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-   -   Woodstove to water heat (big fish tank?) (

Daox 11-25-10 09:04 AM

No problem Ben.

First off we need min and max temperatures. Lets go with a low of 68F and a high of 85F. We can adjust these temperatures as needed.

Now, 1 btu is the energy required to heat 1 pound of water 1 degree. You have 210 gallons and a gallon weighs 8.35 lbs. So, you have 1750 lbs of water as you mentioned before. So, if we heat water from 68F to 85F, you have stored:

85 - 68 = 17

17 * 1750 = 29750 btu

I don't know about your house, but in the worst of winter, my house looses 450,000 btu per day. To get this I just took my worst gas usage in therms and divided it by the number of days. Then I multiplied the therms times 1000 to get BTUs.

So, this would provide me with roughly 6% of the heat required for my home. Dang, I really need more insulation. :) Hopefully yours is better!

If we wanted to get more heat out of it, we could heat it up farther of course. Lets say you take the fish out and are now able to heat it up to 130F.

130 - 68 = 62

62 * 1750 = 108500 btu

Now we're talking some pretty decent heat. Of course, you also now don't want to even touch the tank as its HOT. :) But, thats what you can do with 210 gallons of water.

Ryland 11-25-10 10:49 AM


Originally Posted by AC_Hacker (Post 9706)
But if you had the 210 gal tank of water (1680 lb). and you let it absorb heat from your stove until it got to 85 degrees, then let it cool to 75 degrees, it would release 16,800 BTUs of heat.

On the other hand if you had the same 210 gal tank filled with calcium chloride PCM, and let it absorb heat from your stove until it got to 85 degrees, then let it cool to 75 degrees, it would release over 178,860 BTUs of heat.

So if Ben takes his 40 gallon tank, puts Calcium Chloride in it and hooks that to the wood stove then he could heat it up to a higher temp and get more out of it then if he had heated the 210 gallon tank of water and without killing the fish.

Piwoslaw 11-25-10 12:16 PM

The fish don't need to swim in the phase change material, they can swim next to it.

Find a material with melting point around 80°, maybe some kind of wax. Cut it up into really small pieces, or thin slices (lots of surface area), wrap those in plastic (so it doesn't get messy) and throw it into the tank. When you start pumping hot water into the tank, the temperature will rise until the wax's melting point. You keep pumping hot water, but the temperature in the tank doesn't budge until everything has melted, then it starts to rise again. The temperature curve will flatten out again when cooling off until the wax has become solid again.

So, if you know the melting point of your wax, then you can keep the temperature variations just a few degrees above and below that, so the fishies won't care.

AC_Hacker 11-25-10 12:36 PM

1 Attachment(s)

Originally Posted by Ryland (Post 9719)
So if Ben takes his 40 gallon tank, puts Calcium Chloride in it and hooks that to the wood stove then he could heat it up to a higher temp and get more out of it then if he had heated the 210 gallon tank of water and without killing the fish.

Yes, that would be correct... but I can't be responsible for the fish. The phase change material, in this case a Eutectic Salt Calcium Chloride would store more energy in the temperature range discussed.

40 gallons of water converted to quarts = 40 x 4 = 160 quarts

Since the phase change of hydrated calcium chloride happens at about 82 degrees, and the heat taken up and released in changing from a solid to a liquid is about 237.8 BTU per quart,

160 x 237.8 = 38,048 BTU

I'm not sure what the specific heat of hydrated calcium chlporide is, but there would be heat taken up and released both above and below the phase change temperature. I seem to remember that the specific heat of a similar PCM material was about half that of water. But the serious action (heat storage and release) takes place at the phase change temperature, and since 82 F is very close to the desireable comfort level for humans, it's worth looking into.

There was a ton of work done by Mária Telkes and others on another similar PCM material called Glauber's Salt, but there was a problem with long-term reversability. Turns out that Glauber's Salt (AKA: Sodium sulfate deca-hydrate, or Na2SO4·10H2O) over time would begin to change into Sodium sulfate hexa-hydrate (Na2SO4·6H2O), also a phase change material only with a temperature of phase change that was outside the comfort level of humans. In spite of all the work and all the patents the long-term reversability was not overcome.

Nevertheless, there is a home high in the mountains of Utah, built by a guy named David Allen that uses several solar energy storage techniques, one of which uses Glauber's Salt. He obtained his PCM material here.

He used black ABS tubes with the PCM inside, and glued caps to both ends, and arranged the tubes in a rack that was hit by full sunshine every day (see image above, #5) and released it's stored heat to the rest of the house during the non-sun hours. By the way, the heat is self-circulated to the rest of the house without the use of mechanical blowers due to the intentional design of the house. Really, really clever.

BTW, the image above is hyperlinked to the page that discusses the overall design.

But, regarding PCM and Eutectic Salts, Calcium Chloride Hexahydrate, seems to hold the best promise. That's why I posted the patent. So, since the winter is upon us, and driveway ice-melting products are available in the stores, it's a good time to do some experiments with the stuff.

(* I like the idea of ABS tubes of PCM material stored vertically in a water bath, inside of a chest-type freezer. That would be a cheap way to get the structure, the insulation, the re-purposing of junk, etc. *)



Phantom 12-29-10 02:21 PM

I figure that the heat will still transfer to the water without a pump but not be as efficient just from the difference in temps. With that said when the temp gets high enough and you have the pump turned off the temp of the water will still be rising due to convection when the fire is still going.

For temp control you could place a large jar or something on the line so the heat will be pumped to the tank then after it leaves the tank it will flow into the jar that will have a temp sensor in it and a separate line running out to the stove. This will allow you to set the pump based of how much heat is being pushed through but you will have to set it a little higher than the required temp of the water. Also if you do this on both sides of the line you can see the difference in incoming and out going temp.

I would test the setup with the 40 gal tank to start then add a fish or two if you are not heating the water to much then after it is working move it to the 210gal. It would be a bit more work but you might want to move the fish to the 40gal when testing the 210gal and then after you see how quickly it heats and cools move a fish or two over and see how they do.

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