Down-sizing battery systems [RE-wrenches]

[Walt Ratterman]

Jeff:

I don't do on-grid systems, but primarily off grid systems in developing
world applications.

I try to make all systems hybrid - with a generator backup.  On the
smaller systems, where the generator might be portable (and might not be
there when needed) we generally design for 2 days autonomy.

On larger off-grid systems - 3 kw and up, we go for 1 to 1.5 days.  The
battery banks get pretty big.

Besides - one thing that I think was missed when the fad was to go for 4
and 5 days of autonomy, was that without generators, the PV systems were
still sized for one days' worth of watthours.  This means by definition
that the batteries simply could never get fully charged after the first
couple of cloudy days, and they had shorter lives as a result.  When we
do have longer autonomies (like for vaccine refrigerators) we size the
solar array to charge the batteries fully in one day - which means
generally oversized solar arrays most of the time (but happier doctors
and patients....)

Thanks,

Walt Ratterman



-----Original Message-----
From: Jeff Yago [mailto:jryago at netscape.com] 
Sent: Saturday, January 26, 2008 3:32 PM
To: RE-wrenches at topica.com
Subject: Re: Down-sizing battery systems [RE-wrenches]


Over the years I have noticed we are designing smaller and smaller
battery banks for both grid-connected and total off-grid applications. 

I think this may have come from the pre-90's solar design goals that
off-grid applications should have 4 to 5 day of autonomy.  This was easy
to do when the only electrical loads were a refrigerator, a few lights,
and a small television.  However, this is harder and harder to achieve
these days as more and more homeowners consider multiple computers, a
satellite receiver, high-speed Internet, recessed lighting, a 60"
television with theater-sound, and a dish-washer as must-have
necessities.

To reduce battery space requirements and costs, we are starting to
install off-grid systems with fewer days of battery-only capacity, and
relying on a back-up generator to re-charge when solar is not available.
I also think since it is now easier to sell back to the grid, perhaps
there is less need for huge battery banks in battery back-up type solar
systems to absorb all that excess power when installing a large solar
array. 

Anybody out there doing battery based systems that are starting to
notice these market trends?

Jeff Yago

_____________________________________________________________
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