Loading

Module 1: The Main Source of Clean Energy

Notes
Study Reminders
Support
Text Version

Harnessing Solar Energy

Set your study reminders

We will email you at these times to remind you to study.
  • Monday

    -

    7am

    +

    Tuesday

    -

    7am

    +

    Wednesday

    -

    7am

    +

    Thursday

    -

    7am

    +

    Friday

    -

    7am

    +

    Saturday

    -

    7am

    +

    Sunday

    -

    7am

    +

But you would need to make it black so that the energy is absorbed completely and is not reflected away from this surface. So, it’s the same schematic here shown in grey in fact, here just for ease of observation of what it is, but in principle, it could be black. In addition to this complete setup, we would also need a glass plate on top; which is what I have added in this schematic that you see here. All the rest of it is the same as you just saw before except that to prevent a loss of heat through infrared radiation; from this flat plate collector we place a glass sheet on top which is what you see here, a glass sheet is placed on top. A cross-sectional view of this is shown in the figure below and basically what it shows you are; when you have an arrangement as follows, these would be your copper pipes, these would be the black coloured copper pipes. And this is your copper plate and the incoming radiation which will have a visible ultraviolet as well as infrared. So, from the high frequency that would be ultraviolet-visible and then infrared would arrive at this surface. Because this is glass; the infrared will not enter this surface it will just get reflected away infrared. But visible and a UV will go in and heat the tubes and the water inside. Once this heating is completed or as the heating proceeds as the temperature of this of the copper pipes as well as the copper plate increases; it will try to radiate out infrared because that’s the temperature it is at and at that temperature which says 60, 70 degrees C; much of the energy that leaves this system in the form of radiation; leaves in the form of infrared radiation. But since glass is non-transparent to infrared, it just bounces right back so it just bounces right back and therefore, the energy that has been captured is not lost. And that’s the reason why we place this glass plate on top; the glass plate also takes care of preventing convective losses. So, whatever gas you have; whatever air you have flowed on top, is unable to access the copper pipes and therefore, does not take heat away in the form of convection from the pipes that are below. (Refer Slide Time: 23:09) So, as an assemble setup it would look something like this; you would have the flat plate collector here which is consisting of the copper plate, the copper tubes; the glass tube sorry the glass plate on top as well as the supply and discharge tubes. And essentially it would be attached to the tank that you see out here and basically, cold water is accessed by the pipe at the bottom. And hot water is available at the top and this hot water essentially goes into the reservoir and eventually the water in the reservoir keeps getting hotter and hotter and hotter and hotter. So, this is the basic idea you sometimes see variations on this where you have a tank sitting right on top here. And that is typically what you see in the household situation. So so, for example, it would look something like that and you would have the connection to it out here. So, and that’s the kind of setup that you would typically see in a household situation put on the overhead tank, but that’s the basic idea of a flat plate collector. So, I am going to talk of a couple more things here; one is what needs to be done concerning the orientation of this unit. And also in terms of what needs to be done to extend this idea to capture heat for heating air; so, those are the two things that we are going to discuss. Okay so, we will now look at how we need to orient this unit so that we capture the sunlight effectively. So, to do that we need to consider the orientation of this unit concerning the seasons that we have and also concerning the time of the day, generally for a flat plate collector we are not so worried about the time of the day because it is flat. And so we don’t necessarily track it hour by hour concerning the position of the sun. So, for example, for me, this is due east I have due east out there and this would be due north for me and then that would be due south for me. So, generally, we don’t keep this such that it is pointing towards the east in the morning and then we keep tracking the sun through the day etcetera; you can do it, it is not particularly necessary to do it. It is much more important that we sort of track the seasons; so to speak. So, as I said this is due north for me and that is south the way I am presently oriented. So, for us sitting in the northern hemisphere; the sun is typically in the southern months towards the southern side. So, that’s the arc that it does through the southern part of the during the winter parts of our seasons. And then as the summer comes this slowly comes more vertical and then the sun is moving from east to west more or less directly above us. So, we need to orient this unit corresponding to that; so, for example, I have the pipes this way that’s not the way I would have to orient it given that the sun is going to be moving this way. And then through the winter, it is going to be down that way and then its slowly going to move up as the month's progress. So, we would have to orient it this way and have it pointed downwards. So, when you do that the as the sun moves from morning to evening as it moves from east to west; it keeps on heating the surface and based on the location of the sun; this is oriented correspondingly, based on the season and then you have cold water at the bottom and you have hot water which is the water that gets heated up slowly moves up the tubes. I mean hot water starts going up, cold water comes down and then eventually hot water starts collecting at the top. So, we would have to orient it like this and during winter as the winter comes in this is oriented more and more vertically.
