The photovoltaic effect was observed for the first time by Henri Becquerel in 1839. He observed that the intensity of current between two platinum electrodes immersed in a conductive solution of lead nitrate (electrolytic cell) contained in a glass cylinder, increasing the battery if exposed to sunlight as follows
Therefore the photovoltaic effect is defined as the generation of a current which is to be established between two electrodes attached to a solid or liquid when exposed to sunlight. All photovoltaic devices consist of a pn junction in a semiconductor through which a current is flowing. These devices are known as photovoltaic cells.
In 1954 U.S. study at Bell Labs led to the establishment of the first commercial monocrystalline silicon solar cells. In this period the photovoltaic technology found application in aerospace. Only since 1970 with the emergence of a global energy crisis, began to transfer PV technology also in the field of civil construction.

                                    

Currently on the market photovoltaic cells have different sizes depending on their type:

• Monocrystalline silicon cells: they have a higher degree of purity of the material and ensure the best performance in terms of efficiency since the highest yield (over 15%). The color is very dark blue uniform and have circular or octagonal in size from 8cm to 12 cm in diameter and 0.2 - 0.3 mm thick.

• Polycrystalline silicon cells: they have a lower purity condition that involves a lower efficiency that their performance is between 11 and 14%. They show an intense blue iridescent due to their polycrystalline structure.Have a square or octagonal and thickness similar to the previous type.

• Amorphous silicon: it is the deposition of a thin layer of crystalline silicon (1-2 microns) on surfaces of other materials such as glass or plastic substrates. In this case it is improper to speak of cells, because even large surfaces can be covered continuously. The efficiency of this technology is significantly lower in the order of 5-6.8% and is subject to a significant decline (-30%) of its performance in the first month of life (effect Stabler-Wronsky) and therefore requires an oversize surface installed, so that during operation the power production foreseen in the project.

• Amorphous thin film cells: CIS (Copper Indium Diselenide), CIGS (Copper Indium Gallium Diselinide) AsGa (Cadmium Gallium), CdTe (Cadmium Tellurium). Cells are low cost and thickness of the order of 5 to 10 micrometers. The efficiency is far below the monocrystalline and polycrystalline cells.

The complex of photovoltaic cells connected in series or in parallel to obtain voltage and current suitable for common uses is said photovoltaic module. In form the cells are protected from the elements by a glass on the front and insulating materials and plastic on the back.

The components of a photovoltaic system

                                    

The photovoltaic modules are the major plant as their exposure to solar radiation causes the production of electricity (DC). Inside the module there are solar cells generally consist of thin "slices" of silicon that, properly treated, result in the direct conversion of light energy into electricity. Based on the characteristics of the cell it comes to cell monocrystalline silicon, polycrystalline silicon cells and thin-film cells or thin films.The support structures of the modules are the structures which support the modules and provide guidance to them, giving an angle to the horizontal.In Italy the optimum inclination is about 30 degrees. The structures can be galvanized steel or aluminum, and are placed on the surface installation by means of anchors or ballast.

L 'Inverter is an electronic device that allows you to adjust the electricity produced by the modules to the needs of electrical and network operating the conversion from direct current to alternating current with a frequency of 50 Hz.

The control system is an electronic device that communicates with the inverter and any ancillary sensors (meteorological measurements and electrical).

Gauges of energy are the devices that are installed on power lines and measure the energy flowing through them, for example, are used to count and the energy from the grid.

The electric cables and panels, cables, switches and additional safety devices are electronic components that complete the installation.