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Module 1: Solar Radiator and Semiconductors

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Solar Tower and The Semiconductor

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The same parabolic trough can be oriented in the east-west orientation. So, if this is east and this is west, then you can set it up such that it is in this orientation and then you don’t really have to track the sun through the day, you only need to change the orientation of the overall trough by just rotating it marginally. So, that you to do seasonal tracking concerning the position of the sun or the inclination of the sun based on the season and so,
it would point a little bit more towards the north during I mean or at least vertical upwards during summer, and point more towards the south during winter months. Since we spoke about concentrators through this class, let’s consider a large scale version of it. We spoke about a small dish which concentrates sunlight it is actually very nice because in you know all in two directions it is concentrating the sunlight on to one spot and therefore, you get a lot of concentration effect and of course, you have the challenge of tracking the sun. But suppose you want to make a large scale version of it, that just the small version which I could hold in your hand or you could hold in your hand etcetera but supposing you want to make a large scale version of it, where you want to generate several megawatts of electricity ok. So, then what would be a way to do it? If you can think of it’s like a huge parabola and in the middle, you have this unit which is gathering the current. So, you can think of a tower, a tower on top of it you have a point which is the focal point and this huge parabola which in two dimensional you know in both the axis it’s a parabola and then has this surface, a parabolic dish which is concentrating down to this point. So, just for
I mean demonstration let us assume this tube is a tower, and this is tall tower it is a tall tower I am although I am holding this in the hand, you assume that this is a very talk lower and then you would have parabola all around it. A big parabola all around it from which sunlight is getting concentrated on to this location here and then you have a lot of heat being focused to this location. So, that’s the kind of a concentrator that we are talking about. Now we have some challenges here the first is that of course, this has to be a tall tower. So, that is saying a civil engineering challenge you have to set setting up there, but then what about this reflector, how are you going to create this huge massive reflector that is there all around it so that it concentrates the heat or the sunlight down to this spot. It is not feasible for you to do machining and do metalworking on some sheet of metal which is saying one square kilometre in dimension. So, that is not how they do it, and it is also not necessary and imagine just imagine the kind of engineering challenges that would be there if you to take something like that along with the solid tower and then try to reorient it concerning the sun. So, this is not how it is implemented, it is done in a somewhat different way, but it actually captures the essential concepts of how these concentrations are happening. So, to do this actually all that we need is to have a series of flat plate reflectors. So, for example, I am using this stain steel sheet here as a flat plate reflector, and then this tower would be there somewhere out of the middle, and this sheet would reflect the sunlight on to the top of this tower. (Refer Slide Time: 33:09) So, you can imagine that this single tower in the middle is surrounded by series of flat sheets all around, and a huge number of them semi several concentric circles of these kinds of flat sheets, and then each of them each of the flat sheet can be oriented independently. So, you may have you know thousands of sheets in an in some in a circular fashion arranged around this tower, and each of those sheets can be oriented independently and in the process, you can get the sunlight reflecting off of this sheet to focus on the top of the tower. And as the sun orients as the sun moves in the sky, each of the sheets independently rotates or is reoriented such that its reflection is always at the top of this tower and therefore, you have all these sheets independently oriented concerning the position of the sun, and the focus that is on the top of this tower, and you can concentrate throughout the day using essentially flat sheets of metal. So, you don’t have to worry about too much about the curvature, but the effective curvature of the overall unit is such that it is all focused on this spot, and the sheets can also actually be on the ground itself. You don’t even have to have a sheet up there, you can have a series of sheets which are on the ground which are all oriented accordingly to get to this point. So, that’s the kind of concentrator that is used in a tower kind of a set up where you have a tower concentrator, and this kind of concentration enables you to actually create a situation where you can get several megawatts of power being focused on this spot, and usually, that tower would have a generator at the base and using that you can generate electricity. So, this is how this larger-scale version of the concentration of solar energy is implemented and there are such installations around the tower. (Refer Slide Time: 34:44) This is the solar tower, basically, the idea is that you know if you look at a dish you have how all the energy gets captured in one location. So, you basically have a dish and you have a region where all the energy gets captured, which is what we saw when we looked at the parabolic dish concentrator. So, any incoming radiation reflects off and goes to this region, and collects at this concentrating point. But if you want to extend this to a much larger region, it is not convenient to think of this as a dish, you