SunPower flexible panels in parallel

Mongo

Hello, 
I am installing two sunpower 100w flexible panels on the back of a sailboat, and because of potential shading issues parallel connection would be preferred. Sunpower's installation pdf recommends only serial connection. Can I install a blocking diode to prevent issues with a parallel connection, if so any recommendations on a diode? The panels  will be fed into a Victron  75/15 mmpt charge controller.  Thanks!

mike95490

Before you buy that MPPT controller, (which is designed to work with panels in series and reduce the high voltage to battery voltage)

can you post the specs for your 100w panels ?  200w can deliver 16amps, so you may need a larger PWM controller (20A)

Mongo

The panels are sold by Northern Arizona Wind and Sun. Each panel:

• Nominal power: 100 Watts
• Power tolerance: +6 / -3 Watts
• Vmp: 17.1 Volts
• Voc: 21.4 Volts
• Imp: 5.9 Amps
• Isc: 6.3 Amps
• Module efficiency: 18.8%
• Dimensions: 45.9" x 21.9" x 0.8"
• Weight: 4.4 lbs (2 kg)

I am hoping to install the panels in parallel. 

mike95490

Vmp: 17.1 Volts  in parallel is still 17.1 volts on a chilly day.  Hot days the voltage will sag and batteries may not charge completely, but if you are not using the system daily, it may not matter and you can use a cheap PWM 15A controller.

Mongo

Thanks, but the question in the original posting is about a recommendation for a bypass diode if I install them in parallel. In parallel I should get up to 6 amps per panel, 12 amps total at nominal 12v. Any thoughts, or suggestions on resources to find out?

mcgivor

Accordingto the manual blocking diodes are required if in parallel with shading issues to prevent bypass diode damage, see DANGER section in attached pdf

Mongo

I think we I think we are all in agreement on the problem, sunpower recommends a bypass diode for parallel installation. 
Any recommendations on sizing of bypass diode, or how I might figure it out?
thanks

Estragon

In my admittedly rudimentary understanding of pv diodes, the purpose of a blocking diode would be to prevent the battery (at ~+12v) from discharging reverse current through pv at night (at ~0v). I assume these panels lack this?    

Seems to me adding a blocking diode, with associated voltage drop, isn't going to help with the potential too-low pv voltage into the mppt controller @mike95490 mentioned.  I'm not sure mppt does much for you on a sailboat anyway unless you're at high latitudes in pretty cold weather.  If parallel is a problem, how about a pair of cheap pwm controllers?

mcgivor

As Estragon said using two PWM controllers would eliminate the need for blocking diodes Should diodes be used Schottky diodes would be the preferred choice sized greater than the the maximum current, perhaps 10A. https://www.elprocus.com/schottky-diode-working-and-applications/

Mongo

Estragon- My understanding is that the diode requirement has nothing to with battery flow but with hot spot heating in case of shaded cells in one panel.

 mcgivor- I can understand trying to keep the system simple with two simple Pwm controllers , but I already have one panel in place, with victron controller, and hope to add  another.   

Max possible amperage is about 12amps, if I look at the specs above. 10 amps is probably max realistically, is that how you got your recommendation?

mcgivor

A charge controller inherently isolates the battery from the panels, the purpose of the blocking diodes, I believe, is to prevent one panel backfeeding the other, possibly overwhelming the bypass diodes reverse bias capabilities which protect the cell strings of the module. The blocking diodes would need to carry a forward bias current of a single panel or module, the Imp of 5.9A, realistically it would likely be less ~5A, therefore a diode for each panel would be needed.

 

Estragon

Just to clarify (mostly for me), I think we're talking about adding blocking diodes in series with each panel, not bypass diodes (which I think are in parallel with panels or series strings of cells, and are what the blocking diodes would be protecting)?  If I'm understanding this right, the problem is that absent a blocking diode, current could reverse through bypass diode(s) not only from the partly shaded panel itself, but also current from other parallel connected panel(s).

Mongo

Estragon, that seems correct to me, but my knowledge is limited. So do most solar panels have this diode built in, but these flexible panels do not, under the assumption perhaps that they would be used only singularly?

Mongo

I assume wiring is like this;

Estragon

I assume that's correct.  In general, putting panels in parallel isn't a problem (other than required wire size / voltage drop, and fusing if 3+ panels/strings). 

mcgivor

That's what I believe is the method required, perhaps  you run this by the manufacturer.

mike95490

I again, recommend against using diodes in this case.   I will amend to using 2 small PWM  charge controllers.  I was not aware of the fragility of the panels, which is unusual.

Your panels do not produce enough voltage to waste 0.7V in heating a diode up.  A marine environment is a poor place to kluge in a pair of diodes and their heat sinks (yes, .7v * 6a =4.2watts of heat to dissipate without getting crudded up with salt and corrosion.)

mcgivor

By solving one problem you're potentially creating another. The MPPT controller needs ~19V, or higher, with a Vmp of 17V possibly less when hot, plus the voltage drop across the diodes, the controller may not function properl as @mike95490 said  to in post # 2 and would certainly take away any advantage or benefit  the MPPT would usually offer.

