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."

Re:Neat Idea, but not terribly useful...

[archen]

When an electric car can keep me warm in -40 degrees below zero, while driving against the wind for a 300 mile drive I'll be impressed.

Personally I'd put more stock in a vehicle powered by hydrogen.

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.

Re:thermodynamics, and entropy, and all that

[Phanatic1a]

While that's all very true, it's not tremendously *relevant* here. While you can't break even, you *can* get arbitrarily close to breaking even. Nobody's claiming that thermionics allows you to build an over-unity device, or violate the 2nd law.

What this does do is allow us to design more efficient processes than before. That's a cost savings, a resources savings, and quite allowed by Carnot.

use waste heat as -- heat

[vscjoe]

That's nice, but it seems like a lot of effort for something that, in many cases, has a much simpler solution: use waste heat for heating. A lot of waste heat could be used for heating homes and water for domestic use, and this is largely untapped in the US. (A lot of low-level waste heat could also be avoided entirely if people gave up on their inefficient water heaters and insulated their pipes.)

It's nice when people come up with better technology, but the inefficient use of energy in the US right now is not a technological problem, it's a political problem. Let's hope that we'll eventually be doing well enough that it will really become a technological problem.

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.

Is that right?

[name_already_taken]

Engine manufacturers don't rate their engines based on BTU input (like a water heater or furnace), but on mechanical output (regardless of waste heat output).

Doesn't a 100HP (75kW) internal combustion engine actually consume 300HP of chemical energy to make its 100HP of mechanical energy if it's 33% efficient? So the waste heat would be 200HP or 150kW.

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:is it more efficient than turbines?

[horster]

the warm water is a waste product - not a big deal if it is going into the pacific, but when it goes into a river it can change things enough to alter the ecosystem

or so I've heard.

Cars are great... but that's not all !!!

[prSpectiv2]

It seems so far that most of this discussion focuses on this technology's application to automotives, which are, obviously, an enormous source of fuel consumption. But what about more fundamental wastes of heat?

Quite nearly every home contains dozens of devices that let off lots of energy while in use. Think of your oven, dryer, toaster, refrigerator, furnace... dare I say woodstove?!? Lining these heat-driven devices with such a product could prove valuable.

Consider the open flame of a gas range literally belching heat, much of which escapes into the air or is absorbed by the metal around it. What if the oven and catch-plates below each burner were lined with a hard-coated version of the device? Maybe in the common home this would prove impractical, but surely in commercial kitchens where ovens and stoves are perpetually fired such an implementation would drastically cut down on the total electricity used.

In older homes where radiators are the norm, this might even provide an economical way to prevent burns from leaning up against those pesky pipes!!!

.

This would be useless in automotive..

[Anonymous Coward]

because the car's alternator already produces more than enough power to run everything in the car. Also, on cold mornings, it would produce even less power because it takes longer to warm up. This technology has other uses, but automotive ones are not among them.

Re:thermodynamics, and entropy, and all that

[dragons_flight]

I reread your original post and several others, and I think I may have misinterpreted what you really wanted to know, so I'll try to clarify.

There is energy in everything that has heat. To extract that energy you have to do one of two things: make it colder or decrease it's entropy.

Thermodynamics and conservation of energy guarantee that any mechanical process that makes it colder will cost more energy to perform than the difference between the energy contents in the cold and hot states. Thus you can't have any net gain of energy through a mechanical cooling.

What you can do is bring it into contact with something cooler. Heat energy is transfered from the hot thing to the cool one and in the process you can extract some energy. This is what the devices in the original story do. In fact, ultimately this is what all thermal power sources do, though the details may be obscured by changes in pressure, volume, etc. If you have a convenient hot source, such as "waste" heat, or geothermal power then you can bring it into contact with ambient temperatures and extract power while it cools.

You want to extract heat from the air. Doing it this way, and supposing there is (optimistically) an average differance of 3 degrees C between the ground and the air above it, you could get at most 1% of the energy transfered between the two. This is the thermodynamic ideal. No system will ever do better over so scant a temp difference near room temperature. Air doesn't have that much energy, nor is it a very good conductor of heat, so it doesn't seem like this would ever be worthwhile.

So, yes, you could get energy from the air that way, but that doesn't seem to be what you want. As I said, no mechanical process will give you positive energy gain, and you don't have a cool spot to compare it to, so what else. The other option is to decrease entropy. I don't know how to break the Second Law, so I want to take the entropy and shove it somewhere else. I decrease the entropy of my stuff, which means I get energy out. Unfortunately I increased the entropy of that other stuff, which means I had to put energy in! Thermodynamics tells us that the only time you win in this situation is if that other stuff was colder than the stuff you started with. Yet again you need to have a temperature difference to get any benefit.

So no, you can't extract energy from the room all by itself. You need a temperature difference if you hope to have a net output of energy. Unless of course you know how to build the magic black boxes that lead to a net decrease in the entropy of the universe, in which your Nobel prize and billions await.

Re:Okay, this pisses me off [offtopic]

[xurble]

Could it possibly be that humour is subjective and the person that modded you down didn't find it funny but the person that modded that comment up did?

Nah, it must be a conspiracy.

Jesus, get over it.

Isn't everyone bored of comments complaining about moderation or suggestion moderation?

Everything that says "mod this up/down" or "why did this get modded up/down?" should get modded into oblivion as a matter of course. (He said breaking his own rule, please mod me down :-).

Re:Nice but not the end of entropy

[Alsee]

therefore it'll reduce radiated heat.


True, but only by a limited percentage. It is tremendously easier to keep an engine block cooled to 250C than is to keep cold side of the generator cooled to say 75C. Heat radiates at the 4th power of temperature. It takes several times the radiative area to maintain 75C than 250C.

-

"... and A/C"

[Kymermosst]

Except the air conditioning cooling system in a car runs directly off the drive system, and not on electricity. The heater runs directly off of heat in the car's cooling system. Consequently, this development really has no impact on vehicle air conditioning or internal environment, with the exception of running a couple relays and a control circuit.

You forget, it runs on waste heat

[Spamalamadingdong]

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.

Which doesn't make much difference, because the engine's waste-heat output doesn't change nearly as fast with throttle opening as the crankshaft output does. Even at idle (zero power) you are still burning fuel and still pumping heat out the exhaust pipe. If you can force that waste heat to do some work for you instead of just being diluted to uselessness in the atmosphere, you've accomplished something.

A hybrid vehicle would probably shut down the engine at idle and eliminate that waste-heat stream, so the thermal converter would be more useful as a way to increase the general efficiency level of the powertrain. If you can get an extra 10% off the 40% of the heat which is rejected through the exhaust, that's 4% of your fuel value; added to a 30% engine thermal efficiency, you've gained 13%. That's nothing to sneeze at.