Part one of solar irradiance and characteristics
The use of solar energy is the use of energy generated by solar radiation. How to measure the amount of solar radiation energy, how much it reaches the ground is affected by what factors, and what are the characteristics? Several common units of solar energy are as follows:
(1) Radiant flux: The power emitted by the sun in the form of radiation is called radiant power, also called radiant flux, expressed by Φ, and the unit is W.
(2) Irradiance: The radiant flux projected on a unit area is called irradiance, usually expressed as E, and the unit is W/m2 .
(3) Exposure radiation amount: the radiation energy received from a unit area is called exposure radiation amount, usually expressed by H, and the unit is J/m2.
Solar irradiance can be divided into two types, constant radiation and abnormal radiation, according to the magnitude and stability of energy in different wavelength ranges. Constant radiation includes the three wavebands of visible light, near-ultraviolet, and near-infrared, and is the main part of solar radiation. It is characterized by high energy and stability, its radiation accounts for about 90% of the solar radiant energy, and is little affected by solar activity. The physical quantity that expresses this irradiance is called the solar constant. Abnormal radiation includes radio wave part, ultraviolet part and particle flow part in optical radiation.
At the upper boundary of the earth’s atmosphere, because it is not affected by the atmosphere, the solar radiant energy has a relatively constant value, which is called the solar constant. It refers to the value of the total solar radiant energy obtained on a unit area per unit time at the upper boundary of the earth’s atmosphere and on a plane perpendicular to the sun’s rays at the average distance between the sun and the earth. The common unit is W/m2. The value of the solar constant is (1367±7) W/m2. This value changes very little during the maximum and minimum periods of solar activity, only about 2%.
The solar irradiance mentioned above refers to how much the power emitted by the sun in the form of radiation is projected on a unit area. Due to the existence of the atmosphere, the amount of solar radiant energy that actually reaches the surface of the earth is affected by many factors. Generally speaking, the sun altitude, atmospheric quality, atmospheric transparency, geographic latitude, sunshine time and altitude are the main factors affecting.
1. Sun height
That is, the elevation angle at which the sun is above the ground level. It is often expressed by the angle between the sun’s rays and the horizon, that is, the angle of incidence. The angle of incidence is large, the sun is high, and the irradiance is also large; on the contrary, the angle of incidence is small, and the sun is low, and the irradiance is also small.
Because the earth’s atmosphere absorbs, reflects and scatters solar radiation, the proportion of infrared, visible and ultraviolet rays in the light rays also changes with the change of the sun’s altitude. Figure 1 shows the proportions of infrared, visible, and ultraviolet at various solar altitudes.
The height of the sun also changes continuously throughout the year. This is because the earth is not only rotating, but also revolving around the sun. The earth’s rotation axis is not perpendicular to the orbital plane, but always maintains a certain tilt. The angle between the rotation axis and the plane normal of the revolution orbit is 23.5°. In the first half of the year, the sun rose from low latitude to high latitude day by day, until noon on the summer solstice, reaching its highest point of 90°. Since then, it will decrease day by day until the winter solstice, when it reaches its lowest point. This is why the summer is hot during the year, the winter is cold, and the temperature is higher at noon than in the morning and evening.
For a certain ground plane, because the distance of light passing through the atmosphere is longer when the sun is low, the energy is attenuated more. At the same time, because the light is projected on the ground plane at a smaller angle, the energy reaching the ground plane is less; on the contrary, it is more.