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)
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:
| AM | range due to pollution | formula (I.1) | ASTM G-173 | |
|---|---|---|---|---|
| degree | W/m2 | W/m2 | W/m2 | |
| - | 0 | 1367 | 1353 | 1347.9 |
| 0° | 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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