cannot make say a square kilometre-sized dish, it doesn’t work it's very unwilling to work with. So, instead, what you do is you actually various sections of this dish, and you place it as flat surfaces on the ground. So, that’s basically what we have done. So, if we take, for example, this section here you can think of it. So, it is has been here transferred, you take this section here you can think as though it is been transferred here and so on. So, you have all these sections here, which represent the various locations on that original dish that we had up here, except in a much more enlarged manner and then they are all placed from along the ground. You can see here the inclination of each of these is different. So, this is much more flatly oriented whereas, this is a little bit more inclined more significantly. The idea being that they are all inclined such that the incoming radiation gets reflected off of them and then all arrives at the receiver all of them arrive at the receiver and in this process the heat is concentrated with this kind of an arrangement you can set this up across you know square kilometre in area, and it would still work without any significant difficulty in actually setting it up. So, these flat surfaces which are approximating small segments of the parabola are all basically flat sheets. So, they are flat sheets of metal which have been polished very significantly and so they are highly reflectively, and they could be fairly large reflectors they could be even reflectors the size of a room, which could be laid out on the on a floor. And each of them is programmed so that as the sun moves through the day or as the sun changes position through seasons, always the reflection each of these individual reflectors is oriented to this receiver. So so, based on the time of day right now you are seeing an on your screen you are seeing a situation where all the sun rays are coming vertically down and therefore, the orientations are symmetric on either side of this receiver, you find symmetrically laid out reflectors. So, what you see here is very similar to what you see here in that sense, but if the radiation were coming in a different direction in a bit inclined manner that as you can see here as the time of the day changes, then the orientations may change significantly, it may it will not be symmetric across this centre receiver. So, but again the point being they all have to reflect the radiation to the receiver, and in that process, a lot of energy is stored in the receiver. And from the receiver, this heat is taken straight down and you can actually have a room or some unit down here which then acts as your turbine room, and there you can you know run your turbine and generate electricity. So, that’s how these solar towers work, and they are capable of generating several tens of megawatts or even more very effectively. They are still bit expensive due to the cost of these highly polished reflectors, but in principle, it is an immaculate way in which you can get solar energy and conceptually also it is straightforward to understand what is happening here. In principle, in any desert location you can sort of think of it as some kind of a different version of a solar farm, not the way they currently look at it, but in a way in its own way you can think of it as a farm of some sort where you are capturing solar energy using a solar tower. Fairly high temperatures can be attained at this receiver and using that you can very effectively generate steam and run a turbine. We spoke through this class about a particular class of concentrators, which are the imaging concentrators. So, the parabolic dish that I spoke to you about, the linear concentrator that I spoke to you about, and also the tower kind of a concentrator that I spoke to you about, all of those are trying to concentrate the energy of the sundown to one spot or a very small region a line either a spot or a line. And except for the imperfection in the reflection, the basic idea is to create an image of the sun at that location and therefore, essentially concentrate the energy down to that spot. There is another class of concentrators which are referred to as non-imaging concentrators because they are not focused on where the idea is not about creating the image of the sun. So, the first three that I spoke to you about are imaging concentrates, this one is the non-imaging concentrator. Its actually just a variation on the design, but it actually intelligently recreates a situation where you need not worry so much about the orientation of the sun. And the basic idea is simply this you do have two surfaces curved surfaces one is like this and there is one more you can imagine the front of me thereof a similar nature, and there is a flat region in the middle. So, you have this curved surface a small flat region in front of it, and another curve surface. If you have this kind of a setup and if the profile of these curved surfaces right, you create a situation that any sunlight that falls in here gets reflected multiple times and then arrives at that flat surface. So, when you have that kind of a situation you don’t have to worry so much about tracking the sun carefully. Any light that falls within this region will get will likely undergo multiple reflections, but will always go to that flat area in the middle which is the region where the heat collection is happening. So, when you do that it’s a flat region and you may have you know energy distributed across that region, but all the energy goes there and therefore, it can capture that energy, but it is not focused down to a line it is not focused down to a point. So, it is no image of the sun is being created here. So, this kind of a concentrator is referred to as non-imaging connector and works very well in remote locations, where you don’t have the ability to go and keep changing orientation or perhaps you do not have people there to do it even manually if you wish to do it, but you still need energy with