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Module 1: Solar Energy and Solar Radiation

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The Possibilities of Solar Energy

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So, in all those ways it is already a clean source of energy, we just need to know how to tap it. And you can imagine the level of comfort we have, here it’s not even you know even if all the I mean if all of the worlds start living this fanciful lifestyle, maybe we will go up by a factor of 10 or 20 in our energy usage or maybe even 30. Even if we do that, we have you know; I mean we have a factor of 365 into 24 that is the number of you knows hours that are there in a year right. So, we are looking at something like 7000 and 8000 hours that are there more than 8000 hours in fact, in a year. So, if you look at more than 8000 plus hours in a year, that is the margin of error we have. Every hour we are going to get enough energy for the entire humanity for the year, and you have 8000 plus hours to deliver that energy. So, that much amount of energy. So, even if you put 10 per cent error, 100 per cent error you double the amount of energy you said, you triple the amount of energy, you go up by a factor of 20, go up by a factor of 30 in the energy usage, you still have way more energy arriving at earth than we are possibly using right. So, that is a beautiful piece of information for us to have to understand and latch on to so, that we can you know to build on it. So, this is the point that I wanted to emphasize in the calculation today, that we receive as much energy every hour as we require for the entire year. But that’s not all, there are some more interesting ideas given that this context is there and that’s what I will touch upon briefly in the next slide. What I am going to show you in the next slide is a bit futuristic, but it is it’s more from physicists have thought of imaginary situations, where we can do even better than this and that is all that I am trying to point out. (Refer Slide Time: 34:05) So, you can see here in the 1960s there was a physicist an English man who lived in the US his name was Freeman Dyson. So, he suggested that you know if it were possible to construct a sphere the size of the earth’s orbit, then you can capture all the energy that is coming towards the earth’s orbit. So, whatever energy, so right now we only capture the energy that is coming here right, we only capture the energy that is coming to the earth, but what about all this energy that is going away we are not able to capture it right. So, if we had a hypothetical I mean we managed to if our technology advanced far enough, that we could make a sphere. Some thin thin material sphere which is the size of earth’s orbit, then you can capture all the energy that is coming from the sun. As opposed to only what is coming in this cross-section, all the energy that is coming from the sun can be captured, but this is only a theoretical concept because as of now at least. Because nobody knows how to construct such a sphere, I mean we are no struggling to make we just about making buildings, which are you know say a 100 floors, 150 floors tall and so on. And those are our major accomplishments as of today, and many countries try to no out of prestige issues they will try to build very tall buildings. So, that’s really how far we have come; to build something in earth’s orbit that extends the size of the earth’s orbit, it is itself something that we have not even really imagined in any serious sense, and this may not be the only implementation for it, there may be other implementations where you do only partially this activity to get more energy. But in any case, we have not looked at it, where would we get the material for it, where would we get the material to build such a structure let alone go about building it. We may not have enough material on earth; to build even a thin you know structure which extends the orbit of the earth. But in any case, it is an interesting concept to keep in mind and that’s why it is you know it is the in fact, specifically referred to as the Dyson sphere named after the man Freeman Dyson and you can go look it up you will find in interesting information on this. And incidentally, as an aside as an extension of it, there is something called as the
Kardashev scale, which was sort of postulated in 1964, it is an extension of this it is also by a physicist in this in this instance Russian physicist, where he tried to imagine how civilizations may develop over some time and possibly what you can how you can put a scale to measure the extent to which civilization has developed. So, in this scale, for example, he talks of three types of civilizations, type 1, type 2 and type 3. This is almost mostly a theoretical discussion at this point; type 1 civilization would be a civilization that is harnessing all of the energy that is arriving at the planet okay. So, the discussion that we just had that you know all the energy that is coming from the sun reaches the surface of the earth in whatever energy we receive from the sun even in one hour, takes up I mean can cover the requirement that mankind has for the entire year, right So, if you know how to capture that energy right if you capture that energy or you capture all the sunlight that is coming from the sun to the earth every year, if you manage to do that then you are a type 1 civilization. If you have advanced to the point, where all the energy that is coming from the sun to the earth, all the energy received by the planet is being captured by the planet and harnessed by the planet, and used by the planet as a civilization I mean, of course, it is going somewhere in to be the ecosystem and something in various things are happening. But as an advanced technological civilization that is what we could think of ourselves as human beings. If we have managed to find the devices that capture all the energy that is coming from the sun to the earth the