Solar panel lamination ensures the longevity of the solar cells of a module as they need to be able to withstand outdoor exposure in all types of climate for periods of 25 years and more
Solar panel lamination ensures the longevity of the solar cells of a module as they need to be able to withstand outdoor exposure in all types of climate for periods of 25 years and more. Solar modules need to convert sunlight to electricity at an acceptable cost throughout their lifetime. The encapsulation of the solar cells through lamination is a crucial step in traditional solar PV module manufacturing. Improper lamination can lead to premature failure of these modules. The knowledge of complete lamination process not only helps in making a better product but also lessens losses like cell breakage, air bubbles and delamination, which mostly occurs due to incorrect processing parameters. The lamination process encapsulates solar cells in between a number of substrate layers including top and bottom protective layers. These layers are known as a “lay-up” and this methodology has been successfully employed for over decades. John Kirk, MD, J-Flex notes that the most common shared module lay-ups is tempered glass as the transparent top layer; followed by a layer of encapsulant; the interconnected solar cells; another layer of encapsulant and finally a layer of U.V. stable film as the bottom layer.
The encapsulant is conventionally made from Ethylene Vinyl Acetate (E.V.A.). There have been many advances made to this adhering material due to the very aggressive outgassing created as a bi-product of the continual lamination process.
Crystalline silicon (c-Si) PV modules today are majorly manufactured in accordance with a glass-backsheet (GBS) module layup: 3.2–4mm glass at the front and a polymer-based insulating backsheet. An aluminum frame is applied around the module to increase mechanical stability. The mono- or polycrystalline Si solar cells with busbars (BBs) are electrically connected with tinned copper ribbons using a high-temperature (T > 220°C) soldering process. Antonin Faes, Heng-Yu Li, Christophe Ballif and Laure-Emmanuelle Perret-Aebi in a research report writes that the most popular encapsulant for this PV module design has long been (and still is) the copolymer ethylene vinyl acetate (EVA). This type of module has been functioning in the field for over 30 years, and quite a lot of failures have been reported and investigated. Failure mechanisms are often attributed to moisture penetration into the module through the backsheet and the bulk of the encapsulant notes the researchers.
It has been observed that when these types of modules are exposed to water and/or ultraviolet, EVA molders to produce acetic acid, which speeds up metallization corrosion. According to the researchers under outdoor conditions, EVA suffers yellowing, browning and delamination, which cause considerable power loss. Among the observed failures, there is clear evidence of delamination and yellowing, which lead to a total measured power loss of 15%. Potential induced degradation (PID) has also been linked to EVA formulation and identified as a critical aspect of PV module system reliability.
As a response to the need for longer lasting and higher-efficiency PV modules, significant improvements have been made at the cell and module levels in recent years. In addition, even if the dominance of EVA remains currently uncontested, during the last few years (especially with the emerging cell technologies) non-EVA based products have been proposed as an alternative encapsulant material.
Solar Panel Lamination
Even today, the most common way to laminate a solar panel is by using a lamination machine notes Sinovoltaics. This old-fashioned method has many disadvantages, but is used by the large majority of solar panel manufacturers.
Brij notes that majority of module laminators follow this three step process for proper melting and curing of the encapsulant (EVA) and achieving a good quality lamination, it includes a) heating of the module lay-up to required temperatures to perform the EVA cross-linking step. b) Create vacuum t to remove the air and avoid bubble formation. The time of applying a vacuum as well as the rate of evacuation can be varied to optimize the process and hence the end-result. Reducing the pressure too early or at a high rate will result in significant outgassing of the additives in the EVA like adhesion promoters and/or stabilisers, and hence result in a decreased quality of the PV modules, whereas applying the vacuum too late will lead to air inclusion and hence unwanted bubble formation. c) Application of pressure to ensure a good surface contact and adhesion between the different layers of the PV module.
In order to laminate a solar panel, two layers of ethylene-vinyl acetate (EVA) are used in following sequence: glass / EVA / solar cell strings / EVA / tedlar polyester tedlar (TPT).
