Voltage drop of panels

Dave Angelini

BB. said:

It could also be a failure elsewhere in the panel... At this point, I am not sure that the diode(s) are shorted.

Any chance for Warranty Replacement?

-Bill

Most likely not the diode if these are the C level rejects. I had a client hire me after he bought these. There were so many defects (about 3 per panel) that I had to laugh. You could see through the wafer on some and some had cells cracked in half. Imagine how they will be in 5 years....
They are using them as an intermittent awning. Even the B level rejects were pretty bad. There were footprints on the white backing that Sun Miami got on the panels. Not Sunpower, Sun Miami.

A decent voltmeter with a diode check should allow you to compare the diodes breakdown V's.

BB.

Yea, at $1.00 per watt for first class panels, trying to use factory 3rds is difficult to justify for making a reliable off grid power system.

It may cost more to ship C grade panels than it costs to buy A grade.

-Bill

mike95490

Active FET Bypass Diodes are now in production by several companies (article):
http://www.digikey.com/en/articles/techzone/2012/dec/active-bypass-diodes-improve-solar-panel-efficiency-and-performance

Dave Angelini

BB. said:

Yea, at $1.00 per watt for first class panels, trying to use factory 3rds is difficult to justify for making a reliable off grid power system.

It may cost more to ship C grade panels than it costs to buy A grade.

-Bill

I think he said $.41 and you are right the shipping was about at the panel cost.
Bill, I have an old X x Y HP curve tracer that we built when times were tuff in 1977. The employees gave them as Xmas presents to their tech starved friends. Anyway it is a great curve tracer for a diode and you can easily see the Knee at breakdown. It has this cool 2 inch scope. If you ever get down to the valley the surplus electronics stores sometimes sell the scopes. Kind of geeky but what else was there to do then without the WEB

BB.

Yea... The web and Google has made life sooo muuuccchh easier for technical information. No massive libraries needed full of obsolete parts/product information.

-Bill

Dave Angelini

The old HP Spec Ans and Sig Gen manuals weighed more than a big battery. I know that is where I got my carpal tunnel.

Vic

Dave Angelini said:

BB. said:

Yea, at $1.00 per watt for first class panels, trying to use factory 3rds is difficult to justify for making a reliable off grid power system.

It may cost more to ship C grade panels than it costs to buy A grade.

-Bill

I think he said $.41 and you are right the shipping was about at the panel cost.
Bill, I have an old X x Y HP curve tracer that we built when times were tuff in 1977. The employees gave them as Xmas presents to their tech starved friends. Anyway it is a great curve tracer for a diode and you can easily see the Knee at breakdown. It has this cool 2 inch scope. If you ever get down to the valley the surplus electronics stores sometimes sell the scopes. Kind of geeky but what else was there to do then without the WEB

and Dave Angilini said,   ...   A decent voltmeter with a diode check should allow you to compare the diodes breakdown V's"

One would believe that this is probably not a diode Breakdown issue that continues to occur,  but,  perhaps more likely than that,  might be a Bypass Diode that has failed  -- shorted.   A Curve Tracer would not be needed,  as this is not really a Breakdown,   but Semiconductor Curve Tracers are  very neat instruments.

Bypass diodes have several failure modes,  but,  Thermal Runaway failures is a fairly common one for bypass diodes that have failed,   as reported in the literature.   Schottky diodes exhibit high leakage currents at elevated temperatures (when reverse biased).   As the temperature increases,  the leakage current increases,  and so on,  until the diode suffers catastrophic destruction,  and usually shorts (in varying degrees).   Rapid transition from a PV module being shaded to un-shaded seems to be one worst-case situation. A shorted bypass diode  usually removes one series string of PV cells in a module,   thus reducing the apparent Vmp of that module.

No one here appears to know the real history of these SunPower PVs,  but it has been stated by vince,  that the system performed well in the initial months,  but the performance degraded recently.

I am not an expert in the failure modes of PV modules,  so the above may not be the case at all ...  but,  given what we know,  my bet is that the right-most bypass diode in the photo has shorted (there is some discoloration on the third buss,  and particularly on the fourth buss from the left in the J-box).

FWIW,   Vic

vince

Vic, if it is the bypass diode failure, would a diode 1000V/10A instead of 100v/10A do the job? Only ones I was able to find without going bulk were these http://www.amazon.com/TOOGOO-Molded-Plastic-Rectifier-Diodes/dp/B009IN1KB8?ie=UTF8&ref_=cm_sw_em_r_lm

Vic

vince said:

Vic, if it is the bypass diode failure, would a diode 1000V/10A instead of 100v/10A do the job? Only ones I was able to find without going bulk were these http://www.amazon.com/TOOGOO-Molded-Plastic-Rectifier-Diodes/dp/B009IN1KB8?ie=UTF8&ref_=cm_sw_em_r_lm

Hi vince,

By definition (although could not find the nature of the diodes in the link above),   a 1000 volt diode cannot be a Schottky diode.

