 So, yesterday we discussed about the solar cell and how the solar cell is going to perform under different conditions, under different light conditions, under different temperature conditions. And the next thing is actually to discuss the solar PV module itself and the how to design a module. What are the performance? So, that is what we discussed and now today we will lecture will discuss how to design a module, how many cells are there in module, how they are connected, how the power of the module can be increased and how the performance of the module you can get, what performance you can get in the field. So, that is the most important, how the module is going to behave under different temperature conditions, under different solar radiation conditions and so on. So, before we discuss further, I want to show you some of the modules that are available with me right now. So, for example, let me show you the modules which is here. So, this is just show that there, very long module it can it is a what you can say it is a flexible module you can fold it. There are two power cords that comes out of it plus and minus and this module is made of CIGS technology right. The CIGS the copper indium gallium selenite that is what we have been discussing and this is deposited on the polymer that is why it is flexible. The efficiency of this module will be about 7 percent or so and I told you study this kind of modules can be fabricated in a length kilometers and kilometers. Similar to this you can also get the amorphous silicon flexible module. So, amorphous silicon modules are also available in flexible, but I told you that not all the thin film modules are flexible modules. I will give another example of a flexible module. So, sometime you know you can pick it up and take it with you in the camps this is used for the military applications, but this is another example of a thin film CIGS module again. CIGS module will have typically this color and again these are very flexible you can fold it. So, wherever you require power you can do this other than that there are modules which are of crystalline silicon. So, this is a module of crystalline silicon. So, small I think 18 watt module are there many cells there they are connected in series. If you look at the closely you can find out the front one top of the cell is actually going to the bottom of the cell by the way these modules are there with you and I am sure you will have opportunity to do that. One thing that we should be able to distinguish is between monocrystalline and multicrystalline module. So, look at this module and as I told you yesterday that monocrystalline solar cells and the monocrystalline wafers are actually having the rounded corner. So, this module you can see clearly that all the cells have the square corners perfectly 90 degree if I see here perfectly square or rectangular cell, but if you look at this cell this module actually it is cut from the big silicon monocrystalline silicon solar cell and because of this you can see the corners of this corners of this cells are actually rounded. And because of this you can see by looking at it this is a monocrystalline this is the module made of monocrystalline cell this is the module made of multicrystalline cell and I also showed you the thin film modules. So, these are some of the example also if you look at the multicrystalline cell here I do not know if you can see if you zoom it if you zoom in you can see the different patches yeah probably now you can see that they are different different patches right. So, the whole color is not uniformly blue and this different patches as I told you yesterday that these are the different grains of a multicrystalline silicon. These are the different grains of a multicrystalline silicon and you can see the size of this grain will be order of millimeter or a maximum centimeter. So, this is a multicrystalline sometime you can see this grains very clearly try to see this grains also in the modules that has been provided to you ok. So, these are some of the example that I wanted to show you yesterday of course, we have looked at the silicon ingot kind of very small ingot typically ingot will be large size and this you can use to deposit amorphous silicon yesterday we also looked at the single crystal wafers single crystal wafers are typically comes in a circular shape that is the example of this and multicrystalline will have typically square and rounded shape ok. So, this is this has the some demonstration of the modules and in this lecture we are going to look at the modules in detail design of the modules what are the features of the modules how they are fabricated and how this modules perform in the field ok. So, why do we need a solar PV module what is the need of a PV module and we have seen we have discussed that as a solar cell has a size of you know standard commercial solar cell has a size of 12.5 by 12.5 centimeter square some people are trying to make 15 by 15 centimeter square and if you look at that size the wattage that you can get from such a solar cell will be only couple of watts you can get 2 watt 3 watt 4 watt 5 watt depending on the size that is the kind of wattage that you can get now we know that most of our application. So, for example, this is one example where you can you are getting 5 ampere current 0.5 volt V m. So, your maximum power is going to be 2.5 watt. So, this is what you can get from a single solar cell and many of our applications are requirements are much larger right. So, we need power in the range of you know 100 watt 500 watt kilo watt 1000 kilo watt and so on. So, therefore, one single cell is not going to deliver as that power and therefore, therefore, you need to connect many cells together in order to get the larger power. So, the role of a module is to convert solar cell into a solar PV module where the large cells are large number of cells are connected. So, that you can get the larger power normally a solar PV module the cells are series connected cells are series connected because remember that the seller cell is producing DC power and when the DC power if you want to minimize the I square or losses your current has to be normally kept low in order to keep the current it is the normally the voltage that is increased and current is kept low. That is one reason why kept cells are connected in series, but the other reason why cells are connected in series in a module is the way the PV modules have developed. So, if you look at the early 1970s or even 60s when where the PV modules are used in satellite and normally to charge a battery. So, when we want to use a PV module to charge a battery then your module voltage should be always greater than the battery terminal voltage and the typical battery that you will use is 12 volt battery for large applications or 6 volt battery for small applications. So, if you are using a 12 volt battery a solar PV modules that is being utilized to charge a 12 volt battery must always provide voltage higher than the 12 volt and that is why single solar cell is only giving me 0.5 volt or 0.55 volt or good solar cell even will give me 0.6 volt. So, if I want to get a larger voltage greater than 12 volt I must connect solar cell in series and in series the voltage is added. For example, you can see the 6 cells are connected here positive terminal of one cell is connected to the negative terminal of other cell and therefore, when you connect those so many cells in series you get the voltage of all these cells added up and if one single solar cell is giving 0.5 volt your 3 solar cell will give you 6 volt. So, remember one thing when you are discussing the module design when you are discussing the module design we have two thing we have two voltages defined one is open circuit voltage and other is voltage at the maximum power point. Now, if you are operating your module at open circuit voltage you are not extracting any power. So, if you are operating your module at open circuit voltage you are not extracting your power. So, though your open circuit voltage is very high your maximum power point P a m come somewhere here and therefore, voltage corresponding to maximum power point is referred as V m. Typically, typically for crystalline silicon cells your voltage at maximum power point is about 0.8 to 0.85 of V o c. Typically, voltage at the maximum power point is about 0.8 to 0.85 of V o c. So, even if your solar cell is having open circuit voltage of about you know 0.55 or 0.55 volt or even a 0.6 if this is