Effect of Shading on Solar Panel Operation

Transcription

1 Exercise 5 Effect of Shading on Solar Panel Operation EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the effect of shading on solar panel operation. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: Effect of partial shading on PV panel operation Bypass diodes to mitigate the effect of shading on PV modules connected in series Blocking diodes to mitigate the effect of shading on PV modules connected in parallel DISCUSSION Effect of partial shading on PV panel operation Until now, we have studied the operation of PV panels when their complete surface is illuminated. However, what happens when a PV panel operates with a portion of its surface in the dark due to shading produced by obstacles of all sorts? It is the goal of this exercise to study the effect which shading has on PV panel operation. When a single cell in a PV module is shaded as shown in Figure 59, it no longer contributes to the PV module output, thereby causing a slight drop in the PV module output voltage. Furthermore, the current produced by the illuminated cells must pass through the series combination of the parallel ( ) and series ( ) resistors in the shaded cell as well as through the electrical load connected to the PV module. In other words, the equivalent resistance ( ) of the shaded cell adds to the resistive value of the electrical load connected to the PV module. As a result, the PV module current decreases, and the voltage drop across the series combination of the parallel ( ) and series ( ) resistors in the shaded cell further reduces the PV module output voltage. Festo Didactic

2 Exercise 5 Effect of Shading on Solar Panel Operation Discussion I 0 n th cell 0 n th cell shaded I n-1 cells Electrical load n-1 cells Electrical load (a) All cells in the sunlight (b) Top cell shaded Figure 59. PV module with all cells in the sunlight (a) and with one cell shaded (b). Figure 60 shows an 18-cell PV module operating near MPP and without shading. The output voltage equals 7.9 V and the load current equals 90 ma. The electrical load resistance and output power calculated at this operating point are 87.8 and 711 mw, respectively. 90 ma 18-cell PV module 7.9 V Electrical load resistance ( 87.8 ) Output power = 711 mw Figure cell PV module operating near MPP and without shading. 66 Festo Didactic

3 Exercise 5 Effect of Shading on Solar Panel Operation Discussion When a single cell of this PV module is shaded as shown in Figure 61, the system operating parameters are affected significantly. The output voltage ( ) developed by the remaining 17 illuminated PV cells becomes 7.46 V [(17 / 18) 7.9 V]. The total resistance in the circuit increases to [electrical load resistance (87.8 ) + total resistance of the shaded PV cell ( 80.0 from Figure 61)] causing the circuit current to decrease to 44.5 ma (7.46 V / ). Consequently, the load voltage decreases to 3.9 V ( ma) and the power supplied to the load decreases to 174 mw (3.9 V 44.5 ma). As can be seen, shading only one cell out of eighteen greatly reduces the power supplied to the load (174 mw versus 711 mw). Furthermore, the voltage drop across the equivalent resistors ( ) of the shaded cell is 3.56 V ( ma), leading to a power dissipation in these resistors of 158 mw (3.56 V 44.5 ma). This power, which is mainly dissipated in the parallel resistor of the shaded cell, is much higher than the power normally dissipated in resistors and of any cell when the PV module operates without shading. Since the power in resistors and of any cell is dissipated as heat, shading of a single cell can thus create a hot spot on the PV module and eventually cause permanent damages. In conclusion, shading not only decreases the output power of a PV module dramatically, but it can also cause damage to a PV module. Therefore, some means of mitigating the harmful effects of shading is highly desirable V Power dissipated in the shaded cell = 158 mw 44.5 ma Shaded cell V Electrical load resistance ( 87.8 ) 7.46 V 17 PV cells illuminated Output power = 174 mw - Figure cell PV module operating with one cell shaded. Note that the above calculations should be considered as approximate because the series and parallel equivalent resistances ( and ) vary from one PV cell of the PV module to another (and also because of other phenomena that are beyond the scope of this manual). However, this lack of accuracy has no bearing on the conclusion that shading of even just a single cell considerably decreases the power produced by PV modules and has potentially harmful effects on PV modules. Festo Didactic

