We all know that life is sustained on earth because of the sun as it provides sufficient heat energy to keep the earth warm. This energy is delivered by the sun in the form of Electromagnetic radiation which is usually called solar radiation. Some of the radiation is beneficial to humans while another radiation is harmful to all life.
To reach solar radiation to the earth's surface it must pass through the atmosphere where it gets absorbed, scattered, reflected, and transmitted which results in the reduction of the energy flux density. This reduction is very significant as more than 30% loss occurs on a sunny day and on a cloudy day it goes a high as 90%. So the maximum radiation which reaches the earth's surface through the atmosphere will never be higher than 80%.
Solar flux is very important to measure, as it is the basis of life on earth and is used in building many products whether its related to electronics, crops, medicines, cosmetics, etc. In this tutorial, we will learn about solar radiation and its measurement and will also learn about the two most popular solar energy measuring instruments- Pyrheliometer and Pyranometer.
Beam Radiation and Diffuse Radiation
The radiation which we perceive on the surface is both direct radiation and indirect radiation of the sun. The radiation that comes directly from the sun is direct radiation and it is called beam radiation. The scattered and reflected radiation that is sent to the earth's surface from all directions (reflected from molecules, particles, animal bodies, etc.) is indirect radiation and it is called diffuse radiation. And the sum of both, the beam and diffuse radiation, is defined as global radiation or total radiation.
It is important to differentiate between the beam radiation and diffuse radiation because the beam radiation can be concentrated while the diffuse radiation cannot. There are many solar radiation measuring instruments that are used to measure beam radiation and diffuse radiation.
Now let us have a look at the spectrum of electromagnetic radiation in the below diagram.
In the entire spectrum, we only consider wavelengths from UV rays to IR rays to calculate the solar flux, because most of the high-frequency waves from the sun do not reach the surface and the low-frequency radiation after IR are not reliable. So the solar radiation or flux is usually measured form UV rays to IR rays and the instruments are also designed like that.
Solar radiation measuring instruments are of two types:
Before going into the working of these instruments you need to understand a couple of concepts that are used while designing the devices. So now let’s look into those concepts.
Black body radiation
A black body usually absorbs all radiations without emitting anything back into the atmosphere and purer the black body more perfect the absorption. The fact is, there is no perfect black body present until now so we usually settle for the second-best. After the black body absorbs the radiation its gets heated up since the radiation itself is energy and after absorption, the atoms in the body get exited. This blackbody is used as a core component in sun radiation measuring instruments. Opposite to the black body, a white body reflects all the radiation that falls on it back to the atmosphere that is why we will feel more comfortable wearing white clothes during summer.
The thermocouple is a simple device constructed using two conductors made of different material as shown in the figure.
Here two wires are connected to form a loop with two junctions and these junctions are designated as ‘A’& ‘B’. Now a candle is brought near junction ‘A’ while junction ‘B’ is left alone. With the candle being present junction at ‘A’ its temperature rises considerably while junction B stays cold at room temperature. Because of this temperature difference, a voltage (potential difference) appears at the junctions according to the ‘Seebeck effect’. Since the circuit is closed a current ‘I’ flows through the circuit as shown in the figure and to measure this current we will connect an ammeter in series. It is important to remember that the magnitude of current ‘I’ in the loop is directly proportional to the temperature difference at the junctions, so higher temperature differences result in a higher magnitude of the current. So by getting the reading of ammeter, we can calculate the temperature difference at the junctions.
Now after basics are covered, let’s look into the construction and working of solar radiation measuring instruments.
Pyrheliometer Working and Construction
Pyrheliometer is a device used for measuring direct beam radiation at normal incidence. Its outer structure looks like a long tube projecting the image of a telescope and we have to point the lens to the sun to measure the radiance. Here we will learn the working principle of Pyrheliometer and its construction.
To understand the basic structure of the Pyrheliometer, look at the diagram shown below.
Here the lens is pointed towards the sun and the radiation will pass through the lens, tube and at the end falls on to the black object present at the bottom. Now if we redraw the entire internal structure and circuit in a simpler manner it will look something like below.
In the circuit, it can be seen that the black body absorbs the radiation falling from the lens and as discussed earlier a perfect black body completely absorbs any radiation falls on it, so the radiation falling into the tube gets absorbed by the black object entirely. Once the radiation gets absorbed the atoms in the body get excited because of the increasing temperature of the entire body. This temperature increase will also be experienced by the thermocouple junction ‘A’. Now with junction ‘A’ of the thermocouple at high temperature and junction ‘B’ at low temperature, a current flow takes place in its loop as discussed in the working principle of the thermocouple. This current in the loop will also flow through the galvanometer which is in series and thereby causing a deviation in it. This deviation is proportional to current, which in turn is proportional to temperature difference at junctions.
Deviation ∝ Current in loop ∝ Temperature difference at junctions.
Now we will try to nullify this deviation in the galvanometer with the help of the circuit. The complete process for nullifying the deviation is explained in step by step below.
- First, close the switch in the circuit for starting the current flow.
- The current flows like,
Battery -> Switch -> Metal conductor -> Ammeter -> Variable resistor -> Battery.
- With this current flowing through the Metal conductor its temperature rises to a certain degree.
- Being in contact with the Metal conductor the junction ‘B’ temperature also rises. This reduces the temperature difference between the junction ‘A’ and junction ‘B’.
- Because of the reduction in temperature difference, the current flow in the thermocouple also decreases.
- Since the deviation is proportional to current the deviation of the galvanometer also decreases.
- In summary, we can say- The deviation in the galvanometer can be reduced by adjusting the rheostat to change the current in the Metal conductor.
Now keep adjusting the rheostat until the galvanometer deviation becomes completely void. Once this happens we can obtain voltage and current readings from the meters and do a simple calculation to determine the heat absorbed by the black body. This calculated value can be used to determine the radiation, as heat generated by the black body is directly proportional to the radiation. This radiation value is none another than direct beam solar radiation which we are desired to measure from the beginning. And with this, we can conclude the working of the Pyrheliometer.
Pyranometer Working and Construction
Pyranometer is a device that can be used to measure both beam radiation and diffuse radiation. In other words, it is used to measure total hemispherical radiation (beam plus diffuse on a horizontal surface). Here we will learn about Pyranometer working principle and its construction.
The device looks like a UFO saucer which is the best shape suited for its purpose. This device is more popular than the others and most of the solar resource data nowadays measured using it. You can see the original picture and internal structure of the Pyranometer below.
Here the radiation from the surrounding atmosphere passes through the glass dome and falls onto the blackbody situated at the center of the instrument. Like before, the temperature of the body rises after absorbing all the radiation and this rise will also be experienced by the Thermocouple chain or Thermocouple module present directly beneath the blackbody. So the one side of the module will be hot and another will be cold because of the heat sink. The thermocouple module generates a voltage and this can be seen at the output terminals. This voltage received at the output terminals is directly proportional to temperature difference according to the principle of a thermocouple.
Since we know that the temperature difference is related to radiation absorbed by the black body, we can say the output voltage is linearly proportional to the radiation.
Similar to the previous calculation, the value of total radiation can be easily obtained from this voltage value. Also by using the shade and following the same procedure, we can also obtain the diffuse radiation. With total radiation and diffuse radiation value, beam radiation value can also be calculated. Hence we can calculate both diffuse solar radiation and total radiation using Pyranometer.