Hi Kelly;
I agree the Radian system is appealing. Its basically uses a circuit
breaker that can be controlled like a relay as well, and just
disconnects the GT inverter.
I'm going to offer a brief table of what I've found for my choices:
Brand
Control Method
Pros
Cons
SMA Sunny Island
Frequency shift to disconnect GT inverter
simplest wiring, Trusted GT industry player
most expensive, Freq shift can cause other problems
Outback Radian
Remote controlled Circuit breaker disconnects GT inverter
fairly simple wiring, relay and breaker are combined, better control
of battery charging
still expensive, not available in smaller than 8 kw model
Magnum
Diversion load controls for battery regulation (soon to offer AC load
control)
lower cost, proven technology, freq shift control offered as fail safe
most complicated wiring, diversion loads not always reliable or
available
Modsine Backup system
no AC coupling, just an old fashioned backup system
low cost
complicated, needs customer interaction, not as efficient, GT PV not
used, dedicated PV modules only charge backup system
Generator
No AC coupling, generator runs backup loads
lowest cost, backup power available for as long as outage (no
batteries)
best for week long outages that only occur every few years
Not Renewable, fuel storage issues, doesn't switch fast enough to
maintain computers, etc.
I'm still pricing out the options, but the SI and Radian seem a bit
overkill for smaller systems. Backup inverter power of just a couple
thousand watts is all that most people need. The Magnum is appealing,
but I have quite a bit of experience with load diversion controls from
wind and Hydro systems, and I really feel that its overly complicated
for the less than 1% of the time that the grid is out. Keeping the
battery from being damaged is what's important, but maintaining full 3
stage charging for a couple of days a year is not. The batteries can be
properly charged once the grid returns. May be able to use Outback's
ROCB with the Magnum? That might get the best of both worlds.
If I was building the entire system to start, I'd just use a single
Outback GVFX with its own PV array, and add GT inverters and array that
were independent for the rest of the GT requirements. This project
however is retrofitting an existing GT system.
I've done many backup systems in the days before Grid Tie, and they work
fairly well. Basically either the modules are not used much of the
time, or the customer has a few circuits that are on the backup system
(off grid) all the time. They operate it just like an off grid system,
the only difference is that when its time to run a generator, they can
just switch to the grid. These are not for your average customer, but
can work well for someone that likes to save money and tinker.
Another consideration is that if the battery bank is large enough
relative to the array, over charging becomes less of an issue;
especially with flooded cells that can stand a bit of over charging
("equalization") every now and then. However, I can see that the
average small sealed battery bank hooked up to several KWs of PV could
get ruined in a day.
I'll close with a quote from Joe Swartz in his 2012 Solar pro magazine
article :
"AC coupling is still the Wild West of renewable energy applications"
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 7/30/2013 11:37 PM, Kelly Keilwitz, Whidbey Sun & Wind wrote:
Ray,
The battery-based inverter in an AC-coupled system should not be
grid-interactive. The grid passes through the BB inverter to the
grid-direct inverter(s). The BB inverter is there to create a stable
grid-like signal for the grid-direct inverters when the grid is not
present, and to manage the batteries. Thus, the pass-through AC
capability of the BB inverters normally (there is a complicated
work-around) must be able to handle all of the PV AC output, when
loads are low.
You probably are aware, but to make it clear in this thread: the
system needs to have some method to protect the batteries from
overcharging, as a result of the grid-direct inverter output when the
grid is down and loads can't use all the solar energy available. With
the Sunny Islands it's done by a signal that reduces output from the
Sunny Boys without disconnecting them. With Outback and others it's
done with a installer-designed relay that disconnects the grid-direct
inverter when the battery voltage gets to a set point. We haven't done
an AC-coupled system with the Radian, yet. Sounds like the remote
controlled breaker is Outback's approved and listed way to do it.
About time.
-Kelly
Kelly Keilwitz, P.E.
Principal
Whidbey Sun & Wind
Renewable Energy Systems
NABCEP PV Installation Professional
WA Electrical Administrator
ke...@whidbeysunwind.com <mailto:ke...@whidbeysunwind.com>
PH & FAX: 360.678.7131
On Jul 30, 2013, at 4:23 PM, Ray Walters <r...@solarray.com
<mailto:r...@solarray.com>> wrote:
The Outback rep got me some good info, and I'm liking the Radian with
the Remote controlled breaker to control the GT inverter when the
grid is out. I've heard of quite a few problems related to the Sunny
Island frequency controlled system.
My question is can off grid inverters like the Magnum be AC coupled?
I'm trying to get the price down, and still handle the 240 vac input
from the GT inverter.
I penciled out the Radian and I was topping $10k before installation.
This customer is hoping for a solution under $10k, and closer to
$5k if possible. I think an elegant lower cost solution for Grid tie
with battery back up is in order. A Radian Lite?
For backup, it could even be mod sine, as it wouldn't be much worse
than generator power or the average UPS setup.
R.Ray Walters
CTO, Solarray, Inc
Nabcep Certified PV Installer,
Licensed Master Electrician
Solar Design Engineer
303 505-8760
On 7/30/2013 2:59 PM, Ray Walters wrote:
Hi All;
I know AC coupled systems have been covered before, but I have a new
issue: positive grounding. A customer with an existing 8kw
Sunpower system wants to add backup. He was put off by the high
cost the Sunny Island, so I was steering him towards a single
Outback VFX coupled to just one of his 3 inverters. The issue is
how will the VFX work with positive grounded inverters. I'm
assuming that since the two inverters will only be connected by AC,
that the positive vs negative grounding of their separate DC systems
will not matter. Any experience with this particular scenario:
Sunpower system AC coupled to an Outback?
Also, would I even need a GVFX, since the inverter would only be
used during outages (which might allow a generator to be used later
as well) A VFX should still AC couple to the GT inverter during an
outage, right?
Thanks as always,
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