In order to describe solar energy quantitatively, some concepts need to be introduced. When the earth is located at the average distance between the sun and the earth, the total energy of the full spectrum of solar radiation per unit time that the upper boundary of the earth’s atmosphere is perpendicular to the unit area of the sun’s rays is called the solar constant. The value of the solar constant is 1353W/m2, and the common unit is W/m2. The influence of the atmosphere on the sunlight received on the earth’s surface is defined as atmospheric mass (AM). Atmospheric quality is a dimensionless quantity. It is the ratio of the path of the sun’s rays passing through the atmosphere of the earth to the path of the sun’s rays passing through the atmosphere at the zenith angle, and suppose that at standard atmospheric pressure (101325Pa) and temperature 0℃, the path of sunlight on sea level perpendicular to the incident is 1. The AM value is different, the solar spectrum will have different changes. When the solar radiation intensity is the solar constant, the air quality is recorded as AM0, and the AM0 spectrum is suitable for the situation on artificial satellites and spacecraft. The spectrum of the air quality AMI corresponds to the spectrum of the sun directly hitting the surface of the earth (its incident light power is 925W/cm2). Figure 1 is the solar spectrum under the two conditions of AMO and AM1. The difference between them is caused by the attenuation caused by the absorption of sunlight by the atmosphere, mainly from the absorption of ultraviolet rays by the ozone layer and the absorption of infrared rays by water vapor, as well as air Scattering of dust and suspended matter. In the figure, the solar spectral irradiance Eλ＝dE/dλ, where E is the solar irradiance per unit wavelength interval, given the wavelength λ. These characteristics of the solar spectrum are a very important factor for the selection of solar cell materials.
3Figure 1 – AM0 and AM1 solar spectrum
Solar radiation is a kind of electromagnetic wave radiation, which has both wave and particle properties. The wavelength radiation range of the sun is shown in Figure 2. The main wavelength range of its spectrum is 0.15~4μm, while the main wavelength range of ground and atmospheric radiation is 3~120μm. In meteorology, solar radiation is usually called short-wave radiation, while ground and atmospheric radiation is called long-wave radiation. The wavelength distribution of solar energy can be simulated with a black body radiation, the temperature of the black body is 6000K, and the solar wavelength is distributed in ultraviolet (<0.4μm), visible light (0.4~0.75μm) and infrared (>0.75μm). These bands are affected by atmospheric attenuation to varying degrees. Most of the visible light radiation reaches the ground, but the ozone in the upper atmosphere absorbs most of the ultraviolet radiation. Due to the thinning of the ozone layer, especially in the Antarctic and Arctic regions, more and more ultraviolet radiation reaches the ground. Part of the infrared radiation emitted by humans is absorbed by carbon dioxide, water vapor and other gases, while most of the longer-wavelength infrared radiation from the surface of the earth at night is transmitted to outer space. The accumulation of these greenhouse gases in the upper atmosphere may increase the absorption capacity of the atmosphere, leading to global warming and cloudy weather. Although ozone reduction has little effect on solar energy absorption, the greenhouse effect may increase scattered radiation and may seriously affect solar energy absorption.
3Figure 2 – The wavelength range of solar radiation
The angle between the incident direction of sunlight and the ground plane, that is, the angle between the sun’s rays in a certain place and the tangent of the ground perpendicular to the center of the earth, is called the solar altitude angle, or solar altitude for short. It has daily changes and annual changes. When the solar altitude angle is 90°, in the solar spectrum, infrared rays account for 50%, visible light accounts for 46%, and ultraviolet rays account for 4%; when the solar altitude angle is 5°, infrared rays account for 72%, visible light accounts for 28%, and ultraviolet rays are almost 0. During the day, the sun’s altitude angle is constantly changing; at the same time, it is also constantly changing throughout the year. For a certain ground plane, when the sun’s altitude angle is low, the distance of light passing through the atmosphere is longer, and the radiant energy attenuates more. At the same time, because the light is projected on the ground plane at a smaller angle, the energy that reaches the ground plane is less, and vice versa.