1 00:00:06,190 --> 00:00:07,350 Welcome back. 2 00:00:07,350 --> 00:00:13,260 In the previous block you learnt about the stand-alone PV system and its design. 3 00:00:13,260 --> 00:00:19,100 Now, we will look at the characteristics of a grid-connected PV system. 4 00:00:19,100 --> 00:00:21,590 So let's get started. 5 00:00:21,590 --> 00:00:27,900 As the name suggests, the grid-connected PV system is connected to the electric grid. 6 00:00:27,900 --> 00:00:34,649 There is a continuous exchange of power with the electric grid via the distribution panel. 7 00:00:34,649 --> 00:00:40,689 As discussed before, the grid-connected PV system has these important PV system components. 8 00:00:41,870 --> 00:00:46,999 The PV array is an interconnection of modules that supplies the required photogenerated 9 00:00:46,999 --> 00:00:48,839 power to the system. 10 00:00:48,839 --> 00:00:53,639 The power rating of the array is determined based on the system design. 11 00:00:55,109 --> 00:01:01,449 The grid-connected or grid-tied inverter is the backbone of the grid-connected PV system. 12 00:01:01,920 --> 00:01:08,440 As discussed last week, it is the inverter that is responsible for not only supplying 13 00:01:08,440 --> 00:01:14,100 power to the grid as a current source, but also for tracking the grid voltage and frequency. 14 00:01:15,680 --> 00:01:20,820 The grid and the load are usually connected through a distribution panel. 15 00:01:21,620 --> 00:01:28,780 The interaction with the grid is two-way, while the interaction with the load is unidirectional. 16 00:01:29,040 --> 00:01:33,090 A net meter is also usually found in this type of PV system. 17 00:01:33,090 --> 00:01:37,690 A net meter is nothing but a two way power measuring instrument. 18 00:01:37,690 --> 00:01:44,220 This meter takes into account not only the power consumed from the grid by the consumer, 19 00:01:44,220 --> 00:01:48,670 but also the power fed to the grid by the PV system. 20 00:01:48,670 --> 00:01:53,150 Now let us look at the main features of such a PV system. 21 00:01:53,300 --> 00:01:58,840 What happens when the PV array is producing more power than the load demand? 22 00:01:58,840 --> 00:02:03,920 And what if the PV power is insufficient to power the load? 23 00:02:04,240 --> 00:02:07,600 Let us understand this in the following animation. 24 00:02:08,040 --> 00:02:13,570 On a normal day with the sun out, the PV modules on top of this rooftop are busy converting 25 00:02:13,750 --> 00:02:16,830 the incoming irradiance into photogenerated power. 26 00:02:18,130 --> 00:02:23,220 The grid-connected solar inverters used in the system are also constantly converting 27 00:02:23,220 --> 00:02:28,870 the DC output of the solar modules into usable AC power. 28 00:02:28,870 --> 00:02:33,920 The PV system is able to meet the load demand of the household. 29 00:02:33,920 --> 00:02:40,090 On a different day, if it's a very sunny day, the PV system is providing much higher power 30 00:02:40,090 --> 00:02:41,720 than what the load needs. 31 00:02:41,720 --> 00:02:48,110 Under such a condition, the net surplus power is fed to the grid. 32 00:02:48,110 --> 00:02:53,850 In most countries, the consumer can offset his electric bills in this manner. 33 00:02:53,850 --> 00:02:58,170 This facility is called net metering. 34 00:02:58,170 --> 00:03:04,700 You can see that the net meter is reading a net production as the net power is being fed to the grid. 35 00:03:05,430 --> 00:03:10,490 On the other hand, on a cloudy day, the PV system is providing less power than what the 36 00:03:10,490 --> 00:03:12,209 load is expecting. 37 00:03:12,209 --> 00:03:18,800 Under such a condition, the load demand is fulfilled by taking the excess power from the grid. 38 00:03:19,000 --> 00:03:25,470 Thus, the net meter registers a net consumption as well. 39 00:03:25,470 --> 00:03:31,090 Now, we will look at the important concept of system efficiency. 40 00:03:31,090 --> 00:03:37,520 We know that the PV modules could be as much as 20% efficient in converting the incoming 41 00:03:37,520 --> 00:03:40,650 irradiance into PV power. 42 00:03:40,650 --> 00:03:46,520 We have also seen how the rest of the system components have less than 100% efficiencies. 43 00:03:46,520 --> 00:03:53,520 So, what can be said about the overall system efficiency? 