Hello, in the last several classes we have looked at you know solar energy and wind energy. We focused a lot on those two because they have you know the considerable promise and it’s something that you can install easily relatively easily and you also see it considerably in you know it's required visibly there in many places around the world. And here people also consider you know those two to be quite promising in terms of the ease with which they can be deployed, the number of places they can be deployed, the flexibility of these technologies and so on. So, that’s the reason why there was so much focus on it and there is so much information also available on it and so on. Today we will look at another of our renewable energy technologies that people have worked on two different degrees of success and that is the ocean thermal energy conversion or OTEC it is as it is called ocean thermal energy conversion. As you know two-thirds of the planet is covered by water and clearly if you have energy there which you can tap that is a significant amount of energy that you can get access to. So, that’s the context in which we are looking at the ocean thermal energy conversion or OTEC. (Refer Slide Time: 01:32) So, in this class, for example, are learning objectives are to describe the principle behind the OTEC operation what is the basic idea behind it, because actually from the energy that is available in the sea you can tap it in multiple different ways. So, this is one of how you can sort of getting energy out from the sea and put it to use where you would like it to be put to use. And in the context of how it is done, we will also look at what are the challenges associated with this OTEC technology and also try to understand I mean, therefore, related to those challenges what are maybe some limits associated with it or even some theoretical limits associated with it. So, this is the general context in which we will look at today’s discussion. As I said two-thirds is covered by water there is a lot of energy that’s available in the water and at some fundamental level, just the way you talk of you know energy available in the wind system. The energy that’s available in the water system is also at some fundamental level coming from solar energy. So, the sun beats down on the earth and then heats the land, heats the water, heats the atmosphere and because earth is a sphere we have you know one side facing the sun, one side not facing the sun. So, in any, you know at any given instant of time. So, there are locations which are receiving a lot of heat there are locations which are not receiving a lot of heat this is simply based on the time of the day. On top of it based on the latitude and the season you will have you know some regions receiving less sun and sunlight and more sunlight, during winter and summer and so on and as a result, there is a lot of variation of thermal energy across the planet and there is a lot of variation of the thermal energy that is held in the water across the planet. This, therefore, results in you know differences in density in the water. So, you can have water moving up. Moving down you have a lot of water currents that are existing and on top of it. So, you can potentially you know capture energy from just these water currents if there is a way in which you can do that. And then on top of it, there is also the temperature difference issue. So, if there is a way in which you can utilize the temperature difference between the region that is heated and between water that is heated and water that is not heated and that if that’s the energy that you can tap then that is something that you can utilize. So, the ocean thermal energy conversion as the term indicates you have the word thermal there. So, we are primarily looking at temperature difference. So, the temperature difference is what we are trying to capture here. So, the difference in T in what I just discussed with you there is a temperature difference based on the region that we have, but we can’t pick up the heat from you know the dark side of the planet and the bright side of the planet and then capture the heat to generate a run a heat engine. So, that is not something we are not talking about today. So, there is another way in which there is a difference in heat that is held in the water. So, that is what we are going to look at. In technically if you are out in a satellite you could do that in a satellite you could have the dark side of the satellite and the side of the satellite facing away from the sun and side that is facing the sun, potentially you could run a heat engine there and generate electricity. So, there you could just tap you know the hot side and the cold side, but there you won’t have convective heat transfer, you only have radiative heat transfer. So, within whatever is possible there you can do that. So, this is something that can be done. But on the planet that’s too big, you know the location for us to pull that off. So, we are looking at a thermal difference that is present in a much more local geography local location region, but there is still a difference due to some other parameters. So, we will look at that and on that basis, the temperature difference is being utilized to generate electricity to do a bunch of other things and that energy is being taped. So, this is the OTEC idea. (Refer Slide Time: 05:42) So, what’s the idea here? As I said it’s not region difference it is the difference in temperature between warm water at the surface. So, sunlight comes and we will see that detail there. So, at the surface, there is warm water and much colder water that is approximate, 1 kilometre below the surface ok. So, 1 kilometre is really not that large a distance, but in the context of water that may be significant, look at that in just a moment, but basically, if you look at seawater you have water at the surface and that is warm and then there is significantly colder water about one kilometre