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Solar Energy Analysis
- Jun 07, 2018 -

Solar EnergyAnalysis

Sun radiation

In the process of sunlight reaching the ground through the atmosphere, a portion of solar radiation is dispersed by air molecules, clouds, and various particles in the air into scattered radiation that is non-directional but does not change its monochromatic composition. Another situation, solar radiation is emitted directly from the sun without being redirected by atmospheric scattering, called direct radiation (DNI). Direct radiation and scattered radiation received from the upper 2π solid angle range of the horizontal plane are called total radiation.

The direct radiation received on the horizontal plane is called horizontal direct radiation. Total radiation is equal to the sum of direct radiation and scattered radiation in the horizontal plane.

Reflected radiation refers to radiation where solar radiation is folded back without changing its monochromatic composition.

Solar radiation has characteristics such as periodicity, randomness, and low energy density:

(1)   Periodicity: The periodicity of solar radiation is caused by the rotation of the Earth and the Earth's orbit around the Sun.

(2)   Randomness: The solar radiation received on the Earth's surface is affected by clouds, fog, rain, snow, fog, and dust. The randomness of these factors determines the randomness of solar radiation.

(3)   Low energy density: According to the data released by the World Meteorological Organization in 1981, the direct radiation irradiance at the average distance from the Earth's atmosphere outside the Earth's atmosphere is 1367 W/m2 ± 7 W/m2 (this value is called the "sun constant") ,The total solar radiation received by the ground is generally lower than the solar constant.


Atmospheric quality

Air quality is defined as the ratio of the path of the sun's rays through the Earth's atmosphere to the path of the sun's rays through the atmosphere in the direction of the zenith, expressed in AM. For an ideal uniform atmosphere, it can be calculated by formula (2-1).

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At atmospheric masses of AM=1, the direct radiation Ito sea level under sunny weather conditions decreases from the sun's constant to about 1000 W/m2. For general AM values, the fitting relationship between I and AM can be expressed by the following formula:

2.png

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B0 is the sun constant.

When solar radiation enters the Earth's atmosphere, the spectral components are also affected. Figure 2-2 shows the spectral distribution of AMO (the spectrum of solar radiation received at the average distance over the Earth's atmosphere from the Earth's atmosphere) and the AM 1.5 spectral distribution (clinus of 37°, oriented towards the south on the nucleus). ). If the power density is integrated over the entire wavelength range of this AM 1.5 spectrum, the result is about 970 W/m 2 . The “normalized” spectrum after multiplying the AM1.5 spectral distribution in Figure 2-2 by a factor of 1000/970 is the current standard for classifying photovoltaic products. This is used for standard testing of photovoltaic devices.

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Latitude

At the same time as the Earth rotates, around the sun's revolution, the axis of rotation of the Earth rotates 66°34′ away from its orbital surface. Therefore, the point where the sun's direct contact with the earth changes cyclically between the Earth's Tropic of Captivity and the Tropic of Cancer during the year, for the Northern Hemisphere: at the equinox and equinox, the sun goes direct to the equator; in the summer solstice, the sun goes directly to the Tropic of Cancer; during the winter solstice The sun is directly in the Tropic of Cancer.According to the law of cosines, that is, the irradiance on an arbitrary plane is proportional to the cosine of the angle between the normal and the incident ray of the plane. If the influence of other factors is not considered, the greater the absolute value of latitude, the surface water level will be received. The total annual radiation is lower.


Atmospheric weather conditions

The main factors affecting the total amount of radiation received by atmospheric meteorological conditions on the surface water level are cloudiness, aerosols, water vapor, and atmospheric molecules.Clouds are an important cause of the attenuation and scattering of sunlight in the atmosphere. Cumulus clouds, or large clouds at low altitudes, can block sunlight very effectively. Cirrus clouds, or thin high altitude clouds, are not so effective at blocking the sun. In completely cloudy weather, there is no direct radiation and only scattered radiation reaches the surface of the Earth. It should be noted that in some cases, the total irradiance received by the ground may be greater than the solar constant because it not only directly receives the radiation of the sun but also receives some of the solar radiation reflected by the clouds.


Altitude

The influence of altitude on the total amount of solar radiation received at the surface is first of all reflected in the change of cloud cover caused by differences in elevation. If the conditions are clear and cloudless, the effect of altitude on the radiation received by the Earth’s surface is reflected in the atmospheric transparency coefficient. on. In clear and cloudless weather, the higher the altitude, the greater the atmospheric transparency coefficient and the greater the total solar radiation received by the surface.