Electrical effect and bypass diodes

[cvernay]

Dear all,

I want to modelize PV sheds with this layout: PV panels in landscape, tables of 4 rows, strings connected on a unique row.

How shall I define the electrical effect ? These cells are defined as follows, cf. the enclosed datasheet:

- 6 × 10 pieces polycrystalline solar cells series strings

- With 6 bypass diodes

Shall I define only 4 strings in the width of the row?

Or 4*6=24 strings in order to account for the bypass diodes (the layout can be considered as linear)?

Thank you for your help

Best Regards

Christophe

[André Mermoud]

For your configuration, you should define 4 strings in the width of the shed.

With sheds arrangement (a full row of cells shaded), the by-pass diodes don't recovery any power and the full string production (for beam component) is lost.

Please see our FAQ How to evaluate the effect of by-pass diodes in shaded arrays ?

[cvernay]

Thank you very much for your answer and the useful FAQ.

Regards

Christophe

[oron]

Dear Dr. Andre,

with regarding the question of Mr. Christophe:

is there any other affect of the array? (in case it's array that connected to the same MPPT of the inverter)?

is the voltage of the whole array decrease also, for instance in case of 1 or 2 bypass diodes of the string that installed on the bottom are on? or the inverter will keep the voltage level of the "strongest" strings anyway?

is there any difference between central inverter or a string inverter (with one MPPT)? i guess there is no difference but i'm not sure.... :)

are other strings of the array, which are not shaded ,could produce less energy in that case, or only the shaded string is affected?

Many thanks

Oron

[Wong Wei Chong]

Hi Andre,

Would appreciate your confirmation on my understanding below. I came up with 2 scenarios:

Scenario 1:

Capacity: 2 strings of 24 nos 270Wp

Table: 2x24 Landscape (2 rows, Up and down)

Inverter: SMA Sunny Tripower 12KTL (2 MPPT)

My understanding is if I connect 2 strings separately to the 2 MPPT, I can define 6 strings in the electrical loss because it is essentially 6 "sub-strings" due to the bypass diode.

And if I connect both 2 strings to only 1 MPPT, then I would have to define only 2 strings in the electrical loss.

Am I correct?

Scenario 2:

Capacity: 2 strings of 24 nos 270Wp

Table: 2x24 PORTRAIT (2 rows, Up and down)

Inverter: SMA Sunny Tripower 12KTL (2 MPPT)

My understanding here is because it is portrait, there are no substrings like in the Landscape case. This is because once the bottom row of cells are shaded, the entire string will be affected electrically. The diode can't help in this case. Therefore, irregardless if I connect these 2 strings to the same/ different MPPT, I should still define 2 for the number of strings for electrical loss.

The above 2 scenarios are rather straight forward way of stringing the modules. However, I came across some cases whereby in 2x12 Portrait table (one string one table), the modules are stringed in a U-shape ; or in 2x24 portrait in a double U-shape (two strings are separated into left and right instead) to save the solar cable. In this case, how should I define the number of string for electrical loss then? If in situation where instead of Portrait, the modules are in Landscape but also string in U-shape, isn't it gonna be more complicated?

Thanks in advance.

[André Mermoud]

Portrait, landscape and U-configuration are taken into account with the "Module layout" detailed calculation, applied to each module in the array.

You can have a look on the "pedagogic" I/V curves shown with this tool (button "Calculate > page I/V curves").

[Wong Wei Chong]

Hi Andre,

Thanks for your reply but I can't seem to find "pedagogic" I/V curves explanation in "Module layout" detailed calculation tab. Please enlighten me.

Anyway, I got what you mean that if we define each and single string in "Module layout", everything will be accounted for. However, in most of the cases, I only need to do preliminary design where it is really not necessary to go into this detailed. I am more interested to know in "Orientation",field type "unlimited sheds", how should I define the number of strings for electrical effect that gives a reasonable and realistic simulation for scenarios that I mentioned in my previous posts.

Thank you!

[jasonfoy123]

This use of bypass diodes allows a series (called a string) of connected cells or panels to continue supplying power at a reduced voltage rather than no power at all.

solar panel with bypass diode

Bypass diodes are connected in reverse bias between a solar cells (or panel) positive and negative output terminals and has no effect on its output. Ideally there would be one bypass diode for each solar cell, but this can be rather expensive so generally one diode is used per small group of series cells.

A “solar panel” is constructed using individual solar cells, and solar cells are made from layers of silicon semiconductor materials. One layer of silicon is treated with a substance to create an excess of electrons. This becomes the negative or N-type layer. The other layer is treated to create a deficiency of electrons, and becomes the positive or P-type layer similar to transistors and diodes.

When assembled together with conductors, this silicon arrangement becomes a light-sensitive PN-junction semiconductor. In fact photovoltaic solar cells or PV’s as they are more commonly called, are no more than big, flat photo sensitive diodes.

Photovoltaic solar cells convert the photon light around the PN-junction directly into electricity without any moving or mechanical parts. PV cells produce energy from sunlight, not from heat. In fact, they are most efficient when they are cold!.. Ripple Electrical

When exposed to sunlight (or other intense light source), the voltage produced by a single solar cell is about 0.58 volts DC, with the current flow (amps) being proportional to the light energy (photons). In most photovoltaic cells, the voltage is nearly constant, and the current is proportional to the size of the cell and the intensity of the light.

photovoltaic equivalent circuit

The equivalent circuit of a PV, shown on the left, is that of a battery with a series internal resistance, RINTERNAL, similar to any other conventional battery. However, due to variations in internal resistance, the cell voltage and therefore available current will vary between photovoltaic cells of equivalent size and structure, connected to the same load, and under the same light source so this must be accounted for in the solar panel assemblies you buy.

The silicon wafer of the photovoltaic solar cell that faces the sunlight consist of the electrical contacts and is coated with an anti-reflective coating that helps absorb the sunlight more efficiently. Electrical contacts provide the connection between the semiconductor material and the external electrical load, such as a light bulb or battery.

When sunlight shines on a photovoltaic cell, photons of light strike the surface of the semiconductor material and liberate electrons from their atomic bonds. During manufacture certain doping chemicals are added to the semiconductors composition to help to establish a path for the freed electrons. These paths creates a flow of electrons forming an electrical current which starts to flow over the surface of the photovoltaic solar cell.