1 00:00:05,680 --> 00:00:08,990 Welcome back to the last week of this course. 2 00:00:08,990 --> 00:00:14,049 In week 7 you saw the various PV system components and their characteristics. 3 00:00:14,049 --> 00:00:20,119 Now, we will see how these components come together and make up a PV system. 4 00:00:20,119 --> 00:00:25,580 In doing so, we will not only look at the characteristics of these PV systems, but also 5 00:00:25,580 --> 00:00:29,720 look at some of their basic design rules. 6 00:00:29,720 --> 00:00:33,120 We will first start with the stand-alone PV system. 7 00:00:33,120 --> 00:00:40,120 It is also called the off-grid PV system, simply because it functions independent of 8 00:00:40,190 --> 00:00:43,320 the electric grid. 9 00:00:43,320 --> 00:00:50,320 As discussed before, the stand-alone PV system has these important PV system components. 10 00:00:50,640 --> 00:00:56,879 The PV array is an interconnection of modules that supplies the required photogenerated 11 00:00:56,879 --> 00:00:59,430 power to the system. 12 00:00:59,430 --> 00:01:06,430 The power rating of the array is determined based on the load requirements. 13 00:01:06,899 --> 00:01:13,899 The battery bank is the lifeline of the stand-alone PV system, as it greatly increases the usability 14 00:01:14,250 --> 00:01:16,530 of the PV system. 15 00:01:16,530 --> 00:01:23,130 Without the battery, the system would be unable to meet the load demands outside the available daylight. 16 00:01:24,980 --> 00:01:31,320 As discussed last week, the charge controller is singlehandedly responsible for ensuring 17 00:01:31,330 --> 00:01:33,390 the smooth functioning of the battery. 18 00:01:34,280 --> 00:01:41,140 Given the complexity of the functioning of a battery, the use of a good charge controller can increase 19 00:01:41,150 --> 00:01:43,770 the lifetime of the battery manifold. 20 00:01:44,820 --> 00:01:50,619 This is more than welcome, as the battery is often the biggest bottleneck in a stand-alone 21 00:01:50,619 --> 00:01:53,060 PV system's lifetime. 22 00:01:54,360 --> 00:02:00,060 The stand-alone inverter has to be responsible for efficient power conversion from DC to 23 00:02:00,369 --> 00:02:03,949 AC power over a wide range of the loads. 24 00:02:04,960 --> 00:02:11,039 The better the inverter, the higher the efficiency over a variety of loads. 25 00:02:11,039 --> 00:02:18,039 Together, the charge controller, battery, inverter, distribution panels and the wiring 26 00:02:18,200 --> 00:02:20,709 make up the balance of system. 27 00:02:20,709 --> 00:02:30,600 In general, everything in the PV system minus the PV array constitutes the balance of system, or BOS. 28 00:02:31,450 --> 00:02:36,299 The system could be connected to both DC and AC loads. 29 00:02:36,299 --> 00:02:42,469 Of course, the DC loads would be powered by the DC power before the inversion stage. 30 00:02:42,469 --> 00:02:47,629 Now let us look at the features of such a PV system. 31 00:02:47,629 --> 00:02:53,859 If such a system is used to feed loads that are powered on at different parts of the day, 32 00:02:53,859 --> 00:02:57,769 how will the system cope with such a power demand? 33 00:02:57,769 --> 00:03:02,549 What happens when the PV array is producing more power than the load demand? 34 00:03:03,580 --> 00:03:08,520 Let us understand this using a simple animation. 35 00:03:08,529 --> 00:03:14,569 On a normal day with the sun out, the PV modules on top of this rooftop are busy converting 36 00:03:14,569 --> 00:03:18,549 the incoming irradiance into photogenerated power. 37 00:03:18,549 --> 00:03:24,510 The battery is constantly being charged or discharged through the charge controller, 38 00:03:24,510 --> 00:03:28,199 depending on the difference between PV supply and load demand. 39 00:03:28,199 --> 00:03:34,620 The off-grid or stand-alone inverter is continuously converting the DC power to AC and supplying 40 00:03:34,620 --> 00:03:36,989 this AC power to the loads. 