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Solar Incident Flux

Solar incident flux, or solar irradiance, measures the power received from the Sun at a surface, averaging 1361 W/m2 at Earth's atmosphere. It consists of direct, diffuse, and reflected radiation, with atmospheric conditions significantly affecting the flux. Understanding solar flux is essential for solar energy system design, climate modeling, and optimizing energy efficiency in buildings.

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1K views2 pages

Solar Incident Flux

Solar incident flux, or solar irradiance, measures the power received from the Sun at a surface, averaging 1361 W/m2 at Earth's atmosphere. It consists of direct, diffuse, and reflected radiation, with atmospheric conditions significantly affecting the flux. Understanding solar flux is essential for solar energy system design, climate modeling, and optimizing energy efficiency in buildings.

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lokeshkumaar3421
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Solar Incident Flux: Detailed Notes

June 2025

1 Solar Incident Flux


Solar incident flux, commonly referred to as solar irradiance, is the power per unit area
received from the Sun, measured in watts per square meter (W/m2 ). It represents the
energy flux incident on a surface, typically perpendicular to the Sun’s rays, and is a
fundamental parameter in solar energy systems. At the top of Earth’s atmosphere, at a
mean distance of 1 astronomical unit (AU), the average solar flux, known as the solar
constant, is approximately 1361 W/m2 . This value fluctuates by about 6.9 % due to
Earth’s elliptical orbit and solar activity, such as sunspots and solar flares.

1.1 Components of Solar Flux


Solar incident flux comprises three components: direct (beam) radiation, diffuse radiation,
and reflected radiation. Direct radiation travels in a straight line from the Sun to the
surface, delivering the highest intensity. Diffuse radiation is scattered by atmospheric
constituents like clouds, aerosols, and gases, arriving from multiple directions. Reflected
radiation depends on the albedo of surrounding surfaces, such as snow (0.8) or grass (0.2).
At the Earth’s surface, atmospheric effects reduce the flux significantly, with clear-sky
conditions yielding about 1000 W/m2 at solar noon.

1.2 Mathematical Formulation


The solar flux at the Earth’s surface is modeled as:

I = I0 cos θz τ (1)

where I0 is the extraterrestrial flux (1361 W/m2 ), θz is the solar zenith angle (angle
between the Sun and the vertical), and τ is the atmospheric transmittance (typically
0.6–0.8 for clear skies). The cosine term accounts for the angle of incidence, reducing flux
as the Sun moves away from the zenith.

1.3 Example Calculation


Consider a location at solar noon with θz = 20◦ and τ = 0.7. The surface flux is:

I = 1361 · cos(20◦ ) · 0.7 ≈ 1361 · 0.9397 · 0.7 ≈ 895.6 W/m2 (2)

This illustrates how zenith angle and atmospheric conditions reduce the flux compared
to the solar constant.

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1.4 Applications
Solar incident flux is critical for designing solar energy systems, including photovoltaic
panels and solar thermal collectors. Accurate flux data informs system sizing, efficiency
predictions, and site selection. For instance, regions with high annual flux, such as deserts,
are ideal for solar farms. Flux measurements also support climate modeling and building
energy design, optimizing window placement for passive solar heating.

1.5 Factors Affecting Flux


Key factors include:

• Atmospheric Conditions: Clouds, aerosols, and water vapor reduce τ .

• Geography: Latitude affects θz , with equatorial regions receiving higher flux.

• Time of Day/Year: Diurnal and seasonal variations in solar position alter flux.

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