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Module 1: Solar Energy and Solar Radiation

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The Radiation Process

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So, what would happen? So, now, supposing this is all that happens right supposing this is all that happens, every day you have all these happening and let’s say this is all that happens. Then every day you have 51 per cent of whatever is coming in staying on the planet right. So, if that keeps staying every day you are adding heat to the planet and so, then the heat should keep on rising right. So, the temperature of the planet should just keep on rising every day, we don’t see that that is not what happens. We see that on an average the planet maintains some temperature right we have a seasonal variation, of course, we have seasonal variation we have summer we have winter, we have spring, we have autumn all those things we have. But basically, there is seasonal variation, but in principle, we are expecting we are anticipating that the temperatures will be roughly the same across the once you know which season it is, you can compare it with the same season last year, same date last year, and get a reasonable idea of how the temperatures are holding. ear, same date last year, and get a reasonable idea of how the temperatures are holding. So, this is always our expectation. So, which means the heat is not just collecting on the earth indefinitely. So, this 51 per cent which is half the power that is landing upon earth it shows up on the land and the water, and gets absorbed by the land and the water is being been something is happening to it right it’s not just staying there. If it stays that it will heat the planet. So, it’s not just staying there. So, what is happening to it? So, those are a few of the phenomenon that takes care of that heat or utilize that heat, I should say are what we are going to see in the next few variations or you know developments of what you are seeing on your screen. So, out of this 51 per cent, 45 per cent a very large fraction of it, 45 per cent is used for air that is rising which means it heats the air right. So, the air close to the surface of the land as well as close to the surface of the sea gets heated up, because of the heat that has been absorbed by the land and by the sea right. So, first land and sea absorb the heat and they have a lot of heat available in them, that heat is used to heat the air that is just the layers of air, that are very close to the surface of the land as well as close to the surface of the water. So, when that heats up, you have hot air that’s beginning to move up and cold air from above in the atmosphere that begins to come down. So, that’s typical convection that is going on, and that how our entire weather system works. You have hot air and cold air that’s mixing all the time, hot items are moving up and cold items are coming down and there is a lot of mixing that’s going on and that is how essentially much of our weather phenomena is begins to happen, because of this fundamental set of you know activities that are happening. So, 45 per cent of the heat that first got absorbed by the land and the water, gets used for a few different things, one of them is this rising air it heats the air and then begins to rise. The second very important thing is cloud formation. So, all the clouds that we see including those clouds that are reflecting the sunlight, those clouds are formed due to evaporation of water right. So, water from our rivers, from our seas, from various places, where ever it is evaporating all the time it’s evaporating. It is evaporating because of the sunlight that is falling on those you know those areas of that are containing that water right. So, when that happens for evaporation to happen, you have to provide the latent heat. Latent heat to enable that evaporation to happen and so that’s how the evaporation process is happening and cloud formation is happening. So, every time you see cloud up in the sky, you know that some energy has been absorbed in the formation of that cloud right. So, the sunlight that came in got utilized to create that cloud. If absolutely no sunlight was there, if there is you know a cold then, in that case, you would have no heat, then it would be a cold planet, and then you are not having any scope for cloud formation unless there is some other source of heat which may be coming from underground, but other than that you essentially don’t have any other scope for cloud formation. So, solar energy is very critical for cloud formation on the planet and so, that’s one of the places, where the energy that has been observed by the land, as well as the water, is being used. every time you see cloud up in the sky, you know that some energy has been absorbed in the formation of that cloud right. So, the sunlight that came in got utilized to create that cloud. If absolutely no sunlight was there, if there is you know a cold then in that case you would have no heat, then it would be a cold planet, and then you are not having any scope for cloud formation unless there is some other source of heat which may be coming from underground, but other than that you essentially don’t have any other scope for cloud formation. So, solar energy is very critical for cloud formation on the planet and so, that’s one of the places, where the energy that has been observed by the land, as well as the water, is being used. And finally, there is this idea of radiation, which is again absorbed by the atmosphere. So, what this means is the land as well as the water they also radiate out heat okay. So, what do we mean by that? Well, we will discuss that in just a moment, but you just take it for the moment, that they radiate out heat and that radiated out heat is again absorbed by the atmosphere it’s like another source of heat. So, if received heat it picked up that heat. So, the land let’s say let’s take the example of land, it received heat from the sunlight it became hot and became because it is hot, it is also able to give out some heat. And that giving out of that heat is in the form of radiation, and that is that radiation has to again travel through the atmosphere. And as it travels back through the atmosphere, it can again get absorbed by the atmosphere. Just the way the sunlight that was coming in also got absorbed by the by