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."

Comments

Introducing...

[Iamthefallen]

Introducing Athlon XP 5000 - Now self powered!

Re:Introducing...

[mother_superius]

In this universe, we obey the laws of thermodynamics!

Nice but not the end of entropy

[SysKoll]

According to the article, this "breakthrough" is a reverse Peltier junction with about twice the efficiency of current semiconductor thermoconverters. Nice, but nothing revolutionary.

I think it's quite excessive to claim this will reduce entropy. Although I agree that if it's economically deployed in, say, cars, it will supplement the alternator.

Could this new junction actually replace the alternator for producing electricity in a car? Let's see: assume a car has a 100 HP internal combustion engine. That's 75 kW. Two third of this is wasted in heat. Typically, the radiator gets about half of this heat (the other half is dissipated away in radiant heat or through the exhaust. Assume further that 20 percent of this can be recovered and converted to electricity (for a really efficient semicon pile). That's 75 * 2/3 * 0.50 * 0.20, or 5 kW. That's more than a good SUV alternator. So this could actually work, provided it's reliable and not too expensive.

You'll need a battery for the short runs, though.

--SysKoll

Re:Nice but not the end of entropy

[GMwrench]

I don't think so. First your 100 HP engine will only produce 25-35 HP most of the time. Peak power is only produced during hard accerlation during cruse it's much lower and at iddle almost nonexistant. This is 99% of the time. Also an alternator only produses 1-1.5 KW. And the battery cannot be replaced it's needed to start the engine and supply power at low speed when your charging device is insufficent.

Use on Hybrid cars?

[BlueJay465]

My question is how much more gas mileage could this technology squeeze forth given an array of these attached to the heat producers of a vehicle, like the engine or the brake pads.

Another thing is how do these "thermal diodes" compare to a Peltier Element in heat conversion to electricity?

Anyonw know how much they cost.

[argoff]

It'd be great if we could use this for cheap solar cells. Regular solar cells are pretty expensive. (I'm almost convinced that other industries are screwing with the market to make them cost so much). Anyhow, does anyone know how much this new stuff would cost? PS: nuclear's my favorite, but it's too easy for the govt to regulate.

thermodynamics, and entropy, and all that

[StandardDeviant]

here's the layman's formulation of the things that give chemistry students the cold sweats, the rules of the game as it were:

1. You can't win.
2. You can't break even.
3. You have no choice about playing.

Any closed system ends up in the state of most disorder, and all systems are closed if you look at the boundaries carefully. No matter how hard you try, no matter what ingenous things you do, in the end, the dealer wins and everything is dust. Cold dust, at that. The more energy you expend enforcing order, the more chaos you cause. There are no wins in technology, only a prolonging of the inevitable loss. So while I'm sure this new doohickey is neat, somewhere, Carnot is laughing and his cycle is tapping you on the shoulder snickering to itself.

Re:thermodynamics, and entropy, and all that

[dragons_flight]

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.

is it more efficient than turbines?

[Pyromage]

this truly is the fundamental question: can this be made to be more efficient than a turbine/generator combo?

If this can be more efficient than a turbine, we can have solid-state power plants. Nukes are nothing more than a complex method of boiling water to push a turbine: if we can replace the water, we have an order of magnitude less waste! Not to mention that the core stuff is much easier to deal with than heavy water. Plus, with no pumps or pipes to break, it becomes even safer than it already is.

Or other things, say laptops? PDAs? Naturally all these kinds of applications are XYZ years off, but just imagine what would happen when we get the effiency of these things up? I'd bet that boiling water to turn a turbine is real low efficiency: if we cut out the turbine step alone, that should increase effiency by a whole lot.

This is truly cool shit.

Not enough information yet

[Animats]

Not too much info yet. In particular, there's no indication of how much such devices will cost per watt. This is a basic problem with things like Peltier-effect devices and solar cells; they work fine, but you need an awful lot of them to get serious power levels. If this requires something like a wafer fab to make, it will be a niche device for years to come.

Re:Hmmm... - might ruin smokestack effect

[victim]

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.

Thermodynamic efficiency limits

[Animats]

The law of thermodynamics that's relevant here is that the maximum efficiency of any heat engine is

• (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.