Converting Exhaust Heat to Electricity

[SuperTrooper]

I just read an article about auto industry research into thermoelectrics:

Science News - Researchers work to turn car's exhaust into power

Every little bit helps!

[xOrion73x]

This is something I have been harboring in the back of my head for years. I mean... we are able to harness power from the Sun, isn't heat just another wavelength?

I forget exactly how it works, but I think that the Voyager probes used heat from small thermonuclear generators to supply electrical power. There should be some sort of way to reclaim some of the exhaust energy.

[jamesqf]

Problem is that the thermoelectric diodes aren't all that efficient. Not a problem for a nuclear-powered space probe, of course. Another and more efficient way of reclaiming exhaust energy I've seen is to let it spin a turbine (like a turbocharger), but have a generator in place of the intake side. Called a "TIGER", IIRC.

[c-serpent]

One problem I foresee with putting a turbine in the exhaust stream is that it would increase back pressure on the engine and thus require the engine to do more work and use more fuel. This might be mitigated by running a parallel exhaust line to the turbine, but then why not just do a turbocharger?

Another way to use the waste heat would be to create steam and then run the turbine/generator off of the steam. This is ancient tech and quite doable without placing any added obstructions in the exhaust stream. The trick would be to make sure the steam turbine generates enough energy to pay for its own weight. That shouldn't be difficult.

[FastPlastic]

Another option is the little known Sterling Heat Cycle Engine. Now used in some of the newer solar setups. It's been around forever just never seemed to catch on. With the amount of heat put off by the exhaust you could generate a decent amount of power.

[Bicycle Bob]

BMW has been working on a combined ICE/steam engine recently. The first heat engine to achieve 50% efficiency was the Napier Nomad turbocompound Napier Nomad - Wikipedia, the free encyclopedia
I think a great way to use exhaust heat is for an absorbtion cooling unit to replace the AC compressor. With a bit of redesign to the manifolds, solid-state junctions, as used in heating/cooling portable food service accessories might give enough to replace the generator.

[cfg83]

SuperTrooper -

This is the part I like most :

...
The DOE, which is partially funding the auto industry research, helped develop a thermoelectric generator for a heavy duty diesel truck and tested it for the equivalent of 550,000 miles about 12 years ago.
...
The technology is similar to what NASA uses to power deep space probes, a perk being it doesn't seem to be susceptible to wear. Probes have used a thermoelectric setup for about 30 years.
...

It can take the heat (ha ha). Sounds like it could outlast the car!

CarloSW2

[jamesqf]

Originally Posted by c-serpent

...but then why not just do a turbocharger?

Because the turbocharger only uses a small fraction of the energy in the exhaust gas, maybe?

As for the engine doing more work, it doesn't work that way. Just think about it: suppose you hold the amount of fuel going in constant, then you have X hp out of the engine. Put in the correctly-sized turbo-generator, and the engine produces X hp minus small fraction Y because of back pressure, while the turbo generates several times Y.

This isn't a completely new idea: it was used on some WWII aircraft engines: Turbo-compound engine - Wikipedia, the free encyclopedia It's really a bit of a dead end, though, as that article points out. Cars have used IC engines not because they're efficient (they're not), but because they have the torque & acceleration that a car needs. If you have a Volt-like hybrid, where the wheels are driven by electric motors while the engine turns a generator, you can dump the IC engine and use a much more efficient turbine or Stirling engine. This would at least double the mpg of an IC engine, even before you figure gains from regen or grid charging the battery.

[Bicycle Bob]

I'd sure like to see the reference for a Stirling or a Turbine that is more efficient than IC. They have trouble with Mr. Carnot's equation about peak temperature. Car diesels can get over 35%, and ship diesels hit 50%, same as the Napier Nomad. Electric and steam were both better for simple transmissions with good acceleration; ICE won on range and convenience. Stirlings and turbines are OK to use up waste heat from ICE, if there's enough to pay for them.

[jamesqf]

Originally Posted by Bicycle Bob

I'd sure like to see the reference for a Stirling or a Turbine that is more efficient than IC.

Any basic physics text should cover it.

Electric and steam were both better for simple transmissions with good acceleration; ICE won on range and convenience.

Thermodynamic efficiency doesn't apply to electric motors. Steam is less efficient, which is why we don't have a whole lot of steam locomotives and ships these days.

Stirlings and turbines are OK to use up waste heat from ICE, if there's enough to pay for them.

Nope. That's why the development of the turbo-compounding aircraft engine was dropped: it turned out to be more efficient just to run the turbine, and leave off the IC engine. It's also why you see a lot of gas turbines used for power generation, but very few diesels.