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re: cooling of pv modules (was calculating windload...)
20 dec 2002
jerry krinock wrote:
>there are just a lot of plumbing details...
not really. an unpressurized tank, a $20 pump attached to a pv panel,
a pipe with holes above the panels, and a gutter below.
>and a lot of things which could go wrong,
the pump could fail... but with less than 2 suns, the only consequence
would be a temporary return to the uncooled pv output level.
>...using a lot of water.
it could use less, with a deep pond for 24-hour cooling, or none, with
the water contained in plastic films. an open system in phila with 20%
concentration might evaporate 0.05788x60mx12h = 42 gallons per day.
# of temp w/o temp w/ output
suns cooling cooling gpm watts
1 54.76416 45.65776 .0483204 4476.76
1.2 60.75031 49.84213 .0578811 5257.082
1.4 66.73648 53.03677 .0726934 6030.804...
we get about 45" per year of rain, ie 0.123" or 0.082 gallons per square
foot per day, so a 511 ft^2 roof or pond could supply the water. this is
about the size of the pv array itself...
>> and now for something completely different...
...ie, non-slanted.
>> suppose the 14'x16' array were horizontal, under 2 layers of poly film
>> with some water between the layers, and good insulation beneath, and...
>do you mean the following stack:
>
> top: poly film
> water
> poly film
> pv panel
> good insulation
> bottom: roof
yes, altho i'd probably put the insulation under the roof for better
weather protection, if part of the plan were to heat water for a
swimming pool or showers... if not, i'd leave it out.
>if so, how thick would the "water" layer be, and what would keep the top
>layer of poly film from collapsing?
the water layer might be 1" thick, if the pvs are level to within 1".
the poly films would make a large pillow, sealed at the edges, above
the water level. the top layer of film would rest on the water. i'd
squeeze the edges together with a couple of 1x3s. an alternative is
to lay some greenhouse uv poly film ducts flat on top of the array...
30" diameter duct with a 4-year guarantee costs 62 cents per linear
foot and lays flat to 47.5".
a single layer of poly film with air on both sides has about 10% solar
transmittance, of which about 8% is the fresnel loss, ie about 4% per
poly-air interface, based on the film's higher index of reflection. water
vs air in contact with the films on the inside of the pillow would reduce
the total by something like 8%, from 20% to 12%. would the lower film in
contact with the panel surface also reduce its fresnel loss? some water
or grease might help...
>> a 9' wall above the north edge
>
>a vertical wall?
yes. or possibly taller, or tilted a bit to the south for greater
yearly energy production.
>> the thermal efficiency of the slanted pvs...
the roof-mounted ones...
>>if the films absorb 20% of the sun, the slanted pvs would produce 12% more...
>
>what "slanted pvs? i thought you proposed that they be horizontal?
i was comparing a) roof-mounted pvs tilted to 45 degrees with
b) horizontal water-cooled pvs.
nick
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