It is estimated that at the latitudes of France is about 45 ° the potentially usable energy of the sun is 1500kwh / m² per year.
With current yields of about 10 to 15% we get from 150 to 225kwh / m².an.
Solar panels called "non integrated".
Operating principle of photovoltaics
A photovoltaic cell is composed of semiconductor materials. These are able to transform the energy provided by the sun into electrical charge so electricity because the sunlight excites the electrons of these materials. The absorption curve of these materials starts at low wavelengths up to a limiting wavelength which is 1,1 micrometers for silicon.
Silicon is the main component of a photovoltaic cell.
Physics of a photocell (taken from the CEA website)
Diagram of operation of a photocell.
Silicon was chosen to make photovoltaic solar cells for its electronic properties, characterized by the presence of four electrons on its peripheral layer (column IV of the Mendeleyev table). In solid silicon, each atom is bonded to four neighbors, and all the electrons in the peripheral layer participate in the bonds. If a silicon atom is replaced by an atom of column V (phosphorus for example), one of the electrons does not participate in the bonds; he can therefore move in the network. There is conduction by an electron, and the semiconductor is said to be doped of type n. If, on the other hand, a silicon atom is replaced by an atom of column III (boron for example), an electron is missing to make all the bonds, and an electron can fill this gap. It is said that there is conduction by a hole, and the semiconductor is said p-type doped. Atoms such as boron or phosphorus are dopants of silicon.
When an n-type semiconductor is brought into contact with a p-type semiconductor, the electrons in excess in the material n diffuse into the material p. The initially doped zone n becomes positively charged, and the initially p-doped area becomes negatively charged. An electric field is thus created between the zones n and p, which tends to repel the electrons in the zone n and an equilibrium is established. A junction has been created, and by adding metal contacts on the n and p areas, it is a diode that is obtained.
When this diode is lit, the photons are absorbed by the material and each photon gives birth to an electron and a hole (we speak of electron-hole pair). The junction of the diode separates the electrons and the holes, giving rise to a potential difference between the contacts n and p, and a current flows if a resistor is placed between the contacts of the diode (figure).
Technologies available on the market.
The current modules are distinguished according to the type of silicon they use:
- monocrystalline silicon: photovoltaic sensors are based on silicon crystals encapsulated in a plastic envelope.
- polycrystalline silicon: Photovoltaic sensors are based on silicon polycrystals, which are less expensive to manufacture than monocrystalline silicon, but which also have a slightly lower yield. These polycrystals are obtained by melting scrap of silicon of electronic quality.
- amorphous silicon: "spread" panels are made of amorphous silicon with high energizing power and presented in flexible strips allowing a perfect architectural integration.
The five largest companies manufacturing photovoltaic cells share 60% of the world market. They include Japanese companies Sharp and Kyocera, US companies BP Solar and Astropower, and Germany's RWE Schott Solar. Japan produces nearly half of the world's photovoltaic cells.
Applications of solar electric energy
Currently the main areas of use are isolated dwellings but also for scientific devices such as seismographs.
The first area to use this energy is the space domain. Indeed, almost all the electrical energy of the satellites is provided by the photovoltaic (some satellites would have small stirling engines).
- Non-polluting electrical energy in use and is part of the principle of sustainable development,
- Source of renewable energy because inexhaustible on a human scale,
- Usable either in developing countries without major electricity network or in isolated sites such as mountains where it is not possible to connect to the national electricity grid.
Example of isolated site supply, a seismograph powered by photovoltaic panel of the Soufriere volcano in Guadeloupe.
- Photovoltaic cost is high because it comes from high technology,
- cost depends on the peak power, the current cost of the watt peak is about 3,5 € is about 550 € / m² of solar cells,
- the current yield of the photovoltaic cells remains rather low (about 10% for the general public) and thus delivers only a weak power,
- market very limited but in development
- Electricity production is only during the day, while the strongest demand is at night.
- the storage of electricity is very difficult with current technologies (very high cost of batteries),
- Lifetime: 20 to 25 years, after the silicon "crystalizes" and renders the cell unusable,
- manufacturing pollution: some studies claim that the energy used to manufacture cells is never profitable during the 20 years of production,
- likewise at the end of life: the recycling of cells poses environmental problems.