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re: "magnification is out."
20 apr 1999
anthony matonak wrote:
>...since solar cells seem to dislike heat so much wouldn't it be more
>effective to use them as air heaters rather than water heaters?
the cells might end up warmer in air heaters, even though 80 f air is
useful for solar house heating, and we'd like 110 f water for showers.
an air heater with say, 1 ft^2 of absorber surface per 2 ft^2 of am2 sun
and a 90%-transmitting r1 cover (net solar input 450 btu/h) heating a large
volume of slow-moving 80 f air on a 30 f day (net cover loss 100 btu/h) with
an absorber-air thermal conductance of 1.5 btu/h-f-ft^2 needs an absorber-air
temperature difference of about 350btu/h-ft^2/1.5btu/h-f-ft^2 = 233 f, so
the cell temperature would be about 80 + 233 = 350 f, unless the plate has
some fins and airflow behind it, like the system suggested in richard komp's
1995 practical photovoltaics, with 2 suns. (he says silicone-encapsulated
vs eva cells can handle 3-4 suns, if they are cooled adequately.)
heating water at 110 f under the same conditions, with a cover loss of
160 btu/h and an absorber-water thermal conductance of 30 btu/h-f-ft^2,
the cell temperature might be about 110 + 290/30 = 120 f.
>...you could conceivably use normal unmodified solar pv panels and
>wouldn't have to worry about leaks and how electricity and water are
>such a poor mix.
pv panels are weatherproof anyway. pe norman saunders suggests trickling
some water over their faces. an extra layer of polycarbonate glazing might
help reduce fogging and evaporative loss, and the output might be improved
by adding some 2:1 curved side reflectors made from aluminum roof flashing
covered with self-adhesive metallized mylar film, all in the upper part of
a sunspace, so the heat loss is less to warm sunspace air, which in turn
heats the house in the winter.
>i would imagine that since an air heating solar collector usually is
>some kind of glass faced box with something black inside it and air
>flowing through it that you could just replace the glass face and
>something-black with a solar pv panel and draw hot air off the back.
that might be effective, but air heaters tend to work better with a layer
of dark absorbing porous mesh in the middle, with cooler room air slowly
moving between the mesh and the cover, and solar-warmed air moving through
the mesh from south to north, which doesn't seem to fit well with pv cells.
nick
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