Heat Pump on Exhaust

[jeff88]

For those of us on the EcoRenovator site, we often see that a heat pump is a very popular mod to a house (it's like the grill block of house modding!). I was wondering if one attempted to use the heat of the exhaust for a heat pump if the heat pump could be used for anything useful. Some ideas:

• Cabin heat; don't know what good this would do, since we use the coolant, but maybe it will save electricity off of the alternator to pump the coolant to the heater core?
• Electricity generation; don't know how it could be done, but maybe the heat could be used to make electricity and then delete the alt.
• A/C; a heat pump is often used to cool a house during the summer. Maybe this could be adapted to eliminate the A/C off of the engine's drive belt.
• Powered by a turbo; a heat pump uses a compressor, which could be powered by a turbo (turbine and compressor) so it doesn't take electricity from the alternator (and power from the engine)

Maybe this is another one of those give power back here, but take power from there and thus it's a net unchange or worse net loss. These are just a few ideas, I haven't completely refined my thoughts, but I'm willing to bet there is a diamond in the rough here.

[jeff88]

I had another idea:

What if during warm-up time, the heat pump transfered heat into the engine coolant to help warm up the engine faster? After that period, then it could run other activities like the ones listed above? I think that may actually help efficiency and certainly emissions during the warm-up period.

Thoughts?

[Occasionally6]

The A/C in a car is a heat pump.

The car heater is using otherwise wasted heat so there's nothing to gain by doing it in any other way. You can either pump the coolant through the radiator or through the heater core. It's pretty much the same pump effort required either way.

Exhaust (and coolant) heat might be used to evaporate a low boiling point fluid and useful work extracted from that in a turbine. (Exactly as occurs in a steam turbine, just lower pressure and temperature.) BMW were prototyping that using ethanol as the working fluid a few years ago.

Yes, a gas turbine in the exhaust can be use to extract work. If not connected to an electric generator it can geared to the crankshaft. See turbocompounding.

Exhaust gas from an ICE is at fairly low temperature and pressure so the efficiency is pretty low. That wouldn't matter where the gas is free and otherwise wasted but cost kills it in practice.

In a spark ignited engine, where the gas flow is throttled for much of the engine operating range, there is a lot of variation in gas flow rate so it's not going to be possible to optimise a turbine in terms of efficiency.

Faster warm would be possible with a heat pump but you have to put energy in to get the heat back. Even allowing for the multiplication effect of a heat pump, it's going to cost much more than you can save. Again, engine coolant heat is otherwise wasted anyway.

[jeff88]

I know the A/C is a heat pump, but it runs off of the drive belt. I was thinking about having it run off of the exhaust instead. Conversly, it can also provide heat to the cabin, which means you eliminate the A/C and heater core to have just the exhaust heat pump (eliminate 2 items and add 1 item means a net reduction in car systems = less complex and cheaper... hopefully).

If this is the turbine that you're talking about, I've heard of it and it sounds promising, but like you said, with a throttled engine how useful/efficient would it be?

Originally Posted by Occasionally6

Faster warm would be possible with a heat pump but you have to put energy in to get the heat back.

I'm not sure what you mean by this. The energy in would be from the exhaust which would power the heat pump. A turbo would spin a compressor, but instead of compressing air for the engine, it compresses the warm air from the exhaust (post turbo exhaust is still going to be hundreds of degrees, more than enough for cabin heat). Then run it through a heat exchanger (unless you want to breathe super delicious exhaust fumes in your cabin) to heat the cabin and engine (during warm-up). Where else would I need to put energy in? Maybe a small blower to push the air into the cabin at a desired pace (based on user selection).

I think this might be more efficient, because any pumping can be powered by the heat pump/exhaust. Of course, if one had a TIGERS exhaust generator, then one could just make everything electric and be done with it, but this is another option.

[Occasionally6]

Originally Posted by jeff88

I know the A/C is a heat pump, but it runs off of the drive belt. I was thinking about having it run off of the exhaust instead. Conversly, it can also provide heat to the cabin, which means you eliminate the A/C and heater core to have just the exhaust heat pump (eliminate 2 items and add 1 item means a net reduction in car systems = less complex and cheaper... hopefully).

Using a turbine in the exhaust to run the A/C (& heater?) compressor directly? That might work, subject to the limitations of the highly variable gas flow on a throttled engine.

The same pump that is required anyway, to circulate coolant through the engine, is used to pump coolant through the heater core. It doesn't cost anything to use that heat other than having to carry around an extra heat exchanger (the heater core).

If this is the turbine that you're talking about, I've heard of it and it sounds promising, but like you said, with a throttled engine how useful/efficient would it be?

Yeah, that's it.

I'm not sure what you mean by this. The energy in would be from the exhaust which would power the heat pump. A turbo would spin a compressor, but instead of compressing air for the engine, it compresses the warm air from the exhaust (post turbo exhaust is still going to be hundreds of degrees, more than enough for cabin heat). Then run it through a heat exchanger (unless you want to breathe super delicious exhaust fumes in your cabin) to heat the cabin and engine (during warm-up). Where else would I need to put energy in? Maybe a small blower to push the air into the cabin at a desired pace (based on user selection).

