The intensity of solar radiations that arrive every year to the earth’s surface amounts to 80 thousand billion tons of oil equivalent (the so-called TOE, that indicates a quantity of energy that equals the energy produced by a ton of oil). This quantity is infinitesimal if compared to the energy produced by the Sun thanks to nuclear reactions. But it is also a very large quantity, if you only consider that the world energy demand amounts to 8 billion TOE a year. From the flow of solar energy, the following are derived: biomass energy, through the photosynthesis process; hydraulic energy (the Sun, in fact, is the motor of the water cycle); wind energy from which, in turn, wave energy derives. Anything, starting from what we eat every day, is directly or indirectly linked to it. Even fossil fuels, which derive from chemical-physical alterations of prehistoric living organisms, contain solar energy. Solar radiations, although they only reach a maximum power of 1 kilowatt per square meter (soil irradiation in a clear day, sunny, at midday), are the most abundant and clean energy source on the Earth.

The Sun

The Sun is the closest star, which makes life on Earth possible.  The sun is a sphere with a 1.4 million km diameter (109 times as much as the earth’s diameter) and has a mass approximately 300.000 times greater than the earth’s mass.  75% of it is hydrogen, 23% is helium and only 2% is formed by heavier elements.

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The energy balance of the Earth

A sunray reaches the earth’s surface after travelling for 150 million kilometres in 8 minutes. The solar energy received by the earth amounts to 170,000 TW approximately (the unit of measurement equivalent to 1012 watts, used to measure solar energy).

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Distribution of solar radiations

The sun will illuminate and heat the Earth until its hydrogen reserves are depleted, i.e. in approximately 5 billion years. The sun’s radiation reaches the earth in a non-homogeneous way because of its interaction with the atmosphere and the angle of incidence of sunrays.

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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.

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A bit of history

Mankind has always known what happens when a sunray hits a body. If this is light-coloured or is a mirror, the energy of the sun is reflected. If it is dark-coloured, the sun’s radiation is absorbed and the body heats up.

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Some figures: worldwide

Photovoltaic capacity installed in the world in 2021 reached 942 GW, due to 175 GW installed during the year. In 2020, Italy held the 6th place for installed power, after China, United States, Japan, Germany, India and in front of Australia, Vietnam, South Korea and United Kingdom.

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Some figures in Italy

Italy is the country of sunshine, not only in its popular songs and in the image of Italy that all the tourists have, but also from the point of view of energy. In Italy, mean annual solar radiation ranges from 3.6 kWh per square metre per day in the Po river plain area, to 4.7 kWh per square metre per day in Central-Southern Italy, to 5.4 kWh per square metre per day in Sicily.

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