Solar Irradiance on the Surface of the Earth

Values measured on the surface of Earth are usually lower than the solar constant. Various influences of the atmosphere reduce the irradiance. They are:

• reduction due to reflection by the atmosphere

• reduction due to absorption in the atmosphere (mainly O3, H2O, O2 and CO2)

• reduction due to Rayleigh scattering

• reduction due to Mie scattering.

The absorption of light by different gases in the atmosphere, such as water vapour, ozone and carbon dioxide, is highly selective and influences only some parts of the spectrum. Figure 2.3 shows the spectrum outside the atmosphere (AM 0) and at the surface of the Earth (AM 1.5). The spectrum describes the composition of the light and the contribution of the different wavelengths to the total irradiance. Seven per cent of the extraterrestrial spectrum (AM 0) falls in the ultraviolet range, 47 per cent in the visible range and 46 per cent in the infrared range. The terrestrial spectrum AM 1.5 shows significant reductions at certain wavelengths caused by absorption by different atmospheric gases.

Molecular air particles with diameters smaller than the wavelength of light cause Rayleigh scattering. The influence of Rayleigh scattering rises with decreasing light wavelength.

Dust particles and other air pollution cause Mie scattering. The diameter of these particles is larger than the wavelength of the light. Mie scattering depends significantly on location; in high mountain regions it is relatively low, whereas in industrial regions it is usually high.

Table 2.4 shows the contributions of Mie and Rayleigh scattering and absorption for different sun heights 7S (see section on calculating the position of the sun, p55). Climatic influences such as clouds, snow, rain or fog can cause additional reductions.

Spectral irradiance (W/m2 ^m)

Spectral irradiance (W/m2 ^m)

Note: AM0 is the extraterrestrial spectrum; AM 1.5 is the spectrum on the Earth's surface at a sun height of 41.8°

Note: AM0 is the extraterrestrial spectrum; AM 1.5 is the spectrum on the Earth's surface at a sun height of 41.8°

Figure 2.3 Spectrum of Sunlight

The relationship between the sun height yS and the air mass (AM) is: 1

sin Zs

The AM value is a unitless measure of the length of the path of light through the atmosphere; it is expressed in multiples of the thickness of atmosphere. If the sun is at its zenith, AM is equal to 1, i.e. the light is passing vertically through the atmosphere. The AM value outside the atmosphere is zero. Figure 2.4 shows the highest position of the sun at solar noon and the corresponding AM values for various days of a year for Berlin and Cairo.

The elevation of the sun also influences the irradiation received at the surface of the Earth, which is thus dependent on the time of the year. Clouds

Table 2.4 Reduction Influences at Different Sun Heights

Sun Height

Air

Absorption

Rayleigh

Mie

Total

(Ys)

Mass

(%)

scattering

scattering

reduction

(AM)

(%)

(%)

(%)

90°

1.00

8.7

9.4

0-25.6

17.3-38.5

60°

1.15

9.2

10.5

0.7-29.5

19.4-42.8

30°

2.00

11.2

16.3

4.1-44.9

28.8-59.1

10°

5.76

16.2

31.9

15.4-74.3

51.8-85.4

11.5

19.5

42.5

24.6-86.5

65.1-93.8

Source: according to Schulze, 1970

Source: according to Schulze, 1970

Sun Air Mass
Figure 2.4 Sun Height at Solar Noon and Air Mass (AM) Values for Various Dates in Berlin (top) and Cairo (bottom)

and weather are important as well. The daily irradiation in central Europe can reach values above 7.5 kWh/(m2 day) in summer, whereas single days in winter can have less than 0.1 kWh/(m2 day). Figure 2.5 shows the variation in the irradiance for a cloudless day in summer (2 July) and in winter (28 December) as well as a very cloudy day in winter (22 December) for Karlsruhe in southern Germany.

