Photovoltaic Solar Energy

Photovoltaic solar

It is estimated that in the latitudes of France, around 45 °, the energy potentially usable from the sun is 1500kwh / m² per year.

See the French Seaside Map and DNI solar irradiation from France.

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 made of semiconductor materials. These are capable of transforming the energy supplied by the sun into an electric charge therefore into electricity because the sunlight excites the electrons of these materials. The absorption curve of these materials starts from short wavelengths up to a limit wavelength which is 1,1 micrometers for silicon.

Silicon is the main component of a photovoltaic cell.

Physics of a photoelectric cell (taken from the CEA website)


Operating diagram of a photoelectric cell.

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 table of Mendeleiev). In solid silicon, each atom is linked to four neighbors, and all the electrons of the peripheral layer participate in the bonds. If a silicon atom is replaced by an atom from column V (phosphorus for example), one of the electrons does not participate in the bonds; it can therefore move through the network. There is conduction by an electron, and the semiconductor is said to be n-type doped. If on the contrary an atom of silicon is replaced by an atom of column III (boron for example), it misses an electron to carry out all the connections, and an electron can come to fill this lack. We then say that there is conduction through a hole, and the semiconductor is said to be p-type doped. Atoms such as boron or phosphorus are dopants of silicon.

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When an n-type semiconductor is brought into contact with a p-type semiconductor, the excess electrons in the material n diffuse in the material p. The initially doped n area becomes positively charged, and the initially p doped area becomes negatively charged. An electric field is therefore 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 areas n and p, a diode 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.

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The current modules differ 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.

Cell builders.

The five largest firms manufacturing photovoltaic cells share 60% of the world market. These are the Japanese companies Sharp and Kyocera, the American companies BP Solar and Astropower, and the German RWE Schott Solar. Japan produces almost 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 domain to have used this energy is the space domain. In fact, almost all of the satellites' electrical energy is supplied by photovoltaics (some satellites would have small stirling engines).

Advantages

  • Non-polluting electrical energy in use and is in line with the principle of sustainable development,
  • Renewable energy source because inexhaustible on a human scale,
  • Can be used either in developing countries without a significant electrical network or in isolated sites such as in the mountains where it is not possible to connect to the national electrical network.


Example of isolated site supply, a seismograph powered by photovoltaic panel of the Soufriere volcano in Guadeloupe.

Drawbacks

  • Photovoltaic cost is high because it comes from high technology,
  • cost depends on the peak power, the current cost of the peak watt is around 3,5 € or around 550 € / m² of solar cells,
  • the current yield of photovoltaic cells remains quite low (around 10% for the general public) and therefore only delivers a low power,
  • market very limited but in development
  • electricity is produced only during the day while the highest demand is made at night,
  • the storage of electricity is something very difficult with current technologies (very high econological cost of batteries),
  • Lifetime: 20 to 25 years, after the silicon "crystalizes" and renders the cell unusable,
  • pollution during manufacturing: some studies claim that the energy used to manufacture the cells is never profitable during the 20 years of production,
  • likewise at the end of life: the recycling of cells poses environmental problems.

More:
Energy balance of solar photovoltaic
Map of French solar field
Photovoltaic solar systems integrated into the building (CEA document)

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