your open circuit voltage your V m is going to be less. So, it will be about 0.5 or it is about 0.48 or so. So, your open circuit voltage is normally lower I am sorry your voltage at the maximum power point V m is normally lower than the V o c and because when I am operating my module it operates at V m not at V o c and therefore, when I look at the design when I look at the how much voltage we are generating it is the V m that we should look at not the V o c fine. So, now this side. So, now we have to connect many solar cells in series in order to get the larger voltage. The next different types of PV modules that I have shown you already you can see looking at this right. So, when your pseudo square solar cells are used I will show the pseudo square solar cells are used you see some wide patches here and when multi crystalline solar cells are used you look almost continuous. So, by looking at the view of the module you can say that this module A is made of monocrystalline solar cells this module is made of monocrystalline solar cells, but this module is made of multi crystalline silicon this module is also made of multi crystalline right and the more it will become even clearer from this picture. So, you can see that when you look at this module is made of a monocrystalline silicon because of this rounded corner in this module is made of a multi crystalline silicon because the perfectly squared are the corners which are 90 degree. So, this is the difference in polycrystalline and monocrystalline if you look at the cells it will become your clear in the solar cells are like this. These are the monocrystalline solar cells and these are the multi crystalline silicon cells and in this case you can also see the grains various grains the in the multi crystalline silicon. So, again as a participant of this course you should be able to identify the monocrystalline and multi crystalline cells and modules very clearly. So, difference here you can see the white patches and this which is clear from the module. So, now modules of thin film technology I have shown you yesterday that thin film technology modules are made while the cell is being fabricated. The fabrication of cell and modules same, but in the wafer based technology the cell fabrication technology and module fabrication technology are different. What I also mentioned is the PV modules can be designed for any wattage you want and at the end of this lecture even you should be able to do that. So, if you want a module of 3 watt you can design it, if you want a module of 30 watt you can design it, if you want a module of 100 watt you can design it and if you want even module of 300 watt you can design it. So, modules are different power ratings can be designed. So, normally we have the single solar cell which does not give sufficient powers we connect them together in series. So, we get the module, but even a module does not have enough power. So, the maximum module that you can get is about 300 watt. The maximum wattage of a module that you can get is about 300 watt, but this 300 watt is not enough because sometime your requirement can be 500 watt, 1 kilowatt, 10 kilowatt and in fact people put modules in mega watt. So, therefore, you have to make a cell then you from the cell you have to make a module, but even many modules can be connected together in series and parallel and what you get is a PV array. So, PV cell PV module and PV array this is the terminology that we can use. When we now let us look at the I V characteristic of a module what happens. So, when we this is the I V characteristic of one single solar cell having open circuit voltage of 0.6 volt and having a short circuit current of 0.8 amperes. So, this is the I V characteristic of a single cell when you put two of them in series as shown here when you put two of them in series the current in the in the in the string remain same the current in the string remain same, but your voltage is added. So, what you are trying to do is you are trying to have the series connection of a cells you have the series connection of cells you take a two solar cell plus and minus terminal and then you have the plus and minus terminal. So, they are connected. So, though if this is having voltage V 1 this is having voltage V 2 the total voltage available will be V 1 plus V 2, but the because they are connected in series the current will remain same. So, whatever current is flowing in the first solar cell the same current will also flow in the second solar. So, I will also be out. So, when you put in series the voltage gets added up current remains same that is what I have shown here. When we put cells in parallel the current is get is added up as shown here when you put the cells in parallel the current becomes double. So, if the say current of single cell is 0.8 ampere the the current of two parallel connected cells will be 1.6 amperes and voltage remain same in the parallel connection, but you can make a series and parallel. So, you can actually get the higher current as well as higher voltage. So, in this way what I am trying to show you that by making the cell connection either in series and in parallel or both you can get the any voltage that you want you can get 100 volt, 30 volt, 50 volt, 1 kilo volt whatever you want you can get. Now, these are the conditions of the ideal solar cell ideal solar cell perfectly same open ideal solar cell means all the solar cells are similar because when you are putting them together it is important that they match with each other they match in terms of open circuit voltage they should make each other in terms of the current and so on. In practice when you are making millions of solar cells and when you are starting material may be different from one solar cell to other solar cell the performance of the solar cell is not going to be the same which means there may be a slight difference in the short circuit current of the solar cell there may be slight difference in the V m of the solar cell there may be slight difference in the I m of the solar cell. So, this difference is possible and it can cause because of the many factors for example, solar cell processing may not be same. So, may be you are making one batch of solar cell which is 200 cells which are going under the diffusion the next batch comes you are making another 200 cells in the diffusion and from this one cell to other cell if you want the temperature difference of once degree centigrade is there you are likely to get a different performance also you are you are going to make your metal contacts with a different solar cell and if the contacts are not the same again you are going to get the difference in the performance. Your different outside condition may result in a different performance some cells are shaded some cells are not shaded encapsulate material can result in difference. So, there are many reasons in which the all the solar cell may not be identical and because of that there is going to be mismatch and this mismatch may be important because we are putting the cells in series and parallel. The problem with the mismatch will be that it will result in the loss of power it will result in a loss of power. So, for example, if I am having a one solar cell which is having a power P m 1 other solar cell having power P m 2 ideally if I put them together the maximum power of the both the cells should be P m 1 plus P m 2, but that does not happen because of the mismatch. So, because of the mismatch because of mismatch in the cells what will happen the when the when they put together. So, when you the P m of module is always going to be less than the maximum power point of the cell and so on. P m of the module the maximum power point you can get from the module is always going to be less than the sum of the powers of individual solar cell and that is why if you look at the efficiency there is always efficiency gap between cell and module. So, if your solar cell efficiency is 16 percent because of this reason if your cell efficiency is 16 percent your module efficiency your module efficiency will be somewhere about 14 to 15 percent there is always 1 to 2 percent loss in the efficiency when you go from cell to module. Now, it depends on how best you can match the solar cell from each other. So, definitely there is a loss of the power because of mismatch and as a designer of the module as industry we try to minimize the mismatches in the parameter. So, that you can get the best performance when you are putting the cells in