re: drum wall thermal storage questions....................... SOLAR.KnowledgePublications.com

re: drum wall thermal storage questions.......................
1 apr 2002
i wrote:

>with less floorspace (24'x32', using 4 8'x24' sips for the ceiling and
>14 for the walls, about $17k at $5/ft^2) and more insulation and fewer
>windows and an air-air heat exchanger, you might not need any drums... 

oops... 24'x32' only requires 896 ft^2 of sip walls plus 768 ft^2 of 
ceiling, $8,320 at $5/ft^2 for 8 8'x24' and 2 8'x8' 12" sips. a 24'x40'
shell with 11 8" sips would only cost $7,936 at $4/ft^2.
 
with 4% of the floorspace as r4 windows and 8" r36 sips and 15 cfm of
fresh air and an 80% efficient air-air heat exchanger, g = 40ft^2/r4
= 10 btu/h-f for windows plus 984/36 = 27 for walls plus 1024/36 = 29
for ceiling plus 3 for air, so it needs (77.5-36.2)69 = 2850 btu/h or
68.4k btu to stay 77.5 f on an average january day.

with 50% solar transmission, 20 ft^2 of south windows and 8, 8, and 4
on the e/w/n will collect 0.5(20x1030+16x475+4x230) = 14.6k/day of sun,
reducing the 68.4k above to 53.8k, which might come from 53.8k/3.412x30d
= 473 kwh/month of electrical energy use.

but wait, your mom also needs hot water, right? moms are often very clean.
you might consider a parabolic concentrating water heater on the roof.
nrel says nashville gets 1030 btu/ft^2-day of south wall sun in january,
370 of which is diffuse, leaving 660 btu/ft^2-day of beam sun.

a 4'x12'x8' tall box with a polycarbonate south wall and reflective endwalls
and a parabolic north wall might bounce 0.9x0.9x8'x12'x660 = 51k btu/day of
sun into a shallow 4'x12' pool with a greenhouse poly film cover. the pool
might contain some pv panels as well. at 180 f for 3 hours on a 41 f day,
with uninsulated box walls, it might lose 3h(180-41)4'x12'/r1 = 20k btu,
for a net gain of 31k btu, enough to heat 31k/(110-59.1) = 609 pounds or
76 gallons of 59.1 f nashville groundwater to 110 f for 61 minutes of 1.25
gpm showers per day. (i just measured my shower at 1.25 gpm, with 6% more
leaking into the tub via a brand new diverter valve.) 

a lot of that heat might be recovered with a few free 3' tall 55 gallon
plastic drums acting as a graywater heat exchanger. you might use 2, and
drill a hole near the bottom of each one and connect them together by 
pushing one end of a heater hose into each hole, and make an overflow
drain with another hose near the top of the outlet drum.

or cut off the tops and use $5 3/4" "bulkhead tank fittings." these 3-piece
plastic things have a flange connected to a smaller threaded pipe, a rubber
washer, and a nut that threads over the pipe. drill a hole in each drum and
put the flange inside one with the nut inside the other and the washer 
between the drums. maybe put a submersible pump with a level switch (eg
northern's $47 item# 108981-c151) in one of the drums, adjusted to keep it
nearly full, and pump it back up into the sewer drain or out to the garden.

with 76 gallons of 110 f water entering every 24 hours, warming 70 f air,
this might be an ashrae hof heat exchanger with a min fluid capacity rate
cmin = 76x8/24h = 25 btu/h-f. if each of the n drums has a = 25 ft^2 of
surface with u = 1.5 btu/h-f-ft^2 of slowly-moving air film conductance,
ntu = au/cmin = 25ft^2n(1.5btu/h-f-ft^2)/25 = 1.5n. with a capacity rate
ratio z = 0, the heat exchanger effectiveness e = 1-exp(-1.5n), ie 78%
for 1 drum, 95% for 2, 99% for 3, and 99.8% for 4... 

over 5 cloudy days, the house needs 342k btu. with no electrical energy
use, this might come from 342k/(180-80) = 3420 pounds or 428 gallons or
53 ft^3 of water cooling from 180 to 80 f in 8 1'x8' pipes or a 2'x4'x8'
tall epdm-rubber-lined box below the collector.

nick