Exhaust heat recovery- Exhaust to water exchanger

[jtodd]

I would say 'go for it' and report back what your results are.
This came up recently in the thread I started about activated charcoal exhaust filtering. I think it's a great idea although I can report that based on my truck it doesn't work for removing diesel smell (although it might improve things).
Anyway, one would need to do some tests to see if the cost of the converter is justified by the results obtained.

On a related note, I tested out my heat recovery system the other day (exhaust piped through a shell and tube exchanger) and the results were beyond my wildest expectations. The setup didn't run long enough to take detailed readings of temperatures but the hot side was sizzling hot and the cold side was pleasantly warm, maybe 30 to 40 degrees C. The heat exchanger has a total mass of maybe 150 lbs with water in it and it seemed to heat up faster (minimal water flow) than the engine itself. My cooling water loop needs to be modified based on the results and to be able to test things better. I was surprised by the large amount of water that was recovered on the cold side of the exhaust flow. I had expected some drips of water but not the amount I got. I have not yet tested to see how fast soot gets deposited in the tubes .... this may cause issues yet.

Jens

Got any pictures of it?

JT

[jzeeff]

When I said "$.25/hr", I didn't mean to minimize the idea of waste heat recovery.   I think that ~$2000 saved over 5 years  is well worth putting some effort into.

Plus it might be fun to design and build, save the environment, etc.

[rcavictim]

Hmm. I hesitate to bring the thread back to the top, but........

I wonder if an intermediate storage step would help.

Consider :

- A large drum (or taller) full of loosely-packed aggregate (about 2"). Bits of brick, rocks, steel, whatever.
- A water pipe wrapped around the outside,
- The entire assembly is then wrapped in some insulating substance like your typical water storage heater.
- Exhaust gas is piped in under the aggregate and is directed through it out the top.
- Engine is run for 8 hours (or whatever), heating the aggregate eventually to a few hundred degrees above ambient.
- Water is run through the outer pipe to heat whatever you want to heat.
- Heat transfer out is limited by the gaps in the aggregate and the contact area between water pipe and drum, hopefully alleviating the problem of water flashing to steam in the pipe and allowing a simple thermosiphon arrangement.

Doing the math indicates one could get a usable amount of heat from the hot aggregate for a number of hours after the engine is shut off until its temperature tapers down closer to ambient. A phase change material in place of the aggregate would be ideal, although something with the temperature range needed is hard to find. Waxes are too low for exhaust operating temps. 60/40 solder would be pretty good, except for the quantity needed......

Thoughts?

How about molten salt as the phase change storage media?  They use it now in large Concentrated Solar Power heat systems instead of circulating hot oil.

[biobill]

I was thinking about exhaust heat recovery today and thought about the idea of putting a catalytic converter before the 'recover area' to heat up the exhaust gas a bit more as the converter 'burns' the unburnt fuel in the exhaust gas from the engine, thus more heat; it also would help clean up the exhaust emissions a bit as well. Smiley

  I was thinking the same thing. Anyone know anything about cats? Are gas/diesel the same? Is sizing critical?  Extra heat and cleaner exhaust (fewer deposits?) would certainly be a plus.

How about molten salt as the phase change storage media?

Interesting. How hot do you have to get salt to melt it?
     Bill

[SteveU.]

Hi.
Biobill if you go back one page in the engine section under a thread titled "Activated Charcoal As An Exhaust Filter"  we had quite a long discusion about catylitic converters. Some of your answers will be there.
If nothing else, should be quite entertaining listening to me stuff my foot into my mouth up to my knee.

Enjoy
SteveU.

[oliver90owner]

How hot do you have to get salt to melt it?

Hotter than the exhaust from a 6/1.  You also need to keep it molten, so you are not using the phase change latent heat energy in large Concentrated Solar Power heat systems.

Regards, RAB

[mobile_bob]

a few years backi changed my perspective when it comes to looking at an engine
from an electrical production machine to a heat producing machine

when you stand back and look at the engine as either a furnace or boiler and its ability to make
electricity as a byproduct, things become interesting.

it is often said 1/3 mechanical, 1/3 exhaust heat, 1/3 radiant
so obviously an engine is far better at making heat than it is at making electricity

the reality is closer to a 3 to 1 ratio, 3 parts heat vs 1 part electrical.

personally i am of the thought that one should should choose his prime mover based on
his heat needs and then live with the electrical capacity provided as a secondary concern.

also i don't believe we need to get tricky with phase change stuff, save for maybe refrigerant in some cases.
the heat values are not high with a lister/oid but certainly pretty well matched for domestic hot water, and slab heating
via hydronics or maybe some other thermal storage mass such as charging a trombe wall.

bob g

[jzeeff]

Glaubers salt, not salt.  32C.

Yes, you could use this to store heat from an engine for later release.

http://en.wikipedia.org/wiki/Sodium_sulfate


I say no on the catalytic converter - there shouldn't be significant unburned hydrocarbons in the exhaust.