a series and cells in parallel the mismatch can be can be severe or not severe. So, series connection when you are putting the cells in series there can be mismatch there can be mismatch in current and voltage. Similarly, when you are putting them in parallel connection when you are making parallel connection of cells there can be a mismatch there can be mismatch in current and in voltage. There can also be mismatch in flow factor in other case there can also which is which may be result of current and voltage, but when you are putting the cells in series which of this mismatch will be more dangerous when you are putting the cells in series the voltage is just get it up get added up. So, there is a mismatch in voltage no problem. So, mismatch in voltage in series connected cell will not be a problem it is this which is going to be a problem mismatch in current will problem. Similarly, when you are putting the cells in parallel the currents from the two different solar cell will easily get added even if they are not same. So, the mismatch in current when you are connecting in parallel is not a problem, but mismatch in voltage will create a problem. Now normally in a module as I said our requirement is to increase the voltage rather than increasing the current. So, when our requirement is increasing the voltage we try to put the solar cell in series rather than in parallel. Therefore, because we do not put the solar cell solar modules in parallel this is not so severe this is not so severe case. The most severe case is mismatch in the current the most severe case is mismatch in the current of the solar cell which are series connected this is the more severe case. As I shown here if there is a mismatch in voltage no problem if the series is connected, but if there is a mismatch in current there are two different short circuit current I s c I s c 1 and I s c 2 then there will be some issue I will show you how. What will happen when the combination of these two solar cell operate near short circuit condition? Now when there is a mismatch there is the combination of two can occur near open circuit voltage it can occur near a maximum power point or it can occur near the short circuit current. Now one of the solar cell is having higher short circuit current as compared to the other solar cell one of the solar cell is higher short circuit current. So, if the operation point is somewhere here what will happen this current the solar cell which is having higher short circuit current will actually force this solar cell which is having lower short circuit current to operate at this point. So, this is the common current now. Now what is this? Now if the operating point is here your voltage has become negative your voltage is negative and remember this axis is a negative current axis your voltage is negative your current is also negative. So, your power is positive what does it mean? Power is positive means what? This solar cell which is having a lower short circuit current is now consuming power other than generating power right voltage has become negative current axis is anyway negative. So, this solar cell which is having lower short circuit current is actually consuming power and that is the problem it because the solar cell with the higher short circuit current force it to operate in that way. Now this condition that out of the many solar cell connected in series one of the solar cell is producing less current can occur because of the other reason also. One reason is that well while fabrication the solar cell has got the lower parameter and therefore, it is producing lower current. And therefore, in order to solve this problem what people do in industry is called cell shorting. What people do in industry is? In order to avoid cell mismatch problem mismatch problem people do what is called the cell shorting or many people also call it binning. So, what you do when you are when making let us say 1000 solar cells you fabricated. So, what you do is you create various bins and say solar cell of certain efficiency comes here. So, suppose your range of efficiency is 15 to 15.5 percent that is what you are making. So, you will say all the solar cell which is having 15 percent efficiency will come here all the solar cell having 15.1 percent efficiency will come here cells with 15.2 will come here 15.3 will come here and so on. So, now in this way you are actually separating the cells from each other. So, that all the solar cell which are in this bin are closely matched with each other all the solar cell which are in this bin are closely matched with each other and therefore, you can avoid the losses as much as you can. But this loss can also occur this decrease in the current generation capability of one of the solar cell and remember when the light falling is actually blocked it is the current or when the light intensity changes what we discussed it is the current which changes significantly current changes linearly, but the voltage changes logarithmically the change in current is more significant than change in voltage. And therefore, let us say in a module if there is a some solar cell which is getting shaded right because of the bird dropping or because of the leaf of a tree or because of the shadow coming. So, this can happen not only because of this, but in the real life operating condition also this can happen. So, look at here what happens out of this six solar cells this five solar cells are generating power, but this other solar cell is now getting the reverse voltage this solar cells voltage is added. So, this side is plus and this side minus, but this solar cell because of the shadow this side terminal is getting plus and minus. So, it is getting the reverse bias and because of that it is actually consuming power. Now, when this power then consumption of power happens what can happen is that this may result in a generation of heat because this solar cell which is now shaded getting a reverse polarity of the voltage and because of that it is actually consuming power because of this consumption of power lot of heat will be generated I square or last kind of heat and the power is dissipated and because of that as a very old picture that taken from the PV CD ROM which is available in open source because of this the module can crack the glass because of the temperature difference can actually break what is shown here. And therefore, this problem must be solved in a this is result this is also called the hot spots somebody was asking therefore, yesterday that how the hot spots occur. So, hot spots occurs because of this region, but therefore, we need to solve this problem we need to solve this problem you any idea how we can solve this problem I will not go into the detail because because of this this one solar cell become a load actually the I V character shift to the other quadrant and it is resulting. So, how to solve this problem is by bypassing this diode which diode if somehow I can bypass this diode and if let my current to flow in a parallel path if I let my current to flow in a parallel path. Then I can solve the problem if I have one solar cell another solar cell another solar cell and let us say this my solar cell is shaded. So, light is not falling on here because light is falling everywhere else, but because of some shadow light is not falling. So, this all solar cells are generating power this is plus minus this is plus minus this is plus minus, but this is actually getting reverse bias because of this solar cell bias in the opposite direction. Now, if my current is if my current is is flowing in this direction the I am just trying to look at the direction of the current flow. So, the current flows from N to P and my N side is P side is getting passed. So, my current will flow in this direction. So, my current will flow from N to P and P side becomes positive in solar cell N side becomes negative. So, my current will flow in this direction. Now, this because of this this is this has become a load because of the shading this solar cell has become a load and it is consuming power if of some I put another diode here. If I put a diode which bypasses the current which bypasses the current. So, that the power it is not dissipated here it is bypassing this and if I do this then I can actually avoid the heating of this solar cell and if I avoid the heating of solar cell I can protect my modules. So, by using a bypass diode which can bypass the current I can actually avoid the heating problem, but another problem is that if