deeper into the ocean. And if you can pull that water, so you don’t have to put some other geographical location you are in the same location in that same location if you can pull the water from about 1 kilometre. So, you have to put a pipe down about 1 kilometre-long pipe and then pull the water up once you pull the water up you have cold water available with you, and you also have warm water from the surface of the sea and you can use those two to run a heat engine and in that process, if you generate electricity you know to extract the heat engine heat extract energy from that heat difference in temperature that is what this OTEC is all about, so ocean thermal energy conversions. So this temperature difference between what is at the surface and what is much below the surface about a kilometre below is of the order of 15 to 25-degree centigrade ok. So, this varies from region to region and there, there may be other factors contributing to it, but we are looking at a temperature difference of somewhere between 15 and 25 degrees difference ok. So, this may not be much I mean doesn’t seem like much and. It is not much we will see some numbers relative to this. So, as suppose to the kinds of the temperature difference that let’s say you know engines in our car or trying to utilize etcetera, this is not much, this is a much smaller temperature difference much more modest temperature difference ah. Even relative to say the temperature difference say thermal power plant is trying to use this is not of that order at all it is just that it is abundantly available. I mean it is just you know potentially just there infinitely available and therefore, it can be taped. It’s of course, renewable because you know it is happening in the time scales of in long these are temperatures that are equilibrated in a very large body of material over a long period and they will continue to keep getting equilibrated at those temperatures with time. (Refer Slide Time: 08:07) So, what is the thermal profile of seawater? So, this is of interest to see because you understand what is actually happening in the system and therefore, understand the context within which we can you know tap this energy for example. So, when sunlight arrives at the sea surface of the seawater mostly it is a first 20 meters that absorbs sunlight so in fact, even when you look down into the sea I mean you only find a few places where water. So, clear that you can see a few meters down and you will see the sand below right invariably it is kind of dark after some few meters it is very dark. You don’t see anything if you just stand at the beach and you see the water in front of you will see it light I mean based on which part of the world or how deep the water is and of course, depending on how clean the water is, what you know turbidity is a whatever other factors are there is involved in it you will see a certain level of clarity in the water, but beyond that, it becomes dark right. So, it also means the light just penetrates that far and at that point, it just its all absorb. So, the sunlight more or less in first about 20 meters of depth which is you know not a whole lot it is like say the depth of say two rooms that you may typically be using. So, in about 20 meters of depth, the sunlight gets absorbed ok. So, this gives this surface some temperature. So, this temperature varies from region to region. So, at the extreme when you got closer to the poles you have a temperature, the surface temperature of the water of the seawater that is there could be very close to freezing conditions you may even be frozen or very close to freezing conditions at the polls. But if you go to some warmer region, so the Persian Gulf is considered someone amongst the warmer regions where the seawater is quite hot you can see temperatures of the order 36-degree centigrade at the surface of the water, so in the seawater. So, at the surface itself based on the region, you have a lot of variation and this is primarily got to do with how much sunlight is arriving at what rate it is arriving there in the sense basically what is the power it is delivering there and if it is more obliquely inclined the at the polls that the power being delivered it is a lot less and you may even get it delivered for 6 months of the year and therefore, it becomes very cold there. So, but then you have hot places at about 36 degree C. So, now, if you start in that 20 meters whatever is there that heat energy that has been captured there then as a result of you know both the turbulence caused due to you know mixing of that water because you have now, hot water and you have slightly colder water below it. So, there some density variation you start seeing some mixing going on. You also have a lot of when you go and stand in this sea when you go and stand in the beaches, you see all the waves coming to you that is got to do with the interaction of the wind that is blowing and the surface of the water. So, that surface keeps getting churned up quite a bit by the wind that is blowing across. So, generally, you know water both due to all the activity that is happening at the surface and all you know to what degree that energy transfers downwards variation in density etcetera, you have a lot of mixing going on for a few 100 meters ok, of the depth of a few 100 meters. A significant amount of mixing is going on, starting both due to this energy that is coming from the sun also due to the action of the wind on the surface of the water etcetera, a considerable amount of churning around and mixing and what on is going on for the first 100 meters. So, first few 100 meters this happens. And then you have below that a boundary. So, this top layer is called the mixed layer and from there you have a boundary that boundary itself extends for a few 100 meters ok, it’s not a very sharp boundary, but it relatively sharp in the grand scheme of things, but few 100 meters of the boundary is there in which