUTSIDE the Carnot cycle, but that Exergy is still usable and nothing theoretical prevents me from recovering part of that unused Exergy and convert it to work by other process, thus increasing the "Exergy to work" conversion efficiency. I repeat, nothing.
The limitations we have now are just technical.

4) Because of this we can have a final Exergy to work conversion efficiency that is higher than if the simple Carnot cycle is applied to the starting Exergy. And we can do this without breaking the 2nd law of thermodynamics.

What you don't seem to understand is that a string of heat engines can also be considered a heat engine and is thus limited by Carnot. I've said this before; evidently you didn't get it.

If the waste heat is at a higher temperature than the lowest available, Carnot isn't tapped out and you can use the waste heat to get closer - never to exceed, of course. If it isn't, you're done.

Carnot efficiency doesn't specify what the conversion process is; it's a black box. If the energy input winds up as heat at some point, Carnot applies after that point. Period. Any system or system of systems that breaks this principle can be set up as a perpetual motion machine of the second kind.

but probably is just due to the fact that mechanical Engineers like him and me do understand about this stuff.....

*sigh* I don't like having to throw around my qualifications, never mind twice in the same thread...

I've got two degrees in mechanical engineering and am halfway through a Ph.D. in aerospace engineering, focusing on compressible computational fluid dynamics of non-ideal fluids, specifically combusting sprays thereof as found in jet and rocket engines. During my undergraduate degree I showed a tendency to get perfect scores on thermodynamics exams while under unusual stress and preoccupied with other things.

Also it's classification of engines follows a logic that you are not grasping, probably becouse you are rejecting the concept of Exergy.

I never rejected the concept. Read this carefully:

...quit talking about "exergy" as if it's some brand-new concept that invalidates conventional thermodynamics.

The idea of exergy is not a new one, and Edwards' use of it does not make his thinking a massive paradigm shift.

The trouble is, combustion is inherently irreversible, because thermal energy is inherently high-entropy. This means that combustion destroys exergy. An engine that employs bulk combustion for its entire exergy throughput cannot exceed Carnot, because once the chemical energy is converted to thermal energy, the exergy is equal to the available work under Carnot efficiency.

Edwards ran into this with the low-temperature, "low-irreversibility" combustion research. All it did (aside from practical considerations, which are irrelevant here) was decrease the average temperature of the heat addition, thus decreasing the maximum possible thermal efficiency in accordance with Carnot. You can't make a thermal engine act like a fuel cell just by running it cooler.

As I've said before, you can dodge Carnot, but only if at least part of your system skips the thermal step entirely.

The stuff about the Carnot limitation being an exception seems to mean not that Carnot isn't a limit on a thermal engine, but rather that it is not what's holding us back; other things are in the way and need to be considered too.

He is actually stating something much more different and profound, but if you didn't get it I can't possibly explain it better than the way he did in those pdf.

Of course that's not the only thing he's stating. But it's certainly a component of his work, and there is no indication he's onto anything that invalidates the thermal efficiency limit as defined by Carnot. To beat Carnot, you have to get around the thermal step, and some of the concepts he mentions do this (fuel cells are nice for this - but of course he didn't invent them).

He's not stating anything particularly profound. You just think he is, because you were misled by his marketing due to not being the expert you think you are.

Funniest part is that you think that Exergy should in some way invalidate thermodynamic laws, I am really not following your logic.

...no, you really aren't. I said no such thing; see above.

Keep your convinction, I will wait until technology and material advancements will allow Prof. Edwards to build an engine according 1st thermodynamic law concept, so that people like you will finally understand that Carnot is just a part of a much bigger picture.

Of course Carnot is part of a much bigger picture.

You want a First Law engine that isn't limited by Carnot? Hook a fuel cell to an electric motor. BTDT.

You have to skip the thermal step for at least part of the exergy throughput if you want to beat Carnot. There are ways to do that, and it is indeed a great advantage in many applications - why do you think the space fans on this board are so excited about the possibility of direct conversion in a p-¹¹B Polywell?

I have no problem with what Edwards is saying, and I'm pretty sure I understand it fine. The reason I think you don't... well, take another look at the top of this post.

[Giorgio]

Again, you are mixing what I said with your terminology (and definitions) and you apply it to me to make me say things I didn't say.

Like I said before before, until we have a common terminology and a common process flow this discussion is useless.

If you don't have problems with what professor Edwards is saying that's perfect, it means that we are saying the same thing.



Edited to fix some mistakes and to add the following:

Prof Edwards work is much more than simply stumbling across some "low-temperature, "low-irreversibility" combustion research" like you stated.