44 00:03:53,959 --> 00:03:59,690 In this grid-connected PV system schematic, you see the various system components. 45 00:03:59,690 --> 00:04:04,160 Let's walk through the system to see the efficiency loss at each stage. 46 00:04:04,160 --> 00:04:10,709 I will suppose some efficiency values for each component so that we can get a rough 47 00:04:10,709 --> 00:04:14,509 estimate of how the system efficiency might look like. 48 00:04:15,800 --> 00:04:23,140 Let's assume AM1.5 irradiance levels, and that the PV array is 20% efficient. 49 00:04:23,600 --> 00:04:30,600 Therefore, the power density present at the output of the PV array is 200 W/m². 50 00:04:31,800 --> 00:04:37,000 As the balance of system could be placed at a considerable distance from the PV array, 51 00:04:37,000 --> 00:04:42,460 due to the fact that inverters need to be well protected, it might be that the DC power 52 00:04:42,460 --> 00:04:45,280 loss in the cables is significant. 53 00:04:45,280 --> 00:04:51,410 Depending on the length of the cables and the amount of current through them, this loss 54 00:04:51,410 --> 00:04:54,510 could be anywhere around 1-5%. 55 00:04:54,880 --> 00:05:01,380 In this example, let's assume that we have a cable loss equal to 2% of the PV power production, 56 00:05:01,820 --> 00:05:08,750 or equivalently, the cables have a transmission efficiency of 98%. 57 00:05:09,920 --> 00:05:21,700 Therefore, the DC power per area going to the inverters is 98% of 200 which is 196 W/m². 58 00:05:21,700 --> 00:05:23,979 Now, let's go to the inverter. 59 00:05:23,979 --> 00:05:29,240 With advancements in power electronics, it is rather common to have inverters that reach 60 00:05:29,240 --> 00:05:31,710 greater than 97% efficiencies. 61 00:05:31,710 --> 00:05:38,710 However, usually, the efficiency of the inverter is dependent on the power at which it operates. 62 00:05:39,680 --> 00:05:45,370 It is seen from practice that the inverters typically reach their rated efficiencies at 63 00:05:45,370 --> 00:05:49,900 around 50% of the rated operating power. 64 00:05:49,900 --> 00:05:56,479 For the sake of simplicity, we shall assume an average efficiency of 95% for the inverter. 65 00:05:56,479 --> 00:06:08,000 Thus, the AC power per area present in the system is 95% of 196, which is 186.2 W/m². 66 00:06:08,000 --> 00:06:15,500 Assuming a lossless exchange at the distribution panel and negligible dissipation in the cables 67 00:06:15,580 --> 00:06:24,120 carrying AC power, we get an overall PV system output as 186.2 W/m². 68 00:06:25,100 --> 00:06:34,700 This means, that the incoming irradiance at 1000 W/m² is processed by the PV system to give 186.2 W/m². 69 00:06:34,700 --> 00:06:46,440 In other words, the overall system efficiency is 186.2 divided by 1000 W/m², which is 18.62%. 70 00:06:47,160 --> 00:06:52,880 It can also be said that the overall system efficiency is calculated as the product of 71 00:06:52,889 --> 00:06:55,650 the various component efficiencies. 72 00:06:55,650 --> 00:07:01,970 That is, eta_system equals eta_PV times eta_cable times eta_inverter. 73 00:07:02,700 --> 00:07:09,000 Now let us move on to the design of a simple grid-connected PV system. 74 00:07:09,000 --> 00:07:15,900 As a quick recap, let us look back at the flowchart of the stand-alone PV system design process. 75 00:07:16,169 --> 00:07:22,479 You will remember that the system design process had several steps. 76 00:07:22,479 --> 00:07:27,990 Given that the grid-connected PV system topology is significantly different than that of the 77 00:07:27,990 --> 00:07:34,130 stand-alone PV system, how does the flowchart of the grid-connected PV system differs from 78 00:07:34,130 --> 00:07:36,010 the stand-alone system design? 79 00:07:37,480 --> 00:07:45,140 As expected, the grid-connected system design flowchart looks simpler, owing to the absence 80 00:07:45,259 --> 00:07:47,499 of the battery and the charge controller. 81 00:07:48,400 --> 00:07:54,560 Although the load demand and the equivalent sun hours could shape the sizing of a grid 82 00:07:54,560 --> 00:08:00,420 connected PV system, it is not necessary to base the grid-connected system sizing on the load. 83 00:08:00,990 --> 00:08:02,910 I will come back to this later. 84 00:08:04,380 --> 00:08:10,260 However, for this example, to get a sense of comparison with the stand-alone PV system 85 00:08:10,270 --> 00:08:16,630 design, we shall consider the grid-connected system design process with a load demand. 