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Waste Heat to Electricity?
Darwin_Frog writes: "Recent advances in thermionics at MIT lets waste heat generate electricity, thus pushing entropy one step further down the chain. These devices work at a temperature around 250 deg. C, instead of around 1000, so cars can augment the alternator by using the waste heat in the exhaust system to produce power for onboard electronics and A/C."
I am a physicist and have studied entropy, though it is not my specialty.
At a fundemental level, entropy is a measure of the number of accesible states of a system for a given energy distribution. Presumably you know that temperature is really just a statistical measure of average kinetic energy in a substance. In the simple case of a uniform temperature gas, it's possible to compute the entropy directly, by (a process analogous to) counting the possible ways to arrange the molecules and distribute their kinetic energy such that you still have the same temperature. (Okay it's not really counting cause there is [usually] a continuum of positions and energy values, but the idea is there, only with more integrals.)
Roughly speaking a system is "ordered" or "disordered" based on how much freedom it has in distributing the energy in it's heat. For instance, in highly complicated and stable configurations (e.g. DNA) you can infer that the heat gets distributed only in ways that don't break down the basic structure. Of course with enough heat it will no longer be stable, but that's a different case.
While the number of accesible internal configurations for the heat energy is the basis for entropy, very few people actually use this. What is actually used is a set of laws mathematically derived from this which can be directly applied to macroscopicly measurable quantities. Chemists know more about these areas than I do, but I'll cover a few of the basics.
The most important is known as the Second Law of Thermodynamics, stated simply "Entropy always increases (or stays the same)." Whenever you do anything that moves energy (such as heat) around, the net entropy will increase (except in those rare cases when it stays the same). It is possible to locally decrease the entropy of one system, but you are guaranteed to increase the entropy of everything else by at least the difference.
There is another important trick about entropy. It tells you that it's impossible to transfer energy from heat to any other form with 100% efficiency. Not only that but you can't even do it with arbitrarily close of 100% efficiency unless you have something who's initial temprature is arbitrarily close to 0 degrees Kelvin. Heat engines, any device that changes heat into other forms of energy, depend on having a difference in temperatures available (for instance, cool river water versus hot steam pipe). If you just have a box sitting at room temperature, it can't work.
There is an interesting caveat here. The Second "Law" and most of how we typically apply entropy are based upon something called the Fundemental Assumption of Thermodynamics. Roughly stated: "All possible energy configurations are equally likely". As it turns out this is rarely ever exactly true, but it is so nearly true in almost every concievable macroscopic situation that it makes no difference. Entropy always increases is a mathematically certain law derived from the fundemental assumption and mathematical definitions of temperature, etc, but it is still concievable that their might be systems where the fundemental assumption doesn't apply and entropy might decrease. Over the years there have been a few suggestions for how to build such a thing (mostly at a quantum mechanical level), but no one has ever succeeded.
If someone does build a box that sits on a desk and converts ambient heat into energy output, then they are almost certainly guaranteed a Nobel prize. On the other hand there may be something better than the fundemental assumption, which is exactly true and excludes all possibility of such a wonderful, energy giving black box.
Sapping heat from the smokestack contents will probably cause it to not work correctly.
The goal of a smokestack is to get the harmful exhaust away from the ground long enough that it disperses sufficiently before touching down.
This is done with convection. The hot gas in the tall stack creates the draw that powers it and blows the plume up after it leaves the stack, the hot plume continues to lift itself until it bleeds off too much heat, then it starts coming back down, but presumably dispersed enough to not be too noxious.
The smoke stack was designed with a known gas temperature and dispersal requirement and a desire to minimize masonry. If you take away heat from the gas you will reduce your plume altitude and cause it to come down in a more concentrated region.
I doubt you can use the thermo-generated electricty to run blowers to compensate. The `no free lunch' law of thermodynamics will probably forbid that. (Unless blowers are much more efficient than convection.)
Now, if you are just bleeding off waste steam then it would work, but most of the energy in steam is the expansion from water to steam, there is relatively little left in the puffy clouds.
Mostly unrelated note: I used to live in Pittsburg in a community where all the houses were required to have slate roofs, stone or brick exteriors and no wood trim. Even the window frames were metal. It was a fire-proof community from the days when the steel mills spewed lots of solids including hot cinders. The plume was powerful enough to carry those large distances fast enough that they were still hot enough to start a fire.
- (T1 - T2)/T2
where T1 is the temperature at the hot side, and T2 is the temperature at the cold side. Both of these temperatures are measured from absolute zero.This is why extracting energy from something that's just a little warmer than its environment is very inefficient. With the hot side at 100C and the cold side at 20C, you're limited to about 20% efficiency in theory, and will be lucky to get half that. Power plants generate steam at upwards of 600C, not just above the boiling point, for exactly this reason. Gas turbines run even hotter. Solar plants for power production typically focus enough energy on a target to reach the 600C level, as Solar Two in Mojave does.
You just can't extract much power from things that are merely warm. They have to be really hot.