**3. Solar radiation: Geophysical considerations and energy potential**

Located nearly 150 million km from Earth, the Sun is a huge nuclear power plant—the oldest in the history of mankind—and has a capacity of 25 million kW/h per gram of hydrogen, its main component. The nuclear fusion of one kg of hydrogen releases an energy value of 8.3 million tons oil equivalent (Lhomme, 2004). Since the sun accounts for some two billion tons of material, over 90% being hydrogen of which it uses 600 million tons per second, the energy produced is unimaginable. In fact, it produces 4 x 1017 GW, or the equivalent of 400 million billion nuclear power plants! The Earth receives only a tiny fraction of this energy (Centre National de Recherche Scientifique, n.d.; Lhomme, 2004).

The major characteristics of sun energy, despite a certain ubiquity, are a large regional disparity and more or less marked by seasonal imbalance. For instance, the average energy received by Europe is 1,200 kWh/m2/y vs 1,800 to 2,300 kWh/m2/y in the Middle East (EPIA/Greenpeace, 2011). Latitude, exposure and altitude are parameters that influence the overall daily and seasonal radiation. Tropical regions corresponding to 25–30 degrees latitude are sunnier compared to European countries above the 45-degree parallel.

Climatologists have long endeavoured to assess the solar energy of a given area as thoroughly as possible and even be able to predict the evolution. Statistics on solar radiation were therefore compiled from data collected to input into valuable databases (EPIA/Greenpeace, 2011). Assembling data of a given region based on different criteria is strategic for the design and dimensioning of PV systems, especially their orientations and inclinations (Labouret & Villoz, 2009).

Characterization of increasingly sophisticated global solar energy resources is a sign of PVs' promising potential. Thus the calculations by the International Energy Agency (US IEA) lead to surprising conclusions. Installing PV systems on only 4% of the area of the world's driest deserts would likely be able to provide all of humanity's primary energy needs (EPIA/Greenpeace, 2011).
