Energy

Useful radiation

Only a part of the huge energy flows that gets from the Sun to the Earth can be transformed into useful energy. The quantity of solar energy that arrives to the earth’s surface and that can be usefully “collected” depends on irradiation on the area. Irradiation is the quantity of solar energy that arrives at a surface within a determined time interval, typically one day (it is measured in kW/h by square metre by day). Instead, the value of solar radiation that arrives on the surface unit (at a determined moment) is called radiance (it is measured in kW/square metre). Irradiation is influenced by local climatic conditions (clouds, mist, etc) and depends on the latitude: as it is well known, it increases when it gets closer to the equator. In Italy the average annual irradiation varies from 3.6 kW/square metre in the plain of the Po river to 4.7 kW/square metre in the Centre-South of Italy, to 5.4 kW/square metre in Sicily. In some favourable spots it is possible to collect every year around 2,000 kilowatts for each square metre, which corresponds to 1.5 barrels of oil for a square metre.