Trickling water over PVs, again

 On Saturday I saw 30 BP 175i frameless panels with little airflow beneath
on a 2-story south shingle roof with a gentle slope, about 30 degrees. If
they produce 5.25 kW at 20 C (68 F) and 0.5%/C less when warmer, we might
trickle rainwater over them on hot days to produce more power, and maybe
melt snow off the array with wintertime trickling.

The water might come from a plastic 55 gallon drum or a large pond below
the downspout. The pond might be shaded or filled with rocks, and close to
the wet bulb temp. NREL says Phila is 76.7 F in July on average, with
humidity ratio w = 0.0133, so the vapor pressure Pa = 29.921/(1+0.62198/w)
= 0.626 "Hg, with wet bulb temp Tw = 9621/(22.47-ln(100Pa+Ta-Tw)) = 528.4 R
or 68.4 F, using a Clausius-Clapeyron approximation with some iteration.
The dew point Td = 9621/(17.863-ln(Pa))-460 = 64.8 F, approximately.

If the array is 15% efficient and collects 5.25kW/0.15 = 35 kW (120K Btu/h)
of net sun, we might evaporate 120 lb/h (about 0.25 gpm) of water in full
sun, or less, considering the heat loss to the air and the water's sensible
heat gain. We might use Grainger's $271 170 W 120 V 4PC83 pump, which can
move 3 gpm (1500 lb/h) with a 22' head and trickle whenever the temp in
a small glazed box is over 100 F.

The average daytime temp in July is about 82 F. If 0.85x254 = 216 Btu/h of
net full sun falls on 1 ft^2 of dry panel on a day with no wind (V=0) and
A=.002056*Td+.7378 = 0.871 with a Qr=1.63E-09*((T+460)^4-A*(82+460)^4)
Btu/h-ft^2 radiation loss and convection loss Qc=(.74+.3*V)*(T-82), using
equations from Phil Niles, Qr + Qc = 216 Btu/h makes a panel temp T = 176 F
(80 C), so the array would only produce 5.25(1-0.005(80-20)) = 3.7 kW (or
less, accounting for dust, etc), for a pump gain of 5.25-3.7-0.17 = 1.4 kW,
ie $271/1400 = $0.19 per peak watt :-)

Nick

  I would be careful putting cool water on hot panels.

 

With more logic, we might inhibit starting the pump if the panels are hot,
after a power outage, but I doubt that's needed, since they also have to
survive lots of sun followed by sudden rain.

Nick

 

Isn't there an issue with scale build up over time on the panels with all
that evaporation?  Granted rain water can be relatively low in mineral
content, but is it low enough to avoid any scale at all?

daestrom

 

  I would just filter the water. I have a home 2 um filter that cost $30 that
gets
rid of most of the particles down to the filter size. Takes out lots of calcium,
zinc, etc.