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U S AG E O F P E ROX I D E S
I N S O L A R C E L L C O N S T RU C T I O N
Date: 21st Nov 2016
Mohamed Adam.K([email protected])
A device converting solar radiation into electricity
Silicon solar cells are fragile and are especially sensitive to brittleness failurein tension and bending.
The voltage, and thus the power, of a silicon solar cell is temperature dependent.The efficiency decreases with increasing temperature
at the rate of about 0.5% / ᵒC.
Encapsulation design must remove the energy absorbed and convertedinto thermal energy by the cells.
Front Cover : low cost, transparent, elastomeric materials, UV-screening effect, weather-stable material - acrylics, silicones, fluorocarbons, and glass.
Solar Cell - Construction
Encapsulation materials - construction materials (excluding cells and electrical conductors) required in a PV module to provide
mechanical support and environmental isolation.
Earlier encapsulation efforts to identify a single material that could satisfy all of the encapsulation requirements and needs were unsuccessful.
More than one material would have to be assembled in a composite package to fabricate an encapsulated module satisfying all of the requirements.
Significant difference between the thermal-expansion coefficients of polymeric materials and the silicon cells and metallic interconnects;
stresses developed from the thousands of daily thermal cycles can result in fractured cells, broken interconnects or cracks and separations in the pottant material.
Hence the pottant must be a low-modulus, elastomeric material.
In addition, the mechanical properties of many polymeric encapsulation materials aretemperature-dependent.
Solar ultraviolet radiation can be a major cause or initiator fordetrimental changes in properties of normal polymeric materials.
Encapsulation of Solar Cell
Water: Many polymer encapsulation materials absorb some availablemoisture (liquid or vapor) and causes dimensional and mechanical property changes.
Moisture can participate directly in thermochemical and photochemical reactions with polymer encapsulation materials.
An electrolyte of condensed phase moisture and dissolved salts can participatein corrosion of electrical conductors.
Encapsulation Process: Vacuum lamination, Liquid casting, Spraying, Direct extrusion.
Lamination – dry film materials: Ethylene vinyl acetate, Ethylene methyl acrylate.
Casting – liquid material: Poly-n-butyl acrylate, Acrylic and Aliphatic polyether urathane.
EVA advantages : Good melting fluidity, low melting temperature, toughness, excellent adhesion, high electrical resistivity, low processing and cross linking temperature,
very low water absorption ratio, good optical transmission and high transparency.
EVA disadvantages: Poor heat resistance, low cohesive strength, poor creep resistance and expansion & contraction.
Moreover, unsaturated bonds in EVA molecules will be destroyed by ultraviolet light,and crack, degradation and discoloration, and adhesive failure maybe occurred.
EVA resin is compounded with curing agents (i.e., organic peroxides) and other additives such as UV absorber, UV stabilizer, antioxidant and silane coupling agent .
Cross linked during process.
EVA as an Encapsulation Material
UV absorber: 2-Hydroxy-4-octoxybenzophenone
UV stabilizer (HALS):bis(2,2,6,6-tetramethyl-
4-piperidinyl) sebacate
Antioxidant: Tris(Nonylphenyl)phosphate
Coupling agent:3-trimethoxysilyl- propylmethacrylate
Selection of curing agent: • Negligible decomposition at 85 to 90°C range, the extrusion temperature of EVA• Generates only the chemically inert decomposition residues• No UV-sensitivity for itself and the EVA
During the lamination process, gas bubbles formed from the decomposition of peroxides, inhibit the cure-active chemical species and decreases the curing rate.
It can be avoided by laminating at 150ᵒC and 1 atm lamination pressure
Peroxides, which generate free radicals having energy level ≥418.6 kJ/mol, possess high cross linking efficiency due to their ability to abstract secondary hydrogen,
commonly present in most of the cross linkable polymers.
Peroxides as Cross Linking Agents
Preparation of EVA Compounds:EVA resins are mixed with additives at room temperature.
Those additives are dissolved in 20 ml of acetone before being sprayed over the EVA resins. Then, they are compounded by a twin-screw extruder
(L/D ratio: 16.7, screw dia 19.7 mm, die dia 3 mm), screw speed: 20 rpm, barrels temperature: 80°C, and die temperature: 85°C.
Finally, the compounds are sheeted by a two roll mill at 40°C for 5 min.
Cross linking Characteristics Measurement - Monsanto Moving Die Rheometer4 g sample at temperature range 150°C - 170°C with a measuring time of 15 min.
Determination of Cure time and cure rate index (CRI)– Torque vs Time cure curves
2g sample in 100 ml of toluene is heated for 3 hr at 6OᵒC, the residual gel gives the degree of Cure (in %).
The criterion for acceptable cure is the achievement of mechanical-creep resistance of the cured EVA at 90°C, which corresponded to gel content in excess of 65%.
EVA Compounding and Curing Study
The aromatic by-products or chromophores in the curing step are causes of discoloration in the cured EVA. Discoloration of EVA can reduce PV module efficiency
because of decreased light transmittance
Even though DCP can produce strong free radicals, its half-life temperature is significantly high.
Therefore, it takes a long time to decompose at low temperature, or it must be heated up to higher temperature to produce free radicals in a short cure time.
Moreover, high cure temperature and/or long cure time can generate acetophenone, which causes yellowing in EVA.
Dialkyl Peroxide - Performance
Non-aromatic, non-discoloring, and does not produce decomposition by-productsThe optimum cure times (tC90) of these curves are 7.93, 6.63 and 4.52 minutes;
The CRIs are 14.37, 16.85 and 25.74 / min, respectively.
Peroxyester peroxide - Performance
Peroxyketal peroxide - Performance
The peroxyketal peroxide generates a mixture of weak and high energy free radicals
Decomposition products, such as methane, acetone, t-butyl alcohol, and CO2, are non discoloring
Optimum cure times (tC90) of these compounds were 3.15, 2.29 and 1.30 minutes for cure temperatures of 150°C, 155°C and 160°C, respectively.
Comparison of Peroxide Performance
Conclusion
Dialkyl peroxide is not suitable - has a high half-life temperature. By products can discolor the final product.
Peroxyester peroxide is good for curing in the range of 150°C to 160°C.Ultimate cure within 5 to 8 minutes.
Moreover, peroxyketal peroxide has higher performance, optimum cure time is 3 minutes.