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What Is Solar Irradiance?

Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation as measured in the wavelength range of the measuring instrument. The solar irradiance is measured in watt per square metre in SI units.

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Solar Irradiance

Solar irradiance is the power per unit area received from the Sun in the form of electromagnetic radiation as measured in the wavelength range of the measuring instrument. The solar irradiance is measured in watt per square metre (W/m2) in SI units. Solar irradiance is often integrated over a given time period in order to report the radiant energy emitted into the surrounding environment (joule per square metre, J/m2) during that time period. This integrated solar irradiance is called solar irradiationsolar exposuresolar insolation, or insolation.

Irradiance may be measured in space or at the Earth’s surface after atmospheric absorption and scattering. Irradiance in space is a function of distance from the Sun, the solar cycle, and cross-cycle changes.[1] Irradiance on the Earth’s surface additionally depends on the tilt of the measuring surface, the height of the sun above the horizon, and atmospheric conditions.[2] Solar irradiance affects plant metabolism and animal behavior.[3]

The study and measurement of solar irradiance have several important applications, including the prediction of energy generation from solar power plants, the heating and cooling loads of buildings, and climate modeling and weather forecasting.

There are several measured types of solar irradiance.

  • Total Solar Irradiance (TSI) is a measure of the solar power over all wavelengths per unit area incident on the Earth’s upper atmosphere. It is measured perpendicular to the incoming sunlight.[2] The solar constant is a conventional measure of mean TSI at a distance of one astronomical unit (AU).
  • Direct Normal Irradiance (DNI), or beam radiation, is measured at the surface of the Earth at a given location with a surface element perpendicular to the Sun.[6] It excludes diffuse solar radiation (radiation that is scattered or reflected by atmospheric components). Direct irradiance is equal to the extraterrestrial irradiance above the atmosphere minus the atmospheric losses due to absorption and scattering. Losses depend on time of day (length of light’s path through the atmosphere depending on the solar elevation angle), cloud covermoisture content and other contents. The irradiance above the atmosphere also varies with time of year (because the distance to the sun varies), although this effect is generally less significant compared to the effect of losses on DNI.
  • Diffuse Horizontal Irradiance (DHI), or Diffuse Sky Radiation is the radiation at the Earth’s surface from light scattered by the atmosphere. It is measured on a horizontal surface with radiation coming from all points in the sky excluding circumsolar radiation (radiation coming from the sun disk).[6][7] There would be almost no DHI in the absence of atmosphere.[6]
  • Global Horizontal Irradiance (GHI) is the total irradiance from the sun on a horizontal surface on Earth. It is the sum of direct irradiance (after accounting for the solar zenith angle of the sun z) and diffuse horizontal irradiance:[8]
    {\displaystyle {\text{GHI}}={\text{DHI}}+{\text{DNI}}\times \cos(z)}
  • Global Tilted Irradiance (GTI) is the total radiation received on a surface with defined tilt and azimuth, fixed or sun-tracking. GTI can be measured[7] or modeled from GHI, DNI, DHI.[9][10][11] It is often a reference for photovoltaic power plants, while photovoltaic modules are mounted on fixed or tracking constructions.
  • Global Normal Irradiance (GNI) is the total irradiance from the sun at the surface of Earth at a given location with a surface element perpendicular to the Sun.

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Measurement and Variations

Variation[edit]

Total solar irradiance (TSI)[17] changes slowly on decadal and longer timescales. The variation during solar cycle 21 was about 0.1% (peak-to-peak).[18] In contrast to older reconstructions,[19] most recent TSI reconstructions point to an increase of only about 0.05% to 0.1% between the Maunder Minimum and the present.[20][21][22] Ultraviolet irradiance (EUV) varies by approximately 1.5 percent from solar maxima to minima, for 200 to 300 nm wavelengths.[23] However, a proxy study estimated that UV has increased by 3.0% since the Maunder Minimum.[24]

 
Variations in Earth’s orbit, resulting changes in solar energy flux at high latitude, and the observed glacial cycles.

Some variations in insolation are not due to solar changes but rather due to the Earth moving between its perihelion and aphelion, or changes in the latitudinal distribution of radiation. These orbital changes or Milankovitch cycles have caused radiance variations of as much as 25% (locally; global average changes are much smaller) over long periods.

 

The space-based TSI record comprises measurements from more than ten radiometers spanning three solar cycles. All modern TSI satellite instruments employ active cavity electrical substitution radiometry. This technique applies measured electrical heating to maintain an absorptive blackened cavity in thermal equilibrium while incident sunlight passes through a precision aperture of calibrated area. The aperture is modulated via a shutter. Accuracy uncertainties of <0.01% are required to detect long term solar irradiance variations, because expected changes are in the range 0.05–0.15 W/m2 per century.

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