EVA or POE, how to choose photovoltaic packaging film?

From Global PV

EVA stands for ethylene-vinyl acetate copolymer, and POE stands for ethylene-octene copolymer, which are two kinds of encapsulation films commonly used for double-glass modules at present. Now some manufacturers choose EVA for packaging, some manufacturers choose POE for packaging, and some manufacturers start to use EVA+POE hybrid packaging, so which is the best solution for dual-glass modules to choose EVA or POE? It is worth thinking about by people in the industry.

The solar cell module (also called solar panel, photovoltaic module) is the core part of the solar power generation system. Its function is to convert the solar energy into electrical energy and send it to the battery for storage or to drive the load to work.

PV

Photovoltaic modules, as the core equipment of photovoltaic power plants, can be divided into single-glass modules and double-glass modules from the perspective of packaging. With the development of the photovoltaic power generation industry, bifacial cells have developed rapidly. In the current market, most bifacial cell modules are packaged with double glass, and a few are packaged with transparent backsheets.

According to Bloomberg New Energy Finance (BNEF), by 2022, the market share of bifacial modules will exceed 90%. At the same time, the market share of transparent backsheets is relatively small, and double-glass modules will become the mainstream of the module market.

The quality of battery packaging directly affects the power generation efficiency and service life of photovoltaic modules. Among them, the packaging film has a significant impact on PID and other properties, and the choice of film material is very important.

1. How is PID formed?

The full name of PID (Potential Induced Degradation) is Potential Induced Degradation, which means that a large amount of charge accumulates on the surface of the battery, which makes the surface of the battery fail, resulting in a sudden drop in the power of the battery components.

Due to the needs of lightning protection engineering, the aluminum alloy frame of the module is required to be grounded, and a relatively high DC voltage is formed between the cell and the aluminum frame. For EVA package components, EVA cannot achieve 100% insulation. During use, water vapor penetrates into the components through silica gel, backplane, etc., causing the EVA to decompose, producing free-moving acetic acid, which precipitates on the surface of the acetic acid and glass. Under the action of the applied electric field, the sodium ions move to the surface of the battery and gather to the anti-reflection layer on the surface of the battery, thereby reducing the power of the module.

PID schematic

Moisture enters the module, EVA decomposes with water to generate acetic acid, acetic acid reacts with alkali to generate sodium ions, and the sodium ions are enriched on the surface of the battery. The above four steps are the whole process of the formation of the PID phenomenon, and this process is generally recognized by the photovoltaic industry. One of the causes of PID effect.

2. What are the advantages and disadvantages of EVA packaging film?

The main component of EVA film is EVA, plus various additives, such as cross-linking agent, thickener, antioxidant, light stabilizer, etc. EVA has become the preferred material for photovoltaic module packaging before 2014 due to its excellent packaging performance, good aging resistance and low price. But its PID defect is also obvious.

The emergence of double-glass modules seems to give EVA the possibility to overcome inherent defects. Since the water vapor transmission rate of glass is almost zero, the low water permeability or zero water permeability of double-glass modules makes EVA hydrolysis resistance no longer a problem.

Double glass modules are more resistant to corrosion

However, how to solve the problems of water vapor infiltration at the edge of the double-glass module and the resulting poor anti-PID performance, still cannot get a positive solution.

3. Opportunities and challenges of POE packaging film

Developed from metallocene catalysts, POE is a new type of polyolefin thermoplastic elastomer with narrow relative molecular mass distribution, narrow comonomer distribution and controllable structure. POE has excellent water vapor barrier ability and ion barrier ability. The water vapor transmission rate is only about 1/8 of that of EVA, and the aging process does not produce acidic substances. It has excellent anti-aging performance and is a high-efficiency and high-reliability photovoltaic. The material of choice for component encapsulation films.

POE

Although POE film has better water vapor barrier ability and better anti-aging performance, the market of POE has not been fully opened due to the high cost of particles and inferior processing performance to EVA. Until recent years, backplane companies that felt the challenge of double glass have also joined in the development of POE films.

Since 2017, the volume of POE film has risen and prices have fallen, and the market share has continued to increase. Moreover, since 2020, due to the serious shortage of EVA and the rapid price rise, POE has further gained a good opportunity to occupy the market. At present, POE film has almost become the mainstream choice for double-glass modules, and many owners of double-glass power plants have designated POE film in the bidding stage.

4. The future: The competition is fierce, and it is still unknown!

However, at the moment, EVA film has also ushered in a turning point. Under the pressure of affordable Internet access and subsidy reduction, end customers generally pay more attention to module prices. Some module factories have now begun to use EVA+POE combined packaging to reduce costs or deal with POE supply shortages. At the same time, with the popularity of large silicon wafer modules, double-glass modules have begun to use framed designs, and EVA has better processing fluidity than POE. Some module manufacturers believe that EVA can be used for double-glass modules with frames.

With the further improvement of the anti-PID performance of cells and the production technology of EVA film, it is not uncommon to use EVA film for double glass modules. The benign competition brought by EVA film to POE film will be more conducive to the production of POE film. Businesses will accelerate cost reduction and efficiency enhancement, reduce POE costs, and promote the further development of the photovoltaic industry.

Reference: "Discussion on the Use of EVA and POE in Double Glass Modules" Sun Zeyang, Li Mengmeng, etc.

Ai Bangai finishing