Using Heat From Generator

[JustWood]

I have a 250 KW generator that powers all mill, pallet shop, and firewood processing equipment. Thinking of piping the heat from generator into house and shop. Anyone ever done this ? If so what pitfalls did you run into . The only problem I foresee is the engine running too cold in the winter but if I insulate the building well enough this might not be an issue.

 

[Nofossil]

I want to do this in the worst way (actually, the best way, but I know how these things turn out....)

It should be fine. The thermostat in the generator will throttle the coolant flow to maintain engine temperature. The water temp is usually 180F - perfect for hot water heat. The only risk I can see is if you don't have enough baseboard demand to absorb the heat from the generator. You must be able to dissipate enough heat at all times to keep the generator happy. Maybe leave the radiator in place and use a thermostatically controlled fan to draw air through it only when the coolant temp exceeds 190F?

For back-of-the-envelope purposes, assume 100,000 BTU of available heat for every gallon of fuel that the generator consumes. That will get you in the right ballpark.

Excellent idea and project. I hope others can chime in and help.

 

[JustWood]

My plans is to construct an outbuilding about 4 ' away from my shop out of sand filled 8" cement block and spray foam insulate the walls and ceiling. Then building a plenum around the radiator and ducting the heat into the shop. To the house with Pex or spray foam the ditch and tubing. T off the radiator to a heat exchanger in the plenum of my forced air wood furnace and add a circulating pump .

 

[Nofossil]

I'd be inclined to take advantage of the thermostat and water pump that the generator has if possible - they should do a nice job of circulating water in just the right amount to maintain 180F.

I think you want to remove the fan that normally draws air through the radiator - by itself, that provides more than enough heat loss to keep the generator at temp. You want to have some left over for the house. Having an electric fan on the radiator allows you to use the radiator to shed heat when the shop and house don't need any.

Haven't given much thought about how to divide heat between the shop and the house. Perhaps a cast iron or automotive radiator in the shop with its own thermostatically controlled fan, and plumb that in series with the house?

Make sure you use pex with the oxygen barrier.

Comments from anyone else?

 

[JustWood]

Removing the fan from the engine is totally out of the question ! First and foremost is cooling the engine. 2nd is gaining a little extra heat. An out of frame rebuild is around $14,000.00 . My experience with engines is you dont fool with the fan or size of radiator. On cold days it never drops below 160. Just wondering if the extra cooling capacity of another heat exchanger ( in the house) would further lower the operating temp on cold days. The plenum and duct work off the radiator into the shop is a no brainer. Quick easy heat !

 

[Nofossil]

Understand, but fan is deliberately oversized to provide adequate cooling in worst-case conditions. Ducting to shop will work with no problems, except that the air temp coming out of the radiator will be cooler (maybe much cooler) than the water temp, especially on a cold day. If you're adding other heat extractors (house baseboards or whatever), that will drop the water temp even more, which will close the thermostat more and reduce flow more.

My thought - perhaps harebrained - was that too much cooling is a bad thing. (Cool - the Firefox spell check recognizes 'harebrained'!)

Usually, the engine thermostat restricts water flow to try and get an operating (block) water temp around 180 - 185. If you have the engine fan mounted, I'll bet the thermostat is almost closed tight on a cold day. Most modern cars don't use an engine fan. They use electric fans that come on whenever the engine temp reaches 190 or so. This reduces the load on the engine and allows much faster warmups. Engines are most efficient at higher temps (just like woodstoves).

How big is this engine? About how many gallons per hour does it consume?

 

[par38lamp]

Check to see if there are tapped ports on the jacket water outlet and inlet on the engine. You could use those ports for your heat piping into the building. Use ball valve to shut off the flow in the summer. Don't forget to add antifreeze/water to the now larger cooling circuit.

Make sure you are getting into your jacket water circuit, not the separate circuit aftercooler (doubtful a 250ekW has SCAC).

My first thought was to simply divert the radiator air into your shop. Just insure that your exhaust system is OK.

My other, more radical thought, is to use a exhaust heat exchanger. There is WAY more heat available from your exhaust, so even @ poor transfer efficiency, you still might be able to get good heat out of your engine. Plus, if the hot water loop stops working, your loose nothing on your engine. You would need a circulation pump (Bell and Gossett)

I share the same reservations about removing the radiator from the circuit (if I read that correctly).

