Badiee, Amir
(2016)
An examination of the response of ethylene-vinyl acetate film to changes in environmental conditions.
PhD thesis, University of Nottingham.
Abstract
Photovoltaics are used for the direct conversion of sunlight into electricity. In order to provide useful power, the individual solar cells must be connected together. This electrically connected and environmentally protected unit is termed a photovoltaic (PV) module. The structure of a PV module consists of a number of layers of various materials with different properties. The encapsulation material is one of the critical components of a PV module. It mechanically protects the devices and electrically insulates them, ideally for at least the 20-25 year lifetime of the modules. The lifetime of a PV module is generally limited by the degradation of the constituent parts. The materials degrade and cause a decrease in the efficiency leading to eventual failure, with the encapsulant being particularly susceptible to degradation. The most common encapsulant material is Ethylene Vinyl Acetate (EVA) the degradation of which leads to a significant drop in a PV module’s efficiency, durability and lifetime. EVA undergoes chemical degradation when it is exposed to environmental factors such as elevated temperature, humidity and Ultra Violet (UV) radiation. Although numerous works have been done in this field there is still a gap in knowledge to fully understand the degradation of EVA and develop a predictive tool. This work investigates the chemical degradation of an EVA encapsulant to understand the degradation mechanisms, develop a predictive model and correlate the degradation with changes in the structure and mechanical properties.
To determine the effect of environmental stresses on EVA environmental conditions were simulated in the laboratory in order to accelerate the test program. The ageing was classified into three main groups, namely thermal ageing, UV ageing and damp-heat ageing. In order to investigate the effect of elevated temperature on the mechanical and thermal properties and also to study the thermal degradation, EVA sheets were aged in a dark laboratory oven at 85°C for up to 80 days. To investigate the impact of UV exposure on the properties and photodegradation of EVA the samples were exposed to UV radiation of 0.68 W/m2 at 340 nm and 50°C. To study moisture diffusion and the impact of absorbed moisture on the mechanical properties and morphology, EVA sheets were aged in an environmental chamber at 85°C-85% RH and using a potassium chloride (KCl) salt solution in a sealed chamber to obtain 85% RH at room temperature (22±3°C).
Thermal analysis techniques including Differential Scanning Calorimetry (DSC), Thermo-gravimetric Analysis (TGA), Dynamic Mechanical Analysis (DMA) along with Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and Gravimetrics were used to investigate the structure, degradation kinetics and viscoelastic mechanical properties of the EVA as a function of ageing.
The EVA was shown to have viscoelastic properties that were highly sensitive to the ambient temperature. Thermal ageing was shown to reduce the storage modulus due to the changes in the structure of the EVA and reduction in crystallinity. Over a longer time, chemical changes due to thermal activation also occurred, hence, these were insignificant compared with transient thermal effects. The activation energy of deacetylation was also shown not be affected by the ageing process.
Investigation of photodegradation showed notable chemical changes as a result of UV exposure, with FTIR absorbance peaks related to carboxylic acid, lactone and vinyl exhibiting a sharp increase after UV irradiation. Differences in the ATR-FTIR spectra of the UV irradiated and non-irradiated samples showed that the intensity is depth dependant. DMA results showed UV ageing had a significant influence on the mechanical properties of the EVA and reduces the storage modulus. The predictive photodegradation model showed a good agreement on the UV irradiated surface with the experimental data where it did not agree well with the results on the non-irradiated side which could be due to the presence of UV absorber.
The response of the EVA to damp heat was investigated at two conditions with same the RH level (85% RH) and different temperatures (room temperature and 85°C). The moisture diffusion coefficient was measured via gravimetry and Water Vapour Transmission Rate (WVTR) technique which were well-agreed. Results from DSC indicated that the crystallinity increased due to incorporation of moisture into the structure of the copolymer but decreased as ageing continued, showing the significant influence of elevated temperature and thermal degradation on the structure of EVA.
A comparative study of the impact of the ageing on the structure and mechanical properties indicated that UV has a stronger degrading influence comparing to other degradation factors. DSC results also suggested that property changes could be connected to structural modifications. The impact of different degradation factors can be summarised as UV > T > DH.
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