that location. So, you can set up this non-imaging kind of concentrator, it will still do the job of concentrating solar energy for you and give you good performance, but of course, the imaging concentrators can focus it even better and can get you a much you know the higher degree of concentration so to speak, but this would work just fine. And with without much orientation, your loss is not as much in this case of these kinds of non-imaging concentrators. (Refer Slide Time: 41:50) We can also consider ways of concentrating sunlight, where you are not trying to form an image of the sun. So, both in the parabolic trough, as well as the parabolic dish you're either forming a line or you, are forming a point at which the solar energy is concentrated. This is the reason why you are having to spend a lot of effort in trying to track the sun because if you don’t track the sun the image will not form at the right location and you will not be able to concentrate the energy as effectively as possible. There are ways in which you can actually sort of broadening the area over which the image is effectively being smeared out in to, and what you see here is one such version. So, the way this is set up is and so, these are called non-imaging concentrators, and basically, the idea is that you can set it up such that any incoming radiation sort of bounces off or reflects several times and then finally, arrives at this point. It may even bounce at bounce a few different locations. So, it may bounce this way and then come back this way etcetera, but eventually, it will arrive at this bottom surface. So, it doesn’t matter where the radiation comes as long as its sort of enters this general area, the profiles of this reflecting surfaces have been set up such that after several different you know bouncing events it will arrive at the bottom surface, and in that process, the no heat is captured by the bottom surface. But since then you know the after all these reflections the light can fall anywhere in this area that you see at the bottom, this entire area that you see at the bottom, there is no specific image that is being formed of the sun anywhere and so, this is referred to as a non-imaging concentrator. (Refer Slide Time: 43:42) I am showing you this in the form of a trough here, you can also consider a version for it which is sort of an in the form of a dish, but this is still the non-imagining concentrator, here you set it up such that the overall image could form anywhere here in a smeared sort of fashion and therefore, no there is no specific image being formed, but the energy is being captured. So, this is again a non-imaging concentrator. Thank you. Hello. In this class and the next few classes, we are going to look at the process of capturing solar energy using the photovoltaic approach. We have previously looked at how you can capture solar energy using the thermal approach where you are trying to concentrate the heat that comes from the sun and capture it in some kind of a heat transfer fluid and then use that for some application. Here it could either just be hot water at your home or it could be for running some turbines to generate electricity. So, we looked at you know the flat plate collector, we looked at the concentrators both the dish as well as the trough and that and that entire process we looked at we also looked at the possibility of using a solar tower and also the idea of a non-imaging concentrator. So, all of these things we looked at. We will now look in the next several classes on the possibility and how we go about capturing solar energy directly in the form of electricity or in other words you capture it in a way such that the process creates electricity directly. You don’t need to use a turbine in the middle, you don’t need to generate steam and then run a turbine and then generate electricity you can directly get electricity. So, that process is referred to as the solar photovoltaic process as opposed to the other process which we previously saw in the last two classes which was the solar thermal process. So, this is the solar photovoltaic process and central to the solar photovoltaic process is the use of a material which is the type of material is the semiconductor. So, in this class, we will look at the semiconductor and particularly we will see some of the basics of the semiconductor and some various aspects associated with it how it can you know be used and then build on those basics in the subsequent classes. (Refer Slide Time: 02:08) So, the learning objectives for this class are to plot the band diagrams for materials and particularly this is of interest because you want to get a good feel for how the different materials compare with each other concerning their band diagrams and why a semiconductor is, therefore, of interest to us okay. So, therefore, we will start by looking at by plotting the band diagrams of materials so that will be a learning objective for This class and we will then look at the interaction of these bands with any incoming radiation. So, we have band structure in the material and then there is incoming radiation. So, there is going to be an interaction and through this interaction, some energy gets absorbed and so that’s a process that we would like to get a better feel for. So, we will look at and try to explain the interaction of the bands with the radiation that is incident on the material. And we will also look at in this class the different ways in which band diagrams can be plotted. This is very important because we traditionally through school and in fact, through most of the college there is a certain standard way in which band diagrams are plotted that most of us are familiar with. But when you look at the science of the material in much greater detail you will recognize that if you especially if you have said a physics background and you are looking at these materials in greater detail you will learn that there are different ways in which the band diagram can be plotted and some