Sources of energy

3 - Fossil fuels

Fossil fuels (coal, oil and gas) have been massively used since the beginning of the industrial era. This use, first focussed on coal then oil, was prior to the current boom in gas. Fossil fuels currently account for more than 85% of primary energy requirements. The energy equivalence of fossil fuels is expressed in the oil equivalent metric tonne (tep). 1.5 metric tonnes of coal is needed to obtain as much energy as a metric tonne of oil.

Inventories (relation between proven reserves to date and current production) are estimated at around 40 years for oil, 65 years for gas and 220 years for coal. These deadlines may possibly be extended, but at the price of more expensive extraction and access to sources situated in the frozen areas of Antartica. These resources are patchily spread over the world’s surface. It is estimated, for example that the Middle East will possess 75% of oil resources in 2020. At the same time, 70% of gas reserves will be concentrated in Russia and the Middle East.

The use of fossil fuels leads to the emission of a large quantity of carbon dioxide (CO2) into the atmosphere, which contributes to an increase in the greenhouse effect. Better management of combustion techniques and the use of gas may reduce emissions but will never completely reduce the output (in contrast with renewable power or nuclear power).

4 - Renewable energies

Man has been using renewable sources of energy for thousands of years: wood, horsepower and waterfalls or wind for mechanical applications. In the last 200 years they have been replaced by the use of fossil fuels, which are more suitable for industrial developments. Renewable sources of energy are characterised by low energy density and variable availability.

Hydro-electric energy is the most used of the renewable energy sources. It supplies 3% of worldwide primary energy consumption and about 18% of electrical consumption. It is estimated that to date 15% (2300 TWh/yr) of the technically exploitable potential (around 15 000 TWh/yr) is being used, but the situation is full of contrasts from one country to another. For example, France and Switzerland have exploited 90% of possible sites, whereas Asia and South America exploit less than 20% of their hydraulic potential.

The energy of the oceans is harnessed in various forms: tidal power due to the attraction of the moon, wave power and heat power due to the difference in temperature between the surface and the deep. The two other forms are difficult to exploit at a reasonable cost. The real technical potential of tidal power is estimated at 500 000 GWh/yr (taking into account availability of installations due to the tidal cycle). Technical feasibility has been proven, in particular thanks to the La Rance tidal power station (240 MW installed, 500 GWe.h over a year).

Solar received outside the atmosphere is around 5.5 1024 J (average flow around 1.4 kW/m2). About 30% is reflected into space, 25% is used for the evaporation and precipitation cycle of water and photosynthesis and around 45% is absorbed, and then transformed into heat by the air, the continents and the oceans. This represents around 6000 times world primary energy consumption. The main difficulties in exploiting solar energy rely, on the one hand on considerable variations in weather (daily and annual cycle) requiring storage, and on the other hand low energy density.
Direct conversion of solar energy into electrical energy is possible via photovoltaic conversion. Cell manufacture remains costly, and very energy-consuming. Typical yield of a photovoltaic cell is around 12-13%. World annual electrical production is about 260 kW.h/m²; in one particularly favourable locatio (source World Energy Council).
The production of electricity may be achieved through concepts concentrating the sun’s rays onto a boiler. Experimental power stations have been built (e.g. Themis in the Pyrenees, 2.5 MW) to validate the main technical options but have not led to the construction of stations with greater power, on account of the high kWh cost.
Finally, the use of solar energy may be possible for systems that act as a complement to the heating of a private house (black solar sensor).

Wind power has been used since antiquity (windmills, sails on boats). The main handicap of this form of energy is its great variability (direction, speed, night or day, season). The world wind power market is currently in a period of great development. Forecasts indicate that soon the power installed worldwide will be multiplied by 5 (7200 MW installed to date). Wind turbines may be divided into two main families: vertical axis wind turbines, not requiring any guiding device, but complex and unusual and horizontal axis wind turbines, operating into the wind and therefore requiring a guidance system. These are the most common. A typical 600 kW wind turbine has a rotor of about 45 m. They start to produce electricity from a wind of 13 km/h and must be uncoupled in winds over 90 km/h to avoid damage. A site with an average speed higher than 27 km/h may be considered as sufficient (mainly in coastal areas). Surface area is great(20 kWh/m©&Mac247;/yr) even if effectively used surface areas represent less than 1% of the total area. The "offshore" wind turbine offers considerable potential, but installation and running costs are higher than on land sites. In France, the EOLE 2005 programme is set for a wind turbine potential of 250 to 500 MW by the deadline of 2005.