Matlab/Simulink Based Modeling to Study Effect of Partial Shadow on Solar Photovoltaic Array

Abstract

Background

Solar module is considered as fundamental power transformation unit of Photovoltaic (PV) generation system. The performance of a PV array strongly depends on operating environmental conditions such as operating temperature, solar insolation, shading array configuration. Often PV arrays get shadowed fully or partially by passing cloud, building, poles, trees, etc. Under such partial shading conditions, the operation of PV arrays get more complicated with more than one peak and it is very important to predict the characteristics to obtain possible maximum power. Furthermore, the mismatch losses and hotspot effects caused by partially shading cannot only affect the output power of solar system also can bring security and reliability problem. However, it is quite expensive and takes much time to get operating output characteristics of PV arrays under non uniform working conditions. Therefore, it is necessary to have a simulation model to study the effect of partial shading on solar PV arrays' working characteristics. A 100 W solar panel is used as reference. The study also focuses on position of output peak power with varying location of shaded modules, different levels of solar irradiation as well as role of bypass diodes in this system.

Result

The I-V and P-V output curves of the solar PV arrays under partially shading conditions are given. The simulation results indicate that the higher number of shaded solar modules is, the lower value of power output is and the position of maximum power point does not depend on location of modules under shadow.

Conclusion

The paper provides an easy method to study the response of solar PV arrays under non uniform working conditions: partial shadow. In addition, the following points were also investigated: Impact of varying position of shaded modules on PV array's characteristics. Impact of different levels of solar irradiation accompany with varying temperature on solar PV array's operation under partially shading condition. Role of bypass diode in improving operation of solar system under non uniform condition.

Nguyen Environ Syst Res (2015) 4:20 DOI 10.1186/s40068-015-0042-1

RESEARCH

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Web End = Xuan Hieu Nguyen*Abstract

Background: Solar module is considered as fundamental power transformation unit of Photovoltaic (PV) generation system. The performance of a PV array strongly depends on operating environmental conditions such as operating temperature, solar insolation, shading array conguration. Often PV arrays get shadowed fully or partially by passing cloud, building, poles, trees, etc. Under such partial shading conditions, the operation of PV arrays get more complicated with more than one peak and it is very important to predict the characteristics to obtain possible maximum power. Furthermore, the mismatch losses and hotspot eects caused by partially shading cannot only aect the output power of solar system also can bring security and reliability problem. However, it is quite expensive and takes much time to get operating output characteristics of PV arrays under non uniform working conditions. Therefore, it is necessary to have a simulation model to study the eect of partial shading on solar PV arrays working characteristics. A 100 W solar panel is used as reference. The study also focuses on position of output peak power with varying location of shaded modules, dierent levels of solar irradiation as well as role of bypass diodes in this system.

Result: The IV and PV output curves of the solar PV arrays under partially shading conditions are given. The simulation results indicate that the higher number of shaded solar modules is, the lower value of power output is and the position of maximum power point does not depend on location of modules under shadow.

Conclusion: The paper provides an easy method to study the response of solar PV arrays under non uniform working conditions: partial shadow. In addition, the following points were also investigated:

Impact of varying position of shaded modules on PV arrays characteristics.

Impact of dierent levels of solar irradiation accompany with varying temperature on solar PV arrays operation under partially shading condition.

Role of bypass diode in improving operation of solar system under non uniform condition. Keywords: Matlab/Simulink, Photovoltaic arrays, Shading eect, PV and IV characteristics, Bypass diode

Background

The issue of modeling of PV modules/arrays under non uniform conditions such as shading condition has been largely investigated in literature and gets some certain results.

In initial studies, partial shading condition was just considered as one factor in the eect of environmental

condition (solar insolation, temperature, etc.) on PV arrays operation, for example in the research of Ibbini et al. (2014) so the result is quite limited and does not give reader overview on relationship between solar PV array and shadow eect.

