1 00:00:05,629 --> 00:00:08,660 We now know what a solar cell is. 2 00:00:08,660 --> 00:00:12,219 But how do we make a module out of solar cells? 3 00:00:12,219 --> 00:00:13,959 What is the performance of a solar module? 4 00:00:13,959 --> 00:00:18,830 These are the questions I will answer in this final block. 5 00:00:18,830 --> 00:00:25,789 A wafer based device is usually referred to as solar cell. 6 00:00:25,789 --> 00:00:32,780 A solar module is a larger device in which many solar cells are connected. 7 00:00:32,780 --> 00:00:37,400 If you look at the level of a PV system, we can have several modules connected, 8 00:00:37,400 --> 00:00:40,500 and this is what we call an array. 9 00:00:40,800 --> 00:00:47,570 Now we look at how we make a solar module out of an ensemble of solar cells. 10 00:00:47,570 --> 00:00:52,800 We can connect the solar cells in different ways. 11 00:00:52,800 --> 00:00:57,760 First we have the series connection as shown in this figure. 12 00:00:57,760 --> 00:01:01,070 In a series connection the voltages add up. 13 00:01:01,070 --> 00:01:08,070 It means that if the open-circuit voltage of one cell is equal to 0.6 V, the string 14 00:01:08,460 --> 00:01:14,460 of the three cells deliver an open-circuit voltage of 1.8 V. 15 00:01:14,460 --> 00:01:21,259 If we look at the classic front metal grid, it means that the bus bars at the front side 16 00:01:21,259 --> 00:01:26,689 have to be connected with the back contact of the neighboring cell. 17 00:01:26,689 --> 00:01:32,249 In this slide you see a cross-sectional view of a few solar cells connected in series. 18 00:01:32,249 --> 00:01:39,700 Every solar cell has its back contact connected with the front contact of its neighboring cell. 19 00:01:39,799 --> 00:01:43,950 For series connected cells, the current does not add up. 20 00:01:43,950 --> 00:01:50,300 The current flowing through the solar cell is determined by the photocurrent in each solar cell. 21 00:01:50,300 --> 00:01:57,399 It means that the total current in the string of solar cells is equal to the current generated 22 00:01:57,800 --> 00:02:01,060 by one single solar cell. 23 00:02:01,060 --> 00:02:07,259 Let's look at the J-V curve of solar cells connected in series. 24 00:02:07,259 --> 00:02:13,310 If we connect two solar cells in series, it means that we can add up the voltages. 25 00:02:13,310 --> 00:02:18,400 However, the current remains the same in series connection. 26 00:02:18,400 --> 00:02:24,030 The resulting open-circuit voltage is two times that of the single cell. 27 00:02:24,030 --> 00:02:28,890 If we connect three solar cells in series, the open-circuit voltage becomes three times 28 00:02:28,890 --> 00:02:34,390 as large, whereas the current is that of one single solar cell. 29 00:02:34,390 --> 00:02:38,180 A second way of connecting the solar cell is in parallel. 30 00:02:38,180 --> 00:02:41,690 Here you see three solar cells connected in parallel. 31 00:02:41,690 --> 00:02:47,380 The I-V curve shown is that of a single solar cell. 32 00:02:47,380 --> 00:02:53,070 Parallel connection means that the voltage is the same over all solar cells, however 33 00:02:53,070 --> 00:02:57,860 the currents of the solar cells add up. 34 00:02:57,860 --> 00:03:04,860 If we have two solar cells connected in parallel, the current increases two times, whereas the 35 00:03:05,040 --> 00:03:06,870 voltage remains the same. 36 00:03:06,870 --> 00:03:11,950 If we have three cells in parallel, the current becomes three times as large, while the voltage 37 00:03:11,950 --> 00:03:16,000 is the same as for a single cell. 38 00:03:16,000 --> 00:03:21,880 This means that if we consider a module, you can partly tune the voltage and the current 39 00:03:21,880 --> 00:03:27,370 output by the arrangements of the connections of the solar cells. 40 00:03:27,370 --> 00:03:35,300 Here we see a typical solar panel that contains 36 solar cells connected in series. 41 00:03:35,700 --> 00:03:42,460 If a single junction solar cell would have a short-circuit current of 5 A, and an open 42 00:03:42,460 --> 00:03:49,340 circuit voltage of 0.6 V, it means that the current output of the module is equal to that 43 00:03:49,340 --> 00:03:53,040 of a single solar cell, which is 5 A. 44 00:03:53,040 --> 00:04:02,000 The open-circuit voltage would be 36 times that of a single junction cell, which equals to 21.6 V. 45 00:04:02,500 --> 00:04:07,819 If we would rearrange the connection of this module, we can get a different current and 46 00:04:07,819 --> 00:04:09,400 voltage output. 47 00:04:09,400 --> 00:04:16,239 If we connect 2 strings of 18 series connected solar cells in parallel, we would get a short 48 00:04:16,239 --> 00:04:22,960 circuit current of 2 times that of a single solar cell, which is 10 A. 49 00:04:22,960 --> 00:04:29,960 The open-circuit voltage would be 18 times that of a single solar cell, which is 10.8 V. 50 00:04:31,710 --> 00:04:38,710 Another aspect of modules is that some bypass diodes are integrated into the modules. 