86 00:08:18,240 --> 00:08:23,800 In this design process, we shall first understand the load requirements from the system on a 87 00:08:23,800 --> 00:08:26,000 per day basis. 88 00:08:26,000 --> 00:08:30,460 Then, we shall account for the system losses. 89 00:08:31,960 --> 00:08:36,019 We will then take into account the equivalent sun hours. 90 00:08:36,019 --> 00:08:40,110 Then we will size the PV array. 91 00:08:40,110 --> 00:08:44,000 And finally, we will look at the inverter design. 92 00:08:44,000 --> 00:08:47,070 So let's get started! 93 00:08:47,070 --> 00:08:52,580 Let us consider a similar load demand as shown in the stand-alone PV system design example 94 00:08:52,580 --> 00:08:54,060 of the previous block. 95 00:08:54,839 --> 00:09:01,190 The required quantities of each kind of load, along with their duration of usage are mentioned. 96 00:09:01,190 --> 00:09:05,270 Note that all the loads in this system are AC loads. 97 00:09:05,270 --> 00:09:11,100 The total power and energy requirements have been tabulated here as well. 98 00:09:11,100 --> 00:09:16,600 Another main difference here is that there is no concept of autonomous days, as there 99 00:09:16,600 --> 00:09:17,790 is no storage. 100 00:09:17,790 --> 00:09:22,390 Instead, the electric grid acts as a limitless storage. 101 00:09:23,640 --> 00:09:27,640 Let us first account for the losses in the system. 102 00:09:27,640 --> 00:09:33,240 This would help us find the energy needed at the output of the PV array to successfully 103 00:09:33,240 --> 00:09:34,700 cover the daily load. 104 00:09:35,480 --> 00:09:41,120 Here we assume the same component efficiencies as we saw in the earlier example. 105 00:09:41,120 --> 00:09:48,120 That is, the cables have a transmission efficiency of 98% for the DC power, and the grid-tied 106 00:09:48,200 --> 00:09:51,029 inverter shows an efficiency of 95%. 107 00:09:51,029 --> 00:09:59,669 We also see that the AC load at the inverter end demands a total of 600 Wh during the day. 108 00:10:01,160 --> 00:10:07,600 Given the system component efficiencies, we can then calculate the equivalent energy required 109 00:10:07,610 --> 00:10:09,470 from the PV panels as shown. 110 00:10:09,470 --> 00:10:16,470 This is basically nothing but the transposition of the energy before all these losses occur. 111 00:10:17,209 --> 00:10:24,549 Therefore, the total energy requirement from the PV array is 644.5 Wh. 112 00:10:25,050 --> 00:10:29,140 Next, we consider the irradiance. 113 00:10:29,140 --> 00:10:34,370 Going by the same example as in the stand-alone system case, a location in India is considered 114 00:10:34,370 --> 00:10:38,190 with an average of 4.5 equivalent sun hours. 115 00:10:38,920 --> 00:10:43,820 Let's look at the electrical specifications of an available PV module. 116 00:10:43,830 --> 00:10:50,380 It is a 100 Wp rated module with the given voltage and current parameters. 117 00:10:50,380 --> 00:10:56,800 Now, we need to find out how many of such modules are required to power the loads. 118 00:10:57,980 --> 00:11:04,480 Assuming that the panel would be operated at its MPP, we can find out the required number 119 00:11:04,680 --> 00:11:07,370 of panels as follows. 120 00:11:07,370 --> 00:11:13,740 We can first calculate the amount of minimum PV power required by dividing the total energy 121 00:11:13,740 --> 00:11:19,040 demand at the PV array output with the equivalent sun hours. 122 00:11:19,040 --> 00:11:24,140 Thus the minimum PV power required is 143.2 W. 123 00:11:26,020 --> 00:11:30,220 Also, the number of the panels could be calculated as shown. 124 00:11:30,220 --> 00:11:34,510 In this case, the total number of required panels is 2. 125 00:11:34,510 --> 00:11:40,430 Usually, panels are connected based on their compatibility with the DC ratings of the inverter. 126 00:11:41,020 --> 00:11:47,500 Care should be taken that the worst case current and voltage from the PV array do not violate 127 00:11:47,510 --> 00:11:49,690 the input parameters of the inverter. 128 00:11:51,040 --> 00:11:55,600 Now let us look at the possible PV configurations. 129 00:11:56,300 --> 00:12:01,839 The maximum allowable current and voltage rating can be found by assuming these scenarios. 