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Impact of bypass diode forward voltage on maximum power of a photovoltaic system under partial shading conditions
Journal Pre-proof Impact of Bypass Diode Forward Voltage on Maximum Power of a Photovoltaic System under Partial Shading Conditions
J.C. Teo, Rodney H.G. Tan, V.H. Mok, Vigna K. Ramachandaramurthy, ChiaKwang Tan PII:
S0360-5442(19)32186-3
DOI:
https://doi.org/10.1016/j.energy.2019.116491
Reference:
EGY 116491
To appear in:
Energy
Received Date:
01 November 2018
Accepted Date:
04 November 2019
Please cite this article as: J.C. Teo, Rodney H.G. Tan, V.H. Mok, Vigna K. Ramachandaramurthy, ChiaKwang Tan, Impact of Bypass Diode Forward Voltage on Maximum Power of a Photovoltaic System under Partial Shading Conditions, Energy (2019), https://doi.org/10.1016/j.energy. 2019.116491
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
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Impact of Bypass Diode Forward Voltage on Maximum Power of a Photovoltaic System under Partial Shading Conditions J. C. Teo 1,*, Rodney H. G. Tan 1, V. H. Mok 1, Vigna K. Ramachandaramurthy 2, and ChiaKwang Tan 3 1
Faculty of Engineering, Technology & Built Environment, UCSI University No.1, Jalan Menara Gading, Kuala Lumpur 56000, Malaysia; [email protected] (Rodney); [email protected](V.H. Mok)
2
Institute of Power Engineering, Department of Electrical Power Engineering, Universiti Tenaga Nasional, 43000 Kajang, Malaysia; [email protected]
3
UM Power Energy Dedicated Advanced Centre, University Of Malaya, Jalan Pantai Baharu, Kuala Lumpur 59990, Malaysia; [email protected]
* Correspondence: [email protected]; Tel.: +6017-33395338
Abstract— The maximum power of a photovoltaic system can reduce significantly under partial shading conditions. Bypass diodes can be used in photovoltaic systems to bypass the shaded photovoltaic modules during partial shading. The bypass diode possesses a forward voltage that introduces a voltage drop in the photovoltaic system upon activation. Therefore, the maximum power of a photovoltaic system can reduce further during partial shading due to the forward voltage of the bypass diode. This paper presents an investigation into the effect of bypass diode forward voltage on the maximum power of a photovoltaic system under
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partial shading conditions. The results indicated that the forward voltage of the bypass diode did not necessarily decrease the maximum power of the photovoltaic system. This depends on whether the maximum power is delivered at a lower or higher voltage. When the maximum power is delivered at a higher voltage, it is insusceptible to the forward voltage. Conversely, when the maximum power is delivered at a lower voltage, it is susceptible to the forward voltage. In the worst-case scenario, the forward voltage of the bypass diode reduced the maximum power of the photovoltaic system by 16.48%, which was already subject to partial shading loss.
Keywords— Photovoltaic; bypass diode; partial shading; P-V characteristics; solar energy 1. INTRODUCTION Solar energy has become increasingly popular due to its environment friendly and inexhaustible nature (A. K. Pandeya et al., 2018; C. G. Ozoegwu et al., 2017; E. Kabir et al., 2018; S. K. Sansaniwal et al., 2018; T. Jia et al., 2018). New photovoltaic power systems with a capacity of 75 Gw were installed worldwide in 2016, which could power up to approximately 15 million houses simultaneously (J. D. Bastidas-Rodríguez et al., 2018). The number of new installations was even higher in 2017. Based on the REN21 2018 report, new photovoltaic power systems with a capacity of ~100 Gw were installed worldwide in 2017, recording an approximately 34% growth from 2016 (Renewables 2018 Global Status Report). The rapid growth in the installed capacities of photovoltaic systems has promoted research in the field of photovoltaic power generation. A solar panel or photovoltaic module is the most basic element of a photovoltaic power system, which converts solar energy into electrical power (A. Syafiq et al., 2018; J. C. Teo et al., 2017; M. Belarbi et al., 2018; N. AL-Rousan et al., 2018; P. A. Kumari et al., 2018). The relationship
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between the output current and voltage of a photovoltaic module is denoted by the I-V characteristic, which describes its behaviour (A. J. Bühler et al., 2014; F. Spertino et al., 2015; F. Spertino et al., 2016; N. Boutana et al., 2017). The output current and voltage obtained from the I-V characteristics can be used to calculate the output power of the photovoltaic module at different output voltages. The relationship between the output power and voltage of a photovoltaic module is called the PV characteristic. Fig. 1 shows the typical P-V characteristic curve of a photovoltaic module. The (X,Y) point on the curve corresponds to the highest point of the P-V characteristic, which indicates the maximum power generated by the photovoltaic module (M. R. Maghami et al., 2016). The maximum power of a photovoltaic module is always tracked and harvested during electrical generation (S. K. Das et al., 2017). Hence, the performance of the photovoltaic module depends on its maximum power.
Fig. 1. P-V characteristics of a photovoltaic module To achieve a higher output power and voltage, multiple photovoltaic modules are connected in series to form a photovoltaic string, as illustrated in Fig. 2 (A. Mohammedi et al., 2014; T. Shimizu
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et al., 2001; L. Linares et al., 2009; J. Kwon et al., 2009; H. Kim et al., 2009; M. Forcan et al., 2016; Y. Wang et al., 2016). The P-V characteristic of a photovoltaic string is assessed to harvest its maximum power during electrical generation.
Fig. 2. A photovoltaic string Consider a shaded photovoltaic module in a photovoltaic string (blue box in Fig. 3).The other unshaded photovoltaic modules generate higher currents as compared to the shaded module. However, the total output current is limited to the output current of the shaded photovoltaic module due to the series connection.
Fig. 3. A photovoltaic string without bypass diodes To overcome this problem, a bypass diode is implemented in the photovoltaic module. The bypass diode is connected anti-parallel to the photovoltaic module. It bypasses the shaded photovoltaic module to prevent electrical current generated by the unshaded photovoltaic modules from flowing through the shaded photovoltaic module, as illustrated in Fig. 4.
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Fig. 4. A photovoltaic string with a bypass diode Fig. 5 illustrates the P-V characteristics of a photovoltaic string composed of 20 photovoltaic modules under both non-shading and partial shading conditions. The red line indicates the P-V characteristic of the photovoltaic string under non-shading conditions where all the photovoltaic modules are receiving 1000 W/m2, i.e. during uniform irradiance. The peak represents the global peak, which indicates the maximum power of the photovoltaic string (L. Bouselhama et al., 2017; M. Muthuramalingam and P. S. Manoharan, 2014; Y. E. Abu Eldahaba et al., 2017; A. D. Martin et al., 2018). The blue line represents the P-V characteristic of the photovoltaic string under partial shading conditions, where 10 photovoltaic modules are receiving 1000 W/m2, while the other modules are receiving 300 W/m2. Fig. 5 shows that the maximum power of the photovoltaic string can decrease up to 55% due to partial shading; hence, the impact of partial shading should not be underestimated.
