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So, that’s the direction of the wind and that is also the direction of the tip okay. So, now, the air molecules are physically pushing the wind turbine in the direction that they are moving. Now clearly the only source of energy there is the, you know velocity with which that the kinetic energy of those wind molecules that have come there and so, this they are coming with some velocity and they with that velocity they are pushing this surface that is ahead of them. So, that surface cannot move faster than the rate at which those molecules are coming and pushing them pushing that surface right. So, in this design this flat surface or this curved surface, in this case, cannot move at the tip of that curved surface cannot move at a velocity faster than the velocity with which this wind is flowing because this wind is directly pushing this surface, there is no lift involved here this pushing action that is involved here. So, in this case, if it attempts to go faster then there it’s no longer getting any thrust from the wind, then it loses the ability. So, even for a moment if it were to go faster than the wind, it will pretty much there will be no further push it will slow down till the wind catches up and wind will keep pushing it. So, even if you hypothetically consider that situation. So, you put it in the wind and you turn it faster than the wind, it will slow down. It will slow down and it will go only at the wind velocity. So, you would you use your hand and you rotate it much faster than what the wind can do, and you just leave it there in a few seconds it will line up with the wind velocity, it will not be able to go faster than the wind velocity. So, in this case, the tip a V tip can only be ns equal almost maximum either it can be at it is maximum can only be as much as V wind okay and that’s because they are both moving in the same direction. So, that’s the big difference when you had the lift design they were not moving in the same direction and so, the relationship was very different and Bernoulli’s principle defined what could be done there. Even here there will be energy conservation etcetera. So, that it is not that this violates energy conservation, this is also associated with energy conservation, but there are additional restrictions because there is a push design involved, it is not a lift design and as a result, the v tip at most at it is maximum can only be as much as v wind. So, typically the V tip is less than V wind. So, therefore, you have this tip speed ratio being less than 1 okay. So, now, this is the idea that we have to keep in mind, and this is an important sort of in the context of the windmill designs. (Refer Slide Time: 39:08) So, now that’s a concept we will keep in mind we will use it as we find necessary in the design discussion. So, let’s look at several blades and let’s say to what degree this tip speed ratio or TSR we will call it tip speed ratio we will call that as TSR. Let’s just see to what degree this TSR is of any consequence to us. So, typically we have three blades and it is rotating our most common application is power generation. So, that is why it’s also being referred to as a wind turbine as opposed to simply calling it a windmill. Now the point we have to remember is that as the blade rotates. So, let’s say it is rotating that way okay. So, wind strikes a blade and as a result of wind going past the blade due to a lift action the wind the blade moves away. Now, as the blade moves away it has locally disturbed the wind right. So, there has been an interaction between the wind and the blade and that has resulted in some disturbing wind. So, some turbulent wind is there. So, you had some smooth wind going you had some turbulent wind. Now if you have the next blade of the turbine arrive at that spot before this turbulence goes away, then the next blade cannot completely benefit from the flowing wind okay. So, what to just restate the wind interacts we just say wind interacts with a blade, generates lift okay. So, that’s the first thing that happens. So, what happens? The blade moves away and at the same time wind has been disturbed; wind in that vicinity in the location of that blade where the blade where that interaction happened you know in that vicinity that region of that near that region you know even in that vicinity has been disturbed, and at that point, it is not smooth it is kind of turbulent because it had that interaction and. So, it has been disturbed. So, the blade moves away, but it will take a maybe a few seconds some amount of time is required for wind at that location to re-stabilize. Why it re stabilizes? Because wind is coming fresh wind is coming then the wind is coming from a long distance right. So, it has some stable properties, some velocity some smooth flow that is happening that is coming with which it is coming. So, now, you had a blade, that blade interacted with the wind it got pushed away, but in the process, it sort of messed up the wind there, but if you give it a few seconds this messed upwind will go away fresh wind will arrive here, which would be the same as whatever the original wind that interacted with the blade. So, you need a few seconds before this blade moves away and this fresh wind arrives at that location okay. So, some amount of time is required. So, it is a finite amount of time usually a matter of seconds maybe fractions of seconds to a second or a few seconds before the wind will re-stabilize at that region. So, if I call this region A. So and let me call this blade B call this as a blade C. So, this blade A will move away, but this particular location let me call this location as L okay. So, a