Spec-ing out the voltage for photovoltaic system

 I am trying to determine what voltage range is resonable for a 4kW
photovoltaic array.  I am working on the very low level of the system
design, with modules, not panels.  I am thinking a system with around
48-70V with a current of 30-60A may be a good bet.

Does anyone have any concerns or feedback on this voltage range?

Does anyone know a good resource for spec-ing out the voltage range
and current range?

Jonathan

  On Mon, 21 Jan 2008 12:53:34 -0800 (PST), macdadyj@gmail.com wrote:


Here are some ideas to consider, although I'm no expert.

In general, the higher the better, because you can use smaller (cheaper)
wires to connect.

But you haven't mentioned the type of system.  For your 4kW system, I would
say it depends on whether this is to be an off-grid (with batteries) or
grid-tied system (without batteries).

For a grid-tied system (about which I know little), your limit probably has
to do with the voltage limit on the wiring insulation, as well as the
limits on the inverter.  This is probably regulated, at least in part, by
the NEC.  You don't want to go above 600V at the coldest temperature these
panels might be exposed to (that's also the limit on some of the dedicated
grid-tied inverters, with ZERO margin for error).  So maybe 300V nominal --
more if you're in the tropics, less if you're in cold weather climes.

For an off-grid system, your voltage limit will probably be determined by
your PV controller.  An MX60 has a maximum voltage of 140-150V.  I think it
shuts down over 140, and blows up <g> over 150.  In my system in downeast
Maine, I use a 72V nominal array which, over the past three or four years,
has had a maximum voltage output to the controller of about 141V.

If you are going to use a 48V battery bank, and an MPPT controller, I would
recommend at least a 60V nominal array.  But again, this is weather
(temperature) dependent.


--ron

 
I think you read my mind.  You pretty much confirmed everthing I have
been thinking.  I am designing a solar panel wiring configuration, so
my goal is to have a flexible set up without oversizing components.
Your response was insightful and very helpful.

I am interested in hearing more about what you know about higher
voltage systems.  I am shooting for a nominal voltage around 60V in my
preliminary design configurations, but from what I am hearing that low
of voltage really only has a benefit for off-grid systems.  I haven't
looked much at typical specs for grid-tied systems. But now that I am
writing this perhaps I should design the wiring harness for 600V
maximum photovoltaic system voltage, in order to make it the as
versitile as possible.  I would appreciate any addition feedback you
might have.

Jonathan

 On Jan 23, 6:04 am, macda...@gmail.com wrote:

Oh dear. Your flexible setup will require that the wiring be correctly
sized for the maximum AMPS expected within the range of flexibility
you wish to cover.

You can run 6,000,000 volts down a wire the size of a hair. If you add
an amp to it you will only have a fuse.

As always, design for the worst case.

1000 Watts at 600 Volts  = 1.67A

1000 Watts at 48 Volts  = 20.83

 On Jan 22, 3:06 pm, beal...@gmail.com wrote:

Thanks...that is the conclusion I have reached as well.  This
discussion has facilitated the following of my brain juices.

I appreciate the feedback!