What is ground radiation and direct diffuse solar radiation

What-is-ground-radiation-and-direct-diffuse-solar-radiation

1.1 Ground radiation

While absorbing solar radiation, the earth’s surface transmits most of its energy to the atmosphere in the form of radiation. The way the earth’s surface emits radiation with its own heat H continuously at night is called ground radiation.

Since the surface temperature is much lower than that of the sun (the average surface temperature is about 300K), the ground radiation is mainly concentrated between 1 and 30um. The average wavelength of its maximum radiation is 10um, which belongs to the infrared range. Compared with the solar shortwave radiation, it is called It is ground long wave radiation.

The radiation capacity of the ground mainly depends on the temperature of the ground itself. Since the radiation capacity increases with the increase of the temperature of the radiator, the surface temperature during the day is higher and the radiation is stronger; at night, the surface temperature is lower and the radiation is weaker. Radiation on the surface is long-wave radiation. Except for part of it goes to the universe through the atmosphere, most of it is absorbed by water vapor and carbon dioxide in the atmosphere. The absorption of long-wave radiation by water vapor is more significant. Therefore, the atmosphere, especially the atmosphere in the troposphere, mainly relies on absorbing surface radiation to increase heat.

1.21 Direct solar radiation, diffuse solar radiation and total solar radiation

The sun’s radiation or “insolation” on the ground plane consists of two parts: direct insolation and diffuse insolation. When solar radiation passes through the atmosphere and reaches the ground, the absorption, reflection and scattering of solar radiation by air molecules, water vapor and dust in the atmosphere not only weaken the radiation intensity, but also change the direction of radiation and the spectral distribution of radiation. Therefore, the solar radiation that actually reaches the ground usually consists of two parts: direct and diffuse. Direct radiation refers to radiation that does not change the direction of solar radiation directly; diffuse radiation is solar radiation whose direction changes after being reflected and scattered by the atmosphere. It consists of three parts, namely the scattering around the sun (the sky light around the sun’s surface) , Horizon scattering (bright or dark light in the sky around the horizon) and other sky scattering. In addition, non-horizontal surfaces also receive reflected radiation from the ground. The sum of direct insolation, diffuse insolation and reflected insolation is the total insolation or global insolation. Can rely on lenses or reflectors to focus directly to the sun. If the concentration rate is high, high energy density can be obtained, but diffuse solar radiation is lost. If the concentrating rate is low, part of the diffuse solar radiation around the sun can also be condensed. The range of diffuse insolation varies greatly. When the sky is clear and cloudless, diffuse insolation is 10% of the total insolation. But when the sky is densely covered with clouds and the sun is not visible, the total insolation is equal to the diffuse insolation. Therefore, the energy collected by concentrating collectors is usually much less than the energy collected by non-concentrating collectors. The reflected solar radiation is generally weak, but when the ground is covered with snow and ice, the reflected solar radiation on the vertical surface can reach 40% of the total solar radiation.

The solar radiation reaching the ground is mainly affected by the thickness of the atmosphere. The thicker the atmosphere (the larger the number after AM), the more severe the absorption, reflection and scattering of solar radiation, and the less solar radiation reaching the ground. Obviously, the intensity of solar radiation reaching the ground is different in different regions, seasons, and weather conditions on the earth.

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