entire year, if you managed to that then we are a type 1 civilization. So, if you look at it from that perspective, even today we are not a type 1 civilization. If you think you know we are very, we are so developed, we have all these smartphones, and we do all sorts of fancy things, we fly all around the world, well does become a small place yes all those things are true, all those things are true but from this context of our ability to capture all the energy that is arriving on our planet, we have not even reached there. We have not even reached a place where we are capturing one hour of the energy that is coming to our planet, that is why solar energy is still such an I mean it is seen by the fact of solar energy devices are still such a small fraction of the energy devices that we use in the grand scheme of things. So, you see you saw that in all those statistics, all the renewables everything is falling in that you know just a few per cent of the entire energy is you know supply scheme that is there, and solar energy is a much smaller fraction of that also. So, we have not even come close to capturing what arrives on earth every hour and forget about the 8000 odd hours that are there in the year right. So, if you reach that 8000 odd hours year limit you are a type 1 civilization we have not reached that. Type 2 civilization would be a civilization that has advanced so much, that it can capture all the energy of the star that is powering that civilization. So, a type 2 civilization would be of this nature. One which would have something like a Dyson sphere around its star, and capture all the energy from that star right. So in fact, this is an interesting concept from the perspective of you know science fiction, it is also an interesting concept from the perspective of scientists who are looking for life forms elsewhere in the universe, they are looking to see if there is some massive structure that is a that does not fit the natural scientific evolutional evolutionary processes. So, we know what kinds of stars are there, we know what kinds of planets are there at least we have learnt a lot. There may be still a lot more to learn and then I am sure there is a lot more to learn what we have learnt a lot. So, if you see a structure which is the size of earth’s orbit and we can measure a structure which is the size of a huge orbit, but it is not having the characteristics of a star; that means, it is probably some artificial structure which is blocking a star right. So, this is one way in which you can guess if there is you know advanced life elsewhere in the universe. And once in a way, they will stumble upon something that looks like it, and then they find that maybe it is something else and so on. So, those kinds of things are there in the news. So, you should if you get a chance on the internet, use a search engine and then search for Dyson sphere and get an idea of it. type 3 is an I mean is beyond our imagination certainly, even type 2 is beyond our imagination because I really cannot see how we will build something that covers our entire orbit, but in any case at least notionally that seems like something that that can be considered if at all there is a way to do it, but type 3 would be one which uses all the energy of the galaxy that it is representing. A civilization that is present in a galaxy that uses all the energy of the galaxy. So, this is just a scale it does not mean that there are people who would be who is actually using this or that we are even going to do anything like this, but it is a scale that shows you the kind of energy that civilization might use, and therefore, it is something that is of interest from a scientific perspective and also from you knows technology perspective. So, these are some aspects and as I said the Dyson sphere is interesting to look at. (Refer Slide Time: 41:33) So, we did some calculations in this context and so, to conclude here and if we can briefly look at some of the key things that we sort of saw in this class. The first thing is that the earth receives nearly 5 and a half million Exa joules of energy from the sun each year. So, that’s a huge amount of energy, it is not something that we should I mean ignore or think that it is not much simply because of what we are seeing on the ground. On the ground we see that solar energy devices are just coming up, they have not taken over the market as we would like them to take over. We don’t yet have every house and every automobile powered by solar devices, although that seems like a very interesting concept seems like something that we should be doing because all you need to do is put a solar panel on top of your house right. Once you put solar panel sunlight is anyway already coming on it. So, it’s not like you have to do anything about it you see there is no generator there, you don t have to put a; you don’t have to supply diesel to it anything. You just have to put this panel and wiring to your house that’s all it is. Once you do it the solar energy is already being tapped by your house. So, we potentially we could all do that. Similarly, with automobiles, you could have the roof of an automobile the top of the automobile, the front of the automobile, back of the automobile, except the windows. Other than the windows, every other surface of the automobile could have solar panels pick up solar energy and then power the automobile. So, potentially we could do all this, we don’t do it right we do not yet do it, we do have some street lamps that are powered by solar panels and so on, but we have not reached that stage of usage of solar devices and solar products, where we can feel that you know we are considerably tapping these solar incoming energy right. So, sometimes just looking at the minimal amount of solar products, out there we may have this wrong notion that maybe there isn’t enough in that field, but really if you look at it this is a huge amount of energy that is arriving, which all of a very very large fraction of it we are