By Dr. Sally Humphreys
A wide variety of polymers is already in use including EVA , PVB, silicone, fluoropolymers (such as PVF, PTFE, ECTFE and ETFE), PMMA, thermoplastic elastomers, EPDM, polyamide and PET. The cost per kWh is coming down due to factors such as vertical integration of the supply chain and thus the potential for the module industry is enormous, particularly in sunny regions.
Materials in Demand
Dr. Mohan Narayanan, Vice President of Technology at Hanwha SolarOne in China has reviewed the use of polymers in modules used for rural electrification. Nearly 1.4 billion people across the globe currently have no access to electricity. However, by 2020 it is predicted that 750 million rural customers will be supplied up from 7.5 million in 2007. The barrier to this development is the low disposable income and the upfront investment cost. In India, for example, current per capita use of electricity is low and expected to rise so that by 2020 another 280 GW will be required: solar power is one possible solution as there are over 300 clear sunny days each year. It will require government support for this expansion to be achieved. The demands on materials would be high: encapsulant would need to be highly transparent for maximum efficiency, water repellent, chemical resistant, have low permeability to water vapour, excellent adhesion and flexibility, and perform consistently across a wide temperature range.
Standards & Performance Testing
The first IEC standard for crystalline photovoltaics was published in 1993. In 2010, the first standard for back and front sheets was developed alongside the standard IEC 61730-1, which covers module components. Several aspects are tested including the Relative Thermal Endurance Index (RTI), the flame spread index and weathering resistance. TUV Rheinland is involved in the development of standards and performance testing.
Panels for Rooftop Installation
Many roofs on commercial buildings in the U.S.A. are unsuitable for solar panel installation due to the weight bearing load required, so new lightweight crystalline silicon technology is being developed. Fluoropolymers can replace the heavy glass front sheet: they are lightweight (as low as 0.1 kg per square meter); ETFE and FEP offer excellent light transmittance, and are resistant to chemicals, humidity, light and heat. A rigid reinforcement is required where glass is not used, for example, a rigid back panel. Saint-Gobain Performance Plastics supplies these front sheets: ETFE is already used as transparent roofing structures and has been subject to extensive testing including hail and cut resistance.
Solvay Solexis is another supplier of fluoropolymer materials and has worked on the development of solar films with Ajedium Films (a company division). There is a new transparent grade of PVDF with potential for backsheets and frontsheets, while ECTFE has potential in front sheets including for UV blocking films. A typical backsheet is a multilayer structure with PET and tie layers.
Toray Films produces PET-based backsheets as an alternative to fluoropolymers, which dominated the market in the 1990s. Backsheets need to offer electrical insulation, mechanical strength, UV and weathering stability. In this case, different layers of PET materials provide the different properties required. These Lumirror films have been used in Japan since the 1990s.
Acrylic materials have a role in photovoltaics too: PMMA from Evonik Rohm is already proven for outdoor applications from automotive glazing to signage under the brand name Plexiglass. The light transmission properties can be adjusted to optimize solar module performance. It has been field tested, for example, for more than 12 years in an Amonix Concentrated PV (CPV) system, and in protection of CPV lenses for over 17 years. At Intersolar 2010, the use of Plexiglass in frontsheets was demonstrated in a lightweight module measuring 4.5 m by 1.5 m.
Polyolefin-Based Coextruded Sheet
There are new backsheets under development including the polyolefin-based coextruded sheet from Renolit with integrated adhesive. It can also be combined with EVA as an upper encapsulant. Renolit’s waterproof membrane production has given the company extensive experience of weather-exposed polymer materials. Many backsheets are laminates, this one is coextruded with a reactive PE face for adhesion to module components; soft encapsulating PE layers (including functional fillers like flame retardants); a connecting layer; a PP layer with high concentrations of functional fillers such as reflective pigments; a PP layer with improved heat distortion and a surface treatment or primer layer.
3M supplies adhesives to the solar module industry, such as bonding for junction boxes, cell positioning tape and acrylic foam, frame-bonding tape. It also supplies fluoropolymer backsheets, which are UL and IEC certified.
Sealing is an important aspect of module durability: moisture ingress can cause delamination, leakage of current, discoloration and corrosion. SAES Getters provides sealant tape, which can give 3,000 hours of damp heat stability in thin-film CIGS modules. The breakthrough time is the time required for moisture to break through the barrier sealant. Active barriers based on chemical getters have higher breakthrough times and lower permeation rates.
Encapsulation materials should be transparent, provide cushioning and impact properties, electrical insulation and a high moisture barrier. EVA is the most commonly used polymer and has been in use for over 30 years. More recently silicone rubber, PVB, ionomers and TPU have all been used as alternatives. There are special considerations for the extrusion of encapsulant polymers, as studied by Davis-Standard. A typical EVA encapsulant has a high VA content (33%) and contains additives, which must be correctly mixed: the extruder should have a corrosion-resistant liner and screw, provide adequate torque for low temperature processing and an L/D size which balances residence time against mixing requirements.
