# The second part of the relationship between energy and solar energy

1. Solar constant
The distance between the sun and the earth is constantly changing, which means that the solar radiation intensity of the upper boundary of the earth’s atmosphere will change with time and place. However, since the ratio of the change in the distance between the earth and the sun to the distance between the two is too small, the relative change in the intensity of this solar radiation basically does not exceed ±3.4%. Therefore, we roughly think that the intensity of solar radiation outside the atmosphere is almost constant. That is, the “solar constant”. At the same time, because there is no atmosphere outside the atmosphere, this radiation is defined as the radiation with zero atmospheric mass (AM0).

The solar constant (Isc) is defined as the solar radiation energy received per unit time on the unit area of ​​the earth’s atmosphere perpendicular to the surface of the sun’s rays at the average distance (between the sun and the earth). Its reference value Isc =(1367±7)W/m².

1. Air quality
When sunlight passes through the earth’s atmosphere, it must be attenuated by at least 30%. There are many factors that cause attenuation, mainly Rayleigh scattering or molecular scattering in the atmosphere, scattering caused by suspended particles and dust, and the absorption of the atmosphere, especially its constituent gases. In a sunny day, the energy that the light has when it reaches the ground depends on the distance the light travels through the atmosphere. When the sun is overhead, the distance is the shortest. The ratio of the actual distance to this shortest distance is called the optical air quality.

When sunlight is incident vertically, the optical atmospheric mass is 1, and the radiation at this time is called atmospheric mass 1 (AM1) radiation. When the sun and the vertical are at an angle α, the mass of the atmosphere is: 1/cosα

Therefore, when the sun deviates from the vertical at an angle of 60°, the radiation is AM2 radiation. In practical applications, we usually use AM1.5 radiation, and draw a ground spectral distribution curve based on the air quality at this time. In 1977, the US government’s photovoltaic plan scaled up this distribution as the spectral distribution standard. The purpose of scaling up is to make the total power density 1kW/m², which is close to the maximum value of the power density received on the surface of the earth.