Solar Cells In Photovoltaic Modules Research Articles

Modelling the non-isothermal curing kinetics of peroxide crosslinking polyolefin copolymers for photovoltaic module lamination

This study presents a model for determining the non-isothermal curing kinetics of peroxide crosslinking polyolefin copolymers used for encapsulation of silicon solar cells in photovoltaic modules. Therefore, rheological data of ethylene vinyl acetate copolymer (EVA) and polyolefin elastomer (POE) were analyzed. The curing kinetics were described assuming a temperature dependent Arrhenius rate coefficient and implementing a reaction rate model. Validation was performed under isothermal and non-isothermal conditions. An excellent agreement of experimental and model data was ascertained. The model was used to determine the non-isothermal crosslinking conversion during lamination of EVA or POE based photovoltaic mini-modules. Temperatures were tracked by positioning seven sensors within the mini-modules. Moreover, a thermal simulation model was implemented. Also on module level, the experimental results corroborated the simulated data. A lamination temperature of 150 °C led to insufficient curing of POE based modules. Similar conversion rates were achieved for EVA and POE encapsulants at 150 and 160 °C, respectively.

Photovoltaics literature survey (No. 181)

Photovoltaics literature survey (No. 181)

Improvement in the reliability of crystalline silicon solar cell interconnection by using Nickel Micro-Plating Bonding (NMPB) technology

It is popular to research promoting the light conversion efficiency of crystalline silicon solar cell photovoltaic (PV) modules. However, excellent light conversion efficiency does not mean it could maintain initial performance after long-term running. Long-term reliability of interconnection to assemble crystalline silicon solar cells in PV modules is critical to ensure that the device performs continually for up to 20 years. It is reported that most crystalline PV modules fail from corrosion and breakage of the interconnector with the solder joint. As a result, improvement of interconnection technology is highly necessary. Nickel (Ni) Micro-Plating Bonding (NMPB) is an innovative interconnection technology for crystalline silicon solar cells: copper ribbons bonded with lead or lead-free solder are replaced by copper wires bonded with Ni electroplating film. The NMPB interconnection provides key advantages: low temperature (55 °C) process, enhancement of reliability from strain and stress caused by high temperature, and coefficient of thermal expansion (CTE) mismatch between metal and silicon. Furthermore, the material of NMPB, Ni, possesses excellent corrosion resistance. High reliability was confirmed with about 1.9% degradation of output power for up to 1000 thermal cycling tests and 3.8% for 1000 h of damp heat tests in bare NMPB solar cells, while 64.7% degradation in thermal cycling tests and 23.0% in damp heat tests were confirmed in solder bonding solder cells.

Review of technology specific degradation in crystalline silicon, cadmium telluride, copper indium gallium selenide, dye sensitised, organic and perovskite solar cells in photovoltaic modules: Understanding how reliability improvements in mature technologies can enhance emerging technologies

AbstractA comprehensive understanding of failure modes of solar photovoltaic (PV) modules is key to extending their operational lifetime in the field. In this review, first, specific failure modes associated with mature PV technologies, such as crystalline silicon (c‐Si), copper indium gallium selenide (CIGS) and cadmium telluride (CdTe), are framed by sources of specific failure modes, their development from the early‐developmental stages onwards and their impact upon long term performance of PV modules. These failure modes are sorted by both PV technology and location of occurrence in PV modules, such as substrate, encapsulant, front and rear electrode, absorber and interlayers. The second part of the review is focused on emerging PV technologies, such as perovskites solar cells, dye sensitised and organic PVs, where due to their low to medium technology readiness levels, specific long‐term degradation mechanisms have not fully emerged, and most mechanisms are only partially understood. However, an in‐depth summary of the known stability challenges associated with each emerging PV technology is presented. Finally, in this paper, lessons learned from mature PV technologies are reviewed, and considerations are given in to how these might be applied to the further development of emerging technologies. Namely, any emerging PV technology must eventually pass industry‐standard qualification tests, while warranties for the lifetime of modern c‐Si‐based modules might be extended beyond the existing warranted life of 25 years.

