solar heat for a church? SOLAR.KnowledgePublications.com

solar heat for a church?
6 dec 2004
when offered a chance to do a service on energy conservation at a local
church on 1/30, i said "ok. can we solar heat it for the occasion?" :-)
the board is thinking about this. the mods could be undone, but if they
work and look ok, they might stay. the church is now gas-heated, but
a 2002 iraq resolution called for a reduction in oil and gas use.

the main room is about 40x48x12' tall, with 8' sidewalls and a 16' 
cathedral ceiling. with 12" studs on 16" centers and 10" of fiberglass
insulation, its walls and roof might be about us r-30 (30 f-h/btu.) with
4 8'x6' double pane south glass doors and a thermal conductance of
4x8'x6'/r2+3840ft^2/r30 = 224 btu/h-f. at 70 f, it needs about (70-30)224
= 8960 btu/h of heat on an average 30 f january day near phila, when
1000 btu/ft^2 of sun falls on a south wall. (then again, 50 people could 
make 15k btu/h.)

this might come from 6 4'x8' shallow shelves in 3 4'x8' boxes under the
ceiling with flat water ducts on the shelves and a water-air thermal
conductance of g = 6x4x8x2.1.5 = 576 btu/h-f and a min usable water temp
of 70+8960/g = 85.6 f. if 0.8x1000x4x8x6 = 154k btu/day of sun enters the
windows and warm air from a dark curtain rises and heats the shelves,
which lose heat to a 4'x48' r30 ceiling strip and keep the room 55 f all
day and 154k = 24h((t-30)4x48/r30+(55-30)224), t = 158 f, theoretically. 

keeping the room 70 f for 2 hours requires 2x8960 = 17.9k btu. with 1/2"
drywall, it has at least 3840ft^2/2 = 1920 btu/f of heat capacity. warming
it from 55 to 70 f with c btu/f of shelf capacitance adds about 1920(70-55)
= 30k btu more, for a total of 47.9k btu. c(158-85.6) = 47.9k makes c = 662
btu/f, eg 221 pounds per shelf unit, eg 2 1" layers of water in 2 4'x8'
polyethylene film ducts, an upper one over a layer of poly film for leak
protection over 4'x8' of welded-wire fence and a lower one over a layer of
aluminized mylar for additional leakproofing and ir radiation reduction,
over another 4'x8' welded-wire fence shelf.

the room might warm up in time t, where 70 = ef+(55-ef)e^(-t/rcp) and ef
= (55x1920+158x662)/(1920+662) = 81.4 f and cp = 1920x662/(1920+662) = 492
btu/f and rcp = cp/g = 0.86 hours, so t = -rcpln((70-81.4)/(55-81.4))
= 0.72 hours, ie 43 minutes.

two existing ceiling fans could bring down warm air as needed, using an
occupancy sensor and a 70 f room temp thermostat. the fan near the window
might blow down and the one farther away might blow (air) up.

an 8'x36' piece of 80% black shadecloth on a wire about 2' inside the
window would look like dark window screen. we might need a wall (eg 9
folding vertical 4'x8' foamboard panels) 2' further back from the windows
to keep solar-warmed air from filling the lower part of the room with
convection currents when it is unoccupied.

pe norman saunders wrote that a curtain well back from a sunny window
makes an upward convection current on the opposite side of the room, so
cooler air flows back down near the window. move the curtain very close
to the window, and air flows up near the window and down on the opposite
side of the room. air won't flow much at all if the curtain is at some
certain distance between those distances. what was that distance? if air
is going to circulate through the room, it seems better to have cooler
air falling down near the window and warmer air rising up on the north
side of the curtain.

nick