4 Exercise 5 Effect of Shading on Solar Panel Operation Discussion Bypass diodes to mitigate the effect of shading on PV modules connected in series The negative effect of shading described in the previous subsection can be greatly mitigated by adding a bypass diode in parallel with each PV cell as shown in Figure 62. In Figure 62a, the PV cell is illuminated, current flows through the PV cell and the voltage at the cell terminals equals. When the PV cell is shaded as shown in Figure 62b, the diode in the cell s equivalent circuit is reverse biased by the other cells in the PV module which remain illuminated, and thus, does not allow current flow. The voltage at the shaded cell terminals represents a significant voltage drop caused by the current flowing through resistors and. Adding a bypass diode in parallel with the PV cell as shown in Figure 62c does not affect the operation of the cell when it is illuminated. The bypass diode is reverse biased and no current flows through it, thus the voltage at the cell terminals remains equal to. In Figure 62d the PV cell is shaded but the bypass diode is forward biased thereby providing an alternative path for current flow. Consequently, the voltage at the cell terminals is a voltage drop corresponding to the forward-bias voltage of the bypass diode (0.6 V). This voltage drop is generally much lower than that occurring when the current has to flow through the equivalent resistance of the shaded cell ( ), thereby limiting the decrease in the PV module voltage (load voltage). 68 Festo Didactic

5 Exercise 5 Effect of Shading on Solar Panel Operation Discussion 0.5 V 0 A 0 A Cell 0 V 0 V (a) One cell in the sun (b) Equivalent circuit of a cell when shaded 0.5 V -0.6 V I 0 A Bypass diode is reverse biased (non conductive) Bypass diode is forward biased (conductive) (c) One cell in the sun with bypass diode (d) One shaded cell with bypass diode Figure 62. Bypass diode added in parallel with a PV cell. In actual PV modules, however, it is not practical to add a bypass diode in parallel with each cell. On the other hand, when several PV modules are connected in series, it is common practice to add a bypass diode in parallel with each PV module to help mitigate the negative effect of shading. Figure 63 shows several PV modules connected in series to charge a 60 V battery pack. When there is no shading (Figure 63a), the charge voltage and current are 65 V and 3.3 A. Figure 63b shows that partial shading causes a 33% reduction in charge current. The reduction in charge current is mainly due to the increase of the total resistance of the circuit (caused by the shaded module). Without bypass diodes, the increase in total circuit resistance moves the operating point to the right of the knee in the - curve of the PV modules, causing a large increase in the voltage produced by the unshaded PV modules as well as an important decrease in the current these PV modules deliver. Note, however, that the charge voltage remains unchanged because the increase in PV module voltage is lost across the cells of the partially shaded PV module. On the other hand, the decrease in charge current is only 3% when bypass diodes are added as shown in Festo Didactic

6 Exercise 5 Effect of Shading on Solar Panel Operation Discussion Figure 63c. This is because the bypass diode prevents the total resistance of the circuit from increasing and limits the voltage drop across the cells of the partially shaded PV module to about 0.6 V. Consequently, the operating voltage of the unshaded PV modules only needs to increase moderately to compensate for the loss of the voltage the shaded PV module normally produces. As long as the operating point remains to the left of the knee in the - curve of the PV module, as is the case in this example, the current decreases only a little. 3.3 A 65 V Partial shading 2.2 A 65 V 3.2 A Partial shading 65 V On 52 V 80 V 65.6 V Off 39 V 60 V 49.2 V Off 26 V 40 V 32.8 V 3.3 A 2.2 A 3.2 A Off 13 V 20 V 16.4 V Off 0 V 0 V 0 V (a) Full sun (b) Partial shading no bypass diodes (c) Partial shading with bypass diodes Figure 63. Ability of bypass diodes to mitigate the negative effect of shading on PV modules connected in series. Blocking diodes to mitigate the effect of shading on PV modules connected in parallel Shading has a similar negative effect when several strings of PV modules are connected in parallel. When shading prevents one string of PV modules in the array from producing its normal share of power, this string acts as a resistor and starts sinking current from the other strings of PV modules, thereby greatly reducing the output current provided to the load as shown in Figure 64a. 70 Festo Didactic

