What's the difference between Whole solar cell and Cut solar cell

As solar cell technology updates rapidly,crystalline solar panels are no longer confined to massive solar arrays.

These panels are now more plentiful than ever and come in an endless number of configurations.

 

There are incredibly inexpensive options that are attractive at first blush, but when you dig deeper the quality of these panels and their assembly process leave much to be desired.

 

AHONY utilizes SunPower cells in crystalline application solutions,like glass rigid solar panel,semi flexible solar panel,foldable solar kit etc,as they are the highest efficiency and most reliable cells available now.

 

SunPower produces, tests and grades whole solar cells. They do not make 1/2, 1/3 or 1/6 cut cells. Other panel assemblers and third-party companies will take SunPower whole cells and make 1/2 cut, 1/3 cut and 1/6 cut cells.

 

The disadvantages of using cut cells include loss of efficiency, induced cracks, lower interconnect reliability and lesser long-term stability.As solar panel factory,the producing process will be more complicated:

1. double,3times or 6times connecting points,workers should takes longer time to make;

2. Special location to make,the workers should be more experienced to test the cell connecting,if cell crack meanwhile connecting,AHONY will use the on process EL test machine so can observing if cell crack and replace at once.

3. more precisely cell layout to avoid cell move after lamination.So more tooling are needed.

The advantages of using cut cells include flexibility in output voltage and form factor.There are two pictures of foldable solar kit eMobi F3*40w.

             

pic1: Whole cell solar module,3 panels in 1 string            

pic2: Cut cell solar panel:each modle 1 string,3 panels parallel

From cell itself power side,most customers will choose whole cell,not cutting,no power loss,looks more aesthetics.But considering actual use,the result will be on the contrary.

 

Disadvantages of Cut Cells

 

Loss of Efficiency

 

SunPower produces a 23.7%-24.2% efficient 3.6-3.74w watt cell, which is tested before assembly to ensure it meets specification and is well matched in performance to other cells in the panel.

 

When a solar cell is cut the active area of the cell decreases, due to the kerf (width) of the laser cut, typically 0.05mm. Based on the kerf of the laser used to cut the cell the remaining active area will be about 99.6% of the initial. That reduces cell efficiency from 23.7% to 23.6%. This is a small decrease.

 

For a full cell with two cuts, creating 1/3 cut cells, there will be four defect regions, one on each side of a cut. Taking the 0.05mm *4 for each side of the two cuts the active area is now 99.3% of the total area left. This reduces the cell efficiency to 23.54 %.

 

Mostly based on easy to calculation,we cut cells actually cut the cell into three equal width pieces and do not account for the area missing due to the cropped corners of a full cell. Since the cells are series connected this creates a current imbalance between cells based on cell active area and limits the power output of the cells to the smallest one. The loss of effective area due to cutting the cells in equal widths instead of equal areas is about 0.9%. This reduces the cell efficiency to 23.18%.  

 

These theoretical losses have proven to be significantly greater in field testing. Measuring the output of each of the 1/3 cells in a solar panel shows that the cut cells produce significantly less power than their equivalent full cell.

 

Each of the cells in a panel is connected in series, so the lowest performing cell will limit the current output of the entire array.

Matching cell performance is required to optimize overall panel performance. A weak cell cannot be overcome by putting a few exceptional cells in a series string of cells. A 36 cell string with two to three cells producing less than 0.5 watts cannot be fixed by adding 10 cells producing 1.1 watts.

 

For example, a 36 cell string of 1/3 cut cells will have a power point voltage of 18V and a power point current of 1.9A, however, if one of those cells is only 0.7 watts the current of the panel would be limited to 1.4A at 18V.  At 18V and 1.4A the power is 25.2 watts instead of 34.2 watts. This makes the panel only 14.4% efficient.

 

If cost is a major concern then a supplier of such panels will receive similar lots of cells and potentially use some lower performing cells in finished panels.  

The calculation seems complicated,based on producing experience,cutting cells will makes panel 1-2w decrease for 120w solar panel.

 

Induced Cracks

 

The lamination process places extreme pressure on the cells at a fairly high temperature (140-150C). Any edge defect on cut cells can induce a crack during the lamination process.

 

Most of these cracks are impossible to see with the human eye. Pictures of cracks can be taken by EL machine,which is professonal to test

solar module defects,when the cells is good,the EL shows white,the cells has lower current or have inner crack,there will be a black line or the whole cells shows dark/black.

The calculation seems complicated,based on AHONY producing experience,cutting cells will makes panel 1-2w decrease for 120w solar panel,within 2%.

An infrared picture of a 32 cell panel (purchased online) is shown in Figure 1. This picture shows the defects that were induced mostly during the cutting and laminating process.

 

This was a 26 watt panel mounted on a steel frame that produced only about 20 watts at full sun. At 20 watts the panel's efficiency would only be 16.9%, not 22% as stated on the seller’s website. A picture of a PowerFilm crystalline silicon panel using whole cells is shown in Figure 2.

 

Figure 1: 26 watt panel of 1/3 cut cells. Dark areas show cracks or shorted cells.

Figure 2: 24 watt whole cell panel, no cracks after lamination.

 

Figure 3: 125 watt whole cell panel, no cracks after lamination.

Interconnect Reliability

 

A standard SunPower cell has three contact points on each end of a cell. As Chinese producing developed,more and more tooling are used for producing,at the same size,interconnect reliability are strengthened too.

 

When you cut the cells down to 1/2 cells you have 2 connections per cell, at 1/3 and 1/6 you have only one connection per cell. With one connection per cell, if any of the connections fail in the field the entire panel fails.

In addition, the SunPower “dog bone” connector is designed to provide a low-stress connection to the wafer and if manufacturers do not use the SunPower "dog bone" then undue stress is placed on the connection, which can increase interconnect failure.

 

The "dog bone" has cut out reliefs that allow expansion and contraction from cell to cell without adding mechanical stress to the cells contact points.

 

In addition, 1/2 and 1/6 cut cells will have the one set of contacts cut in half, reducing the size of the soldering pad. Although less critical on 1/2 cut cells this becomes even more significant on a 1/6 cut cell’s interconnect reliability.

Long-Term Stability

 

All the glass panel have 25years output guarrantee,actually full cells solar panel has longer lifespan.

 

Besides lamination, the greatest chance for edge defect induced cracks is due to thermal cycling and thermal shock over the lifetime of a panel.

 

To ensure long-term stability, thermal cycling and thermal shock tests should be performed on cut solar cells.

 

Cut cells advantge

1.Customize voltge

As refered,cut cells either SunPower or other PERC,HJT,bifical cells can make different voltages,power,this will help customers make different projects.That's why more and more field use solar panels to integtrated to their products,like solar bench,solar blinds,solar camera.

2.avoid shading and get maximum power

SunPower cell has the best weak light performance at the moment.

For some application like bimini,RV,when there are cloud or tree shade it,the regular whole cell panel voltage will decrease sharply.

If a panel are made into cut cells,the panel will do parallel,even part of it are shaded,the other part can still keep the voltage high,then do not decrease the whole system voltage.The power will deccrease much less than whole cell system.

                         

                                                   Pic 3:half cut glass solar panel                   pic4:half cut semi flexible solar panel

For example,if use the 200w 20v solar panel to charge 12v battery,when the panel is shading 30% of the panel,the panel can not charge battery anymore,as voltage is too low.For the half cell module,when the 50% of top area are shade,the panel can still charge batteries.

As producing technology is developed,now the half cell solar modules are more and more popular.