Advantages of Half-cut cells
EcoSoch adopted solar panels with half cut cells for the 100kWp project at NTTF, Dharwad due to the benefits offered by this technology. The panels were Waaree 400Wp Mono (WSD-400) which has 144 cells in each module. Now let’s understand the advantages of using half cut cells (a.k.a twin cells) for projects.
They are literally normal solar cells that have been cut in half. Instead of having 60 or 72 solar cells, as most panels put on roofs do, they have 120 or 144 half sized ones while keeping the same design and dimension as conventional modules. 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.
Several large manufacturers are convinced modern production techniques make half cut solar cell panels worthwhile and as per the technology roadmap of PV, twin cell solar market share is expected to increase from 18% in 2018 to 40% by 2028.
How They Work
If a solar cell is cut in half, it will produce half as much current and one fourth as much resistance. However, there’ll be twice as many of them. So, if they are wired up to operate like a standard solar panel, the current would be the same but with half the resistance. This lower resistance reduces electrical losses and improves panel efficiency. Thus half-cut cells divides the internal current in the system and improves its performance, longevity and shade tolerance. As power loss is proportional to current, half-cut solar panels reduce the power loss by a factor of 4 which increases the power output.
Superior price performance
It not only cuts the cells in half but also slashes the overall cost of a solar system, ensuring lower LCOE (Levelized Cost of Energy). Modules with half-cut solar cells have the potential for quicker payback periods as compared to any traditional counterparts. Particularly for sites where shadow cast and limited area are the main constraining elements, it makes the installation even more worth the upfront cost. It is a cost-effective solution where you do not have to compromise on the quality and output.
A conventional solar panel typically contains 60 numbers of 0.5V solar cells wired up in series. Voltages add in series, so the 60 cells 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 (see image below). This would not be appreciated by manufacturers of solar inverters who trying to stay within the 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 like below:
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 dropping) will knock one entire cell string out of action, but not affect the others. Since the half-cut panel has more strings, the effect of partial shade is less severe.
A half cut solar panel has 6 separate cell-strings (but only 3 bypass diodes), offering better partial-shade tolerance. If half of the panel is shaded (e.g the LHS), the other half can still operate.
Less hot spots
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 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. The lower heat production should be less damaging to the panel so there would be an improvement in resistance to hot spot damage and increases the module longevity.
In twin cell panels, the bypass diodes restrict the loss of power from the shaded portion instead of the entire panel. It creates a substitute path for the current to travel in the unshaded part and avoids the current to pass through the shaded part. It reduces the effect of shading and increases its performance in partial shadow conditions as shown below:
Reduced power loss
Resistive losses are one of the reasons for the power loss in the solar system. To fight this issue, twin cell panels enhance the current transport in the panel. The 5-bar bus design restricts the movement of electrons in the circuit, which increases the transmission and reduces the power of internal resistance. Due to this, the rate of resistive losses is way too low.
Use of half-cut cells can offer a big advantage and improved performance in shaded conditions where the shading across a string is generally even, for example where caused through obstacles such as other rows or walls, as long as the modules are correctly positioned to the path of shading. Aligned with other energy generation advancements in half cut cells, a significant increase in overall energy yield is achievable, making the new panel a better option for energy production on rooftops.