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Solar irradiation

Solar irradiation

Solar irradiation is the magnitude that measures the energy per unit area of ​​incident solar radiation on a surface placed in a well-specified place and time range.

The solar irradiation in the terrestrial surface is, from the technical point of view, the addition in a determined interval of the solar radiance filtered by the interposition of the atmosphere.

The value of solar irradiation on the surface depends on the time of year, latitude, local climatology and times of day.

Origin of solar irradiation

The origin of electromagnetic radiation from the Sun. Solar radiation is the reaction of nuclear fusion that occurs constantly inside the Sun. In the nuclear reaction two hydrogen nuclei combine to form a helium nucleus. Through this fusion of nuclei a large amount of energy is released. Hydrogen constitutes 74.9% of the mass of the Sun, and helium 23.8% (in a slight but constant increase) and only the remaining 1.3% corresponds to other chemical elements.

The heat generated by the nuclear fusion reaction is responsible for the Sun being a gigantic incandescent mass, and the outer surface having a temperature of approximately 5,505 ° C. Like any incandescent body, the Sun emits electromagnetic radiation over a wide range of wavelengths (or frequencies) that range from ultraviolet to infrared, whose maximum intensity is in the region that the human eye identifies as the yellow of the rainbow. This spectrum of solar radiation is called the wavelength distribution, most of which is invisible to the human eye.

Types of solar irradiation

There are several measured types of solar irradiance.

  • The total solar irradiation
  • The direct normal irradiation
  • Diffuse horizontal irradiation or diffuse sky radiation
  • The global horizontal irradiation

Total solar irradiation is a measure of solar power at all wavelengths per unit of incident area in the upper atmosphere of the Earth. The perpendicular to the incoming sunlight is measured. The solar constant is a conventional measure of average total solar radiation at a distance of one astronomical unit.

Direct normal irradiation, or beam radiation, is measured at the surface of the Earth at a given location with a surface element perpendicular to the Sun. It excludes diffuse solar radiation (radiation scattered or reflected by atmospheric components). The direct irradiance is equal to the extraterrestrial irradiance above the atmosphere minus the atmospheric losses due to absorption and dispersion. The losses depend on the time of day (length of light path through the atmosphere according to the solar elevation angle), cloud cover, moisture content and other contents. Irradiance on the atmosphere also varies with the time of year (because the distance to the sun varies),

Diffuse horizontal irradiation or diffuse sky radiation is radiation on the surface of the Earth from light scattered by the atmosphere. It is measured on a horizontal surface with radiation from all points in the sky, excluding solar radiation from the solar disk. There would be almost no diffuse horizontal irradiation in the absence of atmosphere.

The global horizontal irradiation is the total irradiance of the sun on a horizontal surface on Earth. It is the sum of the direct irradiance (after taking into account the solar zenith angle of the Sun z) and the diffuse horizontal irradiance.

Propagation of solar irradiation

The electromagnetic radiation of the Sun propagates in a vacuum with the so-called speed of light, which is about 299,792 km / s. This means that your energy flows with that speed. The physical measure of this flow is the power, the amount of energy that flows per unit of time. The standard unit internationally used for power is watt (W).

Solar irradiation can be measured in space or on the surface of the Earth after absorption and atmospheric dispersion. The irradiation in space is a function of the distance to the Sun, the solar cycle and the cross-cycle changes. The irradiation on the surface of the Earth also depends on the inclination of the measuring surface, the height of the sun on the horizon and the atmospheric conditions. Solar irradiance affects the metabolism of plants and animal behavior.

After leaving the surface of the Sun the solar radiation expands almost homogeneously and constantly in time through the surrounding space, except for the cyclical influence of sunspots (solar cycle). The power of solar radiation is thus distributed on the surface of a sphere whose radius increases constantly with the speed of light, decreasing its value per unit area as the inverse of the square of that radius. When this spherical wave front reaches the Earth's atmosphere (where that radius takes the value of the distance from the Earth to the Sun, about 150 million kilometers), the power per unit area of ​​the total spectrum of the radiation (irradiance solar out of the atmosphere) is approximately 1,367 W / m².

Spectral Irradiance

Solar radiation is reflected, absorbed or dispersed by the Earth's atmosphere due to the action of atoms, molecules, ions, dissolved gases and particles in suspension (water droplets, dust, ash from volcanoes...). Molecules such as oxygen (O2), water (H20), carbon dioxide (CO2) and ozone (O3) have a strong impact because they absorb solar radiation in wide ranges (called absorption bands) of wavelength .

Also, in the applications of solar irradiance, it is necessary to take into account the ranges of wavelengths involved, which can be very different in each case. The result is that the direct solar irradiance on the earth's surface, on a clear day when the Sun is in the zenith, is reduced to about 1,050 W / m².

Importance of solar irradiation in solar energy

The solar irradiation figures are used to plan the deployment of solar energy systems. In many countries, the figures can be obtained from a sunstroke map or insolation tables that reflect data from 30 to 50 years earlier. Different solar energy technologies are able to use different components of the total irradiation.

Although solar photovoltaic solar panels can convert both direct irradiation and diffuse irradiation into electricity, concentrated solar energy can only function efficiently with direct irradiation, so these systems are suitable only in places with a relatively low cloudiness .

Because the panels of the solar collectors are almost always mounted at an angle to the sun, the insolation must be adjusted to avoid estimates that are inaccurately low for winter and incorrectly high for summer. This also means that the amount of sun that falls on a solar panel at high latitudes is not as low compared to one at the equator as would be seen if only insolation on a horizontal surface is considered.

Photovoltaic panels are classified under standard conditions to determine the rating of Wp (peak watts), which can then be used with insolation to determine the expected output, adjusted for factors such as tilt, tracking and shading.

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Last review: February 4, 2019

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