This work deals with the simulation of water vapor ingress into wafer-based PV-modules for long-term exposure under different climatic conditions. Measured material parameters together with climatic data sets from four test sites (tropic, moderate, alpine, and arid) were used to calculate the water concentration inside of the encapsulant between solar cell and glass for a lifetime of 20 years. Two back-sheet materials (PET-based and PA-based) combined with EVA as encapsulant were used in respect to their influence on water ingress. The results show faster water ingress for warmer regions, but the highest concentrations were found for the moderate test site. The water ingress was additionally influenced by the used encapsulant and back-sheet combination. In particular the temperature dependency of the mass transfer, which differs from material to material, was the focus of this investigation.
PV-modules are globally installed and, thus, exposed to different climates with continuously changing environmental conditions as, for example, UV-irradiation, temperature cycles, or snow loads over a life time of 20 years and more.
The ambient atmosphere and its containing air humidity constitute one of these loads. Polymers interact with the water vapor in a manner of absorption and desorption. Therefore, water can enter a PV-module and causes hydrolysis of the polymers [
The absorption of air humidity is primarily controlled by the microclimate at the interface PV-module to ambient atmosphere and by specific temperature controlled material parameters such as water vapor transmission rate (WVTR) and diffusion coefficient (
The present paper deals with the investigation of water concentration behavior inside polymeric materials used in PV-modules, depending on different climates, and over a planned lifetime of 20 years. Due to the geometrical structure of wafer-based cSi-PV-modules and the long-lasting life time, measurements of water concentration inside the module are viable. Therefore, a combination of real measured data and 2-D FEM-simulation (finite element method) was used to solve this problem. The measurements include the determination of the temperature dependent permeability of water vapor through different encapsulation and back-sheet materials, but also the microclimate which was measured at four test sites representing different climates (moderate, alpine, tropic, and desert). Compared to prior publications addressing this issue [
In this study, three commercial products—one encapsulant and two back sheets—were chosen for the analysis of their influence on water vapor ingress. The encapsulant was a standard EVA (fast cure) which is well established on the market.
Back sheets generally consist of three layers of polymeric films which are glued together by adhesives. The core material between the two outer layers is, because of its thickness, the dominant part regarding the permeability of gases. Results of a prior work [
Two material parameters, permeability (
Calculated activation energies
Material | Thickness ( |
|
---|---|---|
PET-based (back sheet) | 350 | 39,2 |
PA-based (back sheet) | 350 | 54,1 |
EVA (encapsulant) | 1000 | 35,7 |
The results of
Measured effective diffusion coefficients of the materials used in the simulation [6-modified].
Measured permeability of materials used in the simulation of water ingress [6-modified].
To simulate the water ingress under realistic climatic conditions, knowledge of the local climate the PV-module is exposed to is needed. Therefore, the climate data such as air temperature ( Freiburg, Germany (moderate climate), Negev desert, Israel (arid climate), Zugspitze (altitude: 2650 m), Germany (alpine climate), Serpong, Indonesia (tropic climate).
Figures
Relative air humidity: tropic.
Module and air temperature: tropic.
Relative air humidity: arid.
Module and air temperature: arid.
Relative air humidity: alpine.
Module and air temperature: alpine.
Relative air humidity: moderate.
Module and air temperature: moderate.
Microclimate is defined in this study as the boundary layer between the atmospheric side of the back sheet and the ambient air. This air layer with a thickness of approximately 1 mm holds a temperature balance with the PV-module whose temperature is, compared to the ambient air temperature, elevated by solar irradiation during the day and lowered by thermal emission during the night. Thus, the humidity a PV-module is exposed to at its back sheet is determined by the humidity of the microclimate, which provides dryer conditions at day and wetter conditions at night because of the temperature deviation between PV-module and air.
Figures
Diurnal cycles of measured air and module temperature: arid.
Diurnal cycles of measured air humidity and calculated microclimate humidity: arid.
The FEM-based simulation was carried out with a commercial software tool. The following sections discuss geometry and layout of the used model including boundary conditions and discuss the results of the simulation.
The geometry used for the simulation presents a simple, symmetrical, and 2-dimensional constructed model (Figure
Assembling and geometry of the simulated model including pathway of water molecules.
The possible pathway of a water molecule is illustrated by plotted arrows in Figure
The main advantage of using a 2-D model is that also permeated cross-sectional areas are considered which is given, for instance, by the ratio of space between the cells (here, 3 mm) and the distance of cell to glass (here, ~0.4 mm). Please, mind the unequal dimension of the
The input parameters used for this simulation were given by the measured temperature dependency of permeability of the encapsulation and back-sheet materials which were described in Section
The water ingress was simulated for various back-sheet and encapsulant material combinations which are listed below: PET-based back sheet/EVA (fast cure) encapsulant, PA-based back sheet/EVA (fast cure) encapsulant.
Figures
Simulated water ingress into a PV-module for different climates. Here, an EVA encapsulant and a PET-based back sheet were used.
Simulated water ingress into a PV-module for different climates. The use of the PA-based back sheet causes significantly lower water concentrations.
Both graphs (Figures
In addition to the speed of water ingress, also the maximum water concentration that is reached after several years depends on the climate (Figures
The influence of the back sheet can be determined by focusing on the concentration that is in equilibrium after several years but oscillates around a mean concentration. The mean equilibrium concentration differs from site to site (see above). But within the same site, the equilibrium concentration is significantly influenced by the used back sheet (Figure
Simulated water ingress for the tropic test site and the effect of different back sheets.
A first validation of the simulation of water ingress is shown in Figure
Simulated versus measured permeation through a 1 mm EVA film (at 38°C) which shows a very good agreement.
In this study, the water ingress into PV-modules was simulated for an outdoor exposure of 20 years in different climates. Real measured parameters such as permeability of encapsulant and back sheets, together with monitored data sets of four climatic sites were used as basis for this simulation. The results show the influence of different climates on water concentration inside the encapsulant between solar cell and glass. Interestingly, the tropic test site does not thereby provide the highest concentrations. But also the effect of using different encapsulants in combination with different back-sheet materials on water ingress was shown. In particular the temperature dependency of the mass transfer influences the water ingress, depending on the material combination that is used. Results show, for example, for PA-based back sheets, that a higher
In summary, a method was presented, existing of measured parameters and simulation, which is well suited for material and design studies regarding water ingress—not only for PV-modules.
This work was partly funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU FKz 0329978) and sponsored by the industrial partners Schott Solar, Solarfabrik, Solarwatt, SolarWorld, and Solon.