I mean more and more up in inclination because it has to face the direction that the sun is oriented. So, let’s say once a month you would have to reorient this, so, that your position reasonably optimally concerning where the sun is located. So, if you are going to only change the orientation once a month; you can do this manually. You don’t need to have any sophisticated instruments to do it. As the summer comes from this you know fairly high inclination, you slowly start making it more and more horizontal and then in peak summer you are sitting in a relatively horizontal position. So, you gain a lot in peak summer because strong heat is coming straight down on this surface. So, it heats up a lot the only issue is that if it is relatively horizontal; then you don’t have a natural flow of water from the one side to the other. So, you would have to do something to force the water to continue to flow from one direction to the other to heat up. So, that’s the one small disadvantage if you are in the summer months here in say any place in India, where the sun would then be directly above us. So, that’s one thing that you have to keep in mind, but otherwise, the orientation would be like this and slowly with the onset of summer it becomes more and more horizontal. Again as it goes back to winter, you go back to this kind of orientation. And then the sun is always coming from our east and moving to the west, so through most of the day, this gets heated up. So, this is the type of orientation we would have to keep it in; as I said, of course, there would be a glass plate on top of this which I am not keeping it here right now for our ease of demonstration. But that is the basic manner in which we would have to set this unit up. I will also say that one of the other things that the glass unit does if it were on top of this. As I said the first thing it does is it prevents the IR radiation from escaping from this unit; that would just bounce back after hitting the glass. But the other thing it also does is; it prevents the free flow of breeze on this unit and therefore, prevents the loss of heat using convection. Convection with the form of the blowing breeze taking away the heat from this; so, that is prevented by this glass sheet that is on top of it if it is present there. So, those are the major parts and this is how the orientation is set up concerning various months of the year. We spoke about how this flat plate collector can be used to gather solar energy for water heating. I also told you at the start of this class that two major applications for this kind of flat plate the collector is to heat water as well as to heat air. So, in some ways you can argue that the same implementation can be used for heating air, you simply need to flow air through the tubes instead of water and of course, you would need a blower. Because that would ensure that you have a pretty good movement of air through this system, it would pick up the heat and then transfer it to some other location where you need the dry air the hot air. So, as I said one typical application is in the food industry where they are trying to dry say some various; food to prepare food items using the available hot air. But generally speaking, the ability of air to pick up heat is limited by the amount of cross-sectional area that it sees which is hot. So, you have to increase the surface area that it sees that is hot and it will then be in a better position to pick up the heat; otherwise, it’s kind of slow in picking up the heat. So, usually, a tubular structure like this is not as effective in transferring the heat that is gathered by this unit on to the air. So, in a typical hot plate I mean flat plate collector made specifically for heating air, some modifications will have to be made. The major modification is that instead of these tubes; you would still have copper. But instead of tubes, you would have it as fins; so, they would be vertical fins here. So, a fin here, a fin here, fin here and fin here. So, you can think of vertical pieces of copper of similar plate kind material which are standing at these locations. So, the general orientation would look the same, the general build would look the same except instead of a tube you would have a fin; you would have four fins here in this example. So, when you do that and you blow air from the bottom to the top; the air can pick up heat from the plate, it is also able to pick up heat from the fin. By doing this, you gather a lot of heat into the air and then you take the air away. You would still need this glass sheet on top which would then do those two things that I mentioned; which is prevent the loss of heat through infrared and also prevent the loss of heat to the outside atmosphere through convection. Inside we do one convection, inside the air is going to pick up heat through convection and so that convection inside the unit is necessary, but we don’t want that convection to also occur between this unit and the external atmosphere. So, those two things would be prevented by having this glass sheet in front which I showed you a short while ago. But beyond that, this would be the major other modification that you would have fins. So, this is how you would set it up, you would have a pipe coming in at the bottom which would bring in the air. And then it would distribute itself through this unit with a lot of fins and then those the exit air would also come out through a pipe which would then have the hot air and then that is taken for some application. (Refer Slide Time: 32:00) We looked at how a flat plate collector can be used for heating air and so, this is an implementation of how it will be used for heating air. You will notice that the main difference between the flat plate collector used for heating water versus that used for heating air is that previously we had pipes; we now have fins. Fins which are vertical sheets of copper placed the way they are shown in the figure that you see here. And the idea is that it increases the area of contact between the air that is flowing between these fins and the air itself; so and the fins. So, the air and the fins have a much higher surface area of contact relative to simply air flowing through a pipe. And as a result, you have much better heat transfer from this hot flat plate collector and the air that is above it. Even here you can see in the implementation on your right-hand side that there is a glass plate placed on top, it serves the same purpose that we previously described; which is that? It prevents IR losses from the plate, it also prevents convective losses from the plates. So, both radiation of infrared, as well as the convective losses, are prevented by using the glass plate and in this process, this implementation that you see here can do heating of air. Usually, one other major challenge in this unit is how you would store the heat. So, normally in the case of water you just have an insulated water tank which is holding the hot water. So, you have to think of an analogy an analogous situation which applies to the case of hot air. So, what is typically done is they also have something like a tank, but that tank is filled with pebbles; pebbles of different I mean of different sizes let us say and so there’s a tank full of pebbles; which is otherwise empty. And then you have this hot air go through that tank. So, when