[RE-wrenches] offgrid system question

[Travis Creswell]

Whoa… I think the AC not starting is only part of the problem and I’m not
talking about technical issues.

 

3.5 tons at 13 seer is going to have a running kW of 4kW, give or take and
on hot days that’s going to take the battery to 50% in less then 5 hrs
assuming no other loads are on.  We’ve recently looked into this for a
customer that spontaneously added 3 ton 13 seer unit to his part time off
grid system without telling us until it was already installed.   Of course
it was never part of the original design but that’s an entirely different
story. Thank heavens we have a good relationship with this customer further
bolstered by contract clearly detailing what the system was designed to do.

 

According to everything we can find and the HVAC installer you need a
minimum of 5 to 7.5 times the running current to start an AC unit.  A “hard
start” capacitor will not help because all it does is give the AC unit more
amps to start but do not actually reduce the starting surge.  Apparently as
AC’s age they can actually get harder to start so these units help with
that.  I’ve not come across the “soft start” you mention unless your talking
about 3 phase stuff.

 

The Xantrex XW4024 at this project will not start his 3 ton unit.  But of
course, it’s not that simple.  This is a tiny off grid system with one
string of L16’s.  When the AC attempts to start we observe the battery
voltage dropping on the fully charged battery bank to 21v while the kW
reading on the XW never goes past the mid 2’s.  While this is happening the
XW’s voltage drops L-L voltage drops to 156v.  The inverter does not fault
out and only shows a Low battery voltage error message.  So my theory is the
XW can’t hold the 240 L-L during the surge which inherently reduces the
surge thus self limiting the AC unit and preventing the inverter from going
into a fault mode.  But that’s just my guess. Xantrex technical did pick up
the phone but they were pretty puzzled why the inverter didn’t go into fault
mode.

 

Our solution is to install a 17kW generator and run the AC off the generator
only.  The generator and a propane tank are a 100% solution and they were
cheaper then adding inverters and batteries which we were pretty sure had a
pretty good chance of not being a 100% solution.  Since it’s a rarely used
off grid cabin the propane usage wont’ be terrible.

 

IMHO, Max, I suggest that you don’t even try to run the AC off of the
inverter or even think about adding a 2nd one because it’s always going to
be problematic.

 

Best,

Travis Creswell

Ozark Energy Services

   _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Max
Balchowsky
Sent: Monday, August 18, 2008 3:01 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] offgrid system question

 

 

Dear wrenches, going to look at a system installed by others and would like
some input from some of you with more off grid experience than I have ( 4
systems over the years). the system was put on a new home - off grid 20
sharp ND123UJF panels 4 modules in series - 5 parrallel strings, 2460 total
array dc voltage. mppt 60-150 charge controller, 12 sun extender sealed AGM
batteries, 305 AH at 12 vdc, wired for 915 AH at 48 volt DC. The system is
tripping off when the AC unit trys to start - AC is comfortmaker 3.5 ton 13
seer. They say start up inrush is 104 amps (sounds high to me) and they put
a DST 10 soft start on the unit (I think that's just a big capacitor). I
haven't been on site yet but it sounds to me that they just didn't design
enough inverter capacity into the system...... What are your thoughts?

 

Max Balchowsky

SEE Systems

 

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