On 12/26/2012 12:20 AM,
[email protected] wrote:
There have been ample discussions about the folly
of solar power when you have any alternatives.
I'll not repeat that here. I like to help
people with their hobbies.
I've helped engineer more than one solar system
for mountain-top use in the boonies.
Your statements are worrisome on many levels...
Depending on what batteries you're using
and the tradeoffs you've made for life vs
maximum stored energy,
your battery voltage ranges from 10V to
nearly 15V or so.
To deal with the 20.0A maximum
Sounds logical on the surface, but there's
a troll under that bridge.
Most diodes have a negative temperature coefficient.
If there's any imbalance, one diode takes more
current and warms up which makes it hog more current
which makes it warm up. Pretty soon, you have one diode
in the circuit and the other two just sitting there.
From then on, it's a matter of time until the silicon melts
and shorts or the plastic melts and the device comes apart open.
According to the spec, that diode at 10 amps is gonna
be 150 C above ambient. That's too hot, even if you
could make three share equally. Get something with a bolt hole
and put it on a heat sink.
You're better off with one 40A diode.
And a 20V diode will likely be less lossy and cheaper
than a 100V one. And I don't mean a lower voltage one
selected out of the same bucket. I mean a diode designed
for minimum forward voltage often has a lower reverse voltage.
It doesn't have to be a fast diode.
But there's a more basic question. Many solar panels have built-in
diodes. Are you sure yours don't?
That diode has a forward voltage spec of 1V.
You've got 15V at the battery and you're wasting another volt
or 20 watts.
And it's more complex than that.
Look at the family of curves for the panel.
They're not a straight line. And the 20A is at noon in the desert.
At 3PM when the panel isn't pointed directly at the sun and there's
more atmosphere in the way and there's some haze and some bird poop
on the panel, you're gonna be on another curve of that graph.
That diode might be the difference between your charge current going to
zero at 3PM instead of 3:30 PM. Those lost amp-hours add up.
Also, if I have a
Shunt controller sounds simple, but again, the devil is in the
details.
We built one system that used 4 150W resistors and 4 audio power
transistors. A comparator switched the transistors on at 13.7V and off
at 13.6V.
That worked because
The panel was 20A and the batteries were 800Amp-Hour. They didn't have
any trouble with that current and voltage. And the thing rarely
turned on except on the longest days of the year.
If you had a single car battery and a 20A panel, that wouldn't be a good
idea.
A later version used an oscillator and a crude 300Hz PWM to switch the
transistors.
For a less robust battery system, you'd want a fast switcher and much
better charge management. And today, you'd be able to afford fast
switching transistors at that current.
For starters, take any 300W plus buck converter design that can run on
10-15V. Put 600W of resistors with values that can draw at least
300W from the output of the converter.
Control the converter output voltage to give you the shunt load current
you require. Since you're likely to use multiple resistors in parallel.
It might be easier to use more buck converters of lower current each.
Depends on what you can find in the cheapo bin of the electronic surplus
store.
We built a MPPT controller and a sun tracker. Worked neat and gained
some additional usable power. Problem was that it was deemed unworkable.
It's hard to track the sun when the panel is covered with ice
and the roads to the site are closed eight months out of the year.
For remote stuff that just has to work, it's often better to add another
panel than to try to eek out a few more percent on the one you have.
Reliability trumps efficiency every time.
The hardest part of any project is writing the spec.
Decide exactly what you expect to happen under any and all
conditions of insolation and load current and battery charge level.
Map that all out and decide what to implement. Then figger out HOW
to implement...repeat the cycle until it looks like what you want.
Then start ordering parts.
If you're on the grid and expect to generate power, you're likely to
give up a that point and go play a round of golf.
If you're indulging an expensive hobby, you can have great fun
with solar power. It's a lot cheaper than golf...and less risky
than a mistress.
It's your standard ~0.5v drop power diode.
