- 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.
- 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.
- AM1.5 standard spectral radiation
The total radiation (direct and scattered) AM1.5 solar spectrum distribution conditions: the irradiance is 1000W/m² on an inclined plane 370 with the ground surface, the reflectivity of the ground is 0.2, the water content in the atmosphere is 1.42%, The ozone content is 0.34%, and the atmospheric turbidity is 0.27. - The distribution of solar energy resources in China
The distribution of solar energy resources in various places depends on the latitude, altitude, and climatic conditions of the place, and is generally expressed in terms of the total amount of radiation throughout the year. Because it is difficult to calculate, it can only be measured actually, and sometimes the total sunshine hours throughout the year are used to express the distribution of solar energy resources. The total annual radiation in China is generally within the range of 80-200kcal/(cm².y). Among them, Qinghai and Tibet are the highest. The entire Sichuan Basin, Lianghu area, Qinling and Bashan are low-value areas. The Tarim Basin through the Hexi Corridor to the Inner Mongolia Plateau is another high-value area. Eastern, southern and northeastern China are generally medium-value areas. .
Northern Ningxia, Western Gansu, Southeastern Xinjiang, Western Qinghai, Western Tibet, annual sunshine hours/h: 2800~3300, annual total radiation/kcal/(㎝²·y): 160~200, equivalent to standard coal/ kg:225~235
Northwestern Hebei, northwestern Shanxi, Inner Mongolia, southern Ningxia, central Gansu, eastern Qinghai, southeastern Tibet, southern Xinjiang, annual sunshine hours/h: 3000~3200, annual total radiation/kcal/(㎝²·y): 140~160, equivalent to standard coal/kg: 200~225
Shandong, Henan, southeast of Hebei, southwest of Shanxi, northern Xinjiang, Jilin, Liaoning, Yunnan, northern Shaanxi, southeastern Gansu, southern Guangdong, southern Fujian, northern Jiangsu, northern Anhui, annual sunshine hours/h: 2200~3000, Total annual radiation/kcal/(㎝²·y): 120~140, equivalent to standard coal/kg: 170~200
Hunan, Guangxi, Jiangxi, Zhejiang, Hubei, northern Fujian, northern Guangdong, southern Shaanxi, southern Jiangsu, southern Anhui, Heilongjiang, annual sunshine hours/h: 1400-2200, annual total radiation/kcal/(㎝²·y ): 100~120, equivalent to standard coal/kg: 140~170
Sichuan and Guizhou, annual sunshine hours/h: 1000~1400, annual total radiation amount/kcal/(㎝²·y): 80~100, equivalent to standard coal/kg: 115~140