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re: hot water idea
wed, 1 sep 1999
anthony matonak wrote
> how about this for an idea.
> you take a large stainless steel tank, painted flat black and plumb
> it into your hot water heater as a pre-heater tank.
some objectives: a) gather enough sun (about 100ft^2 for a house,)
b) get the sun into the tank, c) save the heat at night, d) keep the
tank from freezing, and d) do all that simply and inexpensively...
> then enclose that tank in a triangular shaped box with a few inches
> of that foil-faced foam insulation panels (and maybe plywood on the
> outside) with the foil on the inside to reflect the light, on two
> sides and the third side being a sheet of glass.
say 4'x8' of glazing, and 2 4'x8'x2" foil-faced foam (r-10)sides. in phila,
this collects about 0.9x4'x8'x1000 =28.8k btu/day of sun in january, when
the average outdoor temperature is 30 f. the box
has a thermal conductance of about 32ft^2/r1 = 32 btu/h-f for the glazing
and 78ft^2/r10 = 7.8 for the other sides, ie a total of about
40 btu/h-f.
with no water flowing, the tank absorbs about 28.8k btu and
loses 24h(t-30)40 = 960(t-30) btu over an average day, so
t = 30+28.8k/960 = 60 f. not very promising. it would work better indoors,
in a 70 f house.
say we preheat enough 50 f water for 6 110 f 3gpm 10 minute showers per day,
ie 180 gallons per day, so 28.8k = 960(t-30)+180x8(t-50), and t = 54 f, so
the tank provides 180x8(54-50)
= 5760 btu/day of useful heat, 7% of the water heating energy,
with a solar collection efficiency of 5760/(1000x4'x8') = 18%.
> this describes a batch water heater as people have been fond of
> building for years and it works fairly well, usually from old
> (thrown out) electric water heater tanks whose heater elements
> have burned out...
it's my impression that few water heaters have stainless steel tanks, and
they are usually discarded because the tanks are leaky, and their electric
heating elements are replaceable.
this arrangement might work better in an attic, with a long "tank" (a
30' long x 4" diameter black-painted pvc pipe, with a 3/4" copper
pressurized pipe inside?) along the ridge line, and a transparent sloping
south roof, and a concentrating parabolic reflector below the tank, and
foil-faced foam above the tank, which extends down below
the tank bottom to make a secondary concentrating reflector...
the tank might be suspended from ropes, and swung north and south to track
the sun, with a high concentration ratio. nielson's
(253) 941-7281 aluminized mylar film costs about 15 cents/ft^2 in 4'x100'
rolls. it lasts for years indoors, and can be "glued" to curved masonite or
scored foil-faced foamboard with a roller and axle grease, says duane
johnson, who reports it's sufficiently mirrorlike
for 100:1 concentration.
be careful about fires.
nick
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