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- All Subjects: chemical engineering
- Genre: Masters Thesis
- Creators: Tamizhmani, Govindasamy
Description
Encapsulant is a key packaging component of photovoltaic (PV) modules, which protects the solar cell from physical, environmental and electrical damages. Ethylene-vinyl acetate (EVA) is one of the major encapsulant materials used in the PV industry. This work focuses on indoor accelerated ultraviolet (UV) stress testing and characterization to investigate the EVA discoloration and delamination in PV modules by using various non-destructive characterization techniques, including current-voltage (IV) measurements, UV fluorescence (UVf) and colorimetry measurements. Mini-modules with glass/EVA/cell/EVA/backsheet construction were fabricated in the laboratory with two types of EVA, UV-cut EVA (UVC) and UV-pass EVA (UVP).
The accelerated UV testing was performed in a UV chamber equipped with UV lights at an ambient temperature of 50°C, little or no humidity and total UV dosage of 400 kWh/m2. The mini-modules were maintained at three different temperatures through UV light heating by placing different thickness of thermal insulation sheets over the backsheet. Also, prior to thermal insulation sheet placement, the backsheet and laminate edges were fully covered with aluminum tape to prevent oxygen diffusion into the module and hence the photobleaching reaction.
The characterization results showed that mini-modules with UV-cut EVA suffered from discoloration while the modules with UV-pass EVA suffered from delamination. UVf imaging technique has the capability to identify the discoloration region in the UVC modules in the very early stage when the discoloration is not visible to the naked eyes, whereas Isc measurement is unable to measure the performance loss until the color becomes visibly darker. YI also provides the direct evidence of yellowing in the encapsulant. As expected, the extent of degradation due to discoloration increases with the increase in module temperature. The Isc loss is dictated by both the regions – discolored area at the center and non-discolored area at the cell edges, whereas the YI is only determined at the discolored region due to low probe area. This led to the limited correlation between Isc and YI in UVC modules.
In case of UVP modules, UV radiation has caused an adverse impact on the interfacial adhesion between the EVA and solar cell, which was detected from UVf images and severe Isc loss. No change in YI confirms that the reason for Isc loss is not due to yellowing but the delamination.
Further, the activation energy of encapsulant discoloration was estimated by using Arrhenius model on two types of data, %Isc drop and ΔYI. The Ea determined from the change in YI data for the EVA encapsulant discoloration reaction without the influence of oxygen and humidity is 0.61 eV. Based on the activation energy determined in this work and hourly weather data of any site, the degradation rate for the encaspulant browning mode can be estimated.
The accelerated UV testing was performed in a UV chamber equipped with UV lights at an ambient temperature of 50°C, little or no humidity and total UV dosage of 400 kWh/m2. The mini-modules were maintained at three different temperatures through UV light heating by placing different thickness of thermal insulation sheets over the backsheet. Also, prior to thermal insulation sheet placement, the backsheet and laminate edges were fully covered with aluminum tape to prevent oxygen diffusion into the module and hence the photobleaching reaction.
The characterization results showed that mini-modules with UV-cut EVA suffered from discoloration while the modules with UV-pass EVA suffered from delamination. UVf imaging technique has the capability to identify the discoloration region in the UVC modules in the very early stage when the discoloration is not visible to the naked eyes, whereas Isc measurement is unable to measure the performance loss until the color becomes visibly darker. YI also provides the direct evidence of yellowing in the encapsulant. As expected, the extent of degradation due to discoloration increases with the increase in module temperature. The Isc loss is dictated by both the regions – discolored area at the center and non-discolored area at the cell edges, whereas the YI is only determined at the discolored region due to low probe area. This led to the limited correlation between Isc and YI in UVC modules.
In case of UVP modules, UV radiation has caused an adverse impact on the interfacial adhesion between the EVA and solar cell, which was detected from UVf images and severe Isc loss. No change in YI confirms that the reason for Isc loss is not due to yellowing but the delamination.
Further, the activation energy of encapsulant discoloration was estimated by using Arrhenius model on two types of data, %Isc drop and ΔYI. The Ea determined from the change in YI data for the EVA encapsulant discoloration reaction without the influence of oxygen and humidity is 0.61 eV. Based on the activation energy determined in this work and hourly weather data of any site, the degradation rate for the encaspulant browning mode can be estimated.
