Allan:
Welcome to the world of PV design. This is why customers need experienced
designers and installers. The out-of-work electrician with new magnetic
door signs that say "and Solar" will most likely be unable to finesse this
design. A good portion of my work is following these types around and
repairing their designs and installations.
It's all about compromise and working within the constraints you are
given: If you must have strings of three, you either have to install 15 or
18 modules. 15 does not divide evenly between two racks. If you must have
15 modules, then divide the racks into racks of 6 and 9 (DPW can likely
accommodate this).
If you can have 18, then two racks of 9. Continue the decision process
until you have a design that works and the client will approve.
Sure we'd all like to be able to purchase one system that does
everything: equipment that communicates between chassis and the outside
world, has umpteen hundreds of volts PV input, accepts a wide range of
generator power and start schemes, has multiple AC inputs, can grid tie,
can AC couple, is efficient, is reliable and is inexpensive, but we have to
work with what is available.
Good luck,
William
At 10:41 AM 12/16/2012, you wrote:
Wrenches,
Please tell me if I'm overlooking something, in what appears to be a
design weakness:
I would like to use an Outback Radian system for a standard GTBB system.
In order to take full advantage of the system's capabilities, I have to
use all of Outback's main components; in this case the FM60 or FM80 charge
controller. The problem is that the most common (and lower-cost) modules
today are 60-cell, meaning 20 Vnominal. Given the 150V DC hard maximum
voltage limit of the FM-series charge controller, in our cold climate I
can only use these 20V modules in 60 Vnominal series strings; that is, in
multiples of three modules. As the Radian is (wisely) offered in 48V only,
pairs of modules would provide too low a voltage, and series strings of
four modules would exceed 150 Voc in cold weather.
The base Midnite Classic 150 will safely operate to 198 VDC in this
application, but it won't communicate with the Mate3.
Is this a fundamental design limitation in the Radian system, suggesting
that Outback is due for a controller upgrade, or am I missing something
obvious? It appears that arrays and racks have to be sized in ~720-watt
sets of three-module series strings, which can be problematic in some
designs. In the design in question I would like to use 16 240W modules on
two 8-module pole-top racks (for seasonal adjustability); nothing in sets
of three meets the customer's output and aesthetic needs.
Any solutions would be welcome.
Thank you,
Allan
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