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Exercise 4 Storing Energy from Solar Panels into Batteries EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the storage of the energy produced by solar panels using lead-acid batteries. DISCUSSION OUTLINE The Discussion of this exercise covers the following points: DISCUSSION Energy storage Lead-acid batteries Battery charge using a PV module Battery connected to a PV module in the dark Equivalent diagram of a PV cell Blocking diode Energy storage When electric power is produced using PV panels, electrical energy is available only during sunny periods. Therefore, to ensure continuous and reliable energy supply, some means is required to store electrical energy when it is available. This is where batteries enter into action as they serve as a means of storing the excess in electrical energy produced during sunny periods. The energy stored in the batteries during sunny periods can be pumped out afterward to achieve a continuous electric energy supply. In brief, the batteries are charged during sunny periods and discharged during cloudy periods as well as at night. Leadacid batteries are commonly used to store electrical energy produced by PV panels. © Festo Didactic 86352-00 45 Exercise 4 – Storing Energy from Solar Panels into Batteries Discussion Figure 44. Small solar panel battery charger used to recharge standard AA batteries. A single battery can be recharged in two to three hours when the charger is exposed to sunlight (photo courtesy of Peter Halasz). Lead-acid batteries Lead-acid batteries that are commonly available have a nominal voltage of 12 V. Although several lead-acid batteries can be connected in series to obtain higher operating voltages (e.g., 24 V, 48 V, 60 V, etc.), the rest of this discussion deals with storing energy produced by PV panels into 12 V lead-acid batteries. The same principles apply when energy is stored in lead-acid battery systems operating at higher voltages. As Figure 45 shows, when a battery supplies power to an electrical load (discharges), current exits from the positive terminal and enters at the negative terminal of the battery, and the battery voltage decreases gradually during discharge. Conversely, a battery is charged by forcing a current to flow in the opposite direction, i.e., by making current enter at the positive terminal and exit from the negative terminal of the battery (see Figure 45). The voltage of the battery increases gradually during charge. 46 © Festo Didactic 86352-00 Exercise 4 – Storing Energy from Solar Panels into Batteries Discussion I I Electrical load Battery discharge cycle Battery charge cycle Figure 45. Charge and discharge cycles of a battery. Figure 46 shows the relationship between the open-circuit voltage ܧை and the state-of-charge of a 12 V lead-acid battery. The open-circuit voltage ܧை of any lead-acid battery increases with the state-of-charge of the battery. Open-circuit voltage (V) 12.7 12.6 12.5 12.4 12.3 12.2 12.1 12.0 11.9 11.8 0 10 20 30 40 50 60 70 80 90 100 State-of-charge (%) Figure 46. pen-circuit voltage versus state-of-charge of a 12 V lead-acid battery. Battery charge using a PV module Figure 47, shows a PV module connected to a 12 V lead-acid battery. When the PV module is illuminated, it produces current that flows through the battery. The current enters at the battery’s positive terminal and exits from the battery’s negative terminal, thereby charging the battery. The value of the charge current, for a given irradiance, depends on the battery voltage (which in turn depends on the battery state-of-charge) as it sets the operating point on the characteristic ܧ- ܫcurve of the PV module. When the battery is severely discharged (low state-of-charge), its open-circuit voltage ܧை is low, and thus, the operating point is to the left of the knee in the ܧ- ܫcurve and the charge current is close to the short-circuit current ܫௌ of the PV module (see point A in Figure 47). As the battery charges, the voltage across its terminal increases, thereby shifting the operating point toward the knee of the ܧ- ܫcurve of © Festo Didactic 86352-00 47 Exercise 4 – Storing Energy from Solar Panels into Batteries Discussion the PV module (see point B in Figure 47). This causes the charge current to decrease slightly as the battery charges. ܫௌ 12 V lead-acid battery (a) Discharged battery (b) Fully charged battery (C) Overcharged battery ܥܱܧ Figure 47. PV module connected to a 12 V lead-acid battery and ࡱ-ࡵ curve of the PV module. The short-circuit current ܫௌ of the PV module must be selected so that it does not exceed the maximum charge current specified by the battery manufacturer. In other words, the larger the battery capacity, the larger the PV module that can be used to charge the battery. The selection of the PV module size also depends on the electrical load connected to the battery. The higher the electrical load, the larger the PV module (higher the short-circuit current ܫௌ ) required to keep the battery charged. When charging any 12 V lead-acid batteries, the voltage across the battery terminals should be at least 12.6 V but should not be allowed to exceed about 14.4 V (the gassing voltage value) in order to ensure optimal battery life. The open-circuit voltage ܧை of the PV module should be carefully selected so as to satisfy these requirements under normal operating conditions. PV modules consisting of 36 PV cells are commonly used to charge 12 V lead-acid batteries as their ܧ- ܫcurve (at standard test conditions) is well suited for this application. The knee in their ܧ- ܫcurve is at a voltage that is a little above the gassing voltage (about 14.4 V) of a 12 V lead-acid battery (see Figure 48). As long as the operating point is maintained to the left of the knee in the ܧ- ܫcurve of the PV module, the battery is charged without producing gassing. If the electrical load of the system decreased during a significant amount of time for any reason, this could cause the operating point to move to the right side of the knee in the ܧ- ܫcurve as the battery charges (see Figure 47c). This causes the battery voltage to exceed the gassing voltage and gassing to occur in the battery. This condition, which is referred to as battery overcharging, must be avoided as gassing significantly reduces battery life. 48 © Festo Didactic 86352-00 Exercise 4 – Storing Energy from Solar Panels into Batteries Discussion ܧൌ 14.5 V ܫൌ 98 mA ܧൌ 0.98 V ܫൌ 103 mA 100 ܧൌ 15.6 V ܫൌ 95 mA ܴௌ ൌ 0.6 ȍ ܧൌ 16.4 V ܫൌ 80 mA ( ܫmA) 75 ܴ ൌ 79.4 50 ܧൌ 17.7 V ܫൌ 20 mA 25 ܧ PV cell equivalent diagram 0 0 5 10 15 20 RP calculation ( ܧV) RS calculation ߂ ܫൌ 3.5 mA @ 10.0 V ܴ for PV module (36 cells) ൌ ܧȀ߂ = ܫ2857 ȍ ܴ for 1 cell ൌ 2857 ȍ Ȁ 36 ൌ 79.4 ȍ ߂ = ܧ1.5 V @ 70.0 mA ܴௌ for PV module (36 cells) ൌ ߂ܧȀ ܫൌ 21.4 ȍ ܴௌ for 1 cell ൌ 21.4 ȍ Ȁ 36 ൌ 0.6 ȍ Figure 48. Typical ࡱ-ࡵ curve of a 36-cell PV module used to charge 12 V lead-acid batteries. Battery connected to a PV module in the dark So far, we have discussed battery charging with a PV module when it is illuminated. But what happens at night when the PV module is in the dark? Representing a 36-cell PV module using the simplified equivalent diagram of a PV cell shown in Figure 27 suggests that no current (or a very low current) would flow in the system as the current source in each PV cell no longer produces current and the battery open-circuit voltage ܧை is only able to apply a weak forward bias to the diode in each PV cell. For instance, when the battery opencircuit voltage is 12.4 V, the forward-bias voltage across each PV cell is only 0.34 V as shown in Figure 49. Actually, however, this is not exactly what happens as some current does flow from the battery to the PV module, thereby discharging the battery. A more complete equivalent diagram of the PV cell is thus required to explain what happens. © Festo Didactic 86352-00 49 Exercise 4 – Storing Energy from Solar Panels into Batteries Discussion 0.34 V PV module at night 36 PV cells 0.34 V Battery open-circuit voltage ൌ 12.4 V 12 V lead-acid battery 0.34 V Figure 49. Lead-acid battery connected to a PV module (in the dark) represented using the simplified equivalent diagram of a PV cell. Equivalent diagram of