Re: (ET) Solar charging, part 2

["Travis" <ozsolar ipa net>]

Hello All,

The reason I'm using 4 panels is because that is what I had sitting around.
Yep that's right, I had 4 extra 85 panels that I wasn't using for anything
this summer.  I had recently built a new shop to house my business and they
were from an array on the old shop.  I installed almost a kW worth of
modules on my new shop which didn't leave a lot of room since I also have a
160ft2 of thermal collectors tied into the radiant in floor heating system.
At a retail of $5/watt plus the $649 charge controller that comes out
to...well... a lot of money just to charge my tractor.  Because, I wanted 
to
utilize all 4 modules needed to get the OutBack MX60 which allows you to 
run
the solar array at a higher nominal voltages.  I also wanted to test the
product out before I installed it for anyone.  We try to use all of the
products we service and install.

The typical "75 watt" solar module has an open circuit of ~21 volts and a
max power at ~17 volts and 4.4 amps.  17 volts x 4.4 amps = 74.8 watts at
"Standard Test Conditions" (STC).  STC = 1000 watts/ meter2,  77 degrees f.
Once you hook three modules up to your 36 volt battery they  no longer are
75 watt modules.  After mowing lets say the back is down to a rest voltage
of 36.6.  If you hook the solar array up to that pack it going to run at
that same voltage (36.6) gradually climbing as the pack charges.   36.6
volts x 4.4 amps = 158 watts.  Since it's summer, the solar panels will be
at temp of something higher than STC.  Turns out the higher the temp the
lower the voltage a solar panel operates.  That's why they start out at 17
volts else you could never equalize your battery pack in the summer because
a solar panel on a hot roof might only be capable of 15.5 volts or less.
Let's assume their max power voltage is now down to 16.  A max power point
tracking controller will operate the modules at the 16 v.  16 volts x 3
modules in series x 4.4 amps = 211 watts.  So you can see we've gained 53
watts.  Some of that is lost in the conversion to the correct battery
charging voltage. But at a 50 watt gain x $5/watt retail that's $200.  But
then again it's not really about the money to me.  I just think it pretty
darn neat to use solar.

Travis Creswell
Joplin, MO

----- Original Message -----
From: "Christopher Zach" <czach computer org>
To: "Travis Creswell" <ozsolar ipa net>; "ELECTRAK"
<elec-trak cosmos phy tufts edu>
Sent: Tuesday, April 22, 2003 11:07 AM
Subject: Re: (ET) Solar charging, part 2


> Hm. 3 panels should do fine; remember that a "12 volt" panel can put out
> over 21 volts under full sun and open circuit. Three of them plus a
> controller is all you really need.
>
> That being said, I would recommend www.thelastplaceyoulooked.com for a 36
> volt solar charger.
>
> Chris
>