 So, let us so this topic is very different than what you have been discussing so far, but if you want to understand the how much power you can get out of your solar cell or module, you need to know how much power is getting into the solar cell or module right. So, therefore, it is very important to understand the solar radiation and how much of the radiation is available and what form diffuse direct and global, how to estimate how does it varies with the climate and things like that and what should be the inclination of your module so that you can intersect maximum amount of solar radiation. So, invariably whenever you talk about solar radiation you talk about sun and I hope you know lot about the sun right. Anybody who has not seen the sun so far, everybody has seen very good and it looks something like circle right, it is a big and much bigger than earth that much you know right. What else you know about the sun other than this? Anybody have seen the raisins? Sun rise. Sun rise ok. So, this thing you know that it rises in the east and say it is in the west and etc. What else you know other than this the source of energy? High temperature. We are all live because of this it is a high temperature black body, very high temperature black body. What else? It is a thermonuclear furnace, fusion is taking place continuously takes place for some 500 crores of years before it gets exhausted of the source right ok, that is why ok. But that is ok if you know or if you do not know this thing you will anyway have sun rise every day, sensitive every day. But what is important to know is that how it is intensity varies and how it is position varies over the time, over the period of day and over the period of year right. So, you must be aware of all this Uttaran, Dakshina and all this all concepts are very important for this. You need to understand that and if you understand that then you can actually maximize generation of electricity using solar photo type ok. So, one thing that you can definitely do in very simple is to calculate the sun rise and sunset time right, what Pandits keeps on doing all the time, sun rise will happen at that time, sun rise will happen at that time, you can also do it it is very simple calculation that I will teach you in this class and make sure that from tomorrow every day morning you get up do the calculation sunrise time and check whether it is correct or not ok. So, some facts about so there are so one important thing to notice why there is a winter and why there is summer and spring and all, why do you think that happens? Sometimes sun is near to earth, sometimes it is far because sun the rotation of earth by the way not the sun is in electrical form and the earth is sorry sun is at one of its centric and the distances you can see between the sun and earth center and the distance is not same right. So, sometime earth is closer sometime it is farther when it when it revolves in a one year time, but there is not the reason for the change of seasons that is some reason for the change of intensity amount of relation reaching, but that is not the reason for the change of seasons, we will see why it happens. So, because of this because of this change in the distance between the sun and earth the intensity variation is only 5 percent, intensity variation is only 5 percent right, but we know from our experience during the summer and winter intensity changes a lot. So, this is definitely not the reason the distance the change in the distance between the sun and earth is definitely not the reason for the variation in the season. So, one thing before we proceed is one thing is and of course, the sun rays travels through the space and then it comes and heats the atmosphere of the earth and then sun rays travel through the atmosphere and then reaches the surface. While travelling the atmosphere some many things happens scattering happens some absorption happens things like that happens and therefore, the intensity of sun rays outside the earth atmosphere is always constant it is only while it travels different distances through the earth atmosphere it changes. So, how much is the intensity outside the earth or what is the power density outside the earth is known as a solar constant. So, outside the earth atmosphere they have less average solar radiation why we have taken average because the sun or distance also keeps on changing and the variation is 5 percent, but this number this number is average of all that radiation. So, outside we have 1367 watt per meter square that is the power density that you have outside the earth atmosphere or what we can call it extraterrestrial extraterrestrial and what is the number we use for the characterization of solar cell? 1000. We use 1000 watt per meter square right. So, so basically what we get outside earth atmosphere is higher than what we get at the earth surface what happens to remaining 367 watt per meter square? Get absorbed get scattered some of that may be getting reflected in what things happens and eventually what we get is lower than 1030 1367 watt per meter square. So, this solar constant and of course, it varies because the sun earth is revolving in a elliptical and the distance keeps on changing and basically you can from this equation you can estimate how much it will be varying. So, this I s c is this number I s c dash is actually earth or solar constant at any other given day which is given by this 1 plus 0.33 cos of 360 n by 365 what is n here n is the day of the year. So, n equal to 1 means January 1 n equal to 365 means December 31st. So, therefore, n is equal to day of the year if you put n with different n values you can find out how this number is varying over the period. But most of our calculation we do not have to really worry about this we just take this value and you can do our calculation. So, 1367 is the average value of the solar constant and we all are happy to use that value. Any otherwise variation is also not much high person from very right. Then depending on the temperature of a black body sun is a black body. So, depending on the temperature of the black body the radiation that it