1. The structure of the sun
The mass of the sun is very large. Under the sun's own gravity, the sun's matter gathers toward the core. The density and temperature of the core center are very high, enabling nuclear reactions to occur. These nuclear reactions are the energy of the sun. The energy produced continuously radiates to space and controls the activities of the sun. According to various indirect and direct data, it is believed that the sun can be divided into four parts: nuclear reaction zone, radiation zone, convection zone and solar atmosphere from the center to the edge.
(1) Nuclear reaction zone
In the area of 25% of the sun's radius (ie 0.25R), it is the core of the sun, which concentrates more than half of the sun's mass. The temperature here is about 15 million degrees (K), the pressure is about 250 billion atmospheres (1atm = 101325Pa), and the density is close to 158g/cm³. This part of the energy produced accounts for 99% of the total energy produced by the sun, and radiates outward in the form of convection and radiation. Gamma rays are emitted when hydrogen is polymerized. When this kind of rays pass through a colder area, they consume energy, increase their wavelength, and become X-rays or ultraviolet rays and visible light.
(2) Radiation area
Outside the nuclear reaction zone is the radiation zone, which belongs to the range from 0.25-0.8R, the temperature drops to 130,000 degrees, and the density drops to 0.079g/m³. The energy generated in the core of the sun is transmitted by radiation through this area.
(3) Convection zone
Outside the radiation zone is the convection zone (troposphere), which belongs to the range from 0.8-1.0R, the temperature drops to 5000K, and the density is 10-8g/cm³. In the convection zone, energy is mainly propagated by convection. The convection zone and its parts are invisible, and their properties can only be determined by theoretical calculations consistent with observations.
(4) Solar atmosphere
The solar atmosphere can be roughly divided into levels such as photosphere, chromosphere, and corona. The physical properties of each level are obviously different. The lowest layer of the sun’s atmosphere is called the photosphere, and almost all of the sun’s light energy is emitted from this level. The continuous spectrum of the sun is basically the spectrum of the photosphere, and the absorption lines in the solar spectrum are basically formed in this layer. The thickness of the photosphere is about 500km. The chromosphere is the middle layer of the sun's atmosphere, and it is the outward extension of the photosphere, which can extend to a height of several thousand kilometers. The outermost layer of the solar atmosphere is called the corona. The corona is an extremely thin shell of gas that can extend to several solar radii. Strictly speaking, the above-mentioned layering of the solar atmosphere has only formal meaning. In fact, there is no obvious boundary between the layers. Their temperature and density change continuously with height.
It can be seen that the sun is not a black body with a certain temperature, but a radiator that emits and absorbs many layers of different wavelengths. However, when describing the sun, the sun is usually regarded as a black radiator with a temperature of 6000K and a wavelength of 0.3-3.0μm.
2. Solar energy transmission method
In the science of heat transfer, we know that the propagation of heat has three forms: conduction, convection and radiation. The transmission of solar energy is mainly in the form of radiation. Specifically, there are two types: one is the light radiation emitted from the surface of the photosphere. Because it spreads light and heat in the form of electromagnetic waves, it is also called electromagnetic wave radiation. This kind of radiation is invisible to visible light and human eyes. It is composed of invisible light; the other is particle radiation, which is a particle stream composed of positively charged protons and roughly equal amounts of negatively charged electrons and other particles. The radiation of particles is usually weak and the energy is also unstable. Generally speaking, during the long propagation path, it has gradually disappeared without waiting for it to radiate onto the surface of the earth. Therefore, solar radiation mainly refers to light radiation.
To send solar radiation to the earth, not only has to travel a long distance, but also encounter various obstacles. The first barrier is the earth’s magnetosphere, and most of the solar radiation particles will be "captured" by the earth’s magnetosphere; the second barrier is the troposphere, stratosphere, and ionosphere below the earth’s magnetosphere, all of which are against the sun. Radiation has absorption, reflection, and scattering effects. The ionosphere can not only absorb or reflect radio waves in solar radiation, but also block harmful ultraviolet and X-rays; the third block is the ozone layer, which can enter Most of the ultraviolet rays are absorbed.
Because the "three lines of defense" set by the earth eliminate the harmful parts of solar radiation, all kinds of creatures on the earth are protected. However, various substances in the earth's atmosphere also have a great influence on the transmission of solar energy. .
Oxygen, ozone, water, carbon dioxide and dust in the atmosphere all have different absorption effects on solar radiation. Among them: oxygen mainly absorbs the solar radiation band with a wavelength less than 0.2 microns, especially for the radiation band of 0.155 microns, the absorption capacity is the strongest; ozone mainly absorbs ultraviolet rays, and its absorbed energy accounts for about 21% of the total solar radiation energy; water vapor can It absorbs 20% of the total solar radiation energy, and liquid water absorbs more solar radiation energy; carbon dioxide and dust absorb less solar radiation.
In addition to absorbing solar radiation, various substances in the atmosphere also have scattering effects. When solar radiation hits the earth's atmosphere with parallel beams, it will encounter the blocking of particles such as air molecules, dust, clouds and fog to produce scattering effects. This kind of scattering is different from absorption. It does not convert solar radiation energy into internal energy of various particles, but can only change the direction of solar radiation, so that part of the solar radiation becomes back radiation of the atmosphere, which is emitted out of the earth’s atmosphere, of course. This is also an important reason for the weakening of solar radiation energy.
Therefore, due to the presence and influence of the atmosphere, the solar radiation energy reaching the surface of the earth can be divided into two parts: direct radiation and scattered radiation. Generally speaking, on a clear day, direct radiation accounts for the main part of the total radiation; on rainy days, scattered radiation accounts for the main part of the total radiation; at night, it is completely scattered radiation. For most solar thermal equipment, direct radiation is The main utilization part.