All conventional crystalline silicon-based solar panels consist of 5 layers. These layers are
- front encapsulant layer
- matrix-cell/solar cells
- rear encapsulant layer
[1,2]. Silicon solar cells have been embedded in the middle of the other four layers. Although the process of putting silicon solar cells into these layers leads to different optical and electrical power losses, they play a crucial role in protecting PV modules from diverse environmental, electrical, thermal, and mechanical damages. As an example, both encapsulation layers should provide structural and mechanical support for solar cells and their circuit components.
Both encapsulant layers and backsheet not only have the responsibility for the protection of solar panels from harsh environmental conditions but also have an impact on the aesthetic aspect . This article, first of all, will focus on the protective aspect of encapsulation layers. Then, it will describe different encapsulation materials that have been used by PV module manufacturers, and finally, outline the current status and future directions of this segment of the solar PV industry.
Why Encapsulation Layers?
Both the reliability of PV modules for mass production and the durability of them for long service lifetime up to 25-30 years are depending on the careful choice of materials for the encapsulant layers [1,3,4]. It is worth mentioning that the performance analysis of PV modules in the near and long-term is usually referred to as reliability and durability in the literature [1,5-7].
As a matter of fact, the new annual added solar capacity accounted for more than 48% of the total installed renewable energy capacity in 2020 and raised to more than 65% in 2021 [8,9]. Moreover, it is expected that the annual solar capacity will increase to 214GW and 222GW in 2022 and 2023 respectively . Thus, this industrial dominance brings the reliability and durability of PV modules and encapsulant layers into the central focus .
What are the material of an encapsulation layer?
The low cost, lightweight, flexibility, and easier assembly of polymeric materials have made them the most widely used material in the PV module encapsulation . First of all, Polyethylene (PE) was initially chosen as a polymeric encapsulant because of its simple structure and low cost. However, the main drawback was its opaque and translucent appearance [10,11]. After that, PE-based materials in which PE was the backbone along with different side groups were chosen to solve transparency challenges and keep PE advantages. One of these PE-based polymeric materials is Ethylene-vinyl acetate (EVA) in which vinyl acetate (VA) was added to PE to form a copolymer. EVA has become the mainstream material for commercial use due to its low cost, adequate transparency, and flexibility from 1981 up to now [1,5,10].
There is a famous Golden triangle in the PV module production that scientific researchers and PV module manufacturers consider to assess the technical feasibility and commercialization capability of encapsulants in the destination market: Price, Performance, and solar panels Efficiency [12-14].
EVA as the dominant solar panel’s encapsulation material is good at two out of three items, i.e. price and efficiency. Although its performance has been acceptable, some deteriorating factors of EVA functionality including
- Acetic Acid Formation
- Moisture ingression
- UltraViolet (UV) radiation
Sometimes lonely and usually along with each other during the period of field operation lead to different time-dependent failures . Common time-dependent failures in EVA are
- Turned EVA to yellow/brown color (discoloration)
- Delamination and bubble formation
- Potential-induced degradation (PID)
- Snail trails
- Hot spots [1,15,16].
Therefore, the introduction of a new non-EVA type encapsulant with low-cost, high-performance, and high
durability could provide a solution to discoloration, delamination, and PID problems.
Current status and future perspective of encapsulation market
As a dominant type of PV modules, crystalline silicon (c-Si) modules with more than 90% market share are embedded in polymeric encapsulant layers [17,18]. Today, EVA is the most popular polymeric encapsulation material with more than 80% market share [1,19].
Non-cross-linking thermoplastic polyolefin or TPO is a newly emerged encapsulation material which has four distinct advantages over EVA and overcomes the challenges which EVA has faced during field installation. We will deeply dive into time-dependent failures of EVA as well as advantages of TPO over EVA in the second part of this paper.
It is estimated that TPO will reduce the dominant share of EVA in the market and grab close to 20% share within the next 10 years, however, other materials will have kept their low market shares for niche applications [1,20].
Did you know?
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