A model developed from Tag tools in Simulink to study shading effect was proposed in the research of Mantri and Verma (2015) and Bouraiou et al. (2014). In this study, the role of bypass diode and varying levels of solar insolation on PV arrays working characteristics was investigated. However, the gap of this

*Correspondence: [email protected]; [email protected] Faculty of Engineering, Vietnam National University of Agriculture, Trau Quy town, Gia Lam district, Hanoi, Vietnam

2015 Nguyen. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/

Web End =http:// http://creativecommons.org/licenses/by/4.0/

Web End =creativecommons.org/licenses/by/4.0/ ), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

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Fig. 1 A PV cell equivalent circuit (Salmi et al. 2012)

research is the limitation in the number of modules in an array (just two modules were considered) so the results are not adequate because in reality more than two modules combine together into solar arrays. Furthermore, another component called blocking diode which affects to operation of PV array was also not mentioned.

The gap of the researches mentioned above is lled in by Seyedmahmoudian et al. (2013), Belhaouas et al. (2013), Ramasamy et al. (2014) and Ramaprabha and Mathur (2012). From the work of Ramaprabha and Mathur (2012), an overview of dierent types of solar PV system conguration is given and from there a congu-ration is proposed to study eect of using more bypass diodes and varying Rsh. In this research, the study is just carried out with two levels of solar irradiation. Ramasamy etal. (2014) used the graphical user interface (GUI) tool to investigate eect of shadow on maximum power point tracker (MPPT) while Belhaouas etal. (2013) and Seyedmahmoudian etal. (2013) just considered impact of two and three bypass diodes.

Overall, above studies investigate impact of shaded modules on solar PV arrays operation as well as role of bypass diode. However, the location of shaded modules which can aect to the PV system characteristic shape is not emphasized. Furthermore in reality when solar irradiation reduces, the temperature drops accompany so dierent levels of solar irradiation under shading condition needs to be investigated with varying temperature but it is not provided in above studies.

As a result of that, this paper will focus on the gap mentioned above through following points:

A proposed model is developed with Tag tools in Simulink environment. This model is based on the fundamental circuit equations of solar array considering eect of physical and environmental parameters so it indicates advantages in studying the role of these parameters on PV arrays working characteristics.

The relationship between varying location of shaded modules and PV systems characteristic shape is investigated.

The impact of shading condition accompany with varying temperature on PV arrays working characteristics is also investigated.

Methods

Modeling ofsolar PV module inMatlab/Simulink Solar cell, array equivalent circuit

The equivalent circuit of a PV cell is shown in Fig.1. It consists of a current source Iph which represents the cell photo-current, shunt and series resistances of the cell Rsh

and Rs respectively and a diode. Usually the value of Rsh is very large and that of Rs is very small, hence they may be neglected to simplify the analysis. Practically, PV cells are grouped in larger units called PV modules and these modules are connected in series or parallel to create PV arrays that are used to generate electricity in PV generation systems. The equivalent circuit for PV array is shown in Fig.2.

The voltage and current characteristic equation of a solar cell is provided as: (Tu and Su 2008; Salmi etal. 2012).

Module photo-currentIph:

In this equation, Iph is the photo-current, Isc is the short-circuit current, Kiis the cells short circuit current temperature coefficient, Ir is the sollar irradiation. Module reverse saturation currentIrs:

where, q is the electron charge, VOC is the open circuit voltage, NS is the number of solar cells connected in series, k is the Boltzmanns constant, n is the ideality factor of the diode, T is the operating temperature. The module saturation current I0 varies with the cell temperature, which is given by:

Iph = [Isc + Ki(T 298)] Ir/1000

(1)

Irs = Isc/[exp (qVOC/NSknT) 1]

(2)

Fig. 2 Equivalent circuit of a solar array (Tu and Su 2008)

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Case Description

4 Three shaded modules with same irradiation level (500 W/m2), varying position of shaded modules Module 1, 2 and 3 are shadedModule 1, 2 and 4 are shadedModule 2, 3 and 6 are shaded