just wasting. And wasting in the sense we are ignoring we are doing nothing about tapping. A very huge fraction of it is just being not even we are not even trying to tap it, it is all just falling it’s all it’s like you know all the energy coming and we are just falling all around us and we are not doing anything about it. It’s standing it’s like standing in a rain and it is a huge amount of rain coming down on you and you are basically having a small cup which can capture only one drop of water at a time, and then you say this is all the water I m able to get and there is all this water that is falling all around you. We are basically in that situation, there is all this sunlight that’s falling all around us we don’t tap any of it, we have the small panel with which we capture you know one small percentage of the sunlight and that’s all we are capturing. So, today our level of development our level of deployment of the solar panels is like that. Much of it is just going completely wasted we are not capturing it or making use of it. And as I said the great promise of solar energy, the huge promise of solar energy is the fact that the entire energy used by humanity each year is being received by the on the surface of the earth each hour. So, every hour not on some you know long duration, every hour we are receiving all the energy that we need for the entire year that is the biggest promise of solar energy. That you know you get this massive amount of energy at such a short period and it is clean, it is as clean as it gets because that’s how the world has developed that’s how you know entire all the biology that is all the life that has formed on earth has all developed at some fundamental level due to the sunlight that’s been coming into the earth. And then this is where we are the planet that is in equilibrium with the at some kind of dynamic equilibrium with the sun, and all of that energy has arrived here, right. So, this is where we are. And clearly, if we manage to tap this energy we have a very clean source of energy, right. So, with this I would like to conclude; in this class, we have looked at the transaction of energy from the sun to the earth, we looked at what is the power of the sun, we have looked at how that power diffuses as it arrives at the earth’s orbit, we looked at our earth’s cross-sectional area and said you know out of all this energy that the sun is sending out in all directions what is the energy that reaches the earth on the earth’s orbit. And therefore, we know the power of the sun that is arriving on the earth, and then we have you know convert that converted that to joules and joules per second and then looked at how many joules are arriving per year. And then finally, we compared it with the total amount of joules that we need every year as of today. And with that, we find that we are in a very promising ground. We are in a very good situation, we have some lot of promise here, and potentially wonderful things can happen in this arena of solar energy and how it is utilized in our society. Thank you. In the last class, we looked at the solar energy, in the context of the transaction between sun and earth. So, we saw the sun to earth transaction we were looking at you know the idea that the sun puts out so much of energy every second. So, that’s like a powerful equivalent of a new conceptually equivalent of a powerful bulb in the middle of the solar system, and from there energy you know travels all over you know in all the directions faithfully, and then some of it reaches the earth. So, we look that process, we look that possible parameter that is involved in the calculation and we arrived at some number of how much energy arrives per second to the planet earth. So, that was what we did in the last class, in this class what we will look at is the next step. So, since that, once that energy arrives at the outskirts of our planet, what happens to that energy. So, where does it go, how does it get distributed and then what happens to it eventually right. So, all that energy has arrived, what is happening to it eventually in the planet is what we are going to look at, where is it getting distributed, what is being done with it and so on. So, that is what I am referring to as the solar energy budget. How are in what are the various places in which this is you know getting distributed? So, you can think of it notionally as some kind of you know the equivalent of a budget, you put different quantities into different parameters and then see how the total gets distributed into all of these things. So, that’s what we will look at in this class. (Refer Slide Time: 01:46) Refer Slide Time: 01:46) So, what are our learning objectives? So, our learning objectives are these to know how incoming solar energy is distributed across various phenomena on earth okay; so different phenomena that are their natural phenomena on the earth, where is this solar energy getting distributed. So, that’s number 1; and number 2 and number 3 are sort of related, but in any case, there are two they are two separate aspects. First is to become aware of the geographical and seasonal aspects associated with solar energy. So, what are those concepts geographical aspects and seasonal aspects associated with solar energy? So, that’s something that we will look at. And finally, we would also like to become aware of the impact of time of day, on our ability to receive solar energy at any given location. So, where we are what is the intensity of the solar energy that we are receiving then, how is that affected by the time of day. So, these three are our learning objectives for this class. So, as we go through various you know concepts through the class, these are the points that I will try to highlight as we go through it right so. (Refer Slide Time: 02:53) (Refer Slide Time: 02:53) We sort of saw this in one of our earlier classes, the various layers of the atmosphere. So, and perhaps the thing that is of relevance to us is, just to keep in mind