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shelfboxes vs solar closets
9 jul 2003
mike b. writes:

> ...how is the new solar room design different from
>the solar closets you had been working on?

as i construe it, a solar closet is a small room completely surrounded
by insulation and filled with sealed containers of water (eg 55 gallon
drums) with an air gap and glazing over the insulation on the sunny wall
and ventilation holes in the top and bottom of that wall which allow
solar-heated air in the gap to circulate through the closet during the day
and block airflow at night. the closet lives inside a lower-temp sunspace
with its own glazing. sun shines through the sunspace glazing, then on
through the closet "air heater" glazing. on an average day, sunspace air
circulates through the house, and the closet provides little heat. it can 
warm the house for 5 cloudy days in a row.

a shelfbox is basically a repackaging of the thermal mass inside the closet,
replacing the drums with a few inches of water on each of multiple shelves
and an unpressurized water tank below the shelves. the water might be in a
layflat polyethylene film greenhouse duct supported by welded-wire fencing. 
a small pump circulates water between the shelves and tank as needed.

>how is it significantly less expensive?

a 100' roll of 30" diameter poly film duct that lays flat to 47" costs $68.
a 100' roll of 5'x2"x4" welded-wire fence (a standard ag product) costs $50,
about 10 cents/ft^2. a 12x16' folded epdm rubber tank liner costs about $60.
these might replace about 30 $30 55 gallon drums which are bulkier to ship. 

the shelfbox has other advantages. it can store more heat in the same size
closet, while still allowing space for free airflow, which lowers the cost
of the room and the heat loss, compared to a solar closet. a 42 gallon
pressurized galvanized tank inside the shelfbox tank can make water for
showers, replacing lots of fin-tube pipe and a pump in a solar closet.

this works best if the closet provides little heat on an average day, with
an average amount of sun, when the sunspace provides most of the house heat
in the form of warm air. a very efficient greywater heat exchanger can help,
if the closet provides water for showers. increasing the house thermal mass
can help. we might add 12" poly ducts containing 6" of water over welded-
wire fence between joists, above a dropped ceiling.

a 4'x8'x8' tall box with a 4'x8'x4' tank under 12 4'x8'x2" water shelves
on 4" centers would contain about 4'x6'x8'x64 = 12,288 pounds of water and
12x4x8x2 = 768 ft^2 of air-water heat transfer surface with about 1.5x768
= 1152 btu/h-f of air-water conductance. if the house needs, say, 8k btu/h
to stay 70 f, the minimum usable water temp tm = 70+8k/1152 = 77 f. if the
water is say, 130 f on an average day, it can store about 12,288(130-77)
= 651k btu of useful cloudy day heat.

storing 651k btu in a solar closet containing n 55 gallon drums with 440n
btu/f of capacitance and and 38n btu/h-f of air-water conductance requires 
tm = 8k/38n and 440n(130-tm) = 651k, so n = 28. we might stack the drums
2-high in 2 rows of 7 in a 4'x14'x8' tall closet with more wasted space. 

nick




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