According to the Brij due to the relative large temperature difference of about 100°C between the heating plate and the PV module lay-up upon insertion, glass warping (curving) of the 3-4mm thick glass is observed. To avoid this glass warping and achieve homogenous heating profile, the flat-bed laminator is equipped with pins that are used to lift the PV module lay-up about 5mm from the heating plate, which results in a more gentle and homogeneous heating of the lay-up.
During the process of lamination, the prepared 5-layer module is placed in the lamination machine and heated to maximum135°C for a period of approximately 20 minutes. The final product that comes out is completely sealed; the lamination is now ready to protect the solar cells for at least 25 years. Excessive EVA and TPT during the lamination are discarded, and the junction box is attached. Finally the laminate can be framed.
Lamination Process bottlenecks
The mainstream lamination has some disadvantages: the machines are relatively huge, highly expensive, and consumes much electricity. Moreover a lamination machine is slow and is often considered as the PV module production bottleneck with throughputs of 50MW/year or more for any factory. Several advances has been made to address this issue, like larger area laminators, high vacuum, membrane free, multi-stage laminator and stack laminators (multiple lamination chambers in a vertical stack), in which the process steps are split into different processing units. In addition, the laminator can be used to do a lower temperature lamination process, which takes about one third the time of a full lamination plus cure process. In this case, curing is done after lamination in an in-line or batch process in a convection oven. The development of new faster curing and/or lower temperature curing encapsulant materials could have a significant impact on process throughput.
Future of PV lamination
There are numerous encapsulants that are most likely going to replace the old fashioned way of laminating. Lots of companies are constantly working and developing new way of encapsulating solar panels. Canadian manufacturer Qsolar replaced the old fashioned lamination process with a spray machine that equally spreads an adhesive on the solar panel. The major advantage of spray technology is that the solar cell is treated gently. However, this just the first step , lot of innovation which are expected to simplify lamination of panels is still under process.
Solar Encapsulation and EVA Market
Allied Market Research forecasts the solar encapsulation market to reach $4,231 million by 2022, growing at a CAGR of 23.1% from 2016 to 2022.
Solar encapsulation are materials to laminate the photovoltaic solar cells to enhance its efficiency and durability. The solar cell circuits are floated in between the materials such as ethylene vinyl acetate (EVA) and non-ethylene vinyl acetates to soften the effects of any external mechanical shocks and vibrations. EVA solar encapsulation has gained major demand in 2015 owing to its excellent protection against corrosion and delamination. The EVA solar encapsulation materials are expected to grow at a CAGR of 23.0% during the forecast period. In addition, encapsulated solar cells help improve the efficiency of solar modules owing to their excellent barrier protection against humidity and UV radiations. With the growth in need for alternative sources of energy such as solar energy, the market for solar encapsulation is expected to create potential growth opportunities with increase in applications in photovoltaic solar modules.
According to Eswara Prasad, Team Lead, Chemicals and Materials at Allied Market Research “Introduction of low cost, high performance encapsulating materials such as non-ethylene vinyl acetates has emerged into a scientific contribution towards enhancing the efficiencies of PV solar modules.”
Increase in installation of rooftop solar panels positively impacts the growth of the solar encapsulation market. Photovoltaic cells have gained major traction owing to increase in applications in residential and nonresidential sector for electricity generation, which is estimated to generate potential growth opportunities for solar encapsulation materials. Based on solar module, the demand for monocrystalline and polycrystalline silicon cells have witnessed major traction owing to increase in application coupled with low operational cost associated with it. The increase in applications of such solar cells is expected to soar the demand for encapsulation materials. Furthermore, growth in demand for solar installations in construction sector fuels the demand for solar encapsulation in PV modules.
According to the research firm tax incentives for rooftop installations in Asia-Pacific region, such as India, will drive the market growth for solar encapsulation materials. In addition, increase in development of low cost EVA films for solar encapsulation in China is expected to fuel its market growth in Asia-Pacific regions.
Lamination is multifaceted process with interchange of variety of processing parameters like pressure, time and temperature. Systematic research is needed to optimize the lamination process towards the fastest cycle time guaranteeing the highest quality and a robust process window.