net metered system question [RE-wrenches]

[Bill Brooks]

The poor little SW inverter was not designed with this operation
specifically in mind. However, most backup systems (for computers) float
charge the batteries at all times. The trick is doing it with a minimum of
losses. This requires a charging circuit that is highly efficient at nearly
zero load. The SW is not that animal. I think 500 Wh/day is more realistic,
but it is still significant (Trace might tell you it is lower). This loss
only happens at night for PV systems since it is inverter PV during the day
and the PV is essentially providing the float potential. Cycling the battery
is not what you want to be doing since most of the systems have about
between a 500-1000 Watt constant load connected to them. The utility is a
whole lot more efficient. The "silent mode" is the way to go, but we will
see who brings this to market first (this is the way many small computer
UPSs operate). Advanced Energy said they might, but we must wait and see.

Bill.

-----Original Message-----
From: Todd Cory, Bald Mtn. Solar [mailto:toddcory at jps.net]
Sent: Wednesday, March 07, 2001 7:18 AM
To: RE-wrenches at topica.com
Subject: Re: net metered system question [RE-wrenches]


William Miller wrote:

> regarding the problem of inefficiencies of the
> battery connected SW inverters when used for grid interite.  As I
> understand it (and please correct me if I am wrong), the biggest cause of
> inefficiencies is that AC loads connected to the SW output draw the
> batteries down overnight and the SW charges them from the grid.

Actually most grid intertied systems (without TOU meters) do NOT cycle the
batteries. The wasteful (700 watt hours/day) one hell of a huge phantom load
is
from the SW running all the time... continuously floating the batteries AND
it's constant idle current.

The idea is to encourage trace to give the SW enough "brains" to see when
there
is nothing to sell back, and shut itself off (silent mode) until needed
again
when it would wisely go back into sell mode again... that is why it has been
called "silent sell". Both modes provide for grid outage backups, which is
why
people have batteries.

> Why not treat the output of the SW as one might treat a generator AC feed,
> that is, run it through a transfer switch.  Normally, the AC output of the
> SW inverter(s) goes nowhere

Normally the output of a grid intertied SW goes to house loads... having a
transfer switch on the output would negate the reason for having a battery
based system which provides for grid outage backups. The SW's internal
"transfer switch" shifts SW output loads to the grid when it is active and
backs up these same loads up from the batteries/inverter during grid
outages.

Hope this helps...

Todd


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 michael.welch at homepower.com

____________________________________________________________
T O P I C A  -- Learn More. Surf Less. 
Newsletters, Tips and Discussions on Topics You Choose.
http://www.topica.com/partner/tag01