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We hear about cell sizes M2, M6, M12, M10. Is this race for size wise? What does it correspond to? What should we think about it? Does it have a future? Should we choose very large cells? For their part, the manufacturers are each in turn announcing increasingly powerful panels (their lower limit seems to be 600 Wp). Should we trust such powerful panels? These are a few aspects that are examined in the text below.

The different points discussed :

The format of the cells : There are a variety of cell formats on the market. It is getting bigger and bigger. Its diversity hinders the standardisation of production. No format is currently required.

What is the advantage of a larger cell format? The larger the size of the cell, the more sun and energy it receives. There is therefore a logic to using larger formats. In addition, larger panels reduce a number of production and installation costs.

But such formats create new technical problems Larger cells can create too much energy, which would damage the installations. The technicians found the solution by cutting the cells in two or three. This means that each piece receives less energy. The whole thing is saved.

As for the panels, larger sizes make them more fragile if the same architecture is retained as for smaller formats. We don't know if they will be able to withstand high winds and bad weather. In the same way, the power of the panels creates new technical difficulties.

What should we think of this race for size?The promoters of these large sizes (Trina Solar, JinkoSolar) are very confident. Some, including LONGi, are reluctant to propose large formats, insisting on the absence of resistance tests because these panels are very recent.

The engineers will certainly make these cells and panels reliable after having suffered technical setbacks. These large sizes are going to impose themselves in the landscape. Only, they are reserved for ground power plants. Two types of panels are therefore created, one for the power stations and the other for the roofs.

The text

The cell format

The evolution towards larger sizes has been gradual. In 2017, the standard size was still 156 mm (M1) or alternatively 156.75 mm (M2). In the second half of 2018, Jinko Solar will introduce the 158.75 x 158.75 mm size cell. At Intersolar 2018, the first panel with 166 mm x 166 mm (M6) cells is presented. At the time, this format was abnormal as it did not allow the equipment to be used in operation. It seemed more like a prototype than a new format for the profession. All production machines had to be modified to adopt it.

This format was recognised in the first half of 2019 when the wafer manufacturer LONGi recommended it. It believed it was the federator of the sector as it had been a few semesters before. This was not the case, because Zhonghuan Semiconductor sowed discord in the industry by presenting two or three months later a larger format (the M12) of 210 mm by 210 mm.

Since then, confusion has reigned in the industry. Some are still in favour of the M2 format; others the M6 format; some the M12 format. In an attempt to federate the sector, half a dozen manufacturers, including LONGi, launched an intermediate format in June 2020, the M10 with 182 mm sides. However, some manufacturers (notably Trina Solar, ...) have understood the interest of larger formats. They are betting on the gradual introduction of the M12 format by 2022 where, according to PV InfoLink, it should represent more than half of the production! The year 2021 will be the year of transition with still different formats on the market, with the M2 (156.75 mm), M6 (166 mm), M10 (182 mm), M12 (210 mm).

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What is the benefit of a larger cell format?

The larger the surface area of a cell, the more sunlight it receives and the more energy it produces. But then it captures too much energy and the cell size must be reduced to avoid disturbing everything.

At the same time as the cells were taking up millimetres, the panel manufacturers found that the cells could no longer fit into the previous dimensions. By placing the same number of cells, the size of the panels increased. They noticed that it was as simple and quick to produce a traditional 400 W panel (about 2 m x 1 m) as a 600 W panel (2.5 m x 1.5 m). Only in the first case does the usable surface area reach 2 m² and in the second case 3.75 m², almost twice as much for a slightly higher production cost. The calculation is quickly made for the producer who can then increase his prices which makes his very large panels very profitable.

Another advantage is that these large formats of both cells and panels are almost reserved for large manufacturers, the so-called Tier 1 manufacturers. Indeed, it is not enough to simply increase the number of cells and enlarge the panel. You have to review the rigidity of the whole, adopt the cutting of the cells in two or three, and know the gluing technique. Above all, it is necessary to modify the production equipment in order to be able to produce them. This is reserved for manufacturers who have the financial and technical means. All manufacturers who cannot follow, are marginalized and fall into anonymity.