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Fig. 5. P-V characteristics under non-shading and partial shading When partial shading occurs, the bypass diodes are activated to bypass the shaded photovoltaic modules. The bypass diodes introduce a forward voltage drop in the photovoltaic string upon activation, which further decreases the maximum power of the photovoltaic string during partial shading. Industries/companies such as Texas Instruments (http://www.ti.com/product/SM74611) have fabricated smart bypass diodes with a low forward voltage of 26 mV to minimise the power losses. Thus, the impact of bypass diode forward voltage should not be underestimated. However, the impact of the bypass diode forward voltage on the maximum power of the photovoltaic string has not been studied extensively. Most research studies have focused on other aspects of the bypass diode. For example, Suk Whan Ko et al. (2017) investigated the impact of a damaged bypass diode on the photovoltaic module. Zheng et al. (2014) studied the impact of different bypass diode configurations on the energy extraction of a photovoltaic array. Daliento et al. (2016) proposed a new bypass mechanism that implemented an extra series-connected power MOSFET in the photovoltaic module. Silvestre et al. (2009) and Teo et al (2017) studied the impact of bypass diode configurations on the photovoltaic module.
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Bauwens and Doutreloigne (2014) proposed an integral smart bypass diode that almost resembled an ideal diode. Simulation results of a photovoltaic module with normal bypass and smart bypass diodes were presented. The simulation results showed that the photovoltaic module with a smart bypass diode had a higher maximum output power compared to the photovoltaic module with a normal bypass diode under certain degrees of shading. However, very few shading conditions were employed in their simulations, which were insufficient to conclude the impact of the forward voltage of the bypass diode. The simulation results did not show the impact of the forward voltage of the bypass diode on the P-V characteristics. Furthermore, the simulations were conducted at the photovoltaic module level. The authors did not discuss the impact of the forward voltage of the bypass diode on the photovoltaic string. Considering all of these issues, the impact of bypass diode forward voltage on the P-V characteristics and maximum power of a photovoltaic string under partial shading conditions is investigated. 2. METHODOLOGY A photovoltaic string composed of 20 photovoltaic modules connected in series was evaluated. The open circuit voltage, short circuit current, and ideality factor of the photovoltaic modules were 21.6 V, 7.34 A, and 1.5 respectively. In this study, the experiments were conducted at a temperature T = 25 oC. The photovoltaic modules had one bypass diode. The forward voltage of the bypass diode was set to 0.7 V. Four different setups of the photovoltaic string were evaluated, which included 4, 8, 12, and 16 shaded modules, as illustrated in Figs. 6 to 9, respectively.
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Fig. 6. A setup with 4 shaded PV modules
Fig. 7. A setup with 8 shaded PV modules
Fig. 8. A setup with 12 shaded PV modules
Fig. 9. A setup with 16 shaded PV modules TABLE 1 shows the conditions applied to each shaded setup. The P-V characteristic curve and maximum power of each shaded setup under the corresponding conditions were determined.
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TABLE 1. THE CONDITIONS APPLIED TO EACH SHADED SETUP Conditions
Irradiance of the
Irradiance of the Shaded
Unshaded Modules
Modules (W/m2)
(W/m2) Condition 1
1000
900
Condition 2
1000
800
Condition 3
1000
700
Condition 4
1000
600
Condition 5
1000
500
Condition 6
1000
400
Condition 7
1000
300
Condition 8
1000
200
Condition 9
1000
100
Condition 10
1000
0
The forward voltage of the bypass diode was set to 0.6 V. The same experiments were then performed to determine the impact of the forward voltage of the bypass diode on the P-V characteristics and maximum power of the photovoltaic string. To investigate further, the same experiments were also performed by setting the forward voltage of the bypass diode to 0.5, 0.4, 0.3, 0.2, 0.1, and 0.0 V. A photovoltaic string model was developed using Simulink (Mathworks, MA) for the experiments. The solar cell block in Simulink was used to develop the model. This had a five parameter configuration, as defined in Eq. (1) and Eq. (2), where I is the output current, V is the
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output voltage, Iph is the photo-generated current, Io is the diode saturation current, Vth is the junction thermal voltage, Rs is the series resistance, Ns is the number of cells, A is the ideality factor, k is the Boltzmann constant (1.3806503 × 10-23 J/K), T is the cell temperature, and q is the electron charge (1.6021765 × 10-19 C).
I = Iph – Io × exp.((( V + I × Rs ) / ( Ns × Vth )) - 1)
(1)
Vth =( A × k × T) / q
(2)
The open circuit voltage, short circuit current, and ideality factor of the solar cell block were set to 21.6 V, 7.34 A, and 1.5, respectively, based on the experimental setup. Under the above-detailed settings, the solar cell block represents a photovoltaic module with an open circuit voltage of 21.6 V, short circuit current of 7.34 A, and ideality factor of 1.5. The diode block from the Simscape block set was used as the bypass diode in the photovoltaic module, which was connected antiparallel to the solar cell block, as illustrated in Fig. 10.
Fig. 10. A single photovoltaic module with a bypass diode The architecture shown in Fig. 10 represents a photovoltaic module with a bypass diode. This architecture was duplicated to produce 20 units, which were then connected in series to form the photovoltaic strings shown in Figs. 6 to 9. The photovoltaic string was subsequently developed into a single block and termed as a PV string, as shown in Fig. 11. Fig. 11 shows the complete
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developed model, which has been previously published (J. C. Teo et al., 2018) and validated. Thus, it is believed this model will produce appropriate and practical results for the purposes of analysis.
Fig. 11. The photovoltaic string model The Unshaded and Shaded Irr blocks control the irradiance on the unshaded and shaded modules in the photovoltaic string, respectively. The Shaded Module block controls the number of shaded modules in the photovoltaic string. The shaded setups shown in Figs. 6 to 9 can be developed by setting the Unshaded Irr, Shaded Irr, and Shade Module blocks accordingly. At the start of the simulation, the Controlled Current Source block sweeps the output current of the PV string from 0 A to its short circuit current. The Voltage Sensor block measures the output voltages of the PV string under different output currents. The Product block then multiplies the output voltage and output current of the PV string to obtain its output power under different voltages. The output power and output voltage of the PV string are then sent to the Matlab (Mathworks, MA) workspace by the To Workplace block to plot the power-voltage relationship curve or P-V characteristic curve (J. C. Teo et al., 2018).
