No series connection?

System

Hi all, first post

I have a SunPower 90, it has a sticker on the back of it to the effect 'not to be connected in series' http://www.sunpower.de/pdf/SPR-90.pdf

What's with that? The other panel I have, SunTech 120, doesn't have any such sticker.

Is it okay to connect any PV is series to achieve 24 or 48 volts? Or do the panels have to be rated for 24 volt?

The only reason I can think of for this being a problem is manufacture fitted diodes not being rated to suit the voltage? Otherwise, why would a cell care?

I figure series connection of the SunPower 90 will void its warranty, but if I'm going to buy more panels I'd like to able to upgrade to 24 volt at some stage without having to buy new 24v panels.

Confused

mike95490

Re: No series connection?

I see on the datasheet link, that max system voltage is 120V, so I'd assume 2 or 3 in series should be OK. But then why bother to print 7 apply such a sticker ?

Maybe you'll have to call the factory and speak to an engineer (not a sales droid) to get a definitive answer.

BB.

Re: No series connection?

Your in Australia? From the Sunpower US website, they have the UL and IEC papers for their panels:

UL
IEC

From the IEC:

4.0 Electrical Connections

Modules may be connected in series and/or parallel to achieve the
desired electrical output as long as certain conditions are met. Please use
only the same type of modules in a combined source circuit.

4.4 Parallel Connection

The modules may be combined in parallel to produce the desired current
output. Every series string or module must be fused prior to combining
with other strings. Figure 2 lists the maximum fuse size allowed. Bypass

The bigger issue with SunPower may be this one:

Important! For optimal performance, SunPower modules
must only be used in configurations with galvanic insulated
inverters where the positive (+) polarity of the PV array is
connected to ground protected by a fuse. Failure to
comply with this requirement will reduce the
performance of the system and invalidate SunPower’s
Limited Power Warranty for PV Modules.

For more information on grounding the system correctly,
visit our website at www.sunpowercorp.com/inverters to
download a technical note on this subject. For additional
assistance, contact SunPower technical support at
support@sunpower.de.

From the inverter link:

Grounding

SunPower high-efficiency systems must be installed with proper grounding and inverter configuration procedures in order to perform optimally to their specifications. This is to ensure SunPower panels only see negative voltage as referenced to ground.

For optimal performance in North American installations, SunPower solar panels should be used with SunPower positive ground inverters where the positive polarity of the array is connected to ground. The SunPower inverters listed above meet this requirement.

For optimal performance outside the United States, SunPower panels must only be used with transformer-based inverters where the positive (+) polarity of the array is connected to ground.

I would guess their issue is (as best as I can recall), is that with their back side contacts, that a charge can build up between the front and the rear of the cell--and if those cells are negative grounded (average cell voltage is above ground), that this voltage difference can build up in the cell structure and form permanent charge in the cell--causing reduced output in cell voltage (or current/power). If the cells are kept below ground reference, the charge does not build up and remain over time--keeping output voltage/current high (to spec.).

So, does that mean when these systems are used in high voltage DC systems (with normal DC solar charge controllers), that this problem will be worst because many (most?) are negative grounded charge controllers and at higher voltages (battery or using MPPT controllers) that the degradation is more pronounced?

But, if the solar array is positive grounded (either through using an isolated MPPT controller, or a positive grounded battery), then all would be well?

Not sure--but, that is where I think the conversation would head...

OK, looking around, I found an HTML cache (from Google of SunPower's paper about the problem--opening paragraphs):

Introduction

In August, 2005 SunPower announced the discovery of a new performance effect observed inhigh efficiency silicon solar cells. This new effect, called “surface polarization,” creates the non-destructive and reversible accumulation of static charge on the surface of high-efficiency solar cells such as the company’s all back contact A-300.

The high efficiency of SunPower’s A-300 solar cell is obtained in part by covering its front surface with a proprietary coating which prevents the loss of the charge carriers generated by sunlight. This layer performs much like a transistor that is turned off, preventing current flow. If a large enough voltage is applied to the front of the cell, the “transistor” effectively turns on, allowing charge carriers to recombine at the front surface. When this happens, surface polarization reduces the output current of the cell. Also, like a transistor this effect can be fully reversed and current returned to the original level.

How Surface Polarization Occurs

Surface polarization can occur when a module is put into operation at high positive voltages. If the module is operated at a positive voltage with respect to the earth ground, then minute leakage current will flow from the cells to ground. As a result, over time a negative charge is left on the front surface of a cell. This negative charge turns on the surface transistor, attracting positive charge carries to the front surface where they recombine with electrons and are lost. This sequence of events is illustrated in Figure 1.

-Bill

PS: Add some more information on the issue (in case the cache goes away--Note, the graphs and other non-text items "fell out" in the HTML conversion process):

When a module is operated at negative voltage with respect to ground, the surface polarization reverses. In this case the performance of the module is not affected; in fact, it can improve slightly because the front now repels light generated holes, guiding them to the back collecting junction. Figure 2 shows the situation at negative voltage.
...
Figure 3 shows the current-voltage characteristic of a module that has undergone surface polarization after operating in the field at a positive voltage of 160 V. The blue line labeled “before voltage bias” shows that with surface polarization, the module output has declined to 140 W from its initial 200 W rating. After biasing the module at a minus 1000 V for one hour to reverse the surface polarization, the negative surface charge has been bled off and the module completely recovered, as illustrated by the red line labeled “after voltage bias.” The surface polarization effect is completely reversible, with no lingering effect.
...
The polarization effect can be easily avoided by designing systems with proper grounding so that modules only see negative voltage. All of SunPower’s system integration partners have developed grounding procedures that prevent module operation at positive voltage. In Europe, SunPower worked closely with its exclusive distribution partner, SunTechnics, to characterize the polarization effect and to develop system configurations that prevent the effect. SunTechnics also developed a range of system solutions and electronic devices to optimize SunPower system performance and meet European security standards. For European tracker systems, SunPower worked with Solon to characterize and develop system configurations to prevent the polarization effect. In North America, SunPower modules are exclusively installed with SunPower positive-ground inverters which eliminate surface polarization.

If a system is mistakenly installed with the incorrect grounding, modules will temporarily lose power, but no lingering effect remains after surface polarization is reversed. Because the surface polarization effect is completely reversible, once the grounding problem is corrected, modules will quickly recover to their initial performance without any further intervention.

BB.

Re: No series connection?

Here is a more detailed paper (PDF File with graphics) (similar publishing date as previous paper I posted) about the SunPower solar panel polarization issue...

There is an interesting comment buried in this paper:

SunPower is currently modifying the A-300 to include a front surface conductive layer. This layer shunts leakage current and completely eliminates the polarization effect. Production is slated for 2006.

I don't see any indication that this made it into production--but perhaps that is the difference between panels with and without the series warnings...

-Bill

System2

Re: No series connection?

Wow, thanks for the quick and detailed reply. I will read through it all properly tonight or tomorrow.

At a glance, I think my safest bet is to not buy any more SunPower panels, the SunPower 90's are actually a discontinued line - may has something to do with all this...

More later.

BB.

Re: No series connection?

Not trying to drive you away from SunPower panels. Sounds like they are OK (Solar Guppy has discussed them at his site), and a positive grounded battery system is actually very common in telecom industry (the -48 VDC leads don't corrode like a +48 VDC--with respect to ground--would).

The bigger issue is that most current automotive electronics (and many others) tend to have the cases grounded to the negative lead--which would cause a shock/fire hazard if installed in a positive ground network).

-Bill