ContributorsDolia, Kshitiz (Author) / Tamizhmani, Govindasamy (Thesis advisor) / Green, Matthew (Thesis advisor) / Srinivasan, Devarajan (Committee member) / Arizona State University (Publisher)
Created2018
Description
In the past, the photovoltaic (PV) modules were typically constructed with glass superstrate containing cerium oxide and EVA (ethylene vinyl acetate) encapsulant containing UV absorbing additives. However, in the current industry, the PV modules are generally constructed without cerium oxide in the glass and UV absorbing additives in EVA to increase quantum efficiency of crystalline silicon solar cells in the UV regions. This new approach is expected to boost the initial power output of the modules and reduce the long-term encapsulant browning issues. However, this new approach could lead to other durability and reliability issues such as delamination of encapsulant by damaging interfacial bonds, destruction of antireflection coating on solar cells and even breakage of polymeric backbone of EVA. This work compares the durability and reliability issues of PV modules having glass without cerium oxide and EVA with (aka, UVcut or UVC) and without (aka, UVpass or UVP) UV absorbing additives. In addition, modules with UVP front and UVC back EVA have also been investigated (aka, UVhybrid or UVH). The mini-modules with nine split cells used in this work were fabricated at ASU’s Photovoltaic Reliability Laboratory. The durability and reliability caused by three stress variables have been investigated and the three variables are temperature, humidity/oxygen and UV dosage. The influence of up to 800 kWh/m2 UV dosage has been investigated at various dosage levels. Many material and device characterizations have been performed to ascertain the degradation modes and effects. The UVC modules showed encapsulant discoloration at the cell centers as expected but the UVH modules showed a ring-shaped encapsulant discoloration close to the cell edges as evidenced in the UV fluorescence (UVF) imaging study. The PV modules containing UVP on both sides of cells with limited access to humidity or oxygen through backsheet (covered backsheet with adhesive aluminum tape) seem to experience encapsulant delamination as evidenced in the UVF images. Plausible explanations for these observations have been presented.
ContributorsArularasu, Pooja (Author) / Tamizhmani, Govindasamy (Thesis advisor) / Mu, Bin (Thesis advisor) / Varman, Arul M (Committee member) / Arizona State University (Publisher)
Created2019
Description
Ethylene vinyl acetate (EVA) is the most commonly used encapsulant in photovoltaic modules. However, EVA degrades over time and causes performance losses in PV system. Therefore, EVA degradation is a matter of concern from a durability point of view.
This work compares EVA encapsulant degradation in glass/backsheet and glass/glass field-aged PV modules. EVA was extracted from three field-aged modules (two glass/backsheet and one glass/glass modules) from three different manufacturers from various regions (cell edges, cell centers, and non-cell region) from each module based on their visual and UV Fluorescence images. Characterization techniques such as I-V measurements, Colorimetry, Different Scanning Calorimetry, Thermogravimetric Analysis, Raman spectroscopy, and Fourier Transform Infrared Spectroscopy were performed on EVA samples.
The intensity of EVA discoloration was quantified using colorimetric measurements. Module performance parameters like Isc and Pmax degradation rates were calculated from I-V measurements. Properties such as degree of crystallinity, vinyl acetate content and degree of crosslinking were calculated from DSC, TGA, and Raman measurements, respectively. Polyenes responsible for EVA browning were identified in FTIR spectra.
The results from the characterization techniques confirmed that when EVA undergoes degradation, crosslinking in EVA increases beyond 90% causing a decrease in the degree of crystallinity and an increase in vinyl acetate content of EVA. Presence of polyenes in FTIR spectra of degraded EVA confirmed the occurrence of Norrish II reaction. However, photobleaching occurred in glass/backsheet modules due to the breathable backsheet whereas no photobleaching occurred in glass/glass modules because they were hermetically sealed. Hence, the yellowness index along with the Isc and Pmax degradation rates of EVA in glass/glass module is higher than that in glass/backsheet modules.
The results implied that more acetic acid was produced in the non-cell region due to its double layer of EVA compared to the front EVA from cell region. But, since glass/glass module is hermetically sealed, acetic acid gets entrapped inside the module further accelerating EVA degradation whereas it diffuses out through backsheet in glass/backsheet modules. Hence, it can be said that EVA might be a good encapsulant for glass/backsheet modules, but the same cannot be said for glass/glass modules.
This work compares EVA encapsulant degradation in glass/backsheet and glass/glass field-aged PV modules. EVA was extracted from three field-aged modules (two glass/backsheet and one glass/glass modules) from three different manufacturers from various regions (cell edges, cell centers, and non-cell region) from each module based on their visual and UV Fluorescence images. Characterization techniques such as I-V measurements, Colorimetry, Different Scanning Calorimetry, Thermogravimetric Analysis, Raman spectroscopy, and Fourier Transform Infrared Spectroscopy were performed on EVA samples.
The intensity of EVA discoloration was quantified using colorimetric measurements. Module performance parameters like Isc and Pmax degradation rates were calculated from I-V measurements. Properties such as degree of crystallinity, vinyl acetate content and degree of crosslinking were calculated from DSC, TGA, and Raman measurements, respectively. Polyenes responsible for EVA browning were identified in FTIR spectra.