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3. RESULTS AND DISCUSSION Fig. 12 shows the P-V characteristic curve of the setup with 4 shaded photovoltaic modules with an irradiance of 100 W/m2 and the forward voltage of the bypass diode set to 0.7 V. The highest point in the P-V characteristic curve was identified to determine the maximum power of the photovoltaic string. A similar procedure was applied to all the P-V characteristic curves obtained in the experiments to determine the maximum power of the photovoltaic string in each shaded setups and under different forward voltages of the bypass diode.
Fig. 12. The P-V characteristic of the setup with 4 shaded modules with an irradiance of 100 W/m2 and the forward voltage of the bypass diode set to 0.7 V Fig. 13 shows the maximum power of the setup with 4 shaded photovoltaic modules when their irradiance was between 800 and 900 W/m2. The maximum power remained constant as the forward voltage of the bypass diode increased from 0 to 0.7 V. This indicates that the forward voltage of the bypass diode did not affect the maximum power of the photovoltaic string when the irradiance of the shaded modules was between 800 to 900 W/m2.
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Fig. 13. Setup with 4 shaded modules. The irradiance of the shaded modules was between 800– 900 W/m2 Fig. 14 shows the maximum power of the setup with 4 shaded photovoltaic modules when their irradiance was between 0 and 700 W/m2. The maximum power decreased as the forward voltage of the bypass diode increased. This implies that the maximum power of the photovoltaic string was affected by the forward voltage of the bypass diode.
Fig. 14. The setup with 4 shaded modules. The irradiance of the shaded modules was between 0– 700 W/m2
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The forward voltage of the bypass diode introduces a voltage drop in the photovoltaic string during partial shading. The voltage drop is believed to decrease the maximum power of the photovoltaic string, and the effect is more significant if the bypass diode has a higher forward voltage. The results shown in Fig. 14 are in agreement with the theoretical assumption. However, the results shown in Fig. 13 are not aligned with the theoretical assumption, as the maximum power was not affected even when the forward voltage of the bypass diode increased. Fig. 15 shows the maximum power of the setup with 8 shaded photovoltaic modules when their irradiance was between 600 and 900 W/m2. The figure indicates that the maximum power remained constant as the forward voltage of the bypass diode increased.
Fig. 15. The setup with 8 shaded modules. The irradiance of the shaded modules was between 600–900 W/m2 Fig. 16 shows the maximum power of the setup with 8 shaded photovoltaic modules when their irradiance was between 0 to 500 W/m2. The figure shows that the maximum power decreased as the forward voltage of the bypass diode increased. The setups with 4 and 8 shaded photovoltaic modules showed similar trends/results, where the forward voltage of the bypass diode affected the maximum power in certain cases.
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Fig. 16. The setup with 8 shaded modules. The irradiance of the shaded modules was between 0– 500 w/m2 Consider the setup with 12 shaded photovoltaic modules, as shown in Figs. 17 and 18. The trends observed in the setups with 4 and 8 shaded photovoltaic modules were also seen in the setup with 12 shaded photovoltaic modules. Fig. 17 shows that the maximum power remains constant with an increase in the forward voltage of the bypass diode when the irradiance on the shaded modules is between 400 to 900 W/m2. In contrast, the forward voltage of the bypass diode decreases the maximum power when the irradiance of the shaded photovoltaic modules is between 0 to 300 W/m2.
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Fig. 17. The setup with 12 shaded modules. The irradiance of the shaded modules was between 400–900 W/m2
Fig. 18. The setup with 12 shaded modules. The irradiance of the shaded modules was between 0– 300 W/m2 Similar trends were also observed in the setup with 16 shaded photovoltaic modules under a certain irradiance, where the forward voltage of the bypass diode affected the maximum power, as illustrated in Figs. 19 and 20.
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Fig. 19. The setup with 16 shaded modules. The irradiance of the shaded modules was between 200–900 W/m2
Fig. 20. The setup with 16 shaded modules. The irradiance of the shaded modules was between 0– 100 w/m2 Consider the setup with 4 shaded photovoltaic modules shown in Figs. 13 and 14. The forward voltage of the bypass diode affected the maximum power when the irradiance of the shaded modules was ≤700 W/m2. In the setup with 16 shaded photovoltaic modules (Fig. 20), the irradiance threshold level of the shaded modules was ≤ 100 W/m2. This indicates that when a larger number of photovoltaic modules were shaded, the forward voltage of the bypass diode affected the
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maximum power at a lower irradiance threshold. TABLE 2 shows the threshold level at which the forward voltage of the bypass diode began affecting the maximum power in each experimental setup. Eq. (3) was used to calculate the threshold value, which was derived from TABLE 2. TABLE 2. THE THRESHOLD AT WHICH THE FORWARD VOLTAGE OF THE BYPASS DIODE INITIATED MAXIMUM POWER LOSS
Experimental
Irradiance
Setup
Threshold of the Shaded Modules (W/m2)
4 Shaded
≤ 700
Modules 8 Shaded
≤ 500
Modules 12 Shaded
≤ 300
Modules 16 Shaded
≤ 100
Modules
Threshold = −1000 × (number of shaded modules / total number of modules) + 900
(3)
To validate Eq. (3), which was derived from the experiments, consider a photovoltaic string composed of two photovoltaic modules, as shown in Fig. 21. One photovoltaic module in the string was fixed at 1000 W/m2, while the other photovoltaic module was shaded.
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Fig. 21. A photovoltaic string composed of two photovoltaic modules Fig. 22 shows the maximum power of the photovoltaic string depicted in Fig. 21 when the shaded photovoltaic modules were exposed to an irradiance of 500 to 900 W/m2. The maximum power remained constant as the forward voltage of the bypass diode increased. This indicates that the forward voltage of the bypass diode did not affect the maximum power of the photovoltaic string.
Fig. 22. A shaded module with an irradiance of 500 to 900 W/m2 Fig. 23 shows the maximum power of the photovoltaic string when the shaded photovoltaic modules were exposed to an irradiance of 0 to 400 W/m2. The maximum power decreased as the
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forward voltage of the bypass diode increased. This indicates that the forward voltage of the bypass diode affected the maximum power of the photovoltaic string.
Fig. 23. A shaded module with an irradiance of 0 to 400 W/m2 Thus, the forward voltage of the bypass diode affected the maximum power when the irradiance of the shaded modules was ≤ 400 W/m2. It can be observed that the simulation results are similar to the results determined using Eq. (3). This implies that Eq. (3), which was derived from the experiments, was validated with respect to the photovoltaic strings of various sizes. The experimental results indicate that the forward voltage of the bypass diode did not necessarily affect the maximum power, which depended on the shading patterns and shading heaviness over the photovoltaic string. Figs. 24 and 25 were examined to understand the theoretical basis of this finding. Fig. 24 shows the P-V characteristic curve of the setup with 16 shaded modules when their irradiance was 100 W/m2. The maximum power of the photovoltaic string decreased as the forward voltage of the bypass diode increased from 0.2 V to 0.7 V.