7 Exercise 5 Effect of Shading on Solar Panel Operation Discussion 0 (a) Without blocking diodes (b) With blocking diodes Figure 64. Blocking diodes prevent reverse current from flowing down in shaded strings of PV modules connected in parallel. Adding a blocking diode in series with each string of PV modules prevents one string from sinking current from the other strings when it is shaded. This significantly reduces the decrease in output current provided to the load as shown in Figure 64b. Figure 65. Photovoltaic panels used to recharge the battery of electric vehicles, in France (photo courtesy of Tatmouss). Festo Didactic

8 Exercise 5 Effect of Shading on Solar Panel Operation Procedure Outline PROCEDURE OUTLINE The Procedure is divided into the following sections: Setup No-shade operation Operation with one shaded cell Shade mitigation (PV modules connected in series) Shade mitigation (PV modules connected in parallel) PROCEDURE Setup 1. Refer to the Equipment Utilization Chart in Appendix A to obtain the list of equipment required to perform this exercise. a To ensure greater consistency between the results obtained during the various exercises, make sure that you are using the same Monocrystalline Silicon Solar Panel and Solar Panel Test Bench modules as in Exercise 2 (same serial numbers). 2. Install the Monocrystalline Silicon Solar Panel in the Solar Panel Test Bench then install the Solar Panel Test Bench into the Workstation. Adjust the position of the solar panel so that the short-circuit current of the lower PV module is as close to 100 ma as possible at near room temperature. Steps 4 to 9 of Exercise 2 provide detailed directions for installing the modules and setting to about 100 ma. No-shade operation In this part of the exercise, you will connect two PV modules in series to form a 36-cell PV module. You will measure the PV module voltage and current, and estimate the effect that shading one of the 36 cells will have on these parameters. Risk of burns. The halogen lamp and the surrounding components can become very hot during this exercise. 3. Once the Monocrystalline Silicon Solar Panel is properly positioned in the Solar Panel Test Bench and the temperature has stabilized, connect the two PV modules in series to form a 36-cell PV module and connect the circuit shown in Figure 66. Notice that the two PV modules connected in series are represented as a single PV module in this figure. 72 Festo Didactic

9 Exercise 5 Effect of Shading on Solar Panel Operation Procedure 36-cell PV module Figure 66. Circuit used to measure the output voltage and current of the 36-cell PV module. 4. Adjust the rheostat of the Solar Panel Test Bench so that the 36-cell PV module operates as close as possible to the maximum power point (MPP) determined in step 9 of Exercise 4. Enter in the following spaces the PV module voltage and current that you measure. PV module voltage V PV module current A 5. Calculate the load resistance as well as the power supplied to the load (rheostat) for this operating condition. Load resistance Power supplied to the load W 6. Using the total parallel resistance and total series resistance of the 36-cell PV module determined in step 23 of Exercise 4, calculate the values of the parallel resistance and series resistance of a single cell in the PV module. Parallel resistance of a single cell (single cell) Series resistance of a single cell (single cell) 7. Using the resistance values calculated in the previous step, the PV module voltage and current measured in step 4, and the load resistance calculated in step 5, determine the PV module voltage (load voltage) and current that should be obtained when one cell of the PV module is shaded, and the power that should be supplied to the load. Enter your answers in the Calculated row of Table 6. Festo Didactic

10 Exercise 5 Effect of Shading on Solar Panel Operation Procedure Table 6. Voltage, current, and power calculated and measured when one cell of the 36-cell PV module is shaded. PV module voltage (V) PV module current (ma) Power supplied to the load (W) Calculated Measured (trial 1) Measured (trial 2) Measured (trial 3) Measured (trial 4) Operation with one shaded cell In this part of the exercise, you will shade one cell of a 36-cell PV module and observe the effect on the PV module performance. a Take care of not changing the load setting on the Solar Panel Test Bench while removing and installing the Monocrystalline Silicon Solar Panel. 8. Turn the halogen lamp and fan off, and disconnect the power cord. Notice the position of the Monocrystalline Silicon Solar Panel in the Solar Panel Test Bench. Disconnect the cable from the multi-pin connector on the Monocrystalline Silicon Solar Panel. Remove the Monocrystalline Silicon Solar Panel. Using a piece of metallic tape measuring 0.8 cm 5.2 cm (0.32 in 2.05 in), shade one of the 36 PV cells of the PV module as shown in the figure in the left margin. Do not shade a PV cell located at the ends of a PV module section, since these cells can differ slightly in size. a This step requires precision. Make sure that you stay within the boundaries of the PV cell surface to prevent the next PV cell from being partially shaded. Be careful not to damage the transparent surface that covers the PV modules. 9. Reinstall the Monocrystalline Silicon Solar Panel at the same position in the Solar Panel Test Bench. Reconnect the cable from the Solar Panel Test Bench to the multi-pin connector on the Monocrystalline Silicon Solar Panel. Connect the power cord, turn the halogen lamp and fan on, and wait for the temperature to stabilize. 10. Measure the PV module voltage and current, and enter your results in the Measured (trial 1) row of Table Festo Didactic