You have to skip the thermal step for at least part of the exergy throughput if you want to beat Carnot. There are ways to do that, and it is indeed a great advantage in many applications - why do you think the space fans on this board are so excited about the possibility of direct conversion in a p-¹¹B Polywell?

I doubt that more than a couple here cared about Carnot as first thought while thinking of P-B11 fusion. The biggest hope for p-B11 fusion is that it should be aneutronic and that will be it's biggest advantage, followed by direct conversion of produced energy.
This is just to point to you that if you are too much focused on a subject you might only see your point of view and miss the point of view of others.

[krenshala]

I doubt that more than a couple here cared about Carnot as first thought while thinking of P-B11 fusion. The biggest hope for p-B11 fusion is that it should be aneutronic and that will be it's biggest advantage, followed by direct conversion of produced energy.

Most of this thread is over my head (some of it by more than other portions) but even I immediately grasped the advantages of a direct conversion system (via p-B11) vs a thermal cycle (DD or DT) for the amount of "available" energy from a Polywell, or any other fusion device for that matter. While some of us aren't as knowledgeable about the details of Carnot, I think it would be fair to say that most of us at least understand the high-level details of it -- i.e., if you can skip the thermal steps, you end up with better efficiency.

[ladajo]

I bet if you piled up all the higher education degrees and whatnots reading and posting to this thread, and took them to starbucks, you could take them all and $5 and get a cup of coffee.

Something I learned long ago, ironically enough in Nuclear Power, Smart People never agree. Or, in the long ago words of Douglas Buel, "we're not arguing, I'm telling you where you are wrong."

[KitemanSA]

You have to skip the thermal step for at least part of the exergy throughput if you want to beat Carnot. There are ways to do that, and it is indeed a great advantage in many applications - why do you think the space fans on this board are so excited about the possibility of direct conversion in a p-¹¹B Polywell?

Sorry, direct conversion in a p-¹¹B Polywell would still be limited by Carnot. Of course, the temperature differences would be HUGE so the Carnot efficiency would approach 100%.

[93143]

You have to skip the thermal step for at least part of the exergy throughput if you want to beat Carnot. There are ways to do that, and it is indeed a great advantage in many applications - why do you think the space fans on this board are so excited about the possibility of direct conversion in a p-¹¹B Polywell?

Sorry, direct conversion in a p-¹¹B Polywell would still be limited by Carnot. Of course, the temperature differences would be HUGE so the Carnot efficiency would approach 100%.

Nope.

The alpha energy distribution is not thermal, and the velocity profile is almost completely radial. Second Law doesn't apply, never mind Carnot.

[KitemanSA]

Nope.
The alpha energy distribution is not thermal, and the velocity profile is almost completely radial. Second Law doesn't apply, never mind Carnot.

We may be talking a distinction without difference cuz both allow nearly 100% efficiency.

What most people have a problen with wrt Carnot is that most materials only allow relatively low upper temperatures. This limits the efficiency to relatively low percentages. The direct energy conversion process does not have the same limit in that no solid material is encountered until the cold part of the cycle.

[93143]

Nope.
The alpha energy distribution is not thermal, and the velocity profile is almost completely radial. Second Law doesn't apply, never mind Carnot.

We may be talking a distinction without difference cuz both allow nearly 100% efficiency.

What most people have a problen with wrt Carnot is that most materials only allow relatively low upper temperatures. This limits the efficiency to relatively low percentages. The direct energy conversion process does not have the same limit in that no solid material is encountered until the cold part of the cycle.

No, in this case Carnot doesn't apply at all. Carnot is a function of the Second Law of Thermodynamics, and if the energy is not heat in thermodynamic equilibrium, the Second Law itself is inapplicable.

Think of a shock wave. Fluid flowing through a shock experiences a rapid initial rise in entropy, but as it leaves the other side of the shock wave the entropy actually decreases (though not all the way down to what it was upstream of the shock). Why is this? Because the flow inside a shock wave is not in thermodynamic equilibrium. The velocity distribution is not a Maxwellian.

I'm not saying entropy necessarily decreases during the direct conversion process, or that the efficiency is higher than the Carnot efficiency for a 34 billion degree hot side and a 20 degree cold side. I'm just saying that the relevant physical law doesn't apply in this case, so if entropy increases and the efficiency is below Carnot (this last in particular is pretty much guaranteed), it's for other reasons.

I am also not saying that the whole process could ever result in a net decrease in entropy. The Second Law still applies to the start and end states. But the nuclear energy is not present as thermal energy in the initial state, and at no point in the process does it take the form of heat in thermodynamic equilibrium, so its extraction is not governed by Carnot.