hybrid grid tie - SMA / OutBack [RE-wrenches]

Tue Feb 10 10:14:23 PST 2004

Wrenches:

Well, I just couldn't sit on the sideline and let squishy information
go by without a comment.

AC coupling is something SMA Germany has been doing for almost 20 years
now.
This technology is well proven on the Greek Island of Kythnos in both
large (MW scale) and small (kW) scale applications.
(Contact me offline for a marketing paper on Kythnos).
Our experience in hybrids such as Kythnos makes us believe ac coupling
is absolutely the best way to go for larger hybrid systems.
Dc coupling simply does not offer the flexibility and expand ability
needed in larger systems.
Please note that I still think dc coupling is probably the best way to
go due to lower initial costs.

David Katz has indeed been playing with an SMA / Outback hybrid with
some success.
I believe this occurred after talking with Sam Vanderhoof about his
Sunny Boy / Trace PS system.
I also suggested this to Outback over a year ago at the Energy
Outfitters Training seminar in Sacramento.

There a couple potential problems with using the AUX relay (Trace or
Outback) to switch the Sunny Boy off and on.
First is the UL required 5 minute time out before we reconnect to the
grid.
With a smaller battery the inverter can spend a lot of time in the off
state.
For grid tied backup power systems this is probably not critical since
all the power normally flows to the grid.
The "5 minute cycling" will only occur when the grid is down and the
system is operating in stand alone mode.
In a small off grid system I do not suggest this approach and agree
with Christopher to stay with a DC coupled system - for the moment.
For larger systems the Aux relay is still a bit of a brute force method
that is less than elegant.
A better approach is to modulate the power level of the Sunny Boys
instead of simply turning them on or off.
This is what we do in our Sunny Island / Sunny Boy systems.

The efficiency argument is also a bit misleading as well.
We could debate issues of time of use of load versus time of
generation, amount of generator run time, etc.
I do not think anyone has studied these differences especially when we
compare dc and ac coupled approaches.
Fundamentally, I believe the ac coupled approach will yield more energy
in a system that is made up primarily / exclusively of ac loads.
In both cases the energy that will flow in to or out of the battery is
only the difference between the ac loads and the ac generation.
In an ac coupled system the PV energy is converted directly to ac
avoiding the dual conversion of an MPPT charge controller and separate
inverter.
In a dc coupled system all of the energy must go through a dual
conversion, e.g. MPPT and Inverter.

When you run the economics of PV systems, a few percent difference in
energy yield equates to significant dollars in total system costs.
I believe the ultimate metric on which we should judge performance of
PV is $ / AC kWh for the entire system not just for a single components
like an inverter or charge controller. 

Best Regards,

John Berdner

>>> cfreitas at outbackpower.com 2/10/2004 06:28:57 AM >>>
Jeff at GlobalResourceOptions.com wrote:
> If you're talking about back-feeding the AC out on the inverter, this

> sounds like a smoke producing situation to me. Cannot put AC back in

> where it comes out on any inverter I know of.

Actually this has been done already and it does work - this is the same

idea as what SMA calls the "sunny island" system. The inverter's AC 
output becomes the utility grid to the SMA inverter - keeping it 
operating during the power outage. The AC transfer switch in the
OutBack 
inverter keeps the house isolated so we don't cook any lineman. 

If the AC loads are large - then the OutBack inverter adds power from 
the battery automatically to the power from the Sunny Boy inverter. If

the loads are small, then the OutBack inverter will "absorb" the power

produced by the Sunny Boy inverter - converting it into DC and charging

the battery. The issue is what happens when the battery gets full? Use

the AUX output of the OutBack inverter to disconnect the Sunny Boy 
momentarily or use one of those antique PWM charge controllers laying 
around to divert the excess power to a dump load (or do both options
for 
redundancy). 

Credit goes to Dave Katz at AEE for being the first brave soul to try 
this on his own house. He reported that he was even able to manual drop

the grid by throwing the AC disconnect for the house without the SMA 
tripping off - the transfer time of the OutBack inverter/charger was 
fast enough to not trigger the SMA islanding protection... 

So - who's ready to use this in a real off-grid system? 

> I would recommend using (3) 24V modules in series if this is a warm 
> weather application (cannot do this in cold regions as you violate
the 
> 145VOC limit on the MX60). With this voltage level, you can get down
to 
> a "reasonable" wire size. This is much easier than inventing a
custom. 
> And MX60's cost less than SunnyBoys as a charge controller. If you
were 
> 400' away, I might feel differently, but I'd go for the standard
system.

We have had good feedback on using the MX60 with three 24 vdc modules
in 
series even in some pretty cold locations. Some reports of the 
controllers staying off in the morning until the panels warmed up a 
little and the VOC dropped below 140 vdc. 

As fun as the micro-grid SMA/OutBack hybrid system sounds - I agree
that 
the MX60 and some fairly big wire might be the best option. The 3200 
watts DC (nameplate) on the SMA might also cause a bottle neck - and 
there is two complete conversions occurring when battery charging (but

only one when the AC loads are being powered directly from the SMA). 
The MX60 has about 1/2 the losses of the Sunny Boy (97% versus 94%) and

can handle the 3200 watts DC from the PV array (barely). 

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas @ outbackpower.com 
www.outbackpower.com 
Arlington WA USA
Tel 360 435 6030 

- - - -

--
[Non-text portions of this message have been removed]

- - - -
To send a message: RE-wrenches at topica.com

Archive of previous messages: http://www.topica.com/lists/RE-wrenches/

List rules & etiquette: http://www.mrsharkey.com/wrenches/etiquete.htm

Check out participant bios: www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine

Moderator: michael.welch at homepower.com
--^----------------------------------------------------------------
This email was sent to: michael.welch at homepower.com

EASY UNSUBSCRIBE click here: http://topica.com/u/?bz8Qcs.bz9JC9.bWljaGFl
Or send an email to: RE-wrenches-unsubscribe at topica.com

TOPICA - Start your own email discussion group. FREE!
http://www.topica.com/partner/tag02/create/index2.html
--^----------------------------------------------------------------