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. Solar energy, therefore, besides being abundant and well distributed, is also a renewable resource. These characteristics would make the Sun the principal source of energy, only that solar energy has a low power and is intermittent on a local scale. In fact the flow of energy from the Sun depends on the alternating day and night and the variable meteorological conditions.
Not much power from a great energy
A very useful parameter in order to evaluate how much the energy is worth is the power density,  also known as radiance, that indicates the solar radiation per surface unit (Watt per square metre W/sq.m). The amount of solar energy that reaches the Earth’s surface, after subtracting all the reflections and absorptions that take place in the atmosphere, is equal to 85,000 billion W. Knowing that the Earth’s surface is equal to 5.1 billion sq. km, the result is that  every square metre of the Earth’s surface, receives an energy of approximately 170 W/sq.m. This value decreases remarkably when it is converted into power that can be utilized. The present lifestyles in the industrialized Countries require a power density that ranges from 20 to 100 W/sq.m for homes, to 300 to 900 W/sq.m. for steel industries. It is evident that with the current solar technology it is not possible to make most of the large structures which have a high energy demand,  such as the industries and the hospitals, work. The principal technological challenge of our days is to succeed in storing the immense energy that comes from the Sun and make it available at the right intensity where there is a demand for energy.

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