re: help with passive solar hot water storage SOLAR.KnowledgePublications.com

re: help with passive solar hot water storage
2 jun 1996
>dear nick:

hello dan...

>i have been heating my hot tub with 200 ft. of black plastic pipe 
>running back and forth across my black roof on the north west side of 
>my house for 2 weeks.

the northwest side... i wonder if you have a southeast roof you could use.

>the temp comming off the roof at the hotest part of the day is 122
>degrease at 3:00 in the afternoon.

sounds like it works pretty well, in this season. it would be fun to try to
keep the tub warm enough to use in january. seems like that wouldn't be too
difficult, if it had a good cover or moved to some place with a hot spring...
a small solar collector/water store built into the south wall of the base?

>this is to hot for the hot tub at this time of the year.

people like hot tubs around 104 f. there's a pretty narrow range... 100 feels
cool to me, and 106 feels very hot... it's odd that there's all that water,
but it isn't very useful for storing heat for a few cloudy days, because the
usable temperature range is so small. we might start with a $600 6' wide x
5' deep x 12' long 1,500 gal septic tank... if it drops from 106 to 104 f in
5 days, it needs 104 = 36 + (106-36)exp(-5x24/rc) ==> rc = -120 hr/ln(68/70)
= 4140 hours, ie 6 months. wow. the septic tank has 324 ft^2 of surface and
12k pounds of water, so the r-value of the surface would have to be
324x4140/12k = 112, eg 22" of styrofoam...

on the other hand, if it had 6" of styrofoam or a strawbale wall around it,
it would lose about 5x24(106-36)324/r30 = 91k btu over 5 days, which might
be supplied by 91k/50 = 1,800 pounds of water cooling from 154 to 104 f.
another water wall on the south side, 12' long x 5' high x 6" thick? where
i live, that south wall would get about 60k btu/day of sun in december, of
which about 6hr(154-36)60ft^2/r2 = 21k would go back out through 2 layers of
glazing, on an average day, leaving more than enough to keep the tub warm...

>the best place to fine info about this is home power mag or there web site at 
>http://www.homepower.com/hp or e-mail at hp@homepower.org

i remember a solar heater described their using lots of
black tubing in flat spirals on a roof...
worked pretty well, as i recall.
and cheap, vs store-bought,
rooftop, solar
collectors.

nick