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Air-Mass

The term terrestrial refers to the earth’s surface or atmosphere. On the other hand, Air Mass (AM) is defined as the measure of the atmospheric path length of the sun relative to the minimum path length when the sun is directly overhead (at Zenith) as depicted in the diagram below.

Air-Mass

Terrestrial Solar Energy

The solar energy that the Earth’s atmosphere receives from the sun is approximately 1.75 x 1017 W per hour. This one-hour-energy-supply is capable of meeting the world’s energy demand for the whole year. As of the year 2010, the world’s energy consumption stood at 1.61 x 1013 W per annum (R. M. Lamaison, et al., Photovoltaic Solar Energy, 2004). From this statistics we observe that the Sun supplies more than enough energy for the Earth’s need. This calls for improved technology on harvesting and storage of solar energy. However, not all of that energy reaches the Earth’s surface due to the effect of AM that attenuates photons through the atmosphere’s absorption and scattering.

Air Mass

AM varies with the angle of the sun to the vertical (Zenith) position. The more the angle from Zenith, the longer the path and the more the attenuation. The attenuation is mainly through water vapor which absorbs solar radiation in the infrared region, the ozone layer which absorbs radiation in the ultraviolet region, dust, and aerosols which scatter the radiation in the visible region. The attenuation at AM 0 and AM 1.5 is depicted below in comparison to blackbody radiation (Sun).

Air-Mass-Wavelength

Categories of AM

AM is basically divided into 4 levels, namely AM 0, AM 1, AM 1.5, and AM 2.

Mathematically, AM is expressed as the secant of the angle between the zenith and the position of the sun in the sky as shown in the equation below.

One practical way to estimate the AM is to measure the length of the shadow (s) cast by a vertical structure of height (h) using the equation below.

AM-formula
Air-Mass-Graph
Air-mass-formula

The larger the value of the AM, the more the atmospheric attenuation on the solar radiation and the weaker the radiation received. The AM 0 spectrum is only relevant for satellite and space-vehicle applications while the AM 1 spectrum represents the radiation at the earth’s surface when the sun is exactly at zenith i.e. when θ = 0° with an incident power of 925 W/m2. The AM 1.5 conditions (sun at 48.2o above the horizon) represents energy average for terrestrial applications. The total incident power for AM 1.5 is 844 W/m2. On the other hand, The AM 2 spectrum is for θ = 60° and has an incident power of about 691 W/m2

Conclusion

It is important to understand the role played by the atmosphere and AM in particular in order to optimize solar energy harvesting. To maximize solar energy harvesting, there is a need to eliminate dust, moisture, and larger AM. The positioning of the equipment for solar harvesting is best placed at AM 0.

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