A Dual-Transport Model of Moisture Diffusion in PV Encapsulants for Finite-Element Simulations

Stefan Mitterhofer,Gernot Oreski,Marko Jankovec,Chiara Barretta,Marko Topic,Luis F Castillon

doi:10.1109/jphotov.2019.2955182

Stefan Mitterhofer, Gernot Oreski + Show 4 more

Open Access

https://doi.org/10.1109/jphotov.2019.2955182

Copy DOI

Abstract

A better understanding of moisture ingress in photovoltaic modules is crucial for better predictions of their long-term behavior in the field. Current calculations and simulations of moisture uptake in photovoltaic modules are based on the Fickian diffusion model in a homogeneous material. In this article, in situ humidity measurements in four different encapsulants exposed to transient humidity conditions are compared with Fickian simulations. It is found that the model cannot accurately describe the measured moisture ingress and egress curves. Thus, a new model for finite-element simulations based on two transport mechanisms is applied. The mesh is split into two regions, where channels with a high diffusion coefficient lead through a bulk with a low diffusion coefficient. The diffusion in both regions as well as the flow between them is simulated as Fickian. The new model is able to predict the measured ingress and egress curves more accurately than the Fickian model in all four encapsulants. The same simulation parameters can accurately describe ingress for various relative humidity values between 20% and 80%, as well as egress from 40% to 20%. This allows further prediction of moisture ingress after the measurement of a single ingress curve and a corresponding parameter optimization. However, a nonlinearity of the diffusion during egress at higher moisture values in the encapsulants is found.

Highlights

P HOTOVOLTAIC (PV) modules installed in the field experience a wide variety of climatic stresses
1) Variation of the Diffusion Coefficients: In this article, SC is set to 10%, DC to 5·10−10 m2/s, Qmesh to 20, and Hsens to 50 μm
Positron lifetime spectroscopy measurements for ethylene vinyl acetate (EVA) show that the fractional free volume is about 10% of the material, varying a few percent depending on the vinyl acetate (VA) and polyethylene (PE) contents [39]

Summary

Introduction

P HOTOVOLTAIC (PV) modules installed in the field experience a wide variety of climatic stresses. Moisture is one such factor affecting their degradation, in forms of relative humidity (RH) in the air, dew, and precipitation. It can ingress into the polymers used as encapsulants and backsheets, influencing various degradation modes of modules mounted in the field. Examples include yellowing and browning of the encapsulant, delamination, corrosion and potential-induced shunting [1]–[5]. Manuscript received July 18, 2019; revised October 18, 2019; accepted November 19, 2019. Date of publication December 3, 2019; date of current version December 23, 2019.

Results

Discussion

Conclusion