Solar Radiation is extremely important not just to mankind but for life in general on our planet. It is what makes our planet habitable. It provides sustenance to earth’s eco-systems. It is the source of all forms of energy on our planet. One has to bear in mind that even energy from fossils fuels used today was once bio-chemical material. It was produced by photosynthesis and stored in our planets vaults by climate systems that were all powered by the sun. A key fact that epitomizes the importance of solar energy is that the energy received from the sun in one hour is more than what the mankind consumes in one year. Therefore, it is paramount to learn about the nature of this most abundant energy resource on Earth. Note that the earth catches 3.86×1026 Watts of Solar radiation which is 2.2 billion times less than what is emitted in total by the sun.
Solar Radiation is a by-product of a continual fusion reaction on the sun. Two hydrogen atoms get fused to form Helium atom releasing tremendous amounts of energy in the process. The temperature of the sun varies from core to the top layer and at the surface is around 5778 K (~5500 °C). The earth can be considered inside the sun’s extended atmosphere. Our planet is often blasted by solar flares when activity on the sun is high. The earth’s magnetic field and the atmosphere protect is from the ferocity of sun’s turbulent weather.
When it comes to measurement of Solar radiation, there are a couple of terms that are widely used. The first is Solar Irradiance and the second is Solar Insolation. Solar Irradiance is the instantaneous value of the amount of radiation received on a unit area surface. It is measured in Watt/m2. Solar Insolation is the brief form of the term “Incident-Solar-Radiation” and refers to time integrated value of solar radiation. It is measured in kWh/m2/hr or kWh/m2/day. However, both terms Solar Irradiance and Insolation are often used interchangeably in the industry.
The amount of solar radiation received at any particular location on earth depends upon several factors. This includes the time of the day, day of the year, latitude and local atmospheric conditions (cloudy, clear sky etc). The orientation of the receiving surface also has a bearing on the total amount of solar insolation. The orientation of receiver is specified by the azimuth and inclination. Azimuth is the angle of the receiver face from due north. Inclination is the tilt of the surface from ground horizontal.
Solar Constant and Solar Spectrum
Solar constant is the value of the solar energy flux received outside the earth’s atmosphere on a surface perpendicular the sun. Over the years there have been different values reported in literature, the range of which is between 1357-1367 W/m2. The current value measured by satellite that is widely used is 1361 W/m2.
It should be understood that the radiation being received is made up of a variety of electromagnetic waves of different wavelengths. Some of the radiation lies in the ultra violet rays, some of it in infrared region but most of it (over 70%) lies in the visible range (700 nm to 400 nm). The solar spectrum curve as received above the atmosphere resembles that of a black body (theoretical body is an ideal emitter) at the temperature of 5250 °C. The spectrum curve as received on the surface of earth is different because of atmospheric attenuation caused by reflection and absorption by different greenhouse gases and Ozone. The maximum recorded direct solar radiation on the surface of the earth is 1050 W/m2. The maximum global radiation on a horizontal surface at ground level has been recorded is 1120 W/m2.
If this amount of energy is converged on a point than temperatures above 3500 °C can be reached, which is enough to generate heat to not only weld metal but melt rocks. As shown in the video below ( a clip from BBC’s program Bang Goes the Theory), just 2 m2 of concentrated sunshine is enough to melt steel.
Measuring Solar Radiation
Global Solar irradiance is measured by Pyranometer. Direct solar radiation is measured by Pyrheliometer. Pyranometer with shadow band is used to measure diffuse radiation. It should be noted that Solar radiation meters are expensive and are very different to light meters. The latter measure light intensity in Lux while the former measure irradiance in W/m2. There are other handheld devices that can measure solar irradiance, although it has to be mentioned that such devices do not posses the accuracy of pyrano/pyrheliometer.
Due to the tilt of the earth and the surface curvature, the sun rays land normal to the surface of the earth only on a certain region. This relative angle between the surface of the earth and the incident sun rays is called declination. A good educational video explaining the concept of solar declination can be seen below.
The angle of the sun in the sky also influences the intensity of radiation received on earth’s surface. There is a two fold effect of the increasing angle of the sun as measured from Zenith. First is the projection effect, which reduces the intensity by a cosine factor. The second depends on the amount of atmosphere the sun rays have to pierce before reaching the ground level and is measured by an entity called the Air-Mass (AM). The AM value is 1 when the sun is at zenith. When the sun is 48.2º degree from zenith, the AM value is 1.5 and similarly when the sun is 60.1º from zenith, the AM value is 2. This value of AM= 2 means when the sun is low on horizon, the sun rays have to pierce twice the amount of atmosphere before reaching the earth’s surface.
Sun Path Diagram and Solar Data Sets
The sun path diagram is a good snapshot of solar trajectory (locus) through the year. Sun path diagram for regions near the equator indicate very little variation in the path of the sun through the seasons. The sun rises due east and sets due west. This is in contrast to sun path diagram for regions far away from equator. An extremely useful resource to study solar energy in detail for any particular location is NASA’s Data Set. By specifying the latitude and longitude, the monthly and solar insolation values can be obtained.
The study of solar radiation is important to develop an understanding of systems that harness this form of energy for e.g. Photovoltaic Modules of Solar Water Heaters.
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