 

[North of 60]

Most large generation plants scrub it off the exhaust. This way nothing comprimizes the efficient running design temp of the eng. Also you are working with much
higher temps to work with. You build somthing that creates the same back presure the muffler creates & rid the muffler. The water or glycol solution in the scruber will help absorb the noise. Hope this opens up any ideas.

 

[North of 60]

Looks like you got your more radical thought while I was finger pecking. Not radical at all. Its actually the way to go. ;-P

 

[KeithO]

The efficiency of a diesel is typically about 30% or maybe slightly better. About 35% will go out the exhaust and the balance into the cooling system. Some modern engines skew these number a bit, since they may have EGR and oil that is cooled with coolant. It should be possible to find stainless steel heat exchangers to use on the exhaust. Since you will be heating water with exhaust gas, the usual CO dangers associated with exhaust to air heat exchangers is a non issue.

For people in remote locations, co generation (using heat recovery from the IC engine for domestic heating) is a totally viable concept. The only difficulty is in finding appropriate hardware to use for the application and in having suitable controls to protect the equipment and safety. I think nofossil has demonstrated that he is more than capable of handling the control side. So all that remains is to find the appropriate heat exchangers for the coolant and exhaust. The exhaust heat exchanger is the most difficult one to solve since there is not all that much out there. There is marine stuff that could be used else one has to fabricate everything in stainless steel. How one sets it up is another thing. One has to prevent the exhaust heat exchanger from becoming a boiler and exploding. One also has to prevent the exhaust exchanger from running dry and then "quenching" when water finally comes around again. How do you stop the exhaust exchanger from boiling if there is insufficient heat demand ? Does one add a second radiator and electric fan that is thermostatically activiated ?

Lots of issues to overcome, but I think we will see more and more of this technology in the future...

 

[Nofossil]

Lots of ideas now - excellent!

I am a big fan of simple, reliable, and cheap. Automotive parts fit this model very well. Automotive temperature sensors, relays, fans, and heat exchangers are dirt cheap compared to their industrial counterparts. In my home system most of the relays are automotive, for instance. It will take a while for my home boiler to reach 100,000 miles ;-)

Couple of other points: I come out of the aerospace world. You want to design things to fail in a safe way. If any part of my homebrew boiler controller fails, the system reverts to operating the way it did from the factory. If all the things that we're talking about were to fail, it would be good if your generator reverted to cooling itself the way it did originally as much as possible.

I love the exhaust HX idea, but I think I agree that it has to be custom fab in stainless.

Now you've got me thinking, and I don't have time just now....

How about a flat plate exchanger in series with (and upstream of) the radiator? That would protect the generator cooling system from leaks or problems. I still vote for a pair of electric fans instead of the engine fan, but I don't get a majority vote here

Perhaps an unpressurized tank with a coil of stainless exhaust pipe going through it? That way it could boil or go dry with no risk. The water from that could be pumped through radiators or whatever in the shop and/or house, perhaps via another HX coil in the tank.

Sizing all of this properly will take more math than I can deal with right now.

Keep the ideas coming....

 

[JustWood]

Great ideas guys! Now the gears are really turning . Keep them coming. Thanks

 

[DriftWood]

Great idea. Old Volkswagen air cooled engines had aluminium heat exchangers on the exhaust to worm up the bugs. 6 or 8 of these welded parallel to each other might work for a hot airsyystem. Condensate and its disposal will have to be planed for.
The radiator heat could be combined with the exhaust heat and moved where ever.

 

[JustWood]

Understand, but fan is deliberately oversized to provide adequate cooling in worst-case conditions. Ducting to shop will work with no problems, except that the air temp coming out of the radiator will be cooler (maybe much cooler) than the water temp, especially on a cold day. If you're adding other heat extractors (house baseboards or whatever), that will drop the water temp even more, which will close the thermostat more and reduce flow more.

My thought - perhaps harebrained - was that too much cooling is a bad thing. (Cool - the Firefox spell check recognizes 'harebrained'!)