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Fig. 24. The P-V characteristic curve of the setup with 16 shaded modules. The irradiance of the shaded modules was 100 W/m2 Fig. 25 shows the P-V characteristic curve of the setup with 16 shaded modules when their irradiance was 300 W/m2. When the forward voltage of the bypass diode increased from 0.2 V to 0.7 V, the left peak of the P-V characteristic curve decreased. However, the maximum power, which is the highest point of the P-V characteristic curve, remained unchanged.
Fig. 25. The P-V characteristic curve of the setup with 16 shaded modules. The irradiance of the shaded modules was 300 W/m2
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Thus, the forward voltage of the bypass diode did not necessarily decrease the maximum power of the photovoltaic string. The forward voltage of the bypass diode affected the output power only when the photovoltaic string was delivering power at a lower voltage, as illustrated in Figs. 24 and 25. When the irradiance of the shaded photovoltaic module was above the threshold level determined by Eq. (3), the maximum power was delivered at a higher voltage, as illustrated in Fig. 25. This indicates that the maximum power was unaffected by the forward voltage of the bypass diode. When the irradiance of the shaded photovoltaic module was below the threshold level determined by Eq. (3), the maximum power was delivered at a lower voltage, as illustrated in Fig. 24. This indicates that the maximum power was susceptible to the forward voltage of the bypass diode. Thus, the forward voltage of the bypass diode causes power losses only when the photovoltaic string is delivering power at a lower voltage. Furthermore, the forward voltage of the bypass diode does not necessarily cause maximum power loss. This depends on whether the maximum power is delivered at a higher or lower voltage. To validate these new findings, consider the photovoltaic string composed of 4 photovoltaic modules, as shown in Fig. 26. Two photovoltaic modules are shaded, while the other two are unshaded. The unshaded photovoltaic modules generate 7.3 A, while the shaded ones generate 3.65 A.
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Fig. 26. A photovoltaic string composed of 4 photovoltaic modules The shaded photovoltaic modules are unable to sustain when the load draws more than 3.65 A from the photovoltaic string. The bypass diodes are activated to bypass the shaded photovoltaic modules. Hence, the current flow through the two unshaded photovoltaic modules and two bypass diodes is as illustrated in Fig. 27.
Fig. 27. Scenario where the load is drawing more than 3.65 A Thus, the I-V characteristics of the photovoltaic string at currents higher than 3.65 A are determined by the two unshaded photovoltaic modules and two bypass diodes. Variation in the forward voltage of the bypass diode affects the I-V characteristics at currents > 3.65 A, as illustrated in Fig 28.
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Fig. 28. Impact of the forward voltage of the bypass diode on the I-V characteristics The shaded photovoltaic modules are able to sustain when the load is drawing less than 3.65 A from the photovoltaic string. The bypass diodes are not activated. Hence, the current flow through the two unshaded and shaded photovoltaic modules is as illustrated in Fig. 29. Therefore, the I-V characteristics of the photovoltaic string at currents lower than 3.65 A are determined by the two unshaded and shaded photovoltaic modules, as illustrated in Fig. 29. Thus, variation in the forward voltage of the bypass diode does not affect the I-V characteristics at currents < 3.65 A, as illustrated in Fig 28.
Fig. 29. Scenario where the load is drawing less than 3.65 A
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Fig. 30 shows the P-V characteristic curve obtained from the I-V characteristic curve shown in Fig. 28. The figure indicates that the lower voltage part of the P-V characteristic curve changed as the forward voltage of the bypass diode increased from 0.2 V to 0.7 V. However, the higher voltage part of the P-V characteristic curve was not affected.
Fig. 30. The I-V characteristics converted to the P-V characteristics This validates the finding that the forward voltage of the bypass diode causes power losses only when the photovoltaic string is delivering power at lower voltages. Furthermore, it also validates
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that the forward voltage of the bypass diode does not necessarily cause the maximum power loss. This depends on whether the maximum power is delivered at a lower or higher voltage. Fig. 31 shows the maximum power of the setup with 16 shaded modules when the forward voltage of the bypass diode is 0.2 V and 0.7 V. When the irradiance of the shaded modules was 100 W/m2, the maximum power was 245.6 W and 291.3 W corresponding to the bypass diode forward voltages of 0.7 V and 0.2 V, respectively. The maximum power dropped by 15.69% as the forward voltage of the bypass diode increased from 0.2 V to 0.7 V. The maximum power drop was even more significant when the irradiance of the shaded modules was 0 W/m2. The maximum power was 226.24 W and 270.9 W corresponding to the bypass diode forward voltages of 0.7 V and 0.2 V, respectively. The maximum power dropped by 16.48% as the forward voltage of the bypass diode increased from 0.2 V to 0.7 V. Thus, the impact of the forward voltage of the bypass diode should not be underestimated.
Fig. 31. Maximum power of the setup with 16 shaded modules when the forward voltage of the bypass diode is 0.7 V and 0.2 V
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4. CONCLUSIONS A fundamental study was performed to investigate the impact of the forward voltage of the bypass diode on the P-V characteristics and maximum power of a photovoltaic string. A photovoltaic string composed of 20 photovoltaic modules was used in the experiments. Various shading patterns and bypass diode forward voltages were applied to the photovoltaic string. The forward voltage of the bypass diode introduced a voltage drop in the photovoltaic string when activated. Therefore, it was generally assumed that the forward voltage of the bypass diode introduced maximum power losses in the photovoltaic string. However, the findings of this study demonstrated that this was not necessarily true. The forward voltage of the bypass diode only introduced power losses when the photovoltaic string was delivering power at a lower voltage. In a few partial shading cases, the maximum power was delivered at a lower voltage, rendering it susceptible to the forward voltage of the bypass diode. The results and discussion indicated that the bypass forward voltage should be addressed to minimise the losses of the PV system under partial shading. In conclusion, this novel study demonstrated/revealed that the forward voltage of the bypass diode did not cause power loss when the maximum power of the photovoltaic system was delivered at a higher voltage. The fundamental theories of this newly discovered phenomenon were investigated and validated. Declaration of Interest The authors have none to declare.
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Funding: This work was supported by the Ministry of Education Malaysia [grant numbers FRGS/1/2015/TK07/UCSI/02/1].
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights
Bypass diode forward voltage not necessarily causes photovoltaic maximum power loss. Maximum power delivered at high voltage - unaffected by bypass diode forward voltage. Maximum power delivered at low voltage - susceptible to bypass diode forward voltage.
