Solar radiation is one of the most important sources of energy for our planet. It is the energy that is generated by the sun and then travels to the earth in the form of electromagnetic waves. This energy is used in many ways, including but not limited to powering solar panels, powering machines and powering our homes. Solar radiation is a very powerful energy source and understanding its characteristics is important in order to utilize its full potential.
The Sun is a sphere of intensely hot gaseous matter with a diameter of 1.39 × 109 m and is, on the average, 1.5 × 1011 m from the earth. As seen from the earth, the sun rotates on its axis about once in every 4 weeks. However, it does not rotate as a solid body; the equator takes about 27 days and the polar regions take about 30 days for each rotation. The sun has an effective black body temperature of 5777 K.
Solar energy is the result of electromagnetic radiation released from the sun by the thermonuclear reactions occurring inside its core. All of the energy resources on earth originate from the sun (directly or indirectly), except for nuclear, tidal, and geothermal energy.
The sun radiates considerable energy onto the earth. Solar radiation intensity, rarely over 950 W/m2 has led to the creation of many types of devices to convert this energy into useful forms, mainly heat and electricity. Radiant light and heat from the sun is harnessed using a range of ever-evolving technologies such as solar heating, PV, CSP, solar architecture, and artificial photosynthesis.
Fig above. shows schematically the geometry of the sun–earth relationships. The eccentricity of the earth’s orbit is such that the distance between the sun and the earth varies by 1.7%. At a mean earth–sun distance L = 1.495 × 1011 m, the sun subtends an angle of 32 degree. The radiation emitted by the sun and its spatial relationship to the earth result in a nearly fixed intensity of solar radiation outside of the earth’s atmosphere.
The solar constant ISC signifies the energy from the sun per unit time received on a unit area of surface perpendicular to the direction of propagation of the radiation at mean earth–sun distance outside the atmosphere. The mean value of solar constant is equal to 1368 W/m2. Therefore, considering a global plane area of 1.275 × 1014 m2 and the mean radius of the earth being approximately 6371 km, the total solar radiation transmitted to the earth is 1.74 × 1017 W, whereas the overall energy consumption of the world is approximately 1.84 × 1013 W.
The first characteristic to consider when discussing solar radiation is its wavelength. Solar radiation consists of a range of wavelengths, and each wavelength has a different energy. The wavelengths of solar radiation range from about 0.2 micrometers to about 4 micrometers, and the energy contained within each wavelength is what will determine the type of energy that is produced.
The normal measurement of the wavelength of solar and atmospheric radiation is the nanometer (nm, 10-9 m) and for infrared radiation is the micrometer (µm, 10-6 m).
The second characteristic of solar radiation is its intensity. The intensity of solar radiation is measured by the number of photons per square meter that are present. The more intense the solar radiation is, the more energy it will produce. The intensity of solar radiation is also influenced by the amount of clouds and dust in the atmosphere as well as the angle of the sun.
The third characteristic of solar radiation is its frequency. Solar radiation is composed of different frequencies, which are measured in hertz (Hz). The frequency of solar radiation ranges from about 1 Hz to 10 Hz. The higher the frequency, the more energy the radiation will produce.
The sun is an incredibly powerful and complex entity. It emits a wide range of electromagnetic radiation frequencies, ranging from an incredibly high 1018 hertz down to a much more modest 104 hertz. This means that the sun is emitting an astounding 1 million million million waves per second of energy at its highest frequency. At its lowest frequency, this reduces to 10,000 waves per second. In addition to this, the sun also spins on its axis every 27 days, allowing it to spread its energy across the solar system.
Finally, the fourth characteristic of solar radiation is its direction. Solar radiation travels in a straight line, meaning that it is directional. This means that the energy will only be concentrated in a particular direction, and can therefore be used to power solar panels or other devices.
This directional aspect of solar radiation can be leveraged in a variety of ways, such as concentrating the energy on solar panels to maximize efficiency, or using mirrors to reflect the light in specific directions. Additionally, the directional nature of solar radiation can be used to create shadows or light patterns in a particular area, or to direct light in certain areas that would otherwise be inaccessible.