How much sunlight hits the earth

Radiant energy of the sun

The energy of the sun is available almost always and everywhere. We have been using their radiation for heat generation or indirectly for the use of wind or water power for thousands of years. It is the source of daylight and thus the driving force for the production of biomass. Today, physicists, engineers and many other researchers are working on making the solar radiation energy usable directly for the generation of electrical power or through biotechnological processes for fuel supply.

What do we receive from heaven?

The seasonally averaged radiation of the sun arrives at our air envelope, measured perpendicular to the direct line sun - earth, with an intensity of 1361 watts per square meter. On a global average, however, the solar energy flux density at the upper edge of the atmosphere is only about 340 watts per square meter.

The sun's radiation is weakened in the atmosphere by absorption, reflection and scattering and reaches the ground either as direct radiation or indirectly by scattering from air molecules, aerosols or clouds. The latter is perceived as diffuse light that, unlike direct radiation, does not cast a shadow. The direct radiation amounts to about fifty percent of the total radiation on an annual average. The sum of direct and indirect radiation is called global radiation. When the sky is clear, global radiation consists predominantly of direct radiation, and when the sky is cloudy it consists almost exclusively of diffuse radiation. On average over the year, both proportions are roughly the same in Germany.

Solar thermal power plants in Almeria

The amount of radiation energy that reaches the ground depends on the length of the path through the air - i.e. the position of the sun - and the degree of cloud cover. With a cloudless sky, the intensity of the radiation can reach an output of around 1000 watts per square meter. However, on a cloudy day in winter, it can drop as low as 50 watts per square meter or less. In Germany, depending on the location and atmospheric conditions, this radiation output can amount to an amount of energy of up to 900 to 1200 kilowatt hours per square meter and year. The situation is different in the Sahara, for example, where irradiation can reach around 2200 kilowatt hours per square meter per year.

The radiation energy of the sun is not only dependent on the seasons, but is primarily subject to the rhythm of day and night. Energy systems that use solar energy directly are therefore dependent on energy storage if they are to deliver energy uninterruptedly.