there is you know this the level of you know disturbance comes down and then below this boundary there is a relatively undisturbed water ok. So, we have first 100 meters also which is very turbulent and then few 100 meters you continue you come through a boundary below that you have a boundary and that boundary itself lasts a few 100 meters across which this you know disturbance steadily cuts out and then you are at one point it comes completely undisturbed. So, there is water below the boundary that is undisturbed. This boundary is called the thermocline. So this boundary is called the thermocline and below that, you have water that is largely undisturbed and has been you know at certainly not disturbed with relative to whatever water action is going on top. So, this thermocline it turns out that you know more than 90 per cent of the seawater, more than 90 per cent of the seawater exists below this thermocline. So, the volume of water below the thermocline is significant so the surface water which is disturbed and you know mixed up and having higher temperatures etcetera is a very small fraction of the total water that’s available in the sea. So, that is only 10 per cent of the water that is available in the sea right and all the 90 per cent of that water that is a is now below the thermocline where the temperatures are very low and of the order of 3-degree centigrade ok. So, that temperature is there. So, very close to freezing temperatures significantly below the depth of you know from the surface. If you go, so maybe about a kilometre or so, below you go and then it becomes this cold. So, this thermocline is also quite important from various other you know if you read you know books related to say submarine and how submarines operate and so on. This is also considered kind of important from that perspective in terms of how you know sonar behaves there, how calm the water is or how disturbed the water is all these things are related to the thermocline and so, this is also relevant from military aspects so to speak. We are not looking at it and that is not focused here, but for us, the idea is that that this layer below which the water is undisturbed and this layer is called the thermocline. And more specifically we have a temperature of know at depending on where you're above the thermocline close to the surface you have a temperature of 36 degrees C, below the thermocline, you have a temperature of 3 degrees C. So, in this case, we are looking at about nearly 30 degrees, 33 degrees C difference, but this is like an extreme case as I pointed out mostly it is somewhere between 25 and 15 degree C. On average in fact, it’s about 17-18 degree C across the water that you are going to see at various places various coasted areas, but you can have anywhere from 25, 15 to 25 degree C difference. (Refer Slide Time: 14:51) Okay, so this is work that we have to do concerning the sea right. So, this is this technology by definition is something that is associated with the sea with how you can operate something in the sea. So, what are the challenges there, right? So, and we are also looking at as I said we are trying to get cold water and you know to find a way to interact with warm water that is on the surface and process generates energy. So, you have to get that cold water and that cold water is about a kilometre plus deep you have to go further down and then you get that temperature. So, so, in fact, if you plot temperature right and depth, so if you are on the surface and let’s say this is 1 kilometre and this is further down. So, you are looking at say let’s say this is 36 degrees C, this says 3 degrees C. So, you look at something that is like this. Some profile like that this is just a schematic of a profile. The point being it’s not very linear it’s not a very linear profile and you are going to see a significant variation of temperature in this region and then virtually no variation of the temperature below ok. So, here there is the lower region not much in this region right. So, in the lower region there is not much variation in T on top there is significant variation in T. So, it is certainly a non-linear behaviour coming from top to bottom. So now, we have to work with water that is at the surface and work with water with that significantly below the surface which is about a kilometre below. So, just to give you an idea of how complicated it gets working with seawater and especially water that is deep in the sea. If you look at what is you know people have explored the surface of this you know the bottom of the sea bed across the earth some kind of mapping has been done to understand what at what depth you have the sea bed etcetera. So, the deepest point that they have seen in the sea bed is the marina trench ok. So, the marina trench. This is about 11 kilometres deep. So, you have to go 11 kilometres deep into the sea to reach this point it is somewhere near japan somewhere in that area it is called the marina trench. And people got know about this quite some time back that they were, they knew they were aware that this place existed. So, in the if you take the last you know say 100 years, where people have tried to do various things you know in terms of exploration, trying to reach places that others have not gone to etcetera and try to push boundaries of what human people have humans have achieved over the period. Then if you look at the marina trench which is 11 kilometres deep in all this time that people have worked on trying to get there with all the technology that has existed and so on till very recently it has been visited by a total of 3 people ok. And these 3 people did not even step out at this location they were inside a particular kind of vessel which and stayed inside that vessel and that vessel reaches this point, this marina trench it reaches the base of this location and they were able to come to this point. So, they sat inside this vessel and this vessel went deeper and deeper and deeper into the ocean went 11 kilometres deep, reached the bottom of this place a called the marina trench and they were essentially there. So, 3 people did this, 3 people managed to visit this place it includes one of these you know film directors of the movie titanic is visited this place James Cameron he is one of the people was visited this place and so that’s it there are 3 people gone to this place. Now, if you contrast that with the moon that is 384,000 kilometres away ok. So, not half a million, but approaching that you know approaching half a million kilometres that kind of a distance okay. This has been visited by 12 people. 