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re: stock tank/hot tub: plans? ideas?
4 jul 1999
ge  wrote:
>we're planning a hot-tub-for-several on the cheap... we've looked at tanks...
>we like the idea of a 3' by 10' 374 gal galvanized... we live in minnesota
>down in a valley surrounded by forest...

you might remove some of the local forest, and burn it. the worst-case month
for solar heating in minneapolis is december, at 17.9 f, with an average
daily high of 25.5 f; 820 btu/day of solar heat falls on a south wall, and
430 falls on a horizontal surface. 

>among our... ideas...
>3. build big frame for hot tub, build fire under tub.

how about a combining solar and wood? hot tubs have lots of water, but it's
not very useful for thermal storage, since it needs to be close to 104 f
for use. run the tub east and west with some concrete blocks underneath,
insulate the sides with 6" of fiberglass, add a hinged 4" foamboard cover
that's reflective underneath, and build a fire under the tub.

lift up the cover to 45 degrees on sunny days, and control the fire's draft
to keep the tub at 104 f when in use. solar heating might be automated with
a 104 f heating thermostat in the tub and a 130 f cooling thermostat in a
glazed black box, with a windshield wiper motor and some counterweights for
the cover. the draft control might use a small blower in series with the tub
thermostat and a cooling thermostat in the smokestack.

the tub might be 2' deep, with a sidewall surface of 52 ft^2 and 30 ft^2
of top surface. the thermal cover might have another cover underneath with
two layers of r1 transparent plastic film with 90% solar transmission that
stays in place for solar collection and folds up under the top cover when
using the tub. to stay 104 f on an average day, the tub needs to collect
solar energy ein:

    ein = 6h(104f-22f)30ft^2/r2    cover open
       + 18h(104f-18f)30ft^2/r22   cover closed
       + 24h(104f-18f(52ft^2/r20   sides          = 15.5k btu/day.

at an optimum angle, the cover might see sqrt(430^2+820^2)=925 btu/ft^2 of
sun per day. if it reflects 80% of that sun, and 80% of that is transmitted
into the tub through the double clear cover, it needs 15.5k/(925x0.8x0.8) 
= 26 ft^2 of surface. we might make the reflective cover 4' wide, with a
hinged front edge to approximate a parabola when lifted, and give the south
and east and west edges of the tub a 1' wide deck for the cover to rest on
when closed. 

with the cover in place, u = 30ft^2/r22+52ft^2/r20 = 4 btu/h-f. the tub loses
q = 24h(104f-18f)u = 8200 btu/day with the cover down. c = 374x8 = 3k btu/f,
so it cools q/c = 2.7 f on the first cloudy day. on the second, it loses
24h(101.3f-18f)u = 8k btu and cools to 101.3-8k/c = 98.6 f. rc = c/u = 750 h,
so after 5 cloudy days, the tub cools to 18+(104-18)exp(-5x24/750) = 91 f.
heating it back to 104 f requires (104f-91f)374x8btu/f = 38k btu, about
6 pounds of wood, at 10k btu/lb and 60% combustion efficiency.

if close to the house, this could be a wood-fired water heater, with
a few loops of soft copper tubing around the edge, just underwater. 


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