Thanks for the comment. I agree with everything you've said; certainly if you're talking about Carnot cycle (or equivalent) heat engines they do operate on differences of temperature just as you say.
I guess I was just saying that in real physical situations macroscopic definitions of temperature, heat, entroty sometimes don't serve completely well in terms of talking about energy content / availability. As you said in your postscript comments, one has to take into account other 'forms' of energy like electric & magnetic fields, non-thermal pressures, chemical and other kinds of quantum mechanical bonds / energy states, et. al.
If you look at something that's macroscopically fairly thermally uniform / entropic like the interior of a star thermodynamically you wouldn't see any available energy, though of course if you look on quantum scales you'll see photons and atomic level phenomena that carry and create substantial forms of energy.
You could apply "temperature" or "heat" models to chemical bonds of quantum particles but you'd find that there was a great deal of non uniformity of energy / temperature / heat depending on where you looked. It's not uncommon in things like lightning bolts or whatever for the electrons to be at one high effective temperature, the ions to be at a high but MUCH lower temperature, and the surrounding air to be at a still different and MUCH MUCH lower effective temperature still.
So in such cases though you may not be able to so easily apply analyses oriented to macroscopic heat engines, in theory it should be possible to do so if you define (redefine) your conceptions of heat / temperature / entropy sufficiently. In the end you basically are reverting to more generalized forms of conservation laws to understand the whole situation, charge, parity, momentum, energy/mass, et. al.
I believe that the "Maxwell's Demon" thought experiment is a classic example of such kinds of conceptions about how to relate things like thermodynamics, mechanics, statistics, discrete particle physics, information theory, et. al. as one considers simpler and simpler and smaller and smaller physical systems.
Even in more typical heat engines like automobile engines, I gather that the microscopic "real world" non-ideal aspects of combustion have a significant impact on the performance and efficincy of the engine -- how quickly and thoroughly does the air-fuel mixture actually mix, and what droplet sizes are present? How quickly does the flame-front move out from where there's the spark and initial combustion out to the end of the cylinder? What about the non-uniformity of temperature and how quickly / well it comes to equilibrium as your hot combusting fuel exists in the presence of cool as-yet non-combusted fuel and air reactants?
So I guess a purely large-scale physical thermodynamic analysis is a great way to see how well a refrigerator or power plant may work, its generalizations are often too broad and oversimplified to really understand many real world situations.
Photosynthesis is probably the most important energy 'production' (and consumption) process on the planet, and it is non-thermal, though certainly all the same physical conservation and energy analyses apply to its understanding both at the quantum chemical and organic chemical levels.
Originally posted by: BrownTown
I'm not 100% sure what you are trying to say here
QuixoticOne, it sounds like your just referring to the fact that a chemical mixture could have a large amount of potential energy (for example Hydrogen + Oxygen) and that with a little spark you can heat that mixture up a great deal. While this is obviously true it is not related to the issue of heat engines. In a heat engine like a steam turbine you must have a heat source (which could be that burning hydrogen, or geothermal energy, or coal, nuclear etc..) that is at a higher temperature than the heat sink (usually ambient air or water). The problem being stated here is not that there isn't a large amount of geothermal energy underground (there definitely is), the problem is our ability to get that energy to the surface to drive a heat engine to produce electricity.
While it is obviously true that any given object has an absolute temperature in order to derive work from that object by way of a heat engine you need a difference in temperature. For instance even cold room temperature can be a heat source if you use liquid nitrogen as your heatsink. And 1000 degree gas can be your heat sink if your heat source is 2000 degrees. Look for instance at
thisthis diagram of a combined cycle gas plant. The heat sink at the end of the gas turbine is 1000 degrees Fahrenheit!, and you are right there is still TONS of energy left in that stream which is why it goes through the second steam cycle with an inlet temperature of 1000 degrees F and an outlet of 200 degrees F, but in both cases it isn't the absolute temperature but the difference in temperature that matters, this is why the cold end of the gas turbine can be the hot end of the steam turbine.
NOTE: I've never taken thermodynamics or anything like that, so I am no expert here, but in terms of practical energy extraction techniques these are the sorts of things being used.
EDIT: of course more generally in order to extract work from a system you must have an energy gradient of ANY kind, it doesn't *HAVE* to be heat of course. A dam works on pressure gradients, a capacitor works on electrical gradients etc..
