Encapsulation is about as uninteresting as dissecting the design of a solar module as it can be for many investors. However, when assessing the guarantee periods of varied module types, the comprised superstrate and substrate materials play a significant role in how long certain manufacturers (with various encapsulation types) will guarantee a module. With this in mind, considering solar module encapsulation types is influential toward ensuring your investment can outlast in your nominated environment.
Encapsulation is an effective method of improving PV cell operating stability by reducing weather-related (moisture, UV radiation, oxygen, and temperature) degradation and increasing mechanical toughness against external impacts (Kumar Gaddam, Pothu and Boddula 2021).
1.1 Module materials
A clear top surface, an encapsulant, a back layer, and a frame surrounding the outer edge make up the majority of PV bulk silicon PV modules. As indicated below, the top surface of most modules is glass, the encapsulant is EVA (ethyl vinyl acetate), and the back layer is Tedlar (Bowden n.d.).
Figure 1: Materials used in typical bulk silicon models (Bowden n.d.)
1.2 Front Surface Materials
1.2.1 Characteristic Analysis of Front materials
Analysis of Transmittance Characteristic of Front Materials
Transmittance is defined as the fraction of incident light that is transmitted. In other words, transmittance is the amount of light that can “successfully” travel through a substance and emerge from the other side (KGaA 2021). According to (Lim et al., 2020), the front surface material for PV modules is typically low-iron tempered glass which protects the solar cells. The low iron content of 0.02% allows numerous photons to reach the solar cells. Furthermore, when an anti-reflective coating is applied to the entire glass surface to reduce PV module reflection, the transmittance is 90% or higher, and the reflectance is 5% or less. As a result, the low-iron tempered glass PV module has a high electrical output and is more reliable. The PV module with a manufactured front-surface film must pass reliability tests per IEC (International Electrotechnical Commission) 61215 certification standards (Raut et al., 2013).
The transmittance of the PV module’s front surface materials is essential since it impacts the c-Si solar cell’s short circuit current. Both short circuit current and light generated would be equal in an ideal solar cell with very efficient low loss mechanisms. Correspondingly, the maximum current that can be pulled from the solar cell is the short circuit current (Rooij, Short-circuit current n.d.). The short circuit current is reduced when the material on the front surface of the PV module has a low transmittance, resulting in lower electrical output and efficiency of the PV module. Consequently, before applying new front-surface material to a PV module, the transmittance must first be determined (Lim et al., 2020).
1.2.2 UV Characteristic Analysis of Front-Surface Materials of PV module
The reliability characteristics and the electrical output of the PV module can be altered by changes in the front-surface material’s transmittance. Moreover, UV-induced property changes are a considerable risk to PV modules as the front-surface material is continuously exposed to the harsh local environment and is directly exposed to sunshine. A solar module in the field that generates power is constantly exposed to UV from the sun, and if the front-surface materials are not appropriately assessed, this could cause some discoloration and power deterioration to the PV module (Lim. et al., 2020).
1.3 Types of Solar Modules
Solar panel technology has evolved and converged into three basic types: monocrystalline, polycrystalline, and thin-film, as solar has become one of the most dominant renewable energy sources (Winaico 2020).
1.3.1 Monocrystalline Solar Panels
Figure 2: Monocrystalline Solar Module (Winaico 2020)
In next-generation technologies such as PERC, HJT, and TOPCon, the monocrystalline solar cell sector now has the most research and development resources. Solar cells made from wafers cut from a single, pure silicon ingot manufactured by the Czochralski technique were utilised in monocrystalline solar panels. The solar cell’s pure silicone composition allows electrons to travel freely within it, potentially increasing efficiency and performance. Additionally, mono solar modules with high efficiency can use high-density module technologies like numerous busbars, reflecting ribbons, and backsheet to boost module efficiencies above 20% (Winaico 2020).
1.3.2 Polycrystalline solar panels
Figure 3: Polycrystalline Solar Module (Winaico 2020)
Solar cells manufactured from polycrystalline wafers cut from blocks of molten silicon, consisting of melted silicon fragments, were utilised in polycrystalline solar panels to provide cells with a metal flake appearance.
Polycrystalline modules absorb less light than monocrystalline solar panels due to their lighter blue hue, resulting in lower efficiency. Polycrystalline solar panels were once the most popular solar technology at the turn of the century due to their ideal combination of low cost and satisfactory performance. In the last several years, massive monocrystalline wafer manufacturing investments by upstream silicon wafer vendors have irreversibly changed the silicon solar cell market to monocrystalline.
