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re: cooling of pv modules (was calculating windload...)
21 dec 2002
george ghio   wrote:

>> i'd bolt together a 32'x14' frame...

> you have enough for 50 solar garages.

i have enough now for about 15. 

>...50 solar garages soundslike a commercial operation.

yes, on a small scale. it would be nice to build one a week.

>this frame has 5.3 ft centers length wise and 7 ft centers across the 
>width. what make solar panel are you using? do they fit this frame?

sharp 165 w, 32.5" (ns) x 62" (ew), with an extra 1/8" for tape between
ns edges and 1.625 (unistrut)+0.25 (tape) between ew edges...

>>             32'
>> p-----------p-----------p
>> |   |   |   |   |   |   |         using about 400' of 1.625"x2.4375" 
>> |   |   |   |   |   |   |         steel unistrut with the inner beams 
>> |---+---+---+---+---+---| 14'     doubled in each direction.
>> |   |   |   |   |   |   |         
>> |   |   |   |   |   |   |         the next one might have 25% less steel.
>> p-----------p-----------p
>>           south
>> >the tarp frame? how is the tarp kept tight? 
>> the south posts (p) would be 9' tall. the north ones would be 18', ie 9'
>> above the level or 2% tilted array. the tarp would be tied to the posts
>> with light enough twine to let it blow loose before the structure tipped
>> over in a great wind. a few bungee cords might be nice.
>this tarp fixing sounds great - for a sunshade.

in oz...

>as a reflector it sucks...

we don't need specularity or uniform illumination.

>as a fixed angle reflector on what seems to be a fixed angle solar 
>array...well what can i say?...

you can say it's 400x cheaper than pvs :-)

>> >the pipe and fittings that go with the pump?
>> commensurate with the price of the $20 bilge pump.
>i don't thik so...

you might thik more... 

>> >the water recovery and the waste heat recovery?
>> if the water evaporates, there would be no "recovery."
>so you are proposing a total loss system.

that's one version.

>> the water supply would be the pv roof, and maybe
>> a nearby house roof. i can think of 3 applications 
>> for "waste heat recovery": 
>> 1. swimming pool heating, which might only involve
>> a pipe full of holes near the north edge and a gutter
>> below the north edge. maybe no separate pump...

that marginal cost would be low, for a 32'x2" pvc pipe
with some holes and a 32' gutter. if new, the extra 27'
of unistrut for the north poles would cost about $90,
and the cost of the tarp is about $50.

>> 2. space heating, with an insulated unpressurized tank
>> for thermal storage and a water-air heat exchanger, eg
>> a hydronic floor or a fan-coil unit.

more, about $60 for the poly-film pillow. the tank could be
a 4'x8'x8' epdm-lined plywood box in the basement. magic-aire's
$150 2'x2' all-copper shw 2347 duct heat exchanger can transfer
45k btu/hour between 125 f water and 68 f air at 1400 cfm with
a 0.1" h20 pressure drop.

>> 3. making hot water for showers, an extension of (2)
>> with a 42 gallon bare galvanized pressurized tank
>> inside the unpressurized tank to act as a preheater.

my plumbing supply store sells the galvanized tanks for about
$150. seems high, compared to a whole water heater for $200. 

>> >the control for the pump?
>> just wire it up to a panel, basically.
>that the best you can come up with is it? 

read on.

>>a differential thermostat could improve performance...
>> >temperature sensing for the panels? the anti freeze?
>> draindown, with a $6 freezestat to avoid making ice.
>but the pump is wired to a panel and will run any time the sun is out 
>even if it is below freezing outside...

that's the reason for the freezestat.

>> >the nuts and bolts? 
>> i've got about 500 pounds of unistrut nuts and bolts.
>ah yes. steel frame bolted to alloy framed panels what a great idea that is


>> >what will it cost?
>> about the same as the rebate, $25k for 5 kw, including labor.
>seems to me that your 25k would barely cover the cost of 5kw of panels.

the best price i've found so far is $3.50/w, delivered. 

>to turn this concept into a design will take more than you have outlined here.

agreed. i built a model and talked it over with some structural engineers,
and i hope to get some help improving the reflector geometry.
thanks for your helpful questions.


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