Fig. 1: Classification of solar energy. Only slightly over 50% of radiation coming from the sun reaches the earth’s surface . (direct radiation) Credit: (Meteo Ostia)

Solar energy reaches everywhere, it costs nothing and it is renewable . However, it is very diluted in space and it is not continuous - it varies with the alternating of day and night, the different seasons and various weather conditions.

Not all the energy radiated by the Sun reaches the surface of the Earth: some of it is reflected back into space, some is dispersed and diffused in all directions by air molecules and dust particles in the atmosphere, and some is absorbed by water vapour, by carbon dioxide and by the ozone in the atmosphere (Fig. 1).

Fig. 2: Examples of applications of solar panels.

The simplest and most promising use of solar radiation in energy terms is that of solar panels, both for heating and for the direct production of electric energy. On a small and medium scale, both have a limited impact on the environment compared to fossil fuels.

At present there is an enormous number of applications with small  photovoltaic devices. For example, they are used for calculators and wrist watches; bigger panels are used to provide electricity for domestic use, to pump water out of the ground, to supply power to telecommunications systems, for emergency use, etc.

It is worth noting that other renewable sources of energy, such as hydroelectric, wind and biomass power stations are all traced back to solar energy.

Solar energy produces heat which can be exploited in many practical applications: flat panels for producing hot water, heating greenhouses, etc.

Fig. 2 shows some applications of photovoltaic solar panels - used for weather applications (top left), to supply a building with electric energy (bottom left) and for public lighting (right), using batteries which are recharged during the day.

Apart from the normal photovoltaic systems, there are also thermoelectric solar systems made up of panels (mirrors) which "follow" the sun to obtain maximum efficiency, and produce heat and/or energy. In a solar tower the panels/mirrors concentrate light from the sun on a "boiler" placed at the top of the tower. By means of a special system and movement of the panels, the boiler can reach very high temperatures.

In many countries, numerous programmes and various incentives have been approved with regard to the direct use of solar energy with flat photovoltaic panels: Germany has a "100000 solar roofs" programme and in Italy, as already mentioned, ENEA is running a similar project. The United States of America has a programme for one million solar roofs, in part thermal solar, and Japan has a similar programme. The European Community also has a similar programme which includes aid for developing countries.