130 00:12:01,839 --> 00:12:09,800 If the 2 panels are connected in parallel, then a maximum current of 2*Isc is possible, 131 00:12:09,800 --> 00:12:12,900 which in this case is 14 A. 132 00:12:14,490 --> 00:12:21,470 On the other hand, if the 2 panels are connected in series, then a maximum voltage of 2*Voc 133 00:12:21,470 --> 00:12:25,030 is possible, which will equal 40 V. 134 00:12:25,500 --> 00:12:29,720 Now I must clarify one thing about the grid-connected PV system. 135 00:12:29,980 --> 00:12:36,260 Usually, the amount of PV panels or the rating of PV panels need not be exactly as per the 136 00:12:36,260 --> 00:12:41,010 load requirements, like it was in the stand-alone PV system. 137 00:12:41,010 --> 00:12:46,769 This is because any excess or deficit of PV power can be compensated by the grid, that 138 00:12:46,769 --> 00:12:48,970 is present in the scenario anyway. 139 00:12:48,970 --> 00:12:55,790 This was not true for the stand-alone system, as any surplus or deficit of power had to 140 00:12:55,790 --> 00:13:02,470 be met with a variation in the storage size, leading to considerable system costs. 141 00:13:02,470 --> 00:13:09,279 Consequently, in grid-connected PV systems, it is usually seen that the inverter is sized 142 00:13:09,279 --> 00:13:15,379 based on the PV array sizes and not the load size, like in the stand-alone systems. 143 00:13:17,810 --> 00:13:20,769 So the required PV power is 200 W. 144 00:13:20,769 --> 00:13:24,470 The inverter should be able to at least handle this power. 145 00:13:24,470 --> 00:13:30,390 Thus, we say that the minimal nominal rating of the inverter is 200 W. 146 00:13:31,400 --> 00:13:38,000 Let us look at the operational parameters of an example inverter that fits the bill 147 00:13:38,000 --> 00:13:41,100 with respect to the required minimum power rating. 148 00:13:42,200 --> 00:13:49,960 Note that this inverter has an MPPT feature, thereby making the panels work at maximum power point. 149 00:13:50,940 --> 00:13:58,040 Looking at the I-V ratings of the DC side of the inverter, we see that the maximum DC 150 00:13:58,040 --> 00:14:02,660 input voltage for the inverter is greater than the maximum voltage. 151 00:14:04,240 --> 00:14:13,400 On the other hand, the maximum DC input current for the inverter is less than Imax but greater than Isc. 152 00:14:14,400 --> 00:14:19,340 Thus we can say that the ideal panel configuration could be in series. 153 00:14:20,100 --> 00:14:25,760 Note that in practice, sizing of the system is not the only thing, one must take into 154 00:14:25,760 --> 00:14:32,200 account technical and regulatory policies before implementing a grid-connected PV system. 155 00:14:33,040 --> 00:14:38,640 In this video, due to time constraints, we limit ourselves with the sizing of simple 156 00:14:38,649 --> 00:14:40,809 grid-connected PV systems only. 157 00:14:42,280 --> 00:14:48,279 As a final note, I'd like to talk about oversizing the PV system sizing. 158 00:14:48,279 --> 00:14:55,279 Now, should or can a user oversize the PV system, even if he can meet the load needs 159 00:14:55,420 --> 00:14:56,840 for much lesser power? 160 00:14:57,600 --> 00:15:05,800 For instance, in the previous example, what if instead of 200 W, you had installed 500 W? 161 00:15:05,880 --> 00:15:09,440 Well, in simple terms, it is possible. 162 00:15:09,920 --> 00:15:13,480 Any excess you produce is sent to the grid. 163 00:15:14,060 --> 00:15:19,540 Whether or not the grid operator pays you back for this excess depends on the net metering 164 00:15:19,550 --> 00:15:24,530 related policy of the electric utility you are connected to. 165 00:15:25,440 --> 00:15:32,080 If your grid operator pays you back for every watt of PV power you pump into the grid, it might 166 00:15:32,089 --> 00:15:35,429 look beneficial to oversize your PV system. 167 00:15:36,120 --> 00:15:42,699 But then the decision would depend on the system costs, and the expected system yields, 168 00:15:42,699 --> 00:15:49,040 which in turn depend on the irradiance of the place, module level effects, etc. 169 00:15:49,040 --> 00:15:55,600 Thus, there could be quite some optimization that could be done purely at an economical level. 170 00:15:56,500 --> 00:16:00,290 So you have now looked at the grid-connected PV system. 171 00:16:00,290 --> 00:16:05,070 In the next block we shall look at some specific kinds of PV systems. 172 00:16:05,440 --> 00:16:07,040 See you next block!