EVA (Ethylene-vinyl acetate) film as solar PV Encapsulant
Solar PV modules are made up of strings of solar cells that are linked together and capable of generating useful amounts of electricity when exposed to sunlight.
Solar cells are susceptible to brittleness failure under physical stress like bending and tension. Also, environmental factors like rain, dust, and external electric fields can create a negative effect on the performance of solar cells.
This brings the need to protect solar cells through packaging and encapsulation. The goal of encapsulation is to provide the mechanical support and environmental isolation required by the cells and electrical wire system to ensure their electrical performance.
The most common encapsulating material used for this purpose is EVA (Ethylene-vinyl acetate). In fact, EVA encases over 80% of photovoltaic (PV) modules on the market today. Since EVA is inexpensive and has good optical properties, it serves as a good encapsulant for solar cells.
However, EVA is not the only material used to encapsulate solar cells. There are other materials also continuously being tested for their ability to serve as a good encapsulant.
In this blog post, we will specifically look at what EVA film for solar panels is and together with some of its properties that make it such a popular solar cell encapsulant.
Before we delve into the discussion of EVA as a solar cell encapsulant, let’s first take a look at the structure of a typical photovoltaic module.
A photovoltaic module’s packaging is often a five-layer construction: glass front side/EVA for heat and environmental sealing/PV module/2nd EVA sealing film/back face protection. This construction ensures that the solar cells circuit and electrical insulation are protected from environmental damage.
EVA (Ethylene-vinyl acetate) in this construction is employed to protect the cells by offering high mechanical strength, UV resistance, and weatherability.
What is the function of Encapsulation in solar PV modules?
The primary function of encapsulation can be explained as follows:
Encapsulation provides structural and positioning support to the solar cell in the module design layout throughout manufacturing, transport, storage, installation, and use.
Maintain optical transmittance
Encapsulation helps to achieve and maintain the optical connection between the solar cell and the glass while maintaining an incidence of solar radiation transmittance of at least 90% and a maximum loss of 5% over a period of 20–30 years or more.
Maintain proper physical isolation
Encapsulation also provides and maintain proper physical isolation of solar cells and components, as well as circuit protection in the operating environment (i.e. from potentially aggressive and degrading elements)
Maintain electrical insulation
Another function of encapsulation is to maintain the electrical insulation between the solar cells and the circuit elements during the lifetime of the photovoltaic module.
Maintain the integrity of the electric circuit
Encapsulation maintains the integrity of the electric circuit, which generates the necessary current and voltage in the presence of sunlight.
Now let’s take a look at what EVA is and how does it serve as an encapsulant for solar panels.
What is an EVA?
Ethylene-vinyl acetate (EVA) is a thermoplastic copolymer made of ethylene and vinyl acetate. The weight percent of vinyl acetate typically ranges between 10% to 40%, with the remaining being ethylene. There are three varieties of EVA copolymer, which differ in their vinyl acetate (VA) composition and how they are used.
EVA is an elastomeric polymer that provides products with the softness and flexibility of rubber. The material possesses excellent clarity and gloss, as well as low-temperature durability, stress-crack resistance, hot-melt adhesive waterproof characteristics, and UV radiation resistance.
EVA has a characteristic vinegar-like odor and competes in many electrical applications with rubber and vinyl polymer materials.
Why is EVA used in solar panels?
Ethylene/Vinyl Acetate Copolymer (EVA) is widely used for encapsulating solar modules. Because it offers the following properties which are desired for solar modules:
- High electrical resistance makes it a good electrical insulator.
- Low processing and cross-linking temperatures,
- Outstanding melt fluidity
- Low water absorption characteristics
- Excellent optical transmission.
- Good adhesive properties
- High elasticity
EVA is a material that has been used in the PV industry for decades. The photovoltaic module with this type of encapsulation protects cells from mechanical damage, moisture infiltration as well as UV radiation.
It also ensures electrical insulation between layers or components within the same layer to ensure reliable operation over time. In fact, EVA encapsulation is an integral part of the industry standard.
What are some of the problems using EVA films in solar PV?
EVA for solar panels has been around for years and serves as a good encapsulant material because it provides protection to cells from environmental damage by providing necessary mechanical strength, UV resistance, weatherability, etc.
However, there have also been concerns about the reliability of EVA as a solar cell encapsulant.
The main concerns with using EVA are:
Yellowing or discoloration over time
EVA is manufactured with the use of acetate monomer, which gets hydrolyzed to acetic acid over time, which can cause the film to yellow or discolor. Change in color will reduce visible light transmittance of solar modules over time.
But this is not the main problem that comes with acetic acid formation. Acetic acid contacting with the solar cells can speed up the degradation of the silicon cells and can reduce cell efficiency.
The combined impact of temperature and UV radiation content from sunlight has enough energy to disrupt polymeric bonds, causing PV module breakdown, encouraging discoloration (yellowing or browning), and compromising their performance and reliability.
It is important to note that the main cause of discoloration is the encapsulant’s additives (chromophores and luminophores), not the EVA by itself.
UV light can cause photodegradation of EVA and in the presence of molecular oxygen, and higher temperatures result in the formation of acetic acid and other volatile gases.