Suppliers Meeting Demand
The rise in demand for materials from the solar power industry has caused suppliers to divert materials and expand. For example, USI Corp. in Taipei is diverting EVA to the solar industry and Repsol is expanding its EVA production site in Puertollano, Spain, while in the third quarter of 2010, DuPont announced increased production of PVF in North Caroline for Tedlar film for backsheet applications.
The Fraunhofer-Center for Silicon-Photovoltaics (CSP) investigates PV reliability and has examined manufacturing processes such as encapsulation, for example, looking at the temperature profile of different components during vacuum lamination. EVA crosslinking is an exothermic reaction and consumes crosslinking agents: For homogeneity of films, the additives must be mixed in thoroughly and ‘gently’ in the extruder. Ideally, the pressing stage of vacuum lamination should be completed at the gelation point of the EVA. One other aspect that the Fraunhofer CSP investigates is the load-bearing ability of modules as the current test, IEC 61215, is carried out at room temperature, whereas snow loading occurs at much lower temperatures. When stress is applied on the front glass, the polymer layer around the cell can influence laminate stiffness, creep and other aspects. The temperature-dependent behavior of EVA, PVB and Tectosil has been examined.
Specialized Technology Resources, Inc. (STR) has been developing a new High Light Transmission (HLT) range of EVA encapsulants to improve PV efficiency. These have been tested in accelerated aging conditions, using Xenon Arc Weather-o-meters for example, over 30 weeks, and the second generation EVA has been incorporated in 8 modules from 3 manufacturers in field tests in Tempe, Arizona. There were different results from different module manufacturers due to factors such as failure of other components leading to encapsulant deterioration, for example, corrosion in a junction box. The new EVA is faster curing, which can help speed up module manufacturing. In late 2010, a customer achieved the lamination of 4 modules in 3 minutes 15 seconds at 155C using an XL laminator.
Solutia Solar supplies EVA, TPU and PVB and has opened an EVA plant in Suzhou, China. It has developed a new grade of PVB encapsulant to overcome problems of solar panel discoloration, which occur when PVB is exposed to damp heat and electrical bias while in contact with silver-coated glass. The discoloration is due to a silver compound, not to polymer degradation. The new Saflex PS41 shows much less discoloration and passivates the silver surface, it has no voltage-dependent change in color and actively protects metals, like copper, under test conditions.
Machinery suppliers are looking to improve encapsulation, like Meyer Burger Technology (incorporating 3S Modultec). The durability of a module is linked strongly to the encapsulant and there are advantages and disadvantages of all of the different materials available. For example, PVB is proven in window technology but has few suppliers, moisture uptake and some transmission issues; silicone is expensive but has a simple lamination procedure, and another alternative, polyolefin, is widely available but it is difficult to get good adhesion. EVA itself has a matching refractive index to glass giving good transmission, availability, good mechanical strength, known processing, but has a poor moisture barrier, outgasses during processing, and suffers degradation.
Huntsman has introduced several thermosetting materials for PV including a transparent, liquid encapsulant, a liquid dielectric encapsulant/backsheet, and an electrically conductive adhesive. The company is working with Caiten Eco Energy in Austria on experimental modules. The liquid encapsulant plus glass has greater light transmittance than the conventional EVA system, and the material has low temperature processing and cure, as well as excellent adhesive properties. The innovative backsheet is screen printable, which could facilitate manufacturing of cell back contacts using printed circuit board technology.
UV cured polymers are a speciality of Sartomer: a liquid resin is applied to a substrate and UV light is used to initiate polymerisation and ‘curing’. The advantage is the low energy use and speed of processing. These materials can be tailored for properties like adhesion and excellent weathering. They are being tested in laminates for PV applications.
The Energy research Centre of the Netherlands (ECN) has looked at critical factors in encapsulation of thin-film PVs, such as minimizing the Water Vapor Transmission Rate (WVTR) and cutting costs. It has accelerated test facilities where exposure conditions can be tailored. The encapsulation is a very significant proportion of the production costs of a module, and the ECN is looking at making more robust silicon components to reduce the encapsulant requirements as well as looking to improve the polymer. It has found that condensated water conditions are a greater stressor compared to humid, so they could be useful for accelerated testing and lifetime prediction.
With the development of any new industry comes regulation and standards. Underwriters Laboratories (UL) has set up PV testing laboratories in San Jose, the U.S.A., Krefeld in Germany, China, India and Japan, primarily looking at fire and electrical safety. Approving components prior to module specification can speed up the time to market: each aspect has a different set of requirements from flame spread to hot wire ignition and water immersion. ID scans are carried out on materials to set up references including infrared analysis, thermogravimetry and differential scanning calorimetry.
Dr. Sally Humphreys studied science at Oxford University and was a university scholar during her doctorate studies. She has worked in many aspects of science and has specialized in polymers for the past 10 years, including renewable energy applications.
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