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re: solar barn update
20 aug 2004
gary   wrote re:
 
>...solar collectors... with integrated storage...

>a glazing layer, a screen or shade cloth absorber, an insulated 
>partition with high and low vents, a chamber north of the partition 
>with water containers to store heat.  during a sunny day, air 
>circulates through the absorber chamber (heating up), and then into 
>the storage chamber (heating the water).  poly film check valves 
>prevent reverse flow at night.  the back wall of the storage chamber 
>would actually be the existing house wall.  a duct/fan would extract 
>heated air from the storage chamber when needed for the house.  my aim 
>is to store enough heat during a sunny day to carry the house through 
>to the next morning.  i'm not attempting to cover no sun days.
>any thoughts on this?

for overnight heat storage, you might do better just using the thermal
mass of the house... it has a lot more surface, so the air near the
collector glazing could be cooler, with less heat loss to the outdoors. 
if your house has 400 btu/h-f of conductance and 4k btu/f of capacitance, 
rc = c/g = 10 hours. if it's 70 f at dusk and 30 f outdoors, with no
internal heat gain, it would cool to 30+(70-30)e^(-16/10) = 38 f 16 hours
later. brr. making it 60 f at dawn would require that rc = -16/ln(30/40)
= 56 hours, ie c = 56x400 = 22,246 btu/f, eg 2671 1-gallon jugs :-) 

if you heat a ceiling capacitance c near the air heater outlet to 100 f
by dusk and keep the house 50 at night, the house needs 16(50-30)400
= 128k btu over 16 hours, and c = 128k/(100-50) = 2560 btu/f, eg 307
ceiling jugs or a 30" diam x 36' greenhouse poly film duct filled with
4" of water, over some welded-wire fence or 1/2" plywood with a layer
of foil underneath.

>just to get a feel for how well this might work i am converting one 4
>ft bays of the barn collector to this configuration.  i obtained 40
>one gallon jugs of water, and some 2 inch foam board.  i plan to hang 
>the water jugs just north of the collector bay...

in my experience, those cross-linked polyethylene milk jugs will leak
if hung like that.

nick




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