The sun is the main energy supplier. The development of the renewable energy market is largely based on this fact. Most of the green technologies operating today collect solar energy, either directly or indirectly. The direct solar energy source consists of solar rays reaching the atmosphere and these can be collected by two different technologies: solar photovoltaic and solar thermal systems.
Technology
The fundamental difference between photovoltaic solar energy and solar thermal energy can be found in their operating principles. Photovoltaic solar energy is based on the photovoltaic effect, whereby a photon (hitting a surface made of a special material generates the release of an electron. Solar thermal energy, on the other hand, uses sunlight to heat a fluid, which may be water but not necessarily.
The photovoltaic effect only occurs in a small number of materials, called semiconductors (such as silicon - monocrystalline, polycrystalline and amorphous - and cadmium telluride), which, following specific chemical processes, allow the generation of an electric current when exposed to light. These semiconductors take the form of thin layers that form the core element of solar cells, the basic element of a solar photovoltaic system, which produces a direct current.
Solar cells are integrated into larger structures known as solar panels, where the desired values of current and voltage can be achieved. For grid-connected systems - which represent a large part of the market - the use of an inverter is necessary to convert the direct current generated by the cells into alternating current, which is used in the grid.
The principle of operation of solar thermal systems is much less sophisticated, but just as useful for generating consumable energy as the photovoltaic effect. It consists of the direct heating of water, or other fluids, by sunlight. This energy conversion takes place in different devices depending on the temperature range at which the fluid is heated.
Types of photovoltaic solar panels
Solar panels are mainly made of silicon. The purity of this material, as well as the manufacturing processes, determine the quality and performance of solar panels. There are three main types of solar panels:
- Monocrystalline solar panels
They have a yield close to 24%, the theoretical limit being 25%. The purity of the silicon that forms it is higher and therefore ensures higher yields, although at the same time they have a higher cost.
- Polycrystalline solar panels
They are less expensive than monocrystalline but less efficient. They have an approximate efficiency of 18% due to a lower purity of the construction material. Although they have a lower efficiency than monocrystalline panels, the losses when working at high temperatures are lower.
They have half the performance of a monocrystalline panel, and their performance deteriorates more rapidly over the years. They are not recommended for large installations.
Types of solar thermal energy
- Low temperature solar thermal energy
This energy is produced by low-temperature collectors, which allow temperatures of up to 65º C to be reached through heat absorbers. This energy in the form of heat is used for domestic use, allowing to have solar heating in them.
- Medium-temperature solar thermal energy
Medium-temperature collectors are identified with which temperatures up to 300°C can be generated. These collectors work by concentrating energy through mirrors, so they can only operate in places with high amounts of direct radiation.
- High temperature solar thermal energy
They use high-temperature collectors that generate temperatures above 500º C, allowing the generation of solar thermal energy by means of steam turbines. Some of the techniques used to reach these temperatures consist of concentrating solar energy by means of mirrors.
Usage
Regarding the use of these technologies, in power plants, photovoltaic and solar thermal systems share the objective of producing electricity. While photovoltaic systems generate that electricity directly from solar energy, solar thermal systems heat a fluid (water, oil, etc.) that runs a steam engine, gas turbine or similar. This electricity is transferred to the grid, as alternating current and with the required voltage value.
In the case of photovoltaic power plants, the capacity of the largest ones is above 500 MW, while that of the largest thermal power plants is less than 400 MW. Both types are experiencing rapid growth in many parts of the world, and there are even more among those under construction and in planning.
