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re: is a standalone power system more reliable than "the grid"?
28 aug 2000
richard j komp writes:
just a few comments on the question above about standalone power system
reliability.
first, in general the answer is: yes, standalone pv systems are more
reliable than the utility grid, if designed properly. this is why the
coast guard here in maine has gone exclusively to pv power for their
lighthouses. although a number of them still have grid connections, they
all have pv systems as the primary source of power for the light. in fact,
the pv systems are so reliable and need so little maintenance that the
coast guard has been selling off the old lighthouse keepers cottages
in the remote spots.
second, my own experience is probably typical in that i have lived with a
standalone system for almost 13 years now and admittedly don't do as much
maintenance as i should. it has been years since the clock on the car
radio has blinked. (this car radio serves as my main stereo system, with
the addition of a preamp/switiching box i added to get all the necessary
tape deck, phonograph, cd player, etc to work.) this clock will blink
even if the 12 volt power goes off for even an instant, and each time it
has blinked, it has been something stupid i've done. the last time i
lost all the dc power was when i was putting up drywall and drove a nail
right through the main power buss from the battery to the main dc fuse
box. i simply had to replace the fuse right on the battery terminal
(after pulling the nail out, of course). another time, i had temporarily
connected a romex wire directly to the 24 volt battery terminal when i
was shifting some circuits around and "forgot" to go back and connect the
proper kind of lead plated terminal to that spot. years later, i came
back from here in nicaragua and found that some of my lights didn't work
because the wire had corroded clean through and broken off. one can
repair these low voltage dc systems without bothering to turn off the
circuits. this saves having to reset the dc clocks.
third, a failure in a standalone dc system is usually a soft failure,
losing some capacity or part of a circuit, rather than a quick disconnect
of all voltage. for instance, in the math example you give in your
e-mail, failure of only one pv module would mean a (probably unnoticed)
drop in the charging rate. virtually all the failures i've seen in pv
modules have been in corroded terminals, and were easily fixed; and quite
often, the problem had existed for months or years and the homeowner was
completely unaware of the loss of power. the reason manufacturers can
get away with the 25 year warranties is that if a pv module doesn't fail
in the first few months, it will very probably work indefinitely
(remember, nothing ever wears out or gets used up in a solar cell).
this leads us to the most common type of catastrophic failure: the
inverter quitting. you did a very good job of explaining the math of
that in your example. this is why i usually still advocate having at
least some important circuits, like the built in lights in the house, a
radio and maybe the tv, all working directly from the dc circuit. some
people have two inverters always wired up to two different circuits in
their systems, while lots of us have those small, cheap inverters around
for a backup.
i hope this collection of antidotal evidence will help clear up some of
the discussion.
richard j. komp, president
sunwatt corporation
rr 2 box 7751
jonesport me 04649
207-497-2204, e-mail: sunwatt@juno.com
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