1. Solar irradiance
Solar radiation can produce a huge amount of energy. In order to measure the size of radiant energy, we usually use solar radiation to define it. Irradiance refers to the radiant flux projected on a unit area, in W/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 refers to the radiation in the three bands of visible light, near-ultraviolet and near-infrared. It is characterized by large and stable energy, and its radiation energy accounts for about 90% of solar radiation energy, which is the main part of solar radiation; and Anomalous radiation refers to the radiation in the far-infrared part, ultraviolet part, and particle flow part. It is characterized by drastic changes with the strength of solar activity. The energy is very large during the maximum period of solar activity. When the energy is very small.
A term closely related to solar irradiance is the solar constant. It refers to the sun obtained per unit area per unit time on the upper boundary of the earth’s atmosphere at an average distance from the sun on a plane perpendicular to the sun’s rays. The numerical value of radiant energy in W/m². According to the value adopted by the eighth meeting of the World Meteorological Organization’s Instrument and Methods of Observation Committee held in Mexico in October 1981, the solar constant is taken as (1367+7) W/m².
In practical applications, solar irradiance is affected by many factors. Specifically, solar altitude, atmospheric quality, atmospheric transparency, geographic latitude, exposure time, and altitude are the main factors that affect it.
The height of the sun refers to the angle between the sun’s rays and the horizon. For a certain ground plane, when the sun’s height is low, the distance of the light through the atmosphere is longer, the energy is attenuated more, and because the light is projected at a smaller angle To the ground level, so the energy reaching the ground is less, that is, the solar irradiance is lower; on the contrary, it is more.
When the sun illuminates the ground vertically, the distance through the atmosphere is called 1 atmospheric mass. When the sun is in other positions, the atmospheric mass is greater than 1. Therefore, we define the atmospheric mass as the distance between the sun's rays and the sun. The ratio of the distance of the sun’s rays through the atmosphere at the zenith of
As for the other factors, it is relatively simple. High atmospheric transparency, low geographic latitude, long exposure time, and high altitude will all lead to greater solar irradiance; otherwise, it is lower. Of course, when we consider the solar irradiance of a specific area, we must comprehensively consider the various factors mentioned above.
2. Measurement of solar radiation
From the perspective of the use of solar energy, the main thing that needs to be measured is the direct intensity of solar radiation and the total radiation intensity. Direct intensity refers to the solar radiation energy received per unit area per unit time on the surface perpendicular to the sunlight. The instrument for measuring the intensity of direct radiation is called a direct solar meter. The total radiation intensity refers to the solar radiation energy received from the entire hemispherical sky per unit area and unit time on the horizontal plane. Obviously, it includes direct radiation and scattering, and the instrument for measuring the total radiation intensity is called Total Solar Radiometer.
According to the different forms of energy conversion, solar radiometers can be divided into card meter type (solar radiant energy is converted into heat energy), thermoelectric type (solar radiant energy is converted into heat energy and then converted into electrical energy), and photovoltaic type (solar radiant energy is converted into electrical energy) And mechanical type (solar radiant energy is converted into heat energy and then converted into mechanical energy).
Card meter type radiometer is divided into water meter, metal sheet card meter, electric heating compensation type card meter and so on. The sensing device usually uses water or metal sheets such as silver and copper. The mass and specific heat of the water or metal sheets can be easily measured in advance. As long as the temperature rise of the sensing device is measured, the solar irradiance can be determined. As the sensing device has radiation, convection and conduction heat loss, it is necessary to pay full attention to reduce these heat losses during the production process, and introduce a correction coefficient through special experiments to ensure the measurement accuracy.
Thermoelectric radiometer mainly refers to thermocouple or thermopile. The basic principle is that when any two different metals are connected to form a closed circuit, if the temperatures of the two metal contacts are different, electromotive force will be generated, and current will be generated in the circuit. Conversely, if there is no current in the above-mentioned loop, the temperature at the two junctions is the same, and the magnitude of the electromotive force is related to the temperature difference between the two junctions. If you know the temperature of one contact, you can measure the temperature of another point by measuring the electromotive force. If the thermistor is used as the sensing device, it should be connected to a bridge circuit. Since the resistance change caused by the thermistor heating is proportional to the absorbed solar radiation energy, the resistance of the thermistor is also measured by the bridge Changes can determine the intensity of solar radiation.