J.C. Teo, Rodney H.G. Tan, V.H. Mok, Vigna K. Ramachandaramurthy, ChiaKwang Tan PII:
S0360-5442(19)32186-3
DOI:
https://doi.org/10.1016/j.energy.2019.116491
Reference:
EGY 116491
To appear in:
Energy
Received Date:
01 November 2018
Accepted Date:
04 November 2019
Please cite this article as: J.C. Teo, Rodney H.G. Tan, V.H. Mok, Vigna K. Ramachandaramurthy, ChiaKwang Tan, Impact of Bypass Diode Forward Voltage on Maximum Power of a Photovoltaic System under Partial Shading Conditions, Energy (2019), https://doi.org/10.1016/j.energy. 2019.116491
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
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Impact of Bypass Diode Forward Voltage on Maximum Power of a Photovoltaic System under Partial Shading Conditions J. C. Teo 1,*, Rodney H. G. Tan 1, V. H. Mok 1, Vigna K. Ramachandaramurthy 2, and ChiaKwang Tan 3 1
Faculty of Engineering, Technology & Built Environment, UCSI University No.1, Jalan Menara Gading, Kuala Lumpur 56000, Malaysia; [email protected] (Rodney); [email protected](V.H. Mok)
2
Institute of Power Engineering, Department of Electrical Power Engineering, Universiti Tenaga Nasional, 43000 Kajang, Malaysia; [email protected]
3
UM Power Energy Dedicated Advanced Centre, University Of Malaya, Jalan Pantai Baharu, Kuala Lumpur 59990, Malaysia; [email protected]
* Correspondence: [email protected]; Tel.: +6017-33395338
Abstract— The maximum power of a photovoltaic system can reduce significantly under partial shading conditions. Bypass diodes can be used in photovoltaic systems to bypass the shaded photovoltaic modules during partial shading. The bypass diode possesses a forward voltage that introduces a voltage drop in the photovoltaic system upon activation. Therefore, the maximum power of a photovoltaic system can reduce further during partial shading due to the forward voltage of the bypass diode. This paper presents an investigation into the effect of bypass diode forward voltage on the maximum power of a photovoltaic system under
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partial shading conditions. The results indicated that the forward voltage of the bypass diode did not necessarily decrease the maximum power of the photovoltaic system. This depends on whether the maximum power is delivered at a lower or higher voltage. When the maximum power is delivered at a higher voltage, it is insusceptible to the forward voltage. Conversely, when the maximum power is delivered at a lower voltage, it is susceptible to the forward voltage. In the worst-case scenario, the forward voltage of the bypass diode reduced the maximum power of the photovoltaic system by 16.48%, which was already subject to partial shading loss.
Keywords— Photovoltaic; bypass diode; partial shading; P-V characteristics; solar energy 1. INTRODUCTION Solar energy has become increasingly popular due to its environment friendly and inexhaustible nature (A. K. Pandeya et al., 2018; C. G. Ozoegwu et al., 2017; E. Kabir et al., 2018; S. K. Sansaniwal et al., 2018; T. Jia et al., 2018). New photovoltaic power systems with a capacity of 75 Gw were installed worldwide in 2016, which could power up to approximately 15 million houses simultaneously (J. D. Bastidas-Rodríguez et al., 2018). The number of new installations was even higher in 2017. Based on the REN21 2018 report, new photovoltaic power systems with a capacity of ~100 Gw were installed worldwide in 2017, recording an approximately 34% growth from 2016 (Renewables 2018 Global Status Report). The rapid growth in the installed capacities of photovoltaic systems has promoted research in the field of photovoltaic power generation. A solar panel or photovoltaic module is the most basic element of a photovoltaic power system, which converts solar energy into electrical power (A. Syafiq et al., 2018; J. C. Teo et al., 2017; M. Belarbi et al., 2018; N. AL-Rousan et al., 2018; P. A. Kumari et al., 2018). The relationship
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between the output current and voltage of a photovoltaic module is denoted by the I-V characteristic, which describes its behaviour (A. J. Bühler et al., 2014; F. Spertino et al., 2015; F. Spertino et al., 2016; N. Boutana et al., 2017). The output current and voltage obtained from the I-V characteristics can be used to calculate the output power of the photovoltaic module at different output voltages. The relationship between the output power and voltage of a photovoltaic module is called the PV characteristic. Fig. 1 shows the typical P-V characteristic curve of a photovoltaic module. The (X,Y) point on the curve corresponds to the highest point of the P-V characteristic, which indicates the maximum power generated by the photovoltaic module (M. R. Maghami et al., 2016). The maximum power of a photovoltaic module is always tracked and harvested during electrical generation (S. K. Das et al., 2017). Hence, the performance of the photovoltaic module depends on its maximum power.
Fig. 1. P-V characteristics of a photovoltaic module To achieve a higher output power and voltage, multiple photovoltaic modules are connected in series to form a photovoltaic string, as illustrated in Fig. 2 (A. Mohammedi et al., 2014; T. Shimizu
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et al., 2001; L. Linares et al., 2009; J. Kwon et al., 2009; H. Kim et al., 2009; M. Forcan et al., 2016; Y. Wang et al., 2016). The P-V characteristic of a photovoltaic string is assessed to harvest its maximum power during electrical generation.
Fig. 2. A photovoltaic string Consider a shaded photovoltaic module in a photovoltaic string (blue box in Fig. 3).The other unshaded photovoltaic modules generate higher currents as compared to the shaded module. However, the total output current is limited to the output current of the shaded photovoltaic module due to the series connection.
Fig. 3. A photovoltaic string without bypass diodes To overcome this problem, a bypass diode is implemented in the photovoltaic module. The bypass diode is connected anti-parallel to the photovoltaic module. It bypasses the shaded photovoltaic module to prevent electrical current generated by the unshaded photovoltaic modules from flowing through the shaded photovoltaic module, as illustrated in Fig. 4.
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Fig. 4. A photovoltaic string with a bypass diode Fig. 5 illustrates the P-V characteristics of a photovoltaic string composed of 20 photovoltaic modules under both non-shading and partial shading conditions. The red line indicates the P-V characteristic of the photovoltaic string under non-shading conditions where all the photovoltaic modules are receiving 1000 W/m2, i.e. during uniform irradiance. The peak represents the global peak, which indicates the maximum power of the photovoltaic string (L. Bouselhama et al., 2017; M. Muthuramalingam and P. S. Manoharan, 2014; Y. E. Abu Eldahaba et al., 2017; A. D. Martin et al., 2018). The blue line represents the P-V characteristic of the photovoltaic string under partial shading conditions, where 10 photovoltaic modules are receiving 1000 W/m2, while the other modules are receiving 300 W/m2. Fig. 5 shows that the maximum power of the photovoltaic string can decrease up to 55% due to partial shading; hence, the impact of partial shading should not be underestimated.
