At the origin of all the renewable energies that humanity exploits today, there are only two main sources: the Sun and the Earth. However, specialists like to classify these energies into five main types, each with its own specific characteristics.
The term renewable energy is used to refer to energies which, at least on a human scale, are inexhaustible and available in large quantities. There are five main types of renewable energy: solar energy, wind energy, hydropower, biomass and geothermal energy. Their common characteristic is that they produce little or no polluting emissions during the exploitation phase, thus helping to combat the greenhouse effect and global warming.
Photovoltaic or thermal solar energy
Solar energy is the energy we can get from the sun's rays.
A distinction must be made between photovoltaic solar energy and solar thermal energy. Photovoltaic solar energy is electricity produced by so-called photovoltaic cells. These cells receive sunlight and are capable of converting some of it into electricity. Modularity is one of their advantages. Indeed, photovoltaic panels can be used for both domestic purposes and large-scale energy production.
In a solar thermal or thermodynamic system, solar radiation is used to heat a fluid. Water, for example, as in some domestic water heaters. When a concentrating system -- a set of mirrors -- is added, the Sun can heat the fluid to about 1,000°C and the technology becomes usable, for example, for power generation.
The disadvantage of solar energy is that it is intermittent. It can only be used - at least today - when the sun is shining.
The air behind wind energy
The ancestors of wind turbines are windmills. Wind turbines produce energy -- electricity, for example, when coupled to a generator -- from the movement of air masses. They harness the kinetic energy of the wind.
Wind turbines can be installed on land. These are called onshore wind turbines. They are technically the simplest to imagine. Even if the space that can be reserved for them could quickly run out. And the most efficient could be wind turbines installed at sea, which we call offshore wind turbines.
Like solar energy, wind power is intermittent. Wind turbines only produce power when the wind is blowing. However, unlike solar panels, it can be difficult to install a wind turbine in your garden. The technology is more reserved for large installations.
Hydropower from sea currents
The term hydropower refers to the energy that can be obtained by exploiting water. A category of energy that is less subject to weather conditions, but is still reserved for large-scale production. In hydraulic energy, we find :
- Dams that release large quantities of water through turbines to produce electricity.
- Tidal energy, which plays on the potential energy of the tides, an energy linked to the differences in water levels and the currents they induce.
- Tidal power, which exploits ocean currents.
- Wave energy, which relies on the kinetic energy of waves and swell.
- Thermal energy that can be derived -- in a careful way to avoid disturbing the natural flow of the seas -- from the temperature difference between deep and surface waters.
- The osmotic energy that produces electricity thanks to the pressure difference generated by the difference in salinity between seawater and fresh water.
Biomass energy from organic matter
Biomass can become a source of heat, electricity or fuel. Several techniques can be used to extract its energy: combustion, gasification, pyrolysis or methanisation for example.
Biomass energy can be produced locally. However, in some cases, care must be taken to ensure that it does not compete with the food chain.
Biomass energy includes :
- The ancestral source that is wood. It can produce heat, electricity or biofuels (hydrolysis of cellulose into glucose and then fermentation into ethanol).
- Biofuels, liquid or gaseous, resulting from the transformation of plants such as rapeseed or beet (1st generation), from cellulosic materials (2nd generation) or from microorganisms such as microalgae (3rd generation).
It should be noted that biomass can only be considered as a renewable energy source if its regeneration is higher than its consumption.
Drawing energy from the ground, geothermal energy
Geothermal energy is a renewable energy that comes from the extraction of energy from the ground. This heat results essentially from the radioactive decay of the fissile atoms contained in the rocks. It can be used for heating, but also for the production of electricity. It is one of the only energies that are not dependent on atmospheric conditions.
However, it does depend on the depth at which it is taken. Deep geothermal energy -- some 2,500 metres at 150 to 250°C -- can be used to produce electricity. Average geothermal energy -- in water deposits, particularly between 30 and 150°C -- supplies urban heating networks. Very low-energy geothermal energy -- between 10 and 100 metres deep and below 30°C -- is used by heat pumps.
However, for geothermal energy to remain sustainable, the rate at which heat is extracted must not exceed the rate at which it travels inside the Earth.