it does that those pebbles pick up the heat and they have good thermal mass. So, they pick up the heat and they hold the heat; so, you have a box full of pebbles holding hot the heat inside them. And so later after hours if you want to use the heat or later in the evening when the sun is not that hot you still want to use the heat; the air flows through those pebbles picks up the heat from there and then it is used for whatever application it is intended. So, that combination is the combination that we would typically use to heat air using a flat plate collector. There is one final modification of this which is simply that you know although I spoke about fins I told you that you can put four fins here. That is still a limited implementation because that is just four fins; if you want to greatly increase the amount of area through which you know heat transfer is occurring, ideally you want this area to be filled with something that is you know porous and that porous material would then take the heat. And so usually what we do is this is a something like a box; it is filled with say iron fillings. So, you would have a lot of iron fillings filings that you get off when you do all the machine work. So, you have some kind of I know the mesh-like structure that is out here and then the heat gets held there and so when the air goes through it; it has a lot of areas across which it can exchange heat. And that’s the heat that comes out through the top and you utilize it. So, that that would then be the implementation, you would have a box inside which you would have these filings and you would have this copper plate at the bottom, a glass sheet at the top and this heat is transferred. Of course, the more you know such filings that you put in there a greater is the resistance that the air will face; as it goes through this unit and therefore, you have to put in a little extra pressure. (Refer Slide Time: 35:45) So, this a little extra pressure drop between the inlet and outlet, you have to account for that as you transfer this heat onto the air and use it for some application. Heating of air can also be done in a much more effective way; if you essentially increase the area of contact between the air and the surface that is hot. And we saw how by going from a tube to a fin you have increased the amount of surface area of contact. But you can increase it dramatically more; if instead of just having a fin you have a lot of powdered material which is what you see inside this box. A lot of powdered material which is typically metal filings of different forms; which are available even as industrial waste and they are placed within this box. And so air enters the box from one side and then comes out from the box on the other side. But when it is inside, it travels through this box in various routes and finds its way through the box and comes out the other side. In this process, it picks up all the heat that is available inside that box and the box essentially has been heated through solar radiation with everything else being the same as what we are previously described; it is a copper box; copper at least the base is copper it is and the top of it is glass; the top surface is glass, the base is copper. And the copper has been blackened and you have a lot of these metal filings that are present, which pick up the heat and then transfer it onto the air that is forced through them using a blower. The air that comes out can be directly used for some industrial application, but at the same time if you want to store that heat; such that you can use it when the sun is not at its peak, then you also use this kind of a container that you see here which contains pebbles. And then this air is then fed into this container and then it finds its way through this container and you can have an exit. So, you can have an inlet here and you can have an exit here and then you can use this air for any other purpose that you wish. But in the process of it travelling through this container; it heats all these pebbles and these pebbles then store heat and so off-peak hours or later in the night if you want hot air; you simply have to send cold air into this container and it that cold air will pick up the heat from the pebbles and then will be available as hot air at the exit which can be used for some application. So, you can either use the hot air directly; you can directly use the hot air coming off this flat plate collector. Or you can use it to heat this container containing pebbles and then use that heat at a later point in time. So, these are different implementations of this idea of using a flat plate collector to gather heat to either heat water or heat air. So, that’s our class on flat plate collectors and the only other point to keep in mind is that because it is flat and is picking up you know energy from the sun roughly of the order of which is coming down roughly in the order of 1 kilowatt per meter square; the typical heating that happens when you put it in the form of saying a water heater; is that it will get you water from say room temperature kind of situation to say 60, 70, 80 degrees centigrade. So, that’s pretty hot water and so typical household applications that are more than adequate. So, you have like an equivalent of a 1-kilowatt heater if you just have a 1 meter square heating area and it will get you about 60, 70 or 80 degrees centigrade water. So, that’s quite adequate as I said for household applications. But if you want to do power generation; in which case typically you want to take the water to convert it to steam and then use that to run some turbine; if that’s the kind of application you are looking at, clearly reaching you know 70, 80 degrees C is probably not good enough for you; ideally you want to get steam. You want to steam and then that steam will be used to run turbines. So, we have to cross 100 degrees centigrade; so, we have to think of some ways to do that. If you had to use flat plate collectors, you would actually have to use a few of them in series and then you can you know take the hot water from one and then that would again get further heated up in another flat plate collector and so on. So, we can consider such options, but those are not efficient ways of doing it there are better ways to do it. And that is what would be referred to as concentrating the solar energy; so that is a separate topic and we will look at that in greater detail in our next class. But in this class, as I said we looked at flat plate collectors; how they can be used for heating water? How they can be used for heating air and what are its features? How it is put together? What are the major components involved? And also what are you know strong points and limitations of this kind of a design? Thank you.