How does it work?
With photovoltaic technology, solar irradiation can be directly converted into electricity through photovoltaic cells.
Two uses
• Connection to the network: The electric energy is fed into the public EDF grid (eg: large-scale ground photovoltaic plant). This is the most frequent case in countries with a high density power supply network (eg.: Europe).
• Stand-alone site: Photovoltaic energy is consumed locally (eg: panels installed on the roof tops that uses this energy for lighting). This is the case in areas without connection to the nearest power grid.
How a plant functions when connected to the network
• The electricity produced by the solar panels (direct current) is converted into alternating current. An ondulator stabilizes this current, which is then sent to the EDF grid, which in turn supplies it to consumers.
Lifetime of the installation
• The solar panels are operational for more than 30 years.
• The invertors are operational for 10 to 25 years.
How is the electric current produced?
Principles of operation:
- • The top layer of the cell is usually composed of silicon doped with an element with a valence1 higher than silicon (eg: phosphorus). As a result, this layer has an average number of electrons that is greater than a layer of pure silicon. This is an N-type semiconductor (where there is an excess of negatively charged electrons).
- 1: The valence of a chemical element is the maximum number of chemical bonds it can form.
- • The bottom layer of the cell is composed of silicon doped by an element with 1 valence less than silicon (eg: boron). This layer thus has an average number of electrons that is less than a layer of pure silicon. This is a P-type semiconductor (where there is a deficit of electrons and holes that are positively charged as a result).
- • When these two layers contact each other, a P-N junction is created, which allows electrons to move between the two plates. However, in photovoltaic cells, there must be an energy supply in the form of a light photon in order to make an electron leave the N-layer for the P-layer. As a result, global charge distribution in the system is modified.
- • Two electrodes are positioned, one on the top layer and the other on the bottom layer, thereby creating an electric current.
Types of photovoltaic cells
While there are a large number of technologies that can produce the photovoltaic effect, most of them are still in R&D phase. Two of these technologies are widely industrialized today:
- • Crystalline (monocrystalline/polycrystalline);
- • Thin-film.
“Monocrystalline” cells
• Raw silicon is melted into an ingot. When the cooling of the silicon is done slowly and properly, a large monocrystal is obtained. A “wafer” (a slice of silicon) is then cut from the silicon ingot (or brick). After several treatments (e.g., acid treatment of the surface, formation of the P-N junction, deposit of the anti-reflective layer, installation of collectors), the “wafer” becomes a cell. These monocrystalline cells are usually made in the form of round and square panels in a uniform blue.
“Polycrystalline” cells
• The "wafer" is sawed from a silicon ingot (or brick) whose forced cooling creates a polycrystalline structure. This type of cell is also blue, but as there are patterns in its structure, the blue is not uniform as in monocrystalline cells.
• This technology is actually the most common, due to its attractive yield for money and liability. In particular, this technology has existed for several decades, as opposed to thin-film cells, which are more recent. They are produced by many manufacturers in Europe, Asia and America.
Thin-film cells (amorphous)
• “Thin film” refers to a category of cells obtained by applying a thin layer of laminated photovoltaic material on a substrate (eg: glass). Many thin-layer technologies using different alloys (e.g., amorphous silicon, CdTe, CIGS) are industrialized today. While these cells have an inferior yield compared to crystalline cells (thin-layer cells yield 5-10% as opposed to 13-20% for crystalline cells), their production cost is much more competitive.
• Thin-film cells account for about 15% of the photovoltaic market. Crystalline cells account for virtually the rest of the market.