Recovering generator heat

Maine Doug_47

This is for a ski mountain operation that generates their own power for the lodge and snow making machines. I imagine the numbers will indicate that all the heat recoverable would be used to heat the buildings and any excess stored. Certainly would save a bunch of oil.

Maine Doug_47

Has anyone done calculations

on how much heat may be available from 500HP diesel generators? Load will vary from about half to full and they run 24hrs/day much of the winter. A 5000 gal tank of water would be used for heat storage. I assume they would need thermal bypass arrangements/HX's to keep engine temp in proper operating range.

Doug

Barbarossa

that is a lot of heat

The manufacture would be the best source for that info. But a old rule would be if the unit is 35 % electrically efficient the balance of the heat in the input fuel would be divided between the exhaust and the oil/water jacket. Things may be more efficient now days so consider the manufactures input.

Brad White_9

Barbarrossa is correct

check with the manufacturer. Caterpillar, Onan, Kohler, Cummins all vary including between sizes. Good co-gen application.

The manufacturer can verify but the engine coolant circuit thermostat can compensate for the temperatures which you can imagine vary quite a bit from coldest to hottest ambient.

The BTU rejection numbers are often in BTU/Minute, not hour so double-check the units of measurement. Also the rejection numbers reflect "rejection to air", "rejection to coolant" and possibly "rejection to ambient" such as jacket losses; all of these are different.

The distinction I mention because many think that using a remote radiator solves the ventilation problem if in an indoor room. The radiator only cuts it by about half to 2/3 if you are lucky.

Ethylene glycol is often used so a plate HEX may be in your future. If domestic water, double-wall vented.

LarryC

Heat recovery

The engine manufactures also make heat recovery HX for the exhaust systems, turbochargers, and engine oil systems too.

Larry C

Larry Weingarten

Exhaust heat is good!

Not quite as big a system... but you get the idea ;~)

Yours, Larry

Maine Doug_47

Thanks all

for your input. The manufacturer info is interesting, looks like there is more heat rejection to the exhaust than the water jacket. Might be worth collecting both.

Doug

Weezbo

Looks decidedly crafty:) *~/:)

functional And has a real nice swoopy bend

Larry Weingarten

Art AND...

...science ;~) One caveat; don't forget to turn on the little pump when running the generator or the PVC exhaust gets decidedly swoopier. When running normally though most of what you get from the exhaust is condensation.

Yours, Larry

Christian Egli_2

Fever and chills shorten motor life to less than 8000 hours

Whatever you do, worry first about satisfying the every needs of your generator. What you do with the heat may all seem very stupid if you either 1) can't cool off the motor fast enough and cause engine failure, or 2) take away so much heat, your cold engine will fail again.

Like you discovered, most heat gets lost in the exhaust, then the engine water, then the engine oil. For a Diesel engine, from these three, you can harvest about an equal amount of energy, in heat, than that which you got in power. (Gasoline engines render less power).

The harvesting methods, of course, mirror home heating options. You can do it with super performing steam (via flash tanks and such) and avoid the many dangers with either not cooling enough or keeping the engine at too low a temperature, but all this is nothing you can't do with regular hot water piping either, the other way of doing it.

You have a 500 hp engine, if we take this to be (500 hp * 736 W/hp) about a 370 kW electric production and if we follow that that amount is about the same that can be harvested in heat, this machine turns out to have the output of a 370 heating square feet boiler, or about 37 BHP or yet, about 1.25 MMBTU/h.

A serious waste if left untapped. The accumulation tank you mention may have to be surprisingly enormous. Check all this out with your own real numbers.

We have operated several small such groups (~250 hp) to great profits since the seventies... and yes... several of them died of heat exhaustion on the job.

This is not unlike our cars: If we agree they conk out after 200,000 miles, while driving at, say, a lifetime average of 25 miles per hour, your car will have been purring for only as little as 8000 hours of operation. A surprisingly low number in comparison to what we might expect from a stationary engine.

And yet a mere 10 hour road trip will get you the endless: "Are we there yet? --Nope, 7990 more hours!

Jack, CVMS_2

Heat Recovery

More than likely you will want to have a radiator to get rid of excess heat. Just as your heat input will change significantly as the load changes, more than likely your demand will likewise vary with climatic changes.
While I have seen systems set up with large water tanks for buffers, they often create more expense/problems than they offset.
One of the best systems I've set up myself used p/s piping to allow the engine's coolant pump to circulate what it needed from the primary system. The fluid in the primary piping was kept to a sufficiently low temperature to assure adequate cooling for the engine, but still high enough to provide useful heat for recovery.
A general rule of thumb is that from the fuel going into a modern diesel engine you get 30% back as power, 30% as jacket water heat, and 40% goes up the exhaust. If you can recover all the jacket water heat and half of the exhaust heat, you can theoretically achieve 80% efficiency.
Marine exhausts (available for many large diesels) help recover some of that heat, but be careful of overcooling the exhaust. Condensation there is just as harmful as in a chimney.

Constantin

Ditto on the HX...

... make it foolproof so that the engine can run w/o the benefit of the accumulator tank. Then only start diverting water to the accumulator once the engine is warm, the coolant temp exceeds the temp in the tank, etc. In other words, create a parallel cooling system, not a replacement.

A safeguard that kills the diesel pump once a safe working temp is exceeded is also easy to implement. It's a foolproof way to stop a diesel...

As for the exhaust gases, you could go exotic and copy the designs found in the marine field... think exhaust elbows with water injection that cools the gases down to the point where rubber hoses can carry the gases out of the boat safely. In you case, it would necessitate a pretty big chamber to sprinkle the required water quantity into the gases to get any appreciable cooling...

... never mind the engineering challenge of dealing with very humid exhaust gases in cold conditions.