The energy carried by sunrays as a consequence of nuclear reactions (hydrogen fusion) and transmitted to the Earth as electromagnetic radiation is called solar energy. Electromagnetic radiations are made of photons. A photon is a neutral particle that spreads into the air at a speed of 300,000 km/sec, with an energy that depends on its frequency and a mass that is considered as void when at rests (when it is not moving).

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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The solar energy does not make any noise, does not pollute and allows to obtain a hot fluid that can be used as sanitary hot water, for heating, or for different industrial tasks.

The environmental benefits that derive from the installation of photovoltaic systems can be expressed in terms of avoided emissions: the emissions that would have been produced for the generation of an equal quantity of electric power with thermoelectric systems can be calculated. For example it was estimated that a family of four people consumes around 7.7 kWh a day with an electric water heater. In Italy, to produce an electric kWh, thermoelectric plants release into the atmosphere around 0.58 kg of carbon dioxide, one of the main greenhouse effect gases. Therefore, for an electric water heater 4.5 kg of CO2 are produced on average every day. With hybrid solar-gas plants, i.e. solar plants integrated with gas boilers, that ensure hot water all year long, a four-members family in Rome can save up to 0.69 kg of CO2 a day.

Therefore solar energy could significantly reduce the use of fossil fuels, since it would be an electric energy source on a large scale, in particular in Italy, where irradiation levels are high. Directly converting the sun into electric energy is a choice that can be extremely advantageous not only in urban settlements, but also in marginalized and remote areas, especially in the Third World. In those areas the combination of photovoltaic systems with other existing renewable sources can bring electric energy to the most isolated villages and communities, to guarantee lighting, telecommunications, pumps, but also to desalinate seawater and brackish water, to preserve fishing and agricultural products, and to refrigerate drugs and vaccines.

Power density of solar energy

In less than an hour, the Earth receives an amount of energy from the Sun that is equal to the world consumption for a year. Solar energy, unlike the other sources of energy, is present in all over the planet (with some differences depending on the latitude) and it is a source that will accompany us  for billions of years more.

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Impact on the landscape

The environmental impact of a solar power plant must be evaluated considering the entire life cycle and in particular the building stages of the plant, the stage in which the plant is set up and produces energy, and finally the stage when it is no longer used. 

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Solar energy Junior

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