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Fig. 5. P-V characteristics under non-shading and partial shading When partial shading occurs, the bypass diodes are activated to bypass the shaded photovoltaic modules. The bypass diodes introduce a forward voltage drop in the photovoltaic string upon activation, which further decreases the maximum power of the photovoltaic string during partial shading. Industries/companies such as Texas Instruments (http://www.ti.com/product/SM74611) have fabricated smart bypass diodes with a low forward voltage of 26 mV to minimise the power losses. Thus, the impact of bypass diode forward voltage should not be underestimated. However, the impact of the bypass diode forward voltage on the maximum power of the photovoltaic string has not been studied extensively. Most research studies have focused on other aspects of the bypass diode. For example, Suk Whan Ko et al. (2017) investigated the impact of a damaged bypass diode on the photovoltaic module. Zheng et al. (2014) studied the impact of different bypass diode configurations on the energy extraction of a photovoltaic array. Daliento et al. (2016) proposed a new bypass mechanism that implemented an extra series-connected power MOSFET in the photovoltaic module. Silvestre et al. (2009) and Teo et al (2017) studied the impact of bypass diode configurations on the photovoltaic module.
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Bauwens and Doutreloigne (2014) proposed an integral smart bypass diode that almost resembled an ideal diode. Simulation results of a photovoltaic module with normal bypass and smart bypass diodes were presented. The simulation results showed that the photovoltaic module with a smart bypass diode had a higher maximum output power compared to the photovoltaic module with a normal bypass diode under certain degrees of shading. However, very few shading conditions were employed in their simulations, which were insufficient to conclude the impact of the forward voltage of the bypass diode. The simulation results did not show the impact of the forward voltage of the bypass diode on the P-V characteristics. Furthermore, the simulations were conducted at the photovoltaic module level. The authors did not discuss the impact of the forward voltage of the bypass diode on the photovoltaic string. Considering all of these issues, the impact of bypass diode forward voltage on the P-V characteristics and maximum power of a photovoltaic string under partial shading conditions is investigated. 2. METHODOLOGY A photovoltaic string composed of 20 photovoltaic modules connected in series was evaluated. The open circuit voltage, short circuit current, and ideality factor of the photovoltaic modules were 21.6 V, 7.34 A, and 1.5 respectively. In this study, the experiments were conducted at a temperature T = 25 oC. The photovoltaic modules had one bypass diode. The forward voltage of the bypass diode was set to 0.7 V. Four different setups of the photovoltaic string were evaluated, which included 4, 8, 12, and 16 shaded modules, as illustrated in Figs. 6 to 9, respectively.
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Fig. 6. A setup with 4 shaded PV modules
Fig. 7. A setup with 8 shaded PV modules
Fig. 8. A setup with 12 shaded PV modules
Fig. 9. A setup with 16 shaded PV modules TABLE 1 shows the conditions applied to each shaded setup. The P-V characteristic curve and maximum power of each shaded setup under the corresponding conditions were determined.
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TABLE 1. THE CONDITIONS APPLIED TO EACH SHADED SETUP Conditions
Irradiance of the
Irradiance of the Shaded
Unshaded Modules
Modules (W/m2)
(W/m2) Condition 1
1000
900
Condition 2
1000
800
Condition 3
1000
700
Condition 4
1000
600
Condition 5
1000
500
Condition 6
1000
400
Condition 7
1000
300
Condition 8
1000
200
Condition 9
1000
100
Condition 10
1000
0
The forward voltage of the bypass diode was set to 0.6 V. The same experiments were then performed to determine the impact of the forward voltage of the bypass diode on the P-V characteristics and maximum power of the photovoltaic string. To investigate further, the same experiments were also performed by setting the forward voltage of the bypass diode to 0.5, 0.4, 0.3, 0.2, 0.1, and 0.0 V. A photovoltaic string model was developed using Simulink (Mathworks, MA) for the experiments. The solar cell block in Simulink was used to develop the model. This had a five parameter configuration, as defined in Eq. (1) and Eq. (2), where I is the output current, V is the
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output voltage, Iph is the photo-generated current, Io is the diode saturation current, Vth is the junction thermal voltage, Rs is the series resistance, Ns is the number of cells, A is the ideality factor, k is the Boltzmann constant (1.3806503 × 10-23 J/K), T is the cell temperature, and q is the electron charge (1.6021765 × 10-19 C).
I = Iph – Io × exp.((( V + I × Rs ) / ( Ns × Vth )) - 1)
(1)
Vth =( A × k × T) / q
(2)
The open circuit voltage, short circuit current, and ideality factor of the solar cell block were set to 21.6 V, 7.34 A, and 1.5, respectively, based on the experimental setup. Under the above-detailed settings, the solar cell block represents a photovoltaic module with an open circuit voltage of 21.6 V, short circuit current of 7.34 A, and ideality factor of 1.5. The diode block from the Simscape block set was used as the bypass diode in the photovoltaic module, which was connected antiparallel to the solar cell block, as illustrated in Fig. 10.
Fig. 10. A single photovoltaic module with a bypass diode The architecture shown in Fig. 10 represents a photovoltaic module with a bypass diode. This architecture was duplicated to produce 20 units, which were then connected in series to form the photovoltaic strings shown in Figs. 6 to 9. The photovoltaic string was subsequently developed into a single block and termed as a PV string, as shown in Fig. 11. Fig. 11 shows the complete
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developed model, which has been previously published (J. C. Teo et al., 2018) and validated. Thus, it is believed this model will produce appropriate and practical results for the purposes of analysis.
Fig. 11. The photovoltaic string model The Unshaded and Shaded Irr blocks control the irradiance on the unshaded and shaded modules in the photovoltaic string, respectively. The Shaded Module block controls the number of shaded modules in the photovoltaic string. The shaded setups shown in Figs. 6 to 9 can be developed by setting the Unshaded Irr, Shaded Irr, and Shade Module blocks accordingly. At the start of the simulation, the Controlled Current Source block sweeps the output current of the PV string from 0 A to its short circuit current. The Voltage Sensor block measures the output voltages of the PV string under different output currents. The Product block then multiplies the output voltage and output current of the PV string to obtain its output power under different voltages. The output power and output voltage of the PV string are then sent to the Matlab (Mathworks, MA) workspace by the To Workplace block to plot the power-voltage relationship curve or P-V characteristic curve (J. C. Teo et al., 2018).
