Half Cut Solar Panels: Higher Efficiency & Better Shade Tolerance
Silicon solar cells of the type now used for almost every residential solar installation in Australia have been around for a long time. They are 64 years old. This makes them same age as nuclear power generation. They’ve come a long way since they were first made at Bell Labs in the United States. Originally they were only able to convert around 6% of the energy in sunlight into electrical energy but now the most efficient solar panels on the market manage 22%.
Unfortunately, the days of large improvements in efficiency have long been over. Now we only see small incremental improvements and one of these in use that will be much more common in the future are panels that use half cut solar cells.
Luckily, explaining what half cut solar cells are doesn’t involve complex scientific explanations involving quantum mechanics. They are literally normal solar cells that have been cut in half. Instead of having 60 solar cells, as most panels put on roofs do, they have 120 half sized ones. This results in lower electrical resistance that improves efficiency. An additional benefit is half cut panels resist the effects of shade better than standard solar panels. This isn’t directly due to the cells being cut in half but because of the way they are wired together.
While the increase in efficiency is only small, several large manufacturers are convinced modern production techniques make half cut solar cell panels worthwhile. At the moment REC Solar‘s TwinPeak panels are the only half cut ones widely available in Australia, but with many large manufacturers either starting to produce them or planning to we will soon be spoiled for choice.
How They Work
If you cut a solar cell in half it will produce half as much current and one fourth as much resistance. But you’ll have twice as many of them so, if they are wired up to operate like a standard solar panel, you will have the same current but with half the resistance. This lower resistance reduces electrical losses and improves panel efficiency.
But it’s not that simple because there will also be twice as many connections that will have their own resistance as well. All up, the decreased electrical losses can increase the output of a panel by up to 3% over what it would be with full size cells. That’s enough to bump a panel from 18% efficiency to 18.5%. In terms of panel wattage, it will increase a 300 watt panel to 309 watts.
A conventional solar panel typically contains sixty 0.5V solar cells wired up in series. Voltages add in series, so the solar panel operates at 30V.
If half cut cells were wired together as in a standard panel, they would produce half the current and twice the voltage. This would not be appreciated by installers using normal solar inverters or trying to stay within Australian standards for residential solar voltage.
To make them operate like standard panels they are wired together differently. There are 2 lots of sixty series-connected cells that operate at 30V each. These two 30V halves are then connected in parallel. Voltages in parallel stay the same, so the panel remains at the standard 30V.
Instead of having 3 panel cell-strings like a standard solar panel, the half cut panel has 6 panel cell strings making it a 6 string panel. Thanks to bypass diodes (shown in red below), one small spot of shade on a panel, caused by say a leaf or bird poop, will knock one entire cell string out of action, but not affect the others. Because the half-cut panel has more strings, the effect of partial shade is less severe.
On a roof without shade in an area where the birds are not particularly incontinent, this will make very little difference in overall generation, probably less than 1%, but that’s still an advantage. On a roof with a significant amount of shading I would expect a modest improvement in output. But it won’t be great because shade is basically solar panel kryptonite.
Hot Spots Not So Hot?
When one solar cell in a panel cell string is shaded, all the preceding unshaded cells can dump the energy they produce into the first shaded shaded cell as heat. This creates a hot spot that can potentially damage the solar panel if it lasts for a long time. Twice as many panel cell strings means only half as much heat, but as the shaded cell only has half the area to radiate heat as a normal cell, I’m not sure there will be much of an improvement. But the decreased total amount of heat produced should be less damaging to the panel so there is likely to be an improvement in resistance to hot spot damage.
Split Junction Box
Standard solar panels have one junction box that cables come out of located on the back of the panel near the top. Panels with half cut solar cells can have junction boxes that are split into three, as you can see in this picture of a REC Twinpeak half cut panel. The middle box is for the middle bypass diode.
At first I thought that since the junction box, or boxes, were in the middle of the panel it made no difference which way up it was installed. But then I was reminded that one cable is positive and the other is negative and it’s probably best not to confuse them. While there is enough length in the cables to cross them over if necessary, the thought of doing that gives me the creeps. It is not an elegant solution.
More Resistant To Heat?
I have read that half cut cells are more resistant to the effects of heat and so won’t suffer as much of a decrease in efficiency when they get hot. And they will get hot if they have been installed properly, which is outside in the sun. But this may just be the result of the slightly higher efficiency of half cut panels causing more more sunlight energy to be converted into electricity and less into heat. Looking at solar panel datasheets I can’t clearly see better heat tolerance compared to standard panels of similar efficiency. As information on more half cut panels becomes available I might be able to arrive at a conclusion.
Half Cut Panel Disadvantages
Half cut panels aren’t new. They’ve probably been around ever since someone accidentally broke some cells and decided to wire them together anyway. Solar cells used to be incredibly expensive so it must have happened a long time ago.
But despite having some advantages, panels with half cut solar cells also had some disadvantages that prevented them from being put into widespread production until now. These problems were:
- Higher cost
- Twice the potential for soldering defects
- Internal cell defects become doubly dangerous
Half cut panels have the expense of cutting solar cells in half with a “laser”.
The laser that is used doesn’t even cut all the way through. It only puts a groove in the cell and then it’s snapped in two. This is apparently cheaper than just using a bigger laser.