So, for example, I also in the same context in terms of cycle life; if you look at cycle life you can see that capacitors are almost infinite in their cycle life you can keep on cycling a capacitor for all practical purposes it nothing will happen to it. Because you are not doing any significant change to the material it just remains whatever it is so nothing much is happening to it. So, it works for you to know millions of several millions of cycles it will run essentially it’s infinite from for our practical applications. Batteries, on the other hand, good batteries I am not even talking of ordinary batteries good batteries will probably give you about 1000 cycles. So, what is the real reason for this restriction where several reasons are there usually what is happening because there is a lot of chemical reaction that is going on the electrochemical reaction that is going on across the interface and a few different things happen for? For one thing for example, at lithium-ion batteries, for example, you will have an eventual degradation of the electrolyte that may happen, but more importantly, the electrode itself is changing from one phase to another phase you are having some reaction that occurs. So, some chemical that is there in the electrode changes to some other chemical. So, there’s a change in you know one reactant becomes some product and so in terms of a phase, one phase becomes another phase each phase has a different you know specific gravity and. So, it’s volume will be different. So, invariably if you look at battery electrodes their dimensions even though you don’t see it externally you are seeing the same battery that is there. Internally the dimensions of those electrodes could be changing, they could be swelling, they could be getting compressed, etcetera. So, there’s a lot of structural change that happens at that level it is not visible to us, but at the level of those particles, it is a lot of structural change that is happening the crystal structure level there is a lot of change that is happening so that puts a lot of stress at that you know size scale. So, many of those particles that are present in that electrode can break, can you know deform or particularly if they break off they may lose contact with the rest of the electrode. So, you have an electrode, and you have some two particles there one particle here and another article here. So, this particle the second particle may at some other particle break away from the electrode and so now, this is no longer in contact. So, if it’s no longer in contact it is no longer in a position to participate in the reaction ok. So, all these things happen in a battery so because there is swelling there is a change in dimensions of the particles etcetera lot of damage happens to the electrode over some time ok. So, it happens over some time. So, normally that is the reason why by the time you have done up say a 1000 cycles, the battery does not behave that well it doesn’t behave that particularly well and then eventually you give up on it get yourself a new set of batteries. On the other hand, supercapacitors are only charging on the surface there is nothing there is no chemical reaction that’s happening. So, there are no species chemical species in the form of ions no ions are entering the electrode structure I mean the sense if you go back here and look at these particles, if I take an individual particle here the ions just come to the surface. So, for example, if I say so, this is let’s say it is positively charged here, the ions come to the surface, but these ions are not entering the particle this holding position just around the surface they don’t enter the particle. So, in the particle nothing is happening the particle doesn’t swell, does not contract, nothing happens it just stays more or less intact. So, therefore, in many ways it is undisturbed by the cycling process and therefore, if you go down here it is also in a position to give you a million cycles, a million cycles is a lot I mean in the context of you know electric vehicles and so on. So, a million cycles work out very well because even though you know if you are doing a let’s say a battery that is running the electric vehicle is charged once overnight. So, you do one full charge and then the next day you run for 150 kilometres and you do a full discharge. So, if you used a battery for 24 hours of an electric vehicle you do 1 charge, and 1 discharge, but if the braking is regenerative braking which is using you know supercapacitors and you also let’s say you take the assistance of supercapacitors to help in the acceleration and deceleration and so on of the vehicle. then especially in conditions where there is high traffic and heavy traffic and so on let’s say typical Indian conditions where there is a lot of start of the car stop of the car you know you have to slow down the car, accelerate the car and so on. In the same 24; 24 hour period you may have I mean several 1000, several 100 to a few 1000, charge-discharge you know acceleration, deceleration events right. So, you have several 100 to 1000 maybe a few 1000 acceleration-deceleration events. So, a charge-discharge of the battery happens only once, but the acceleration-deceleration of the vehicle happens several 100 times maybe 1000 times it happens. So, therefore, it makes sense that you use some device which can handle I mean you would utilize this the fact that it cannot be it can be cycled a million times is useful to you and the fact that the battery can be cycled only if I mean a thousand times is still for you. So, a 1000 times if you can charge and discharge a battery that already means 1000 days you can use the battery so; that means, that that is over 3 years right, something like around 3 years you can run the battery, but a million times you are charging and discharging ability to run this 3 orders of magnitude