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3. RESULTS AND DISCUSSION Fig. 12 shows the P-V characteristic curve of the setup with 4 shaded photovoltaic modules with an irradiance of 100 W/m2 and the forward voltage of the bypass diode set to 0.7 V. The highest point in the P-V characteristic curve was identified to determine the maximum power of the photovoltaic string. A similar procedure was applied to all the P-V characteristic curves obtained in the experiments to determine the maximum power of the photovoltaic string in each shaded setups and under different forward voltages of the bypass diode.
Fig. 12. The P-V characteristic of the setup with 4 shaded modules with an irradiance of 100 W/m2 and the forward voltage of the bypass diode set to 0.7 V Fig. 13 shows the maximum power of the setup with 4 shaded photovoltaic modules when their irradiance was between 800 and 900 W/m2. The maximum power remained constant as the forward voltage of the bypass diode increased from 0 to 0.7 V. This indicates that the forward voltage of the bypass diode did not affect the maximum power of the photovoltaic string when the irradiance of the shaded modules was between 800 to 900 W/m2.
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Fig. 13. Setup with 4 shaded modules. The irradiance of the shaded modules was between 800– 900 W/m2 Fig. 14 shows the maximum power of the setup with 4 shaded photovoltaic modules when their irradiance was between 0 and 700 W/m2. The maximum power decreased as the forward voltage of the bypass diode increased. This implies that the maximum power of the photovoltaic string was affected by the forward voltage of the bypass diode.
Fig. 14. The setup with 4 shaded modules. The irradiance of the shaded modules was between 0– 700 W/m2
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The forward voltage of the bypass diode introduces a voltage drop in the photovoltaic string during partial shading. The voltage drop is believed to decrease the maximum power of the photovoltaic string, and the effect is more significant if the bypass diode has a higher forward voltage. The results shown in Fig. 14 are in agreement with the theoretical assumption. However, the results shown in Fig. 13 are not aligned with the theoretical assumption, as the maximum power was not affected even when the forward voltage of the bypass diode increased. Fig. 15 shows the maximum power of the setup with 8 shaded photovoltaic modules when their irradiance was between 600 and 900 W/m2. The figure indicates that the maximum power remained constant as the forward voltage of the bypass diode increased.
Fig. 15. The setup with 8 shaded modules. The irradiance of the shaded modules was between 600–900 W/m2 Fig. 16 shows the maximum power of the setup with 8 shaded photovoltaic modules when their irradiance was between 0 to 500 W/m2. The figure shows that the maximum power decreased as the forward voltage of the bypass diode increased. The setups with 4 and 8 shaded photovoltaic modules showed similar trends/results, where the forward voltage of the bypass diode affected the maximum power in certain cases.
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Fig. 16. The setup with 8 shaded modules. The irradiance of the shaded modules was between 0– 500 w/m2 Consider the setup with 12 shaded photovoltaic modules, as shown in Figs. 17 and 18. The trends observed in the setups with 4 and 8 shaded photovoltaic modules were also seen in the setup with 12 shaded photovoltaic modules. Fig. 17 shows that the maximum power remains constant with an increase in the forward voltage of the bypass diode when the irradiance on the shaded modules is between 400 to 900 W/m2. In contrast, the forward voltage of the bypass diode decreases the maximum power when the irradiance of the shaded photovoltaic modules is between 0 to 300 W/m2.
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Fig. 17. The setup with 12 shaded modules. The irradiance of the shaded modules was between 400–900 W/m2
Fig. 18. The setup with 12 shaded modules. The irradiance of the shaded modules was between 0– 300 W/m2 Similar trends were also observed in the setup with 16 shaded photovoltaic modules under a certain irradiance, where the forward voltage of the bypass diode affected the maximum power, as illustrated in Figs. 19 and 20.
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Fig. 19. The setup with 16 shaded modules. The irradiance of the shaded modules was between 200–900 W/m2
Fig. 20. The setup with 16 shaded modules. The irradiance of the shaded modules was between 0– 100 w/m2 Consider the setup with 4 shaded photovoltaic modules shown in Figs. 13 and 14. The forward voltage of the bypass diode affected the maximum power when the irradiance of the shaded modules was ≤700 W/m2. In the setup with 16 shaded photovoltaic modules (Fig. 20), the irradiance threshold level of the shaded modules was ≤ 100 W/m2. This indicates that when a larger number of photovoltaic modules were shaded, the forward voltage of the bypass diode affected the
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maximum power at a lower irradiance threshold. TABLE 2 shows the threshold level at which the forward voltage of the bypass diode began affecting the maximum power in each experimental setup. Eq. (3) was used to calculate the threshold value, which was derived from TABLE 2. TABLE 2. THE THRESHOLD AT WHICH THE FORWARD VOLTAGE OF THE BYPASS DIODE INITIATED MAXIMUM POWER LOSS
Experimental
Irradiance
Setup
Threshold of the Shaded Modules (W/m2)
4 Shaded
≤ 700
Modules 8 Shaded
≤ 500
Modules 12 Shaded
≤ 300
Modules 16 Shaded
≤ 100
Modules
Threshold = −1000 × (number of shaded modules / total number of modules) + 900
(3)
To validate Eq. (3), which was derived from the experiments, consider a photovoltaic string composed of two photovoltaic modules, as shown in Fig. 21. One photovoltaic module in the string was fixed at 1000 W/m2, while the other photovoltaic module was shaded.
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Fig. 21. A photovoltaic string composed of two photovoltaic modules Fig. 22 shows the maximum power of the photovoltaic string depicted in Fig. 21 when the shaded photovoltaic modules were exposed to an irradiance of 500 to 900 W/m2. The maximum power remained constant as the forward voltage of the bypass diode increased. This indicates that the forward voltage of the bypass diode did not affect the maximum power of the photovoltaic string.
Fig. 22. A shaded module with an irradiance of 500 to 900 W/m2 Fig. 23 shows the maximum power of the photovoltaic string when the shaded photovoltaic modules were exposed to an irradiance of 0 to 400 W/m2. The maximum power decreased as the
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forward voltage of the bypass diode increased. This indicates that the forward voltage of the bypass diode affected the maximum power of the photovoltaic string.
Fig. 23. A shaded module with an irradiance of 0 to 400 W/m2 Thus, the forward voltage of the bypass diode affected the maximum power when the irradiance of the shaded modules was ≤ 400 W/m2. It can be observed that the simulation results are similar to the results determined using Eq. (3). This implies that Eq. (3), which was derived from the experiments, was validated with respect to the photovoltaic strings of various sizes. The experimental results indicate that the forward voltage of the bypass diode did not necessarily affect the maximum power, which depended on the shading patterns and shading heaviness over the photovoltaic string. Figs. 24 and 25 were examined to understand the theoretical basis of this finding. Fig. 24 shows the P-V characteristic curve of the setup with 16 shaded modules when their irradiance was 100 W/m2. The maximum power of the photovoltaic string decreased as the forward voltage of the bypass diode increased from 0.2 V to 0.7 V.
