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re: hot water idea
tue, 31 aug 1999

> > wrote

> >what if:
> >
> >you purchased a large stainless steel tank and painted it flat black,
> >and it sat out in the sun right before entering your hot water heater?
> that's a nice idea in principle, but wouldn't be very effective the way >
you have presented it-

it might work reasonably well in a warm sunny climate, altho the wind could
blow away lots of heat, with no glazing, and it would lose lots of heat at
night and on cloudy days, with no night insulation. even with no sun, a
tempering/preheating tank in the house could help.

> there wouldn't be enough steel to water contact to
> quickly transfer the heat.

i disagree. you might check this out with a double boiler on your kitchen
stove, and a couple of thermometers and a clock...
i measured a still water film thermal conductance of about
60 btu/h-f-ft^2 this way.

a gallon of water in contact with a square foot of steel has an rc
time constant of about 8 btu/f/60btu/h-f hours, ie 8 minutes, so starting at
say, 50 f, in contact with 130 f steel, it would warm to
100 f when 100 = 130-(130-50)exp(-t/8), ie when t = -8 ln(30/80)
 = 7.84 minutes.

>you'd be much better off with a network of copper tubing (say
>3/8" o.d.), painted flat black and placed in a glassed-over box
>oriented to catch most of the day's light.

rubbish. this might work better with fins and a pump and an insulated tank,
or larger plastic tubing, altho that's unlikely to survive normal water
pressure. that copper tubing's too small to hold much water or work well by
thermosyphoning, and it needs more sun-gathering surface.

>...personally, i'm much more in favor of building a heat storage
> building, and keeping a hot water tank in it-

sounds nice.  big polyethylene tanks are cheap, eg $899.99 for tractor
supply's 2100 gallon 87" diameter x 87" tall version. put it inside an 8'
x2" foamboard cube surrounded by bags of leaves?
that makes rc about 2100x8btu/fxr28/(6x64ft^2) = 1225 hours,
or 51 days. yum :-)

> then you just burn your trash, and whatever wood you need
> to keep things hot, and no worries about burning down
> your house.

wood pollutes, and it's lots of work. how can we solar heat that sucker? for
potable water storage, one might efficiently bubble a
little air through the higher temperature water it to keep it pure
and free of bacterial nutrients, as in a sewage treatment plant.


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