1.3.3 Thin-Film Solar Panels
Figure 4: A thin-film solar system for commercial use (Winaico 2020)
Thick layers of photovoltaic material (CdTe, CIGS, or a-Si) are deposited on glass plastic, or metal to create thin-film solar panels. The most common thin-film solar panels are designed with two glass panes sandwiching the photovoltaic film and can work without frames.
In the case of thin-film solar modules, an experiment was conducted by (Lim et al., 2020) to study the UV characteristics of suitable films as the front material. The UV test was carried out using a 15 kWh/m^2 irradiance from 280 to 385 nm (nanometre) based irradiance and a module temperature of 60 ◦C (± 5 ◦C) alining with IEC 61215. The transmittance of the front materials’ was measured before and after the UV test. After the UV test, each sample is shown in the figure below.
Figure 5: Front material after UV test (size 4cm2) (Lim., et al. 2020)
A modest discoloration of the polymer protective film could be seen with the naked eye after the UV test, however, the EFTE and EFCTE films did not alter much (Lim., et al. 2020).
- ETFE (Ethylene Tetrafluoroethylene) is a lightweight material that works well as a replacement for glass. It’s a common choice for applications that demand a lot of light and UV transmission, such as:(ETFE|Fabritecture 2021)
- Large-span roofing
- ECTFE (ethylene chlorotrifluoroethylene) is a fluoropolymer substance that is more robust, stiffer, and more dimensionally stable than many other fluoropolymer materials (Plastics n.d.).
Figure 6: (A) Ultra-barrier film, ETFE, ECTF transmittance before and after the UV test (B) ETFE transmittance before and after the UV test (C) ETFE transmittance before and after the UV test (Lim., et al. 2020).
As a result of the experiment, the lightweight films (EFTE and EFTCE) had an excellent transmittance and UV characteristics as can be seen in Figure 3. Meanwhile, Figure 4 depicts the front-film PV module’s lay up structure.
Figure 7: The front -film PV module’s lay-up structure for a thin-film pv module (Lim., et al. 2020).
1.4 EVA (Ethylene Vinyl Acetate)
Ethylene Vinyl Acetate or also known as “EVA” is a solar cell/module encapsulant. It is a copolymer film that ensures the reliability and performance of photovoltaic solar modules by acting as a sealer (Mango n.d.). Solar EVA sheets are essential for extending the life and performance of solar panels, and since the 1980s, EVA has been the preferred encapsulant material for solar modules (Montgomery 2013). They allow the solar cells to float between the glass and the backsheet, absorbing shocks and vibrations while protecting the cells and their circuits (Targray n.d.).
1.4.1 Why EVA?
Ethylene-vinyl acetate is the most popular encapsulant material for PV modules (EVA) (F. Pern 1997). Its low cost, strong adhesion strength, and excellent transparency, with glass-like transmission qualities in the 400nm to 1100nm range, have made it popular in the PV sector (A. C. Pern 1996) (P 1983) (E.F.Cuddihy 1983). EVA also has a high electrical resistivity, a low polymerisation temperature, and a low water absorption ratio, all of which lead to it being an excellent, cost-effective option for encapsulating PV modules (Amrani., A and Y.Boukennous 2007).
1.4.2 Who manufactures EVA?
International Polymers Company (IPC), a Sipchem subsidiary since 2014, produces ethylene vinyl acetate (EVA) as a solid copolymer from ethylene and vinyl acetate with a production capacity of 200,000 metric tonnes per year (mtpa) (Sipchem 2021).
The primary purpose of the solar panel backsheet is to create electrical isolation between internal circuitry and the external environment (Surveying n.d.). International 2018 would go a step further to describe the primary function of the backsheet as being a shield for the PV module from UV rays, moisture penetration, and system electrical insulation and provide durability to the PV module. The backsheet is commonly constructed of a polymer or a mixture of polymers. Backsheets for solar panels play an important role in preventing damages for PV modules, allowing for long-term endurance and quality guarantee.
Module failures can occur because of backsheet failures, including catastrophic failure, unacceptable power degradation, and safety problems. The consequences can be devastating, ranging from damage to a company’s image and reputation to injuries (International 2018).
1.5.1 Backsheet manufacturers
Module manufacturers typically have a variety of backsheets within the scope of their certification for any given model number. These, however, are not of equal quality, even though they have all passed IEC testing (Pulsford 2020).