that is the case actually I need to put a bypass diode everywhere. If I am that is the case I need to put bypass diode every by the I am sorry the bypass diode should be in other direction. So, bypass diode should be in this direction. So, that when this terminal gets positive the current should flow like this. So, current I am sorry the current is going to current should be flowing in the opposite direction. So, that this gets because of the shading this becomes what the current will actually flow like this and then go and come like this. So, therefore, my bypass diode can actually help me my bypass diode can help me to do that and that is what I shown here this is the model of the solar cell that we have created earlier that this is the current source and then the reverse direction there is a diode and if the light is not falling on this solar cell the current which was flowing like this will not be able to flow or even if it flows it will result in dissipating the powers. If I add a voltage I am sorry if I add a diode here called the bypass diode my current will actually go like this will like this will go like this and it will go through the bypass diode and here this diode is a normal this solar cell is a normal. So, current is flowing normally I have shown this with the direct line I hope everybody can see it the current is flowing like this is going like this and it is going. So, this is the use of the bypass diode. Now, the problem is that you know for in order to protect each and every solar cell you have to connect that many bypass diodes right in in a module typically there are 36 solar cells you have to connect 36 bypass diodes now 36 bypass diode putting 36 bypass diode can be very expensive and therefore, what people do is they put a bypass diode for some bunch of solar cells in the current practices people put two bypass diode in a 36 cell of a module. So, there are 36 cells in the module people only put two bypass diode and people have found out that this is good enough to actually have that. So, again if you go back to your experiment today make sure that you actually notice that there are bypass diode. If there is a problem in the parallel connection cells connected in parallel then it is the mainly the problem voltage current is no problem and that is not a major issue because the difference in the voltage cannot it does not result in that severe problem as a difference in current in the series connected from modules. Now, somebody has also asked that now this module if I am connecting this module to the battery. So, if I am connecting this module to the battery if there is a module this is a normally the symbol of a module in if it is connected to the battery in the daytime in the daytime current is going to flow like this. So, the battery is getting charged, but when there is no sunlight and there is no sunlight a current can also flow in this direction. When the current flows in this direction will the module become load. Now, obviously that module can actually become a load and all the power that is there in the battery will get dissipated in the module. Therefore, this condition should not occur and people put what is called the blocking diode. As the name suggests this blocking diode blocks the reverse flow of current into the module. This blocking diode blocks the reverse flow of the current into the module and therefore, other than the bypass diode blocking diodes are also utilized. Now, my next important very important slide is how to design a solar PV module how to design a solar PV module. Now, I will show is that design of PV module basically what we want to design we want to find out how much power I can get, how much voltage that I need to do, how much current I will get etcetera. These are my design parameters and also other thing is for what application I am doing it for what application I am doing. So, suppose I am doing the design for my battery charging. So, if I am doing a design for a battery charging and typically battery plus and minus and if I am doing for the 12 volt battery right. Typically, the voltage of a 12 volt battery when it is fully charged when the under the full charge condition it actually can go up to 13.4 or 13.5. So, that is the fully charged battery will have the terminal voltage of 13.4 I am talking about lead acid battery. Similarly, there will be other conditions I mean for other batteries. So, even if you want to design a module for 12 volt battery charging your module should always be able to supply you at least 13 and half to 14 volt right or in fact larger. So, larger difference. So, your module should be able to supply about 14 to 15 volt in all operating condition. So, for a 12 volt battery module should supply about 14 to 15 volt in all operating conditions. What is the all operating conditions? This is very important in all operating conditions. What are the all operating conditions? All operating conditions is all solar radiation condition in all temperature condition in all solar radiation and all temperature. So, the operating condition our radiation varies and our temperature varies this keeps changing and this also keeps changing. So, for example, when you are when a very hard climate still you should be able to get 14 to 15 volt when you are in the early morning or late evening still you should be able to get 14 or 15 volt. So, that you can charge the battery. Now, one thing you have noticed that because of the change in the current I am sorry because of the change in the radiation the voltage of a module is always quite high because the voltage decreases very slightly because it is a logarithmic function of the light intensity. But it is the temperature which actually results in the significant change in the voltage right. As we have discussed yesterday that the cell temperature can change significantly because of the change in the temperature and therefore, we have to design so that my module will give me sufficient voltage. Now, when your module is encapsulated in a glass and we will discuss in later when your module is in the glass the cell temperature in a module cell temperature in a module is typically almost 15 to 25 degree centigrade higher than ambient temperature. So, if your ambient is 35 degree centigrade your cell may be 60 degree centigrade or may be 65 degree centigrade. If your ambient is 25 degree centigrade your cell may be 50, 55 degree centigrade. So, your cell temperature is higher and therefore, we must take that into account. So, your module should be able to supply higher. So, how much voltage you can if you are considering crystalline silicon solar cell I am doing the reverse calculation here. So, if I am if I am using crystalline silicon solar cells and let us say I am considering the worst case scenario the cell temperature I am assuming is 70 degree centigrade worst case scenario 70 degree centigrade which is possible by the way. How much voltage drop per cell will occur? My per cell voltage drop will be as we discussed the drop in voltage is 2.3 milliold per degree centigrade as compared to standard test conditions 70 degree centigrade minus 25 degree which is S 3 second condition. So, this is 35 in 2.3 milliold into 35 this is a drop per solar cell. So, 0.08 0.08 is the is the drop in the voltage that you will get. So, now this is the voltage drop per cell right this is the voltage drop per cell and remember what we are discussing that in a module many solar cells are connected in series. So, suppose there are I am just taking a random number. So, suppose there are 30 cells connected in series how much is the total voltage drop? 30 cells connected in series the voltage drop is 0.08 into 30. So, that will become 2.4 volt almost 2.4 volt will drop because of just increase temperature right. So, coming back to the design that first of all we require 15 volt out of the battery to charge the battery plus we should actually take care that additionally we get about 2.4 to 2.5 volt. So, that the temperature losses is taken. So, in all operating conditions in all operating conditions your module should supply you about about 17 to 18 volt is not it 14 plus 16 about 17 to 18 volt is what your module should supply you will all operating condition. Now, what is this voltage? What is this voltage? This is V o c or V m this voltage is V o c or V m this should not be V o c this should be V m because your module has to operate right. So, this is your V m. So, your module voltage should be about 17 to 18 volt in all operating