As already seen, Biomass is, by far, the largest renewable energy source, followed by Hydropower, the largest renewable electricity source. But on the long term, Solar energy has the greatest potential as a source of heat and power. Wind power, quite fashionable, is the fastest growing renewable source, but it is handicapped by its intermittence and raises local protests.
There are several other renewable energy sources with various levels of development. After geothermal energy, the only significant source today, one can mention OTEC (Ocean thermal Energy), tidal and wave power.
A tiny fraction of the energy radiated by the Sun toward our Earth – say, the energy received by one tenth of the Sahara Desert area – would theoretically be enough to supply all Mankind energy needs, and without any risk of depletion for billions of years. But this energy is very diffuse and, of course, available only during daylight.
Solar energy is quite suitable for heating purposes: is intermittence is not a heavy handicap because heat can easily be stored. Building materials give back during the night the heat they accumulated during daytime, and a well insulated hot water tank does not cool down appreciably over a few hours.
Insulation is the necessary complement of solar energy. A separate house built according to the French norm RT 2000, the thermal leakage of which ranges from 100 to 120 kWh/m2.year, can fulfil with solar energy 33% of its heating needs and 67% of its hot water consumption. Some “Passive Buildings” go farther and loose only 15 to 20 kWh/m2.year.
As far as renewables are concerned, France’s Achilles heel is certainly its poor deployment of solar water heating units, a "low tec" technology which requires only the presence of well trained plumbers.
To produce "thermal solar" power, an array of mirrors reflects solar rays and focus them on a boiler. In this boiler, the concentrated solar heat vaporizes a fluid, and the vapour actions a turbine coupled to a generator, before being condensed back. The rays may be focussed on a point in power plants where the array of mirrors heats a boiler atop a tower, or on a transparent linear pipe, partially surrounded by parabolic reflectors, where the cooling fluid circulates.
In any case, the site selected must be cloudless to allow for direct sunlight reflection (the French prototype solar plant Thémis, for instance, was built on a much too cloudy site in Targassonne, Pyrénées Orientales, and proved to be a failure).
Photovoltaic electricity (PV) is produced by converting part of the solar radiation in a photovoltaic cell, a semiconductor component which, when exposed to light, generates a 0.5 volt electric voltage.
Current PV cells are made of silicon, but materials still under development appear more promising. An array of cells constitutes a PV module, often called solar panel. Solar panels generate DC current, which must be converted to AC for use with standard appliances.
Such modules were first used to power orbiting satellites, then to power isolated sites (with storage batteries), and finally to produce power on the grid.
In 2006, 1 500 MW of new solar photovoltaic facilities have been installed throughout the world, and the total installed power has reached 6 700 MW: the growth rate is fast but the total remains marginal.
Wind energy is a concentrate of solar energy with a little help from the kinetic rotation energy of the Earth. By heating the atmosphere unevenly, the Sun causes differences in temperature and pressure of the air which produce winds. The nature of the ground, its elevation and 3D structure modulate locally the large air movements by influencing their velocity, their regularity and their turbulence: a hill will shield the wind, a mountain pass will accelerate the wind (venturi effect), a cliff will induce turbulences and so on.
Somehow the heirs of the windmills attacked by Quixote, modern wind turbines have become real giants, products of high technology. The main wind turbine builders produce engines rating 1 MW to 5 MW. The rotor of the latter is larger than a Boeing 747 and the machine room towers more than 130 metres above ground.
In 2005, windpower produced 0.5% of the world electricity. According to IEA, its share could reach 3.4% in 2030. Windpower is still subsidized in every country, usually via the obligation to buy the power at a high fixed price, irrespective of the demand from the grid. But the production cost of the largest turbines in the best sites is close to competitiveness.
Nevertheless, one cannot compel the wind to blow on demand. On a given territory, the amount of windpower can drastically vary from one day to the other! This creates significant strains on the grid and limits the maximum share of windpower.
