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re: minnesota solar power
22 mar 1999
wrote:
>has anyone had any experience with making their own solar panels for hot water
>or heating, i'm looking for ideas and designs to use on my farm, any help or
>advice would be greatly appreciated..thanks in advance..alan harvey
minnesota isn't an easy place for solar heating. about like seattle,
with more sun, but colder, 14.1 on an an average december day in st.
cloud, with an average daily max of 23.1 and 810 btu/ft^2 of sun that
falls on a south wall and 410 that falls on a horizontal surface. the
wall component might increase 30% with a frozen pond or snow in front
of a housewarming sunspace, and the sunspace might work better with 2
layers of r1 glazing, say replex polycarbonate, which comes in 49" x
100' rolls for $1.25/ft^2 + $10 ups from rimol greenhouse systems in
nh at (603) 425-6563.
let's see. a 1 foot slice of an 8' diameter quarter-cylindrical lean-to
sunspace with r1 glazing with 90% solar transmission and a frozen pond
in front gains about 1.3x810x0.9x8' = 7,600 btu/day from the south and
410x0.9x8' = 2,900 from above. if 80 f inside 6 hours a day, with 12' of
curved glazing, it loses about 6h(80f-20f)12ft^2/r1 = 4,300 btu, making
the net gain about 6,200 btu, starting with about 12,000 btu of sun, for
a solar collection efficiency of 6,200/12,000 = 53%. not bad. i guess
we don't need 2 layers of plastic, which would lower the solar input to
about 9,500 btu and lower the loss to 2,200, for a net gain of 7,300,
not much of an improvement.
for water heating, you might consider something like big fins in the
sunspace, with normally pressurized water inside and a thermosyphoning
loop through an insulated tank above. these are bare solar water heating
collectors, with no glazing or insulation of their own, and no pumps or
antifreeze or heat exchangers. i think they can also be homemade from
some copper pipe and brown aluminum roofing coil stock and a few nuts
and bolts, for about $1/ft^2, as described in previous postings. you
might have a tiny pump in a separate loop that keeps the fins from
freezing when the sunspace is very cold at night.
don't fill up your housewarmer with thermal mass, as psic brick salesmen
suggest, unless it's inside a closet completely surrounded by insulation
(even then it's more efficient to use sealed containers of water than
bricks or rocks). putting lots of thermal mass in the sunspace makes it
coolish 24 hours a day vs hotter during the day and colder at night.
that wastes lots of solar heat by storing it during the day and letting
it leak out all night through the low thermal resistance of the glazing.
heat the attached structure by circulating warm sunspace air during the
day, with no air movement at night, and an insulated wall between the
sunspace and the structure.
filling up the housewarmer above with thermal mass makes 7,600+2,900 =
10,500 btu = 24h(t-14.1)12ft^2/r1, for a fairly constant sunspace temp
t = 14.1 + 10,500/(24x12) = 50.6 f, not very useful for house heating.
nick
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