5 Three shade modules with dierent levels of irradiation: 500, 300 and 100 W/m2Module 1, 2 and 3 are shadedModule 1, 2 and 4 are shadedModule 2, 3 and 6 are shaded

Eect ofpartial shadow andoperating temperature

(4) In reality, when cells or modules are shaded partially or fully, the operating temperature of modules reduced so in this study real characteristic of power output according to eect of shadow and temperature together is also investigated. The study will be carried out in following cases:

Case Description

6 Modules 1, 2 and 3: 500 W/m2

and operating temperature: 25 C, others have 1000 W/m2

(T = 25 C)7 Modules 1, 2 and 3: 500 W/m2

but operating temperature: 15 C, others have 1000 W/m2

(T = 25 C)

Eect ofbypass diode

In order to study eect of bypass diode to operating function of solar PV system, a developed model was proposed in Fig.5. The impact of bypass diode on power output of solar PV array will be discussed in two cases:

CaseDescription

8 Modules 2, 3 and 6 are shaded (Ir = 500, 300 and 100 W/m2 respec

tively), T = 25 C; No bypass diodes for these three modules9 Modules 2, 3 and 6 are shaded (Ir = 500, 300 and 100 W/m2 respec

tively), T = 25 C with bypass diodes for these three modules10 All modules are shaded (Ir = 100 W/m2), T = 25 C with bypass

diodes

I0 = Irs

T Tr

3 exp

~q Eg0

nk

1T

1 Tr

(3)

Here, q is the electron charge, Eg0 is the band gap energy of the semiconductor, RS and Rsh are the series and shunt resistors of the cell, respectively. The current output of PV module is:

with

and

Here, NPis the number of solar PV modules connected in parallel, Vtis the thermal voltage.

Modeling ofsolar PV array inMatlab/Simulink

Base on Eqs. from (1) to (6), a model of solar module is developed by using Tag tools in Simulink environment. The model includes six cells connected in seriesand it is given in Figs.3 and 4.

As a result of that, six solar PV modules combine together to form a solar PV array. The proposed model is shown in Fig.5.

Reference model

A 100W solar PV module is taken as the reference module for simulation and the detailed parameters of module is given in Table1.

Case study

Eect oflocation andnumber ofshaded modules

Case Description

1 No shaded PV module (full irradiation on solar PV array): 1000 W/m2

2 One shaded PV module: the position of shaded module changes from 1 to 6 respectively (shaded module has: Ir = 500 W/m2, other modules in array

have Ir = 1000 W/m2)3 Two shaded modules with same irradiation level : 500 W/m2 and others have 1000 W/m2Module 1 and 2 are shadedModule 1 and 3 are shadedModule 2 and 3 are shaded

I = NP Iph NP I0

~exp

V /NS + I Rs/NP

n Vt

1 Ish

Vt = k

T q

(5)

Ish = V

NP/NS + I RS

Rsh

(6)

Results anddiscussion

Eect oflocation andnumber ofshaded modules

The simulation results were shown in Figs.6, 7, 8 and 9.

In case of 1 shaded module, the IV curve were recorded with two steps while the PV curve has two peaks and the highest peak point is tended to 6065% VOC and belong to rst step in the curve.

As can be seen from Figs. 6, 7 and 8, two-steps IV curves are seen in cases of 2 and 3 shaded modules but in these cases peak points are recorded in second peak of the curves. Furthermore, these peak points belong to 8085% Voc.

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Fig. 3 Simulation model of a module consisting of 6 cells connected in series (modeling of Eqs. (1) to (3))

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Table 1 Electrical characteristics data of DS-100 M PV module

Name DS-100M

Rated power (Vmp) 100 W

Voltage at Maximum power (Vmp) 18 V

Current at Maximum power (Imp) 5.55 A

Open circuit voltage (VOC) 21.6 V Short circuit current (ISC) 6.11 A

Total number of cells in series (NS) 36

Total number of cells in parallel (NP) 1

Maximum system voltage 1000 V

Range of operation temperature 4080 C

The electrical specications are under test conditions of irradiance of 1kW/m2 ,

spectrum of 1.5 air masses and cell temperature of 25C

In case of 3 shaded modules, 4-step curves are seen in Fig.9 (case 5) and the peak point is given in the second step of the diagram.