a PV cell Comparing the simplified equivalent diagram of a PV cell shown in Figure 27 with the actual equivalent diagram of a PV cell shown in Figure 50 reveals that the latter includes a resistor ܴ in parallel and a resistor ܴௌ in series with the current source and diode. ܴௌ ܴ Figure 50. Actual equivalent diagram of a PV cell. The value of resistors ܴ and ܴௌ can be evaluated from the ܧ- ܫcurve of a PV cell as shown in Figure 51. The presence of the parallel resistor explains why the PV cell current decreases a little with voltage in the constant-current region of the ܧ- ܫcurve – the lower the value of ܴ , the higher the decrease of cell current 50 © Festo Didactic 86352-00 Exercise 4 – Storing Energy from Solar Panels into Batteries Discussion with voltage. The presence of the series resistor explains why the PV cell voltage decreases a little with current in the constant-voltage region of the ܧ- ܫcurve – the higher the value of ܴௌ , the higher the decrease of cell voltage with current. ߂ @ ܫvoltage ܧ Current (A) ൌ 0 ܧ Voltage (V) Evaluation of resistor ܴ ܧை 0.6 ߂ܫ ͳ ൌ ܧ ܴ ܫௌ Current (A) ܫ ܫௌ ൌ ܫ ߂ܧ ൌ ܴௌ ܫ ߂ @ ܧcurrent ܫ Voltage (V) ܧை 0.6 Evaluation of resistor ܴௌ Figure 51. Evaluation of resistors ࡾࡼ and ࡾࡿ from the characteristic ࡱ-ࡵ curve of a PV cell. When the ܧ- ܫcurve of a PV module (series–connected PV cells) is used to calculate the value of resistors ܴ and ܴௌ , the calculated values correspond to the total parallel resistance and total series resistance of the PV module. The total parallel resistance and total series resistance are distributed over the cells in the PV module. The total parallel resistance is simply divided by the number of cells in the PV module to obtain the mean value of the parallel resistance ܴ of each cell. Similarly, the total series resistance is simply divided by the number of cells in the PV module to obtain the mean value of the series resistance ܴௌ of each cell. Figure 48 of this discussion shows the calculations of the values of resistors ܴ and ܴௌ made with the ܧ- ܫcurve of a 36-cell PV module implemented with the Monocrystalline Silicon Solar Panel. Figure 52 shows a 36-cell PV module at night represented using the actual equivalent diagram of the PV cell (see Figure 50). This representation reveals that at night time current flows in the PV module through the series combination of ܴ and ܴௌ in each PV cell, thereby discharging the battery. The value of the discharge current ܫ௦Ǥ . can be calculated using the battery open-circuit voltage ܧை and the total parallel resistance ܴ and total series resistance ܴௌ of the PV module (ܫ௦Ǥ ൌ ܧை Τሺܴ ܴௌ ሻ. Note that the discharge current is usually small with respect to the battery capacity. © Festo Didactic 86352-00 51 Exercise 4 – Storing Energy from Solar Panels into Batteries Discussion Battery discharge current ܫ௦Ǥ ܴௌ ܴ PV module at night 36 PV cells 12 V leadacid battery Figure 52. At night, current flows in the PV module through ࡾࡼ and ࡾࡿ in each PV cell, thereby slowly discharging the battery. Blocking diode Although the battery discharge current at night is usually low, it is common practice to add a diode (blocking diode) in series with the PV module to avoid battery discharge at night (see Figure 53). When the PV module is illuminated, the diode is forward biased and charge current flows into the battery. At night, the PV module stops producing current and the battery voltage applies a reverse bias to the diode. The blocking diode prevents current flow thereby avoiding the battery discharge. 