Recovery of potential-induced degraded p-type multicrystalline Si PV modules by application of a reverse DC bias

Potential-induced degradation (PID) of photovoltaic (PV) modules based on p-type crystalline Si occurs in large-scale PV systems. The development of a low-cost PID recovery technique is important to maximize the power generation capacity of PID-affected facilities. In this paper, we study a simple PID recovery method for multicrystalline silicon solar cells. PID recovery was conducted by application of a reverse DC-bias voltage to the p-n junction of solar cells in PV modules, without temperature control devices. By using this method, the conversion efficiency of solar cells reduced to approximately 20% of their initial value via a PID test was recovered up to 92%. Additionally, nonuniform temperature increases of PV modules by leakage current were observed during the voltage application.

Close examination of localized hot spots within photovoltaic modules

For photovoltaic modules, hot-spot phenomena are very common and influential, affecting device performance and causing irreversible damage. Researchers mainly pay attentions to hot-spot phenomena from a large-scale view that hot spots result from module failures, i.e., abnormal solar cells in photovoltaic modules are heated by other normal cells as loads. However, this heating process may not be the start of hot-spot phenomena. The large-scale hot-spot phenomena may develop from localized temperatures anomaly within a unit cell in the module while current researches generally ignored this small-scale but important problem. In this paper, close inspection of localized hot spots within photovoltaic modules is conducted with a xenon lamp of simulating the solar irradiation. An electronic load and an infrared thermal camera are utilized to detect and analyze electrical and thermal characteristics of the tested cells under different irradiation and surface conditions. With this experimental system, the localized hot-spot phenomena within a unit cell are experimentally revealed and the transient forming and variation processes of hot spots resulted from different mechanisms are explored, which are seldom reported to our knowledge. Attributed to localized hot spots, power conversion efficiency may fall to as low as 1.58% from 15.61%. In addition to the non-uniform irradiation, environmental obstructions falling on the cell surface may also form hot spots of very high temperature. Dust covering on the cell may lead to a hot spot above 64 °C, reducing cell efficiency by as much as 5.7 percent. The easily ignored localized hot-spot phenomena must be handled properly in case they get exacerbated and expand to the whole module.

Stress and Fracture of Crystalline Silicon Cells in Solar Photovoltaic Modules – A Synchrotron X-ray Microdiffraction based Investigation

ABSTRACTFracture of crystalline silicon (c-Si) solar cells in photovoltaic modules is a big concern to the photovoltaics (PV) industry. Cell cracks cause performance degradation and warranty issues to the manufacturers. The roots of cell fractures lie in the manufacturing and integration process of the cells and modules as they go through a series of elevated temperature and pressure processes, involving bonding of dissimilar materials, causing residual stresses. Evaluation of the exact physical mechanisms leading to these thermomechanical stresses is highly essential to quantify them and optimize the PV modules to address them. We present a novel synchrotron X-ray microdiffraction based techniques to characterize the stress and fracture in the crystalline silicon PV modules. We show the detailed stress state after soldering and lamination process, using the synchrotron X-ray microdiffraction experiments. We also calculate the maximum tolerable microcrack size in the c-Si cells to sustain the residual stress after lamination. We further demonstrate the effect of these residual stresses on the cell fractures using the widely accepted fracture (4-point bending) tests. These test results show that the soldering and lamination induced localized residual stresses indeed reduce the load-carrying capacity of the c-Si cells.

Evaluating Power Loss and Performance Ratio of Hot-Spotted Photovoltaic Modules

The impact of photovoltaic (PV) hot-spots is assessed through the analysis of 2580 polycrystalline silicon PV modules distributed across the U.K. PV hot-spots were categorized into eight different groups using the percentage of power loss. All hot-spots groups were modeled using the cumulative density function, state-of-the-art geographical mapping, and performance ratio (PR) analysis. Significantly, it was found that 92.15% of the PV modules affected by hot-spotted PV string are located in northern U.K., where the effect of low-temperature levels, heavy snow, and hoarfrost are more significant. Finally, it was found that the distribution of PV modules affected by only one hot-spotted solar cell are likely (82.41%) located in coastal locations. Hence, coastal locations expect to have lower risks for causing multiple hot-spotted solar cells in PV modules, compared to central and colder locations. The PR of all examined PV modules was analyzed. It was evident that the mean PR is significantly reduced due to the existence of hot-spots in the PV modules. The least difference in the PR between healthy and hot-spotted PV modules is equal to −0.83%, whereas the most difference is calculated at −15.47%.

