Solar irradiance

Solar irradiance is the power per unit area (surface power density) received from the Sun in the form of electromagnetic radiation in the wavelength range of the measuring instrument. Solar irradiance is measured in watts per square metre (W/m²) in SI units.

"Insolation" redirects here. Not to be confused with insulation.

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 irradiation, solar exposure, solar 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.^[2] 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.^[3] Solar irradiance affects plant metabolism and animal behavior.^[4]

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, climate modeling and weather forecasting, passive daytime radiative cooling applications, and space travel.

Total solar irradiance (TSI) is a measure of the over all wavelengths per unit area incident on the Earth's upper atmosphere. It is measured perpendicular to the incoming sunlight.^[3] The solar constant is a conventional measure of mean TSI at a distance of one astronomical unit (AU).

solar power

(DNI), or beam radiation, is measured at the surface of the Earth at a given location with a surface element perpendicular to the Sun direction.^[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 cover, moisture 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.

Direct normal irradiance

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).^[7] There would be almost no DHI in the absence of atmosphere.^[6]

[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 of the Sun z) and diffuse horizontal irradiance:^[8]
${\text{GHI}}={\text{DHI}}+{\text{DNI}}\times \cos(z)$

solar zenith angle

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 or modeled from GHI, DNI, DHI.^[9]^[10]^[11] It is often a reference for photovoltaic power plants, while photovoltaic modules are mounted on the fixed or tracking constructions.

[7]

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.

There are several measured types of solar irradiance.

Units[edit]

The SI unit of irradiance is watts per square metre (W/m² = Wm⁻²). The unit of insolation often used in the solar power industry is kilowatt hours per square metre (kWh/m²).^[12]

The Langley is an alternative unit of insolation. One Langley is one thermochemical calorie per square centimetre or 41,840 J/m².^[13]

Maps of GHI potential by region and country (Note: colors are not consistent across maps)

Sub-Saharan Africa

Latin America and Caribbean

China

India

Mexico

South Africa

Willson, Richard C.; H.S. Hudson (1991). "The Sun's luminosity over a complete solar cycle". Nature. 351 (6321): 42–4. :1991Natur.351...42W. doi:10.1038/351042a0. S2CID 4273483.

Bibcode

. U.S. Geological Survey Fact Sheet 0095-00. Retrieved 2005-02-21.

"The Sun and Climate"

Foukal, Peter; et al. (1977). . Astrophysical Journal. 215: 952. Bibcode:1977ApJ...215..952F. doi:10.1086/155431.

"The effects of sunspots and faculae on the solar constant"

Stetson, H.T. (1937). . New York: McGraw Hill.

Sunspots and Their Effects

Yaskell, Steven Haywood (31 December 2012). . Trafford Publishing. ISBN 978-1-4669-6300-9.

Grand Phases On The Sun: The case for a mechanism responsible for extended solar minima and maxima

- browse or download maps and GIS data layers (global or per country) of the long-term averages of solar irradiation data (published by the World Bank, provided by Solargis)]

Global Solar Atlas

updated every 10–15 minutes. Recent, live, historical and forecast, free for public research use

Solcast - solar irradiance data

updated every Monday

Recent Total Solar Irradiance data

San Francisco Solar Map

European Commission- Interactive Maps

Yesterday's Australian Solar Radiation Map

Solar Radiation using Google Maps

software to compute solar insolation of each date/location of earth Solar Resource Data and Tools