[FoRK] Solar power

Russell Turpin deafbox at hotmail.com
Mon Feb 9 07:38:34 PST 2004


Eugen Leitl:
>Why can't I use a small generator for overnight/
>spike use, and avoid batteries altogether?

(1) The general principle behind PVs or some
other form of passive generation is to bank the
spare amps from peak times, for use at other
times. Note that the amps don't just turn on
at dawn. The amount supplied varies depending
on the height of the sun, its angle to the
panels, and how much it peaks from behind the
clouds. On a sunny day, you get peak amps only
for a couple hours either side of solar noon.

(2) Generally, engines work inefficiently and wear
out faster if they are constantly turned on and
off. Most generator engines are designed to run
at a constant speed and relatively constant load.
Their use model is precisely that they are either
(a) matched to a proportionate load, e.g., some
piece of machinary they power, or (b) they
dump their amps into a battery bank, and then
come on again when it is drawn down.

(3) You're going to need at least a small battery
to turn on the generator when it is needed,
unless you're planning to hand-crank the
generator every time. So really, you're talking
about eliminating the "house" bank, not all
batteries in the system.

(4) The generator doesn't come on instanter.
The time it takes for some sensor to detect
that your PV amps have dropped, to signal
the ignition, to throw the solenoid, to turn
over the starter, to catch the engine, to
start generating amps is plenty for your PC
to reboot from loss of juice. Of course, you
can solve this with some kind of UPS. But now
you're opting for expensive, small capacity
batteries over cheaper, big capacity batteries.

(5) What are you going to do for light when
your generator needs some work? It will.
And if sailing experience is anything to go by,
it usually chooses the most inconvenient
time to stop working, say a cold, cloudy
morning before you've had your coffee.

Bottom line: lots of people have lots of
experience powering homes, work posts,
and boats with a generator and/or PVs, not
because they have chosen to live "off the
grid," but because the grid doesn't go
where they are. Universally, they use some
kind of deep-cycle battery bank to store
amps and buffer the need for generated
amps. What you're proposing runs against
broad engineering and practical experience.

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