emits is not one particular wavelength, but it is range of wavelengths it is spectrum right it is not one wavelength it is a spectrum and that spectrum can be given by Planck s black body radiation model which is given here ok. So, this is playing black body radiation one so power emitted per for a given wavelength and then you have the h and c and lambda function of that. So, of course, you plot this what you get you get that kind of solar spectrum type of graph right you must have done in your physics 10 12 kind of thing. The plot of this Planck s black body model and what kind of spectrum it will emit and depending on the temperature it will have a peak certain peak and for so obviously in this slides I will show some of the one particular spectrum of the sun and where the peak is occurring. If you integrate this over the whole wavelength then you get the total power emitted if you integrate this over the you get the total power emitted and it is of course, have the proportionality of the temperature to the power 4 and if you look at the what kind of power density that is there at the earth sun surface is very high 10 so 11 watt per meter square huge ok. Thanks for 11 watt per meter space so just if you take a like 100 square meter this is good enough for the whole earth surface that kind of power density you are getting. The spectrum that I have we have already seen that it way the sun spectrum which is reaching the earth surface varies from part of ultraviolet visible and part of infrared right it varies from part of ultraviolet about 300 nanometer, 380 nanometer and this is the visible and then some part there here I have already corrected so this conversion of energy with the wavelength is 1.24 ok so when suns radiation travels earth atmosphere so what the solar constant is the power density available outside the earth atmosphere which is here how much we have seen it 1367 that is here right and that time the sun the rays has not travelled to any distance of atmosphere ok but then there is a condition when rays travels perpendicular and come to the earth surface ok. So if we call this air mass mass of the air right in the earth atmosphere if sun rays travels straight then we say that the air mass travelled is 1 air mass travelled is 1 right because the rays have crossed 1 air mass equivalent distance. When it is outside the earth atmosphere we call it air mass 0 ok so 1367 is the power density where air mass 0 and of course when it goes through the atmosphere it comes down here it results in loss of some energy some power and the standard that we use is actually more than that the 1000 is not air mass 1 right how much is that air mass 1000 what 1000 watt per meter square is corresponding to what air mass air mass 1.5 not 1 ok what does it mean so if it is 1367 when it is coming straight the amount of power here is should be more than 1000 because when it comes some angle air mass 1.5 which is 1.5 times of the vertical air distance right air mass 1.5 means the radiation so sun rays are travelling longer distance than the vertical distance how much longer 1.5 time. So that 1000 is corresponding to 1.5 air mass it is corresponding to 1.5 air mass ok. So if sun rays is coming at any other angle which is this then whatever the that ray makes an angle with the vertical which is theta so air mass is 1 over cos theta air mass is 1 over cos theta. So now you know air mass is 1.5 our normal standard test condition is for solar cell standards to test condition is air mass 1.5. So what is the theta corresponding to the calculation to the calculation if air mass is 1.5 what is theta if air mass is 1.5 what is theta near 40 anybody having closer answer 45 to 48 ok. If you use your scientific calculator it will tell you 47. something ok. So air mass is 1.5 so when sun is at 47 degree where of the normal if sun is overhead exactly then it is air mass 1 if sun is somewhere there it is even if that angle is 47.5 or something like that it is air mass 1.5. So our standard test condition is for air mass 1.5 and you should remember why you should remember why air mass 1.5 is chosen as a standard for testing we will come back to that again ok. So then the radiation reaching a surface earth surface can be of several type one is one is a direct radiation without going any interaction at the earth atmosphere sometime what happen the scattering will occur in and the radiation will either get absorbed absorption and finishes there itself or it changes its direction and reaches the earth surface called diffuse radiation ok. So the diffuse radiation will come from all angles diffusion will occur everywhere and the direct radiation will come from the direction of the sun right. So the radiation reaching the earth surface will be of two type one is direct and other is diffuse and if we combine direct and diffuse together it is called global radiation ok. If you combine direct and diffuse this called the global radiation. So global radiation is some of the direct radiation plus diffuse radiation. What is the difference in two? What is the difference in the two type of radiation? One is directional and other is unidirectional comes from all everything is diffuse right. So if you actually shut down all the lights and open the curtain some light will come in what is what kind of that light is all diffuse light. So all diffuse light comes from any direction ok. Is it clear concept of air mass is clear air mass 0 air mass 1 1.5 2 3 ok. So when sun is actually setting all almost in the evening what do you think will be the air mass? It will be some 6 7 8 10 another the amount of distance travelled by the sun will be very large almost coming. So the air mass can be very high ok. So at any given day the diffuse radiation is some friction of the global radiation. Global radiation is some of the direct and diffuse. So the diffuse radiation is some portion of the global radiation and that portion actually varies in the range of 15 to 20 percent ok. 15 to 20 percent radiation that is reaching earth surface of course it will also vary depending on the season right. If the very cloudy sky all the radiation