Physicists help needed!!! Current technology limitations.

Well, there you go again. Physical laws (except in the relativistic sense which doesn't apply on the surface of the Earth where we all live) are totally invariable . They only vary in the non- or semi-educated minds of those who do not understand them.

 

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Yeah, that would be you all over the place. You aren't tracking your own inconsistencies and I'm not paid enough to do it.

 

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While it doesn't apply much to cars overall thermal efficiencies of stationary engines utlilzing cogeneration processes can be pretty good. Specifically I'm talking about large diesel engines running generators in the 1 - 5 MW range utilizing heat recovery from jacketwater, aftercooler and exhaust systems. The heat recovered is used in other processes a manufacturing or chemical plant may require, ambient heating, or to produce steam, etc. The engines themselves are getting pretty good - especially spark-ignited engines running on natural gas - up to a claimed 43% efficiency on a low speed Caterpillar 3500.

 

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You aren't being paid anything and I've presented no inconsistencies at all. You, however...

 

That's interesting, Flunch.

I've also wondered a few times about using additional heat recovery in ordinary power generating plants. It would have to be done carefully, though, so as to not back-throttle the turbine exhaust.

Normally, at least in the ones I've seen, they use water sprayed on the turbine exhaust and the resulting hot water is first used to pre-heat the water being pumped to the boilers and is then routed to the cooling towers where the temperature is knocked down enough for discharge into a nearby river.

It's always seemed to me that between the inlet pre-heating stage and the cooling towers, a fair part of that waste heat could be used to drive something like a very large bank of Stirling's or something similar. Perhaps even vapor-driven turbines that use a low-boiling point working fluid in a sealed system.

 

Light,

This sort of heat recovery system I was talking about is making more sense now that energy is no longer dirt cheap. It's used quite a bit in remote situations like mine sites where multiple large diesels or heavy-fuel engines run all the electrical power for the mine. The heat is recovered using heat exchangers instead of being dumped to a radiator bank and is used to heat the buildings at the mine site.

 

Yes, that would be very apporiate indeed.

Another class of situations where that would make sense are industries that use large amounts of heat in their routine daily business. Without giving it a great deal of though, one that comes to mind immediately for co-generation is a bakery. Generate the electrical power they need from natural gas (which currently fires many power generating stations) and use the "waste" heat in their ovens. I'm sure there are many more applications very similar to that. (Breakfast-food cereal companies, any process that requires drying, etc.)

 

yup, those would work. Just supplying residential heat through a hot water distribution system is also an option within a small radius of the plant. This is done in Europe I believe. I am aware of a Hospital that is supplied heat and steam from an operating plant here in Canada.

 

There's a fair amount of that done here in the states. Not residential, but commercial in major cities where urban growth has expanded and, in many cases surrounded local generating facilities. I've seen it in NYC, Chicago, Atlanta, and at least a few others. They could probably do it in others as well. I'm not sure, though, if it would be practical for individual homes. Dense apartment neighborhoods might work. Laying any kind of underground utilities here is very expensive and the payback period might extend into decades.

 

Agreed. I think much of Europe has the population density to make it worthwhile.

Laying underground utilities is perhaps even more expensive up here on Canadian shield - and virtually impossible in the arctic - but there they use insulated above-ground piping in utilidors.

 

So, Flunch, what do you think the Carnot equation says about fuel efficiency?

 

The Carnot equation for power cycles states that thermal efficiency can never be greater than unity. n = 1 - Qout/Qin. Practically, because of entropy and other losses a thermal efficiency of unity is not attainable. Most modern conventional power cycles (engines) have thermal efficiencies ranging up to around 40%

The maximum theoretical efficiency dictated by the Carnot equation for ANY reversible (meaning ideal, no entropy) power cycle can be expressed as n(max) = 1 - Tcold/Thot. Thus, the efficiency would approach 100% as the difference between the hot and cold reservoirs approaches infinity.

For heat pumps, the coefficient of performance (CoP) = Qout/(Qout-Qin) so it can never be less than unity. The coefficient of performance is a way of describing the performance of the heat pump but it is a different kind of ratio than is expressed by the efficiency for power cycles above.

For a heat pump, the maximum (Carnot) efficiency of a reversible process is CoPmax = Thot / (Thot-Tcold)

That is basically what the Carnot equation says.

 

So what does it say about fuel efficiency? I'm giving away a whole store here today, you might want to pick up on it.

 

I don't know what you're trying to get at... but I think Light was right... I don't think you know what you're talking about.

 

What does thermal efficiency mean about fuel efficiency? Show your work.

 

What are you trying to ask me, if there is a difference between thermal and fuel efficiency?

Fuel effiency is loosely used to indicate how much fuel it takes to produce a certain amount of work. Thermal efficiency is the correct terminology when looking at a thermodynamic system (could be any kind of IC engine, a heat pump, a refrigerator, etc) - indicating how much useful work (or heat) is produced compared with how much energy was available to the system.

Again, I don't know if that's what you were after...

 

Fuel efficiency is what matters. What percentage of the heat supplied by the fuel does your engine use?

 

Hello Flunch,

I think I can explain. I'll keep it very civil, too.

The problem seem to be that he has learned some very fundamental principles and terms - like the Carnot cycle, basic thermodynamics, fuel efficiency, etc. and doesn't yet quite understand how they translate into the real world.

He's still down at the lower level where every single exercise he's encountered still says things like "neglecting friction, wind resistance, and so forth. And that's why I had so much difficulty with him on heat pumps. He insisted - even at the very end - that they violated the 2nd law of thermodynamics.

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And that's why I tried to explain to him that he would understand in a few years - implying that once he got beyond the basics taught in high school and closer to the practical applications. I well remember being at that stage once, and I assume we all were. As in, "The principle says 'this' - so you are wrong!"

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Addendum:

To show one of the reasons I said what I just did, Flunch, take a look again at this quote from him,"Originally Posted by MetaKron
Have you read this discussion? I'm the one who has recited doctrine on these "well established laws." I realize that in the eyes of many physical laws are variable depending on the authority who spake them."

See what I mean? "Variable."

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"In the eyes of many." Dolt.