51 00:04:39,620 --> 00:04:43,190 Why do we need bypass diodes? 52 00:04:43,190 --> 00:04:46,710 For that we have to look at a solar module in real life. 53 00:04:46,710 --> 00:04:51,350 In real life, the solar module can be partly shaded. 54 00:04:51,350 --> 00:04:59,300 This can be the shade of an object nearby, like a tree, a chimney or a neighboring building. 55 00:04:59,900 --> 00:05:05,910 The shading can be caused by a simple leaf that has fallen from the tree. 56 00:05:05,910 --> 00:05:11,750 This can have significant consequences for the output of the solar module. 57 00:05:11,750 --> 00:05:18,800 Let's consider the situation in which one solar cell in the module is for a large part shaded. 58 00:05:19,639 --> 00:05:26,600 For simplicity we assume that all 6 cells, part of this small module, are connected in series. 59 00:05:26,700 --> 00:05:33,419 This means that the current generated in the shaded cell is significantly reduced. 60 00:05:33,419 --> 00:05:39,639 In a series connection the current is limited by the cell producing the lowest current. 61 00:05:39,639 --> 00:05:45,290 This cell dictates the maximum current flowing through the module. 62 00:05:45,290 --> 00:05:51,500 In this J-V curve we show the theoretical J-V curve of the 5 non-shaded solar cells 63 00:05:51,500 --> 00:05:55,780 and the 1 shaded solar cell. 64 00:05:55,780 --> 00:06:01,389 If we have a constant load, like the resistance R in the previous picture, it means that the 65 00:06:01,389 --> 00:06:07,169 voltage over the module is dropping due to the lower current generated. 66 00:06:07,169 --> 00:06:13,200 However, since the 5 non-shaded solar cells are forced to produce high voltages, 67 00:06:13,200 --> 00:06:18,650 they act like a reverse bias source on the shaded solar cell. 68 00:06:18,650 --> 00:06:23,900 The dashed line represents the reverse bias load put on the shaded cell. 69 00:06:23,900 --> 00:06:31,500 It is the J-V curve of the 5 cells, mirrored into the vertical axis, equal to 0 V. 70 00:06:31,520 --> 00:06:38,300 This means that the shaded solar cell does not generate energy, but starts to dissipate energy. 71 00:06:38,300 --> 00:06:43,400 It means the solar cell is getting warmer and warmer. 72 00:06:43,400 --> 00:06:49,110 The temperature can increase to such a critical level, that the encapsulation material cracks, 73 00:06:49,110 --> 00:06:51,330 or other materials wear out. 74 00:06:51,330 --> 00:06:58,330 In general, high temperature conditions lead to decrease of the PV output as well. 75 00:06:58,820 --> 00:07:03,460 This can be prevented by including bypass diodes in the module. 76 00:07:03,460 --> 00:07:07,960 In this figure you can see bypass diodes included in the electric circuit. 77 00:07:07,960 --> 00:07:13,820 A diode, as discussed in week 2 and 3, blocks the current in a direction when it's under 78 00:07:13,820 --> 00:07:19,000 negative voltage, but conducts a current when it's under positive voltage. 79 00:07:19,000 --> 00:07:25,410 If no cell is shaded, no current is flowing through the bypass diodes. 80 00:07:25,410 --> 00:07:31,060 In the case that 1 cell is shaded, due to the biasing of the other cells, the bypass 81 00:07:31,060 --> 00:07:33,710 diode starts to pass current through. 82 00:07:33,710 --> 00:07:39,810 As a result the current can go around the shaded cell and the module can still produce 83 00:07:39,810 --> 00:07:43,560 the current equal to that of a non-shaded single solar cell. 84 00:07:43,560 --> 00:07:50,280 As you can see, going from a solar cell up to module level gives rise to a whole new 85 00:07:50,280 --> 00:07:53,220 set of technical considerations. 86 00:07:53,220 --> 00:07:58,820 In week 7 we will discuss the operation of modules and the design considerations in the 87 00:07:58,820 --> 00:08:01,610 PV system in greater detail. 88 00:08:01,610 --> 00:08:07,009 So this week we have discussed crystalline silicon wafer based PV technology. 89 00:08:07,009 --> 00:08:12,340 I hope it was a helpful introduction into this technology. 90 00:08:12,340 --> 00:08:19,169 You have to realize that in view of time limits, we only covered the most rudimentary concepts. 91 00:08:19,169 --> 00:08:22,360 In the next week we will discuss different PV technologies: 92 00:08:22,360 --> 00:08:31,500 technologies which are based on thin-film materials, such as amorphous and nanocrystalline silicon, 93 00:08:31,500 --> 00:08:40,600 cadmium telluride, CIGS, dye-sensitized and organic materials and the III-V semiconductor materials. 94 00:08:41,400 --> 00:08:48,400 You will discover that every technology will have its own specific design rules and challenges. 95 00:08:49,300 --> 00:08:50,580 See you next week!