11 Exercise 5 Effect of Shading on Solar Panel Operation Procedure 11. Using the PV module voltage and current measured in the previous step, calculate the power supplied to the load when one cell is shaded and enter your answer in Table 6. a Because of the variance between the cells and the precision of the shading, the measured values can substantially differ from the calculated values. 12. Compare the power supplied to the load without shading (step 5) and with shading (step 11). What can you conclude from the difference between the values? 13. Calculate the voltage produced by the 35 cells of the PV module (35/36 PV module voltage measured in step 4) that are still illuminated: Voltage produced by the 35 illuminated cells V 14. Determine the voltage drop across the shaded cell using the voltage produced by the 35 illuminated cells and the PV module voltage measured when one cell is shaded. Voltage drop across the shaded cell V 15. Calculate the power dissipated in the shaded cell. Power dissipated in the shaded cell W 16. If time permits, remove the piece of metallic tape from the PV panel surface then repeat steps 8 to 15 to shade different cells of the PV module. Record your results in the subsequent rows of Table 6. You should obtain results that vary significantly, depending on which particular cell is shaded. The variation is caused by a variance between each specific cell in the PV panel. However, even if the results show a certain variation, they should consistently lead to the same conclusion: shading even just one cell of a PV module causes a significant decrease in power that is provided to the load. Shade mitigation (PV modules connected in series) In this part of the exercise, you will observe the effect of bypass diodes to mitigate the effect of shading on PV modules connected in series. 17. Turn the halogen lamp and fan off, and disconnect the power cord. Notice the position of the Monocrystalline Silicon Solar Panel in the Solar Panel Test Bench. Festo Didactic

12 Exercise 5 Effect of Shading on Solar Panel Operation Procedure Disconnect the cable from the multi-pin connector on the Monocrystalline Silicon Solar Panel. Remove the Monocrystalline Silicon Solar Panel. Remove the piece of metallic tape from the PV panel surface to restore no shade operation. 18. Reinstall the Monocrystalline Silicon Solar Panel at the same position in the Solar Panel Test Bench. Reconnect the cable from the Solar Panel Test Bench to the multi-pin connector on the Monocrystalline Silicon Solar Panel. Connect the power cord, turn the halogen lamp and fan on, and wait for the temperature to stabilize. Adjust the rheostat of the Solar Panel Test Bench so that the 36-cell PV module voltage is 8 V. This operating point is widely off the maximum power point (MPP). Measure the PV module current and then calculate the power supplied to the load. PV module current A Power supplied to the load W 19. Turn the halogen lamp and fan off, and disconnect the power cord. Notice the position of the Monocrystalline Silicon Solar Panel in the Solar Panel Test Bench. Disconnect the cable from the multi-pin connector on the Monocrystalline Silicon Solar Panel. a Take care of not changing the load setting on the Solar Panel Test Bench while removing and installing the Monocrystalline Silicon Solar Panel. Remove the Monocrystalline Silicon Solar Panel. Using a piece of metallic tape, shade 9 cells of the upper PV module as shown in the figure in the left margin. 20. Reinstall the Monocrystalline Silicon Solar Panel at the same position in the Solar Panel Test Bench. Reconnect the cable from the Solar Panel Test Bench to the multi-pin connector on the Monocrystalline Silicon Solar Panel. Connect the power cord, turn the halogen lamp and fan on, and wait for the temperature to stabilize. 21. Measure the PV module voltage and current then calculate the power supplied to the load. 76 Festo Didactic