that 80 per cent of the mass of the earth’s atmosphere is in this lowest section called as the troposphere. And essentially all our commercial activity, commercial air everything sort of happens here and beyond that you do have all these other regions and in fact, we are doing some you know let’s say satellite-related in that region and so on. So, it’s not that we don’t use these things or that they are irrelevant to us, but 80 per cent of the mass of the earth atmosphere is in the troposphere, and at least be; so, we need to be aware of it. So, naturally, correspondingly many more phenomena tend to happen in this troposphere. So, that is something that we will keep in mind. Of course, the energy that comes from the sun essentially comes through these layers. So, when we stand out there in you know in the sun, and we feel the heat of the sun. So, that’s energy from the sun, that has left the sun you know several minutes ago, typically I, think it’s about 9 minutes or something that it leaves the sun, and then travels through space goes through this layers of, these layers of atmosphere and makes it to us makes it to the surface of our skin and that’s how you feel the heat. So, these are all there in the pathway by the time the energy from the sun reaches you. So, this is something we will keep in the back of our mind, we will use it as we find necessary. (Refer Slide Time: 04:24) Refer Slide Time: 04:24) And this is what we did last class, where we looked at the idea that the sun is putting out power equal to 384 Yotta watts. So, it is like a very powerful bulb you can think of it that way in the middle of the solar system, which is lighting up the whole solar system. That light gets radiated in all different directions, and then some of it arrives at the earth. So, that’s basically what we saw, and we run the calculation of you know as it goes away from the sun, how effectively because it is getting distributed over the larger and larger surface area, the intensity that is the energy per unit area has decreasing or the power per unit area is decreasing and the. So, there is a certain power per unit area that arrives at the surface of the earth. (Refer Slide Time: 05:13) (Refer Slide Time: 05:13) And that value we calculated to be about 1.755 into 10 power 17 watts or joules per second ok. So, that’s what is arriving on these on this the earth from the sun after it has travelled the entire distance that it has to travel between the; I mean the sun and the orbit of the earth okay. So, this is 1.755 into 10 power 17 watts or it is about roughly 175 into 10 power 15 watts or we would refer to that as 175 petawatts. So, 175 petawatts and again if you have any doubt you please look at the quantities that we saw in our first class on you know the scale of quantities, and this is the kind of number that we are dealing with. So, this is the energy that comes to the outer reaches of the earth’s atmosphere. So, that’s where it arrives. So, now in this class, we start from there, and we see how this energy penetrates the earth and what is happening as the process proceeds right. (Refer Slide Time: 06:19) Refer Slide Time: 06:19) So, we have here, you can think of this as you know this is the sun and 175 petawatts have arrived at the outer reaches of our atmosphere. So, we have the atmosphere and down below we have earth the solid ground of earth. So, at what happens as you will see through this process is that, as the suns energy arrives at the earth and begins to penetrate the atmosphere, it gets to participate in several different phenomena and. So, some of that energy gets used for all of these phenomena. So, that’s the process that we will see as we proceed through this stage by stage okay. So, out of these 175 petawatts, as it penetrates the atmosphere, we find that right at the top. (Refer Slide Time: 07:06) So, in the top layers of the atmosphere itself, you have about 6 per cent that is just getting reflected by the atmosphere. in the top layers of the atmosphere itself, you have about 6 per cent that is just getting reflected by the atmosphere. So, this simply means it just reflects 6 per cent of the sunlight coming into the planet, it is just getting reflected off right. So so, that never even reaches into the interiors of our atmosphere, it just goes off the top from the outer reaches of our atmosphere, it’s just getting reflected in it’s on it’s way out. 16 per cent is absorbed by the atmosphere okay. So, 16 per cent is getting absorbed by the atmosphere and. So, this can be used for a variety of different things, but it is there in the system. So, if you see the overall process we have to account for this 16 per cent somewhere, this 6 per cent that did not reach earth and simply basically bounced off the atmosphere, is not something that we need to account for in any great sense, in the sense that we don’t have to worry about what it can do in terms of heating of earth or cooling down earth etcetera. So, because it does not participate in anything that is happening on you know on the earth it’s just on it’s way out, but this other 16 per cent is there somewhere in our atmosphere. So, it has arrived it is sitting somewhere in our atmosphere. So, that is something we have to keep in mind. If you take this step further and you try to see what is happening after this these two processes have happened. So, there is already you have removed 6 plus 16, 22 per cent of what was coming in. So, you have 78 per cent remaining. 