[RE-wrenches] Voltage Degradation with Uni-Solar laminate

Mon Jun 1 09:48:41 PDT 2015

Hi:

With the rapid increase in the number of new installs and installers, the 
question of determining the lowest Voc for a system which uses PWM or MPPT 
charge controllers has come up more often recently.

I did a slide for it in our training,

a. PWM and MPPT charge controllers cannot boost the incoming voltage.  The 
input voltage always has to higher than the desired charging output 
voltage.
b. Charge controllers have a loss in the range of 2-4%, depending on array 
voltage, battery voltage, charging current, temperature etc.
c. The charge controller loss will cause a voltage drop from input to 
output which will be proportional to the loss percentage.
d. Additional losses will be present due to system wiring, terminations, 
etc.
e. The total system loss will typically be in the range of 4 -6 %, 
depending on the particulars of the design and operating conditions such 
as temperature, sun intensity etc.
f. The lowest possible Vmp of any module is approximately 75% to 90% of 
the highest possible Voc for virtually all modules depending on module 
type and whole bunch of other environmental and operating variables.

In other words, by the time you go through the math, the Voc of typical 
PWM or MPPT systems needs to be in the range of 1.3 X the highest desired 
charging voltage, (1.3X equalize voltage if that is to be used, 1.3X bulk 
if that's the highest desired).

If you don't have enough margin, then the effect is that the module/array 
will still provide charging current, but it will forced to operate at a 
voltage somewhere between Voc and Vmp.  It will therefore produce a 
reduced amount of charging current less than Imp and possibly approaching 
zero if the module/array is asked to operate at or very near Voc. 

Marginal design along with the guaranteed variable nature of sun intensity 
and ambient temperature throughout the year, very likely can result in a 
system which will provide a charging current, but wont be able to quite 
reach bulk voltage and that of course is not a good thing.

Whatever the cause, you need more Voc.

JARMO
_____________________________________________________________________________________ 

Jarmo Venalainen  |   Schneider Electric   |  Xantrex Brand  |   CANADA  | 
  Sales Application Engineer 
Phone: +604-422-2528  |   Tech Support: 800-670-0707  |   Mobile: 
+604-505-0291 
Email: jarmo.venalainen at schneider-electric.com  |   Site: www.Xantrex.com 
|   Address: 3700 Gilmore Way, Burnaby, BC V5G4M1 

*** Please consider the environment before printing this e-mail

From:
Luke Christy <sgsrenewables at gmail.com>
To:
"re-wrenches at lists.re-wrenches.org" <re-wrenches at lists.re-wrenches.org>, 
Date:
05/31/2015 10:55 AM
Subject:
[RE-wrenches] Voltage Degradation with Uni-Solar laminate
Sent by:
"RE-wrenches" <re-wrenches-bounces at lists.re-wrenches.org>

Hello Off-Grid Wrenches,

I recently made a periodic maintenance visit to a residential off-grid 
system I take care of here in Southern CO, and discovered an interesting 
problem. 

This system has an array of 48x Uni-Solar PVL-68 laminate modules applied 
directly to standing-seam metal roofing. 

The data logging on the controller (Outback MX-60) indicated that the 
system was having trouble completing a charge cycle and going to float. 
Initially I assumed was due to heavy electrical use by the occupants 
(renters). After a little checking, I noticed that the module string 
voltage was only about 0.5V above the battery voltage! 
This was a cool late morning, with full sun, and the array was charging 
almost 2kW at 54.5V. The module strings were running at 55V. I've noticed 
the module voltage being lower than I'd like before, but it has never been 
this low. I suspect this is the real reason the system hasn't been able to 
complete a charge cycle.

By removing the loads I was able to get an open-circuit voltage of 71V, 
but that still seems pretty low. 

Has anyone else experienced a voltage degradation issue with the Uni-Solar 
laminate? What was the expected annual voltage degradation for this 
technology? 

These modules do have some age on them. The system was installed about 15 
years ago (by a competent installer who is now retired). The system was 
installed before MPPT controllers were common, so I assume the modules are 
wired in strings of 4 for 48V nominal. 

Any experiences or opinions are appreciated.
Thanks in advance. 

-Luke

Luke Christy

NABCEP Certified PV Installation Professional™: Certification #031409-25 
NABCEP Certified Solar Heating Installer™: Certification #ST032611-03  
CoSEIA Certified PV Installer 

Solar Gain Services, LLC
PO Box 531
Monte Vista, CO. 81144
SGSRenewables at Gmail.com
719.588.3044
www.sgsrenewables.com

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