ETFE and PET: Two key materials in the field of semi-flexible panels

Published : 05/08/2017 09:59:22
Categories : Photovoltaic Panels

In the field of semi-flexible photovoltaic panels, which are widely used in the nautical field, as in any other situation where the available space or the permissible load load are two very stringent limits, the choice of materials that will coat the photovoltaic cells is perhaps the most important aspect in characterizing the mechanical properties of the panel.

In fact, postponing later in the article the discussion about the type of cells present and their ability to influence the efficiency of the panel and consequently its size according to the desired output power, the coating materials mainly distinguish this product category in two distinct types: 7-layer semi-flexible panels and 5-layer flexible panels.

The most widespread model on the market, that is the 5-layer model, can be schematized in its structure b the following image:

As you can see, there are not many layers of cell protection: the first is the EVA, a plastic copolymer very common in the industry, that is used in the photovoltaic sector for PV cells encapsulation through a thermal lamination process of the cells to make them completely isolated from any contact with liquids, powders, vapors or other gases.

In the outer layer is added the PET, another plastic polymer widely distributed in virtually all fields of industrial production, which in our case is used as an outer coating for flexible panels since it has good mechanical resistance, malleability during production and keeps its flexibility once worked.

Given the degree and duration of the protection offered by the usage of these materials, it is difficult to suggest indiscriminate use in areas with constant or persistent adverse weather conditions. It is well known that PET has a low thermal resistance in the short term and therefore usage in very hot environments is not recommended. In addition, PET is unable to withstand chemical attacks caused by the various acid and alkaline compounds dissolved in the water which will, in the long run, cause yellowing and then delamination or breakage of the plastic layer.

PET PANEL DELAMINATION

The strengths of this type of product thus focus on the flexibility of the panel, its low weight and small dimensions (for both panel and the junction box) and above all in the economical saving due to the use of widely used and easy to fabric plastic polymers.

However, if we look at a field of application such as the nautical one, we realize that these properties, though surely beneficial, are not sufficient to guarantee a high durability of the product.

At sea, exposure to deteriorating agents is practically constant, and in some cases physical stress caused by a very rough sea conditions can also cause damages to the photovoltaic cells if they do not have adequate protection.

In order to meet such strict resistance requirements while trying to preserve the flexibility and compactness of the panel, a 7-layere structure has been conceived, schematized in the drawing below:

Leaving aside the polymer layers that we have already found in the 5-layer panels, this product differs from the first by the adoption of ETFE as additional outer protective layer. ETFE is a fluoropolymer, or a polymer (a macromolecule made up of a chain of the same molecules) that contains fluorine atoms. The molecules that composes the polymer are chemically very stable and therefore can withstand high levels of thermal stress and chemical aggression, much more than PET can do. Another fundamental feature of ETFE, which makes it an excellent polymer for this field of application, is its total UV permeability: numerous tests carried out also on outdated samples (to prove the durability of this feature) show a permeability of 95 % (Compared to an average of 80% of PET) with a direct light percentage of 88% and an irradiance ranging from 400 to 600 nm of the visible light spectrum.

Not only is ETFE a durable and "UV-friendly" material, but is also "intelligent": thanks to the chemical properties of its molecules, ETFE is a self-cleaning material that maintains its transparency for its whole life cycle, which in optimal conditions can reach even 20 years (Hightex data). Another sign of its "intelligence" lies in the environmental sustainability of this polymer: in fact it is 100% recyclable. Even its production process respect the environment, as it does not require the usage of chemical solvents or petroleum products and respects the Montreal Treaty, so it does not fall into the group of materials that damage the ozone layer of the atmosphere. Finally, unlike polymers such as PET or TPT, ETFE is also recommended for usage in particularly hot and dry environments and climates due to its low flammability given by the high flame-retardant factor.

In conclusion to the 7-layer structure of the FLY SOLARTECH panels there is a glass fiber back film that is drastically more resistant to mechanical stress than the PET + TPT combination and can exert a heat-drowning effect on the cells above, in addition to confer greater stiffness.

However, the usage of very versatile and refined polymers is not the only difference that distinguishes the two types of semi-flexible panels: although in both types of panels are used the same photovoltaic cells from Sunpower american company, it is good to keep in mind That there are 3 distinct variants of these cells based on their percentage content of silicon. The first and most expensive category is A, which stands for a minimum silicon content of 98% and a cell efficiency of more than 20%; Only this category of Sunpower cells is adopted in the 7-layer FLY SOLARTECH semi-flexible panel. Next, in order of cost, there are then the "B" and "C" cells containing silicon percentages from 92 to 96% which can deliver an efficiency ranging between 15% and 18%, with a consequent 17% increase of the surface required to match the output power compared to A-series cells. Normally, these lower class cells are used in the semi-flexible 5 layer PET panels.

It should be clear that, apart from the structural differences mentioned above, as we have seen both types of panels have their advantages and defects, both economically and practically, and it is up to the buyer to evaluate his choice according to his needs. While it's true that the cost of an ETFE panel is 60% higher than the equivalent of PET at the same power output from the panel, it's also true that the PET version requires additional care and attention you want to extend its useful life as much as possible. Both products therefore are, within their optimal application field, qualitatively very good because both have both CE and RoHS safety certifications.

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