52 © Festo Didactic 86352-00 Exercise 4 – Storing Energy from Solar Panels into Batteries Procedure Outline Diode is forward biased Diode is reverse biased ܫ ܫൌ0 PV module (36 cells) PV module (36 cells) 12 V leadacid battery During day 12 V leadacic battery At night Figure 53. A diode is added in series to avoid battery discharge at night. PROCEDURE OUTLINE The Procedure is divided into the following sections: PROCEDURE Setup Open-circuit voltage and short-circuit current of a 36 cell PV module operating at near room temperature Characteristic E-I curve of a 36-cell PV module operating at near room temperature Battery charging using a PV module Battery discharge at night time Evaluation of the parallel and series resistances of the PV module Operation of the circuit with a blocking diode when the PV module is in the dark Operation of the circuit with a blocking diode when the PV module is illuminated 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 ܫௌ 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 ܫௌ to about 100 mA. © Festo Didactic 86352-00 53 Exercise 4 – Storing Energy from Solar Panels into Batteries Procedure Open-circuit voltage and short-circuit current of a 36 cell PV module operating at near room temperature In this part of the exercise, you will connect the two PV modules of the Monocrystalline Silicon Solar Panel in series to form a 36-cell PV module. You will measure the open-circuit voltage ܧை and short-circuit current ܫௌ of the 36-cell PV module when it operates at near room temperature. 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 and measure the open-circuit voltage ܧை , short-circuit current ܫௌ , and PV panel temperature. Refer to Figure 34 and Figure 35 if necessary. Open-circuit voltage ܧை ൌ Short-circuit current ܫௌ ൌ PV panel temperature ൌ V A °C (°F) 4. Compare the open-circuit voltage ܧை measured with the two PV modules connected in series with the open-circuit voltage ܧை measured with a single PV module in step 12 of Exercise 2. What can you conclude about the difference between the values? 5. Compare the short-circuit current ܫௌ measured with the two PV modules connected in series with the short-circuit current ܫௌ measured with a single PV module in step 9 of Exercise 2. What can you conclude about the difference between the values? 54 © Festo Didactic 86352-00 Exercise 4 – Storing Energy from Solar Panels into Batteries Procedure Characteristic E-I curve of a 36-cell PV module operating at near room temperature In this part of the exercise, you will plot the characteristic ܧ- ܫcurve of a 36-cell PV module operating at near room temperature. 6. Set up the circuit shown in Figure 54. Figure 54. Circuit used to determine the characteristic ࡱ-ࡵ curve of a 36-cell PV module (built with two 18-cell PV modules connected in series) operating at near room temperature. 7. Using the potentiometer as a variable load, vary the output voltage from minimum to maximum by increments of about 0.5 V. For each voltage setting, record the output voltage, output current, and temperature in Table 5. Voltage (V) Table 5. Characteristic ࡱ-ࡵ curve of a 36-cell PV module. Current (mA) Voltage (V) Current (mA) Voltage (V) Current (mA) Temperature of the PV panel during the measurements ൌ © Festo Didactic 86352-00 Voltage (V) Current (mA) °C (°F) 55 Exercise 4 – Storing Energy from Solar Panels into Batteries Procedure 8. Using the values in Table 5, plot the characteristic ܧ- ܫcurve of the 36-cell PV module in Figure 55. 120 110 100 90 Current (mA) 80 70 60 50 40 30 20 10 0 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Output Voltage (V) Figure 55. Characteristic ࡱ-ࡵ curve of a 36-cell PV module. 9. Using the values in Table 5, determine the PV module voltage and current at the maximum power point (MPP), and indicate the point in Figure 55. PV module voltage at MPP ൌ V PV module current at MPP ൌ A Battery charging using a PV module In this part of the exercise, you will charge a lead-acid battery using the 36-cell PV module used in the previous subsection of the exercise. You will check the state-of-charge of a battery. Then you will observe the voltage and current at the beginning of the charging cycle, and after fifteen minutes of charge. You will locate each system operating point on the ܧ- ܫcurve of the PV module. 10. Measure the open-circuit voltage ܧை of the lead-acid battery located at the right in the Lead-Acid Battery module. Open-circuit voltage ܧை ൌ V 11. Determine the state-of-charge of the battery (expressed in percentage) corresponding to the open-circuit voltage measured in the previous step using the open-circuit voltage versus state-of-charge curve shown in Figure 46. State-of-charge ൌ 56 % © Festo Didactic 86352-00 Exercise 4 – Storing Energy from Solar Panels into Batteries Procedure a The state-of-charge of the battery should be between 40% and 70% at this moment to observe the behavior of the battery during the charge and discharge cycles. Ask your instructor for instructions if the state-of-charge of your battery does not match this range. 