These products become trapped within the module at various interfaces, causing delamination or the formation of bubbles, lowering the performance of the PV module. Generated acetic acid during these processes has the potential to corrode the photovoltaic module by attacking the metal contact.
Mediocre moisture resistance
Although EVA has an acceptable moisture resistance for solar applications, it is still not good enough to protect solar cells over a long period of time. The absorption of moisture by EVA can cause the delamination and corrosion of cells.
Water vapor forces EVA to hydrolysis which then creates acid, causing corrosion of metal contacts. This moisture can also create bubbles within the encapsulant layer and cause a reduction in the module’s power output efficiency due to increased light scattering among other things.
A certain amount of moisture is expected to be absorbed by the encapsulant because of the humidity in the air and the exposure of the modules to rainfall, water spray, and dew.
However, the needs of long-term solar cell encapsulation were not being fulfilled with the use of EVA, so industry players have been looking for alternative materials.
Inefficient use of the entire light spectrum
EVA has UV blockers that absorb the UV radiation and prevent it from reaching the solar cells. This is perfectly fine since traditional cell designs cannot use UV light spectrum.
However, there are newer cell designs that can utilize UV light for increasing efficiency. In this case, EVA acts as a barrier to the cells by preventing UV light from reaching them which in turn reduces the cell efficiency.
Are there any EVA encapsulation improvements?
As the solar industry is growing, there has been a strong push towards finding better solutions for EVA encapsulants.
One way to address the problems with EVA is by improving its properties in one or more areas of importance such as UV resistance, moisture barrier property, mechanical strength/flexibility, etc.
For example, surface crosslinking of EVA can improve its barrier properties by reducing the diffusion of gases and moisture. In this process, the EVA is treated with a silane coupling agent and then exposed to ultraviolet radiation.
This leads to the formation of covalent bonds between polymer chains at their surface, making them more resistant against moisture penetration and improving their resistance towards UV degradation.
Another way to improve EVA is by adding inorganic additives that are known for their UV-absorbing properties. Incorporating these additives can increase the photostability of encapsulants, which allows them to absorb higher levels of light energy without breaking down when exposed to high heat and sunlight.
The next step is looking into ways that the EVA encapsulant can be made more efficient at preventing corrosion and delamination, which leads to better performance over time.
A few ideas include using nano polymers to prevent corrosion and developing new adhesives with better heat resistance. In fact, this is exactly what so many research teams are studying.
But the main problem no combination of additives creates a perfect material. There is almost always a trade-off improving one property of the encapsulant while sacrificing another.
More research is needed to improve the current methods of EVA encapsulation. However, there are positive things on the horizon as more and more companies start looking for new ways to make solar modules better overall.
Are there any EVA alternatives?
Although EVA is still the most widely used encapsulation material it is not perfect. So, the solar industry has been looking for alternative materials to use as encapsulants that will improve upon EVA’s weaknesses and retain its strengths.
Some companies are already using EVA alternatives in their PV encapsulation material, and there are many more in the research phase. We are going to discuss a few of the common alternative materials being used and what makes them different from EVA.
Here are some of the most promising EVA alternatives:
Thermoplastic Polyolefines (TPO/POE)
POE is a thermoplastic polymer made from polyethylene and propylene monomers. It exhibits a similar performance to EVA with the added benefit of improved thermal stability and UV resistance. This is because POE doesn’t contain any acetyl groups like EVA does.
That means POE won’t be as susceptible to degradation from heat and UV radiation, so it will keep the solar cells encapsulated for much longer periods of time.
Thermoplastic polyurethane (TPU)
TPU film is superior to EVA film for encapsulation because it is more flexible and can bond with relatively hard materials. These films can be treated at room temperature without crosslinking or emissions.
Polyvinyl butyral (PVB)
PVB offers high optical transparency, good heat resistance, good adhesion to solar cells, glass, and other plastics, enhanced bond durability, and compatibility with module components. PVB is being employed as an encapsulating layer for thin-film solar cells.
EMA (Ethylene methyl acrylate)
EMA is also called ethylene methacrylate. It provides superior light transmission and durability properties than EVA. It has excellent mechanical properties, high light transmission, and optical clarity. EMA is used as an encapsulant for thin-film solar cells.
PIB is a type of synthetic rubber. It has the ability to encapsulate organic and perovskite solar cells.
Are EVA films recyclable?
EVA film is recyclable but currently recycling rates are very low and there are few available recycling options. Therefore most EVA waste ends up in landfills which is environmentally unfriendly.
Despite the certain concerns we have mentioned about long-term reliability, EVA has a long track record of being the most widely used encapsulant material for PV modules. This is because it provides an affordable solution that meets industry-standard requirements in terms of mechanical strength, weatherability, and stability.
The current market trend shows that most companies are sticking with the use of EVA as their encapsulant material. But, a few companies have already started using POE and EMA as alternatives to EVA in their solar modules which shows promise.
The replacement of EVA with new materials is a promising direction for the industry to take, but it will be years before we can fully replace EVA.
In the meantime, it is important to address the concerns of long-term reliability and efficiency with current PV encapsulation materials.
Finally, standard crystalline silicon module manufacturers are unlikely to switch to new EVA alternatives as they need time and more cost-effective materials that can fulfill their needs. Because EVA has performed satisfactorily in-field at an attractive $/W price, it will remain the most widely used encapsulant material for now.