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Module 1: The Main Source of Clean Energy

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Solar Flat Plate Collector

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Hello there, Namaste; [FL] and welcome to this Southern Indian state of Tamilnadu; the city of Chennai. I am sure you are aware that Tamilnadu and Chennai are popular for many things ancient temples, South Indian food and of course, the ancient language of Tamil. One of the things we are very well known for is the amount of sunlight we get here and the amount of heat that we get here through our summers which are very hot and humid. In this class we are going to look at solar energy and some ways in which we can harness it, capture it and use it for energy in our day to day applications. So, that will be the focus of today’s class. (Refer Slide Time: 01:07) I hope you are ready; I am sure this is familiar to all of us, this is our planet where we live. And briefly as we look at solar energy let’s get some idea of what it represents concerning where we are on the planet. So, here we are in India that’s our country; this is our equator and due north of the equator some way up north of the equator, we have tropic of cancer. So, if you look at India, for the most part, a fair bit of India is above the equator and below the tropic of cancer. So, in our seasons that we see through the year; we get a lot of sunlight through summer we get a considerable amount of sunlight. And in most places in India, you are going to have the sun beating down on us through the summer months. Even during winter in some places in India, it doesn’t become too cold; it only becomes marginally cold and so the potential for us to pick up solar energy and utilize it is very high; relative to many other places in the world. Of course, if you are looking at the inclination of the planet, then during the winter months we are a little bit up north and the sun is a little bit concerning the axis of the earth it’s a bit below the equator and so we are getting sunlight in a little bit of an oblique manner. But as the earth goes around the sun, we will soon come to an orientation where the earth tilts this way and then the sun is more or less vertically down on us. And so there are months in the winter half of our year; that basically, the sun is coming to us in an oblique angle and during the summer it's coming more or less vertically down on us. So, this is the range of sunlight that we have; some places in the northern hemisphere as well as the southern hemisphere, as the world goes round the sun; sometimes for a certain number of months in the year, they will receive no sunlight. So, if you look at the regions which are closer to the poles; both the north pole as well as the south pole, there are places where you are not going to receive any sunlight for some months in the year. And this means that those are places where you cannot utilize solar energy throughout the year whereas, we are relatively fortunate from that perspective that we do receive sunlight throughout the year. Of course, for the most part, we grumble about it when we are out in the hot sun saying you know it’s so hot and that is why you need to have some coverage for your eyes and as well as for your head. But basically, it is something that we should not look at as a disadvantage, it is something that we should look at as an advantage. It is something that we can capture, utilize and really benefit from and so it is something that we should apply our minds to and spend some energy trying to harness this energy. So, this is what we are trying to do in the next several classes we are going to look at solar energy. And today I am just going to show you one particular manner in which we capture solar energy. Broadly solar energy consists of thermal energy and a photovoltaic kind of energy. In other words, you can capture solar energy as heat; you can also capture solar energy as electricity. So, these are two possible ways in which you can capture solar energy for use. In today’s class, I am going to look at the first version which is the thermal energy; how we capture thermal energy. And in particular, I am going to look at a particular manner in which we capture this solar energy. So, that is what I am going to describe; briefly going to set up something that looks like the unit which will capture solar energy for us. And I will explain various aspects associated with how that unit functions? What are the parameters that we have to keep in mind as we utilize that unit? And what are some you know limitations and capabilities of that unit? (Refer Slide Time: 05:04) So, we have solar energy coming down to our planet and if you look at the top part of the atmosphere of our planet what arrives from the sun is about 1400 watts per meter square. So, this is some calculation that we did in our class; so you have seen how it comes down to about 1400 watts per meter square or about 1.4-kilowatts per meter square. So, if you look at the earth’s atmosphere and the various layers of the earth’s atmosphere; about 30 per cent of what arrives at the top part of the atmosphere, actually gets reflected away. And so only about 70 per cent of what arrives at the top part of our atmosphere comes down to our planet which is what we feel when we step out in the sun and you have you know this harsh sunlight coming down on you or the heat that you feel as you walk down on a road without any cover. The heat that you feel coming down at you is about 70 per cent of what comes to the top surface of the atmosphere. And so, if you do the calculation 70 per cent of about 1400 watts is about 1-kilo watt. So, 1-kilo watt per meter square is what we feel on the surface of the planet. And so that’s a number that is relatively easy for you to remember; it is just a number 1; 1-kilo watt per meter square; that’s the amount of solar energy or the intensity with which the solar energy comes down to our planet. So, that is what we will feel on the surface of the planet. And, if you are trying to capture solar energy; that’s the kind of energy that we are trying to capture; so, one of how we do that is to use something that is called as a flat plate collector okay. So, it’s a flat plate collector it’s easily the simplest way in which you could capture