Capturing waste heat from Changfa diesel

[daymik]

This is my first post to the Board though I've followed it off and on for 7 years. I'm finally getting around to mating a Changfa 190 (that I bought from Veggie 7 years ago!) to a TGE-DG-5KW gen head. Can anyone tell me the BTUs produced by the Changfa under load? I'm wondering about plumbing the cooling system into a 1500 gal. heat sink the surrounds my wood fired boiler and am wondering if there is enough heat produced to go this route as opposed to an external radiator.

[Ronmar]

Rule of thirds...  Of the fuel burnt(BTU) a third does the work, a third goes out the exhaust and a third goes out the cooling system. So if you know the fuel burn of your engine at a given load, convert that to BTU and divide by 3...  in a bit rougher terms the heat output in BTU from the cooling system will approximate the electric load on the generator...

[veggie]

To expand on what Ronmar stated...

That engine produces about 7HP at 3000 ft. altitude.
7HP equates to ~5.2 KW's.
... at 3415 BTU per KW, your engine cooling system should reject about (3415 X 5.2) 17,758 BTU per hour.
(Not every bit of heat makes it to the cooling loop. Some will be lost to surrounding air via the engine block surface).
The same amount of BTU's would be rejected by the exhaust system.

That's a healthy amount of heat.
To put that into perspective, an electric domestic hot water tank with a 3000 watt element produces 10,245 BTU's per hour of heat.

The contribution to your 1500 gallon tank will be dependent on how long the engine runs each cycle.

Veggie

[glort]

You will also be lucky to put about 50% of that heat to actual work heating the water.

There are significant inefficencys in the heat exchange, particularly from the exhaust to the liquid. Depends a lot on the setup of course but you'd need something pretty elaborate and expensive to get the efficeny numbers of 50% I'd reckon.  You will need a higher final exhaust heat than the water temp for a start and you won't be able to extract all the heat from the coolant either. You will also have losses from plumbing .

Not saying it can't be done or it won't be worthwhile, just making you aware that what you start out with and what goes to actual work will be very different.
Don't say I have xxx Btu to play with and expect that to heat your water up as a straight calculation. You are going to have to down rate the numbers a LOT to get actual Vs calculated output.

A simple setup for this may be a gas water heater with teh gas burner removed and output the exhaust through it instead. Have a HE and a couple of circ pumps on the coolant side.

[Ronmar]

Exhaust heat from a diesel is also difficult in that the cool surface of the heat ex will condense the carbon very quickly and require constant cleaning to maintain efficiency.  As for coolant heat recovery I disagree with glort.  I have measured it from my 6/1 when I did my fuel consumption measurements.  I have a brazed flat plate heatex in place of a radiator, and it follows right along with the rule of thirds pretty closely with btu recovered vs btu of fuel burnt.  Recovering just over16KBTU at full load if memory is working right(was quite a few years ago now). 

The funny thing was I built the genset to keep the lights on and maintain a reasonably civilized lifestyle and keep a pellet stove operational during extended power outtages.  But the heat I recover to a hot water tank and then a fan coil unit exceeds the output of the pellet stove...

[mobile_bob]

it has been a bit over 10 years ago, so forgive me if my memory is a bit shady, i have slept since doing the testing
on my trigen.

taking heat off the cooling system was done with a hot water maker, one of those blue tanks that have a copper heat exchanger built in, double wall constructions so if there is ever a leak you don't end up poisoning yourself... at least that is theory. 

i ran my coolant temps between 205 and 214F
the engine was never allowed to run under anything by full load, as defined as smoke limit, where there was just a hint of gray color to the exhaust, 8.1 kwatt electrical output.

the exhaust coming out of the head of the s195 was measured to be 640F,  all testing was done with ambient temps between 70 and 73 F

the exhaust exchanger was and still is of my own design,  it is designed to harvest the heat from 640f to 240f, that allows for no condensation within the exchanger leaving enough heat to get the exhaust out of the shed without condensing in the pipe downstream.

the design theory of operation was what is known as "batch" fire, that being intermittent operation of about 1 hour per run, the amount of exhaust carbon buildup was small due to short run times and relatively high exhaust heat in the exchanger.

after an hour of run time, the unit would fully cool down, the cold inner parts of the exchanger would then be cold enough to allow significant condensation when the next run started, this condensation was used by design to provide for, in effect a steam cleaning effect and was very effective at removing what little carbon that had built up during the previous run.  this self cleaning ability was something that worked out far better than i had expected.

the exhaust heat recovery was the ability to make right at 60 gallons of 140f water having started at 70F at the beginning of the run.   i don't recall the btu's calculation, but i remember figuring the recovery to be something over 80% of the exhaust heat.

as we all know engines are actually better as furnaces than making rotational force.  with 1/3 going out the crankshaft, about another 1/3 out the cooling system and another 1/3 going out the exhaust.   which mean about 2/3 of the btu's available from burning fuel is available for us to harvest.

about the time i was doing all my testing there was a company in the marine industry, that did some testing of several different marine generators that were fitted for heat recovery.  they did three classes, iirc up to 3kw output, 3-8kwatt and 8-12kwatt.... more or less. 

i was very pleased to find my results putting the trigen right in the middle of efficiency in its class. there was only one or two that did better, but only marginally, and there were a couple that did significantly worst.

the s195 changfa based trigen, with coolant and exhaust heat recovery is actually overkill for my then intended use,  it makes far more heat and power that the plan had use for.

i still have it sitting in the shop, under a bunch of dust, dirt, sawdust and cobwebs.  i am thinking of getting it setup for use where i am now living as it is a better match for this place.  i have access to natural gas and going dual fuel is something i have done a lot of thought on over the years.   if i get that worked out, this place and the cold winters of kansas could use all the harvested heat i can produce, to be sure.

about 4 years ago now i built a kid brother to the s195 based trigen, using a 175 changfa....  i downsized the design of the exhaust heat exchanger, but haven't got back to testing it to see what the efficiency might be.  it is really too small for this place, but might be useful during the months where  there is little need for heat other than domestic hot water production.

there just is not enough time!

i wish there was a younger guy around here with time and a healthy back, and an interest in this sort of thing... i think if i could find someone like that, he would certainly get the better end of a labor swap, or at least take from what i have learned and expand upon it...  God knows i still have a lot of iron in crates takin up space around here.

bob g