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Fig. 24. The P-V characteristic curve of the setup with 16 shaded modules. The irradiance of the shaded modules was 100 W/m2 Fig. 25 shows the P-V characteristic curve of the setup with 16 shaded modules when their irradiance was 300 W/m2. When the forward voltage of the bypass diode increased from 0.2 V to 0.7 V, the left peak of the P-V characteristic curve decreased. However, the maximum power, which is the highest point of the P-V characteristic curve, remained unchanged.
Fig. 25. The P-V characteristic curve of the setup with 16 shaded modules. The irradiance of the shaded modules was 300 W/m2
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Thus, the forward voltage of the bypass diode did not necessarily decrease the maximum power of the photovoltaic string. The forward voltage of the bypass diode affected the output power only when the photovoltaic string was delivering power at a lower voltage, as illustrated in Figs. 24 and 25. When the irradiance of the shaded photovoltaic module was above the threshold level determined by Eq. (3), the maximum power was delivered at a higher voltage, as illustrated in Fig. 25. This indicates that the maximum power was unaffected by the forward voltage of the bypass diode. When the irradiance of the shaded photovoltaic module was below the threshold level determined by Eq. (3), the maximum power was delivered at a lower voltage, as illustrated in Fig. 24. This indicates that the maximum power was susceptible to the forward voltage of the bypass diode. Thus, the forward voltage of the bypass diode causes power losses only when the photovoltaic string is delivering power at a lower voltage. Furthermore, the forward voltage of the bypass diode does not necessarily cause maximum power loss. This depends on whether the maximum power is delivered at a higher or lower voltage. To validate these new findings, consider the photovoltaic string composed of 4 photovoltaic modules, as shown in Fig. 26. Two photovoltaic modules are shaded, while the other two are unshaded. The unshaded photovoltaic modules generate 7.3 A, while the shaded ones generate 3.65 A.
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Fig. 26. A photovoltaic string composed of 4 photovoltaic modules The shaded photovoltaic modules are unable to sustain when the load draws more than 3.65 A from the photovoltaic string. The bypass diodes are activated to bypass the shaded photovoltaic modules. Hence, the current flow through the two unshaded photovoltaic modules and two bypass diodes is as illustrated in Fig. 27.
Fig. 27. Scenario where the load is drawing more than 3.65 A Thus, the I-V characteristics of the photovoltaic string at currents higher than 3.65 A are determined by the two unshaded photovoltaic modules and two bypass diodes. Variation in the forward voltage of the bypass diode affects the I-V characteristics at currents > 3.65 A, as illustrated in Fig 28.
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Fig. 28. Impact of the forward voltage of the bypass diode on the I-V characteristics The shaded photovoltaic modules are able to sustain when the load is drawing less than 3.65 A from the photovoltaic string. The bypass diodes are not activated. Hence, the current flow through the two unshaded and shaded photovoltaic modules is as illustrated in Fig. 29. Therefore, the I-V characteristics of the photovoltaic string at currents lower than 3.65 A are determined by the two unshaded and shaded photovoltaic modules, as illustrated in Fig. 29. Thus, variation in the forward voltage of the bypass diode does not affect the I-V characteristics at currents < 3.65 A, as illustrated in Fig 28.
Fig. 29. Scenario where the load is drawing less than 3.65 A
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Fig. 30 shows the P-V characteristic curve obtained from the I-V characteristic curve shown in Fig. 28. The figure indicates that the lower voltage part of the P-V characteristic curve changed as the forward voltage of the bypass diode increased from 0.2 V to 0.7 V. However, the higher voltage part of the P-V characteristic curve was not affected.
Fig. 30. The I-V characteristics converted to the P-V characteristics This validates the finding that the forward voltage of the bypass diode causes power losses only when the photovoltaic string is delivering power at lower voltages. Furthermore, it also validates
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that the forward voltage of the bypass diode does not necessarily cause the maximum power loss. This depends on whether the maximum power is delivered at a lower or higher voltage. Fig. 31 shows the maximum power of the setup with 16 shaded modules when the forward voltage of the bypass diode is 0.2 V and 0.7 V. When the irradiance of the shaded modules was 100 W/m2, the maximum power was 245.6 W and 291.3 W corresponding to the bypass diode forward voltages of 0.7 V and 0.2 V, respectively. The maximum power dropped by 15.69% as the forward voltage of the bypass diode increased from 0.2 V to 0.7 V. The maximum power drop was even more significant when the irradiance of the shaded modules was 0 W/m2. The maximum power was 226.24 W and 270.9 W corresponding to the bypass diode forward voltages of 0.7 V and 0.2 V, respectively. The maximum power dropped by 16.48% as the forward voltage of the bypass diode increased from 0.2 V to 0.7 V. Thus, the impact of the forward voltage of the bypass diode should not be underestimated.
Fig. 31. Maximum power of the setup with 16 shaded modules when the forward voltage of the bypass diode is 0.7 V and 0.2 V
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4. CONCLUSIONS A fundamental study was performed to investigate the impact of the forward voltage of the bypass diode on the P-V characteristics and maximum power of a photovoltaic string. A photovoltaic string composed of 20 photovoltaic modules was used in the experiments. Various shading patterns and bypass diode forward voltages were applied to the photovoltaic string. The forward voltage of the bypass diode introduced a voltage drop in the photovoltaic string when activated. Therefore, it was generally assumed that the forward voltage of the bypass diode introduced maximum power losses in the photovoltaic string. However, the findings of this study demonstrated that this was not necessarily true. The forward voltage of the bypass diode only introduced power losses when the photovoltaic string was delivering power at a lower voltage. In a few partial shading cases, the maximum power was delivered at a lower voltage, rendering it susceptible to the forward voltage of the bypass diode. The results and discussion indicated that the bypass forward voltage should be addressed to minimise the losses of the PV system under partial shading. In conclusion, this novel study demonstrated/revealed that the forward voltage of the bypass diode did not cause power loss when the maximum power of the photovoltaic system was delivered at a higher voltage. The fundamental theories of this newly discovered phenomenon were investigated and validated. Declaration of Interest The authors have none to declare.
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Funding: This work was supported by the Ministry of Education Malaysia [grant numbers FRGS/1/2015/TK07/UCSI/02/1].
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights
Bypass diode forward voltage not necessarily causes photovoltaic maximum power loss. Maximum power delivered at high voltage - unaffected by bypass diode forward voltage. Maximum power delivered at low voltage - susceptible to bypass diode forward voltage.