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Solar Irradiation

Solar Irradiation

Solar irradiation is the magnitude that measures the energy per unit area of ​​solar radiation incident on a surface placed in a well specified place and time range. That is to say, not all the solar radiation generated by the Sun reaches the Earth. The magintud that describes the solar radiation that reaches the earth is the irradiance. The irradiance is expressed in units of power per surface, generally watts per square meter. When we talk about solar irradiation, we mean the amount of irradiance received in a given time.

Because irradiance is measured in watts per square meter, solar irradiation is usually measured in watt-hours per square meter.

Solar irradiation on the earth's surface is, from a technical point of view, the addition in a given time 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 the year, the latitude, the local weather and the hours of the day.

Origin of solar irradiation

The origin of electromagnetic radiation from the Sun. Solar radiation is the nuclear fusion reaction that occurs constantly inside the Sun. In the nuclear reaction two hydrogen nuclei combine to form a helium nucleus. A large amount of energy is released through this fusion of nuclei. Hydrogen constitutes 74.9% of the mass of the Sun, and helium 23.8% (in 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 that the outer surface has an approximate temperature of 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 human sight identifies as the yellow of the rainbow. This wavelength distribution is called the solar radiation spectrum, most of which is invisible to the human eye.

Types of solar irradiation

There are several measured types of solar irradiance.

  • Total solar irradiation
  • Direct normal irradiation
  • Diffuse horizontal irradiation or diffuse sky radiation
  • Global horizontal irradiation

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

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

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

Global horizontal irradiation is the total irradiance of the sun on a horizontal surface on Earth. It is the sum of direct irradiance (after taking into account the solar zenith angle of the Sun z) and 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 approximately 299,792 km / s. This means that your energy flows with that speed. The physical measure of this flow is power, the amount of energy that flows per unit of time. The internationally used standard unit for power is Watt (W).

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

After leaving the surface of the Sun, solar radiation expands almost homogeneously and constantly over time through the surrounding space, except for the cyclic influence of sunspots (solar cycle). The solar radiation power is thus distributed on the surface of a sphere whose radius constantly increases with the speed of light, decreasing its value per unit area as the inverse of the square of that radius. When this spherical wavefront reaches the Earth's atmosphere (where said 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 outside 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 suspended particles (drops of water, dust, ash from volcanoes ...). Molecules such as oxygen (O 2 ), water (H 2 0), carbon dioxide (CO 2 ) and ozone (O 3 ) have a strong impact because they absorb solar radiation in wide ranges (called absorption bands) of wavelength .

Also, in the applications of solar irradiance the ranges of wavelengths involved must be taken into account, which can be very different in each case. The result is that 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

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

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

Because the solar collector panels are almost always mounted at an angle to the sun, the insolation should 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 on the equator as would be seen if only sunstroke is considered on a horizontal surface.

The photovoltaic panels are classified under standard conditions to determine the Wp (peak watts) rating, 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