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In PV modules,cross-linked EVA encapsulant is commonly used because it has its transparency,thermal creep resistance,softness and good adhesiveness to PV module materials such as glass,cells and backsheets.SOLAR EVATM of Mitsui Chemicals Tohcello,which is carefully designed EVA encapsulant,has proven long term reliability on the field exposure over 20 years.Potential-induce degradation (PID) issue is the loss of output associated with system bias voltage and Sodium migration from the cover glass.One solution for PID is using high insulation encapsulant which keeps high volume resistivity under severe conditions i.e.high temperature,high humidity and high voltage.Polyolefin encapsulant show better insulation property than EVA.But in case of thermoplastic polyolefin encapsulants which are already available in the market,there are some concern about thermal creep resistance and decreasing transparency at high temperature.SOLAR ASCETM,which is our brandnew polyolefin encapsulant,is based on high electrical resistivity polyolefin resin specialized for PV encapsulant and is cross linked during lamination like EVA encapsulant.Fig.1 shows temperature dependence of volume resistivity for EVA,polyolefin and SOLAR ASCETM.SOLAR ASCETM is 1000 times as high as EVA encapsulant and higher than conventional polyolefin at wide range temperature.Fig.2 shows comparison of DMA curves between thermoplastic polyolefin and SOLAR ASCETM.SOLAR ASCETM keeps modulus over 120 deg C.PID stress tests of 1 cell modules using PID-prone cells were carried out for SOLAR ASCETM and EVA encapsulant under 85degC,85%RH,-1000V,200 W/m2(Fig.3).A power degradation of SOLAR ASCETM is less than 0.1% within 250 hours.For comparison,output of the module using conventional EVA degraded more than 90 % in less than 50 hours.SOLAR ASCETM is capable encapsulant with reliability suppressing PID power loss and can afford no constraints for selecting cells for PV modules.