41 00:03:36,989 --> 00:03:42,879 The system is designed and sized such that during the day with the sun shining, the system 42 00:03:42,879 --> 00:03:48,359 generates enough power to at least meet the load demand for the rest of the day. 43 00:03:48,359 --> 00:03:53,799 Therefore, the PV system acts such that the battery is net charged over the day. 44 00:03:53,799 --> 00:04:00,799 After daylight, the rest of the power demand is fulfilled by the battery. 45 00:04:00,809 --> 00:04:06,999 Now you might be thinking what happens if it's not a sunny day, or even worse, there 46 00:04:06,999 --> 00:04:11,409 is a string of dark days, like in many parts of the world. 47 00:04:11,409 --> 00:04:18,389 In such a case, if you have to meet the load demand, then there is no option but to oversize 48 00:04:18,389 --> 00:04:23,749 the battery so that the expected days of autonomy can be dealt with. 49 00:04:25,240 --> 00:04:31,820 Now, let's go on to the more interesting aspect of designing an off-grid system for a house 50 00:04:32,120 --> 00:04:34,720 in a remote location. 51 00:04:34,720 --> 00:04:40,870 We shall look at the designing and sizing of a PV system, based on the load demands and 52 00:04:40,870 --> 00:04:43,500 the available components. 53 00:04:43,500 --> 00:04:45,280 Let's go to this example. 54 00:04:45,280 --> 00:04:52,280 I will introduce you to a simple flowchart that helps to design an off-grid PV system. 55 00:04:52,710 --> 00:04:59,710 We will walk through all the stages of this flowchart while considering an example. 56 00:05:00,270 --> 00:05:06,439 Let us first define the load demand that has to be powered with an off-grid PV system. 57 00:05:06,439 --> 00:05:13,439 The total days of autonomy required should also be specified. 58 00:05:13,939 --> 00:05:20,000 Consider a house that is to be powered independent of the electric grid by means of a stand-alone 59 00:05:20,000 --> 00:05:22,680 PV system. 60 00:05:22,680 --> 00:05:29,680 Let's assume that 3 simple kinds of loads are required to be powered: light, TV and 61 00:05:29,860 --> 00:05:32,699 desktop computer. 62 00:05:32,699 --> 00:05:39,620 The lights are supposed to be DC powered, while the TV and the desktop computer are 63 00:05:39,620 --> 00:05:41,379 AC powered. 64 00:05:41,379 --> 00:05:48,800 The required quantities of each kind of load, along with their duration of usage are also mentioned. 65 00:05:49,509 --> 00:05:54,780 The total power and energy requirements have been tabulated here as well. 66 00:05:54,780 --> 00:06:03,700 Both the DC loads and the AC loads require a total energy of 300 Wh each per day. 67 00:06:05,259 --> 00:06:11,590 It is required that the system enjoys a total autonomy of 2 days, that is, a fully charged 68 00:06:11,590 --> 00:06:18,590 battery should be able to feed the load when there are 2 days without solar power. 69 00:06:18,860 --> 00:06:23,780 Now we must account for the various losses in the system. 70 00:06:23,780 --> 00:06:30,780 In doing so, we want to estimate the minimum power that the PV array is expected to deliver. 71 00:06:31,960 --> 00:06:38,259 Here we have the various efficiencies of the system components mentioned. 72 00:06:38,259 --> 00:06:45,199 The inverter is supposed to have an efficiency of 90%, while the combined efficiency of the 73 00:06:45,199 --> 00:06:51,139 charge controller, battery and cables is 85%. 74 00:06:51,139 --> 00:06:55,639 First we have the DC and AC load energy required. 75 00:06:55,639 --> 00:07:01,939 Now, we must transpose these energy needs to the input of the inverter. 76 00:07:01,939 --> 00:07:08,249 The DC load energy remains unchanged, while the AC load energy is changed into the equivalent 77 00:07:08,249 --> 00:07:15,249 DC load energy based on the inverter efficiency as shown. 78 00:07:15,569 --> 00:07:22,569 Now these equivalent load energies should be transposed again, to the output of the 79 00:07:22,639 --> 00:07:24,499 PV array. 80 00:07:24,499 --> 00:07:31,439 This is so that we can get the actual amount of PV energy expected by the system per day. 81 00:07:31,439 --> 00:07:37,229 Again, we arrive at this total energy need by using the combined efficiency of charge 82 00:07:37,229 --> 00:07:42,729 controller, battery and the cables as shown. 