The third advantage is that the energy production per panel is immediately increased and also the installation cost is reduced. Thus, in the example above, one large panel represents almost two traditional panels. This saves space (less land to rent), reduces wiring and junction boxes and reduces installation time because one panel is installed faster than two!

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But such formats create new technical problems

The first is the format of these new panels. Their rigidity and weather resistance must be maintained. Here, these larger consequences are easy to identify and conjure up. However, glass manufacturers used to produce 2 m² sheets, now they have to produce 4 m² sheets, which forces them to solve new technical problems. Above all, they have to change their production equipment, and probably also the thickness of the glass, which used to be 2 millimetres thick. Their rigidity was suitable for small surfaces; larger sizes make these thin glass plates more fragile. Bad weather can tear or destroy them.

If the rigidity of the panels is weakened by large surfaces, the fixings on the racks must be reinforced, the wiring that receives more energy must be re-examined; the junction boxes redesigned. Thus, the embrittlement of large surfaces (and cells) is increased compared to a small format. The difficulty is physical. It is not insurmountable.

The second is that the larger the size of a cell, the more sun it receives, the more energy it produces, the higher the electric current flowing through it, the greater the losses due to resistance. Here, manufacturers are concerned that this intensity could disturb or destroy the cell, hot spots or junction boxes. To reduce the disturbance, engineers have used the division of large cells into two or three, which are then welded together. This results in a reduction in series resistance and halves the electrical current in the cells (in half the surface area of a cell there is half the current, but due to the doubling of the number of half cells there is a doubling of the voltage).

The third problem is the arrangement of the panels : Either we reduce the number of panels connected to each other to reduce the amount of energy produced, but we increase the wiring, the combination boxes, the incidental costs, or we put the panels in a separate location.The number of panels connected to each other can be reduced to reduce the amount of energy produced, but the wiring, combination boxes and ancillary costs are increased. The larger the number of panels in series, the greater the loss of efficiency. Because of the increased energy produced, the risk of fire is increased.

Panel manufacturers are looking for the best solution

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What should we think of this race for the waist?

The use of larger cells and larger panels has created a formidable debate between those who are in favour of this novelty and those who are still reluctant. The latter underline the fragility of these new panels, the technical problems and even difficulties of transport and installation. They argue that these panels are recent and have not yet undergone the usual qualification tests. The most well-known manufacturer among the reticents is LONGi, which limits its panels to 540 W, while its competitors offer powers of 600 to 800 W.

As always for observers from outside the profession, the distinction between true and false, between the technically possible and the unreasonable, between financial benefits and costs are particularly difficult to make. While technical problems can always be solved as soon as they are located, there is no financial evaluation to determine the gain in production on the panels, or at the time of installation. Conversely, the additional cost of technical modifications and the cost of replacing production equipment is not known (although manufacturers are obliged to modernise very often).

Trina Sola is therefore well advised to state that "large cells and panels will be essential as soon as the equipment has been put into service". The inverter manufacturer Ingeteam confirms that "these panels will make it possible to further increase density and energy efficiency. They offer better costs per kWp. Baywa r.e. and Krannich believe that these panels are a step towards greater photovoltaic efficiency and that the trend towards large surface areas will continue.

It therefore seems that if the first intention of the manufacturers was to draw attention to them, to create a technical distance with manufacturers unable to adapt, or to create a new product while the production technology remains mono-PERC, they have re-examined the issue.They have been able to provoke novelty through the format while waiting for the production cost of new technologies (heterojunction, TOPCon, IBC, etc.) to decrease and allow their generalised adoption. On the other hand, these large sizes are reserved for ground power plants. They cannot be used (unless the size of the panels is adapted) for residential installations. Therefore, this increase in size has above all the merit of creating two types of products, one for power stations, the other for roofs, whereas until now, the same panel was used!