of those processes some of those approaches have a lot more detail in a lot more information about the material in the band diagram. And therefore, the diagram that you are used to is an approximation and that’s the point that I will try to highlight the diagram that you are used to is not necessarily wrong it is just that it’s an approximation and therefore, it is not having some very important detail in it which is relevant from the perspective of application for capturing solar energy as in the form of a photovoltaic, cell capturing the solar energy. So, since it is missing that information it is important to see how that diagram comes about and what is that additional information that is available in the diagram which is then being approximated by the kind of diagrams that you are familiar with? So so, these are the 3 learning objectives for this class and so we will go over this as we progress through the class. (Refer Slide Time: 04:21) So, in this slide you see the various types of materials that you are most often familiar with we have insulators, we have semiconductors and we have metals. So, these are at least from the perspective of electrical properties these are the 3 broad classes of materials that we are familiar with insulators, semiconductors and metals. So, if you look at it essentially from the perspective of a band diagram and again this is the kind of band diagram that we are more familiar with we are used to from high school days there is a valence band and there is a conduction band. And so we typically explain the material properties using these two bands the valence band and the conduction band and it is at least when you look at insulators and semiconductors we talk of a filled valence band and then empty conduction band. So, this is how we look at it and then there is an important parameter here called the bandgap which you see here band gap E g. So, there is a bandgap there Eg and that is a forbidden energy gap where there are no allowed energy levels for the electrons. So, electrons can either be in the valence band or they can be in the conduction band and typically in an insulator or a semiconductor especially if it’s in a pure state, at 0 Kelvin this is the situation you will find where all the electrons are sitting in the valence band and the valence band is full and the conduction band is empty. So, this is how you will see. So, the real difference between the insulator and the semiconductor is actually in the value of this bandgap E g. So, if you see here I have put here if the bandgap is less than two electron volts it gets referred to as a semiconductor and if it is greater than 2 electron volts it gets referred to as an insulator. Now, this is not a very hard and fast rule, but this is a guideline which is used. So, just for easy reference, we are referring to roughly about two electron volts as being the cutoff if it is around 2 electron volts or less the bandgap would be referred to as a semiconductor if it is more than 2 electron volts you refer to it as an insulator. Metals, on the other hand, have a very different band structure here also you can think of various bands being present in the material, but the important thing is that in metal there is a half-filled band. So, you can see here this band is partially full. So, within this band itself, you have empty states and within the same band, you have full states, filled states. So, within the same band, you have both filled states as well as empty states or in this case which you see here on in this diagram here you have a brand that is full this one and an empty band, but they are overlapping over a range of energies okay. So, you either have a, you either have an empty band and a full band that are overlapping. So, there is no band gap it is bandgap is essentially I mean 0 in this case in fact, less than 0 if you want to just look at the idea that it’s overlapping, but or in another case wherein this case where the band is half full. So, that is what you are looking at out here. So, the main difference is that since you have these there is no gap there is no forbidden gap that is just above the highest occupied level of electrons here, that’s the highest occupied level of electrons in a metal and there is no forbidden gap immediately above it. So, you still have continuous energy levels that electrons can occupy in both cases both sides both possibilities. So, this is very critical to the behaviour that the metals display, that there is no gap and therefore, the electrons which are on the top of this level can occupy energy levels that are immediately above them very easily okay. As opposed to semiconductors and insulators where if you look at the topmost occupied level which is sitting here or here in this case immediately above the topmost level there are there is a forbidden gap and that gap is relatively large, it’s a fairly large gap that you see here right several electron volts is what you are looking at. Whereas, here you have virtually nothing in the metal you have virtually no gap it is a continuous set of energy levels that are there just above the highest occupied energy level that is the reason why metals behave very differently from semiconductors and insulators because these electrons have immediately available energy levels and so they can do certain things which semiconducting which electrons in semiconductors and insulators are unable to do because they do not have that freedom of seeing those energy levels just badly above their highest occupied level. So, this is the sort of the band structure of these materials that you see. Incidentally, I kept referring to the highest occupied energy level in metal at 0 Kelvin they give it a name it is called the Fermi energy level. So, this is E f the Fermi energy level, E subscript f Fermi energy level it is the highest occupied energy level for by electrons in a metal at 0 Kelvin and so here again would be E f. So, E f is there in these two cases I have just indicated the E f