What is solar irradiance?

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How to measure the amount of solar radiation energy, what factors affect the amount of sunlight it reaches the ground, and what are its characteristics? This is a fundamental question that must be clarified in understanding and utilizing solar energy.

First, a few common units of solar energy are introduced as follows.

① Radiant flux. The power emitted by the sun in the form of radiation is called radiant power, also known as radiant flux, which is commonly expressed in Φ, and the unit is W.

② irradiance. The radiant flux projected on a unit area is called irradiance, commonly expressed as E, and the unit is W/m2.

③ exposure to radiation. The radiation energy received from a unit area is called exposure radiation, which is usually expressed by H, and the unit is J/m2. Here, by the way, some solar literature materials often confuse irradiance and radiant intensity.

It should be noted that radiation intensity refers to the radiant power leaving the radiation source within a unit solid angle, and its unit is watts per steradian (W/sr), which should not be confused.

The solar irradiance can be divided into two categories: constant radiation and abnormal radiation according to the magnitude and stability of energy in different wavelength ranges. Steady radiation, including the visible light part, the near-ultraviolet part and the near-infrared part, is the main part of the solar radiation. It is characterized by large and stable energy, its radiation accounts for about 90% of the solar radiation energy, and is little affected by solar activity. The physical quantity that expresses this irradiance is called the solar constant. Anomalous radiation includes the radio wave part, the ultraviolet part and the particle flow part of the optical radiation. Its characteristic is that it changes drastically with the intensity of solar activity, the energy is very large in the maximum period, and the energy is very weak in the minimum period.

What is the solar constant? In scientific and experimental work using solar energy, people are very concerned about such a value, that is, how much solar energy the earth can receive per unit area and per unit time. In the upper bound of the earth’s atmosphere, because it is not affected by the atmosphere, the solar radiation energy has a relatively constant value, which is called the solar constant. It refers to the value of the total solar radiation energy obtained on a unit area per unit time in the upper bound 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. According to the latest value adopted by the Eighth Session of the Commission for Instruments and Methods of Observation of the World Meteorological Organization held in Mexico City in October 1981, 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 the amount of power emitted by the sun in the form of radiation projected onto a unit area. Due to the existence of the atmosphere, the amount of solar radiation that actually reaches the earth’s surface is affected by many factors. Generally speaking, the sun altitude, atmospheric quality, atmospheric transparency, geographic latitude, sunshine time and altitude are the main factors.

The next article will detail the main factors affecting ground solar irradiance.