Adding battery backup to existing grid-tie PV system [RE-wrenches]
William Korthof
wkorthof at earthlink.net
Thu May 18 23:58:41 PDT 2006
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We have a customer from a several years back who bought a larger system
(~7kW AC system) which used 3 SMA 2500 and 1 SMA 1800 inverters... they
always wanted battery backup, but given their high end, low maintenance
lifestyle, I wasnot eager to throw a pile of batteries, miles of heavy
48 volt wiring, and dual SW inverters at this application. But when they
called a couple months ago discussing their extended and unseemly LADWP
power outage frequency, and apparent willingness to let go of the
whirlpool, sauna, wine chiller, outdoor light show, and multi-level HVAC
refrigeration system (at least during power outages), I took note.
Rather than dig in and rewire the PV array (or alternately), install a
backup power system that didn't use the PV system... we came up with an
interesting compromise:
install (1) new Outback GVFX inverter with (4) 8D deep cycle batteries,
this inverter is directly grid-tied, like the existing SMA inverters.
On the (backed-up) load side of the inverter, we conneced the frige, a
few lights, computers, and a couple other important items. In addition,
we reconnected the (1) SMA 1800 inverter onto the load side of the
outback. When the outback is online (whether grid-tied or standalone)
the SMA inverter feeds in power to offset emergency loads (and even
charge the batteries if the power draw is lower than solar output).
This approach requires redundant inverter capacity (extra cost), but
allows the PV array to be wired at high voltage, enabling a larger array
and longer electrical feeder distances if needed.
I'm thinking that this is a reasonable compromise solution (cost, labor,
performance...) for us to use as a retrofit/upgrade on older systems, as
well as a decent way for us to furnish battery backup power with systems
that wouldn't otherwise be compatible (Sunpower).
I'm sure others have already done this... were you satisfied with the
results? Any issues of note?
/wk
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