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paulstef

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Reply with quote  #11 
 I will have to digest these numbers. It starts by getting different solar MJ/ft2/day all over the internet. As for the low hanging fruits, you are certainly right. I built an air collector which I will test this winter. And the basement can definitely use better insulation.

thanks

paulstef

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Reply with quote  #12 
http://www.theatlantic.com/technology/archive/2014/04/scientists-discover-how-to-generate-solar-power-in-the-dark/360679/

This is what I need. Maybe in 20 years....
Homer

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Reply with quote  #13 
My plan is to build a well insulated tank in my basement and have 5,000 gallons of water heated all summer hopefully reach 160° then heat my R40 /R50 1200 sf house as long as I can while replenishing heated water on sunny days. I'll let you know how that goes.
stmbtwle

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Reply with quote  #14 
You can do it but I'm not sure it would be cost-efficient. The larger your storage is, the greater the surface area and the greater the heat loss in total btu/hr, and the longer you try to hold that heat the more you lose. The loss also goes up with the temperature differential. You can reduce the loss with insulation of course, but my guess you'll need a couple FEET or more of insulation. You'll also need a certain amount of solar input just to balance the remaining loss, which means a bigger collector. You reach a point of diminishing returns.

Having this big hot mass in the basement would be great in the winter as the "lost" heat will naturally help heat your home, but in the summer it could mean you have to dump that heat somehow.

Just make sure you do the math first.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Strus

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Reply with quote  #15 
Some guy in Alaska did this, but he has build house around its tank -  not the other way.
It has to have round base, high but rather thin. It has to be heavy insulated, foam is not the best insulation. Not 6'' but 15'' at least. That shape makes advantage of water stratification.
I have 2 tanks made from 30'' steel pipe, which have total capacity of 570 gallons. It has copper pipe coils. I can heat it to 200F. In worst case at -5F I have 16 hours time to resume heating before temperature start to drop. During typical heating zone day (40F average temperature) it can cover heating for 40hours -but really onty 36h because some heat is used for hot water demands. The tank can be as hot as 190F in the upper part and as low as 60F.
My house heaiting demands are about 2500kWh per year

edited for typos&miscalculations
Rick H Parker

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Reply with quote  #16 
The larger your storage is, the greater the surface area and the greater the heat loss in total btu/hr, and the longer you try to hold that heat the more you lose.

If the geometry ratios stay the same, the surface to volume ratio declines as the volume goes up.

A cube 1'X'1'X1' has a surface area of 6 sq ft and volume of 1 cubic ft . That is 6.0 sq ft surface area per cubic ft of volume, ratio 6.0:1.
A cube 2'X'2'X2' has a surface area of 24 sq ft and volume of 8 cubic ft. That is 3.0 sq ft surface area per cubic ft of volume, ratio 3.0:1.
A cube 4'X'4'X4' has a surface area of 96 sq ft and volume of 64 cubic ft. That is 1.5 sq ft surface area per cubic ft of volume, ratio 1.5:1.

The bigger the tank the more thermal efficient it is.
With a bigger tank one can run a lower ΔT, that would reduce the rate of heat loss also.




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Rick H Parker
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stmbtwle

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Reply with quote  #17 
In terms of VOLUME, yes the larger tank is more efficient.  But let us say we "load" them with the same BTUs...  the larger tank will at a considerably lower temperature which makes it more difficult to get usable heat out of it.  

Somewhere there is an optimum tank size for a given input, but I'm not sure what it is.

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Solar is like the wind. It may be free, but putting it to work isn't!
Willie, Tampa Bay
Strus

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Reply with quote  #18 
If you put a coil under upside down funnel with attached long pipe with big holes, then you can storage higher temperatures on the top. Hot water will gathen under the funnel and wiill be lift through the pipe. But if you just put a coil into tank...  only small part of the water under the coil will not get heated and temperature will be much lower on the top. But mine is quite another type of tank, made of steel pipe, not from wood& rubber. Coil is from copper and your solars are mostly non- pressurized, and if you don't use brine-  you don't need the coil.  My tanks are much higher(8'+) than broader. This type of tank can last a lifetime. And they are heavy- both are ~1000lbs each.
You can make a lot of lukewarm water with high efficacy or  a little hot water which is useful. But you need the coil, the funnel with pipe and high tank to hav this choice.
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