Mongo

Voltage drop is in the range of .2 to .46 volts for most shottky diodes.  I found mc4 connectors with built in 15 or 10 amp diodes, The alternative is wiring the panels in series to get 24 volts nominal.

mcgivor

Wiring the panels  in series would provide 42.8V Vmp the MPPT controller will reduce the voltage to whatever a 12V battery needs to change and  convert excess voltage to increase charging current. The downside is if one panel is shaded, the whole string is compromised without the use of an external panel bypass diode. On a boat PWM makes so much sense with regular panels which can be in parallel without the issues associated with the flexible somewhat fragile panels on hand.

Strangely the website shows a marine application, perhaps they are counting on constant orientation changes, who knows, if it were me I would be asking the manufacturer for their suggestions.

mike95490

Mongo said:

Voltage drop is in the range of .2 to .46 volts for most shottky diodes.  I found mc4 connectors with built in 15 or 10 amp diodes, The alternative is wiring the panels in series to get 24 volts nominal.

The OP wants to avoid one panel being shaded and shutting off charging, hence parallel panels.  Even with schottky diodes, loosing half a volt and reduced heat issue is still an avoidable problem with dual PWM controllers.  The panel voltage is marginal for charging in hot weather, diodes lessen the margin of usability 

Mongo

Any recommendations on a PWM controller?

BB.

If you put solar panels in series, without a bypass diode every 24 cells or less, A shaded cell/panel will have current "forced through the shaded portion" by the unshaded panel(s) in series (shaded solar cells go "high resistance" and break down if the voltage is over ~12 volts).

I am not sure why blocking diodes on paralleled panels---In the olden days, lower Vmp panels and blocking diodes were used instead of of a solar charge controller (blocking reverse current flow/leakage current at night). These panels were known as "self regulating".

A solar cell in shade is just a reverse biased diode and if the reverse voltage gets too high, the diode fails.

MorningStar makes a nice series of PWM charge controllers (various sizes/configurations/options).

https://www.solar-electric.com/search/?q=morningstar+pwm

And there is the Midnite Brat:

https://www.solar-electric.com/midnite-solar-brat-pwm-solar-charge-controller.html

-Bill

Mongo

Thanks for all the input everyone. I will most likely put the next panel on it's own PWM controller.  It's interesting that the Sunpower installation manual does not really have a problem with serial connection of panels, except to warn of  increased voltage. Apart from the loss of power from shading, maybe the cells are internally protected?

BB.

Most panels do have internal bypass diodes... Specifically because if you put panels in series, a shaded cell/panel will be damaged by the higher voltage from the serial connected panels.

If you have "low voltage panels" (Vmp~18-20 volts or less), you don't need the bypass diodes if they are never put in series.

From the "glossy":

https://www.solar-electric.com/lib/wind-sun/sunpower_SPR-E-Flex-100-110_spec_sheet.pdf

Max system voltage is 45 VDC. So, at best, you can put two panels in series... Which would seem to indicate that one shaded panel (no voltage) and one panel under sun, that would not be more than ~22 volts Voc across a single cell...

The installation guide talks about bypass diodes in the junction box:

https://www.solar-electric.com/lib/wind-sun/sunpower_flexible_panel_installation_guide.pdf

DANGER!
• SunPower does not recommend connecting these panels in parallel without proper system and safety protection.
• Connecting panels in parallel will double the current output of the panels. DC Currents may be greater than 12 amps for 100 W, 110 W and 170 W, and greater than 6amps for 50 W, in high illumination conditions (>1000 W/m2).
• If shading occurs without additional electronic components, such as a blocking diode, the by-pass diodes may be damaged leading to further panel damage and unsafe conditions including fire.
• If Paralleled panels are attached to a battery for charging, the high current may cause damage to the battery and subsequent safety issues.
• The charging characteristics of any battery should always be checked for compatibility with the current and voltage output of the panels prior to use. 

Electrical Connections
The connectors used in these panels allows for modules to be connected in series or parallel. However, connecting the panels in this manner can put the by-pass diode in the panel and the battery at risk for damage and could create a safety issue. Please read the DANGER! note below.

Series Connection
Connecting the panels in series increases the voltage of the system, so the two panels produce double the voltage as compared to one panel. This high voltage can cause damage to the battery and could cause a safety issues. Please read the DANGER! note below. To connect panels in series, connect the negative (-) plug of panel #1 to the positive (+) plug of the panel #2. See Figure1 left.

Parallel Connection
Connecting the panels in parallel increases the current of the system, so the two panels produce double the current as compared to one panel. This high current may cause damage to the battery and the by-pass diode in the junction box and cause a safety issue. Please read the DANGER! note below. To connect panels in parallel, connect the positive (+) plug of panel #1 to the positive (+) plug panel #2 Connect the negative (-) plug of panel #1 to the negative (-) plug of panel #2. See Figure 1 Right. In this configuration, cable adapters will be needed.

And they talk about blocking diodes if you put panels in parallel... In the 'olden days', blocking diodes were used to parallel solar strings together--But these days, using a fuse/breaker per string is the standard practice to prevent a panel short circuit from being damaged by the other parallel connected strings (from looking at older installation photographs--Use of blocking diodes).

-Bill

Mongo

As an experiment I may try the series connection out to see how much power I get in average use. Based on the discussion above, it could make the existing mppt controller more efficient. I know that for a sailboat I have to end up with a parallel connection or two separate controllers, and with these relatively low voltage panels, PWM is the better choice.  Thanks.