blade A is at L because of the interaction it moves away. Now, blade B is also going to come to the same location L. By the time the bay blade B arrives at L, we want the wind to be stabilized at the location L. So so, we say wind interacts with blade generates lift for blade A at location L, okay, the blade moves away and wind in the vicinity has been disturbed in that vicinity has been disturbed near around L. Some amount of time is required for wind at that location to re-stabilized at L. So, this restabilization we want at L. So, then four by the time blade B arrives at L wind should be stable there okay. So, this is the kind of thought process that is here by the time the blade B arrives at L the wind should have restabilized at that location L. Now, the point is if the wind has not restabilized it does not mean that your windmill will stop operating or that it will stop generating energy, it is just that that process will not be efficient. You will lose the energy you are not capturing all the energy in the wind, a wind has much more energy you are capturing disturbed energy only some part of that energy you will capture from the wind you are not capturing all the energy that is there the free-flowing wind. So, the better design is such that you want to set it up such that the tip speed ratio is such that, by the time that blade comes to that location vacated by the previous blade the wind has restabilized okay so, that is how we do this calculation. So, if you want to extract the maximum power out of that is available in the wind, we need to consider the tip speed ratio of this windmill and based on that only we decide how many blades are possible okay. So, if the tip speed ratio is high then you can use less number of blades ok. If you use less number of blades then is just let’s say. So, you have less number of blades. So, for example, let’s just look at this we will look at possibilities here. (Refer Slide Time: 45:43) So, we could have two blade design, we could even have a one blade design we will just talk about this. So, we have a two-blade, three-blade design, two-blade design, one blade design. So, supposing the tip speed ratio is some value v 1 in this case and v 2 in this case and v 3, in this case, these are tip speed ratios. So, if the tip speed ratio is some value v 1 which is medium, you want it such that in that at that velocity by the time it comes here this the wind has stabilised here. If the steep speed ratio is higher than you need the blades to be further separated, so 2 blade system would work. So, this tip would work travel this entire distance before it comes here, and by the time that wind would have stabilized and of course, in this case, this travel the full circle and come right back here right. So, that is the full circle that it is going to do, to come right back to that starting point. So, the higher the tip speed ratio you can get away with lesser and lesser number of blades. The lower the tip speed ratio you can if you wish used more and more blades and still capture more and more energy from the windmill. So, in principle you can have a three-blade design, two-blade design, and one blade design based on tip speed ratios and also you have to keep in mind that if you are using this primarily for generating electricity, then having higher rpm’s is what is more useful to us. Generally, the torque requirement is not as high whereas, the rpm requirement is much higher and therefore, high tip speed ratios is important because that decides the rpm the faster the tip is moving you will have a higher rpm right. So, generally, these designs are based on that thought process, you should also keep in mind that as the blades move ever. So, often one blade will line up with the tower. So, you have a tower here. So, that blade the instant that it is lined up with that tower it is not going to face the same kind of environment that the other blades are facing. So, usually, the blade that is in front of the tower has a, you know disturbing kind of behaviour relative to the other blades and that affects the stability of this wind turbine or affects the stability of this whole process. So, if you have only two blades, you have one blade facing a drastically different situation compared to the second the blade that is diametrically opposite to it. Every time the blade is you know one of the blades is lined up with the tower right. So, that’s a very unstable situation relatively speaking. So, three-blade designs in that sense are considered to be very good because you have two blades which are you know sort of stabilizing the wind turbine, when only one blade is facing this circumstance of that tower okay, of the involvement of the tower. So, generally, three-blade designs are preferred; one blade design can also do the job you simply have to have a counterweight here, can also do the job if you are only interested in looking at rpm if torque is not an issue. If torque is not an issue when one blade design can also you know at the end of the day that blade also is about rotating as a result of the interaction with the wind. So, it will also have the same tip speed ratio and or something to that based on that behaviour of the wind interacting with that blade. So, you can get the rpm using one blade also, but generally, this is considered very people where ever this has been installed although functionally it works fine, it is considered as something that a lot of people object to because it seems to affect them to see this single blade rotating. So, it’s more psychological reason why people don’t use this design. It seems to bother a lot of people to see this single blade repeatedly rotating in the sky there, something