### The relationship between solar photovoltaic power system and solar radiation

Solar radiant energy is most measured by the radiant power projected by the sun onto a unit area, the unit is watt square meters (W/m2), and the sun is projected onto a unit area within a period of time (such as hourly, day, month, year, etc.) The radiation energy is called the irradiance, the unit is kilowatt·hour/(m2·day) or kilowatt·hour/(square meter·month), kilowatt·hour (square meter·year) [kW·h/(m2· d) or kW·h/(m2·mon), kW·h/(m2·a)]. The units of radiant energy also include joule (J), megajoule (MJ), calorie (cal), kilocalorie (kcal) and so on. The unit conversion is as follows:
1kW·h=3.6MJ
1cal =4.186J =1.16278mW·h
1MJ/m2 =23.889cal/cm2 =27. 8mW·h/cm2
1kW·h/m2 =85.98cal/cm2 =3.6MJ/m2 = 100mW·h/cm2
During the construction and operation of photovoltaic power station systems, AM1.0, AM1.5 and AM0 are often heard. These professional terms are closely related to solar radiation.

②Solar constant
The solar radiation intensity per unit area in the vertical direction of sunlight outside the earth’s atmosphere can be regarded as a constant value. This radiation intensity is described by the “solar constant”, which is also called the radiation of atmospheric mass m=0 (AM0). Expressed by Isc, the unit is W/m2, and the value was adopted by the Eighth Session of the World Meteorological Organization’s Instrument and Methods of Observation Committee held in Mexico in October 1981
Isc=(1367 ±7) W/m2
③ Solar spectrum
Solar cell modules can generate electricity and provide electricity for human beings. In addition to using the unique photoelectric effect of solar cell semiconductor materials, they mainly use the radiant energy of sunlight. The distribution of the radiation emitted by the sun with wavelength on the top of the atmosphere is called the solar spectrum. The solar spectrum is very important for the application of solar cell power generation systems. Figure 1 shows the solar spectrum distribution. The solar radiation capability is shown in Figure 2. Different materials absorb the solar spectrum in different ranges. For example, silicon crystal (c-Si) solar cells mainly absorb the spectrum in the 400 ~ 1000 wavelength range; amorphous silicon (a-Si) solar cells mainly absorb the spectrum in the 400 ~ 700 mm wavelength range. In terms of spectroscopy, the absorption wavelength range of CIS thin-film solar cells is relatively large.

④Air quality
Before solar radiation reaches the earth, it is weakened by the absorption, scattering and reflection of gas molecules and aerosols in the atmosphere. This weakening is also related to the distance through the atmosphere. The thicker the atmosphere, the absorption, reflection and reflection of solar radiation. The more severe the scattering, the less solar radiation will eventually reach the ground.
Air Mass (AM) is a “dimensionless unit”, which is the ratio of the actual distance of solar radiation through the earth’s atmosphere and the distance of the sun through the earth’s atmosphere when the sun is incident perpendicular to the zenith. As shown in Figure 4, the solar radiation received outside the earth’s atmosphere is not reflected and absorbed by the atmosphere. It is called atmospheric mass as m=0, usually written as AM0; m=1 (usually written as AM1.0) means 1 At standard atmospheric pressure and 0°C, the atmospheric mass at sea level when the sun’s rays are incident perpendicularly: m=1.5 (AM1.5) and m=2 (AM2) respectively represent the atmospheric mass obtained in different directions. 1.5 is the distance/vertical distance traveled by the light in the atmosphere, reflecting an angle relationship.
Because the sun’s zenith angle (the angle between the incident light and the ground normal) at different angles when the sun is incident on the earth is different, that is, the optical path is different, the relative equivalent atmospheric mass is also different.
As shown in Figure 4, the air quality is expressed as
AM=O’A/OA=1/sinαs=1/cosθz （1）
In the formula, AM is the air quality: αs is the sun’s altitude angle: θz is the zenith angle.
From equation (1), it can be seen that AM1.5 corresponds to the celestial angle of 48.2°, and most countries including China, Europe, and the United States are in this mid-latitude region. Therefore, the solar spectrum on the earth’s surface is generally represented by AM1.5. The irradiance is 1000W/m2. AM2 corresponds to the zenith angle of 60°.
Standard lubrication conditions (STC) of light-generating components: Air quality is AM1.5, irradiance is 1000W/m2, and temperature is 25°C.