The basic principle of the photoelectric radiometer is to use the short-circuit current of the silicon solar cell to have a linear relationship with the solar radiation intensity projected on the cell to measure the solar radiation intensity. In order to make the solar cell work in a short-circuit state, a resistor with a small resistance value can be connected across its two ends, and the milliamp meter is used to indicate the intensity of solar radiation, and its range is consistent with the exposure of the battery. Match the area.
As for the mechanical radiometer, it usually uses a bimetallic sheet made of iron and alloy as the sensing device, and the alloy generally does not deform in a considerable temperature range. When the sensing device receives solar radiation energy, the iron is heated and expands, while the alloy hardly deforms, which causes the bimetallic sheet to bend, causing the pointer connected to the free end of the bimetallic sheet to shift, and the pointer is offset by the pointer. The scale can get the solar radiation intensity.
However, none of the above-mentioned radiometers can be used for absolute measurement and needs to be calibrated with standard instruments. As the direct radiometer used for calibration, there is an Ehrlich electric compensation direct radiometer. Since 1957, it has been specified as the universal solar radiation standard in the world.
3. Analysis of the pros and cons of solar energy utilization
As a new energy source, solar energy has the following characteristics compared with conventional energy sources:
(1) Universality: The sun is shining all over the earth, there is no geographical limitation, no matter land or sea, no matter mountain or island, it is everywhere, and it can be directly developed and used without mining and transportation. Especially in rural areas, islands and remote areas with underdeveloped transportation, it has more use value.
(2) Harmlessness: the development and utilization of solar energy will not pollute the environment. It is one of the cleanest energy sources. During development and utilization, it will not produce waste residue, waste water, exhaust gas, no noise, and will not affect the ecological balance. Will cause pollution and public hazards. Today, when environmental pollution is getting more and more serious, this is extremely valuable.
(3) Hugeness: The solar radiant energy that reaches the surface of the earth every year is equivalent to about 130 trillion tons of coal, and its total amount is the largest energy source that can be developed in the world today.
(4) Longevity: Estimated based on the current rate of nuclear energy produced by the sun, the hydrogen storage is sufficient to last for tens of billions of years. It is estimated that in the past 1.1 billion years, the sun consumed 2% of its own energy. In the future, it will be enough to supply the earth for billions of years. It is really inexhaustible and inexhaustible.
Although solar energy has the above-mentioned characteristics, there are still many shortcomings in the process of human utilization of solar energy. The details can be summarized as follows:
(1) Dispersibility: Although the total amount of solar radiation reaching the earth's surface is large, the energy flow density is very low. On average, near the Tropic of Cancer, when the weather is clear in summer, the irradiance of solar radiation is the largest at noon, and the average solar energy received on an area of 1 square meter perpendicular to the direction of sunlight is about 1,000; Average year and night, it is only about 200W. In winter, it is only half, and cloudy days are generally only about 1/5. This kind of energy flow density is very low. Therefore, when using solar energy, to obtain a certain conversion power, a set of collection and conversion equipment with a relatively large area is often required, and the cost is relatively high.
(2) Instability: Due to natural conditions such as day and night, seasons, geographic latitude and altitude, as well as the influence of random factors such as sunny, overcast, cloud, and rain, the solar irradiance reaching a certain ground is intermittent. Yes, it is extremely unstable, which makes the large-scale application of solar energy more difficult. In order to make solar energy a continuous and stable energy source, and finally become an alternative energy source that can compete with conventional energy sources, the problem of energy storage must be solved well, that is, the solar radiation energy during the sunny day should be stored as much as possible for the night or rainy days. Days use, but currently energy storage is also one of the weaker links in solar energy utilization.
(3) Low efficiency and high cost: At the current level of development of solar energy utilization, some aspects are theoretically feasible and technically mature. However, some solar energy utilization devices have low efficiency and high cost. In general, the economy cannot compete with conventional energy sources. For a considerable period of time in the future, the further development of solar energy utilization will be mainly restricted by economy.