some parts of the atmosphere, the radiation that is coming back from the earth can also get absorbed by the atmosphere right. So, that happens and all of this then eventually gets radiated back to space. So, what are all of those things, you had 45 per cent which is rising cloud, rising air, cloud formation and radiation absorbed by atmosphere etcetera they all contribute to this energy and then the whatever got observed by the clouds also arrives here and whatever got absorbed by the rest of the atmosphere also arrives here. So, you have 45 plus 348, 48 per cent and then another 16 per cent. So, that is 64 per cent. So, 64 per cent gets radiated to space by atmosphere and clouds. So, everything comes to the land and water gets absorbed by the land and the water, I mean whatever comes to the land and water gets absorbed there except for the percentage of 4 per cent that’s reflected back and then it transfers the heat back to the atmosphere as well as the clouds, and then from the atmosphere and the clouds it goes back into space. So, 64 per cent is radiated back to space by the atmosphere and the clouds. And there is also a small percentage which is remaining here which is about 6 per cent, that is getting directly radiated by the earth to space the difference between what I am referring to here and what I am referring to here is the fact that this radiation here is going straight into space, it is not getting absorbed by the atmosphere it is going straight into space and going out. But some of the radiation is first getting out of the land getting into the atmosphere, getting absorbed by the atmosphere and then again being radiated out by the atmosphere. 64 per cent is radiated back to space by the atmosphere and the clouds. And there is also a small percentage which is remaining here which is about 6 per cent, that is getting directly radiated by the earth to space the difference between what I am referring to here and what I am referring to here is the fact that this radiation here is going straight into space, it is not getting absorbed by the atmosphere it is going straight into space and going out. But some of the radiation is first getting out of the land getting into the atmosphere, getting absorbed by the atmosphere and then again being radiated out by the atmosphere. So, it’s just like a two-stage process whereas, what is happening here is a single-stage process it just goes straight off. So, this is. So, if you take this total 6 per cent here and 64 per cent here that is 70 per cent and we have already seen that what was reflected was the remaining 30 per cent. So, that’s your 6 per cent, 20 per cent and 4 per cent here. So, already 70 per cent I mean 30 per cent just got reflected. So, we only have to account for the remaining 70 per cent and that remaining 70 per cent we are accounting for here. 64 per cent is getting radiated to space by atmosphere and clouds, and 6 per cent is getting radiated to space directly from earth to space. So, this is what is happening ok. So, now I would also add a couple more details before I talk a little bit more about this whole idea of radiation, but and that simply that you know although I have stated all this in front of you, why should I mean why should we know have things being absorbed by the earth, why should we have things being radiated by the earth, and what are the circumstances that are permitting these phenomena to happen. So, quite simply whatever you saw here which is the incoming part of the process, are all essentially happening during the day okay. So, it is all essentially happening during the day, you have continuously sunlight that is coming in and that sunlight is doing all of these things. It’s getting reflected, it’s getting absorbed, it’s heating the land, it’s heating the water, it’s creating the clouds, it’s creating you know weather patterns lot of things are happening during the day due to the incoming solar radiation. And then you look at the essentially the situation in the night. So, when we are not facing the sun, the side of the earth which is not facing the sun now has an opportunity to release this energy back into space okay. So and that’s how the radiation process happens. So, it just radiates everything back into space and it continues. So, I mean you can sort of think of it in that sense, that you know in one case you are in a position to put out the radiation and so, that is happening. And another case the radiation is just continuously coming in. So, this is essentially what is happening, and when we talk of radiation it has got basically to do with the idea that anybody that has a particular temperature, we will give out radiation consistent concerning the temperature. and that’s how the radiation process happens. So, it just radiates everything back into space and it continues. So, I mean you can sort of think of it in that sense, that you know in one case you are in a position to put out the radiation and so, that is happening. And another case the radiation is just continuously coming in. So, this is essentially what is happening, and when we talk of radiation it has got basically to do with the idea that anybody that has a particular temperature, we will give out radiation consistent concerning the temperature. So, the sun has a high temperature and because of all the nuclear reactions that are going on there, it has a temperature internally because of those nuclear reactions. And as a result of that, it keeps out keeps on giving out radiation which is electromagnetic waves and light that we see visible light is part of that electromagnetic spectrum. But it has a wide spectrum, there is a wider spectrum through which you can have to give out radiation and the spectrum simply means different-different different wavelengths wide range of wavelengths. Some of those wavelengths we can see, and that is that forms the visible spectrum. And we will talk about the visible spectrum essentially I think in our next class we will be spending time on the visible spectrum, but the radiation is what is coming off the sun. So, it is coming off the sun, and we have a wide range of wavelengths over which this radiation is coming out, and there is a lot of different intensities for each of that radiation. So, there is a spectrum that’s called a spectrum, what wavelength and what intensity is coming on that wavelength. So, that is the spectrum. So, that’s