A PV panel consists of a number of interconnected solar cells (typically 36 connected in series) encapsulated into a single, long-lasting, stable unit. The key reasons for encapsulation are to prevent mechanical damage to the solar cells and to prevent water or water vapour from corroding the electrical contacts.
Panel lifetimes and warranties on bulk silicon PV panels are over 20 years, indicating the robustness of an encapsulated PV panel. A typical warranty will guarantee that the panel produces 90% of its rated output for the first 10 years and 80% of its rated output up to 25 years.
Most PV bulk silicon PV modules consist of a transparent top surface, an encapsulate, a rear layer and a frame around the outer edge. In most panels, the top surface is glass, the encapsulant is EVA (ethyl vinyl acetate) and the rear layer is Tedlar, as shown below.
Typical bulk silicon panel materials.
The front surface of a PV panel must have a high transmission in the wavelengths which can be used by the solar cells in the PV panel. For silicon solar cells, the top surface must have high transmission of light in the wavelength range of 350 nm to 1200 nm. In addition, the reflection from the front surface should be low.
In addition to its reflection and transmission properties, the top surface material should be impervious to water, should have good impact resistance, should be stable under prolonged UV exposure and should have a low thermal resistivity. Water or water vapour ingress into a PV panel will corrode the metal contacts and interconnects, and consequently will dramatically reduce the lifetime of the PV panel. In most modules the front surface is used to provide the mechanical strength and rigidity, therefore either the top surface or the rear surface must be mechanically rigid in order to support the solar cells and the wiring.
There are several choices for a top surface material including acrylic, polymers and glass. Tempered, low iron-content glass is most commonly used as it is low cost, strong, stable, highly transparent, impervious to water and gases and has good self-cleaning properties.
An encapsulant is used to provide adhesion between the solar cells, the top surface and the rear surface of the PV module. The encapsulant should be stable at elevated temperatures and high UV exposure. It should also be optically transparent and should have a low thermal resistance. EVA (ethyl vinyl acetate) is the most commonly used encapsulant material.
The key characteristics of the rear surface of the PV panel are that it must have low thermal resistance and that it must prevent the ingress of water or water vapour. In most panels, a thin polymer sheet, typically Telaar, is used as the rear surface. Some PV panels, known as bifacial modules are designed to accept light from either the front or the rear of the solar cell. In bifacial panels both the front and the rear must be optically transparent.
A final structural component of the module is the edging or framing of the module. A conventional PV panel frame is typically made of aluminium. The frame structure should be free of projections which could result in the lodgement of water, dust or other matter.
Source: PV education
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