[Greenbuilding] solar heat collection

Clarke Olsen colsen at fairpoint.net
Wed Jul 4 18:19:25 CDT 2012


   Cold water indeed. I assume a narrower temp range, maybe 60* to 120*, giving me 480,000 btu, or 10,000
a day for 2 days. The house is quite well insulated, with a passive solar quotient and a wood stove.
   About the tilting aspect: when I draw the summer and winter angles, I get a third more exposure. I realize
that these are extremes: if you don't tilt, you use a compromise angle higher then the depths of winter. 
This should mean that adjusting will improve not only the summer efficiency, but winter also. Latitude 42*.
Clarke Olsen
373 route 203
Spencertown, NY 12165 
USA
518-392-4640
colsen at fairpoint.net


On Jul 4, 2012, at 5:27 PM, Ethan Goldman wrote:

> 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).
> 
> 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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