78 per cent off 175 petawatts, that started at the outer reaches of our atmosphere is remaining now (Refer Slide Time: 08:54) Refer Slide Time: 08:54) So, that is 70 per cent 78 per cent is remaining. Out of that 78 per cent, a further 20percent is being reflected by clouds okay. So, as I said you know the original 16 per cent is being reflected by the atmosphere itself, it’s the various layers of the atmosphere just the fact that you know it has some material in it, it has an air it, it has air molecules in it and it may even have some fine dust in it, a lot of different things will be there at the end that is bouncing light off of the atmosphere. But 20 per cent is being reflected off the clouds. So, that is quite significant. This is quite important to keep in mind because this reflection plays a critical role in you know temperature control. If this reflection were not there all of this energy would still be coming into the earth right. So, and then correspondingly this is the energy that we would have to deal with inside the planet. So, it means it’s going to heat things further. So, that is something that we have to keep in mind and 20 per cent reflection is quite significant. If you ever travel by flight and you are on flying over a layer of clouds. So, you cross the layer of clouds, you are just above the layer of clouds and you are flying. If you look out and you see the clouds it’s quite glaring the sunlight that reflects off the clouds it’s quite glaring and quite blinding, and it can affect your eyes quite a bit. You just watch for a little while and you look away you can feel the darkness in your eyes, I mean because it is just been overwhelmed by the amount of light coming from the reflected light from the clouds. This is just even for a few you know several seconds you look out the window and you watch that glaring light getting reflected off the clouds, you get that sense that it’s too harsh for you. So, you tend to look away. This is just even for a few you know several seconds you look out the window and you watch that glaring light getting reflected off the clouds, you get that sense that it’s too harsh for you. So, you tend to look away. So, that’s a significant amount of light that is being reflected, incidentally, the same thing happens even if you go to you know regions that are receiving a lot of snow. So, of course, when there is snowfall you have you know it’s overcast kind of condition through which the store is coming. So, it looks all gloomy like you would have on a heavy rainy day kind of a thing so. But once the snow is on the ground and then and let’s say the sky clears after that, you have this bright blinding sunlight that you experience when you step out during the day, because there’s so much sunlight that is getting reflected from that snow. People who live in this extremely snowy regions where you know let’s say they are living in Alaska or you know Greenland or Iceland places like that where there’s a significant amount of snow for significant periods in the year. They have to wear necessarily wear what we would call what you know cooling glasses or you know some sunglasses of different kinds which are some coated glasses which reflect a significant amount of the light away. Otherwise, the light is simply too harsh for the eye. Many places there are in fact, there’s a specific term for it there is a time which is referred to as snow blindness. So, people who are in such regions if they don’t take appropriate care to you know protect their eyes, their eyes can become blind simple because you know overdose of light into their eyes. Just the way people can become deaf if they have excessive sound going to their ear all the time, people can go blind if they have excessive light going their eye all the time. So, snow blindness is something that people have to deal with. If they live in regions where there’s a considerable amount of snow. So, that leaves us with one very interesting question given that this is an aspect that we should be aware of, how did people who lived in snowy regions manage in early days. In early days in the sense when before they had cooling glasses available to them okay. So, that’s a question you keep in mind we will briefly address it towards the end of the class, think about it people who lived in cold regions which had a lot of snow, had to deal with snow blindness, had to deal with the fact that there was extremely harsh sunlight coming to their eyes all the time and. So, they had to do protect their eyes from it. And in fact, they couldn’t see I mean that would it would hurt their eyes to see anything. It had to do protect their eyes from it. And in fact, they couldn’t see I mean that would it would hurt their eyes to see anything. So, they had to do something equivalent of cooling glasses to or sunglasses to help themselves see, which you may not be completely familiar with. So, you need to think about it as to what they did to protect their eye and we will discuss the answer to that towards the end of the class. But in any case, this reflection is a very critical aspect that we have to keep in mind. This reflection is a very important aspect even from an overall planets perspective, of course, this is already in the perspective of the planet that we are looking at, a significant amount of reflection happens off of the ice that is there in the polar regions, ok. So, that also actually helps control the temperature of the planet. A significant amount of heat bounces of that significant amount of sunlight incoming radiation just bounces off these huge ice caps that are around, and that decreases the amount of heat that is penetrating and staying in the planet from the sun okay. So, so these are all important things. So, what we are doing now is looking at what is the planet doing if the sunlight that is coming in, and trying to get a sense for it because we need to understand if you are doing something that is disturbing this. Then we need to be careful because we may abruptly disrupt this equation of how the energy that is arriving from the sun is being handled by the planet okay. So, that is one aspect of it, the other aspect is we are trying to see in this context, how are we going to use the solar energy how much of this is energy is available for us to use and so on. So, these are two-three aspects that I will touch upon as we go through with this class. So, anyway, 20 per cent of the sunlight is being reflected by clouds, and 3 per cent is being absorbed by the clouds. 20 per cent