reaching is a right. If it is a cloudy sky everything that is reaching earth surface is all diffuse. If it is a clear sky typically in the clear sky the contribution of diffuse radiation is in the range of 15 to 20 percent. It depends on what is the constant range of the earth atmosphere whether there is a dust particle or not dust particle whether there is a moisture or no moisture what is the humidity level and things like that. The parameter changes but typically you will have 15 to 20 percent diffuse radiation in the earth atmosphere. And that I am focusing I am kind of emphasizing on the diffuse radiation because in some technology the diffuse radiation is of no use. Some technology diffuse radiation is of no use. What is the technology in a case? Concentrator application right. Concentrator what what the concentrator does if the rays are coming parallel then it concentrates. If rays are coming from all direction it cannot concentrate ok. So whenever you are actually looking at the concentrated technology. Concentrator can be a solar photovoltaic technology or it can be a solar thermal technology. But whenever you are concentrating diffuse light is of no use you are not using that ok. So for example, if you are in a region somewhere in the east where diffuse light can contain may be 30 percent you should what you should not do is you should not go for a concentrator based technology right. So it is very important to know the diffuse component. In India unfortunately there is a very strong component the diffuse component is very high in India ok not so much in Europe for example. So this this is can be a matter of concern for a some technology. But otherwise if it is just a flat module it does not matter light is coming from this side or this side of photon is the photon. Any photon of higher energy if it is coming from the angle angle will always get absorbed. So it does not matter that much otherwise ok. What kind of intensities you will get AM you can anyway calculate 1 over cos theta that is ratio of y over x right. Air mass is ratio of y any other distance when sun is at angle over x. If sun is perpendicular y is equal to 1 x equal to 1. So y or x is or 1 over cos theta where this angle is theta. So extra terrestrial radiation AM 0 is 376. Air mass 1 is 1105 ok air mass 1 is 1105. What does it mean? There may be situation when the power density at the earth surface is more than 1000. 1000 is not maximum right. And then air mass 1.5 is 1000 and air mass 2 is 894. You can also do do the calculation for air mass 3, air mass 4 and things like that ok fine everybody so far so good. So this is how the spectrum looks like. This is the Planck's black body radiation model if you the solid black line ok. What is the temperature of the sun? 5700 something Kelvin right 5700 Kelvin. If you draw if you put t equal to 5700 Kelvin you will get this kind of spectrum black bodies ok. Then you have yellow line yellow is the spectrum which is available outside the earth atmosphere. If you go to outside earth atmosphere extra terrestrial spectrum will be shown by yellow curve. But when it reaches earth surface it goes through some interaction with the atmosphere. And because of that interaction what is available at earth is what is given by the red ok. What you see? What is the differences? Lot of UV get absorbed ok. This much UV is not reaching which is good for us right. UV is good who does it? Ozone absorbs it. If ozone is not there we are in problem. So good that that much UV is not reaching us. Then the overall intensity decreases and outside earth atmosphere this much it comes this much. Why? Interaction absorption in the earth atmosphere reflection is scattering all those happen. And then some special absorption by the oxygen water vapors water vapors water vapors water vapors CO2 H2O. So these are the things get absorbed into that water vapour particles are O2 or CO2 the gaseous atmosphere. And therefore whatever is reaching to you is what is given by red curve. Is that clear? Where is the peak intensity? What wavelength is the peak intensity? Here little higher. So, it is about 550 or so this one. What does it mean? All my solar cell design you know because the most of the radiation is coming at 550 nanometer. Therefore my solar cell should be working best for that radiation right. I should not make my solar cell working best for this radiation because the energy coming at this part is very small. Energy coming at this part is very small. Most of the energy is coming at this part and therefore I should design a solar cell such that it works best for that kind of radiation right. Question, how many photons of 500 nanometer wavelength are reaching earth per second? If this is the radiation right and there are lot of photons right starting at about 300 nanometer all the way 2500 nanometer. And they are coming different numbers. So how many photons of 500 nanometer are reaching earth surface? So, all that got the problem. What do you have here? What? So, with joule per second right you have joule per second and you know the energy of the photon. In terms of electron volt you can convert into joule just divide the energy you will get the number of photons. Got it? Let me put it again. What was the y axis of the spectrum curve? What per meter square? Per micrometer. Now, what per meter square is the power density? What is the unit of what? What is the unit of what? Rate of energy right. It is a joule per second. So, joule per second per meter square right. Now, what is my question? How many photons of 500 nanometer are reaching the earth surface? So, 500 nanometer is what? Wavelength. Can I convert this into energy? I can convert into energy E in electron volt is equal to 1.24 lambrine wavelength. So, 0.5 micrometer. So, how much is this energy? 4 8. 2.4 8 electron volt. Here what is it joule per second meter square? I need to say electron volt is unit of energy I need to convert into joule. I need to convert into joule. What is the relation between electron volt in joule? 1.6 into 10 to the power minus 19. 1 electron volt is 1.6 into 10 to the power minus 19 joule. Electron volt is a very small unit right. 