re: hair brained solar cistern??? SOLAR.KnowledgePublications.com

re: hair brained solar cistern???
14 jan 2003
it's 27 f outdoors on an average december day in worchester, ma...

>...we need 18h/24hx193k = 145k btu of overnight heat. with a 10 f daily
>temp swing, 7k btu/f of inherent house thermal mass and furnishings with
>a short (2 hour) time constant can store 70k btu...

if 100 f air from a sunspace passes along the underside of a basement
ceiling for 6 hours with the help of a large ceiling fan in an upstairs
closet, how much mass does the basement ceiling need in order to make
the house 60 f at dawn?

we might omit most of the usual cross-bracing and screw 10 cent/ft^2 2"x4"
welded-wire fencing under the joists with 1x2 cap strips and lay l feet of
38 cent/ft 12" greenhouse poly duct over the fencing and raise the ends and
fill it with 5.4" (about 32 lb/ft) of water. with slow-moving air, a foot
of duct with 32 btu/f and 1.5pi btu/h-f makes rc = 32/(1.5pi) = 6.8 hours 
or "charging."

if the water temp tc = 70 f at dawn, and it reaches 100-(tc-100)e^(-6/rc)
= 87.6 f 6 hours later and cools to 70 f again by dawn, releasing 75k btu, 
32l(87.6-70) = 75k, so l = 133', with a 1.5pil = 628 btu/h-f conductance
from water to house air.

at 60 f, the house needs (60-27)200 = 6600 btu/h of heat, which makes tc 
= 60+6600/628 = 70.5 f, so th = 87.8 and l = 136' and tc = 60+6600/639
= 70.3 f. once more makes th = 87.7 and l = 135' and tc = 70.4...

nick