The results from the characterization techniques confirmed that when EVA undergoes degradation, crosslinking in EVA increases beyond 90% causing a decrease in the degree of crystallinity and an increase in vinyl acetate content of EVA. Presence of polyenes in FTIR spectra of degraded EVA confirmed the occurrence of Norrish II reaction. However, photobleaching occurred in glass/backsheet modules due to the breathable backsheet whereas no photobleaching occurred in glass/glass modules because they were hermetically sealed. Hence, the yellowness index along with the Isc and Pmax degradation rates of EVA in glass/glass module is higher than that in glass/backsheet modules.
The results implied that more acetic acid was produced in the non-cell region due to its double layer of EVA compared to the front EVA from cell region. But, since glass/glass module is hermetically sealed, acetic acid gets entrapped inside the module further accelerating EVA degradation whereas it diffuses out through backsheet in glass/backsheet modules. Hence, it can be said that EVA might be a good encapsulant for glass/backsheet modules, but the same cannot be said for glass/glass modules.
ContributorsPatel, Aesha Parimalbhai (Author) / Tamizhmani, Govindasamy (Thesis advisor) / Green, Matthew (Committee member) / Mu, Bin (Committee member) / Arizona State University (Publisher)
Created2018
Description
This is a two-part thesis assessing the long-term reliability of photovoltaic modules.
Part 1: Manufacturing dependent reliability - Adapting FMECA for quality control in PV module manufacturing
This part is aimed at introducing a statistical tool in quality assessments in PV module manufacturing. Developed jointly by ASU-PRL and Clean Energy Associates, this work adapts the Failure Mode Effect and Criticality Analysis (FMECA, IEC 60812) to quantify the impact of failure modes observed at the time of manufacturing. The method was developed through analysis of nearly 9000 modules at the pre-shipment evaluation stage in module manufacturing facilities across south east Asia. Numerous projects were analyzed to generate RPN (Risk Priority Number) scores for projects. In this manner, it was possibly to quantitatively assess the risk being carried the project at the time of shipment of modules. The objective of this work was to develop a benchmarking system that would allow for accurate quantitative estimations of risk mitigation and project bankability.
Part 2: Climate dependent reliability - Activation energy determination for climate specific degradation modes
This work attempts to model the parameter (Isc or Rs) degradation rate of modules as a function of the climatic parameters (i.e. temperature, relative humidity and ultraviolet radiation) at the site. The objective of this work was to look beyond the power degradation rate and model based on the performance parameter directly affected by the degradation mode under investigation (encapsulant browning or IMS degradation of solder bonds). Different physical models were tested and validated through comparing the activation energy obtained for each degradation mode. It was concluded that, for the degradation of the solder bonds within the module, the Pecks equation (function of temperature and relative humidity) modelled with Rs increase was the best fit; the activation energy ranging from 0.4 – 0.7 eV based on the climate type. For encapsulant browning, the Modified Arrhenius equation (function of temperature and UV) seemed to be the best fit presently, yielding an activation energy of 0.3 eV. The work was concluded by suggesting possible modifications to the models based on degradation pathways unaccounted for in the present work.
Part 1: Manufacturing dependent reliability - Adapting FMECA for quality control in PV module manufacturing
This part is aimed at introducing a statistical tool in quality assessments in PV module manufacturing. Developed jointly by ASU-PRL and Clean Energy Associates, this work adapts the Failure Mode Effect and Criticality Analysis (FMECA, IEC 60812) to quantify the impact of failure modes observed at the time of manufacturing. The method was developed through analysis of nearly 9000 modules at the pre-shipment evaluation stage in module manufacturing facilities across south east Asia. Numerous projects were analyzed to generate RPN (Risk Priority Number) scores for projects. In this manner, it was possibly to quantitatively assess the risk being carried the project at the time of shipment of modules. The objective of this work was to develop a benchmarking system that would allow for accurate quantitative estimations of risk mitigation and project bankability.
Part 2: Climate dependent reliability - Activation energy determination for climate specific degradation modes
This work attempts to model the parameter (Isc or Rs) degradation rate of modules as a function of the climatic parameters (i.e. temperature, relative humidity and ultraviolet radiation) at the site. The objective of this work was to look beyond the power degradation rate and model based on the performance parameter directly affected by the degradation mode under investigation (encapsulant browning or IMS degradation of solder bonds). Different physical models were tested and validated through comparing the activation energy obtained for each degradation mode. It was concluded that, for the degradation of the solder bonds within the module, the Pecks equation (function of temperature and relative humidity) modelled with Rs increase was the best fit; the activation energy ranging from 0.4 – 0.7 eV based on the climate type. For encapsulant browning, the Modified Arrhenius equation (function of temperature and UV) seemed to be the best fit presently, yielding an activation energy of 0.3 eV. The work was concluded by suggesting possible modifications to the models based on degradation pathways unaccounted for in the present work.
ContributorsPore, Shantanu (Author) / Tamizhmani, Govindasamy (Thesis advisor) / Green, Matthew (Thesis advisor) / Srinivasan, Devrajan (Committee member) / Arizona State University (Publisher)
Created2017