the highest occupied energy level in both these cases for metal is the Fermi energy level. In the case of semiconductors especially and insulators especially what are referred to as intrinsic semiconductors and we will talk about them in just a moment intrinsic semiconductor simply means it is pure, it does not have any impurities in it that have been deliberately added it is not a doped semiconductor it is a pure semiconductor. So, in those conditions, you can still define something called a Fermi energy level and that definition is necessary because it helps us figure out certain other behaviour of the materials which I will talk about in just a moment. But for insulators as well as for pure semiconductors which is the two band diagrams that I am showing you the Fermi energy level is exactly halfway between the top of the valence band and the bottom of the conduction band. So, if you can imagine the energy level here and here. So, that would be the Fermi energy level for semiconductor and another Fermi energy level for the insulator that I have shown you. So, of course, it is by definition it is like this and as you can see it is sitting in the forbidden energy gap. So, actually, at that instant, there is not going to be an electron sitting at that position, but that is how it is defined the Fermi energy level is defined there and in the middle of the gap. So, there is a variation in the definition for the Fermi energy level for metals and metals as well as the definition that is used for semiconductors and insulators. So, this is just something that you keep in mind we will use it later. The Fermi energy level is very important when we look at all these materials because that is the electrons which are sitting close to the Fermi energy level are the electrons that interact with anything okay. So, when you put two materials together even when you make two materials come in contact it is the electrons that are sitting at the Fermi energy level that decide the behaviour of the contact that is formed when those two materials come together and therefore, that energy level is important to at least know to know where is that energy level and then figure out what is going to happen when two materials come in contact it decides which direction electrons will flow from one material to the other when you put two dissimilar materials in contact. In many ways, you can think of it as the chemical potential of electrons if in thermodynamics they will talk of the chemical potential of any you know species in an in a phase which is the amount of energy that is required to add a small delta extra of that material to that phase and essentially it represents the energy level of that component in that phase. And therefore, when two know samples with the same component available at two different chemical potentials come together wherever it is at the higher chemical potential it will start flowing into the other material okay. So, that is how diffusion occurs. Normally, we say you know something at high concentration comes in contact with something at low concentration if you put them in contact from high concentration the species will go to, the sample which higher where it is there in low concentration that’s our general deferred general way in which we describe it. But concentration does not completely capture the detail of what is the driving force for the process. So, in some cases, concentration works perfectly fine, but in many cases, the correct term to use is a chemical potential if you are not familiar with it you can look up some thermodynamics book, but the idea is the same in some Examples the chemical potential will be the concentration in some other examples chemical potential may have additional detail which will not be fully captured by the concentration. But the basic idea being if your concentration is different it flows the same way chemical potential is different species flows and the Fermi energy is the chemical potential for electrons. So, if you say Fermi energy is high in material then it’s the electrons are holding at higher energy in that material Fermi energy is low in another material it’s at a lower level when they come in contact the electrons will flow from wherever it is in high energy to wherever it is in low energy okay. So, that is the reason why this Fermi energy is of importance to be aware of and to follow as you look at various materials. So, anyway, so this is the background for these materials and we will try to understand a little look a little bit more and how these materials will interact with Radiation. (Refer Slide Time: 14:18) So, if you look at the spectrum that comes to us from the sun then we have some parameters that are of interest for us to see. So, the wavelength range for the visible spectrum is about 400 nanometers which is the violet end of the range to 700 nanometers which is the red end of the range. So, this is in microns this is 0.4 microns to about 0.7 microns and you can ascribe energy to it. So, you simply have to do E equals h nu and that is the same as h c by lambda this lambda is the wavelength that I am just referring to here on your screen you have the wavelength put down there and c is simply sped of light which is 3 into 10 power 8 meters per second. So, once and h is the Planck’s constant. So, you can look up the Planck’s constant. So, once you know the Planck’s constant and you know the speed of light if you know the wavelength of the radiation. So, its 400 nanometers, we convert it to, so 400 into 10 power minus 9 meters if you use that and you use this formula you will get the energy corresponding to it. That energy will be in Joules I see I have shown I am showing you here energy in electron volts. So, that again is a simple conversion 1 electron volt is the amount of energy that is required to move 1 electron across 1 volt and an electron has a charge that is 1.6 into 10 power minus 19 coulombs. So, when you take 1.6 into 10 to power minus 19 coulombs and move it across a volt