Motorbike generates 200W of power from its own exhaust fumes
 

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Motorbike generates 200W of power from its own exhaust fumes | Science! | Geek.com

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Motorbike Generates Electricity Using Its Exhaust Gas -- Tech-On!

CarloSW2

Would it be light enough?
200 Watt is not a lot. A 9 kg 1kWh LiFePO4 battery pack can provide 2,000 W for half an hour, like my commute. I'd need to charge it of course, but it would have much more of an impact than this.

And would it be cheap enough?
The thermoelectric conversion elements I've seen were all expensive, ineffective and heavy. What you'd gain you'd lose on having to haul the extra weight, never mind earning anything back.
If they'd work on cars they certainly would work in fixed applications that run 24/7. So, where are they?
For just 200W it needs to be very light and quite cheap. If so, why not.

I have yet to find the first system that does effectively harvest thermal heat from exhaust systems that is of any use for cars.
Green turbine states it: I have got a hybrid so I mailed them.
The answer I got was (iirc) that it was still experimental and not ready to fit to a car at all. It would not take the exhaust gas directly; that should be used to heat a medium gas to drive the turbine and be cooled by an extra radiator. Thanks for your interest, keep reading the news letter...

They may be on to something, but I doubt it would ever be practical for a car. 20% total FC reduction from just exhaust gas heat seems very unlikely to me.

There are also memory-wire motors that can be used to power a low-speed alternator to provide electrical power from the exhaust waste heat. Much cheaper than thermoelectrics.

It would need to provide at least 50 Watt per kilo (25 Watt per pound) or its mass alone would hurt acceleration even at full output, assuming a typical 1000 kilogram econobox with an 50 kW engine.

Can you show a memory wire motor that can provide more than 25 Watt for every pound of its own weight?

It could displace the alternator. The alternator isn't much but taking any load off the engine = good, especially if it can be picked up by harvesting energy from waste heat.

I think what everyone is waiting for is suitable high temperature thermoelectric modules that are cheaper. Because of the requirement for an onboard generator to be light and the fact that electrical energy isn't as useful in a gasoline powered car, the main problem is cost. I think Alphabet Energy or something has newer TEGs that are maybe viable but I don't think you can buy them.

It shouldn't be hard to remove most of the alternator's load, you can generate 300W with commercially available units for about 1500 dollars and something like 10lbs weight which isn't too bad, but the cost has to be trimmed down a lot if it's ever going to pay for itself.

The problem is not so much the weight of the unit itself.
To make the cooling part effective it needs to be big, and probably heavy.
Unless we could use the return of the radiator or something like that.

The system that has the most chance of being effective would be one that employs the difference in coolant temperature to and from the radiator, never mind the exhaust.
It would reduce the effectiveness of the radiator, but then again it would also reduce the heat production of the engine.

Half of every gallon of gas you burn goes directly out the tailpipe without doing usable work and up to 75% of the gallon that does work may also be wasted in other ways.

I have no doubt that energy recover from exhaust is usefull, but like you wonder will it ever be cheap enough?

Correct, coolant is the way to go. At 370K coolant vs. 600-1100K engine exhaust that's plenty of room to play with. You could have extra cooling systems like BMW and their Turbosteamer thing but that would cost a ton more money. A slightly bigger radiator from the factory doesn't add much cost, an extra radiator on the other hand just for the generator would be.

As for how much energy is in the exhaust, well at full throttle it's something like 40% heat energy going out the exhaust, at light throttle it's proportionally not much less but the temperature is lower so you can't collect as much. The TEGs nowadays can collect maybe like 2% of total fuel energy (adding to the ~30-35% engine efficiency) for modest single digit percentage improvement. That said, any load you take off the alternator counts extra because alternators are inefficient.

Well, considering that nitinol has a force of as much as 55 tons per square inch, and given that a one-foot length of the wire of 0.020" diameter weighs just 0.06 pounds, we get ~16 wires per pound, and approximately 50 wires per linear inch or 2500 wires per square inch. So to produce 55 tons of force, we'd need approximately 156 pounds of the stuff in foot-long wires. That'd produce about 41 watt-hours with a lever length of one foot.

So, with just 16 wires (one pound), we'd get a pull of 727 pounds. At a lever length of 1 foot, giving 727 pound-foot of force, that equates to 0.27 watt-hours.

That's a couple orders of magnitude below your benchmark.