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re: solar heat storage
4 mar 2001
m russon  wrote:

>   i am in the process of making a rock bin approx. 128 cubic feet in
>size (4x4x8) my collector area is also 128 cubic feet so i am using a
>1 to 1 ratio. my question is, since water has a storage capacity 3
>times higher than rock, would it be smarter to fill the bin with water
>filled containers of some kind rather than rock?

i'd say so. and the water containers would probably have less airflow
resistance than rocks, so they wouldn't need as much fan power. what
kind of smallish containers can you find locally? there's 4" pvc sewer
pipes with rubber stoppers and end caps, 6" square x 8" high 1 gallon
milk jugs on shelves, 4" diameter x 12" tall 2-liter soda bottles, or
(my favorite) 4 gallon 10" square x 3" tall plastic tubs made by ropak
of fullerton, ca. they nest for shipping and stack (at least 20 high)
for storage. used for powdered soaps and dried cherries, inter alia.
our local recycling center has thousands of them.

it would be nice to have at least 1280 ft^2 of thermal mass surface for
128 ft^2 of collector, to minimize the air-water temp diff and make solar
collection efficient. that would be 51 55 gallon drums, but you can only
fit 16 of them in a 4x6x8' box. the tubs have about 5 ft^2 of surface.
a 4x4x8' box would hold 108 of them with 540 ft^2 of surface. you have
to top up these containers with water every few years, which makes big
ones more attractive than small ones. 

a cubic foot of d" rocks has about 12^3/d^3 rocks with about 12pi/d ft^2
of surface, eg 12pi/(3/4) = 50 ft^2/ft^3 for 3/4" clean (not "modified")
stone. you might lay 6 4"x8' perforated pipes with an up-elbow at one end
on the bin bottom (for easy airflow), cover them to about 9" depth with
4x8x9/12-6x8xpi(2/12^2) = 20 ft^3 of stone with about 1000 ft^2 of surface,
then stack the 108 tubs on top. a low-power fan might move 500 cfm of air
through them with a velocity of 1000 lfm in each pipe. this would flow up
through the rocks at 16 lfm, ie 0.08 m/s, with 60 c air weighing 1.06 kg/m^3,
which makes the mass velocity go = 0.084 kg/m^2-s.

the dunkle and ellul correlation says the air pressure drop through the
rock bed is dp = lgo^2/(rhod)(21+1750mu/(god)) pa, where l is the bed
length in the direction of flow (8" is 0.2m), rho is the air density,
mu is air viscosity (2x10^-5 pa-s at 60 c), and d is the rock diameter
(3/4" is 0.019m), so dp = 0.0703(21+2.08) = 1.6 pascals or 0.0065 "h20.

nick

   in our home solar heating system we used water as the thermal storage
   medium for an air-transfer unit, the water being contained in 1000
   one-gallon polyethylene bottles stacked so that air could flow between
   them. they worked satisfactorily until some desert pack rats invaded
   the storage bin, making nests of the insulation and chewing holes
   in the water bottles. 
                                 p 468, _applied solar energy_, by
                                 aden b. meinel and marjorie p. meinel
                                 addison-wesley, 1976




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