solar energy. So, what we are going to do in the rest of this class is to look at this flat plate collector. And I am going to show you various parts associated with this flat plate collector, how it is put together? And give you some brief idea of you know how the energy is captured. So, as the name suggests the main part of this collector is a flat plate collector. So, if there’s something that’s a flat plate; so, I have here a flat plate, it is it’s a copper sheet that I have here. You could use any material; potentially you can consider using any material for doing this process, but clearly, we have some material restrictions on what can be selected. So, the point is that first, you have solar energy coming down on this surface. And this energy has then got to be picked up by something that is a heat transfer fluid of some sort. Usually, we use water or we use air; so it often the solar heaters; solar thermal heaters are trying to do one of those two things. They are trying to heat water or they are trying to heat the air; so, many of the units that we buy for our homes or maybe even the hostel that you stay in or dorm that you stay in; those are units which are typically trying to capture solar energy for heating water. But if you go to many of the food industry applications there; they use hot air to dry various items. So, there the solar energy is used to heat air okay; so broadly those are the two different things that; we are trying to do with solar energy heat water or heat air. And I will start by first describing what you would do to heat air am sorry water and then we will also add the detail that is required to understand how you would go about heating air. So, the first thing that we need is a flat plate and as I said you know you could use any material. But first of all this, the heat that falls here should get absorbed by this material easily and it should get transferred by conduction; it typically it is going to get transferred through this material by conduction to the heat transfer liquid which is or fluid which is either water or air. So, since you need it to get conducted from the location where it is falling to the heat transfer fluid; the material that you select needs to have good thermal conductivity okay. So, that’s a very important criterion for this material, so you could consider using something like if you were to use a thermocouple board; that will not work because that has very poor thermal conductivity. So, you need something that has good thermal conductivity. So, typically metal is a good idea to start with and a good thermal conducting metal is copper. You could use stainless steel and so on, but copper is typically much better conductor than stainless steel. Of course, there are better conductors than copper, but then it becomes a question of economics. So, if you were to use silver that would be even better than copper, but that’s a very expensive material to use. So, typically we work with a copper plate; I will also add that this copper plate is since it is the colour that it is also going to reflect sunlight. All metallic surfaces tend to be very reflective that is a property of the metallic surface and so that is not a great feature to have in a unit which is trying to capture solar energy. You don’t want to reflect the solar energy, you want to capture it and therefore, having a shiny surface is not a good thing for a solar capturing unit okay. So, therefore typically even though they use copper and which is what I am going to show you through this class; in the actual implementation; they will paint this with something black in colour. So, it will be coated with something black in colour to enhance the extent to which it is absorbing the radiation and reduce the extent to which it is reflecting the radiation. So, even though I will show you a copper version of it; please keep in mind that at the end of it; it’s all going to be black, the unit that you see in a lab setting or a commercial setting will typically be black because that absorbs all the radiation that is falling on it. So, what we are going to do now; are we are going to assemble some of the parts that go in towards making a flat plate collector. So, primarily on top of this plate, there are going to be some tubes and those are the tubes through which the heat transfer fluid is going to flow. In this case, it’s going to be water. So, I am just going to assemble those tubes and then I am going to talk a little bit on how the system works and then we will talk about how it works as a unit to heat air. So, we have with us some of the parts for this flat plate collector. So, we have the copper sheet as I said and we have a bunch of copper tubes here; four copper tubes that I have. And basically, all I am going to do is; I am going to place these on this copper plate. So, you have here one, the second one is out here; roughly evenly distribute them; so that’s three and four. So, basically in the real setting, this would need to be fixed to the plate and in fact since I am just doing a demo here; I am going to find a simple way to get them to stay on the plate. And so I am just going to use a sticky tape here to get them to stay there and so first we will do that. The idea is that there has to be good contact between the plate and the tubes that are there on top of the plate. And therefore, it needs to be fixed pretty well to the plate only then any heat that is coming to the plate will then also arrive at these tubes. So, here for our just for our explanation; I am just using a sticky tape to get them to stay in position so that I can explain the process to you. In reality, they would either weld it together so that it stays put and also improves the kind of contact between the plate and the tubes. Or you could also use instead of this kind of a tape; you could also use some kind of a good heat-conducting compound which could then be used to hold this together. So, you do have certain compositions of these compounds; so, for example, those of you who are familiar with how to say desktop computers are used. Often you know that there is a specific part called the heat sink; which is then fixed on top of the processor in the computer. And this heat sink needs to stick; convey the heat away from the processor so that the computer doesn’t processor doesn’t heat up too