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Photovoltaic module

Photovoltaic module

Photovoltaic modules or solar panels are devices that are used to capture the energy of the sun's light. Photovoltaic solar panels contain a set of solar cells that convert light into electricity. It is called solar because the sun is one of the strongest energy sources for this type of use. Solar cells are sometimes called photovoltaic cells, and photovoltaics literally means "light-electricity". Solar cells have the photovoltaic effect to absorb the sun's energy and cause electrical current to flow between two charged layers in the opposite direction.

Currently, the costs associated with solar modules become cheap in applications where the power of the electric stations is available. The cost of fossil fuels is increasing, and the production experience is reducing the costs of solar cells, this may not be seen in the very near future, but in the long run the trend is an increase in the use of this type of energy renewable.

A photovoltaic module is a set of interconnected photovoltaic cells protected from the outside by a structure composed basically of a glass and a rigid frame.

Photovoltaic cells are elements that, thanks to the properties of silicon, can transform solar radiation into electrical energy at very low voltage through the photovoltaic effect.

The photovoltaic panel has the function of grouping all these small voltages generated to provide a higher nominal voltage to the system.

The photovoltaic modules provide a DC voltage. The rest of the elements of the photovoltaic system will be responsible for managing and transforming this voltage into alternating current, if necessary.

Solar collectors are solar panels that, through the laws of thermodynamics, take advantage of the sun's heat to heat a liquid.

Construction technologies for photovoltaic modules

Of the many materials that can be used for the construction of photovoltaic modules, silicon is the most used. The silicon is obtained in wafers that are then joined to form a photovoltaic module.

The types of construction of the most common photovoltaic cells are:

  • Monocrystalline silicon: the cells have an efficiency of 18-21%. They tend to be expensive and are also present, they are cut with cylindrical ingots, it is difficult to cover them with extended surfaces without wasting material or space.
  • Polycrystalline silicon: cheaper cells, but less efficient (15-17%), whose advantage lies in the ease with which it is possible to cut them into suitable shapes to join in modules.
  • Amorphous silicon deposited by vapor phase: photovoltaic cells have a low efficiency (8%), but they are much cheaper to produce. Amorphous silicon (Si-a) has an important band of crystalline silicon (Si-c): this means that it is more efficient in absorbing the visible part of the spectrum of solar radiation, but less efficient in the collection of the infrared part. Since nanocrystalline silicon (with crystalline nanometric domains) has approximately the same Si-c band range, the two materials can be combined to create a layered photovoltaic cell, in which the upper Si-a layer absorbs visible light and leaves the infrared portion of the spectrum to the lower nanocrystalline silicon cell.
  • CIS: the cells are based on chalcogenide layers (for example, Cu (InxGa1-x) (SexS1-x) 2). They have an efficiency of up to 15%, but their cost is still too high.
  • Photoelectrochemical cells: these photovoltaic cells, first built in 1991, were initially designed to mimic the process of photosynthesis. This type of cell in a photovoltaic module allows a more flexible use of materials and production technology seems to be very convenient. However, the dyes used in these cells suffer degradation problems when exposed to heat or ultraviolet light. Despite this problem, this is an emerging technology with an expected commercial impact within a decade.
  • Hybrid photovoltaic cell: combines the advantages of organic semiconductors and various types of inorganic semiconductors.
  • Concentrated photovoltaic cell: the use of this cell in a photovoltaic module combines the aforementioned technologies with solar concentration lenses that significantly increase efficiency. They represent the promising new generation of panels still in development.
  • Monocrystalline silicon, in which each cell is made of a wafer whose crystal structure is homogeneous (monocrystal), appropriately doped to form a pn junction;
  • Photovoltaic module with polycrystalline silicon, in which the wafer mentioned above is not structurally homogeneous, but is organized in locally ordered grains.

Crystalline photovoltaic modules

  • Monocrystalline silicon, in which each cell is made of a wafer whose crystal structure is homogeneous (monocrystal), appropriately doped to form a pn junction;
  • Photovoltaic module with polycrystalline silicon, in which the wafer mentioned above is not structurally homogeneous, but is organized in locally ordered grains.

