Guide to the operation of photovoltaic systems: technologies, types of cells, installation advantages, types and sizes plants

The Solar Photovoltaic and 'a technology that allows you to use the energy from the sun to produce electricity. A photovoltaic device is, in fact, capable of producing electric energy using solar energy directly through specific devices (photovoltaic cells) using the so-called "photoelectric effect."

The photoelectric effect is achieved within some materials suitably treated (eg. silicon) in the moment in which they are affected by solar radiation, the physical result of this effect is the production of electricity. The elementary device of a photovoltaic system for the production of electrical energy is the "solar cell", a series of connected cells constitute a "photovoltaic module".

The power of the photovoltaic modules is measured in Wp (watt peak) and that 'equal to the power delivered by the modules in standard conditions at room temperature of 25 ° C and in the presence of solar radiation that is of 1kW per square meter in a day serena at noon.

Principle of solar cell

The principle of operation of a solar cell is essentially the dual of that of the LED (Light Emitting diodes, light emitting diodes), devices that emit light when they are crossed by electric current. In an LED, electric energy is converted to (partially) into light energy (although not an incandescent lamp). In the photovoltaic cell is the process of "reverse" light energy can be partially converted into electrical energy. The cell becomes a "stack" electricity that can power an external electrical circuit. The operation of photovoltaic cells is based on the electronic phenomena that occur on the surface of contact of two different materials (in particular two semiconductors).

Principle of solar cell: Photovoltaic cells are photosensitive diodes that work in the fourth quadrant of the VI characteristic. Since the levels of light energy strikes the cell of the solar panels with value R = 0 behaves as a normal diode. When R is different from zero and is increasing the reverse current will form a potential difference across the junction which gives rise to the photovoltaic effect, and the diode supplies current to a load. In the maximum power point of the theoretical yield of a photovoltaic cell is about 22%. This falls to 15% in an actual performance. Normally from one square meter of solar panels can be produced at most 130-160 Watt. Each solar cell has an output voltage of about 0.5 volts. To produce electrical voltages higher photovoltaic cells are connected in series, vice versa to produce an output current greater photovoltaic cells are connected in parallel.


Obviously the current produced varies with the intensity of the radiation that reaches the cell, as can be seen in the example of the graph above. More intense is the light that arrives more electrons are liberated within the semiconductor and, consequently, greater SAR to the current flow. The most common material used to fabricate a photovoltaic cell is silicon.


The photoelectric effect

Photovoltaic conversion is a physical phenomenon that allows you to directly convert light energy into electrical energy in elementary structures, photovoltaic cells.

These devices are made from semiconductor materials, ie materials which conduct electric current and whose resistivity decreases with increasing temperature and the presence of impurities.

The photovoltaic effect consists in the generation of an electric potential difference, through the integration of a flow of radiant energy with matter.


Why install a photovoltaic system

Through the sun you can find up to 100% of the electricity needs of a family, the yield associated with these plants is, however, considerably lower than the one linked to the production facilities hot water for domestic use, that is to say that for the same energy produced must be a surface of upper panels. Photovoltaic systems may have a "cut" above the closely neceesaria to supply the needs of the individual user of the system in that any surplus can be sold to ENEL.


Photovoltaic systems - Technology

The operation of a simple photocell is based on the exploitation of the properties of some semiconductor materials that appropriately treated and connected together produce electricity when struck by solar radiation. The voltage produced by such elementary components is "channeled" through specific electrodes properly connected. To give an idea, each cell of dimensions 10cm x 10cm if well irradiated generates a voltage of about 0.5 V and a current of about 3A, namely a power (P = V x I) of about 1.5 W. To increase the power produced, the individual solar cells are connected appropriately to form the photovoltaic modules.


What type can be photovoltaic cells?

Depending on their production processes, we distinguish the following types of solar cells:

Monocrystalline cells (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 cells are usually characterized by a homogeneous blue staining.

Cells poly (multi-) crystalline (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).

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).


The photovoltaic modules

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 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.


Environmental Benefits

PV systems produce no emissions of any kind and during exercise does not emit gases with greenhouse effect. The production of one kWh of electricity from solar energy, when compared with equivalent energy production from fossil fuels, avoids the emission of 0.53 kg of carbon dioxide and that 'one of the main gases responsible for the emissions, but the same considerations can be repeated for other types of pollutants.

It is true that, at present, the total installed capacity of photovoltaic systems on a global scale is not 'enough to ensure an appreciable reduction of air pollutant emissions. 'Cause what happens, and' need for greater diffusion of technology in order to obtain concrete benefits. If the production of energy from photovoltaic cells has a very low impact on the environment, and that it 'restricted to the occupation of the land or of visual impact, the production phase of the photovoltaic panels involves a certain energy consumption and the use of chemicals. It should be recognized, however, that the majority of manufacturers of photovoltaic components is ISO14000 certified, and committed to recover and recycle all its effluents and industrial waste under careful control.

In the phase of decommissioning of the plant, the basic materials such as aluminum, silicon, or glass, can be recycled and reused under other sources. As regards the energy consumption necessary for the production of panels, what is called energy pay back time, ie the time required by the plant to produce as much energy than is required during stages of their industrial production, fell dramatically in the past years and is currently at about 3 years. For thin-film modules, the energy pay back time even reduced to one year. This means that, assuming a useful life of the solar panels of about 30 years, for the remaining 29 years the plant will produce clean energy.


What types of plants are there?

To determine which type of system to choose between different solutions on the market must, first and foremost assess the solar potential of the place where you live. That said, there are two types of installation:

 Plant photovoltaic modules for isolated users (stand alone) - this type of photovoltaic solar systems produce energy, which is used to charge the batteries, typically 12-24 volts, this energy is then used at the time of use. Normally this type of system is used where the operator does not come with their own lines, in particular for very isolated locations. Although in reality, completely detach ENEL, can create serious problems when there are no faults, or cloudy days, particularly, besides the fact that in any case, in principle, from an ecological point of view it is more convenient and practical yield Enel energy produced directly, rather than store it in expensive batteries and that once disused become highly polluting material.

 Plant photovoltaic modules with connection to the electricity grid Enel low voltage (grid connected) - These types of solar photovoltaic plants produce electricity that is fed once converted into alternating current at 220 volts, the Enel grid, to be used by any other user. This is done through a special counter installed by Enel. The solution "grid connected" to the network allows you to assign the surplus energy produced and to absorb in case of need, so we could pay only the difference between the energy produced and consumed. The cost of hiring the counter above is about 30 euro per year and this constitutes an economic advantage (but also environmental) when compared to the cost of the batteries it would take to store the energy produced, the cost is around 1000 € approximately every 10 years; under the environmental point of view it must be emphasized, also, the fact that the used batteries are material very polluting and difficult to dispose.


What size do I need a photovoltaic system for home use?

The average annual consumption of electricity for a family of 4 in Italy and 'of about 4000 kWh. As seen above to meet these needs it is necessary a plant of about 3000 kWp this means that, for a domestic user should be able to have an inclined surface of approximately 24 m2.



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