1.1 Air mass

Generally, when the sun is at the zenith and illuminates the ground vertically, the distance traveled by the sun’s rays is called 1 air mass. The solar radiation received outside the earth’s atmosphere is not reflected and absorbed by the atmosphere, and the atmospheric mass is called 0, that is, outside the earth’s atmosphere (atmosphere), the atmospheric mass is 0, expressed as AM0. AM is used to indicate direct access into the atmosphere The distance traveled by light. AM is the abbreviation of air mass, which means air mass.

Before solar radiation reaches the earth, it is absorbed by gas molecules and suspended particles in the atmosphere (mainly including the absorption of ultraviolet rays by the ozone layer, and the absorption of infrared rays by water vapor), scattering and reflection. This weakening is obviously related to the penetration of the atmosphere. Distance is related. Therefore, the longer the distance the light travels through the atmosphere, the more severely the solar radiation energy will be weakened, the less solar energy reaches the ground, and the lower the power generated by solar cells. Therefore, the longer the distance the light travels in the atmosphere, the more atmospheric The worse the mass.

When the incident angle of sunlight and the ground form an angle B, the air mass is:

AM=1/cosθ

Since the sun has different zenith angles at different latitudes of the earth (the angle between the incident light and the ground normal), that is, the optical path is different, the relative equivalent atmospheric mass is also different, including those in China, Europe, and the United States. Some countries are in this mid-latitude region. Therefore, the solar energy on the surface is generally expressed by AM1.5, and the energy is 1000/m^{2} (the solar energy test standard is also AM1.5).

1.2 Sunshine hours and hours

(1) Sunshine time

The actual number of hours of sunlight in a day is called sunshine time (Figure 1-11), in hours. The sunshine time can be divided into the maximum possible sunshine time and the geographical or terrain possible sunshine time. Maximum possible sunshine duration: The period between the rising and falling of the edge of the sun. Geographical or topographical possible moonshine time: the longest period of time during which solar radiation can reach a given plane. Long-day sunshine refers to the daily sunshine time greater than 14h. The effective time of sunshine is determined according to the orientation of the building.

The sunshine time can be divided into astronomical sunshine time and actual sunshine time. Astronomical sunshine time refers to the amount of sunshine time under this condition if a place is always sunny, that is, the upper limit of actual sunshine time. The ratio of actual sunshine time to astronomical sight time is the sunshine rate, which can be used to measure the incidence of sunny days in a place. The percentage of sunshine is determined by the following formula:

Percentage of sunshine = actual daily sunshine time ÷ astronomical sunshine time × 100%

(2) Sunshine hours

Sunshine hours refers to the length of time that the sun’s radiant intensity on a plane perpendicular to its rays exceeds or equals 120W/m^{2} per day.

(3) Annual average sunshine time

That is, the ratio of the annual sunshine time of several years to the number of years is one of the evaluation indexes of the solar energy utilization value.

(4) Daily peak sunshine hours

Sometimes it is necessary to convert the radiation amount into the peak sunshine hours.

Annual peak sunshine hours = total annual radiation in a certain place × 0.0116

In the formula, 0.0116 is the conversion factor for converting radiation (kcal/cm^{2}) into peak sunshine hours.

Daily peak sunshine hours = total annual radiation in a certain place×0.016÷365

The total annual radiation in a certain place is generally the radiation on the solar array (the square array is inclined), which is 5% to 15% higher than the solar panels placed on the horizontal plane.

[Example] The total annual radiation in a certain area is 130kcal/cm^{2} (1kcal=4.18kJ), and the radiation on the solar array is 143kcal/cm^{2}, then the annual peak sunshine hours = 143×0.0116=1658.8(h ), the daily peak sunshine hours=1658.8÷365=4.54(h/d), that is, 4.54 hours/day.