12 humans have landed on the moon ok. So, 12 humans have landed on the moon and not just landed on the moon, they were able to come out of that spacecraft, of course, they were wearing a special space suit they were not you know wearing our kind of clothes they have a special space suit which you know moderated, their temperature and atmosphere that they could breathe normally etcetera because there is no air there. But they were able to step out of the spacecraft wearing that suit walk around on the moon ok. So, 12 people, 12 human beings have set foot on an object that is outside earth these are the only 12 people who have ever done that ok. So, we have only explored the planet these are the only 12 people who have 12 human beings who have ever stepped on any object outside of earth, and you know any other celestial object other than the earth. So, only 12 people have done that. But 12 people have done that, they have been able to land on the moon, they may able to get out, walk around, even drive vehicles they have had some lunar modules something rovers they had they drove those vehicles on the moon. They did various things various activities that humans could do on earth they try to mimic doing all those activities there. So, this is 384,000 kilometres away. And you know the amount of difficulty it is you mean every time there is a rocket launch people are. So, anxious that everything has to go right. So, many technical challenges are there it’s by no means easy. Even to spend an unmanned you know satellite to space it is an extremely technically challenging activity there are only a few countries, even today there are only a few handfuls of countries that can regularly send unmanned satellites India is of course, proudly one of those countries that can do so quite regularly, but still that is the only handful of countries that can do that. In that, the man-machine has been done only by 3 3 countries in which only 2 have prominently done it only 1 the china has just started doing it. But only the Soviets and the Russian people of the Soviet Union and the Americans have been able to do manned machines regularly and in that, only the Americans have managed landed on the moon. So, has been quite a complicated set of activities that are there and that is very complex, very difficult, very challenging and a lot of danger there. People have died in the process of trying to land on the moon. So, the very first Apollo we are blowing up, I mean the very first such rocket. So, which was on a test mode blew up I mean or the people were trapped in a fire they got killed. So, it is quite a dangerous activity, it is hardly a safe activity, with despite all those challenges, despite that challenges of you know what is required to get out of the gravity of the earth to pull out do so many complicated things have to be done before you reach the moon. Despite all those challenges 12 people landed on the moon okay. So, that is the phenomenon of accomplishment. But contrast that with the marina trench that’s just 11 kilometres, 11 kilometres is nothing. I mean 11 kilometres if you were on you know road outside 11 kilometres would take you no point I mean if you and if the traffic were flowing you would probably cover 11 kilometres you know if you were doing say 60 kilometres on earth, then every 6 minutes you would be doing 10 kilometres right. So, 6 7 minutes is what you are looking at to do 11 kilometres, I mean something like that you know in under 10 minutes in about 10 minutes on a road you would cover 11 kilometres based on the traffic conditions. So, 11 kilometres is not at all a significant distance in the context of human activities. In the context of human activities, 384,000 kilometre is a significant amount of distance it will take you forever to travel this distance if you had done so, still, 12 people managed to reach the moon step out, walk around, do various activities only 3 people managed to reach the bottom of the marina trench and nobody stepped out, nobody stepped they just sat inside that vessel looked out through whatever port was available in the vessel and they can right back up ok. So, they just took pictures some inside the vessel, they saw through the port, they flash the light around and then came right back. So, this is all they were. So, what is the challenge there? The challenge is the pressure or the pressure difference ok, the pressure difference that is the challenge. So, what is a challenge? When you go from the earth to the moon even though it’s such a complicated activity so many dangers and so many difficult things involved that pressure difference when you got from the surface of the earth into space is one atmosphere ok. So, the difference in pressure, so earth to space. So, one difference in pressure meaning, when you are on the earth you have one atmosphere of pressure that you are experiencing that when you stand you experience one atmosphere of pressure. If you just lift off from earth and come out into space there is essentially vacuum out there. So, there is the pressure has dropped to 0. So, you are used to one-atmosphere pressure, pressure outside is 0. So, whatever you know vessel that you create whatever space ship you create should maintain one-atmosphere pressure inside the space ship while facing 0 