Summary

1. When solar PV system is shaded partially, the power output decreases.

2. The number of shaded modules increases, the number of peaks in power output increases.

3. With same number of shaded modules, similar curves are recorded with varying location of shaded modules. In other words, in case of same number of shaded modules similar output characteristic curves are given with varying position of these modules in the array.

4. Position of maximum power point is independent with varying number of shaded modules.

5. In cases of shaded modules with similar levels of solar insolation, PV curve peaks occur nearly around multiple of 80% of Voc. Nevertheless, displacement between successive peaks comes nearly around 80% of arrays Voc.

6. However, in case study of shading modules under varying solar irradiation, PV curve peaks are tended to be around 50% of Voc.

Fig. 4 Simulation model of a module consisting of 6 cells connected in series (modeling of Eqs. (4) to (6))

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Fig. 5 a Solar PV subsystem model. b Simulation model of PV array

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Fig. 6 Operating curves in cases 1 and 2

Fig. 7 Operating curves in cases 1 and 3

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Fig. 8 Operating curves in cases 1, 3 and 4

Fig. 9 Operating curves in cases 1, 3, 4 and 5

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Fig. 10 Operating curves in cases 6 and 7

Fig. 11 Operating curves in cases 8, 9 and 10

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Eect ofpartial shadow andoperating temperature

The simulation results are given in Fig.10.

Summary

1. Similar IV and PV curves are recorded with and without considering varying temperature.

2. However, focusing on power output, higher power output is captured in case of lower operating temperature. Similarly, higher value of open voltage is also recorded when operating temperature reduces.

Eect ofbypass diode

The results given in Fig.11 show that:

1. A number of peaks may appear when the number of strings is connected in parallel with diodes. The number of peaks cannot be greater than the number of strings connected in parallel with bypass diode.

2. With bypass diodes, solar PV system gets higher power output comparing with the system without these diodes. This claims that bypass diodes improve operating function of solar PV system including series modules.

3. The shaded system under dierent levels of insolation without bypass diodes provides higher value in open circuit voltage and power output comparing to the system where all modules are shaded with irradiation of 100W/m2.

Conclusions

A Matlab/Simulink model for the solar PV cells, modules, and array is developed and presented in this paper. This model is based on the fundamental circuit equations of a solar PV array taking into the eects of physical and environmental parameters such as cell temperature, solar irradiation and shading condition. As a result of the study, the proposed model seems to be easy for other researchers to follow to build the model and study by themselves because it is based on the Eqs. from (1) to (6) and each equation has each model (shown in Fig.3 and 4). Furthermore, such a model would provide eective tool to predict the behavior of solar array underchanging climate and physical parameters due to all these parameters are included in the simulation model.

In addition, the proposed model is also used eectively to study the eect of shadow on operating characteristics of solar PV system and give clearance in some point as follows:

1. The higher number of shaded modules is, the lower value of power output is.

2. With a xed number of shading modules, similar IV and PV characteristic shape is recorded under varying position of modules shaded.

3. In cases of 1 or 2 shading modules, power peaks occur nearly about multiple of 80% of Voc. However, in case of three modules under shading condition, peaks tend to be around 50% of Voc.

4. Considering the appearance of operating temperature, solar PV array generates more energy under lower temperature.

5. Bypass diodes help to improve PV arrays operating curve and enable them to provide more energy.

Acknowledgements

The authors are grateful to the support by this work through the project Study, design and manufacture a solar PV system using SPV technology severed for chicken farms in Faculty of Animal Science, Vietnam National University of Agriculture, Vietnam (20142017).

Compliance with ethical guidelines

Competing interests

The author declares that they have no competing interests.

Received: 7 August 2015 Accepted: 17 September 2015

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