The photovoltaic cell is the basic element in the construction of a photovoltaic module, but it can also be used individually in specific uses.
The most common version of the photovoltaic cell, that in the crystalline material, is constituted by a lamina of semiconductor material, the most widespread of which is silicon, and typically occurs in black or blue and varying in size from 4 to 6 inches .
Small specimens of photovoltaic cells in the amorphous material are able to feed autonomously consumer electronic devices, such as calculators, watches and the like.
The photovoltaic modules are made up of individual solar cells. Each module has a size ranging from 0.5 to 1 m² with power from 50Wp to 200Wp. A set of modules assembled together form a solar panel. A set of modules connected electrically constitutes a string, a set of strings connected in parallel between them, form the photovoltaic field that, together with other components, allows to realize the photovoltaic systems usable for the production of useful electricity.
Classification of photovoltaic cells: Crystalline Modules
Depending on their production processes, we distinguish the following types of solar cells:
Cells (silicon) monocrystalline (1): are produced by cutting a single crystal bar. The main advantage is a high efficiency (up to 16%). This type of cells, however, is very expensive due to the complicated production process. The type monocrystalline solar cells are usually characterized by a homogeneous blue staining.
Cells (silicon) polycrystalline or multicrystalline (2) are poured into blocks and then cut into disks. The yield is lower (10-12%), but also the price. This type of cell is recognizable from a drawing clearly distinguishable (because of various crystals contenutivi). There are cells with contacts drowned in silicon (back-contact) with higher yields of 1.5 to 2%. Above the cell is deposited a layer of antireflective (titanium oxide).
Classification of solar cells: Thin-film modules
Amorphous cells (3): are produced by sputtering of silicon atoms on a glass plate. This type of cell has a lower yield (ca. 4-8%), but also adapts to the case of diffused light (overcast sky, etc..). The cells thus produced are recognizable by a characteristic dark color, also can be made in any geometric shape (circular shapes, octagonal, irregular, and even convex are feasible). There are different types made of different materials:
amorphous silicon, in which the atoms are chemically deposited silica in amorphous form, or structurally disorganized, on the surface of support. This technology uses very small amounts of silicon (thickness of the order of microns). The amorphous silicon photovoltaic modules typically show less constant efficiency of other technologies from the nominal values, despite guarantees in line with the market. The most interesting concerns the EROEI, which provides very high values (in some cases also coming to 9), attesting to the profitability of this technology.
Cadmium telluride (CdTe)
Cadmium sulfide (CdS) microcrystalline, which has production costs very low because the technology used for its production does not require the attainment of high temperatures necessary instead to the fusion and purification of silicon. It is applied to a metal support for spray-coating, that is literally sprayed as a paint. Among the disadvantages associated with the production of this kind of solar cells there is the toxicity of cadmium and the low performance of the device.
Gallium arsenide (GaAs), a binary alloy with semiconductive properties, capable of ensuring very high efficiency, due to the property of having a direct gap (unlike the silicon). It is used mainly for military applications or advanced science (such as planetary exploration missions automated or particularly sensitive photodetectors). However the prohibitive cost of the monocrystalline material, from which the cells are made, it is intended to use a niche.
copper indium diselenide (CIS), with variable opacity from 100% to 70% obtained by means of holes formed directly in the film.
copper indium gallium diselenide (CIGS)
Certification of photovoltaic modules
The photovoltaic modules, when used in a photovoltaic system connected to the network within the European Union are obliged to be certified according to IEC 61215, which determines the characteristics of both electrical and mechanical. Among the most important tests to determine what is mentioned in the power insolation conditions standard, expressed in watt peak (Wp).
Performance and efficiency of photovoltaic modules
The performance of photovoltaic modules are susceptible to substantial variations in base:
the performance of materials;
the manufacturing tolerance percentage of the nominal values;
radiation to which its cells are exposed;
angle with which this comes to its surface;
at the operating temperature of the materials, which tend to "struggling" in hot environments;
the composition of the light spectrum.
Due to its construction, the performance of PV modules is generally less than or equal to their worst performance of the cell.
With yield refers to the percentage of energy captured and transformed with respect to the total reached on the surface of the module, and can be considered an index of correlation between watt disbursed and area occupied, without prejudice to all other conditions.
Some panels, for use in aerospace, have nominal yields that can reach 40%, and are produced with rare and costly materials and highly toxic; typical values found in commercial products based on silica are around:
16% in monocrystalline silicon modules;
13% in polycrystalline silicon modules;
10% in the silicon modules microsphere;
8% in the amorphous silicon modules.
It follows that such a parity of power production demand, the surface occupied by an amorphous photovoltaic field will be more than double compared to an equivalent crystalline photovoltaic field.
Due to the natural fatigue of materials, the performance of a photovoltaic panel joint decreased by about one percentage point on an annual basis. To ensure the quality of the materials used, the legislation requires a minimum of two years of guarantee against manufacturing defects also also on the decline in yield of silicon over time, this gets at least 20 years. The warranty today in the forms of good quality is 90% of nominal for 10 years and 80% of nominal for 25 years.
The photovoltaic modules today have an estimated life of 80 years or so, although it is plausible to assume that they are discontinued after a cycle of life of 35-40 years, due to the power loss of the modules.
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