[Greenbuilding] Cooling PVs

[nick pine]

Reuben Deumling <9watts at gmail.com> asks:

>... is it ever sensible to use PV to heat water?

I'd say no, but it seems sensible to water-cool PVs...

I like these $159.99 500 W Chinese grid-tie inverters 
http://www.amazon.com/Small-Inverter-Converter-24-52v-90v-130v/dp/B005N2B0C2/ref=sr_1_1?ie=UTF8&qid=1331847121&sr=8-1

The least expensive PVs I've found so far are AUO panels which make 240 
watts at 46 C in 1kW/m^2 sun with a 14.4% efficiency and a -0.44% power 
tempco and an 80 C max operating temp, for $1.28/Wp, including shipping. I'd 
like to raise their output to at least 250 watts so 2 panels can drive a 500 
watt inverter. I could lay them flat on the north edge of a deck and lay a 
piece of greenhouse poly film on top to make a shallow water trough. At my 
40 N lat, 6/21 beam sun at noon has a 90-40+23.7 = 73.5 degree elevation. 
The film and water would act as an index matching layer to raise the output 
about 6%. The cooler temp and a reflector to augment the sun can raise it 
more.

In July, Phila has an average 0.0133 outdoor humidity ratio, with Pa = 
29.921/(0.62198/0.0133-1) = 0.654 "Hg. At the wet bulb temp, Bowen's 1926 
equation says 100(Pw-Pa)/(Twb-Td) = -1, ie Twb = 
9621/(22.47-ln(Td-Twb+100Pa) = 9621/(22.47-ln(606.8-Twb), with Td = 460+81.4 
R and Twb (R). Plugging in Twb = 70 F (530 R) on the right makes Twb = 530.7 
on the left. Repeating makes Twb = 530.4, then 530.5 R, ie 70.5 F or 19.2 C, 
so the panel output would increase by 100(46-19.2)0.0044 = 11.8% if cooled 
at the wet bulb temp, eg 240x1.118 = 268 watts under 1 kW/m^2 sun.

How tall should a vertical north reflector be to make 250 watts per panel at 
noon on 6/21? If P = (800+1.9h800)268/1000 = 214(1+0.9h) with h in meters, h 
= 0.0052P-0.9 meters, eg 0.39 meters for 250 watts or 0.826 for 333 watts. A 
4'-tall (1.22 m) 10 cent/ft^2 aluminized Mylar film reflector would make 450 
watts peak, or 2.7 real kW for 6 panels, enough for 4 or 5 $159.99 500 W 
grid tie inverters. They have overcurrent protection, so they probably don't 
mind overdriving.

With 5 inverters, this system would cost about $2638.60/2.5kW = $1.06 per 
real peak watt.

I see 240 watt panels for 80 cents/watt at http://www.soldist.com/clearance/

Nick