more becomes necessary because you may use it a few 100 times each day. So, you will have to multiply it by a few 100 and then that the million works out comfortably for you right so that is; how these two taken together are useful for electric vehicle applications. The battery supercapacitor combination. (Refer Slide Time: 44:35) So, let’s look briefly at the materials used for this supercapacitor, as I said, in general, the idea is that there is no reaction as such happening on the electrode it is just a highly porous structure which can interact very well with the with good electronic conductivity which can interact well with the electrolyte. So, the electrode-electrolyte combination is typically critical usually people are using things like activated carbon, carbon nanotubes, and Graphene this is these are the kinds of materials that people are actively researching and working on. So, activated carbon, for example, is simply natural carbons that you get from you know let’s say sawdust, or you know some waste material from the agricultural sector, and then or you can take polymers artificially derived polymers and then you heat treat this in an inert atmosphere. So, you remove all the other species from it you will get carbon that carbon is typically called activated carbon it has high surface area so that is what is this activated carbon. Then you have graphene is like this it’s a single layer of a hexagonally bonded carbon atom. So, this has a very high surface area because you just have a single layer of atoms and you have a high surface area. So, if you put a lot of graphene particles together you have a lot of a very high surface area with which the electrolyte can interact, but the only issue is that the graphite is much more you know stable kind of a structure. So, if you give it enough chance the graphene will realign and form restack and start forming graphite. So, you have to keep that in mind so you have to do something to stabilize it so to speak then, of course, there are carbon nanotubes which are cylindrical in structure. (Refer Slide Time: 46:27) So, they essentially look something like that, A long cylinder we can end cap this side and an end cap that side and in this you have all those hexagonally bonded carbon atoms right. So, this is how you have the structure. So, you have hexagons like that and then you get the so these hexagons are there all over the surface. So, this continuous in all directions and this is how you get this structure you get cylindrical structures based on how you generate this the nanotube you can get it straight the way I have drawn it or you may even get highly curved coiled tubes like that ok. So, you will have you know some extremely coiled tubes also you can form so usually the CVD, CVD based synthesis of chemical vapour deposition based synthesis of a carbon nanotube, which is the typical kind of carbon nanotube you get when you commercially buy it when you go to you know some internet site where they sell carbon nanotubes and you purchase it mostly they are providing you chemical vapour deposition synthesized carbon nanotubes that will give you extremely coiled structure. If you look under an electron microscope you will see the structures that look extremely coiled whereas, if you do arc discharge based carbon, nanotubes or CNTs as they accord then you get this straight structures. So, this straight structures or the coil structures you can use in an electrode. So, for example, if this were your current collector you could line all of these up on top of them ok. So, I have just drawn some for example, so if you look at it charge wise let’s say this is positively charged so all of this will get a positive charge, all the surface of this will get positive charge and correspondingly you will get negative charge in all the negative charge ions will come around it. So, like this, you will get right. So, you get all this negatively charged ions which are collecting in the gaps between these carbon nanotubes, because you have the charge that has been built on the carbon nanotube and the positive charge is sitting in the carbon nanotube. So, this way you now have a very three-dimensional structure over which you have a lot of changes that have been held together ok. So, this is how you take a flat structure and generate a lot more charge and store it in this structure. (Refer Slide Time: 49:27) So, in terms of electrolytes; so, that is the main thing the electrode. The electrolytes are usually of three kinds aqueous, organic electrolytes, or ionic liquids. Generally, aqueous electrolyte means water is I mean by the by using the term aqueous we mean water is present if the water is present it breaks down at an at 1.23 volts, into hydrogen and oxygen. So, it means if you are using an aqueous electrolyte the voltage of that capacitor cannot be more than about 1 volt. So, if you go more than 1 volt you are increasing the chance that the electrolyte will break down so voltage gets restricted. therefore, the more commonly used electrolytes are these on organic electrolytes almost half of what is out there consists of in terms of product is organic electrolyte based structures. They have lower conductivity are usually propylene carbonate is the solvent used and some salt is used there lower conductivity, but still, if you see some better stability under some circumstances so that is used. More recently people have been looking at I mean a researching these things called ionic liquids, where you are taking an organic salt and you are not putting any solvent in it ok. If you put a solvent you have to do a lot of purification of the solvent and so on to remove any moisture present etcetera. Here you put no solvent the except that you pick an organic salt which has a melting point that is less than 100 degrees C so that you can just keep the capacitor at that temperature and you