Solar irradiance

Solar Irradiance

Global distribution of solar irradiance.

Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. It is measured in watts per square meter (W/m²) in SI units. Solar irradiance is an important factor in the study of climate, solar energy, and atmospheric science.

Types of Solar Irradiance

Solar irradiance can be categorized into different types based on the path it takes to reach the Earth's surface:

• Direct Normal Irradiance (DNI): This is the solar radiation received per unit area by a surface that is always held perpendicular (or normal) to the rays that come in a straight line from the direction of the sun. It is important for concentrating solar power systems.

• Diffuse Horizontal Irradiance (DHI): This is the solar radiation received from the sun after its direction has been changed by scattering by the atmosphere. It is measured on a horizontal surface.

• Global Horizontal Irradiance (GHI): This is the total amount of shortwave radiation received from above by a horizontal surface. It is the sum of direct and diffuse solar radiation.

Global Horizontal Irradiance map.

Measurement of Solar Irradiance

Solar irradiance is measured using instruments such as pyranometers and pyrheliometers.

• A pyranometer measures global solar irradiance on a planar surface and is used to measure both direct and diffuse solar radiation.

• A pyrheliometer measures direct beam solar irradiance and is used to measure DNI.

A pyranometer used to measure solar irradiance.

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A pyrheliometer used to measure direct solar irradiance.

Factors Affecting Solar Irradiance

Several factors affect the amount of solar irradiance received at a given location on Earth:

• Solar zenith angle: The angle between the sun and the vertical directly affects the intensity of solar radiation. The smaller the angle, the more intense the radiation.

• Atmospheric conditions: Clouds, dust, and pollution can scatter and absorb solar radiation, reducing the amount of solar irradiance reaching the surface.

• Earth's orbit and axial tilt: The Milankovitch cycles describe the collective effects of changes in the Earth's movements on its climate over thousands of years, affecting solar irradiance.

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Diagram showing the solar zenith angle.

Milankovitch cycles affecting solar irradiance.

Solar Spectrum

The solar spectrum is the distribution of solar radiation as a function of wavelength. It includes ultraviolet, visible, and infrared radiation. The solar spectrum is crucial for understanding the energy balance of the Earth and the design of solar energy systems.

The solar spectrum.

Applications of Solar Irradiance

Solar irradiance data is essential for various applications, including:

• Designing and optimizing solar power systems.
• Studying and modeling climate change.
• Understanding and predicting weather patterns.

Related Pages

• Solar energy
• Photovoltaics
• Climate change
• Atmospheric science