atmospheres on the outside of the space ship. So, the wall of the space ship we will see one atmosphere inside it will see 0 atmospheres on the outside right. So, that’s the difference. So, the difference in pressure across the wall of that space ship is one atmosphere. It has to see one atmosphere inside 0 atmospheres outside. So, that is the difference and that space ship should stay intact in that difference it should not break up, it should not explore you know you blow air into a balloon, why it is expanding its expanding because the pressure inside is more than the pressure outside or at least it is initially it if you just maintain it like that pressure is higher. So, it expands still the air pressure outside as well as you know the elastic properties of that balloon together counteract the pressure inside. So, that’s how it takes some shape, but if the balloon is weak it will explore right. So, that is because there is a pressure difference between inside and outside. Same thing, if the space ship which is a manned machine should enable people inside to lift. So, it has to have oxygen it has to have if it is just air it has to have a mix of oxygen and nitrogen and the pressure should be one atmosphere if the pressure is too low you will be struggling to breathe. So, pressure should be one atmosphere because our bodies used to that pressure outside is 1 0. So, that pressure difference across the wall of the vessel is 1 atmosphere. Now, from the surface of the earth into sea one-atmosphere pair 10 meters ok. So, remember it is not just one-atmosphere pressure difference it is a difference of pressure is one atmosphere for every 10 meters that you go into the sea ok. So, you started the surface of the sea and you go down 10 meters depth you go the difference in pressure is one atmosphere. So, your body is now, used to one atmosphere, but the pressure outside is two atmospheres ok. So now, if you want to compare spaceship that has landed on the moon with something underwater and you want to create a situation that is very similar, similar to a spaceship sitting in the moon and is an underwater vessel. So, what is the situation ah? The pressure difference across this one atmosphere and pressure inside one atmosphere. So, in the moon pressure difference is one atmosphere between inside and outside and the pressure inside is one atmosphere. If you go down just 10 meters you have recreated the same situation except that the pressure outside is excess there the pressure outside is 0, in the sea, you just go down 10 meters or anywhere even you know in a deep enough swimming pool if you go down 10 meters your body is feeling the pressure of one atmosphere of air and 10 meters of water the 10 meters of water is equal to 1 atmosphere of air. So, two atmospheres you are facing right.
So, just 10 meters down the vessel already faces the same situation that vessel that is landed on the moon faces. So, you can now, imagine if you got on 11 kilometres ok. So, 11 kilometres is 11000 meters and it is one atmosphere for every 10 meters. Therefore, pressure differences total pressure differences 11000 divided by 10 more than 1000
atmospheres the difference in pressure between the inside and outside of that vessel is more than 1000 atmospheres. So, that is why even though this is only 11 kilometres and technically you know on a straight road you will reach this in about 10 minutes, you would never do it in 10 minutes if you are going under the sea and the pressure difference you faces 11000, I mean 1000 100 atmospheres that are 1000 times more difficult from just the pressure of perspective of pressure this is 1000 times more difficult to handle than the than landing on the moon. If do not look at any other technology change the scenario completely I am just saying concerning pressure going 11 kilometres under the sea is 1000 times more difficult than reaching the moon ok. So, that’s the point and that’s a reason why this is a problem. And the OTEC technology requires you to deal with water that is at least 1 kilometre down. So, therefore, you have to send a pipe down there, 1 kilometre down you have to send a pipe you have to work with the fact that there is water is moving the topwater is moving there may be some currents of water below. So, you have to work with all that ok. So so, there is a lot of challenges associated with dealing with water pressure is a major challenge, depth is a challenge, water currents are a challenge. So, these are all extremely difficult to deal with and therefore, OTEC even though people have been working on the successful demonstration that has lasted a period have been very few where they have managed work with it. (Refer Slide Time: 29:18) So what is happening here? So, you have as you recall 175 petawatts of energy coming from the sun. So, that arrives into the sea and then it heats the top surface of the sea. So, we said you know say let’s say this is the top 20 meter. So, this is one kilometre. So, the top 20 meters is a very thin layer. So, in this context, this is a very thin layer. So, we are looking at this is 1000 meters. So, this is 1000 meters. So, 10 per cent of this would be 100 meters. So, you are looking at 20 meters we should just be this much ok. So, to the first 20 meters is where you are having much of the heating that is going on and then below that you have actually you know several 100 meters over which things are cooling down quietening down and then when you go down, 1 kilometre down everything is quiet. So, this is 1 kilometre and that is where you have this temperature difference of 15 to 25-degree centigrade. So, what are we doing? In principle, this OTEC system which is what we have here simply taken water from the surface of the sea and water from deep below and both of them arrive here and with this, we generate electricity. So, this is the basic idea of the OTEC. So, you have picked up water.
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