12. Connect the output of the 36-cell PV module (two 18-cell PV modules in series) to the 12 V battery at right in the Lead-Acid Battery module as shown in Figure 56. Note that the two 18-cell PV modules connected in series are represented as a single PV module in this figure. Halogen lamp PV module (36 cells) 12 V One battery in the Lead-Acid Battery module Figure 56. Battery being charged by a PV module. 13. Measure the PV module voltage and current. PV module voltage at the beginning of the charge cycle ൌ PV module current at the beginning of the charge cycle ൌ V A 14. Does the polarity of the current indicate that the battery is being charged or discharged? Explain. 15. Let the battery continue to charge. Using the PV module voltage and current measured in step 13, place the system operating point on the PV module characteristic ܧ- ܫcurve in Figure 55. © Festo Didactic 86352-00 57 Exercise 4 – Storing Energy from Solar Panels into Batteries Procedure 16. Is the system operating point located to the left of the knee in the characteristic ܧ- ܫcurve (i.e., in the constant-current region)? Yes No 17. Is the charging current close to the short circuit current ܫௌ ? Yes No 18. Wait for the battery to charge for about 15 minutes. You should observe that the PV module voltage slowly increases as the battery charges while the current is relatively constant or decreases a little. As the battery charges, the system operating point moves toward the knee of the PV module characteristic ܧ- ܫcurve. Measure the PV module output voltage and current and place the operating point on the PV module characteristic ܧ- ܫcurve in Figure 55. PV module voltage after 15 minutes of charge ൌ PV module current after 15 minutes of charge ൌ V A Battery discharge at night time In this part of the exercise, you will observe the voltage and current when the PV module is not illuminated by the halogen lamp. You will observe that the battery discharges via the PV module in this condition. 19. Turn the halogen lamp and fan off in the Solar Panel Test Bench to simulate night-time conditions. Measure the PV module voltage and current. PV module voltage ൌ PV module current ൌ V A 20. Is current flowing even though the halogen lamp is off? Yes No 21. Does the polarity of this current indicate that the battery is being discharged? Explain. 58 © Festo Didactic 86352-00 Exercise 4 – Storing Energy from Solar Panels into Batteries Procedure Evaluation of the parallel and series resistances of the PV module In this part of the exercise, you will evaluate the parallel and series resistances of the PV module. 22. Using the voltage and current measured in step 19 during night-time conditions, calculate the resistance of the 36-cell PV module. This resistance includes the total parallel resistance and total series resistance of the 36-cell PV module. Resistance of the 36-cell PV module ܴ ܴௌ ൌ ȍ 23. Using the characteristic ܧ- ܫcurve plotted in Figure 55, determine the total parallel resistance and total series resistance of the 36-cell PV module as is shown in Figure 51. For better accuracy, do not use values near the knee of the curve to calculate the parallel and series resistance values. Total parallel resistance ܴ ൌ Total series resistance ܴௌ ൌ ȍ ȍ Total resistance of the 36-cell PV module ܴ ܴௌ ൌ ȍ 24. Compare the total resistance value determined using the voltage and current measured during night-time in step 22 to the value determined using the characteristic ܧ- ܫcurve in step 23. Are the resistance values similar? Yes a © Festo Didactic 86352-00 No The two methods used to determine the resistance values give approximate results. A difference of up to 30% between the calculated resistance values is considered normal. 