about that is very disturbing to a lot of people when they see it in action, it’s not something that seems to fit with our you know the psychology of what we are expecting in nature and therefore, a lot of people object to this design when it is put up. So, there is no real objection to two-blade designs and three-blade designs, in that context, the three-blade design seems to have a good mix of various factors taken into account, the stability, you know dynamics of the whole process etcetera seems to come off much better the three-blade design. So, that is what is commonly used and like I said if torque is important where you are where you don’t mind the speed of the windmill. But you are more interested in torque. So, rpm is not important, but torque is important than having more blades is useful because you can get more energy from the wind that is going through that process and then push that use that to generate that torque. So, the older design windmills which were primarily aimed at you know grinding things or pumping up water, where torque was a critical thing they typically have more blades that are put up. So, while we have discussed all this, I would like to take the finish of this class by looking at a completely alternate design for windmills and which is going to be quite surprising for you and that is referred to as a wind tree. (Refer Slide Time: 50:16) So, what I want to draw your attention to is the fact that people are looking at totally different designs. So, what you see here it looks nothing like a windmill right. So, one of the objections to the windmill is that you know that you have these huge structures which are blocking up space and are you know blocking up your view I mean you go to a nice beautiful landscape, you look around you what you would like to say you know let’s go let’s say you go into the dunes of Rajasthan or someplace and some lovely place you go you want to see nature in pristine condition. You see the nature and then ever so, often you see this big windmill that is the parked there which you know is not natural I mean you see this huge manmade object that is parked there, it seems to become you know an eyesore. So, to speak in that landscape and the more you know deploy these windmills you are going to see more and more of them. So, people have come up with some interesting design where it does not even look like a windmill. So, it just looks like a tree. So, I mean, of course, this is coloured artificially just to you know to give you an idea of what is expected here, but basically, what you have here you have a serious of vertical axis small vertical axis windmills, wind turbines. These are all vertical axis wind turbines and you can make you know this is just a schematic, you can make like a tree with several branches that look very natural and you can dot you can set it up in streets and whatnot or in some open parking areas that are enough breeze blowing. And so, these windmills are rotating this way okay. So, that’s how they are rotating, and in that process, they are generating electricity. So, instead of having one large wind turbine, you have a lot of small wind turbines which are continuously interacting with the breeze and generating electricity, and they look and the whole thing looks nothing like you know like a wind turbine, and you know distributes it makes the whole you know deployment process very easy, you don’t have to create this huge massive structure that you have to go and put up. In many ways, it is not very different than putting up a lamppost. So, you can put up a lamppost on top of it you can even put up this tree if you want and you can keep generating electricity. So, this is a very lovely concept and since this is not solar power you can do this during the day during night etcetera and it works fine okay. So, those are the interesting design aspects that I wanted to discuss today as we close our discussion on wind energy. (Refer Slide Time: 52:27) So, in conclusion, I just wanted to say that the lift based design is more efficient than the drag based design, indeed can have tip speed ratios higher than 1 and can capture more energy than the drag based designs. For power generation, rpm is important and even one blade may be sufficient if torque is not important and boundary rpm is important, one blade may be sufficient. For other mechanical purposes, torque is very important, and in that case, multiple blades may be desired. In any case in all these cases, you want to check for the tip speed ratio is and then understand that you are you know not overdoing the number of blades that you can you are effectively using you know interaction between the blades in the wind. So, that’s something that you have to look at. And finally, that I just showed you that there are very interesting alternate designs, you can check on this on the internet they have a lot of videos on this kind of wind trees you can easily just you know the search for it, you will see videos or animations of those wind trees. They look nice they look very similar to trees and you can maybe design one with that looks almost exactly like a tree and then generate twin. So, that’s alternate designs are their people are looking at all those alternative designs, and so, if you look years down the line if you say certainly wind energy is either everything or a very significant fraction of our you know energy supply, then you may have all these kinds of interesting gadgets distributed across our cities. So, that’s something that we can look forward to. So, in any case with this, I would conclude this class, and also our discussion on wind energy in subsequent classes we look at other technologies. Thank you.