condition all operating condition means low light condition high temperature condition any condition. Now, this is your V m. So, what should be your V o c? So, your module I am talking about the module V m should be about 17 to 18 volt and I am talking about this crystalline silicon solar cell and actually this should be the similar calculation can be done for other also. Now, this is V m what I what is the other thumb rule I told you that how much is the V m as a percent of V o c V m is a 0.8 to 0.85 of the V o c. So, if I divide this number if I divide this number 0.85 I will get what should be my V o c. So, if I divide 18 volt by 0.8 let us say I am dividing. So, this should be my V o c. So, this goes to 22.5 and if I take the lower side 17 by. So, lower side is 17 higher side is 20. So, 21.25. So, my voltage open circuit voltage of a crystalline silicon module which is designed to charge a 12 volt battery should be able to have open circuit voltage about 21 in all operating conditions in all operating conditions. And if you look if you go back again to your laboratory today and you see at the back side of the module your rated value of the open circuit voltage is this. So, now we have solved the one design problem and this kind of methodology you can use to design a module of any solar cell. Even if you want to design a module of cadmium telleride if you want to design a module of c i g s or any other module you should be able to design like this. So, this has a. So, therefore my open circuit voltage let us say 21 is what normally considered. Now, how many cells should I connect in series? How many cells in series that is the question how many cells in series that is the question mark and this is my V o c. So, suppose if I get a solar cell which is having if I get a solar cell which is having open circuit voltage of 0.55 volt. So, then I should divide this is 0.55 volt how much you get 21 by 0.55 38.18 and if I get the higher size if I get higher voltage 0.6 volt if my V o c is 0.6 volt as the technology in improving people are getting as the technology in improving people are getting higher and higher open circuit voltage. So, now you get close to 0.6. So, now you can see that your number of cells that should be connected in series is 35 to 38 and typically if you look at the PV modules people use 36 cells in series. 36 cells in series are connected together in order to in order to make a crystalline silicon module which is suitable for 12 volt. Again definitely this is this has come from the consideration that I am going for 70 degree centigrade my cell temperature, but suppose I am going to install my module in Ladakh for example, where the temperature ambient temperature is never going to go above 15 degree 20 degree and therefore, I am sure that my cell temperature inside the module is not going to go above about let us say 45 degree centigrade and therefore, I know that my voltage drop is not going to be 2.4 and therefore, in that case instead of instead of putting 36 solar cell I may have to put only 33 solar cell or 32 solar cell or 35 solar cell. So, depending on the temperature of operation your number of solar cells in series to be connected in series is determined. In India most of the country there is a sufficiently high temperature. So, 36 solar cells is a common commonly connected and nowadays because your open circuit voltage of a solar cell is getting higher and higher because your technology is becoming better and better sometime people use connect even less number of solar cells are also. So, you can now onwards whenever you look at the module please measure please note down the number of solar cells that are connected in series. So, this is how we design a solar cell I am sorry this is how we design the solar PV module. Now, similar to the I V characteristic of a solar cell you can also write the I V characteristic of a solar PV module it is very simple. When you are going to write I V characteristic of solar PV module what you have to do? Voltage gets connected voltage is actually added in series what else added in series? Series resistance right. So, if your series resistance of one solar cell is some value R S when you are putting 36 solar cell or N number of solar cells in series your series resistance should also be multiplied by the factor N. So, what are the things you should take care while writing the equation of a module I V equation of PV module when you write I V equation of module you take proper. So, suppose N is the number of cells connected in series M is the number connected in parallel. Now, this equation can also be applied to the PV array when you are a N number of modules are connected in series then M number of modules connected in parallel you can modify equation like that also. So, what are the things you do when you are talking about series? If the open circuit voltage of one cell is V O C or the voltage of one cell if voltage of one cell is V series connected cells we have voltage of N V. If the voltage of one cell is R S series connected solar cell or the module will have voltage total series resistance of N R S same thing when you are, but the current. So, when you are doing series connection when you do series voltage becomes N V resistance become N R S, but what happens to the current remains I current remains I only. When you are doing parallel connections when you are doing parallel connection your voltage remains V what happens to series resistance it becomes a R S by M, M is the number of cells connected in parallel when you are what happens to the current, current becomes M times I. So, these are how you can actually replace the current voltage and resistance by various whether series are parallel connected and you can actually you can modify the equation for a PV module or even PV array for that matter. So, you can actually modify the equation for PV module in array and actually you can get and that is what we have done here the same thing we have done this is this equation of a single solar cell shunt resistance has been neglected because I told you shunt resistance normally is not a problem series resistance is a problem. So, this is the equation of a single solar cell this equation has been modified in terms of the voltage same equation here. Here this equation is written in terms of the current here the equation is written in terms of the voltage you get this. Now, when you put this cells in series and parallel as I said you modify your current and voltage appropriately you get this. When you put cells in series you multiply the series resistance by N S when you put them in parallel you divide the series resistance by N P when you put both series in parallel you multiply with the both factor. So, now this is the slide I will stop and will have the time tomorrow hopefully to do that. So, now once I got the voltage how we will get the module of the sorry how we get the power of the module. So, power of the module similar to solar cell will be V m times I m power of the module is V m times I m. So, if my for example, if I am using crystalline silicon solar cells let us say my current density which I am taking regularly 35 milli ampere per centimeter square is the current density, but this can be very different if I have solar cell of 15 centimeter by 15 centimeter. How much is this total current? 15 milli ampere multiplied by 15 centimeter by 15 centimeter. So, this is total area this is current. Now, this is the area of one solar cell and there are let us say 36 solar cells are connected in series and those solar cells have the V m of 0.5 let us say 4 volt. What is it? What is 0.78? No, but there is a milli and then you have to multiply with this. So, you will get about I think 7.8 milli amperes. So, you get about 7.8 amperes. Now, this 35 let us consider it is not I am considering it is I m not I s c not J s it is I m. So, very high current very good solar cell actually. Typically I showed you thumb rule is that V m is about 0.82 0.85 of V o c. Similarly, I m is about 0.92 0.95 of I s c right. So, these are the thumb rule for crystalline silicon solar cell. So, this is my I m this is my V m. Now, this is V m of one single solar cell and suppose there are 36 cells connected in series. So, what how much voltage you get? So, 0.54 into 36 cell which are connected in series. So, you get 19.4 volt very nice module. So, this is your I m what is the maximum power of this module and this are we always assume that this all values are under standard test condition. If not mentioned we always talk about standard test condition. So, P m is going to be a 7.8 ampere and 19.4 volt. How much is the maximum power? 