A Quantitative Comparison Between Double Glass Photovoltaic Modules Using Half-Size Cells and Quarter-Size Cells

The use of half-size silicon (Si) wafer solar cells in photovoltaic (PV) modules can enhance the output power compared to full-size Si wafer solar cells. In this paper, an optimal combination of cutting parameters based on the cutting surface, the cutting repetitive time, and the parameters of the Nd:YAG nanosecond laser is achieved. The optimized method consists of a double cutting process on the aluminum back side of the solar cells. The optimized condition was identified with a laser cutting power of 16 W, a laser repetitive frequency of 40 kHz, and a scribing speed of 110 mm/s. Quarter-size Si wafer solar cells in PV modules were also investigated. We compared the output power of full-size, half-size, and quarter-size cells of a double glass transparent PV module quantitatively, finding cell-to-module values of 96.79%, 98.91%, and 99.73%. The total power gain of the half-size cell module and quarter-size cell module compared to the full-cell large-size PV module is 1.46% and 1.92%, respectively. Finally, electroluminescence measurements were carried out to test unconventional size cell PV modules.

A global/local approach for the prediction of the electric response of cracked solar cells in photovoltaic modules under the action of mechanical loads

A numerical approach based on the finite element method to assess the impact of cracks in Silicon solar cells on the electric response of photovoltaic modules is proposed. A global coarse-scale finite element model of the composite laminate is used for carrying out the structural analysis. The computed displacements at the edges of each solar cell are passed via a projection scheme as boundary conditions to a 3D local fine-scale finite element model of the cells which accounts for cohesive cracks. The evaluated crack opening displacements along the crack faces are finally used as input to an electric model characterizing the grid line/solar cell ensemble. The identification of the relation between the localized electric resistance due to cracks and the crack opening, to be used as a constitutive model of cracks, is finally discussed in reference to experimental tests performed in the laboratory.

Migration of Sn and Pb from Solder Ribbon onto Ag Fingers in Field-Aged Silicon Photovoltaic Modules

We investigated the migration of Sn and Pb onto the Ag fingers of crystalline Si solar cells in photovoltaic modules aged in field for 6 years. Layers of Sn and Pb were found on the Ag fingers down to the edge of the solar cells. This phenomenon is not observed in a standard acceleration test condition for PV modules. In contrast to the acceleration test conditions, field aging subjects the PV modules to solar irradiation and moisture condensation at the interface between the solar cells and the encapsulant. The solder ribbon releases Sn and Pb via repeated galvanic corrosion and the Sn and Pb precipitate on Ag fingers due to the light-induced plating under solar irradiation.

301 Thermal Deformation Analysis of Solar Cells Considering Thermal Profiles of both Manufacturing and Working Processes

Solar cells in photovoltaic modules are generally bonded by soldering which requires heating to a temperature of 220℃ or higher. As a result the different thermal expansion coefficients of different materials in the system cause mechanical stress, leading to deformation that reduces expected field lifespan. Therefore, the conductive adhesive film (CF) can be used as a bonding material which enables low temperature bonding at 180℃ to reduce residual stress. In this study, two finite element models are employed to analyze the thermal deformation of solar cell using CF and solder. Thermal deformation during manufacturing process and the real used conditions has been analyzed and compared considering viscoelastic properties of CF and creep properties of lead free solder.

Performance of low series-resistance interconnections on the polycrystalline solar cells

The performance of four ribbons of lead SnAgPb (SAP) and lead-free SnAgCu (SAC) photovoltaic (PV) ribbons in patterned (pat-) and high-conductivity (con-) types soldered on silicon solar cells are investigated by electron characteristics and microstructure analyses. The electrical performances of the solar strings were characterized before and after the soldering process by a solar flasher, and strings with commercial ribbons of lead and lead-free are compared. In general, lead ribbon solar strings show lower power loss value than lead-free counterparts. And the commercial lead and lead-free ribbon strings show highest power loss around 6.66%–10.65% of all the solar strings. Furthermore, the power losses for patterned lead (pat-SAP) and lead-free (pat-SAC) solar string are 3.93% and 5.51%, respectively, implying that a patterned structure should improve the soldering process and result in lower power loss. Yet, significant low output power loss values are detected in the solar strings which are soldered with high-conductivity ribbons. It is estimated that the power loss for high-conductivity lead (con-SAP) and lead-free (con-SAC) solar strings are 3.89% and 4.43%, respectively. And a similar condition was revealed when the solder materials were practically interconnected the solar cells in PV modules. According to the microstructure analyses, the low power loss of con-SAP solar string should be due to the good electrical performance in low series resistance ( R s) and high shunt resistance ( R sh), thus resulting in a high fill factor of the solar device.