Extraterrestrial Solar Radiation

Extraterrestrial Solar Radiation

DOI: 10.4018/978-1-5225-2950-7.ch003


Extraterrestrial solar radiation is the main source of terrestrial solar radiation components. Data on the spectrum of this radiation is available and a value of 1367 Wm-2 for the solar constant is accepted in solar literature. The knowledge of extraterrestrial solar radiation is essential for solar applications, within them is the Sun tracking. This radiation on horizontal surface is widely treated and a simple formula, for calculating it, is widely used. On Equator facing solar receivers, the appropriate equations for obtaining this radiation are also available, but the application of these equations by different authors was found to be not evident mainly on calculating the sunset hour angle on such surfaces. This question becomes problematic for some authors when treating surfaces of different orientations. The term of characteristic day is widely used in solar literature. The ambiguity of this term with regard to extraterrestrial solar radiation, declination angle and extraterrestrial solar radiation on a horizontal plane is described. In order to completely solve the above-mentioned problems, extraterrestrial solar radiation is calculated on surfaces of different orientations and the required relations are given for each case. The introduction of the energy gain on the basis of extraterrestrial solar radiation could be formulated mathematically very precisely. Therefore, this question is treated in details in this chapter. The difference between short term and long term Sun tracking is described also and the maximum possible energy gain of these two types of tracking is characterized.
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The sun, our ultimate source of energy, is just an average-sized star of average age, located in one of the spiral arms of the Milky Way galaxy. To astronomers, it is a main sequence star of spectral class G. This means that it has an apparent surface temperature around 6000K and is of average brightness. Other known main sequence stars have luminosities up to 1000 times greater and 1000 times less and temperatures ranging from 3000K to 16000K. At the center of the Sun it is presumed that hydrogen nuclei are combining to form helium nuclei in a thermonuclear fusion process where the excess binding energy is released into the body of the Sun. This energy is released at the rate of 3.83 × 1026 W.

Most of the electromagnetic radiation reaching the earth emanates from a spherical outer shell of hot dense gas called the photosphere. This region has a diameter of approximately 1.39 × 109 m and appears as a bright disc with some “limb darkening” (brighter near the center) since radiation coming to us from the outer edges comes from higher and cooler layers of gas. Observations of sunspot movement indicate that the Sun does not rotate uniformly. The region near its equator rotates with a period of about 27 days, whereas the polar regions rotate more slowly, with a period of about 32 days.

Beyond the photosphere are the chromosphere and the corona. These regions are characterized by low-density gases, higher temperature, and time wise variations in energy and diameter. Because of the low density and thus minimal energy emission from these regions, they are of little significance to Earth-based solar thermal applications. They do, however, produce uniform cyclic variations in the X-ray and ultraviolet (UV) components of the solar spectrum, having approximately 11-year periods, coincident with the sunspot cycles. Appendix A summarizes the important characteristics of the Sun.

The Sun sits at the center of the solar system and emits energy as electromagnetic radiation at an extremely large and relatively constant rate, 24 hours per day, 365 days of the year. The rate at which this energy is emitted is equivalent to the energy coming from a furnace at a temperature of about 6000 K. If we could harvest the energy coming from just 10 hectares of the surface of the Sun, we would have enough to supply the current energy demand of the world. However, there are three important reasons why this cannot be done: First, the Earth is displaced from the Sun, and since the Sun’s energy spreads out like light from a candle, only a small fraction of the energy leaving an area of the Sun reaches an equal area on the Earth. Second, the Earth rotates about its polar axis, so that any collection device located on the Earth’s surface can receive the Sun’s radiant energy for only about one-half of each day. The third and least predictable factor is the atmosphere that surrounds the Earth’s surface. At best, the Earth’s atmosphere accounts for another 30 percent reduction in the Sun’s energy. As is widely known, however, the weather conditions can stop all but a minimal amount of solar radiation from reaching the Earth’s surface for many days in a row.

The rate at which solar energy reaches a unit area at the Earth is called the “solar irradiance” or “insolation”. The units of measure for irradiance are Watts per square meter (W/m2). Solar irradiance is an instantaneous measure of rate and can vary over time. The maximum solar irradiance value is used in system design to determine the peak rate of energy input into the system. If the storage is included in a system design, the designer also needs to know the variation of solar irradiance over time in order to optimize the system design. The designer of solar energy collection systems is also interested in knowing how much solar energy has fallen on a collector over a period of time such as a day, week or year. This summation is called solar radiation or irradiation. The units of measure for solar radiation are Joules per square meter (J/m2) but often Watt-hours per square meter (Wh/m2) are used. As will be described below, solar radiation is simply the integration or summation of solar irradiance over a time period.

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