Photovoltaic systems: What are they, how do they work and what are the advantages?
The basic guide to understanding how a an electricity generation system works and what are the benefits for residential installations.
The Photovoltaic effect is a natural phenomenon that occurs when solar radiation passes through a semiconductor material, such as crystalline silicon.
Silicon and the photovoltaic effect
What happens when the structure of a silicon crystal, treated with appropriate materials so as to create a PN Junction, is hit by sunlight (electromagnetic radiation)?
The sunlight is composed of energy particles called photons. When these photons hit the photovoltaic cell’s semiconductor material, a part of the energy is reflected, a part becomes heat and the remaining part causes an electron displacement in the atoms of the semiconductor material.
As the electrons move across the semiconductor’s structure, an electric current is generated in proportion to both the cell’s surface and the solar radiation intensity.
For example, a 10cm square cell, exposed to sunlight that hits it perpendicularly, under full sunlight, can deliver an electrical power of about 1.3 Watts at a voltage of 0.5 Volts.
Most photovoltaic energy applications require high power outputs with specific current and voltage ratings. These can be achieved by using multiple cells simultaneously, either in series or in parallel.
By assembling several cells together a photovoltaic module is formed.
To obtain higher voltages and currents, series or parallel connections between multiple photovoltaic cells are used.
Connecting modules in series form a string and the set of strings constitutes the photovoltaic generator.
What is a photovoltaic system?
A photovoltaic system uses solar radiation to generate a direct current (DC) by means of the photovoltaic effect.
By connecting a set of these photovoltaic modules in series/parallel, a photovoltaic generator is obtained with the desired current and voltage characteristics.
Modularity allows great flexibility of use.
In the first phase, a direct current is generated, which is then transformed into alternating current by an inverter.
This current can be used to power electrical appliances or simply fed to the grid.
There are two main types of photovoltaic systems:
- stand alone PV systems
- grid-connected PV systems
Stand alone PV systems
Stand alone systems are autonomous PV generators normally used to electrify households that are difficult to reach or connect to the grid because of their location.
A Stand Alone system is characterized by the need to cover the entire energy demand and is formed of the following elements:
- photovoltaic modules
- charge controller
- energy storage system (batteries)
These systems are economically convenient in cases where the power grid is absent or difficult to reach.
Grid connected systems
Grid-connected PV systems exchange the produced energy with the national power grid.
Power exchange can take place in two directions: if the PV generator output exceeds consumption over a period of time, the energy surplus is fed to national grid.
In the hours when the generator does not supply enough electricity to satisfy the users, the energy is taken from the Grid. Generally there are two energy meters that measure the amount of energy exchanged in both directions. However, there is an inverter that transforms the DC current produced by the alternating current photovoltaic system.
Systems connected to the network obviously do not need batteries because national grid supplies electricity solar radiation is unavailable.