will have a liquid electrolyte, but the liquid electrode is a molten salt, so that is; how we operate this material the supercapacitor. (Refer Slide Time: 50:53) So, the electrodes used are carbon-based either activated carbon or graphene or carbon nanotubes and the electrolyte used is either an aqueous electrolyte or an organic electrolyte or an ionic liquid so this is basically what is used. So, that’s our discussion on supercapacitors both what is special about it? How is it different from a capacitor? How a difference how it differs from a battery and what are the applications it can be used for what are it is capabilities and; what are some materials that are used to create this supercapacitor? So, in conclusion, supercapacitors bridge the gap between capacitors and batteries so they offer you a; you know a new realm of operation which neither the battery would offer nor the capacitor could offer so it gives you a good mix of the positive aspects of both the battery and the capacitor. It is typically based on high surface area carbon being used as electrodes with the charge being distributed into ions in the electrolyte ok. So, the ions in the electrolyte; distribute the charge and hold the charge and that is how the electron-ion combination helps you hold that charge and typically you have aqueous organic as well as ionic liquids which are being considered as electrolytes for the supercapacitor. So, that’s our summary for of supercapacitors and it is an interesting topic has a very niche application in you know specific technologies such as electric vehicles and as you know something that you will keep hearing about more and more. Hello, in this class we will talk about a particular form of energy storage device, which is referred to as the flywheel. We have spoken about other energy storage devices particularly we are very familiar with batteries because we use them in several applications including you know remotes, toys or mobile phones. So, a variety of places we end up using batteries. So, we are and we replace the batteries. So, as the battery gets exhausted we tend to replace the battery we recharge the battery. So, there are so many activities that we do that bring us or make us very aware that there is a battery in you know the device that we are using. So, this is how we are you know quite comfortable and quite conscious of the fact that there are batteries in many of the things that we use. Interestingly flywheels are also there in several of you know devices that we use or at least that we have used. And it is just that we don’t there is no formal recharging of the flywheel and there is no formal replacement of the flywheel we don’t do those kinds of things. So, many times we don’t even know that there is a flywheel inside, we just use it we don’t realize that there is a flywheel and we take it for granted. I guarantee you that I mean I am pretty sure. Other and that is the reason I say I guarantee you that for sure you have used something that had a flywheel in it, and as we look through the examples you will understand why that is the case. I am going to show you some commonplace examples where we use them and also the more you know sophisticated possibilities that we are looking at. So, that’s the thing that that’s the thought that I would like you to have in the back of your mind, that it doesn’t matter you know what your background is or where you are from, this it is almost guaranteed that you have used a flywheel of some sort. So, that is the point that I wanted to highlight here ok. (Refer Slide Time: 02:14) So, in this class our learning objectives are, of course, to indicate what is a flywheel. So, first, let’s get some clarity on what is a flywheel and because I am going to keep telling you that you are using it anyway and we will see will describe how it operates, what is the basic idea behind its operation, we will try to understand what are some limits of the flywheel operation. So, that we get some sense of what’s possible in it, and you know up to with what kind of range we have to stay in this. So, and then finally, we will finish up with some material aspects associated with flywheels, We look at you know what is the kind of material that is used, what are the possibilities and what are some you know pros and cons of those materials. So, this is the basic set of learning objectives we have. What is a flywheel, how it operates, what are some limits in it for it and what are the materials associated with it that is our learning objectives? (Refer Slide Time: 03:15) So, what is a flywheel? So, a flywheel is an energy storage device ok. So, it’s an energy storage device except that it is a mechanical energy storage device ok. So, when you look at a battery, that stores energy using chemicals. So, that’s the battery is also an energy storage device, but there the energy is stored using chemicals. So, there is some chemical reaction in one direction and if it’s a rechargeable battery there is a chemical reaction the opposite direction so, but there are chemicals there that that store the energy and we understand that you know there is a delta h associated with the reaction and from that you can get some information about you know kind of energy that is there in the reaction, and then from there you can get you to know delta g and as well as the open-circuit voltage for that reaction, the standard electrochemical potential for the electrodes all of that we can do. But that’s got to do with the fact that there are chemicals there and we have electrochemical reactions. Whereas in flywheels there are no chemicals in the sense that there is no reaction that is happening there, there is no chemical reaction that is happening there, there is only something mechanical that is happening there. So, there is energy in something that we are already doing, and this mechanical event picks up