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re: reflectors/solar cells
31 jan 2002
n. thornton wrote:
>> have you priced ss?
>
>no, i guess i was hoping it would be worth while if it was thin enough.
>i dare say you have.
i was unable to find "very thin" stainless steel. as i recall, the thinnest
available sheets (22 gauge?) were still very expensive. you might find it
cheaper in coils, but it seems we have to buy a very large quantity that
way, eg a single coil that might weigh a ton. perhaps you could buy a small
quantity from a manufacturer who uses a product like that.
home depot sells aluminum roof flashing in 2' wide coils for about
50 cents/ft^2. one version is painted brown on one side and white on
the other... the brown might make a nice "big fin" water heater if
we could attach it well to a pipe.
>greased mylar is no good for commercially installed systems. any
>relfector would have to reliably last as long as the rest of the
>system,
i'm not sure about that, altho homeowners (vs businesses) do not
like maintenance.
>and be pyhsically tough enough to withstand birds pecking, storms etc.
maybe 5 years is enough.
>that's why i doubt silvered mylar film.
mylar (less than 10 cents/ft^2 in 4' wide rolls) might last 5-10 years
if the metal surface is greased to some poly film (the grease makes it
easily-replacable), and the other surface faces down. this might be
something like a commercial plastic film greenhouse "polytunnel" with
parabolic steel pipe bows on 4' centers. greenhouse poly film comes
with a 4 year guarantee. it might last 10 years if it is whitewashed
or covered with shadecloth on top. it costs about 5 cents/ft^2, and
it is available in huge folded rolls, eg 40'x150'. the labor cost to
replace it is 1-2 cents/ft^2.
we might just put the panels under a whitewashed poly film reflector.
>>...downwards, which also facilitates cooling the cells in a water bath.
>
>two problems with a water bath:
>
>you know what gutrtering looks like in 20 years, mud caked. that could
>be a real problem with outdoor water. and it would dry out when most
>needed. plumbing adds cost.
how much? hoses and tiny bilge pumps are cheap. norman saunders, pe,
suggested just trickling some water over some panel faces.
we might use rainwater from a parabolic trough roof, with a reservoir
and float valve. no minerals... and scrub the panels once a year if
needed. corrosion is another possible problem if they are immersed.
they are made for weather, but i can picture connectors rusting off
and pinholes in potting. maybe the panels need to live inside plastic
film drum liners. this isn't rocket science.
>so it'd have to be mains water, ie plumbed. plumbing the cells to the
>house's hot water tank would work, but again it's more expense.
collecting hot water would be nice, altho a poly film cover would add
some expense (10 cents/ft^2?) and reduce pv efficiency by about 10%.
>we really need something super cheap that's going to turn solar around,
>if possible!
how many $/ft^2 can something cost, if it triples a panel's output?
>so i'm thinking of 2 points: first put them onto sheet metal to
>partially cool them...
water and plastic film seem cheaper.
>and second let them run hotter. silicon can run at higher temps than
>they are used at now. higher temp reduces efficiency...
it also reduces lifetime.
>maybe there's a better solution, but i don't think we've found it...
you might look at my "silver tarp posting" in alt.solar.photovoltaic.
mylar is much more reflective (over 90% vs 50%?), but those tarps are
cheap, and the structure (something like a tent, with lots of ropes?)
might serve some other purpose, eg a place to park a car in the rain,
and diffused sun is less likely to cause hot spots and fires and
shadowing inefficiencies.
nick
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