83 00:07:42,729 --> 00:07:47,300 This is calculated to be 745 Wh. 84 00:07:47,300 --> 00:07:58,000 Thus, the PV array on a regular day should be able to supply 745 Wh of photogenerated energy. 85 00:07:58,189 --> 00:08:02,870 The next thing we have to consider is the equivalent sun hours. 86 00:08:02,870 --> 00:08:07,789 This will of course depend on the irradiance that the chosen location enjoys. 87 00:08:07,789 --> 00:08:13,779 In this example, suppose that the place is in a location within India, and that the place 88 00:08:13,779 --> 00:08:17,979 enjoys an average of 4.5 equivalent sun hours. 89 00:08:19,639 --> 00:08:25,939 Knowing the load demanded at the PV output, and the equivalent sun hours, we must now 90 00:08:25,939 --> 00:08:30,179 estimate the rated PV power required in the PV system. 91 00:08:32,479 --> 00:08:38,400 We have been given a particular type of PV panel that has the electrical parameters as shown. 92 00:08:38,589 --> 00:08:42,169 The panel is rated at 100 Wp. 93 00:08:42,880 --> 00:08:50,500 Assuming that the panel would be operated at its MPP, we can find out the required number 94 00:08:50,500 --> 00:08:53,320 of panels as follows. 95 00:08:53,320 --> 00:08:58,540 We can first calculate the amount of minimum PV power required by dividing the total energy 96 00:08:58,540 --> 00:09:03,889 demand at the PV output with the equivalent sun hours. 97 00:09:03,889 --> 00:09:08,860 This gives us a power of 165.6 W. 98 00:09:08,860 --> 00:09:15,360 Also, the number of panels could be calculated as shown, which in this case is found to be 99 00:09:15,360 --> 00:09:22,360 2, as the number of panels would always be the upper approximation to a whole number. 100 00:09:22,990 --> 00:09:26,970 Now let us look at the possible PV configurations. 101 00:09:26,970 --> 00:09:33,970 The maximum allowable current and voltage rating can be found by assuming these scenarios. 102 00:09:34,050 --> 00:09:41,590 If the 2 panels are connected in parallel, then a maximum current of 2*Isc is possible, 103 00:09:41,790 --> 00:09:44,290 which in this case is 14 A. 104 00:09:45,520 --> 00:09:52,680 On the other hand, if the 2 panels are connected in series, then a maximum voltage of 2*Voc 105 00:09:52,779 --> 00:09:56,079 is possible, which will equal 40 V. 106 00:09:57,880 --> 00:10:03,080 Now let's look at the specifications of an available charge controller. 107 00:10:03,089 --> 00:10:08,440 In this case, we need to ensure that the charge controller parameters would conform with the 108 00:10:08,440 --> 00:10:12,960 maximum parameters found in the different PV configurations. 109 00:10:14,430 --> 00:10:20,009 The operational voltage is the battery voltage that is supported by the controller. 110 00:10:20,009 --> 00:10:26,120 It is also the voltage at which the DC side of the PV system would operate; in other words, 111 00:10:26,120 --> 00:10:29,640 the DC loads would be operated at this voltage. 112 00:10:30,880 --> 00:10:38,120 Thus the nominal operating DC voltage of the system is dictated by the load rating as well 113 00:10:38,199 --> 00:10:39,419 as the battery bank. 114 00:10:41,100 --> 00:10:44,759 In this case it is given as 24 V. 115 00:10:44,759 --> 00:10:49,860 This is not to be confused with the maximum voltage, which simply specifies the amount 116 00:10:49,860 --> 00:10:57,580 of maximum voltage as provided by the PV output that the charge controller can handle at the input. 117 00:10:59,600 --> 00:11:05,050 We see that the maximum allowable voltage is greater than the series limit of the 2 118 00:11:05,050 --> 00:11:14,000 panels of 40 V, while the maximum current doesn't support the parallel configuration current of 14A. 119 00:11:14,980 --> 00:11:18,269 So we select the series configuration of the panels. 120 00:11:18,269 --> 00:11:25,100 In general, given a choice between series and parallel, series configuration is preferred 121 00:11:25,100 --> 00:11:32,100 to keep the current levels down, thereby minimizing the DC cable loss. 122 00:11:32,690 --> 00:11:36,649 Now let's move onto the battery design. 