that energy and holds it, and then when we want to we want that energy back it releases it back to us ok. So, therefore, it’s a sort of a mechanical event or a mechanism that pulls up the energy and then stores the energy and therefore, it’s a mechanical energy storage device ok. And the basic idea is that energy is stored by increasing the rpm. So, that is revolutions per minute. That is what I am referring to as rpm which of course, you are familiar with it. So, energy is stored by increasing the rpm of a rotating wheel. So, there is a wheel that is rotating, you increase the rpm of that wheel and then in the process, it stores up energy and then you can extract that energy back from the wheel. As needed and of course, as you can imagine when you extract the energy back from the wheel it will slow down the way. Of course, in the extreme case, the wheel comes to a complete halt. The in which case you have extracted all the energy that the wheel had, so you have extracted the energy out of the wheel and it comes to a complete halt. So, you are already doing something, where you have a lot of energy in the form of say kinetic energy or whatever it is, and then that energy gets transformed transferred to a mechanical device which consists of a rotating wheel ok. So, so that wheel could have been stationary also, to begin with; it could have been stationary and then you have this energy, then it picks up that energy and therefore, starts rotating. So, that’s also possible or it could already be rotating and you simply increase the rpm of that rotating wheel and therefore, that energy gets stored because of the angular you know the velocity of that wheel which is being, which is either going from zero to something higher or is already at some value and goes to an even higher value and then as you extract it out it is handing it back to whatever system that you want and then it, therefore, gradually slows down. So, this is the basic idea and so, that’s how the Flywheel operates. (Refer Slide Time: 06:34) So, for example, what is the Flywheel used for. So, some of the things it gets used for is smoothened smoothing the smoothing of the power output of an energy source. So, many energy sources don’t have like a continuous you know delivery function so to speak. So, if you look at energy as a function of time, it may be it may have a cyclic you know a manner in which it delivers the energy. So, there may be if I were doing something where it delivers energy for part of the cycle it is not delivering energy, again part of the cycle it delivers energy. So, this kind of a thing might be going on based on what is happening in that energy source, how it is generating the energy, how is it conveying that energy out into the external system right. So, whereas, many times when we are using the energy, when we are utilizing the energy we want smooth availability of the energy. So, we have lights. Lights fall on us and so, in our house, we are using a house you know lighting system. So, but this lighting system we want to know steady light it is very distracting if the light keeps flickering on and off in fact, it is worse than not having the light right. So, at least not if you don’t have the light your eyes are if you keep on having this flashing light which is going on and off that is extreme you are knowing distracting possibly even very bad for the eyes. So, if your energy sources an on-off on-off profile and you simply connect lights to it, then this is what we are going to have we would not have lights that go on-off. So, on the other hand, if you have something like a flywheel in the system, and you if you figure out a way in which you can implement the flywheel in the system then the flywheel will take this on-off energy and make it some smooth average value ok. So, that’s the nice thing about it, it makes it a smooth average value. So, maybe your light will not be as bright as you can have it when it is fully on, it will be a little less bright, but it will stay steadily on. And as I said for most of us that’s a much better situation much more desirable situation, we do not want a flickering light. So, we want a steady kind of light and so, that’s the kind of thing that as flywheel does. And that’s just an example I am just telling you about a light that is flickering on and off, but many other activities also you have an engine that is running, you don’t want the engine to go on off it is a very jerky kind of movement you are sitting in a vehicle you keep on getting jerked back and forth because the engine is switching on and switching off repeatedly. So, the flywheel then ensures that you get may be less powerful than the peak power of that engine that’s putting out, but it will stay steady. You will have it steadily available to you, then the vehicles run smoothly, you don’t you know get pushed around back and forth and it runs smoothly. It also helps us extend the ability of an energy source to operate outside of it is rating ok. So, in other words, something may not be able to give as much power, but because you have a flywheel, you can store up much more energy in the flywheel than is being given by you know energy source at a given point in time and then deliver it faster than the energy source. So, the original energy source may not be able to give this much energy in such a short period, but it can give you energy steadily. So, the flywheel stores up that energy steadily and it releases it to you at a faster rate then the energy source can do. So, in some sense, it extends the operating window of some energy storage device, and we will see for example, how it is used for something like regenerative braking. So, that is something that we look. So, there we are doing the opposite where you already have energy out there and you are trying to you know to remove that energy from the system, the regenerative braking