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re: a stupid question
8 feb 2004
anthony matonak wrote:
>mirrors are fairly inexpensive.
especially this kind, with 10 cents/ft^2 mylar greased to
35 cents/ft^2 foil-faced foamboard in 1'x16' strips, about
100x cheaper than pv panels at $40/ft^2.
dynaglas polycarbonate (to protect the mylar from weather
and trap more heat) costs about $1/ft^2, but we might use
less of something clear and cheaper (polyurethane varnish?)
directly over the mylar (vs a window 6' away), with more
desirable heat loss, in a heat-wasting application.
then again, heat can be valuable...
>i would imagine that you could extend the mirrors both
>east and west past the size of the pv panels so that
>they can reflect the sun on them for more of the day.
i'm looking into that. reflective endwalls may help.
>perhaps if we limit ourselves to sunlight from solar noon +/- 3 hours
>we will have workable sun angles both horizontally and vertically.
that could fail to collect lots of summer sun, if we
are aiming for a maximum yearly electrical output.
>i don't think a small single panel demonstration system would be
>terribly expensive or complex to build.
agreed.
>i'm most interested to learn how pv panels, even under water,
>deal with the irregular illumination such a system is likely
>to produce during early morning and late afternoon hours.
the panels themselves wouldn't be underwater, just under
a plastic film duct containing an inch or two of water...
they might work better in parallel than in series. a couple
of years ago, roland winston gave a paper about using an
integrating sphere (a reflective shoebox with a port on
one side) as an "optical mixer" at an ases conference,
suggesting it might work well with concentrating pvs because
the intensity on one dark internal wall of the shoebox was
fairly constant (+/- 10% or so.)
dynaglas is clear but corrugated. if the corrugations are
vertical, they will refract light east and west, but they
may not alter its downward path much, so the reflector may
still work well, with better-mixed light. approximating
a parabola with a few sections also tends to mix light...
below is a calc that suggests that 990 w stc panels might
produce about 2000 kwh/year in phila, when cooled to about
90 f. at this temp, it looks like the pumps will run about
200 hours per year (vs 35 hours at 130 f), so they might
wear out at the end of attwood's unconditional 3-year
guarantee ("...unless you drive over the pump with a truck" :-)
attwood's 600-hour lifetime involves pumping salty hot
mud containing hair and rocks and so on.
of course it would be more interesting to use the heat.
nick
10 pi=4*atn(1)
20 data 620,310,1000,340,0.0025'nrel phila weather data
30 data 870,420,1080,410,.0027
40 data 1200,550,1070,480,.0036
50 data 1520,700,950,540,.0050
60 data 1760,840,830,570,.0080
70 data 1940,880,790,580,.0111
80 data 1890,870,820,580,.0133
90 data 1710,760,940,570,.0130
100 data 1380,610,1070,520,.0104
110 data 1020,450,1150,450,.0070
120 data 680,330,990,360,.0048
130 data 530,280,900,310,.0032
140 beta=71.2/180*pi'reflector tilt (90 = vertical)
150 tup=90.5'upper limit for pv panel temp (f)
160 for month=1 to 12
170 read hg,hd,vg,vd,w
180 floorheat=.9*6*16*(hg-hd)'beam sun from above (btu/day)
190 floorheat=[error].9*6*16*hd*(1+cos(beta))/2'plus diffuse sun from above
200 floorheat=floorheat+.81*12*16*(vg-vd)'plus south beam sun
210 floorheat=floorheat+.81*12*16*sin(beta)*vd'plus south diffuse sun
220 psh=floorheat/6/16/317'peak sun hours
230 elec=.99*psh'elec output per day (kwh)
240 floorheat=floorheat-3412*psh'net floor heat
250 pa=29.921/(.62198/w+1)'ambient vapor pressure ("hg)
260 pp=pa+floorheat/24/100/8/16'pond vapor pressure ("hg)
270 tp=9621/(17.863-log(pp))-460'pond temp (f)
280 dt=tup-tp'output-input cooling water temp diff (f)
290 tpv=(tp+tup)/2'average pv panel temp (f)
295 tpvc=(tpv-32)/1.8'average pv panel temp (c)
296 elec=elec*(1-(tpvc-25)*.005)'correct pv output for temp
300 g=floorheat/dt/8'gallons of cooling water per day
310 ph=g/41.67/60'pump hours per day
320 print month,tp,30*elec,30*ph
330 telec=telec+30*elec:tph=tph+30*ph
340 next month
360 print "kwh/yr =";telec-70*tph/1000,"ph/yr =";tph,"pkwh/yr = ";70*tph/1000
month pond temp (f) kwh produced pump hours
1 64.20423 166.1452 8.630029
2 66.9729 183.1281 10.67264
3 69.37134 187.6356 12.21757
4 70.76556 177.0364 12.36596
5 74.70636 165.5716 14.53121
6 78.93201 160.7463 19.37662
7 82.16669 163.9006 27.55221
8 82.76392 175.5979 31.8245
9 80.39826 187.6883 25.96215
10 75.88226 192.6182 18.29517
11 68.87262 164.5063 10.45724
12 63.45789 149.7092 7.553842
yearly totals:
kwh/yr = 2060.323 ph/yr = 199.4391 pkwh/yr = 13.96074
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