123 00:11:36,649 --> 00:11:44,000 The battery size is greatly affected by the days of autonomous operation expected from the system. 124 00:11:44,149 --> 00:11:48,609 A single battery is given with the specifications as shown. 125 00:11:49,500 --> 00:11:55,399 The depth of discharge or the DOD is the depth until which the battery can be effectively used. 126 00:11:56,720 --> 00:12:00,720 We must now choose how the battery bank must be configured. 127 00:12:01,640 --> 00:12:08,840 That is, how many batteries of the given specification should be there in the battery bank, and how 128 00:12:08,940 --> 00:12:11,340 should they be interconnected? 129 00:12:14,420 --> 00:12:17,960 The minimum battery capacity can be derived as follows. 130 00:12:19,220 --> 00:12:25,699 Based on the operational voltage, depth of discharge, and energy demand, the battery 131 00:12:25,699 --> 00:12:27,639 capacity can be sized as follows. 132 00:12:28,240 --> 00:12:35,240 Note that the days of autonomy, and the system losses tend to increase the battery bank size. 133 00:12:35,250 --> 00:12:43,250 Also, note that the lower the battery DOD, the higher the battery bank capacity requirement. 134 00:12:44,000 --> 00:12:50,149 On the other hand, the higher the battery DOD, the battery can be discharged more, and 135 00:12:50,149 --> 00:12:54,940 therefore, lower the battery bank capacity needed. 136 00:12:54,940 --> 00:13:00,380 So this gives us a minimum battery capacity of 103.5 Ah. 137 00:13:01,840 --> 00:13:06,839 Now this brings us to the number of batteries required. 138 00:13:06,839 --> 00:13:16,100 An operational voltage of 24 V requires 2 batteries of 12 V each in series, whereas a battery capacity 139 00:13:16,100 --> 00:13:21,300 of 103.5 Ah requires 5 batteries in parallel. 140 00:13:21,300 --> 00:13:28,300 Therefore, a total of 10 batteries are required to create the battery bank for the PV system. 141 00:13:30,580 --> 00:13:33,240 Lastly, we come to the inverter sizing. 142 00:13:34,060 --> 00:13:39,620 Based on the efficiency of the inverter, the power rating of the inverter needs to be calculated. 143 00:13:41,149 --> 00:13:47,009 An inverter is available with an efficiency of 90%, and operational voltage of 24 V. 144 00:13:47,920 --> 00:13:52,200 What should be the minimum nominal power rating of this inverter? 145 00:13:53,649 --> 00:13:58,569 We know that the power demand of the AC load is 200 W. 146 00:13:59,200 --> 00:14:05,060 Given the rated efficiency of the inverter, we can estimate the minimum nominal power 147 00:14:05,060 --> 00:14:08,260 rating as 222.2 W. 148 00:14:09,080 --> 00:14:14,740 Note that the inverter may be less efficient than the rated efficiency, and under such 149 00:14:14,750 --> 00:14:20,860 conditions it may be safer to have a margin for the power ratings of the inverter. 150 00:14:20,860 --> 00:14:27,800 So, we have seen a simple method to design a stand-alone PV system based on the load requirements 151 00:14:27,800 --> 00:14:29,910 and available system components. 152 00:14:29,910 --> 00:14:35,670 Note, that this is definitely not the most exhaustive way to design the PV system. 153 00:14:36,480 --> 00:14:42,720 This method works better for places with more or less consistent irradiance levels. 154 00:14:42,720 --> 00:14:48,980 For places with severe disparity between the summer and winter irradiances, the number 155 00:14:48,980 --> 00:14:55,750 of autonomous days would increase for winter, and the system costs would consequently rise. 156 00:14:55,750 --> 00:15:01,949 Also, the component margins considered for various parameters could differ from place 157 00:15:01,949 --> 00:15:04,749 to place based on the system requirements. 158 00:15:05,440 --> 00:15:12,000 Nonetheless, this method is good enough to get a basic estimate of the stand-alone PV 159 00:15:12,000 --> 00:15:13,060 system sizing. 160 00:15:14,079 --> 00:15:18,759 You will be able to practice more on this in the exercises of this week. 161 00:15:19,700 --> 00:15:21,260 See you in the next block!