2 to 155 151 watt peak. So, this module will actually give a wattage of 151. Now, I have really taken very good numbers typically you will find in industry will have modules of 140 watt 135 watt that is the number because this values not so high normally in this current is also not so high. So, these are the very higher this are the number of current at maximum power point and voltage at maximum at a higher side typically industrial industry you will find modules of 130 watt peak 140 watt peak. Now, this is the highest size of the solar cell. This is the highest size of the solar cell you cannot get the larger values of this and then people are then trying to go parallel connection solar cells instead of 36 solar cell there are now modules available where to put 72 solar cell or sometimes 70 solar cell or even 65 solar cell and so on 66 solar cell. So, this is how once you know the current and voltage you can actually design a module. So, I I given you I think in the tutorial but you can solve this problem. So, suppose if you given that you want to design a module of 10 watt peak you need to design a module of 10 watt peak which is suitable for 6 volt battery suitable for charging syscall battery design a module what should be the size of the solar cell and what should be the number of cells connected in series. So, do it do this problem in the tutorial if you want to design a module of 10 watt peak what should the size current and voltage. Now, let me stop here now there are many slides to be discussed let me discuss I think you can finish it just hold on. So, modules are encapsulated in the glass and and encapsulation layer. So, that modules has to be protected by the moisture for the whole life. So, there is an encapsulant there the solar cell there is an encapsulant on the top and bottom and there is a glass cover on the top and the tedlar on the bottom and this glass because it is a glass that is a it transmit the visible light, but it does not allow the far infrared light. So, whatever heat that is generated remains strapped inside the cell and therefore, normally the cell temperature is much higher than the ambient temperature. And this is how the cells are connected together somebody was asking that there is a reflection from the back sheet that happens the tedlar at the back side the white sheet that you can see is a tedlar sheet it is called the some kind of plastic called tedlar. So, between the solar cell also there is some reflection that can take place and you get the advantage the solar cells are connected in series and they are encapsulated in polymer called EVA. So, immediately on the top of the cell and the bottom of the cell there is a polymer called EVA. Now, this EVA ethyl vinyl acetate is a transparent polymer it is a transparent polymer and you cannot see it, but it is there between the glass between the glass cover and the solar cell there is a polymer sitting on the top and bottom. And many people have been asking what is the reason for degradation of the efficiency and one of the reason for degradation of efficiency is the EVA itself. EVA is a polymer and the lot of ultraviolet light keeps on falling throughout the life of the module and because of that ultraviolet light the transparency of the EVA can decrease. Other thing is that connection between this cells the contact between this cell and this cell the series connection this itself can degrade over a period of time and there is another reason why modules get degraded over a period of time. Then this is how the modules get that fabricated cells sorting I told you is one of the important process this is the process in which you identify the solar cells of different efficiency. It goes to the framing encapsulation and this is how the module is created and that is it. So, these are the main thing about the design of a solar PV module. Now, the as we discussed with the solar cell we should also discuss how the output of a module how the power of the module changes with respect to temperature how the power of the module changes with respect to irradiance. What are the other typical parameters or advantage and disadvantage of crystalline silicon module and thin film module. Tomorrow I will have two lectures one I think I will discuss the design and performance of PV module and another I will discuss the PV system design. So, once your module is there and if you want to fulfill we want to use PV module for a given application for example, a design of a home lighting system then how to go for the design. So, that we will discuss tomorrow. Let me ask let me take some questions V I T Vellore. Hello. Yeah. So, is it possible to buy the solar cells and make a PV module? Is it possible to buy solar cells and make PV modules on our own? So, what happens the making of a PV module is relatively simpler technology and that is why in the country in India there are something like 40 module manufacturers, but there are only 10 solar cell manufacturers and the investment that is required to make a module is much lower than the investment that is required to make a solar cell. So, in practice it is possible to make a module at your home it requires little perfect little arrangement particularly the polymer sheet and encapsulation and that is why you will see that this modules which are used in solar lanterns are very very small modules and this modules can easily be manufactured with a small facilities. Sir, are you providing training to make these modules? No, no, I am sorry. We are not providing training to make the modules. Pandit Dindayal, University, Gandhinagar. Yes sir, my question is that sir, in slide number 32 it is seen that if two more series connected cells having two different values of I S E then the value lower value lower value of I S E if the cell is having then it is working in the second quadrant then how it will happen? So, that is why the because the module because in the series connection both the cells have to carry the same current right both the cells have will have to carry the same current. Now, what will happen if I am having two cells? So, if I am having one cell is like this and other cell is like this. Now, all the cells will have to carry same current. So, suppose my operating point is here. So, this current must be conducted in both. Now, so this current is here, but it is not appearing in this graph, but what will happen if it goes to the reverse direction it will actually make that is current. So, when this point occur at this point both this cell with the lower short circuit current and the cell with the higher short circuit current will have the same will have the same current and that is the requirement of a series connection that in the series connection both the cells should have the same current and it is and because of that because of that this cell will actually force this. So, the only possibility that the same current will flow in the both the cell is that this cell which is having lower short circuit current is actually forced into the second quadrant it is forced in this quadrant and that is how that condition of a constant current or the same current in both cells is satisfied. The operating point should be the constant. I mean when the cells are connected in series the same current must flow right the same current must flow in the. Is the temperature increases because of green house effect? Yes, temperature of the module temperature of the cells when side the module increases because of the green house effect. NIT Warangal. My question is that if the surge condition is there in the environment. So, during rainy season so much surge will occur and the what will happen to the performance of our PV module is there any change will occur or? You mean surge of the radiation. So, if there is a surge of radiation like in the case of lightning if suddenly there is a lot of light falling on a solar cell or solar PV module will it affect the performance is that the question? So, that is the question I think the solar cell actually can take larger current for a short period of time only problem is that because of larger current some series resistance losses I square losses will increase and therefore, your performance at that particular instant or the efficiency at that particular instant will decrease. But otherwise I think because of the generation of higher current and