A Schottky diode is important for this application,  as the forward voltage drop (when it is forward biased,  and protecting a series of PV cells)  is lower than that of standard silicon diodes.   But Schottky diodes are a bit fragile,  as the maximum  reverse voltage breakdown available in Schottky is less than standard silicon diodes,  and at elevated temperatures,  the reverse Leakage current is significantly higher than that of Silicon diodes.

It I quite possible that for Schottky diodes with axial leads with the ratings needed for this application,  that diodes from Asia (most likely China)  may be the only game in town,   The major reluctance in suggesting Asian sources,  is that it cold be possible that the Schottky diodes available to we retail buyers might be out-of-spec,  and possibly susceptible to failure in a period of time.   And,  of course,  it is possible that this might not even be the issue with your two suspect PV modules.

If you were to approach the supplier of your SunPower PVs for a refund for all three of these PVs,  could you manage changing to different PVs?

Four Kyocera 325 W PVs in two strings of two would yield 1300 watts STC with a string Vmp of just over 80 volts and give you about the amount of PV power that the SP PVs had,  although would require more space,  as the SP PVs appear to be very efficient.  Here is a Link to the Kyo 325s (although wind-sun does not have them in stock):
http://www.solar-electric.com/solar-panels-mounts-kits-accessories/solarpanels/kyocera-solar-panels/kyocera-kd325gx-lfb-325-watt-polycrystalline-solar-panel.html

One could run 72-cell PVs in strings of two,  but IMO,  their Vmp is slightly too low to be ideal for MPPT CCs.

60 Cell PVs in strings of three for a string Vmp of about 90-ish volts would be fine,  if you have the space and willingness to redo your racking and wiring.

The SP PVs are quite unique,  so upgrading or replacing those PVs can be difficult,  as there are few.  if any alternatives to them.   It is possible that you could contact a reputable installer of PV systems in your area to see if they have any full-spec SP PVs that you could use with your existing setup,  or perhaps order three more than needed for an upcoming job,  and perhaps sell them to you ...   SP may not favor this,  so this may be a long shot.

Later  Good Luck,    Vic

mike95490

There are now some new "active FET" bypass diodes for PV panels
http://www.digikey.com/en/articles/techzone/2012/dec/active-bypass-diodes-improve-solar-panel-efficiency-and-performance
less power (heat) then standard schottky diodes

Vic

mike95490 said:

There are now some new "active FET" bypass diodes for PV panels
http://www.digikey.com/en/articles/techzone/2012/dec/active-bypass-diodes-improve-solar-panel-efficiency-and-performance
less power (heat) then standard schottky diodes

Right,  you are.    However  it would seem to be a bit of an engineering project to convert from Axial Lead Schottky diodes to the FET-based diode replacements,   IMO.

FWIW,   Vic

mike95490

Just solder a "L" shape solid #14 wire across each side - all J leads on each side are internally connected.
and heat sinking requirement is greatly reduced

Vic

Hi Mike,

Did look at  that article,  and a number of others in the past.

IMO  the photo that is in your Post,  above,  is for a part that is not in a Gull-Wing package.  It IS a Surface Mount package,  with its contacts on the under side of the package,  and is designed to be reflow-soldered onto a PCB.  This gives mechanical support for the device,  especially under temperature-cycling conditions:
http://www.digchip.com/datasheets/download_datasheet.php?id=4417502&part-number=LX2400ILG

Flying the device on soldered leads is certainly beyond the design spec for that device/package,  IMO.

Have looked at some other Active Diode solutions,  and those,  also,  are not in DO-type (Axial Lead)  packages  --   usually  are in a TO-2XX type pack,  like a TO-263,   TO-247,  or perhaps TO-220,  all of which would like SOME heatsinking:
http://www.ti.com/lit/ds/symlink/sm74611.pdf

Not to put too fine a point on it ...  just about anything can work,  at least for a while ...

More Later,   Vic

vince

I hooked up a 12V module in series to the panels that were only producing 60V. The Imp was within 5% (5.92 A and 5.65 A).  Now system is running fine and batteries are happy. Seemed the best option with the limited space although with some modification of the racking,  I could accommodate 6 60 cells panels also. May go that route if I can get a refund on all three panels. So far, he has only been willing to refund the two panels.

Vic

OK vince,   Thanks for the update.

Hope that you can get a refund on all PVs.   Sounds like your added 12 V PVs are doing the trick for now.

Does that supplier want the two bad SP PVs returned to them?   Hope that the supplier of the SunPower PVs is close to your location   ...

Good Luck,  and please let us know how things are going.   Thanks,   Vic