[Greenbuilding] solar heat collection

Ethan Goldman ethan at buildinggreen.com
Wed Jul 4 16:27:54 CDT 2012


A friend of mine has proposed a similar system, but I had concerns about
the usable capacity. Out of curiosity, I ran your 1,000 gallon scenario
through the calculations to see how much that could hold. The basic theory
is that the specific heat of water is 4.2 kJ/kg*K (kilojoules per kilogram
degree kelvin), meaning that you must add 4.2 kJ to raise 1 kg of water by
1 degree K (or degree C) and vice-versa. Convert 1,000 gallons to 3,800 kg,
and assume that you can raise the temperature all the way to 200F (93C) and
continue using it to heat your floor all the way down to 90F (32C).
Therefore, you could extract a total of 970,000 kJ from a fully-charged
tank. This is equivalent to 0.9 MMBTU (million BTUs) or 1 mcf of natural
gas (10 therms or ccf if you prefer). A home the size of yours in a
northern climate will typically require tens of thousands of MMBTU for a
full winter's heating; you can convert last year's fuel bills for an
estimate.

If I didn't drop a few zeros along the way, I'd say that your 1,000 gal
tank should be able to get you through a cold night if you have good
insulation. How much capacity do your panels have? You can use that info to
calculate how long it will take to charge the tank. If you can get 10^9 kJ
out of your panels in a typical winter day, this system might be able to
handle the diurnal swing.

Feel free to check my calculations here:
https://docs.google.com/spreadsheet/ccc?key=0Aly4mhoYtT8jdHpEZnZnNmFkWm01Skdta01FUlE5SXc
or try this calculator:
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html

Filling the tank with a phase-change solution could store much more energy,
but would add considerable cost and complexity.

Sorry that I don't have any advice about the mechanics of the system, and I
hope I didn't throw (pardon the pun) cold water on your plans!

Ethan

On Wed, Jul 4, 2012 at 4:24 PM, Corwyn <corwyn at midcoast.com> wrote:

> On 7/4/2012 3:36 PM, Clarke Olsen wrote:
>
>>      Corwyn,
>>      Thank you for your considered advice.
>>       1. 1000 gal was the biggest (5'x10'x4' low boy concrete septic)
>> tank that I could squeeze in the crawl,
>>            before the house went on.
>>       2. Bottom to top is obvious, though it seems that loops pulling
>> heat out should be high in the water,
>>           and one putting heat in could stay on the bottom.
>>
>
> You will get better stratification if the incoming water from the
> collector, is heating the warmest water at the top of the tank, it will
> also be a more efficient heat exchanger.
>
>
>        3. I have no calculation.
>>
>
> I suggest you get some.  You could probably give enough information to
> this list to get someone to do most of the heavy lifting for you.
>
>
>        4. My collectors will mount on ground level frames against the
>> south wall, making adjiustments easy,
>>           and inviting drain back.
>>
>
> My concerns are weight, and water connections.
>
>
>  Tilting back to catch summer sun inceases the effective area by 50%.
>
> Not really.  For my location, adjusting tilt by +/- 15 degrees, twice a
> year, gets me 5.05% more solar (804 kBTUs vs 765 kBTUs per square foot per
> year).
>
>
>        5. The house went through last winter with just the booster to heat
>> the floor. Oh, yeah, and a wood stove.
>>
>
> Well, if you already have the booster, you might as well keep it, but
> controlling it is still going to be tricky.  I would expect to only use the
> wood stove for supplemental heat.
>
>
> Thank You Kindly,
>
> Corwyn
>
> --
> Topher Belknap
> Green Fret Consulting
> Kermit didn't know the half of it...
> http://www.greenfret.com/
> topher at greenfret.com
> (207) 882-7652
>
>
>
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