Photovoltaic is an adjective to identify everything that is related to photovoltaic energy and the photovoltaic effect, more specifically. This word is a composition of photon and volt. A photon is the carrier particle of all forms of electromagnetic radiation, including visible light. On the other hand, one volt is the unit of electric current. In this way, it is easy to dedicate the relationship between visible light or solar radiation with the generation of electricity or electricity.
We refer to the photovoltaic effect in everything related to obtaining energy through the action of light. The most used light source for photovoltaic installations is that coming from the Sun, that is, solar energy. Although there are small devices, such as calculators, that can work with artificial light.
What is the photovoltaic energy?
A solar panel is a set of photovoltaic cells. These photoelectric cells are semiconductor devices that receive solar radiation when excited, cause electronic jumps and a small difference in diode-like potential at their ends (electric current). This property is called photovoltaic effect.
Classification and types of photovoltaic energy
Photovoltaic systems are mainly divided into 2 large families:
- "Isolated" systems (also called "independent"): they are not connected to any distribution network, so they directly exploit the electricity produced and stored in an energy storage device ( batteries) on site;
- "network connected" plants (also called network connected): these are plants connected to an existing electricity distribution network managed by third parties and, often, also to the private electrical system to be served;
A particular case of an island system, called "hybrid", remains connected to the distribution network, but mainly uses its sources, only one, or it can have a combination, for example, photovoltaic, wind, generator, even with the help of an accumulator. If none of the sources is available or the battery is discharged, a circuit connects the system to the mains for continuity of supply.
From the structural point of view, the "architecturally integrated" installation should be mentioned. The architectural integration is obtained by placing the photovoltaic modules of the plant within the profile of the building that houses it. The techniques are mainly:
- local replacement of the covering mantle (eg, tiles or tiles) with a suitable coating on which the photovoltaic field is superimposed, so that it drowns in the covering mantle;
- use of appropriate technologies for integration, such as thin films;
- structural use of photovoltaic modules, which also perform the function of frame, with or without glazing.
The costs to achieve an integrated photovoltaic system are higher than the traditional method, but the aesthetic result is favored by the energy bill legislation, with the recognition of a significantly higher incentive rate.
Off-grid photovoltaic systems (autonomous)
This family is at the service of those electrical companies isolated from other energy sources, such as the national alternating current network, which are supplied by an electrically isolated and self-sufficient photovoltaic system.
The main components of an island photovoltaic system are in general:
- photovoltaic field, designated to collect energy through photovoltaic modules conveniently arranged in favor of the sun;
- the storage battery or accumulator, which consists of one or more suitably connected rechargeable batteries (series / parallel) MEP / and retain the electric charge supplied by the modules in the presence of sufficient sunlight to allow deferred use by the user electrical equipment. With the use of a rear control unit, it can triple its life.
- Management automation: a type rear unit can automatically switch between different renewable energy sources (PV panels, wind, generators, etc., etc.) from one to another or for accumulation batteries and finally also to the supplier.
- charge regulator, designated to stabilize the collected energy and manage it within the system according to several possible situations;
- inverter otherwise said direct current / alternating current converter, designated to convert the output direct current (direct current) voltage from the solar panel (usually 12 or 24/48 volts) into a higher alternating voltage (typically 110 or 230 volts for systems up to a few kW, 400 volts for systems with a power exceeding 5 kW).
The photovoltaic field generally used for island systems is optimized for a specific system voltage, evaluated during the design phase. The most commonly used voltages are 12 or 24 V. Consequently, since most of the photovoltaic modules used in this type of plant have output voltages equal to 12 or 24 V, the so-called electric chains that make up the field are made up of a few modules, up to the limit only module by chain . In the latter case, in practice, the PV field is composed of a simple electrical parallel between the modules, equipped with a diode to protect against so-called reverse currents that will be discussed further.
The accumulator is generally composed of monoblocks, or individual elements designed specifically for deep and cyclic charges and discharges. In systems that must guarantee the continuity of the service even in the most severe conditions, they are not generally used to use automotive accumulators, which, despite functioning correctly, have a low "useful life", that is, tolerate a lower number of charge cycles and discharge compared to accumulators designed and built specifically for this type of use. In the case of pole or height accumulation facilities (e.g., street lighting or photovoltaic street lamp), car batteries cannot be used because any electrolyte leakage (which is composed of a highly corrosive solution) could cause harm to people, Animals and properties. In these facilities special accumulators are used in which the liquid electrolyte is replaced by a special gel.
The charge controller is an electronic device that has the following minimum characteristics:
- automatic disconnection of the photovoltaic field (understood as a set of all modules) from the battery of accumulators in the case where the voltage supplied by the modules is less than the minimum of the charge accumulators (very cloudy sky, night, failures , maintenance interruptions, etc.); in this case, in fact, the modules would behave like resistive loads by discharging the accumulators;
- automatic disconnection of the photovoltaic field from the accumulators in case of full recharge and possible derivation of the current produced by the modules to be sent directly to the inverter in case there is a request for energy from the user's devices;
- automatic disconnection of the photovoltaic field from the accumulators in case of total discharge of the latter (battery now depleted) and possible derivation of the current produced by the modules to be sent directly to the inverter in case of power demand of the user's devices.
In autonomous systems, the combination of photovoltaic solar panels with solar collectors is also frequent. The dollar collectors allow to capture solar radiation to increase the temperature of a liquid. The heat energy of this liquid can be used for sanitary hot water or heating. It is about solar thermal energy.