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6-7 Factors Affecting Terrestrial Solar Irradiance

Author: Source: Datetime: 2016-12-22 14:08:00

There are many factors that affect the solar irradiance of the ground. The solar irradiance of a specific site is determined by the following comprehensive results.


1. The height of the sun

For a point on the earth, the solar elevation angle is the angle between the direction of the sun's rays and the ground plane, ie the angle between the sun's rays and the ground's tangent to the earth's surface perpendicular to the earth's center. As the Earth's atmosphere of solar radiation absorption, reflection and scattering, therefore, infrared, can light and ultraviolet light in the proportion of the proportion of light with the solar altitude angle changes. When the solar elevation angle of 90 °, in the sunlight. Infrared accounted for 50%, 46% visible light, UV accounted for when the solar height of 5 °. Infrared Mountain 72%, see 28% of the light, UV Close to zero. - In the sky, the solar elevation angle is constantly changing: at the same time, in the year is also changing. For a given ground plane, when the solar elevation angle is low, the light travels through the atmosphere for a longer distance. At the same time, because the light to a smaller angle projection to the ground plane, so to reach the ground plane can pull less. On the contrary, more.

2. Atmospheric quality

The magnitude of solar radiation attenuation is related to the length of solar radiation through the atmosphere. The longer the distance, the more energy loss; shorter distances, the less the energy loss. Atmospheric mass is the dimensionless travel of solar radiation through the atmosphere and is defined as the ratio of the distance traveled by the sun's rays through the atmosphere to the distance traveled by the sun's rays in the zenith direction. So that the sun at sea level perpendicular to the incident path is 1, that is, non-dimensional distance is slow. The figure shows that when the solar elevation angle is greater than or equal to 30 °, the dimensionless distance is calculated as .Atmospheric quality shown below,

fdsfa

In the formula, αsthe solar elevation angle.

solar elevation angle

The relationship between solar altitude and air quality is shown in the following figure 1-2

Relationship between Solar Altitude Angle and Air Quality

Solar elevation angle αs

90°

60°

45°

30°

10°

5°

Air quality1/sinαs

1.00

1.155

1.414

2.000

5.758

11.480


3. Atmospheric transparency

The solar radiation is essentially a constant in the plane perpendicular to the atmosphere. However, in the Earth 5 surface, the solar irradiance is constantly changing. This is mainly due to the different levels of atmospheric transparency caused. Atmospheric transparency is the table
A parameter for the degree of solar light penetration. In clear weather, high transparency, to reach the surface of the sun blessing can be more. A lot of sky clouds or dusty sand, the atmosphere is very low transparency, to reach the ground less solar radiation. Visible, atmospheric transparency is the number of clouds in the sky Yun Gu and the dust contained in the atmosphere and other impurities are closely related. Force to consider the impact of atmospheric transparency on solar radiation, the cumbersome formula is derived, its made in Table 1-3. The direct solar irradiance at different altitudes and altitudes is shown in Tables 1-3.

1-3 Relationship between direct solar irradiance and solar angle under different atmospheric transparency (solar-terrestrial mean distance)


Transparency

P2


Solar elevation angle αs /(°)

7

10

15

20

25

30

40

50

60

70

90

Very turbid 0.60

0.17

0.26

0.41

0.54

0.63

0.70

0.83

0.94

1.00

1.04

1.06

Turbid 0.65

0.25

0.38

0.55

0.67

0.76

0.84

0.98

1.08

1.13

1.16

1.17

Low 0.70

0.35

0.49

0.67

0.79

0.88

0.96

1.08

1.16

1.21

1.25

1.27

Normal 0.75

0.48

0.63

0.81

0.93

1.02

1.10

1.21

1.27

1.32

1.35

1.37

High 0.80

0.61

0.76

0.93

1.06

1.15

1.22

1.32

137

1.41

1.44

1.46

Very transparent 0.85

0.77

0.90

1.08

1.20

1.29

1.35

1.42

1.47

1.51

1.53

1.54

4. Geographic latitude


Solar radiation energy 8 is from low latitude to high latitude gradually weakened. The atmospheric transparency is assumed to be the same in different latitudes. In this condition, the comparison, as shown in the figure, the vernal equinox at noon time of the sun perpendicular to the Earth's equator F point on the same longitude on the other two points B, D latitude than D latitude high. As can be seen, the sun shot to point B through the atmosphere of the distance AB than the sun hit the point D through the atmosphere of the distance CD long. Therefore, the vertical radiation energy at point B will be smaller than point D. The vertical radiation field at the F-point is the largest at the equator, because the distance EF in the atmosphere passes through the EF is the shortest. For example, at a high latitude in St. Petersburg (60 ° N) in Russia, only 335kJ of heat is available per 1cm2 of land per year. In China's capital, Beijing, is located in the middle latitude (latitude 39 ° 67 '), you can get 586kJ of heat in the low latitudes of the Sahara desert area can get 921kJ of heat.relationship between vertical solar radiation flux and geographical latitude diagram

5. Sunshine time

Sunshine time is also an important factor affecting the solar irradiance on the ground. If a region has a day 14h, if the cloudy time ≥ 6h, while the sun time is less than or equal to 8h, then you can call the region the day of sunshine time is 8h. The longer the sunshine, the ground the total amount of solar radiation obtained by the more.

6. Altitude

The higher the altitude, the better the transparency of the atmosphere, and thus the higher the direct solar radiation. China's Tibet Plateau, the average elevation of up to 4000m above, and the atmosphere clean, dry air, low latitude, so the total solar radiation sites ranging from 6000-8000MJ / m2 direct radiation than the major.

In addition, the sun and earth distance, topography, terrain and other solar irradiance also have a certain impact. In the same latitude, the temperature of the basin is higher than that of the plain, and the temperature of the sunny slope is higher than that of the shady slope.



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