Date: Sun, 17 Jun 2007 02:18:56 -0500
Reply-To: Matt Roberds <mattroberds@COX.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Matt Roberds <mattroberds@COX.NET>
Subject: Re: Solar panel for top of Westy
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed
> From: Frank Condelli <RAlanen@AOL.COM>
> Date: Sat, 16 Jun 2007 07:01:25 EDT
>
> I'm looking for a solar panel for use on the top of my Westy.
David Etter has some good information. I'm not an expert either, but
to add a little to what he said:
> From: David Etter <detter@MAIL.AURACOM.COM>
> Date: Sat, 16 Jun 2007 12:07:21 -0300
>
> Now the included controller, I had to ditch because it used a
> mechanical relay to turn the charging circuit on and off.
You definitely need a charge controller of some description, unless you
wire the solar panel directly to the 12 V resistor in the fridge and
nothing else. The raw output of a "12 V" solar panel will actually vary
from maybe 9 V to over 20 V, depending on how sunny it is and how much
the load is. Anything designed to be plugged into a cigarette lighter
outlet should be able to cope with about 11 V to 15 V, but wider swings
than that are a problem. Also, for a given amount of sunlight and a
given load, there is a most efficient spot to operate the panel at, and
the controller will help you find it. What I mean is that if you want
12 V at 1 A (12 W) coming out, you could draw 12 V at 1 A from the
panel, or 16 V at 0.75 A, or 24 V at 0.5 A, or several points in
between. The solar panel might be 20% efficient at 12 V, 25% efficient
at 16 V, and 22% efficient at 24 V - the controller "knows" this and
will try to keep the panel voltage/current around the most efficient
spot.
> And then add a digital ammeter and a voltmeter, but arrange your circuit
> so that you can monitor current in and out of the spare (from the panel-
> not the alternator) battery; you will be surprised by the effect of a
> single cloud, haze or a tree branch.
If you already own a digital multimeter, it can be instructive to hook
it up to the battery when you're setting up the solar panel - you can
easily see the variations David is talking about. Small LCD panel
meters are cheap these days ($10), work well, and are easy to set up as
a voltmeter (for example, some of the meters at
http://www.allelectronics.com/cgi-bin/category/385/Meters_(Panel).html
). Get one that can measure voltage to two decimal places (12.34 V).
These LCD meters can be used as ammeters as well, but the connection is
a little harder to set up - an ammeter is really a voltmeter that is
measuring the voltage drop across a small fixed resistance, called a
"shunt". You can buy a ready-made shunt, or use one of the lengths of
cable in your system as a shunt - but then you have to calibrate it
against another meter.
Aside: Late air-cooled VWs that had the central test socket had a wire
from both ends of the battery ground strap - battery terminal and body -
to the test socket. I always figured this was redundant, until I found
out that the analyzer at the VW dealer was using the strap as a shunt
to measure the battery current. This gave a check on the alternator/
generator performance, but they also used it to do a compression test.
What? The analyzer could run the starter and watch the current pulses
as the starter pushed each cylinder to compare the cylinders against
each other. There was a TDC sensor connected to the check plug, too,
so it knew which one was #1 and could tell you which cylinder was low or
high. Galatic Institute Prize for Extreme Cleverness stuff.
> Use 10 gauge wire and connectors from the panel into the main circuit
> which itself should be 8 gauge. Fuse (10 amp) the panel inside the
> connector box under the panel itself.
Agreed. Use as thick of a wire as you can stand to work with, and don't
be stingy with the fuses. Dumping 120 W from the panel into an unfused
piece of wire can make it hot enough to set something on fire, and in
certain failure modes of the charge controller, it's possible to turn
loose the entire short-circuit energy of the battery, which is
approximately 1.22 gigawatts, Great Scott!
As has been mentioned, if you can work out a simple way to rotate and
tilt the cell, it can help you get more power. You can still strap it
flat to the roof of the van for transport, or in case you happen to park
with the pop-top facing the right direction. I have seen (home made)
vehicle-top solar cell mounts that were essentially hinges between the
cell and the roof on all four sides of the cell, plus an adjustable prop
rod. When moving, the pins are in all four hinges, the prop rod is
stowed, and the cell is flat on the roof. When parked, you take the
pins out of three of the hinges, and tilt the cell on the fourth hinge,
using the prop rod to set the angle. You can do a lot of trig and work
out the optimum angle and adjust it every two or three minutes as the
sun moves, or you can cheat and give it a latitude tilt - make the angle
between the panel and the ground equal to your latitude. This is pretty
close to optimal and allows you to spend time on more pressing matters,
such as studying the effect of salt-water or fresh-water corrosion on
thin-wall aluminum cylinders in the presence of ethanol. (AKA sitting
on the beach and drinking beer.)
Having given the practical comments above, here are some theoretical
ones. Photovoltaic solar does have the advantages of quiet operation
and not needing to be near shore power. However, it is (at this point)
still very expensive power. With David's example of a 120 W panel and
controller for $525, you'd need to run it for about 43,800 hours (almost
five years) at full output to get the price down to ten cents a kilowatt
hour. If you go camping every day and get full output for six hours a
day, it takes twenty years to get the costs down to that point. Also,
the solar cell factories do not yet run on solar cells. Again, for
portable power where you have no grid and lots of sun, it's one of the
least bad options around. The solar-cell people, just like the internal-
combustion engine people, are chipping away at it - we're not going to
wake up tomorrow and have 70% efficient PV solar cells or 70% efficient
engines - but they are making a percent or two a year. My point here is
that PV solar might not be as "green" as it first sounds - but if you
understand the limitations, it can still be a useful choice.
Don't forget, every internal-combustion engine on the planet is solar
powered. Most of them use solar power from several million years ago
instead of a few minutes ago, though. :)
Matt Roberds
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