is a way in which you take up that energy and store it in some you know some fashion without just wasting it as heat So, this is the thing. So, these are some interesting ways in which the flywheel gets utilized in various energy applications. (Refer Slide Time: 10:25) So, now even though flywheel is something that you know sounds like a very strange concept, I told you that it is something that all we are almost guaranteed that we have all used at some point in time. This is not even the example that I will tell is the most common example, but we will get through some examples and then you will have some sense of what I mean by saying that is a very commonplace thing. So, if you look at you know manual sewing machines. Of course, these days if you go to many shops, they are not using manual sewing machines, they have a motor because electricity is available and they want to remove the drudgery of you know regular work where people keep on doing this with their like moving their foot back and forth because that is a repetitive movement and can actually over some time do damage to their people's feet right. So, therefore, that it is not desired, but at the same time if you look at the old designs of sewing machines, this is what they had they had one will giant wheel at the bottom and then you had a foot-operated pedal. There is a foot-operated pedal here. So, that is what you have out here, and there’s a large wheel right. So, you keep you operate the pedal a few times and this wheel picks up speed ok. So, it starts rotating fast after a while you can take your foot off the pedal ok. So, when this wheel rotates. So, let’s say it is rotating like this, then because of this belt that is here the belt goes up and belt comes down it makes the smaller wheel also rotate like that ok. So, that's the basic action that we have, and then there is some you know some axle some shaft here, which is connected to this wheel that is out here and that rotates. So, that rotates this whatever is the machinery inside this machine. So, this is the sewing machine. So, it operates something inside it and then you have some needle out here, which gets your job done. So, some needle out there which goes back and forth and get some job done. So, that’s how the sewing machine operates right. So, now, the point is that you keep pressing this pedal a few times and then this you know there is some mechanism which connects the pedal to the wheel and that gets the wheel to rotate. So, some mechanism which gets the wheel to rotate and then that that wheel keeps rotating and then the machine runs. The point being that when you take your foot off the pedal, the wheel continues to rotate and that is because it has picked up angular momentum there is inertia, there is inertia in that wheel and that inertia, as you know by definition, implies that if an object is stationary, it will continue to remain stationary if it is if the object is moving, it will continue to remain moving until it is acted upon by some external force right. So, either stationary or moving object will continue to be in a state of rest or state of motion until it’s acted upon by some external force that’s the basic idea of inertia we are just using it in the form of you know inertia of a rotating wheel. So, we have a real wheel which has some mass associated with it, it has the moment of inertia associated with it, which is the equivalent of mass in an in the in a rotating object and then as you rotate it, it continues to rotate. So, even if you stop take your foot off, you are no longer providing energy for it, it continues to rotate, it rotates the small wheel on top and you continue to be able to operate the sewing machine. So, this idea that you have this wheel that continues to rotate even after you have stopped providing it with energy, that is the idea that the idea of or implementation of this flywheel. So, that’s the basic idea right. So, this is an example that many of you I am sure to have seen, but like I said it is not just about what you have seen, it is also about what you have used. So, there is there are some versions of these flywheels, which you have for sure used and we are going to see that. (Refer Slide Time: 14:28) And that comes from your toy ok. So, almost all of us have used toys where you have no battery in the toy, you can have various versions of it, you have one where you just basically push it and then it just has because you pushed it, it continues to run freely there is no motor in it, there is no mechanism in it, it just rolls it rolls till friction stops it right. So, that is one version of it. There is another version of the toy, where you have to push it I mean you know to push it against the ground a few times and the wheel picks up some speed, then you release it on the ground it will continue to run ok. So, then there is something inside which seems to have picked up the energy. So, initially, you have to push it hard on the ground you have to push it a few times hard, then you will find even if you take it off the wheels are rotating and it is you can feel that even if you put your hand to stop the wheel there is a resistance. The wheel continues to push your hand and continue to work so; that means, it has got something more than the free wheels that are rotating in the front right. There will be two wheels in the front which are freely rotating. So, let me say these are freely rotating wheels. So, you have some toys, some car or whatever it is and the toy car the front two wheels may be freely rotating. So, you. So, when you push the car on the ground and then you pick it up, the front wheel rotated the back wheel also rotated. But once you take it off the ground will find that the front wheel more or less immediately comes to a halt, but the back wheel continues to rotate okay that is because the back wheel is attached to a mechanism.