59 Exercise 4 – Storing Energy from Solar Panels into Batteries Procedure Operation of the circuit with a blocking diode when the PV module is in the dark In this part of the exercise, you will observe that a diode can be used to prevent the battery from discharging via the PV module when it is in the dark (night-time conditions). 25. Add a blocking diode between the PV module and the battery, and modify the voltmeter connections as shown in Figure 57. Blocking diode Halogen lamp PV module (36 cells) 12 V One battery in the LeadAcid Battery module Figure 57. Circuit used to observe the effect of a blocking diode when the PV module is in the dark (night-time conditions). 26. Describe how the diode will affect the circuit operation. 60 © Festo Didactic 86352-00 Exercise 4 – Storing Energy from Solar Panels into Batteries Procedure 27. Measure the voltage at the diode terminals when the PV module is in the dark. Voltage at the diode terminals ൌ V 28. From the voltage measured in the previous step, determine if the diode is forward or reverse biased when the PV module is in the dark. 29. Is current flowing from the battery to the PV module? Yes No 30. Do your observations confirm that the blocking diode prevents the battery from discharging when the PV module is in the dark? Yes No Operation of the circuit with a blocking diode when the PV module is illuminated In this part of the exercise, you will observe the effect of the blocking diode when the PV module is illuminated by the halogen lamp. 31. Turn the halogen lamp and fan on to simulate daytime conditions and modify the voltmeter connections as shown in Figure 58. Blocking diode Halogen lamp 12 V One battery in the LeadAcid Battery module PV module (36 cells) Figure 58. Circuit used to observe the effect of a blocking diode when the PV module is in the light. 32. Measure the PV module voltage and current. PV module voltage ൌ © Festo Didactic 86352-00 V 61 Exercise 4 – Storing Energy from Solar Panels into Batteries Conclusion PV module current ൌ A 33. Using the PV module voltage and current measured in the previous step, place the system operating point on the PV module characteristic ܧ- ܫcurve in Figure 55. Does the operating point remain to the left of the knee in the characteristic ܧ- ܫcurve (i.e., in the constant-current region) of the PV module? Yes No 34. If you answered yes to the last question, determine if the new operating point is nearer or farther from the knee of the curve, and explain why. CONCLUSION In this exercise you learned that energy produced by photovoltaic solar panels during sunny periods can be stored in batteries to ensure a continuous reliable energy supply. You learned that the open-circuit voltage of a battery varies with the state-of-charge. You saw that current enters at the battery’s positive terminal when the battery is being charged, and that it exits the battery’s positive terminal when the battery is being discharged (supplying a load). You saw that during the charging cycle, the system operating point moves toward the knee in the ܧ- ܫcurve of the PV module, but that it is maintained at the left of the knee in the curve to prevent gassing. You learned that the actual equivalent diagram of a PV cell adds a parallel resistor and a series resistor to the current source and diode in the simplified equivalent diagram of a PV cell. You saw that these resistors cause the battery connected to the PV cell to discharge at night if no blocking diode is added in the circuit. REVIEW QUESTIONS 62 1. In which direction does the current flow when a battery is being charged by a PV module. © Festo Didactic 86352-00 Exercise 4 – Storing Energy from Solar Panels into Batteries Review Questions 2. Describe the relationship between the open-circuit voltage and the state-ofcharge of a lead-acid battery. 3. Explain why PV modules consisting of 36 PV-cells are well suited to charge 12 V lead acid batteries? 4. What causes a battery to discharge via a PV module during night? 5. What can be done to prevent a battery connected to a PV module from discharging during night? © Festo Didactic 86352-00 63