13 Exercise 5 Effect of Shading on Solar Panel Operation Procedure PV module voltage (9 cells shaded without bypass diodes) V PV module current (9 cells shaded without bypass diodes) A Power supplied to the load (9 cells shaded without bypass diodes) W 22. Compare the power supplied to the load without shading (previous step) and with shading (step 18). What can you conclude about the difference between the values? 23. Connect a bypass diode in parallel with each PV module as shown in Figure 67. Shaded module (upper) Figure 67. Shaded PV module with bypass diode. 24. Measure the PV module voltage and current then calculate the power supplied to the load. PV module voltage (9 cells shaded with bypass diodes) V PV module current (9 cells shaded with bypass diodes) A Power supplied to the load (9 cells shaded with bypass diodes) W Festo Didactic

14 Exercise 5 Effect of Shading on Solar Panel Operation Procedure 25. Compare the power supplied to the load when there is no shading (step 18) with the load power obtained when 9 cells are shaded (without and with bypass diode (steps 21 and 24). What can you conclude from the difference between the values? 26. If time permits, remove the piece of metallic tape from the PV module surface then repeat steps 17 to 25 to shade 9 cells of the other PV module. You should obtain similar results. Shade mitigation (PV modules connected in parallel) In this part of the exercise, you will observe the effect of blocking diodes to mitigate the effect of shading on PV modules connected in parallel. 27. Turn the halogen lamp and fan off, and disconnect the power cord. Notice the position of the Monocrystalline Silicon Solar Panel in the Solar Panel Test Bench. Disconnect the cable from the multi-pin connector on the Monocrystalline Silicon Solar Panel. Remove the Monocrystalline Silicon Solar Panel. Remove the piece of metallic tape from the PV panel surface to restore no shade operation. 28. Reinstall the Monocrystalline Silicon Solar Panel at the same position in the Solar Panel Test Bench. Reconnect the cable from the Solar Panel Test Bench to the multi-pin connector on the Monocrystalline Silicon Solar Panel. Set up the circuit shown in Figure 68. In this circuit, the two 18-cell PV modules (PV1 and PV2) in the Monocrystalline Silicon Solar Panel are connected in parallel, and the output of the PV modules is connected to the rheostat of the Solar Panel Test Bench. One voltmeter and two ammeters are used to measure circuit parameters. a Note: Make sure that the voltmeter is connected directly across PV module PV2 at all times. The multimeter used to measure load current will also be used to measure later in the exercise. 78 Festo Didactic

15 Exercise 5 Effect of Shading on Solar Panel Operation Procedure PV1 (upper) PV2 (lower) Figure 68. Circuit used to observe the effect of shading when PV modules are connected in parallel (without blocking diodes). 29. Connect the power cord, turn the halogen lamp and fan on, and wait for the temperature to stabilize. Adjust the rheostat of the Solar Panel Test Bench so that the voltage across PV module PV2 is equal or near the voltage at the maximum power point (MPP) determined from the values in Table 3 of Exercise 2. This causes the two PV modules connected in parallel to operate at the MPP. Measure the following parameters: Voltage across PV module PV2 ( ) V Load current A Output current of PV module PV1 ( ) A 30. Determine the output current of PV module PV2 ( ) using your measured load current and output current of PV module PV1 ( ). Output current of PV module PV2 ( ) A 31. Determine the power supplied to the load using your measured load current and PV module PV2 output voltage. Power supplied to the load W 32. Turn the halogen lamp and fan off, and disconnect the power cord. Notice the position of the Monocrystalline Silicon Solar Panel in the Solar Panel Test Bench. Festo Didactic

