[RE-wrenches] Voltage correction (was: large array string sizing)

Ryan Mayfield ryan_mayfield at earthlink.net
Tue Dec 8 12:46:43 PST 2009


This brings up some interesting questions I had yesterday on this subject.

 

1. If the percentage change for Vmp is greater than Voc but the manufacturer
only lists the temperature coefficient for Voc, what would be considered the
best practice to calculate the Vmp loss? Use the temperature coefficient for
power (typically ~-0.5%/C for crystalline)? Evergreen is the only
manufacturer I have found that lists all these coefficients, and as much as
I would like it to, one data point doesn't constitute a trend.

 

2. If I have a module with power tolerance of +-3%, and I presume the power
will be at or near the -3%, should I apply -1.5% to the voltage and -1.5% to
the current out of the box? Or will one be weighted more than the other?

 

To me it makes sense to do what  have heard Bill Brooks advocate for,
determine the minimum string length and then add one module. That would take
the issue of dropping out of the inverter's window out of the equation. But,
as Jeff has brought up, we don't always have the liberty to do that.


Ryan

 

 

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kent
Osterberg
Sent: Monday, December 07, 2009 10:25 PM
To: jryago at netscape.com; RE-wrenches
Subject: Re: [RE-wrenches] large array string sizing

 

Jeff,

The maximum voltage of the PV modules will probably be a little less than
the spec'd value and it will decrease slightly as the modules age.  But
going over the inverter's voltage rating can (probably will) be
catastrophic. It looks like your leaning towards avoiding that possibility
by using 12 modules and I think that is the right approach.  While inverters
do have a minimum operating voltage, the more likely circumstance is that
the array voltage drops outside the MPPT window.  The inverter will still
operate and the output power will drop off but by less than the percentage
by which the maximum array voltage drops outside the MPPT window.  So if you
are loosing 5% of your potential output for 60 hours a year, that's not a
serious problem.  But you still need to make this decision carefully.  In 10
to 20 years the system voltage will drop 5 or maybe 10%.  How far are you
going to be outside the MPPT window then?  Is the inverter still going to
operate?  

Another potential gotcha working against you is that some inverter
manufacturers string calculators assume that the peak power voltage
coefficient is essentially the same as the open circuit voltage coefficient;
that is to say the percentage change applies to both numbers.  It doesn't!
In fact the peak power voltage changes almost as many volts as the open
circuit voltage.  Take a look at the voltage/temperature coefficients for
the Evergreen modules; Evergreen publishes both coefficients.  While most
manufacturers only publish one value, the numbers are similar for all poly
or mono crystalline cells. 

Sounds like this module/inverter combination may not be a good match.

Kent Osterberg
Blue Mountain Solar, Inc.




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