Solar energy: silicon solar cells

EN With oil reserves running out, silicon solar cells offer an alternative source of energy. How do they work and how can we exploit their full potential? Indirectly, the Sun is the source of most of the energy we use on Earth: not only of fossil fuels and biomass, but also wind and tidal energy, to mention just a few. Increasingly, there is interest in capturing the energy from the Sun more directly, using photovoltaic cells. A relatively old, medium-sized star made of hot plasma, the Sun radiates energy as electromagnetic radiation over a wide spectrum. At a distance of 150 million kilometres, our planet receives an irradiance of around 1366 W/m2 (1 W= 1 J·s) from the Sun, but not all of this actually reaches us because Earth’s atmosphere reflects and absorbs about 30 % of this energy. Nonetheless, every square metre of Earth’s surface receives an average of nearly 1000 Joules per second from the Sun. To put this into perspective, the total energy consumed globally in 2010 was around 5 x 1020 J. If we assume that our planet is a perfect sphere with a radius of 6370 km, Earth receives 1.8 x 1017 J/s, of which about 1.3 x 1017 J/s reaches Earth’s surface. Thus in one hour, the Sun provides Earth with all the energy we need for a whole year. It isn’t quite that simple, however. Due to meteorological factors, the Sun’s declination and Earth’s rotation, the irradiance is actually closer to 230 W/m2. If we repeat the last calculation using that figure, the time needed to power Earth with energy from the Sun for a year is about five and a half hours – still an impressively short time. Solar radiation is therefore a promising energy reservoir, but how can we collect it and use it?