Renewable energies (hydraulic, solar, wind, geothermal and biomass) are developing intensely all over the world, driven by the need to combat global warming by reducing greenhouse gas emissions.
Energy is said to be renewable when it is produced by a source that nature constantly renews, unlike energy that depends on depleting sources. Renewable energies are very diverse, but they all come from two main natural sources:
the Sun: it emits radiation that can be converted into electricity or heat, it generates areas of unequal temperature and pressure at the origin of the winds, it generates the water cycle, it allows plant growth and the generation of biomass;
the Earth, whose internal heat can be recovered from the surface.
The renewable nature of these energies, their low emissions of waste, pollutants and greenhouse gases are advantages. But their energy power, which is relatively scattered, is much lower than that of highly concentrated non-renewable energies. They can be – in the case of solar and wind energy – “intermittent” and difficult to store because they are immediately transformed into electricity, requiring the occasional contribution of other energies. Their use also requires heavy investment, even if they become increasingly competitive with fossil fuels over time.
Hydropower from large dams is today the leading renewable energy source. China, Brazil, Canada, the United States, and perhaps Africa tomorrow, are the leaders in the sector.
Solar energy is produced in two forms: photovoltaic solar energy, which transforms the sun’s light radiation into electricity using panels made up of semiconductor cells, and thermal solar energy, which captures the sun’s heat and uses it as such or transforms it into mechanical energy and then into electricity.
Wind power, the energy of the wind, has progressed, both offshore and onshore, with technology steadily improving. The highest wind turbines reach 170 metres in height, with rotors over 150 metres in diameter!
The different types of marine energy used come from the force of waves, currents and tides, differences in ocean temperature and certain characteristics of the saltwater/freshwater couple (osmotic energy). They are still at an early stage of development.
Biomass consists of all organic matter of plant (including micro-algae), animal, bacterial or fungal (fungi) origin. For centuries, wood was the main source of energy through its combustion, before being replaced by coal and then oil and gas. But there are other forms of biomass use. Methanisation produces biogas from our household or agricultural waste. The refining of plant biomass allows the production of biofuels.
Geothermal energy uses the heat from underground aquifers, or even dry rocks, captured at varying depths, to supply urban districts, buildings or factories, or to produce electricity via power plants. Some countries with favourable geological conditions use it on a massive scale, such as Iceland and the Philippines, two volcanic countries. Heat from other sources can also be captured and used in networks or industrial processes.
Nuclear power is not considered a renewable energy as it is based on a limited raw material, at least at acceptable operating costs, uranium. Nuclear fusion, on the other hand, if carried out at an industrial stage, would provide an inexhaustible source of energy.
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.
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 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.
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 :
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 :
It should be noted that biomass can only be considered as a renewable energy source if its regeneration is higher than its consumption.
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.
The nuclear accident in Fukushima is rekindling the debate on alternative energy sources. The European Union wants 20% of its energy to come from renewable sources by 2020 – up from the current 9%. On 22 March members of the Industry, Research and Energy Committee discussed a study on available technologies. A brief overview of the energy sources of the future.
Wind turbines can be built at sea, where the wind is stronger than on land. Wind farms are being developed offshore, but this technique is still at a very early stage of commercialization. In the medium term, the cost of this electricity could fall below the cost of traditional wind power, due to better yields. Currently, the cost of construction is higher.
Mirrors concentrate sunlight into a heat transfer fluid, whose heat is then transformed into electricity by a generator. Some experts estimate that this technique could supply up to 3% of European consumption by 2020.
This is the use of energy stored by biomass. For example, it is possible to recover the gas emitted by decomposing organic waste. However, its potential is lower than wind or solar power.Wave energy. The ripples of the ocean swell or waves produce energy that can be captured either by machines on the surface or by underwater pressure variation sensors. The energy produced could be of the same order of magnitude as the world’s total consumption. Nevertheless, this technology raises many environmental issues.
The difference in temperature between the Earth’s surface and the heat reserves located deep underground (the geothermal gradient) causes the release of thermal energy. However, the costs of capturing this energy are too high for it to be commercialized – unless major technological developments take place.