16 Exercise 5 Effect of Shading on Solar Panel Operation Procedure Disconnect the cable from the multi-pin connector on the Monocrystalline Silicon Solar Panel. Remove the Monocrystalline Silicon Solar Panel. Using a piece of metallic tape, shade all cells of the upper PV module PV Reinstall the Monocrystalline Silicon Solar Panel at the same position in the Solar Panel Test Bench. Reconnect the cable from the Solar Panel Test Bench to the multi-pin connector on the Monocrystalline Silicon Solar Panel. Connect the power cord, turn the halogen lamp and fan on, and wait for the temperature to stabilize. Readjust the rheostat of the Solar Panel Test Bench so that is still equal to the voltage set in step 29 of this exercise (determined from the values at MPP in Table 3 of Exercise 2). This ensures that the illuminated panel still operates at or near the MMP. Measure the load current and the output current of PV module PV1 ( ). Load current (all cells of PV1 shaded, without blocking diodes) A Output current of PV1 (all cells of PV1 shaded, without blocking diodes) A 34. Compare the polarity of the output current of PV module PV1 ( ) obtained with and without shading. What do you observe? 35. Explain why the load current has dropped by a little more than 50%. 36. Determine the power supplied to the load using your measured load current and PV module PV2 output voltage. Power supplied to the load without blocking diodes W 80 Festo Didactic

17 Exercise 5 Effect of Shading on Solar Panel Operation Procedure 37. Compare the power supplied to the load obtained with and without shading. What do you observe? 38. Connect a blocking diode in series with each 18-cell PV module as shown in Figure 69. Blocking diode PV1 (upper) PV2 (lower) Figure 69. Circuit used to observe the effect of shading when PV modules are connected in parallel (with blocking diodes). 39. Readjust the rheostat of the Solar Panel Test Bench so that is still equal to the voltage set in step 29 of this exercise (determined from the values at MPP in Table 3 of Exercise 2). This ensures that the illuminated panel still operates at or near the MMP. Measure the load current and the output current of PV module PV1 ( ). Load current (all cells of PV1 shaded, with blocking diodes) A Output current of PV1 ( ) (all cells of PV1 shaded, with blocking diodes) A 40. What do you observe about the output current of PV module PV1 ( )? Festo Didactic

18 Exercise 5 Effect of Shading on Solar Panel Operation Conclusion 41. Compare the load current measured with blocking diodes in step 39 with the load current measured when the two panels were operating normally (i.e., without shading) in step 29. What do you observe? 42. Modify the voltmeter connections to measure the load voltage. Load voltage (all cells of PV1 shaded, with blocking diodes) V 43. Determine the power supplied to the load using your measured load voltage and load current. Power supplied to the load P L with blocking diodes W 44. Compare the power supplied to the load obtained with and without blocking diodes. What do you observe? 45. If time permits, remove the piece of metallic tape from the PV module surface then repeat steps 32 to 44 to shade all cells of the other PV module (lower PV module). You should obtain similar results. CONCLUSION In this exercise, you were introduced to the effect of partial shading on solar panel operation. You saw that, because of the internal resistance of PV cells, partial shading of a PV module considerably decreases the power produced by the module and has potentially harmful effects on the PV module. You saw that bypass diodes can be used to mitigate the negative effect of shading on PV cells or PV modules connected in series. You learned that blocking diodes can be used to mitigate the negative effect of shading on PV cells or PV modules connected in parallel. However, you observed that blocking diodes do not provide a significant increase in load power when shading occurs in low-voltage PV modules connected in parallel because of the decrease in load voltage due to the voltage drop in the blocking diodes. You learned that a blocking diode in series with each PV module (or string of PV modules) helps 82 Festo Didactic

19 Exercise 5 Effect of Shading on Solar Panel Operation Review Questions significantly in mitigating the effect of shading when PV modules are made of a high number of PV cells (or PV modules) connected in series to form high-voltage (>25-V) parallel strings of PV modules. REVIEW QUESTIONS 1. Shading not only decreases the output power of a PV module dramatically but it can also 2. Explain why adding a blocking diode in series with each string of PV modules connected in parallel is a good method for mitigating the negative effect of shading. 3. Explain why adding a bypass diode in parallel with each PV module connected in series is a good method for mitigating the shading effect. 4. Explain why the increase in load power is small or null when blocking diodes are added to parallel-connected strings of low-voltage PV modules. 5. When can the addition of blocking diodes to parallel-connected PV modules or strings of PV modules still be worthwhile? Explain briefly. Festo Didactic