1 electron volt is 1.6 into 10 to the power minus 19 joule. So, how much you are getting? 2.4 8 into 1.6 3.75 3.96 into 10 to the power minus 19 joule. That is the energy of 1 photon. What is my question? How many photons? So, we simply divide, is not it? So, what was the number there? The red curve, the peak point of the red curve. 1.4. 1.4 what? So, divide with this number you will get this. 1.4 joule per second per meter square you are dividing with 3.96. So, minus 19 joule will get cancel you get number per second per meter square. What is the unit number per second per meter square per micrometer? 3.5 into 10 to the power minus plus 18. Oh, we are getting large number of photons of only 500 nanometer. That is the whole spectrum. Got the feeling of a number? You are getting 10s for 18 photons of 500 nanometer per second. Every second so many photons of 500 nanometer. There are photons of 550, 600, 700, 700. Everything is coming together. Everything is coming together. So, you put together in a peak radiation condition we get something like 10s for 22 photons per second, 10s for 22 photons per meter square per second. That is kind of the photon flux. What is it? Per meter square per area per unit time means what? Flux. So, this is the photon flux we get in the peak power condition. Is that clear? How to calculate? Very simple. Everybody? Yes, no? Yes, very good. Ok fine. So, now we know our spectrum little bit better. So, we know our sun little bit better now, right? More than the common man, right? That is what you should know as a solar expert. You know sun, you should know the sun little bit more than the common man, more than the sun rises and sun sets and all this. So, we know the spectrum exactly from where it starts, how many photons are coming, how much energy of those photons, you know why there are dips because of the absorption due to the water, vapor, moisture and some dips are there because of the oxygen. Ozone is really helping us so that the ultraviolet rays are not coming and things like that. And we know the intensity also. Let us move forward. So, how much energy is there? So, we have some energy about 7.6 percent in the ultraviolet radiation 0.15 to 0.38. Lot of energy is coming in the visible radiation 48.4 percent, infrared is also significant 43 percent 0.572 micrometer to 4 micrometer and very small percentage of energy is coming beyond 4 micrometer, ok. So, these two are important component, right? And we have studied we have seen that the for corresponding to silicon the cutoff is 1100 nanometer, right? We had at the calculation study corresponding to silicon the cutoff is 1100 nanometer. So, we use infrared portion only from 0.72 to 1.1, from 1.1 to 4 that we are not using. So, lot of this percentage is not being used for silicon that we have seen again and again, right? That is one of the major loss in the solar cell, the loss of low energy photons is one of the major loss, ok. How do you measure them? You have the pyranometer, principle is very simple of the pyranometer, whenever sunlight is falling on a black body it gets heated, ok. More the light, higher the temperature, less lower the light, lower is the temperature. So, there is a double glass, I do not know if you can see here, there is a one glass sphere and there is a inside that there is another glass sphere. Why do you use double glass to insulate it from the atmosphere, ok? So, between this glass sphere there is a black body, some black coated film is sitting, when light falls on it gets heated and it is insulated from the atmosphere because of the double glass layer coating and that rise in the temperature of that particular black body is giving an idea about the how much is the radiation intensity. So, what kind of radiation it will give? Direct or diffuse or global? Direct or diffuse or global? Light can come from any direction, right? So, therefore, it is also collect global, it is called collect the diffuse as well as direct, right? Light can come from any direction, it is any hemisphere. So, light can come from here, come here, any light which is going inside will contribute to the increase in current and therefore, pyranometer will measure the global radiation, ok. Now, what also people do is, if I somehow connect a strip of metal on the top of this pyranometer such that the direct light is not falling, direct light comes from very small angle, right? If I block the direct light then what is the light it is measuring? Diffuse. Diffuse. So, I can measure the global radiation, I can measure the diffuse radiation and therefore, I can also measure the direct radiation. So, global minus diffuse will give me the direct. So, pyranometer is very useful device, it can measure both global and diffuse radiation and therefore, direct radiation, right? There is another equipment called pyrelometer which is particularly dedicated for the direct solar radiation. So, whenever you are measuring direct solar radiation, first of all you have to follow the sun because sun rays are coming from the directly from the sun, you have to collect those rays. So, there is a tube here as you can see and this tube is following the sun. So, there is a motor, two axis motor, it will always follow the sun, principle is same at the back of the tube you have some kind of black coated device. So, that the radiation which is coming can heat the dead black and heat it and the temperature increases give rise to the idea about the direct radiation. Why do you think the tube is very long in this case? So, the diffuse light does not reach there, any diffuse light which is coming from slightly higher angle will actually hit the wall of the tube and get absorbed. So, it will not reach the sensor which is sitting at the bottom and therefore, tube has to be long and it is always following the sun. So, this equipment is more expensive of course, because of this it is called pyrelometer. Now, let us try to estimate, now let us try to see any questions so far. The pyranometer, the pyrelometer will give more accurate reading, but you can use pyranometer for global as well as diffuse. Any other?