What you are describing is a variation on perpetual motion; expand and cool the exhaust gas in a turbine then re-compress it back to the initial state in a compressor.

Heat from the exhaust has been used to heat the cabin (air cooled VW's) via a heat exchanger.

You were discussing using a heat pump to encourage faster engine warm up. A heat pump uses a compressor, condenser and changes of state (condensation and evaporation) in the working fluid to extract low grade heat from an external source.

Driving that compressor takes energy, even if you can get more heat energy out (taken from the surroundings - air usually, or earth for a building) than you put in to drive the compressor.

I guess it's possible to use a turbine in the exhaust to drive the compressor for the heat pump but that'd be a lot of hardware just to save a tiny bit of fuel.

If you wanted to use the energy in the exhaust gas to aid engine warm up, it would be much easier to do it directly with a heat exchanger between exhaust gas and engine coolant.

Even better is to store some of the otherwise wasted energy from the cooling system (as latent heat) and feed that back into the cooling system in a cold start situation, as is done in Prius.

[RedDevil]

The big problem with using exhaust heat to power anything is not so much in aquiring the heat - it is right there - but shedding it on the other side of whatever device you'd use.
As you cannot power anything with just heat. You need a temperature differential.

Take the simplest form: Electric energy directly from your exhaust. Attach a Seebeck effect element between the exhaust pipe and a big metal plate. That will easily produce several Watt of electricity that you can feed back into the 12V system. Great!

Except that the plate needs to be quite big to shed enough heat to make it work effectively. And you cannot just attach it to the car body; you do not want to heat that up. The exhaust pipe trajectory is not isolated for nothing.
All in all you'd add quite some weight to the car, just to get a few Watts, less than 0.1% of whay you'd use on the highway. ROI? Not in a lifetime.

The same applies for other approaches; the Stirling generators, the steam turbines; they all need cooling, and that's where it stops being practical for cars.

There are stationary applications that produce heat 24/7. Weight is not an issue there. Size is generally not an issue. They work all the time. They could use waste heat for power generation so much better than any car ever could.

But they don't generally. There are exceptions, but they seldom get a follow-up.
There must be a reason for that.
The reason is that even in ideal circumstances the power generated hardly makes up for the initial cost and maintenance, if at all.
If it does not work there, it has no place on a car. Sadly.

[Tesla]

There's no free ride on turbo's, the added restriction in the exhaust puts a higher load on the engine, the question is can you extract more energy from it than the amount that is added back by the additional load.

Extracting waste heat is difficult in the sense of getting to a point of usable heat differential, outside of making hot water or heating the cabin which only requires a differential in the order of 50-100°C, to produce mechanical motion you generally require a differential of 300°C +. There have been some attempts to use lower temps with specially designed stirling engines, but they require highly volatile working fluids and are generally operated at a partial vacuum, so not really something an ecomodder is going to do with an old twostroke and some PVC pipe.

As for the heat pump, again it's efficiency is governed by a heat differential, it can effectively move heat from one place to another, but to use it to increase heat potential it's efficiency begins to fall rapidly, it is nothing more than a compressor and is governed by the same laws.

I applaud you for putting ideas out there, but I have yet to see any workable proposal for utilizing waste heat from the ICE.

[JRMichler]

Turbocompounding is barely worth the cost, complexity, and reduced reliability in an airplane engine that runs 100% power for 5 minutes, then 75% until almost there.

In a road vehicle that sees 100% power for 5 or 10 seconds, then 10% power, it's useless. In a vehicle driven for gas mileage, the average power is even lower, and it's even more useless.

Turbocompounding only works at relatively high power settings.

[Occasionally6]

Originally Posted by RedDevil

The big problem with using exhaust heat to power anything is not so much in aquiring the heat - it is right there - but shedding it on the other side of whatever device you'd use.
As you cannot power anything with just heat. You need a temperature differential...

There's no heat transfer required with the turbine in the ICE exhaust. It's a bottoming process, solely based on the reduction in pressure across the turbine. The work out through the turbine shaft is equal to the loss of internal energy in the gas as its pressure drops. It cools adiabatically (ideally).

An alternative would be to extend the expansion stroke relative to the compression stroke in the engine and extract the energy from the gas in that way, through the crankshaft i.e. use Atkinson or Miller cycles. Same thing done in a different way.

[RedDevil]

That´s exhaust pressure you´re talking about. I was reffering to the heat part.

Even so, most of the time you do not want exhaust pressure. Performance goes up when exhaust pressure gets down. At WOT there is some residual pressure at the bottom of the power stroke that could be of use, but not at light load.

That´s why (exhaust) turbo´s (compressing intake air) work so well: they do a lot at WOT and nothing much at light load, just what you need.

At light load running the A/C on exhaust pressure means the pressure differential needs to be high enough to spin the turbo as the volume is low. Then it will work against the engine, requiring much more load.
At WOT the pressure differential can be lower, and tapping that would not matter much as there is an overage. Even so, you are not at WOT for nothing; you would not like to do anything that reduces power then.
A turbo that uses a high pressure difference at low speed and can still function efficiently (having relatively low drag) at high speed would be a golden nugget.