how the radiation comes off electromagnetic wave leave anybody that is at some finite temperature. Even our own body gives out electromagnetic radiation, except that the frequency range at which it gives out that radiation is in the infrared region of the spectrum. That is why you know when you look at military applications, in military applications they use these you know infrared goggles IR goggles, which allow them to see in the night right in the night how do they see? They see anything that you see you is because some radiation coming off of that body is reaching your eye, and you can recognize that radiation okay. So, it during the day visible light is falling on some object getting reflected from that object, it reaches your eye. That’s how you can see that object. In the night if the lights are off you are not and there is no light reflecting off of a person who is standing in front of you and. So, nothing is reaching your eye. Visible light is not coming from that person and or any object that is in front of you, and therefore, you not able to see it, but if it is a human being they have they are standing there and they have somebody temperature right. So, nothing is reaching your eye. Visible light is not coming from that person and or any object that is in front of you, and therefore, you not able to see it, but if it is a human being they have they are standing there and they have somebody temperature right. So, that body temperature corresponds to radiation in the infrared spectrum and. So, if you have a glass that is sensitive to that radiation, it will convert that to an image that you can now see in your eye okay with your eye. So, that is how infrared goggles are made, which are used for seeing in the night vision. So, the point being a body at any temperature is giving out radiation. So, similarly, when the earth gets heated through the sun, it also is in a position to give out radiation, when it sees temperatures which are less than it’s own temperature. So, in the night it is seeing the dark side of the space and at that time the temperatures on the dark side with space are much much lower than the temperature of the earth and. So, the radiation now goes off the earth, and then it can send off the radiation. During the day it is facing the sun. So, it is facing an atmosphere that is hotter than itself. So, radiation is coming into it okay. So, that’s sort of the way you have to think about it and. So, when we talk of you know radiation here that is basically what we mean. In this entire diagram wherever we talk of radiation that is basically what we mean. The idea that a body at some temperature will give out electromagnetic waves, that are consistent with that temperature and that set of electromagnetic waves are being referred to as this radiation. And that is why even solar energy that comes to us is referred to as solar radiation. So, what you see here is a range of phenomena that are occurring on the planet, which correspond to this sunlight that’s coming in, and how it is distributed through the planet and how it leaves the planet. So, some few general aspects associated with this that we should you know sort of be better aware of because it impacts the overall picture of what is happening here is that this is a very delicate balance okay. So, we now saw how all the numbers add up to 100 per cent. Supposing for some reason any one of this is disturbed slightly, let’s say whatever is the amount of energy that is leaving the earth is not 100 per cent, but it is 99.5 per cent. So, you have 100 per cent energy coming in, only 99.5 per cent is going out, then that 0.5 per cent is just collecting steadily in the system, and then it keeps heating the system and that’s how the temperature of the planet begins to rise right. So, it is critical to keep in mind that this is you know I mean it’s a continuous process, every day we are receiving light from the sun and. Some side of the planet is essentially receiving sunlight all the time, I mean at any given point in time there is at least one side of the planet that is always receiving that is receiving sunlight. light from the sun and. Some side of the planet is essentially receiving sunlight all the time, I mean at any given point in time there is at least one side of the planet that is always receiving that is receiving sunlight. So, we are continuously getting energy from the sun, and the system the planet is in a very delicate situation because whatever we consider as our habitable conditions it seems to have worked out just right for us. I mean in the sense those we are now in exactly those conditions even when we go from say the equator to the North Pole or towards the north port we find that the conditions change so, significantly right. So, and that’s not a huge distance, if you take the relative distance of the earth to the sun which is 149 million kilometres, this is nothing we are just moving about 6300 kilometres that’s it. If you are moving 6300 kilometres away from you know the point that is closest to the sun to the point that is towards the pole, that’s the distance the radius is how far we move away in the direction of the sunlight. Just that makes the difference between a desert and you know a huge amount of stone. So, that is a very delicate balance, we are just at the right region where you have ice at one end, you have a dessert at the centre and then in the middle, you have all these rivers that are flowing. So, that’s a very delicate range of temperatures over which the planet is sitting. And this range seems to have worked out very conveniently or worked out in a manner that is appropriate for life as we know it and. So, the life that has evolved on the planet has evolved with this range of activities being present on the planet. So, anything that we do that disturbs this, has the potential to have many ramifications. So, that is why there is so much concern about CO2 in the atmosphere. Because if you have a slightly higher CO2 in the atmosphere how this balance of how much energy comes in and how much energy goes out is altered slightly. As you will slightly increase the amount of CO2 and as you can see we are not just slightly increasing amount of CO2 we are incrassating, increasing it in ways wherein a matter of 8 years you can double the amount of CO2. So, when you do that the balance is being twisted quite significantly, and it is not just one parameter that gets affected that’s the point that you have to remember. Every time one parameter gets affected many other parameters get affected, and they build on each other, that’s the that’s a much more dangerous problem. So, for example, as I said you know even if it’s just a tiny fraction extra COI said you know even if it’s just a tiny fraction extra CO2, and let’s say that leads to a