much. So, this is a part of the process that is typically there in the computers that are present; the typical desktop computers that are present. And so that is a pretty common requirement in most of these cases. So, when they do that; when they attach that heat sink to the processor, one of the things that they apply there is a heat sink compound; it’s called a heat sink compound. It does help the transfer of heat between the processor and the fan that is on top of it; so, that is done. So, that’s just a possibility that you could use, but that’s typically not necessarily the kind of item that is used, but the point being that you have to use something that has got good conductivity and can also hold these copper tubes to that copper plate. So, for example, I have now got this assembly here which essentially has these four copper pipes; which have been fixed to the copper plate. So, we have now moved from just a plate which is gathering heat to a small assembly here. Just you know somewhat crudely put together, but primarily to demonstrate the process; where essentially you get heat on the plate and it absorbs the heat. And then it transfers the heat, so it laterally transfers the heat. That laterally transferred heat comes to these pipes and then when you have water flowing through these pipes, they pick up that heat and then they can be stored. So, in a typical application, you will have a feeder tube here and you will also have a tube out here; which would then pick up the water. So, usually, you will have cold water at the bottom and it will just gradually as it heats up; it will move up this pipe and it will come to the top and then from there it gets gathered. You may have a tank a separate tank in which the hot water collects and then the water is piped from the tank to the bottom. So, if you just set this up as the day progresses; it keeps getting hotter and hotter the water keeps getting hotter and hotter. And typically this kind of a unit with the black colouring is what can get used as a flat plate collector. So, there are a few more aspects of it that we have to keep in mind; when we talk of this kind of a collector. There is at least one more critical part that is not yet here; which is what I am going to show you now and then I am going to discuss what that part does to this unit. So, you will typically find that wherever you go and you look at this kind of a unit. First of all that it’s not copper its black, but there is a glass sheet on top. So, for example, I am just going to put a glass sheet on top of it just to show you how it is going to look. And then we are going to discuss what does that glass sheet do for us? So, that’s what we going to discuss in the next few minutes. Okay so, here we have a glass sheet and this is the unit that we have put together so far. So, in the typical application; installation you are going to have this glass sheet sitting on top of this unit okay. So, this is how the unit would look like there will be a glass sheet on top of this copper unit. The purpose of it is not just cosmetic; it has got a very specific application which is why this glass sheet is put like this. And the basic idea is that when you have sunlight falling on this unit right; it goes in and it heats those the copper plate and the copper tubes. Now any hot surface; any surface that is not at absolute 0; it is giving out energy as well and so when this is hot; it is typically giving out energy in the infrared part of the spectrum. So, heat is leaving this unit in the infrared part of the spectrum as radiation and so that is not something that we want we want the heat to remain here; we don’t want the heat to be lost. And therefore, we want that infrared radiation which is leaving this copper plate does not leave the copper plate to get stuck inside this unit. And that is the actual application of this glass. So, this glass typically if you take a glass its property is that it is transparent to visible light, but it is opaque to infrared. So so, when you put this glass on top what you find is that even in the incoming solar radiation there is infrared. So, that infrared does not get in; so we are losing part of the spectrum by putting this glass on top. But once the radiation goes in heats up the water and heats the copper plate; the infrared that is coming out of it which is the loss of heat from that water is minimized or in fact, eliminated. That heat stays within this unit and just gets reflected repeatedly within this unit and therefore, stays within this unit. And therefore, the heat that you have gained in the system stays with the system and so that’s a very important application for this glass that I have placed on top. So, together this is the unit that would then actually serve; these are the main parts of the unit that would then serve as a flat plate collector. And this is very good at connecting heat for heating water to get hot water. (Refer Slide Time: 18:45) These are the parts of a flat plate collector you can see here the copper sheet. So, that is your starting point on these are going to be placed a series of copper pipes, which will be placed and therefore, any solar energy that comes onto the copper sheet is then distributed to the pipes closer to it. And then water flowing through the pipe picks up this heat. So, this is a flat plate collector that is being designed for picking up heat in into the water and. So, it is a water heater and so, therefore, it will have pipes as the conduit through which the water will flow. So, this is how those parts would come together and then we would also have an arrangement by which let’s say you will have incoming water that is available. (Refer Slide Time: 19:33) And then the hot water can be trapped from the tube on the top and the arrangement in the middle then does the heating of the water through the pipes that are there. And if you incline it such that the top side is inclined upwards; then the water flows in the direction that I have just pointed in this using these arrows; simply because the hot water will start moving upwards and cold water would remain at the bottom. So, in this process, you can get hot water to start collecting at the top and then take it away to a reservoir of some sort. This is all, of course, shown as a schematic here in copper colour. (Refer Slide Time: 20:24).