[RE-wrenches] Radian design limitation - am I missing something?

William Miller william at millersolar.com
Mon Dec 17 11:15:20 PST 2012


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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