ETFE – Ethylene Tetrafluoroethylene as superstrate: design and applications
First a bit of linguistics. What is meant by superstrate literally and how was this word coined? Well, super is superior or upper, and “strate” is derived from stratum or layer. So, superstrate would mean an upper layer. When PV engineers are talking, superstrate should mean the upper layer of a solar panel. That does not, of course mean that vertically mounted solar panels have no upper side and hence no superstrate! The outermost layer facing the sun is the superstrate.
Why A Superstrate and What does it Do?
The superstrate in a solar panel must do certain functions.
- Transparency: It must be transparent to solar energy so that it allows sunshine to go through and meet the solar cells which are sitting behind and waiting to extract some energy. Glass is a popular material for a superstrate, but one serious disadvantage is the weight.
- Weather Protection: The Superstrate must provide protection to the electronics underneath against elements of weather:
- Penetration of water and water vapor. Water is enemy of most engineering materials although it may be an inherent part of them.
- Bending and flexing by wind. Wind may be particularly strong in some localities. Bending and flexing of the panel can result in hidden microcracks which reduce panel output.
- Protection Against Abrasion: A superstrate must protect the panels against damage by sand and dust. This becomes especially important in deserts and open areas where solar farms are likely to be. Wind speeds are likely to be high in such places. High-speed sand blasting can destroy the panel if not protected by the superstrate.
- Protect against traces of chemicals present in the atmosphere due to nearby industrial activity.
- And electrical insulation, of course.
- It must withstand ultraviolet irradiation
- Be physically tough to resist like scratches, bumps etc, which may occur during installation and maintenance.
- Withstand temperature and large variations of temperature. The superstrate material must remain stable at temperatures in the range between -40°C to +85°C.
- It should be preferably self-cleaning (non-stick) which means it should not readily collect dust smoke, silt, snow etc.
- It should be eco friendly and be recyclable as far as possible.
- And it should be (like most stuff and ladies!) as light weight as possible.
Glass is Excellent But!!!
Glass has most of these attributes but suffers from weight issues. In addition to being heavy, it is also inflexible. It is not suitable for uses where flexibility is required or where excessive transportation is required. As flexible and more portable PV panels are now increasingly in demand, the weight and inflexibility features of glass becomes a handicap. The currently favorite alternate for a superstrate is Ethylene tetrafluoroethylene (ETFE) is a fluorine-based plastic.
Ethylene tetrafluoroethylene (ETFE)
Ethylene tetrafluoroethylene (ETFE) is a fluorine-based plastic. It is produced by many chemical companies and sold under various brand names. DuPont offer it under the name Tefzel, Asahi Glass Company as Fluon, Daikin as Neoflon ETFE, and Vector Foiltec sell under the trade name Texlon. ETFE has been in use for buildings. What made it so popular in the PV market recently? One reason is that it has most of the attributes of glass. To list:
- Transparency: ETFE transparent material offering a light transmission coefficient approximately 85-95%.
- Weather-Proof: It prevents water or vapor penetration. Withstands weather effects. It has been claimed that accelerated test of exposure equal to 30 years showed little signs of deterioration. In fact, the oldest construction using ETFE is said to be in Europe and is almost four decades old now.
- Tensile Strength: ETFE has good mechanical strength. Tensile strength is approximately 42 MPa (6100 psi),
- Electrical Insulation: It is a good insulator for electricity with a dielectric strength of 59 kilovolt / mm or 59 Mega Volt /meter.
- Ultraviolet Radiation: Very good resistance to high energy light (ultraviolet). (For reference, please see “Fluoroplastics, Volume 2: Melt Processible Fluoropolymers-The Definitive User’s Guide and Data Book, S Ebnesajjad– 2015”). One reason for this is that it does not absorb ultraviolet light and passes it through.
- Wide Temperature range: It has a wide enough working temperature range for solar panel applications. Operational temperature range extends from −185 °Cto +150 °C, which is more than sufficient.
- Eco Friendly: Ethylene tetrafluoroethylene ETFE is 100% recyclable, which is a great attribute in this age when disposal of unusable stuff is a major consideration. Non-recyclable and non-biodegradable stuff simply add to pollution and litter the earth. Also, the production process of Ethylene tetrafluoroethylene is water-based. That means it does not require the consumption of either chemical solvents or petroleum derivatives. Thus, EFTE production does not deplete the ozone layer.