Usually, the engine thermostat restricts water flow to try and get an operating (block) water temp around 180 - 185. If you have the engine fan mounted, I'll bet the thermostat is almost closed tight on a cold day. Most modern cars don't use an engine fan. They use electric fans that come on whenever the engine temp reaches 190 or so. This reduces the load on the engine and allows much faster warmups. Engines are most efficient at higher temps (just like woodstoves).

How big is this engine? About how many gallons per hour does it consume?

250 HP 4 GPH under light load 6 under heavy load

 

[Nofossil]

OK, a little back-of-the envelope calculations here - rough order of magnitude only. Just thinking out loud....

6gph of diesel is about 800,000 BTU/hr. Making a very rough energy budget, we'll assume that about 1/3 goes to making electricity, 1/3 out the exhaust, and 1/3 to the radiator. That's about 270,000 BTU/hr at full load and 180,000 BTU/hr at light load going to both the radiator and the exhaust.

A house can generally absorb about 100,000 BTU/hr when it needs heat. Depends on baseboards or radiators, of course.

I'm beginning to think that a really big storage tank makes sense - something you can route your stainless exhaust pipe through, and someplace you can dump extra heat when the house and shop don't need it.

If you can capture 50% of the waste heat from the generator, you could heat a 2000 gallon tank from 120 to 170 in under 5 hours. Assuming that you can get usable heat out of the tank at any temperature above 120, that gives you about 800,000 BTU of stored heat to use when the generator isn't running.

If you duct your engine fan to the shop, you'll need some thermostatic option to limit how much heated air you get. Must be a big fan on the engine - you might also want to consider whether you draw outside air or shop air through the front of the radiator, or perhaps have some damper that lets you select.

 

[KeithO]

I was trying to calculate through a domestic size scenario and to begin with, the monthly operating cost had me floored. I finally figured that if one goes with a 5kW generator running 12 hours a day, the diesel bill would be about $500 / month ! Of course that assumes full load operation. From my current utility bills, my present electricity demand is below 1kw continuous. If I scaled down that far (maybe 3kW peak) I would have to use gasoline or propane since there are no diesels of that size on the market. The rejected heat would go up in relation to electricity output and exhaust gas temp would go up too, so it would be better for heating stored water.

To have 24/7 lights and electricity one would need a battery pack which would need re-charging during the day. To use the rejected heat for heating the home, one would need to be able to store the heat as pointed out by nofossil. Even the 5kW genny is going to provide a significant amount of heat output for home scale heating, but at $500/month I think the cost would be beaten by solar PV and solar hot water. I guess climate would be a big factor and I'm thinking Colorado. Locations with greater amounts of cloud cover would require a bigger investment.

 

[JohnnyBravo]

i like the idea of keeping the radiator and fan in the system. i would move it to the end of the line. go heat the house first then let radiator take up the slack. the t-stat will open when the engine is up to temp, and the fan will kick on if your house does not meet cooling requirements. i also agree that the exaust can not go to waste, and carbon monoxide is scarry. i would like to see the exaust go hydronic also. on average 60% of heating costs goes to domestic hot water, all that energy to heat water only to have it splash you or some dishes and then go down the drain. i was thinking of building some sort of radiator system to recapture some of this heat, like an HVAC air exchager. it would have to be easy to get to cleanouts and filters.

 

[mayhem]

Seems like you would want to simply move the radiator to a remote spot where you can connect it to a short duct that can channel air into your area you want to heat...when it gets warm enough and the thermostat kicks open you turn on a small electric fan to pull sool air into the radiator. Keep the existing shroud and fan on the generator as it sits now to make sure you keep air moving over the block (though you mgiht find it runs too cool this way since its designed to have the hot exhaust air from the radiator running over it and not ambient temperature air.

 

[JustWood]

I'm going to scrap my idea of taping the radiator for heat because of the extra plumbing that could fail and overheat my genset. Keith Os idea of an exhaust heat exchanger is the way to go. I can fabricate an aluminum fuel tank (myself which will save a ton of $) to use as a storage tank with a coil of copper in it plumbed to a heat exchanger in shop and garage.

 

[Tony H]

If you were to use a flat heat exchanger to xfer heat (as NoFossil suggested) to a small tank and pipe with a small pump running to the shop and thru a heat exc with a thermo controlled fan almost all of the extra plumbing would be isolated from the radiator and engine.