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Category Archives: Irradiance
The air mass coefficient defines the path length of sunlight through the atmosphere (e.g. the column depth), and is a key input for estimating solar extinction and the irradiance intensity on the Earth’s surface. The challenge in modeling air mass and solar extinction is an atmosphere that is highly variable, exhibiting unique behavior at different altitudes. Atmospheric models seek to overcome some of the errors in the interpolative models of air mass. Specifically, atmospheric models:
“I’d put my money on the Sun and solar energy, what a source of power! We shouldn’t wait until oil and coal run out before we tackle that.”
– Thomas Edison in conversation with Henry Ford and Harvey Firestone, 1931
Zenith, Azimuth and Elevation Angles
The position of the Sun relative to a point on the ground is an important input needed to model solar air mass and solar system performance. The inputs used to describe solar position include:
- Zenith angle
- Azimuth angle relative to the North point on a compass
- Elevation angle or altitude , where
Total Solar Irradiance and the Solar Constant
Extraterrestrial irradiance refers to solar irradiance outside the earth’s atmosphere. Total Solar Irradiance (TSI) is the amount of incoming solar electromagnetic radiation per unit area at the top of the Earth’s atmosphere (TOA). The solar constant is the mean TSI.
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Welcome to Sunlight and Weather, a technical introduction to solar irradiance, the Earth’s atmosphere, and the basic principles which define the performance of solar power systems.
The Earth’s atmosphere has several effects on terrestrial radiation. The figure below depicts the relative importance of atmospheric impacts on the sunlight striking the Earth’s surface. The process poster also depicts the solar energy balance of the Earth-atmosphere system.
The major impacts of the atmosphere on sunlight include:
- A reduction in solar radiation and change in spectral content given atmospheric absorption;
Aerosols directly and indirectly effect the Earth’s radiation budget and climate. As a direct effect, aerosols absorb and scatter sunlight, affecting the spectral intensity of solar radiation reaching the earth’s surface. As an indirect effect, atmospheric aerosols modify cloud formation processes and how clouds affect the energy budget.
A common question concerning the safety of photovoltaic (PV) power systems is the impact of reflected sunlight. PV modules have the potential to impact neighboring structures or activities, notably aviation. It is important to know where the reflected light will go and what the intensity of the light will be at any point in time.