all I do not think there will be any damage to the cellular module ok Slampur. The polycrystalline silicon that you have shown at the beginning the polycrystalline silicon that you have shown at the beginning each cell has the surface area around 5 to 10 centimeter square right and then we are connecting the cells in series to make one module and we are getting the problems of mismatch and at the same time the bypass diodes also the number of bypass diodes also increasing right. So, instead of that if we directly make a cell of bigger surface area because CZ process produces around 300 millimeter size of monocrystalline silicon. So, cannot we make a wafer of bigger size directly ok the no. So, only making a cell bigger will not is not going to help you because you know one of the reason why many cells have to be connected in series is because we want higher voltage and if you want higher voltage you have to connect many cells in together ok. Now, one reason why for a small voltage module the why the cells are smaller is because you know that is that is the good enough small solar cells is good enough to generate the current that you require and that is why making a bigger cell will not solve the problem because you need to create a higher voltages. Also what happens many of the big industry which are making you know 50 megawatt module 100 megawatt module they are many times the wafer breaks during the processing and so on. So, all those wastage of the wafers on the solar cell is actually given to the smaller player and what the smaller player do is they actually cut down the cells in a smaller size and then try to make a small power module. So, no big industry actually makes a modules of 5 watt, 10 watt, 15, 20 watt size it is all the smaller people who actually buy this you know wastage wafers from the big industry and make that. So, in that way we are also utilizing the kind of you know rejected solar cells. Amal Jyothi college. Sir, actually the question is that this typically this ARC material is it IR resistant or not? Sir, this ARC material is typically IR resistant or not? ARC is the IR resistant. If temperature increases efficiency decreases, if ARC material is IR resistant then we can minimize the reduction in efficiency. See the ARC anti-reflective coating is a dielectric material they are the oxides and the nitrides for example, silicon nitride is used as a ARC, silicon oxide is also used as a ARC and all these materials are a high band gap material. When the material is high band gap they do not interact with the low energy photons and your IR photons are a low energy photon. Even they will not interact with the UV photons ultraviolet and therefore, they are perfectly resistant to the IR radiation and visible radiation. So, no problem with the ARC material as such. The degradation of ARC occurs because of the ambient condition. So, if ARC getting is a moisture somehow and if the interface between the ARC layer and the silicon layer is changing because of the change in the temperature then degradation occur, but the degradation of ARC layer is not because of the light. Vianite in Akpur. Sir, how will the bypass diodes are present in the module? So, how do we know the bypass diodes are present in the module? If you look at the backside where the two connections are coming out of the module you know there is small box that box it is called the junction box, the black color box at the backside look open that box and you will see that there are two diodes sitting there. So, at the backside of modules after the lunch when you do your experiment you try to identify the bypass diode. So, sir that will be the blocking diode or bypass diode? They are mainly normally the blocking diodes are not connected and that we should try to connect, but bypass diodes are essential because the shadow of any module can occur and therefore those are the normally the bypass diodes. Sir, one more question suppose we have 36 cell module. So, there will be two bypass diodes? Yes. So, normally optimization people have done that one bypass diode should be connected for every 15 to 18 solar cell. So, when there are 36 solar cell you should put two bypass diodes. Now, KG Sumaya Mumbai. In between the system was down here sir. So, we did not hear that explanation of mismatch in parallel connection can you just summarize it? So, mismatch in parallel connection is not that severe you know because mismatch in parallel connection of course, when you are doing the parallel connection your currents. So, let me go back to that slide where discuss. So, in parallel connection the mismatch can be because of the current and because of the voltage or because of the fill factor. Now, because the currents in the parallel connection the currents are additive right. So, even if you have two different current from two different solar cell or module you are not going to have a problem. So, mismatch in current is no problem in the parallel connection the mismatch in voltage could be a problem, but mismatch in voltage is not that severe in the parallel connection as the mismatch in the current in the series connection is because mismatch in current in series connection can actually force a solar cell in the to become a load and there can be lot of power dissipation and because of that there can be heat generated and the damage to the module. That is it is a mismatch in series connection mismatch in current in series connected cells and module is more a bigger problem. When you are connecting the on the voltage it is only you are going to lose the power, but otherwise there is no damage cause to the modules. Sir that blocking diode thing we did not here actually. Somebody was asking a query about blocking diode. The blocking diode is actually is to block the its block the current flowing back into the module. Now and that blocking diode is normally not connected in the in the module typically every manufacturer will supply you the bypass diode, but not the blocking diode and many times if required the blocking diode has to be added. Now this blocking diode is taken care by many electronic circuits. So, when there is a charge controller sitting there or when there is a inverter designed for solar PV the blocking diode will be part of the electronics and not normally added in the module, but the bypass diode is added in the module. Question from VNIT in Akpur. Sir in a series connected cells if the short circuit current of two cells are different then how will ensure that the total short circuit current will be same? No we do not have to ensure it is the property of the circuit that we do not have to do anything it is the property of the series connected circuit that the current same current can only flow that is the property of the circuit itself we do not have to ensure. What we have to do is we have to try to use the cells and connect them series only those cells which have the same short circuit current that is what we should try otherwise we are going to lose in terms of the power. So, if your solar cell efficiency like there are out of 36 solar cell if 35 solar cell have the efficiency of 15 percent and one solar cell has the efficiency of 14 percent this one particular solar cell will actually bring down the efficiency of the whole module. So, we are going to lose the power of those 35 solar cells and that is why it is our responsibility to try to match the current as much as we can. Amrita school, Kolam. So, in this solar panel will there be any circulating current between the cells if the voltage is different? Is there any circulating current between the cell if the voltage is different no actually so what happens as we have seen that if the current is not same in the series connected cell immediately the voltage of a solar cell will be different. So, that the same current is maintained. So, it is the voltage of individual cells will get change and if it is really blocking if really the light is shaded or not falling on a solar cell the voltage polarity itself will change. So, the change in the voltage across the solar cell in a series connected module will try to manage. So, that the current is constant in all the solar cells and how this can be maintained you know by changing the voltage what will happen which current component will change it is the forward bias current will change. So, that the net current from the cell is same in the all series connected the forward bias current component will change. Last question on the audio from the salem and