Air Mass (solar Energy) - Solar Intensity

Solar Intensity

Solar intensity at the collector reduces with increasing airmass coefficient, but due to the complex and variable atmospheric factors involved, not in a simple or linear fashion. For example, almost all high energy radiation is removed in the upper atmosphere (between AM0 and AM1) and so AM2 is not twice as bad as AM1. Furthermore there is great variability in many of the factors contributing to atmospheric attenuation, such as water vapor, aerosols, photochemical smog and the effects of temperature inversions. Depending on level of pollution in the air, overall attenuation can change by up to ±70% towards the horizon, greatly affecting performance particularly towards the horizon where effects of the lower layers of atmosphere are amplified manyfold.

One approximate model for solar intensity versus airmass is given by:

I = 1.1 \times I_\mathrm{o} \times 0.7^{(AM)^{(0.678)}}
\,

(I.1)

where solar intensity external to the Earth's atmosphere = 1.353 kW/m2, and the factor of 1.1 is derived assuming that the diffuse component is 10% of the direct component.

This formula fits comfortably within the mid-range of the expected pollution-based variability:

Solar intensity vs zenith angle and airmass coefficient AM
AM range due to pollution formula (I.1) ASTM G-173
degree W/m2 W/m2 W/m2
- 0 1367 1353 1347.9
1 840 .. 1130 = 990 ± 15% 1040
23° 1.09 1020
30° 1.15 780 .. 1100 = 940 ± 17% 1010
45° 1.41 950
48.2° 1.5 930 1000.4
60° 2 560 .. 970 = 770 ± 27% 840
70° 2.9 710
75° 3.8 620
80° 5.6 200 .. 660 = 430 ± 53% 470
85° 10 85 .. 480 = 280 ± 70% 270
90° 38 20

This illustrates that significant power is available at only a few degrees above the horizon.

Read more about this topic:  Air Mass (solar Energy)

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