cost per kWh [RE-wrenches]

[Bill Brooks]

Hugh and Eric,

I think the fill factor of the array is critically important when making
assumptions about how far off max power a PV-battery system operates. I have
nearly 5 years of experience with my own battery-based, grid-tied PV system,
and I have to say that the majority of the energy is delivered at voltages
very close to max power voltage.

I recently set up a SunnyBoy next to my old Trace SW. The SunnyBoy has a
little better conversion efficiency and does not have to maintain a battery
at float voltage, but the operating voltage of the SW is nearly identical
(on a module-by-module basis) to the SunnyBoy output. I have AstroPower 120s
and they don't have the highest fill factor. I higher fill factor module
(like a BP585) would reveal a different story.

Using a differential voltage (14/17.6=0.8) is not accurate to determine
difference because the current is absolutely changing with the increase in
voltage. This is a common mistake made by manufacturers of max power
tracking controllers. The max power voltage of most 12-volt crystalline
modules at typical operating temperatures is 14 to 15 volts, not 17.6.
Stand-Alone systems are different and depend on the state of charge of the
battery.

I think the correct method is to use watts and an appropriate adjustment
factor rather than amps.

Bill.


-----Original Message-----
From: Hugh Piggott [mailto:hugh at scoraigwind.co.uk]
Sent: Friday, May 30, 2003 2:14 AM
To: RE-wrenches at topica.com
Subject: RE: cost per kWh [RE-wrenches]


Thanks Eric for the nice simple approach.

>I think it makes more sense on a battery based system without MPPT to
>use peak sun hours and array mpp amps (Imp). Amps are pretty much
>proportional to sun light intensity and the modules are going to produce
>Imp or greater at battery charging voltage and elevated temperatures.
>For simplicity, I keep all my calculations on battery based systems in
>terms of amps and amp hours. Working in watts means using two different
>"typical" battery voltages -- one for charging and one for discharging
>-- which is necessarily vague.

Looking back at my previous reports, I see that I have used peak amps
and then assumed a certain charging voltage.  That's because I was
interested in the energy input to the battery. This approach gives a
factor of  about 14/17.6=0.8 between rated output and useful output.

Now I have been pointed at Bill Brooks' pv installation guide
http://www.solarexpert.com/grid-tie/CEC-PV-Guide-2001-09-04_500-01-020.pdf
and I find that he has many more factors based on manufacturing
tolerance, temperature, dust, and wiring loss.  I hope I don't have
to worry much about wiring loss, because the module is capable of
much higher voltage than the battery is assumed to be at (without
loss of current) so I hope I have already paid that particular toll.
( I don't understand why people size for 3% wiring loss in battery
charging systems.)  But the other loss factors make 0.95 x 0.89 x
0.93 = 0.8

Multiply my 0.8 by Bill's o.8 and we have 0.64!  And that's before we
even look at losses in the battery/inverter system, or at energy lost
because it is rejected or dumped by the charge controller.

Does anyone have data to confirm this estimated factor?

This is all very interesting.  Thanks for the inputs :-)
--
Hugh

hugh at scoraigwind.co.uk
http://www.scoraigwind.co.uk/

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