Measuring moisture related properties of Ethylene Vinyl Acetate

Authors: S. Rashtchi

Polymer based encapsulant materials such as Ethylene Vinyl Acetate (EVA) are commonly
used in silicon-based PV modules and play a central role in the structural integrity
and hence long term performance of the device. In spite of many advantages, however,
water can diffuse through them, leading to swelling and chemical degradation that can
make the device susceptible to accelerated corrosion and failure.
This part of research was developing a novel methodologies to investigate the optomechanical
response of EVA due to water diffusion and on the quantification of water
concentration in deployed PV modules. A novel non-contact time-resolved measurement
technique based onWavelength Scanning Interferometry (WSI) is proposed to determine
thickness, refractive index, thickness change and refractive index change in EVA during
water uptake see Fig. 1.
This is preceded by numerical simulations to inform the design of the experimental
methodology and set-up, such as thickness of EVA layer, maximum surface velocity
during measurements, type of data filters to use for data analysis and also for validating
the proposed approach. The measured time-resolved thickness profiles provide a unique
opportunity to study the swelling behaviour of EVA. Using a novel inversion technique,
the hygroscopic expansion coefficient (HEC) as a function of water concentration is
established. A hygro-mechanical Finite Element model is also developed to predict timeresolved
hygroscopic expansion of EVA and compare it with experimental measurements.
The hygro-mechanical stress is further studied through a set of deflection tests on EVAglass
bi-material strip specimens immersed in water and intermittently measured with
an optical profilometer. Finally, the feasibility of combining FTIR and spectroscopic
optical coherence tomography (SOCT) to map water concentration in EVA through
the front glass in a PV module is also studied through numerical simulations and an
experimental system is proposed see Fig. 2.

SOCT provides depth resolved spectral information and thus has the potential of
measuring water absorption in EVA through the front glass in a reflection modality (i.e.
without requiring transmission through the PV module). It is expected that this would
enable in-situ assessment of PV modules for predictive estimates of durability.

Figure 2: Schematic diagram of SOCT set-up

Figure 2: Schematic diagram of SOCT set-up

Figure 1: WSI set up for thickness and refractive index change measurement in EVA

Figure 1: WSI set up for thickness and refractive index change measurement in EVA