then we will take the chat questions. Sir, we are having a solar panel, but it is not connected to the load for about 6 months what will happen will there be any faults are occurring? No, nothing will happen to the module it is well designed to operate in the ambient condition outside field condition which has much harsher and it works for 25 years if you keep in your covered for 6 months it will be perfectly safe no problem. How can we track the maximum power from the PV any algorithm sir sir sir can you explain? How to track the maximum power from PV is there any algorithm answer is yes I guess professor Fernandez will take care of that question. So, yeah there are many algorithm which are implemented there are devices available in the market which are called MPPT device maximum power point tracking device which tracks electrically the maximum power point and there are many algorithms available and I think professor Fernandez is better suited to answer that question. So, let me go to the chat and try to look at some of the question. Can you arrange a local solar industrial visit in future programs? Nice suggestions yes we can arrange bringing thousand people together will be difficult, but in your local area we can do that that is very nice if we can do that, but normally it is very difficult. In the fourth example today tutorial we are getting required number of solar cells 17 can you please solve the query the 17 you are getting what is open circuit voltage requirement. So, it may be possible you know first of all I will check what is the solution, but if your voltage is very high of a solar cell if you are using cadmium telluride solar cell which is giving a higher voltage because of the higher band gap of or if you are using amorphous silicon solar cell which is again having high band gap and therefore, high voltage and if your voltage requirement is low it is possible that you get less number of solar cells. So, that may be possible condition yeah go ahead. Hello sir I have question that when the two cells are connected in series there are short circuit current should be identical, but if they are not matching then you told that the voltage becomes negative output voltage becomes negative and hence the power becomes negative. So, why the voltage becomes negative that? So here you see when you are when you are connecting two of your solar cell in series the current if I current is entering same current should flow here same current should come out right. So, whatever is the condition the voltage and now this voltage can be different right voltage across the two solar cell can be different. Now in our solar cell there are two current component I equal to I 0 e raise to power q v by k t minus 1 minus I 0 I am sorry minus I l. So, this is light generated current and this is what is this current this is the forward bias diode current right this is the forward bias diode current. So, we have to do something. So, now this two current components will automatically get adjusted now because of the shading this current component changes and because of the voltage this current component should change such that the total current flowing in the series is constant and that is why when when you are putting two solar cell which are not having the same current in series what will happen is the voltage of the other solar cell will change in such a way that actually it goes into the diode mode rather than the p v solar cell mode it goes into diode mode and because of that the voltage across will change such that the two currents in the both the solar cell are the same right. So, this is the characteristic the same current must flow and therefore, the voltage across the solar cells should change in a such a way that it actually allows the same current to flow and that is why the voltage of a second solar cell will change and that is why you will and that is how you will get the same current flowing in the circuit. Negative voltage how come it compensates for this one? Now, your current will change. So, let me draw let me draw the I v normal I v current how is the normal I v curve your normal I v curve is like this right this is a normal I v curve. So, normally you want your voltage to operate here you want your solar cell to operate here, but your solar cell can operate here also. So, by going from positive it becomes negative and by doing this actually current also increases little bit right this curve is not flat here the current also little bit increases. So, as compared to this point where the current will be I 1 at this point your current is I 2 and in this case what is required this current solar cell which is having lower shock circuit current should increase its current and that can be happen by this way. So, if your voltage goes from positive to negative your current actually now increases and that is how you fulfill the condition of a same current in the circuit. Now, for charging the battery of say 12 volts the the solar pv that we have to use has been the voltage of say 40 to 15 volts, but what about a current rating for the pv cell or a battery? If the current rating of the battery is greater than the current rating of the pv module then will it load the module ok. So, I think as for as as for as long as the voltage of a module is higher than the voltage of the battery that is good enough condition even if the current rating of your ampere hour rating of your battery is much larger that is not going to affect only it will affect is the slow charging of the battery, but the main important the potential difference of your module has to be higher than the potential of your battery that is enough. So, that it can actually force the charge to go inside the battery, but if your the current rating is not the same it is it is not it is not a problem. You have solved last question power calculation you have your answer is 130 watt, but it is 130 watt per square meter it should be 130 watt per square meter and you have calculated it for 15 centimeter by 15 centimeter module. So, I think there is some somewhere mistake. There is no mistake answer is perfectly calculated. Sir that the wattage is for 15 by 15 centimeter. This wattage is for 15 by 15 15 by 15 centimeter square cell and there are 36 cell connected in series. Now, this module area may be 1 meter square or may not be 1 meter square irrespective of that this kind of arrangement is always going to give you 151 watt. Sir understood. While calculating VOC change in the temperature we took change in the temperature to be positive what happen if the that if the delta change is the negative. So, this I think I have answered that normally we say that the cell temperature increases. So, there is a positive difference what will happen if the cell temperature decreases. So, I would say the power will increase. Let me give an example of that this question people have asked couple of times. So, if my if I have module if my PV module is there which is having a power rating of let us say 100 watt peak. Now, this 100 watt peak is as we know as 1000 watt per meter square and 25 degree centigrade. Now, what will be the power what will be the power output for temperature of for temperature of 50 degree centigrade answer is low lower than 100 watt lower than 100 watt we can calculate, but what if my temperature is 15 degree centigrade will the 100 watt peak module will give me more than 100 watt peak answer is yes your power is going to be more than 100 watt. So, at 15 degree centigrade because your temperature change is negative you are actually going to get higher power than 100 watt peak. So, 25 degree centigrade is your bar if your temperature increases above that your power decreases if your temperature decreases below that your power increases by the same fashion. Where is the bypass diode situated in a PV module provided by you the bypass diode as I explained to some center is at the back side if you look at the back side where your two wires are coming from the module that is called the junction box your bypass diode will be available in junction box. So, look at the junction box carefully it is a small black box at the back side you can open it actually there is a sliding sheet you can open it and you can look at the bypass diode. Normally the block normally the blocking diode is not connected as I told blocking diode is a part of the electronics many times. So, you will find your blocking diode functions in charge controller or inverter not in the module. Thank you very much.