[RE-wrenches] Bi-facial Open Circuit voltage testing

Tue Nov 3 10:50:39 EST 2020

Hey Jay and Brian,

System: 2p SAT
Mods/String: 27

Measurements taken on average of CB
Ambient: ~23ºC
Module: 21.4ºC
Wind: moderate
Irradiance: 500W/m2

Expected Voc: 1,320.37

Testing based on IEC 62446
Measured Voc on a range of 9 string combiner: 1251 - 1271. 1251 being the only string in the CB at less than 5% of expected, which I am sure the wind had some effects. All voltages in the CB were +/- 1.5% of the adjacent.

I’ll share more results as the come in from the field. 

The questions:
Which of the two Vocs should we use? The average of the two?
Since we are not taking the BOM temp measurements from a back sheet, but rather a cell. What does the thermal effect of glass play on the temperature reading vs. the back sheet. (May be wording this wrong, but I think y’all get the point)

I have searched the inter webs and not found much guidance/support in particular to this query.

Jason Kechijian

> 
> 
> Hey wrenches,
> 
> Does anyone have any guidance to which values to use when testing open circuit voltage on bi-facial modules, considering you take the temperature reading from the cell vs. the back sheet? Also, what would be the correct module Voc to use since there are values for both the front and back?
> 
> 405W Module: Front/Back
> Pmpp: 405 / 285
> Vmpp: 40.89 / 41.10
> Impp: 9.91 / 6.94
> Voc: 48.42 / 46.99
> Isc: 10.38 / 7.28
> Module efficiency: 19.7% / 13.9% 
> Temp coefficient Pmpp: -0.3528%/?C
> Temp coefficient Isc: +0.0400%/?C
> Temp coefficient Voc: -0.2769%/?C
> Nominal: 44+/-2?C
> 
> I was using the front Voc and Voc temp coefficient, but I am getting some results which are not in line with the 5% of expected.
> 
> Jason D. Kechijian, LEED AP
> NABCEP Certified PV Installation Professional?
> 
> 
> 
> 
> 
> ------------------------------
> 
> Message: 2
> Date: Mon, 2 Nov 2020 08:25:20 -0700
> From: Jay <jay.peltz at gmail.com>
> To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
> Subject: Re: [RE-wrenches] Bi-facial Open Circuit voltage testing
> Message-ID: <50543918-A062-456F-AC6D-6970C793F71A at gmail.com>
> Content-Type: text/plain; charset=utf-8
> 
> Hi Jason,
> 
> What are you seeing in terms of measurements?
> 
> Temp air/module 
> Voc
> Irradiance
> Etc
> 
> Jay
> Peltz power
> 
> 
> Message: 3
> Date: Mon, 02 Nov 2020 08:41:21 -0800
> From: Dave Angelini Offgrid Solar <offgridsolar at sti.net>
> To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
> Subject: Re: [RE-wrenches] Bi-facial Open Circuit voltage testing
> Message-ID: <4af9d868a899895ed1fd8ad15ba0ff2e at 127.0.0.1>
> Content-Type: text/plain; charset=UTF-8
> 
> I am curious why you care?  Is it close to maxing out cold weather Voc for
> one more panel in the string? If so I would be conservative and use one
> less panel.
> 
> I have an old Sanyo Bi Array and it pretty much has never gone outside the
> cold weather Voc that is logged by the controller. The 600v mppt just
> cranks out the power when the snow is off the array. It does pretty well
> also when the snow is on it. 
> 
> Just curious what you are doing? Testing used panels to make sure they are
> good? If so I would line them all up exactly the same and have a second set
> of eyes to record it all.
> 
> 
> Dave Angelini Offgrid Solar
> "we go where powerlines don't"
> http://members.sti.net/offgridsolar/
> e-mail offgridsolar at sti.net
> text 209 813 0060
> 
> 
> ------------------------------
> 
> Message: 4
> Date: Mon, 2 Nov 2020 09:53:43 -0700
> From: Brian Mehalic <brian at solarenergy.org>
> To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
> Subject: Re: [RE-wrenches] Bi-facial Open Circuit voltage testing
> Message-ID:
> 	<CAPKGs9Tz6yeHzWeOarDp0MVaMHuxMFhcLUoPROwEtXYNeW04gQ at mail.gmail.com>
> Content-Type: text/plain; charset="utf-8"
> 
> I second Jay's request for some numbers, I'm curious about this not having
> participated directly in any bifacial projects yet.  The difference in Voc
> and Vmp ratios is interesting.
> 
> Brian Mehalic
> NABCEP Certified Solar PV Installation Professional? R031508-59
> National Electrical Code? CMP-4 Member
> (520) 204-6639
> 
> Solar Energy International
> http://www.solarenergy.org
> 
> SEI Professional Services
> http://www.seisolarpros.com
> 
> 
> 
> On Mon, Nov 2, 2020 at 8:55 AM Jay <jay.peltz at gmail.com> wrote:
> 
>> Hi Jason,
>> 
>> What are you seeing in terms of measurements?
>> 
>> Temp air/module
>> Voc
>> Irradiance
>> Etc
>> 
>> Jay
>> Peltz power
>> 
>> 
>>> On Nov 2, 2020, at 8:18 AM, Jason Kechijian <solarj at me.com> wrote:
>>> 
>>> ?Hey wrenches,
>>> 
>>> Does anyone have any guidance to which values to use when testing open
>> circuit voltage on bi-facial modules, considering you take the temperature
>> reading from the cell vs. the back sheet? Also, what would be the correct
>> module Voc to use since there are values for both the front and back?
>>> 
>>> 405W Module: Front/Back
>>> Pmpp: 405 / 285
>>> Vmpp: 40.89 / 41.10
>>> Impp: 9.91 / 6.94
>>> Voc: 48.42 / 46.99
>>> Isc: 10.38 / 7.28
>>> Module efficiency: 19.7% / 13.9%
>>> Temp coefficient Pmpp: -0.3528%/?C
>>> Temp coefficient Isc: +0.0400%/?C
>>> Temp coefficient Voc: -0.2769%/?C
>>> Nominal: 44+/-2?C
>>> 
>>> I was using the front Voc and Voc temp coefficient, but I am getting
>> some results which are not in line with the 5% of expected.
>>> 
>>> Jason D. Kechijian, LEED AP
>>> NABCEP Certified PV Installation Professional?
>>> 
>>> 
>>> 
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