The photovoltaic backsheet market is dominated by Europe, which accounts for roughly 30% of the total market share. The photovoltaic backsheet market in Europe is expanding throughout Central and Eastern Europe. The photovoltaic backsheet market in the Asia Pacific is crucial due to rising hotspots such as China, India, Taiwan, Japan, and South Korea, which have considerable future market development potential.
DuPont, Isovoltaic, Coveme, Arkema, 3M, Toyo Aluminium, Madico, Hangzhou, Taiflex, Krempel, Targray, Toray, Dunmore, Astenik, and ZTT International are among the leading competitors in the worldwide photovoltaic backsheet market (Kasat 2021).
1.5.2 List of Manufacturers
Table 1: List of Backsheet manufacturers (ENFSolar 2021)
1.5.3 Types of backsheets
Backsheets are divided into three groups based on the layers that make up the backsheet (International 2018):
- Double Fluoropolymer – This is made up mostly of Tedlar polyvinyl fluoride (PVF) or Kynar polyvinylidene fluoride (PVDF) outer layers and a polythene terephthalate core layer (PET). These types of backsheets are the most expensive in terms of pricing.
- Single Fluoropolymer – Reduce the number of fluoropolymer layers from two to one which lowers the cost of the backsheet while retaining appropriate behavior and durability.
- Non-fluoropolymer – This is the cheapest option and comprises of two PET and one primer or EVA layer. Backsheet lamination is made possible by Arkema’s tri layered Kynar PVDF film. PVDF is one of the strongest compounds in fluoropolymer chemistry, according to Arkema Manager Sachin V Upadhye. Because of its chemistry, it provides the backsheet with excellent chemical resistance, sand abrasion resistance, and endurance. He also stated that Arkema’s KPK and KPE chemicals outperform the other chemistries (International 2018).
1.5.4 How to choose the backsheet?
TPT (Tedlar film-PET-Tedlar) is commonly used as a solar backsheet by most solar panel manufacturers. Tedlar is a brand name for polyvinyl fluoride (PVF), a thermoplastic fluoropolymer material manufactured by DuPont’s American chemical corporation. PET stands for Polyethylenterephthalat, a thermoplastic polymer that is widely used (Sunlight 2017).
Because TPT is protected by a patent asserted by the DuPont Company, only the solar backsheet containing Tedlar film-PET-Tedlar film can be named TPT (Sunlight 2017).
According to (Sunlight 2017), it is suggested that you apply the following guidelines when selecting a new backsheet:
- Choose a Tier 1 solar panel manufacturer (e.g., DuPont) since they are industry leaders and employ high-quality materials in their solar PV panels.
- To reduce risk, solar PV panels with non-UV resistant solar backsheets should be avoided.
- Inquire about the solar panel certifications, which are usually UL (UL1703) or IEC (IEC60601) (IEC61215 and IEC61730).
- When and how were the solar PV panels made, and how are they stored?
- Ensure that the solar modules have serial numbers.
- What is covered by a solar PV panel warranty and what is not covered by a warranty(Sunlight 2017).
Solar panels made of thin-film standard glass are frequently utilized as the substrate of a thin film panel since it is inexpensive. The amount of solar radiation transmitted is directly influenced by the type of solar glass used. Solar panels are often made with low-iron oxide glass to ensure excellent solar energy transmission (SinoVoltaics n.d.). However, cheap solar panel glass might become cloudy over time which could considerably reduce the solar panels’ efficiency. When choosing a monocrystalline, polycrystalline, or amorphous solar panel, pay attention to the type of glass used on the panel. There should be an explicit statement to this effect (Matters n.d.).
- Anti-reflective (AR) coating can be combined with solar glass by plating one layer of anti-reflective film on top of the glass before it is tempered. The coating will enhance transmittance by lowering reflectance on the glass’s surface. The addition of an AR coating on the glass surface can improve the amount of solar irradiation that is efficiently exploited for power generation by more than 2.5%. A module with a power rating of 250Wp equates to a gain of 6>Wp (SinoVoltaics n.d.).
- Light-Trapping is an alternative to an AR coating where direct sunlight enters the solar cells, and the less advantageous, flat-angle radiation is absorbed. A solar panel with this surface has the ability to catch more solar radiation (SinoVoltaics n.d.).
Figure 9: Standard solar glass (Left) vs Light Trapping (SinoVoltaics n.d.)
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