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Advantages Of Cut Cells For Solar Module Encapsulation
Jan 30, 2019

Why cut cells in half

 

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Most of the advantages of half-cut cell panels is attributed to decreasing by half the internal current of the panel. Halving the current lowers the resistive losses, which is why the performance is enhanced. This results in many benefits like higher output, better production per m² and better performance in heat. Shading benefits, better durability and some other performance gains are also realised with the necessary revised panel configuration.

 

Reduction in resistive losses

In a solar module, power losses occur as the electrons travel through the cell interconnections and bus bars. Since power loss equals resistance multiplied by the current squared (P loss = R x I²), a reduction in current would reduce the loss. Splitting the cell into two halves the current (not the voltage) of the cell, so when you apply this change to the equation the losses are reduced by 75%. As current is highest in peak production times, this is when the benefit is greatest. Reducing current to reduce losses is nothing new, we’ve done it for over a century in power transmission. However having twice the amount of half current cells doubles our voltage, which would have unwanted consequences on system design. This is solved in the revised panel configuration.

 

Standard 60 cell vs 120 half-cut cell panel

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The new, better configuration

To understand how this works, you need to know the following:

  1. Adding cells in a string (series) accumulates voltage, not current

  2. Adding a second string of cells (in parallel) accumulates current, not voltage

So if the 120 half-cut cells were wired in one string, we’d have twice the voltage and half the current of a normal 60 cell panel. To fix this, manufacturers have redesigned the cell layout to have two strings of 60 half-cut cell panels joined in parallel. The overall result is quite clever, as the voltage and current coming out is identical to a standard 60 cell panel, but the internal current is halved. This results in a 1.5-3% increase in efficiency, which is more profound than it sounds. It also has some desirable side effects.

 

Shading handling improvements

As mentioned, the change in layout allows the panel to perform better under certain shading scenarios. Before digging into this, please note a couple of things:

 

  1. Shading is still going to have a significant impact on your system, even if it’s the scenarios favoured by these panels.

  2. The panel  may behave differently on its own than it would in a string or with an MLPE device (Module Level Power Electronics, like micro-inverters or optimisers).

One of the things the manufacturers push is the ability for the top half of the panel to perform unaffected if the bottom half is in shade, or vice versa. To understand this, we need a quick refresh on shading.

 

Why shading can be managed better on a half-cut cell panel

When you have two strings connected in parallel (like the top and bottom half of these panels are), you can isolate the lower current cell to just that side. So one half can be producing at 10% capacity and the other producing at full. This is quite handy, but it comes with a drawback.

Remember my comment, "The panel may behave differently on its own than it would in a string or with an MLPE device"? This is why it's important.

Let's say you have a string of 10 panels (quite common), on a string inverter, all in perfect sunlight - except one panel which has complete shade on the bottom half. In this case, that panel could produce at 50%, but then so would all the other panels. This is not ideal. The inverter's MPPT won't let this happen though. Instead, the current will remain high and the bypass diodes on that panel will activate and bypass that entire panel.

If you had an optimiser or micro-inverter in the above scenario, it's a different (better) story. That panel could then produce at 50% while the others continue unaffected.

 

Here's an illustrated version: 

Two Scenarios

Both have 10 half-cut cell panels in a string, using a string inverter, varying shade conditions.

Scenario1, suppose 90% shading bottom half of one panel (as illustrated)

Bypass diodes on panel 1 will activate

Despite the half-cut cells, the system is still better off dropping one panel entirely than having a lower current. See below a rough and simplified overview of why. Note:

1 power(P)=Current(I)xVoltage(V)

1 Let’s say panels is producing approx. 30 V and 9 Amps

3 Voltage increases when you odd panels in a string, current does not string operates at lowest current.

Option 1 – Diodes active, drop panel 1 entirely:

P= 9 amps x 270 Volts (9 panels@30Volts), P=Approx. 2430 Watts

Option 2 –Diodes inactive, reduce current of all panels:

P= 4.95 amps x 300 Volts (9 panels@30Volts), P=Approx. 1485 Watts

Scenario 2, suppose 90% shading on bottom of all panels

All bypass diodes will remain inactive

This is where half-cut cells are excellent. Bypass diodes would not active and production would be option 2 above. With a standard panel, almost all production would have been lost.


Cut Cells For Solar Module Encapsulation 7

In the two scenarios, one where where half-cut cells won't help and the other where they will help immensely.


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    E-mail:info@dsneg.com

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