slight increase in temperature we are already seeing temperatures are going up by a degree or 2 right across you know after 100 years expectation, is another degree or so, it will climb up. When that happens you will see the melting of ice and many places where that you know the region over which that ice is there will now shrink. Because there is a region which is the boundary, that boundary is where that 0 degrees centigrade is between you know much below than 0 degrees centigrade, 0 degrees centigrade and then the heat goes up as you come from the pole towards the equator. So, there is a boundary where the natural temperature is hovering around 0 degrees centigrade. If you raise the overall temperature of the planet by one-degree centigrade, that boundary moves closer to the poles okay because; that means, there is more area over which the heat is available and therefore, the boundary moves closer to the poles. Many things happened because of this just because the boundary moves, first of all, the amount of water suddenly goes up and that make some impact in terms of how much the water encroaches into land etcetera that’s one aspect of it. But another interesting aspect is that when the boundary moves closer to the poles there is less ice present on the planet. If there is less ice present on the planet, there is less reflection of sunlight from the planet into space. If there is a less reflection of sunlight from the planet to space, more of the sunlight is sitting absorbed in the planet ok. So, by doing a small one-degree difference in temperature, you have opened the gate to increase the heat of the planet in a much more significant way right. It’s a delicate balance you are sort of sitting on a peak and you have pushed it off the peak. If you push it off the peak that is a major rundown hill so that is sort of the kind of situation we are dealing with and so, and that is the reason why that needs to be more careful. Because if you push it a little to the hotter side, you increase the chances that you are pushing it much more into the hotter side because you are adding factors that were not related to this directly. You suddenly melted the ice because the ice melted, you reduce the amount of reflection because the amount of reflection got reduced, you are increasing the amount of heat even further. So, this is the kind of issue that is there in this balance and that’s why this balance is so critical, so important for us to be so alert to, what this balance is okay. So that is one aspect of this whole discussion that we should keep in mind. The other aspect is, of course, the fact that we are now trying to harness this energy, you want to harness this energy and therefore, we need to get an understanding what is this energy that is arriving at the planet where is it situated. So, that if you want to harness it, we have an option of harnessing it right. So, we now got a sense through this discussion so far, that how much sunlight is coming, where all is it going. So, say we found that 30 per cent just gets reflected. is, of course, the fact that we are now trying to harness this energy, you want to harness this energy and therefore, we need to get an understanding what is this energy that is arriving at the planet where is it situated. So, that if you want to harness it, we have an option of harnessing it right. So, we now got a sense through this discussion so far, that how much sunlight is coming, where all is it going. So, say we found that 30 per cent just gets reflected. But still, 70 per cent is somewhere in the system either in the form of heat in the atmosphere or the form of heat in the land, a form of heat in the water, and then this is again also getting radiated forms clouds lot of things are happening, but there is a lot of scopes for us to capture the energy. And even with 70 per cent of heat arriving off the heat the come to the top part of our atmosphere. If only 70 per cent of it reaches the surface of the planet we still find as per our calculations in the last class, that every hour we are receiving enough sunlight from this enough energy from the sun equal to everything that all of humanity uses in today’s modern world for an entire year right. And that gives us such a large room to work with and satisfy all our energy requirements using such a clean form of energy which is solar energy. And that is why there is such an interest in it and that’s why we are spending a lot of time, in trying to understand it look at devices associated with using solar energy and we are looking at it as our first and most prominent non-conventional source of energy. (Refer Slide Time: 38:23) Okay so we will look at the how it is distributed; as you come from the top of the atmosphere top reaches of the atmosphere, down to the surface of the planet. Now our planet as we are aware is essentially you know nearly spherical and I also just described you how you know the temperature varies as you move from one region to the other in the planet. So, let's we will look at how it is distributed; as you come from the top of the atmosphere top reaches of the atmosphere, down to the surface of the planet. Now our planet as we are aware is essentially you know nearly spherical and I also just described you how you know the temperature varies as you move from one region to the other in the planet. So, let’s look at that also as a factor, because that gives us an idea of what is which are places wherein the planet where there is greater scope to do something with solar energy and what all parameters that we have to keep in mind. So, what are the geographical aspects associated with this, what are the seasonal aspects associated with this? So, we now have the I mean an outline map of our world, and we find that if you look at how solar energy comes to our planet. We find a big band here which is where let’s say we are looking at the entire geographical and seasonal aspects I have got to do with what time of the year that we are talking about.