Like Input: Storing The Heat

Like Input: Storing The Heat

PostBy: Robby On: Sun Nov 23, 2008 5:19 pm

I use a Harman SF360 boiler, and burn bituminous and wood. Both seem to be very inefficient when idling as most of the heat goes up the chimney. Looking at Garn (and others) that store the heat in large volumes of water, I am looking at 1000 gal. propane tank, welding in outlets, bottom and top, a couple for aquastats and storing water at 180 to 190 degrees. That should be short of 300,000 btu. Then I could run boiler in most efficent mode and heat would last from 24 hrs to 2 or 3 days depending on weather. It could still be a closed system, 20 psi, etc. Anyone have experience doing this? Ideas?

ps; the tank is free, of course welding,plumbing and insulating will cost.

Robby
Robby
 
Stove/Furnace Make: Harman & Elmira
Stove/Furnace Model: SF360, TLC2000, PC45

Re: Like Input: Storing The Heat

PostBy: LsFarm On: Sun Nov 23, 2008 5:36 pm

If your SF360 is way oversize for your house and BTU needs.. running it hard and storing the heat may work.. but you will have standby losses, and yu do lose a lot of heat up the chimney when the boiler is running hard too.. I'm not sure you will gain or not..

The only way to find out is to try.. I know the woodburning folks like storing heat,, but that is because burning wood cool and slowly is very inefficient, But burning coal is not.. you can slow down a coal fire and not waste much heat up the chimney at all..

I'd document a month or two of burn rates. temperature, and degree-days, and wind.. overcast or sunny.. then hook up the storage tank.. burn till you get it hot.. discount this initial heating of the water.. then document the same weather conditions and see how you do with coal burn quantities..

Take care,, good luck with the science experiment.

Greg L
LsFarm
 
Stoker Coal Boiler: Axeman Anderson 260
Hand Fed Coal Boiler: Self-built 'Big Bertha' SS Boiler
Baseburners & Antiques: Keystone 11, Art Garland

Re: Like Input: Storing The Heat

PostBy: rberq On: Sun Nov 23, 2008 6:25 pm

I have thought about doing something like that. One big problem is, when you have drawn down half of the stored heat, your water temperature will be half of the original 180 degrees, which will not be adequate for radiators or baseboards. I think you would need something other than water for the heat storage, though water might be the heat-transfer agent. For example, a phase-change salt that melts at 180 degrees and absorbs (stores) large amounts of heat as it melts. Then as it solidifies it would release that heat at the 180 degree melting point. It's analogous to a gram of ice at 0 degrees C changing to water at 0 degrees -- 80 calories are absorbed in the change to water, and released when it changes back to ice, with no temperature change. I don't know whether such a substance is available, for the temperatures we are considering.

The other possibility I thought of was to put lots of rock or iron or something in the tank, that is, something with higher specific heat per unit of volume compared to water. That way the tank could store more total heat than with just water. My physics is not up to date enough to know what metals, if any, would be suitable. And it still would not solve the problem of the storage water decreasing below useful temperatures as you draw on the stored heat. I suppose you could get around that by adding a heat pump to extract the heat and boost the temperature, until the water temp was reduced close to freezing -- but now we are REALLY getting complicated and expensive.

Anyhow, I never did pursue the idea beyond the thoughts I have outlined. If you do, be sure to post it! Or just post your ideas for discussion!
rberq
 
Hand Fed Coal Stove: DS Machine 1300
Coal Size/Type: Nut -- Kimmel/Blaschak/Reading
Other Heating: Oil hot water radiators, propane

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Re: Like Input: Storing The Heat

PostBy: Robby On: Sun Nov 23, 2008 6:44 pm

My house is fairly new and quite well insulated, good windows etc., but large. 4400 sq.ft including basement. I do not circulate water except when heat called for and noticed that the boiler, on idle, did not increase temp, even over a long period of time. Therefore it must be going up the chimney. I was only expecting to cool water from 180 to 150 degrees. I tried it and my fan coils and radiators still produce reasonable heat at more than 140 degrees. Less than that pretty poor performance.

I was surpised to see Tarm and others rate their BTU storage using temp down to 120 degrees. OK for infloor I suppose.
8300 lbs @ 1 btu per lb x30 degrees =249000 btu? Is this correct.

Just seemed to worth giving serious thought to.

Robby
Robby
 
Stove/Furnace Make: Harman & Elmira
Stove/Furnace Model: SF360, TLC2000, PC45

Re: Like Input: Storing The Heat

PostBy: rberq On: Sun Nov 23, 2008 8:37 pm

Robby -

I didn't mean to rain on your parade. As I said, I gave this a lot of thought some years ago and was disappointed that there wasn't a straight-forward solution. I was working with wood heat at the time, which has a much narrower range of efficient burn rates than coal (see Greg's comments).

Again, I think the flaw in this idea is using straight water as the heat-storage substance. The fact you want to work only with a range of 150 - 180 degrees makes matters worse: you will have perpetual stand-by losses on the 150 degree water, probably more than offsetting the "most efficient" burn rate of your coal.

But I still think your concept is good. Do a Google search on "phase change materials" and "latent heat storage". There may be economical materials available now that were not around when I was considering this. (Not to mention that the Internet was not available at the time, either, so researching was a lot harder.) Your thousand-gallon tank would contain the phase-change solution, and you would have coils within it to deliver the heat from your boiler, and other coils to absorb it from the tank and transfer it to the baseboard heaters.

Another type of storage you might think about -- if you are in the mood to rebuild big parts of your house -- is the Russian Fireplace or Thermal Mass type of storage. Essentially, it is a very-high-mass structure of rock, concrete, brick, or whatever, maybe as an internal wall of the house. You heat it to a comparatively low temperature but because of the great mass it stores a lot of heat and radiates it steadily back to the living area of the house. So you can produce and store the heat quickly, shut down your heater, and then use the stored heat over many hours or even days. In a sense it is like concrete-slab radiant floor heat, only vertical instead of horizontal.
rberq
 
Hand Fed Coal Stove: DS Machine 1300
Coal Size/Type: Nut -- Kimmel/Blaschak/Reading
Other Heating: Oil hot water radiators, propane

Re: Like Input: Storing The Heat

PostBy: coaledsweat On: Mon Nov 24, 2008 9:44 pm

Go to Hearth.com's forum, the wood guys have heat storage down to a science over there, it isn't really needed with coal. Plenty of info there for you.
coaledsweat
 
Stoker Coal Boiler: Axeman Anderson 260M
Coal Size/Type: Pea

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