The Sun emits energy in the form of different frequency electromagnetic waves, essentially within the visible, ultraviolet and infrared ranges. A small portion of this energy is intercepted by the Earth. The amount of energy received by our planet -before reaching the atmosphere- per unit of area is called "solar constant" and its value is approximately equivalent to 1367 W/m2 if measured as perpendicular to the solar rays.
An important portion of the radiation that reaches the atmosphere is absorbed by the existing water vapor, carbon anhydride, oxygen, and ozone, with the intensity of that radiation decreasing as it gets across the atmosphere. At sea level, solar radiation is approximately equivalent to 1000 W/m2 if measured as perpendicular to the solar rays. This is the standard value utilized as a basis to express the electric characteristics of solar energy generators and to calculate systems.
The photovoltaic cells are devices whereon solar radiation is converted into electric energy, and the photovoltaic effect is the process created by said transformation.
Most of the photovoltaic cells of commercial application consist of wafers (less than 200 microns thick ) of different semi conductors materials like pure, mono or polycrystalline silicon , and some thin films of other semiconductors like amorphous silicon, Cadmium Telluride (CdTe), and Copper, Indium and Gallium Selenide (CISS).
In this report we will only refer to the mono- or poly-crystalline silicon cells that are utilized by SOLARTEC S.A. for manufacturing its photovoltaic modules.
A silicon crystal consists of silicon atoms arranged in an orderly way, so that each atom is bonded with another four neighbor atoms sharing electron pairs (co-valence links). In this condition, the electrons are under a minimum energy condition called "valence band".
The interaction of solar radiation with the silicon atoms in the photovoltaic cell can be considered to be as the interaction of particles, called "photons", with the silicon electrons. Each photon's energy depends on the wavelength of the solar spectrum whereto such photon belongs.
Under this energy condition, an electron moves freely within the crystal. Should there be no electric field inside the cell that may arrange the movement of all the released electrons in a same direction, the generated electric current would be null.
For getting this electric field, Phosphorus and Boron atoms are incorporated to the upper and lower portions of the cell respectively. The joint area between the upper and lower portions, which is parallel to the cell's faces, is called "Junction".
On the junction a difference in potential is generated that is approximately equivalent to 0.7 Volts, i.e. sufficient to achieve the photovoltaic effect.
The upper face of the cell has a conducting grid (negative electrode) that enables solar radiation to get inside the cell, and the lower face of the cell is fully covered by a conducting surface-area.
If a load (e.g. a lamp) is wired to a photovoltaic cell the operation would be as follows:
- The released electrons inside the cell reach the grid due to the electric field of the juncture, and they get out of the cell towards the lamp.
- The electrons deliver the energy to the load, keeping the lamp "on".
- The electrons return to the cell and get into it by its lower face.
- The electrons getting into the cell are absorbed once more by the crystal structure, taking-up the places ( holes) left vacant by other released electrons.
Not all the radiation incident upon a photovoltaic cell is transformed into electric power. This is so for the reasons as follows:
- A portion of the incident radiation is reflected again to the space.
- Some photons have no energy enough to break the electron bonds.
- Not all of the energy from the photons is utilized for the photovoltaic effect. This excess of energy is transformed into heat and lost for the process.
At present, the best silicon cells present a percentage of solar energy transformed into electric energy lower than 20 percent.
For a photovoltaic cell, the nominal current delivered by this cell is proportional to the cell area and to the insolation, and its nominal voltage depends on the junction electric field and its operation temperature.
For a square 156 mm-side silicon cell, the generated nominal current and the nominal voltage are approximately equivalent to 8 A and 0.48 V respectively under standard insolation conditions of 1000 W/m2 and at an operation temperature of 25 ºC.
A photovoltaic cell is a fragile element that cannot be exposed directly to the atmospheric conditions. On the other hand, its voltage is not sufficient for most of practical applications.
The basic element of a solar electric generator is its photovoltaic module, which is constituted by connecting individual cells in series (the positive face of a cell is connected with the negative face of the next one, and so on).
A module consisting of 36 series-connected cells will deliver a nominal current equivalent to that of each individual cell (approximately 8 A) and a nominal voltage equivalent to the product of 36 cells x 0.48 V = 17.3 V.
This module will deliver a maximum power of approximately 138 watts, which results from the product of 8 A x 17.3 V. This is the so-called "Nominal Power" or "Peak Power" for solar radiation standard conditions of 1000 W/m2 and an operation temperature of 25 ºC.
For enabling the connected cells to undergo extreme environmental conditions, they are encapsulated with a thermoplastic material, between a glass plate facing the sun and a composed plastic sheet that constitutes the rear face of the module. This process is called lamination and the product: a laminate. An anodized aluminum frame is fitted to this laminate and the electric terminals (positive and negative) of the set of cells are connected to a watertight junction box that is stuck to the rear face of the module.
Characteristics Curves and Values
The electric behavior of a photovoltaic module is expressed by means of a characteristic curve that represents the current-intensity values (measured in A) generated by the module according to the voltage (measured in V) at which it may be operating.
A given module model is defined by the characteristic values as follows:
- MP Maximum Power (Peak Power).
- IMP Current at Maximum Power point.
- VMP Voltage at Maximum Power point.
- ISC Current in short circuit.
- VCC Voltage in open circuit.
For instance, for SOLARTEC KS 65T module (see module list on PRODUCTS respective page) these values are as follows:
- PP 65 W
- IMP 3.75 A
- VMP 17.40 V
- ISC 4.08 A
- VCC 21.70 V