The more regular and frequent the wind, the better will the wind turbine operate. Indeed, turbines begin to generate power when the wind velocity exceeds a value somewhere between 10 and 20 km/h, but when the wind velocity exceeds 90 km/h, they must be shut down for safety reasons. Similarly, the rotation axis must remain as long as possible parallel to the wind direction. It is therefore preferable to select a location where the wind direction is as stable as possible.
Often, windpower advocates quote only the statistics of installed power (in MW), while the important data is the amount of electricity generated (in MWh) over a year, i.e. the number of equivalent full power hours (efph). In a good land based site, one can expect 2500 efph/y, but a good offshore site can exceed 3500 efph/y. In a less favourable location, the number may be quite low. The average performance in Germany hardly exceeds 1600 efph/y while the French figure is 2320 efph/y. Under the circumstances prevailing in mainland France, a wind farm of 30 GW may replace between 4 and 6 GW worth of thermal power plants.
In summary: a renewable energy source on the fast track, with a strong support from most European governments, but subject to strong local opposition here or there (landscape degradation, noise, electromagnetic interferences, etc...).
Geothermal energy is the by-product of our planet’s radioactivity which keeps the magma at high temperature. In some favourable sites where the magma is close to the surface and overheats “naturally” underground water, it is possible to make boreholes and directly extract the steam to produce power in a turbine. This high temperature geothermal energy is exploited notably in Iceland, California (the Geysers), in Italy (Lardarello) and the Guadeloupe island (Bouillante). The heat is free but thermal water is often corrosive for the power plants piping.
If the magma is not near the surface, one can drill deep enough to find dry hot rock. At 5000 metres deep, it is not unusual to find temperatures above 200°C. If a "doublet" is drilled, it is possible to inject cold water in one borehole and recover steam from the other hole provided the rock has enough fractures to allow for the water circulation. This form of HT geothermal energy is costly and not really renewable, but it can be a very sizeable energy source.
There are many underground aquifers from which warm water can be extracted for district heating, with or without an additional heat pump. The water must be injected back into the aquifer, far enough not to cool it down (about 1 km away). At a depth between 800 and 1500 m under the Paris Basin, there is such a warm (80°C) aquifer, called the Dogger. It is used to heat several housing districts in the suburbs.
The la Rance tidal power plant is the only significant example of the use of tides to produce electricity. It provided the opportunity to develop the "bulb group" electrical generators now equipping most of the downriver dams. In the whole world, only a handful of sites would be promising (Bay of Fundy, Severn estuary, Mont St Michel bay) but the environmental impact would be very significant. There was in the 70s a project to build a huge tidal plant in the Mont St Michel bay. It would have required the construction of a 40 km long levee, with a serious risk of silting the bay and destroying the landscape of a unique natural and architectural wonder.
It is possible to produce energy from the temperature difference between deep water and surface water in the warm oceans. Georges Claude made a well known but unsuccessful attempt before World War II. The potential amount of recoverable energy is huge but the conversion efficiency is very low: the process requires very large facilities able to withstand the ocean’s hazards. Often tried, this process has never been economical.
If wind power is a concentrate of solar energy, similarly wave (or swell) power is a concentrate of wind energy. Many devices have been tried to recover this energy, notably in the UK and in Portugal. The process involves using the wave motions to somehow action pistons, the mechanical energy of which is transformed into electricity. The principle appears promising, but the devices must survive storms (or flee from them), which is far from obvious.
There are many ideas being floated to recover solar, wind and hydraulic energies, and some of them are quite interesting, but none has reached the industrial prototype stage:
Anchored "Hydroturbines" would operate under the surface in rivers or tidal undercurrents without requiring the construction of dams. Tidal currents can locally be both powerful and regular.
Tethered high flyer balloons could carry wind turbines at high altitudes where winds are stronger and more regular.
Parabolic mirrors can heat in their focus a Stirling engine generating power locally.
Most futuristically, orbiting solar power plants could radiate power to Earth via microwaves.