Is light a kind of warmth

How warmth is created from light

The mean annual irradiation of the sun in our latitudes is around 1000 kWh / m2

"Solar thermal energy" is the term used to describe the energetic use of sunlight by means of absorption and conversion into heat. Basically, a distinction is made between two areas, namely the low and high temperature range.

  • In the low temperature range, the sun's radiation is used directly or via a heat exchanger to heat water or another heat transfer medium.
  • In the high temperature range, the main focus is on converting solar heat into electricity. However, process heat and extremely high temperatures can also be generated for research purposes.

The conversion of sunlight into electrical energy by means of solar cells is not part of solar thermal energy. This area, the so-called photovoltaics, is dealt with in "Electricity from sunlight". "Electricity from hydropower and" electricity from wind "provide information about the indirect use of solar energy by hydropower plants and wind converters.

Black materials absorb best

From a physical point of view, sunlight consists of electromagnetic waves, the length of which is between 0.3 and 2.5 millionths of a meter. If this radiation hits matter, it is more or less "absorbed" and converted into heat. How much energy is absorbed depends on the type of absorbent material. It is well known that white materials are poor absorbers because they reflect practically the entire visible spectrum of sunlight. Black materials absorb best. They appear black to us because they absorb the entire spectrum of sunlight. Dark materials heat up correspondingly more strongly when they are exposed to the sun, because with the absorption of sunlight, the molecules of the irradiated matter sometimes change into excited energy states.

The earth's surface is also an absorber to the sun. Through the radiation of the sun, the earth's surface receives an energy of up to 2500 kilowatt hours per square meter per year. In Germany it is 1000 kilowatt hours a year. This radiation causes the soil, water and air to be heated.

Earth absorbs over two thirds of the sun's energy

The earth's air envelope plays a particularly important role. Without the atmosphere, the solar energy absorbed by the earth's surface and converted into long-wave infrared radiation would be reflected back into space. The mean temperature on the earth's surface would only be minus 18 ° Celsius. However, the air envelope acts as a "heat trap": On the one hand, it largely lets sunlight through. Their gases only "swallow" large parts of the ultraviolet and infrared radiation as well as narrow bands of optically visible light, which can be detected as "Fraunhofer lines" in the spectral analysis. The atmosphere absorbs 17.4 percent of the solar radiation on the earth and converts it into heat. On the other hand, the atmosphere prevents the immediate reflection of the 52 percent of the solar radiation that reaches the earth's surface. It absorbs the long-wave thermal radiation that arises from the absorption of light on the earth's surface, and thereby warms up again. This is the reason why the surface of the earth is not freezing cold, but actually a mean temperature of plus 15 ° Celsius.

Overall, the earth - more precisely the system of the earth's surface and air envelope - absorbs 69 percent of the radiated solar energy. The remaining 31 percent is reflected in the atmosphere and is "lost" to the planet's energy balance.

"Greenhouse Effect" and "Ozone Hole"

Regardless of the temperature on its surface, the earth radiates just as much energy into space as it receives from the sun. For the overall balance, it is irrelevant what temperature prevails on the earth's surface. For the biosphere of our planet, however, which has adjusted to the aforementioned mean temperature of plus 15 ° Celsius, even slight temperature fluctuations within the system of the earth's surface / air envelope can have devastating effects. That is why today we fear the "greenhouse effect" - more precisely, an unwanted intensification of the already existing and absolutely vital greenhouse effect as a result of the increase in carbon dioxide and other absorbing gases in the atmosphere. But the decrease in absorbent gases in the atmosphere can also have dire consequences. That is why we are afraid of the "ozone hole" or the thinning of those ozone layers that absorb most of the life-threatening ultraviolet radiation from sunlight.