Thin film modules

Thin film photovoltaic modules are manufactured by depositing the semiconductor material on a glass-like substrate, so that the rigid solar panels are used outdoors; or plastic, in the case of flexible panels for less conventional uses.

The thin film module is manufactured monolithically and does not require the assembly of several cells, as in the case of crystalline silicon panels, in addition, the amount of semiconductor material present in the panel is considerably smaller than the panels made with photovoltaic cells standard, which reduces the production costs, on the other hand, the deposited material seems to have a high defect and, therefore, the thin film panels will have a lower efficiency compared to their monocrystalline equivalents.

The thin film modules are subdivided into several categories according to the semiconductor materials deposited therein, among the most common we find:

  • Amorphous silicon, in which the silicon atoms are chemically deposited in amorphous form, or structurally disorganized, on the supporting surface. This technology uses very small amounts of silicon (thicknesses of the order of microns). The amorphous silicon modules generally show a less constant efficiency of the other technologies compared to the nominal values, despite having guarantees in line with the market. The most interesting data refers to EROEI, which provides very high values (in some cases it even reaches 9), which demonstrates the economic efficiency of this technology.
  • Cadmium telluride (CdTe): these are thinner solar panels with a lower price and lower thermodynamic efficiency.
  • Microcrystalline cadmium sulfide (CdS), which has very low production costs because the technology used for its production does not require the achievement of the very high temperatures required instead of the fusion and purification of silicon. It is applied to a metal support for spray coating, that is, it is literally sprayed as a paint. Among the disadvantages associated with the production of this type of photovoltaic cells is the toxicity of cadmium and the low efficiency of the device.
  • Gallium arsenide (GaAs), it is a binary alloy with semiconductor properties, able to guarantee very high yields, due to the property of having a direct gap (unlike silicon). It is mainly used for advanced military or scientific applications (such as automated planetary scouting missions or especially sensitive photodetectors). However, the prohibitive cost of the monocrystalline material from which the cells are manufactured has been used for a specific use.
  • Indian copper diselenide (CIS), with an opacity ranging from 100% to 70% obtained through holes made directly in the film.
  • Indian copper gallium diselenide (CIGS)
  • Heterojunction, literally union between different substances, in which a layer of crystalline silicon is used as support surface of one or more amorphous or crystalline layers, each of which is optimized for a specific radiation subband;
  • Microspherical silicon, in which reduced polycrystalline silicon is used in spheres with a diameter of approximately 0.75 mm caged in an aluminum substrate;

Proprietary variants

Of the mentioned technologies, only the amorphous and the microspherical allow the flexing of the module: in the case of the amorphous there is no crystalline structure of the material to prevent it from bending, in the case of the microspherical is not the cell (sphere) that bends, but the honeycomb grid in which it is placed.

Construction of photovoltaic modules

Crystalline silicon and gallium arsenide are the typical choices of materials for solar cells. Gallium arsenide crystals are created especially for photovoltaic applications, but silicon crystals are also produced for consumption by the microelectronics industry.

Polycrystalline silicon has a lower percentage of conversion, but at a reduced cost.

Photovoltaic panels

When exposed to a direct light of 1 AU, a silicon cell 6 centimeters in diameter can produce a current of 0.5 amperes at 0.5 volts. Gallium arsenide is more efficient.

The sets of solar panels can produce electricity for isolated places that have good lighting.

The glass is cut into small discs, polished to eliminate the danger of cutting, dopants are inserted into the discs and the metallic controllers are deposited on each surface: a small connector on the surface facing the sun and a connector on the other side. The solar modules are constructed with these cells cut into appropriate shapes, protected from radiation and damaged by applying a layer of glass and cemented onto a substrate (either a rigid or flexible panel).

The electrical connections are made in series-parallel to determine the total output voltage. The protective layer must be a thermal conductor, because the cell enters when it absorbs the infrared energy of the sun that is not converted into electrical energy. As the heating of the cell reduces the operational efficiency, it is desirable to reduce this heat. The result of this construction is called photovoltaic module or solar panel.

A solar panel is a set of solar cells. Although each solar cell provides a relatively small amount of energy, a set of solar cells scattered over a large area can generate enough energy to be useful. To receive the greatest amount of energy, solar panels must be directed directly at the sun.

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Last review: October 30, 2018

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