[RE-wrenches] calculating low string voltage

Bill Brooks billbrooks7 at yahoo.com
Tue Nov 29 16:45:39 PST 2011


Folks,

 

One factor often overlooked when calculating minimum system voltage is
degradation of the array and rating variations of the modules.

 

If you assume 0.5% loss in voltage per year, you need to factor a 10% higher
voltage at year one.

 

Manufacturers tolerance on voltage and current can be 5-10% (rare in real
life). It is healthy to add another 5% for voltage tolerance. Voltage
tolerance can be much larger than power tolerance that may only be +/- 3%.

 

The voltage tolerance numbers used by Ray are for Voc not Vmp. Vmp numbers
are closer to 0.45% to 0.5%. This why the power loss due to temperature is
about 0.5%/C-all of it is in voltage, not current. Therefore, for a 40C rise
in temperature, there will be a 20% loss in voltage. Factoring another 15%
loss for degradation and module tolerance, the total loss in voltage would
be:

 

Vmin = 35V x 0.8 x 0.9 x 0.95 = 23.94V => 24 Volts, not 30 Volts. It is
always lower than you think unless you are Eeyore. This is one reason why
the most common string sizing mistake is not enough modules in series.

 

Bill.

 

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Ray Walters
Sent: Tuesday, November 29, 2011 2:55 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] calculating low string voltage

 

I'm prepping for a class on this very subject tonight, so I'll just wrap up
everybody's good points:
V min = Vmpp x (1- temp diff x Temp coefficient)

So first, I'd take your ambient temp from ASHRAE Extreme Annual Mean
Maximum, then add Scott's adder (ie roof mount off 6"  = +30C) so let's say
the ASHRAE # is 35C, then you have 30 + 35 = 65C cell temp
then, 65C - 25C(STC) = a differential of 40 deg C from standard test
conditions

The Aug/ Sept 2010 edition of Solar Pro shows the coefficients for numerous
modules, 
Solar world for example is -0.33 %/degC

With a 40 degC differential then, the voltage would change by 40 x .33, or
13.2%

So just assuming Vmpp =  35v, the adjustment would be 35v x (100%-13.2%), or
86.8% of 35v, 
which gives a Volt min of 30.4 v

Somebody please correct my math if I got that wrong.
Also probably more interesting is the actual Power coefficient, which shows
the total power losses from higher temperatures, not just voltage. The
calculation would be similar.

Ray

On 11/29/2011 2:07 PM, Darryl Thayer wrote: 

I am on road so I have no texts with me, but the same formula applies about
0.4% per degree (less voltage) or use the module label  voltage coefficent.
However you must use the MPPT voltage and the temperature = (temperature
ambient (2% high) plus roof temperature warming)  The roof temperature is
hard to find good values because it depends upon spacing between roof and
module.  I add 15 deg C for most situations.  

 

Best I can do from memory.

DT

 

From: Kirk Herander  <mailto:kirk at vtsolar.com> <kirk at vtsolar.com>
To: RE-wrenches  <mailto:re-wrenches at lists.re-wrenches.org>
<re-wrenches at lists.re-wrenches.org> 
Sent: Tuesday, November 29, 2011 1:48 PM
Subject: [RE-wrenches] calculating low string voltage

I am in debate with a PE over calculation of low voltage of a series string
on a hot day. He insists that an arbitrary high cell temp is factored in,
not just ambient temperature. Could someone please give an accepted formula
for this calculation? Thanks. I cannot find a clear reference to low voltage
calculation on a hot day (but every reference material is clear on how to
calculate high voltage on a cold day).

 

Kirk Herander

VT Solar, LLC

dba Vermont Solar Engineering

NABCEPTM Certified installer Charter Member

NYSERDA-eligible Installer

VT RE Incentive Program Partner

 


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