The sun, which is about 150 million km away from the earth, is a huge nuclear fusion reactor in which hydrogen fuses into helium. The energy released by this fusion is emitted into space in the form of electromagnetic radiation.
At the outer edge of the Earth's atmosphere, the radiation power, which was defined as the solar constant in 1982, is around 1,365 W/m² (incident vertically).
However, the earth's sphere is not irradiated evenly, which is why only about 342 W/m² reach the earth as a global annual average.
But even this radiation energy, at 173 million GW per hour, is still enormous and the question arises why we still need so much additional fossil energy on Earth to generate heat?
Of the solar radiation energy that reaches the earth in different wavelengths, about 19 % is already absorbed by the atmosphere and clouds. Around 26 % is reflected directly in the atmosphere and 4 % from the Earth's surface and sent back into space. Around 51 % of the incident radiation is used for photosynthesis or is absorbed by the Earth and, in particular, the short-wave radiation is converted into heat (The radiation energy stimulates the atoms to move).
We all know the pleasant warmth in spring when the sun shines on our skin, but also the feeling of how quickly it can become cool when the sun is covered by a cloud. But heated objects also give off energy again by emitting radiation. All in all, about the same amount of energy is emitted from the Earth into space every day as is radiated onto the Earth. If this were not the case, the Earth would heat up continuously.
The fact that this does not happen - despite the amount of energy exchanged every day of around 4,156 million gigawatts (4,156,000,000 GW) - is actually bordering on a miracle.
It is especially the lower layers of the troposphere, and here in particular the water vapour, which prevents the earth from cooling down quickly. Depending on the temperature, the air can contain up to 30 g/m³ of invisible water vapour. The water vapour enters the atmosphere through evaporation. In the process, an enormous amount of warmth is stored in the atmosphere. With the amount of water stored in the atmosphere of approx. 13,000 km³, this latent warmth amounts to 32,000 exajoules, which is equivalent to the energy of 73 hours of solar radiation.
However, this amount of water is not evenly distributed over the earth. The amount of water is highest around the equator. For this reason, the temperatures there are almost constant all year round. In the deserts, on the other hand, the water vapour content is very low, therefore it gets very hot there during the day (evaporative cooling is missing) and cools down strongly at night.
For more information, see the page "How the weather works".
The aim should therefore be to use as much as possible of the radiant heat, which we get for free, so to speak, directly thermally for hot water production by means of simple solar collectors without using heat pumps. In this way, large amounts of fossil energy could be saved.
A gigantic field lies fallow here.
In Germany in particular, photovoltaics have unfortunately become widespread (because of financial support), with which electricity is first generated from the sun and then this electricity is converted back into heat by means of a heat pump or heating element. However, the efficiency of this solution is considerably worse than direct use!