In the field of photovoltaics, a photovoltaic module is a packaged interconnected assembly of photovoltaic cells, also known as solar cells. An installation of photovoltaic modules or panels is known as a photovoltaic array. Photovoltaic cells typically require protection from the environment. For cost and practicality reasons a number of cells are connected electrically and packaged in a photovoltaic module, while a collection of these modules that are mechanically fastened together, wired, and designed to be a field-installable unit, usually with a glass covering and a frame and backing made of metal, plastic or fiberglass, are known as a photovoltaic panel or simply solar panel. A photovoltaic installation typically includes an array of photovoltaic modules or panels, an inverter, batteries (for off grid) and interconnection wiring.
A photovoltaic module is composed of individual PV cells. This crystalline-silicon module has an aluminium frame and glass on the front.
Theory and construction
The majority of modules use water based Crystalline silicon cells or a thin film cell based on cadmium telluride or silicon (see photovoltaic cells for details).
In order to use the cells in practical applications, they must be:
connected electrically to one another and to the rest of the system
protected from mechanical damage during manufacture, transport and installation and use (in particular against hail impact, wind and snow loads). This is especially important for water based silicon cells which are brittle.
protected from moisture, which corrodes metal contacts and interconnects, (and for thin film cells the transparent conductive oxide layer) thus decreasing performance and lifetime.
electrically insulated including under rainy conditions
mountable on a substructure
Most modules are rigid, but there are some flexible modules available, based on thin film cells.
Electrical connections are made in series to achieve a desired output voltage and/or in parallel to provide a desired amount of current source capability. Diodes are included to avoid overheating of cells in case of partial shading.
Since cell heating reduces the operating efficiency it is desirable to minimize the heating. Very few modules incorporate any design features to decrease temperature, however installers try to provide good ventilation behind the module,
New designs of module include concentrator modules in which the light is concentrated by an array of lenses or mirrors onto an array of small cells. This allows the use of cells with a very high cost per unit area (such as gallium arsenide) in a cost competive way.
Depending on construction the photovoltaic can cover a range of frequencies of light and can produce electricity from them, but cannot cover the entire solar spectrum. Hence much of incident sunlight energy is wasted when used for solar panels, although they can give far higher efficiencies if illuminated with monochromatic light. Another design concept is to split the light into different wavelength ranges and direct the beams onto different cells tuned to the appropriate wavelength ranges.This is projected to raise efficiency to 50%. Sunlight conversion rates (module efficiencies) can vary from 5-18% in commercial production. It normally cost $1,000,000.
Crystalline silicon modules
The most common design of modules contains cells made from silicon wafers. The wafers and cells are manufactured separately from the modules, sometimes in different factories and by different companies.
These cells are connected using conductive ribbons into one or more 'strings'. The module consists of a transparent top surface, an encapsulant, the strings of cells, another layer of encapsulant and a rear layer, and a frame around the outer edge. Typically, the top surface is low iron solar glass, the encapsulant is crosslinkable Ethylene-vinyl acetate (EVA), and the rear layer is a Tedlar- PET-Tedlar laminate.Glass
The front glass must have a high transmission in the wavelengths used by the cells, ie in 350 to 1200 nm range. Tempered, low-iron glass is the material of choice. Starting in 1990, cerium was added to absorb UV light and thus protect the encapsulant from degradation. Today, due to improvements in encapsulant stability, Ce is rarely used.
Typically, 8% of light is reflected by the outer surface of the glass i.e. at the air-glass interface. This reflective loss can be reduced to allow more light to reach the cell and more electrical power to be generated. Antireflective coatings (ref) or texturing the surface will reduce the reflective loss. However in some cases the textured modules collect dust and the increased light transmission through lower reflective loss is outweighed by light lost due to soiling.
The glass also serves the purpose of keeping out water and rigidifying the module, protecting the cells from damage from hail impact and bending and impact during manufacture, transport and installation. It must be stable under long-term exposure to ultraviolet radiation.
Encapsulant
The encapsulant serves two main purposes. The first purpose is to mechanically bond the strings of cells to the glass and thus maintain their positions over the life of the module. The second purpose is to provide an optical bridge between the glass and the cells. Otherwise there would be another air-glass interface and an air silicon interface each causing >=8% loss. The material must, like the glass, have excellent light transmission and be durable enough to last at least 20 years without degrading or debonding from the glass or the silicon. The material of choice is crosslinkable EVA, although Polyvinyl butyral (PVB) is used in modules with a glass backsheet.
Backsheet
The material on the back side of the module provides protection from UV degradation, electrical resistance, and moisture penetration. The majority of modules use a Tedlar-PET-Tedlar sheet. Glass and coated PET are also used.
Process
The encapsulant is melted and crosslinked in a vacuum laminator (except for glass front-glass back modules which are made in an autoclave. The glass, strings and backsheet are now attached to one another and form a 'laminate'. A frame made of aluminium profile is fitted around the edges of the laminate with an elastomeric seal (e.g. silicone) to seal the edges against moisture. The frame provides rigidity and the means to attach the module to a supporting structure. Finally, the strings are electrically terminated into a junction box usually glued to the back of the module.
Crystalline silicon modules have an efficiency of 13-18%.
1. Cells connected to make string
2. encapsulant film ready
3. ready for lamination. Note ribbons terminating 2 strings
4. after lamination
5. Aluminium profiles added to make the frame
Rigid thin-film modules
In rigid thin film modules, the cell and the module are manufactured in the same production line.
The cell is created directly on a glass substrate or superstrate, and the electrical connections are created in situ, a so called "monolithic integration". The substrate or superstrate is laminated with an encapsulant to a front or back sheet, usually another sheet of glass.
The main cell technologies in this category are CdTe, amorphous silicon, micromorphous silicon (alone or tandem), or CIGS (or variant). Amorphous silicon has a sunlight conversion rate of 5-9%.
Flexible thin-film modules
Flexible thin film cells and modules are manufactured in the same production line. They are created by depositing the photoactive layer and other necessary layers on a flexible substrate. If the substrate is an insulator (e.g. polyester or polyimide film) then monolithic integration can be used. If a conductor then monolithic integration cannot be used, and another technique for electrical connection used. The cells are converted to a module by lamination to a transparent colourless fluoropolymer on the front side (typically ETFE or FEP) and a polymer suitable for bonding to the final substrate on the other side. The only commercially available (in MW quantities) in a flexible module is amorphous silicon triple junction (from Unisolar).
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