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re: electricity generated by solar hot water system
17 jul 2002
andy gee wrote:
>...a lot of solar water heaters i've seen are glass tubes in a flat roof
>panel painted black. instead of the black painted frame, what would
>happen if you had a pv cell of the same size? would you be taking
>advantage of the ir to heat the water and the uv to generate the
>electricity at the same time, getting more work from the same footprint?
sure. you might bolt some extra aluminum angle around the frame and
cover the front surface with a single layer of polycarbonate plastic
with 90% solar transmission, using silicone caulk and more angle as
a cap strip, with 2 bulkhead fittings for water flow. the panels might
be above a horizontal white surface to the south.
another alternative would be to lay them flat on the ground under a 2-sun
east-west downward-reflecting linear parabolic concentrator (something
like a commercial plastic film greenhouse covered in mylar film and
sliced in half lengthwise) and cover them with a cooling blanket, eg
a uv greenhouse poly film duct (62 cents per foot for a 48" flat duct)
containing some flowing water in a drain-down system. geometrically, this
might concentrate sun only in wintertime, when outdoor temps are lower.
>would the water being heated help cool the pv cells?
maybe. the on-line data sheet and 15 page 4/11/02 technical description
for kyocera's kc158g pv module has thermal parameters a = 6.08x10^-3 a/c
for isc and b = -1.1x10^-1 for voc and a 47 c nominal operating cell temp.
the electrical specs say the 158 w max power point occurs at vmp = 23.2 v
and imp = 6.82 a, measured at 1 kw/m^2, am1.5, and a 25 c cell temp.
page 7 of the technical description explains how to find the output at
another cell temperature:
i2 = i1+isc(isr/imr-1)+a(t2-t1) and
v2 = v1+b(t2-t1)-rs(i2-i1)-ki2(t2-t1),
where i1, v1, i2 and v2 are points on the iv curve, t1 is a reference temp,
t2 is a different temp, isc is measured for the test specimen, imr is "the
measured short-circuit current of the reference device," isr is "the short-
circuit current of the reference device at the standard or other desired
irradiance," rx is the internal series resistance, 0.2 ohms, and k is
a curve correction factor, 2.02x10^-3 ohms/c.
if isr = imr and t1 = 25 c and t2 = 50 c, it looks like
i2 = 6.82+0.00608(50-25) = 6.972 a and
v2 = 23.2-0.11(50-25)-0.2(6.972-6.82)-0.00202x6.972(50-25)
= 23.2 -2.75 -0.0304 -0.352 = 20.07 v,
with i2v2 = 139.9 w. not bad. is this also a max power point?
on a 100 f day with 1 kw/m^2 (317 btu/h-ft^2) of sun, a 13.76 ft^2 kc158g
panel with insulation behind it and 50 c (122 f) water in front and a cover
with 90% solar transmission and no load might gain 3925 btu/h and lose
(122-100)13.76x1.5 = 454 btu/h to still air. it needs 3471 btu/h of cooling,
eg 3471/8/60/(132-112) = 0.36 gpm warming from 112 to 132 f. this water
might circulate naturally through an unpressurized overhead tank.
in still 100 f air, with 1 kw/m^2 of sun and no load, we might have
317 = (t-100)3, losing heat from both sides, ie a panel temp t = 206 f or
96.5 c. page 6 of the kc158g description specs a max 90 c operating temp.
at 90 c,
i2 = 6.82+0.00608(90-25) = 7.215 a and
v2 = 23.2-0.11(90-25)-0.2(7.215-6.82)-0.00202x7.215(90-25) = 15.02 v
with i2v2 = 108.4 w.
a 50 c (122 f) panel could make another 32 watts while providing hot water
for showers. it might last longer. would a little more sun void kyocera's
guarantee? it mentions abuse and misapplication, but i've heard it doesn't
explicitly mention mirrors or concentration. most people say wearout (eva
browning, and so on) depends more on temperature than solar intensity. we
might glue a max temp test strip to the back of a panel to verify that
it's never been more than 90 c...
nick
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