The annual irradiation varies significantly throughout the world. For instance, in Europe there are large differences between north and south. In the north, differences between summer and winter are much higher than in the south. In Bergen (Norway, 60.4°N) the ratio of global irradiation (total irradiance on a horizontal surface on Earth) in June to global irradiation in December is 40:1, whereas in Lisbon (Portugal, 38.72°N) this ratio is only 3.3:1. Central and northern Europe have annual global irradiation values of between 700 kWh/(m2 year) and 1000 kWh/(m2 year). In southern Europe this irradiation can be more than 1700 kWh/(m2 year) and in desert regions of

Figure 2.5 Global Irradiance throughout the Day in Karlsruhe (Germany) for 2 July and 22 and 28 December 1991

the Earth's sunbelt the figure is around 2500 kWh/(m2 year). However, the latitude can only give a rough indication of the annual irradiation because local effects have a major impact on the energy reaching Earth's surface. For instance, the annual irradiation in Stockholm (Sweden) and Berlin (Germany) are nearly the same, although Stockholm's latitude is 7° higher than Berlin. On the other hand, the annual irradiation in London, which is south of Berlin, is significantly lower.

Table 2.5 gives an overview of monthly average global irradiation values for some locations around the world. It clearly demonstrates that there are significant variations between different locations. However, the precise irradiance at the given site is required for planning solar energy systems. This can be estimated using existing databases. Some free Internet databases offer monthly irradiation values for many locations in the world; some even offer hourly irradiance datasets for some sites (e.g. www.satellight.com, eosweb.larc.nasa.gov/sse or rredc.nrel.gov/solar). Computer programs (see also CD-ROM) such as the Meteonorm program can also be used for interpolating the meteorological parameters of a given site based on measurements taken at locations close to the proposed site.

The annual irradiation in the Sahara is about 2350 kWh/(m2 year). The total annual irradiation received by the surface of the Sahara (around 8.7 million km2) is nearly 200 times higher than the global annual primary energy demand; in fact the global primary energy demand could be provided by collecting the solar energy received by 48,500 km2 of the Sahara, an area slightly larger than Switzerland, or one-ninth that of California. These numbers clearly show that it is possible to provide the whole global energy demand solely by solar energy.

Table 2.5 Monthly Average Values in kWh/(m2 day) of the Daily Global

Irradiation

Table 2.5 Monthly Average Values in kWh/(m2 day) of the Daily Global

Irradiation

Bergen

Berlin

London

Rome

LA

Cairo

BombayUpington Sydney

Norway Germany

UK

Italy

US

Egypt

India

RSA

Australia

Latitude 60.40°N 52.47°N 51.52°N 41.80°N 33.93°N 30.08°N 19.12°N 28.40°S

33.95°S

Jan

0.20

0.61

0.56

1.70

2.88

3.09

4.74

8.08

6.41

Feb

0.72

1.14

1.10

2.54

3.97

4.00

5.56

7.45

5.57

Mar

1.71

2.44

2.07

3.78

5.14

5.15

6.29

6.26

4.72

Apr

3.27

3.49

3.04

4.99

6.47

6.27

6.72

5.19

3.47

May

4.13

4.77

4.12

6.03

6.55

7.03

6.77

4.26

2.63

June

4.85

5.44

4.99

6.59

6.57

7.56

4.99

3.72

2.38

July

4.15

5.26

4.38

6.86

7.38

7.34

3.84

4.04

2.52

Aug

3.49

4.58

3.62

6.16

6.82

6.76

3.86

4.95

3.47

Sep

1.86

3.05

2.71

4.69

5.26

5.87

4.65

6.09

4.66

Oct

0.94

1.59

1.56

3.29

4.24

4.69

5.11

7.21

5.63

Nov

0.30

0.76

0.81

2.02

3.22

3.45

4.73

8.27

6.40

Dec

0.12

0.45

0.47

1.51

2.72

2.86

4.46

8.49

6.69

Average 2.15

2.81

2.46

4.19

5.10

5.34

5.14

6.17

4.55

Source: data from Palz and Greif, 1996; NASA, 2003

Source: data from Palz and Greif, 1996; NASA, 2003

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Responses

  • samppa
    How and by what mechanism does the atmosphere reduce the solar irradiance?
    3 months ago
  • Doreen
    How to see hourly solar irradiation in meteonorm?
    2 months ago

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