- Weight Advantage: ETFE is much lighter than glass. The specific density of glass is 2.5. That means a slab of glass weighs 2.5 times as much as a mass of water having the same volume. Thus, a flat sheet of glass having an area of one square meter just one mm thick will weigh 2.5 Kg. In comparison, the specific density of Ethylene tetrafluoroethylene is 1.7. Hence, ETFE sheets of equivalent thickness will be nearly 1/3rd lighter. Add to it the fact that ETFE sheets used are much thinner. Thus, ETFE has a many-fold weight advantage over glass.
- ETFE films are self-cleaning. That is, they tend to shed liquids dropped on them. That is a great advantage, particularly in areas where rainfall is frequent. A layer of water sticking to the film after rain will not only affect energy capture but will allow atmospheric dust to bond to the surface. This will reduce light penetration further and demand physical cleaning.
- Thermal Coefficient of Expansion: Ethylene tetrafluoroethylene ETFE has a reasonably low linear coefficient of thermal expansion, i.e., 9.5 x 10-5 per degree centigrade
- Fire Rating: ETFE Films are rated as self-extinguishing with no burning drops under different international standards.
- Nothing in this world can be perfect, and the wonder material ETFE also has limitations.
- Scratch-resistance: ETFE layers will not be able to compete with glass as far as scratch performance is concerned. Scratches are quite likely during installation and maintenance.
- When Burned: ETFE does not catch fire easily, but when it is burned, it releases hydrofluoric acid which is highly corrosive. Extreme caution is necessary in operations where this acid may be given out. However, when used in solar panels, there is little chance of a fire hazard given all the protective arrangements in vogue. Further, Ethylene tetrafluoroethylene ETFE melts at about 270 degree centigrade and the film will split and shrink away from a hot spot thus eliminating chances of catching fire.
- Flexural Strength– Glass is rigid and will not allow much flexing. In contrast, ETFE sheets are very flexible. They can be rolled and stretched. Hence, they will not be able to provide the PV cells any protection against flexing stresses.
Weakness Turns an Advantage
The lack of flexural strength in Ethylene tetrafluoroethylene as superstrate is a weakness in the context rigid PV panels. But it is precisely the desired property for use in many other applications.
- Electrical Wire Insulations: High quality fire-resistant insulation for wires in critical systems like aircraft cabling have been using Ethylene tetrafluoroethylene ETFE for long.
- Roofing material in Outdoor Structures– ETFE single or multiple layered sheets are popular as a transparent, strong, fire-resistant and light weight roofing material in outdoor structures like greenhouses, stadiums, and other large buildings. It is claimed that ETFE roofing has just 1% of weight of glass over the same area.
- Flexible Solar Panels: The latest application for ETFE is as superstrate in flexible PV panels which are gaining popularity in multiple applications. Glass, being rigid, is just unfit, and there is no other real match for ETFE as superstrate for flexible panels.
Flexible Solar Panels
Flexible photovoltaic panels are being increasingly integrated into building designs, reducing costs and improving energy performance. This practice is broadly called building-integrated-photovoltaics (BIPV). Flexible photovoltaic panels are also considered good for portable solar power. Panels are available in the market which can be rolled. Vehicle windows could also use flexible solar panels. Future applications are unlimited, and these may find applications eg in aircraft windows or even personal wear.
Ethylene tetrafluoroethylene ETFE is a very versatile material. It has multiple uses outside the solar PV domain.
- ETFE is very suitable for outdoor architectural structures because of its response at ultraviolet as well as IR frequencies. It is valuable because of its high transparency, light weight, chemical stability and strength. Although very transparent it can also be used as solar control material by changing color with additives.
- In the automobile field it is used in construction of gaskets, O rings, and seals. It resists permeation by petroleum and derivatives and hence is used in lining of hoses and tubing of transmission, power steering, and fuel.
- Because of its insulation properties it is used in electrical cable and wire jacket and insulation, and in wire coatings. It is also used in coil formers, battery parts, and in switches and sockets as insulator.
- It is used as lining in many parts of machines.
- In the medical field it is used as parts in centrifuges, respirators, evaporators, blood analyzer valves, etc.
- It is also used as film in the food and pharmaceutical industry, and as heat shrinkable tubing and in electroluminescent displays in the relevant industries.