Air mass (solar energy)
The air mass coefficient is commonly used to characterize the performance of solar cells under standardized conditions, and is often referred to using the syntax "AM" followed by a number.
By the time it reaches the Earth's surface, the spectrum is strongly confined between the far infrared and near ultraviolet.
[3] The greater the distance in the atmosphere through which the sunlight travels, the greater this effect, which is why the sun looks orange or red at dawn and sunset when the sunlight is travelling very obliquely through the atmosphere — progressively more of the blues and greens are removed from the direct rays, giving an orange or red appearance to the sun; and the sky appears pink — because the blues and greens are scattered over such long paths that they are highly attenuated before arriving at the observer, resulting in characteristic pink skies at dawn and sunset.
is the path length at zenith (i.e., normal to the Earth's surface) at sea level.
The air mass number is thus dependent on the Sun's elevation path through the sky and therefore varies with time of day and with the passing seasons of the year, and with the latitude of the observer.
The above approximation overlooks the atmosphere's finite height, and predicts an infinite air mass at the horizon.
[6] Modelling the atmosphere as a simple spherical shell provides a reasonable approximation:[7] where the radius of the Earth
To avoid taking the difference of two large numbers, this can be written as which also shows the similarity to the simple
If x is the distance along the light ray from where it meets the ground, divided by the equivalent thickness of the atmosphere (approximately 9 km), then the height of a point is: The air mass is then: where
However, neither this model nor the previous take into consideration the bending of light rays due to refraction (see Levelling).
Solar cells used for space power applications, like those on communications satellites, are generally characterized using AM0.
The spectrum after travelling through the atmosphere to sea level with the sun directly overhead is referred to, by definition, as "AM1".
=25°) is a useful range for estimating performance of solar cells in equatorial and tropical regions.
Many of the world's major population centres, and hence solar installations and industry, across Europe, China, Japan, the United States of America and elsewhere (including northern India, southern Africa and Australia) lie in temperate latitudes.
While the summertime AM number for mid-latitudes during the middle parts of the day is less than 1.5, higher figures apply in the morning and evening and at other times of the year.
The specific value of 1.5 has been selected in the 1970s for standardization purposes, based on an analysis of solar irradiance data in the conterminous United States.
The latest AM1.5 standards pertaining to photovoltaic applications are the ASTM G-173[10][11] and IEC 60904, all derived from simulations obtained with the SMARTS code.
=70°) is a useful range for estimating the overall average performance of solar cells installed at high latitudes such as in northern Europe.
The relative air mass is only a function of the sun's zenith angle, and therefore does not change with local elevation.
Conversely, the absolute air mass, equal to the relative air mass multiplied by the local atmospheric pressure and divided by the standard (sea-level) pressure, decreases with elevation above sea level.
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.
Furthermore, there is great variability in many of the factors contributing to atmospheric attenuation,[12] such as water vapor, aerosols, photochemical smog and the effects of temperature inversions.
[13] This formula fits comfortably within the mid-range of the expected pollution-based variability: This illustrates that significant power is available at only a few degrees above the horizon.
One approximate model for intensity increase with altitude and accurate to a few kilometres above sea level is given by:[13][19] where
Alternatively, given the significant practical variabilities involved, the homogeneous spherical model could be applied to estimate AM, using: where the normalized heights of the atmosphere and of the collector are respectively
And then the above table or the appropriate equation (I.1 or I.3 or I.4 for average, polluted or clean air respectively) can be used to estimate intensity from AM in the normal way.
By contrast much of the attenuation of the high energy components occurs in the ozone layer - at higher altitudes around 30 km.
This apparently counter-intuitive result arises simply because silicon cells can't make much use of the high energy radiation which the atmosphere filters out.
As illustrated below, even though the efficiency is lower at AM0 the total output power (Pout) for a typical solar cell is still highest at AM0.
This illustrates the more general point that given that solar energy is "free", and where available space is not a limitation, other factors such as total output power Pout, and Pout per unit of invested money (e.g. per dollar), are often more important considerations than efficiency (Pout/Pin).
