Analysis on the Effect of Shading on the Characteristics of Large-scale on-grid PV System in China

doi:10.4236/epe.2013.54B042

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Energy and Power Engineering, 2013, 5, 215-218

doi:10.4236/epe.2013.54B042 Published Online July 2013 (http://www.scirp.org/journal/epe)

Analysis on the Effect of Shading on the Characteristics of

Large-scale on-grid PV System in China

Yunlin Sun, Xiangzhi Li, Ruijiang Hong, Hui Shen*

School of Physics and Engineering, Sun Yat-Sen University, Guangzhou, China

Email: [email protected]

Received February, 2013

ABSTRACT

Because of rich solar resource and low land cost, a lot of large-scale ground-based grid-connected PV systems have

been built in Northwest China. In this paper, some shading phenomena on a grid-connected PV system in Northwest

China are classified and analyzed. Through the I-V curve test of PV modules, it can be seen that dust influence system

performance of the grid-connected PV system. And the experimental results have shown that shading could affect the

electrical properties of PV modules. Meanwhile, same shading area on different shading positions could have different

impacts on the identical PV module.

Keywords: Large-scale; Ground-based on-grid PV System; Shading; System Performance

1. Introduction

In the process of actual installation of PV system, there

are many influencing factors. Among them, shading

which could influence the performance of PV system has

aroused common concern and research. PV modules are

generally installed in the areas of rich solar resource and

vast land. Exactly, Northwest China can provide an ideal

environment for PV modules installation because of its

widely distributed Gobi, desert and shoals. However,

under natural conditions, PV modules would be covered

by certain blocks such as leafs and guanos after longtime

use. These kinds of objects will form shadows on the

modules that would affect the performance of PV mod-

ules so that the output power reduces directly [1, 2]. On

the other hand, owing to the difference between prelimi-

nary design and actual terrain of PV system, inappropri-

ate distance between front and rear PV arrays would

cause shading phenomena too. Like the uneven and rug-

ged surface would also lead to this kind of shading in the

actual situation [3].

Obviously, partial shading will change the I-V charac-

teristics of a PV module [4]. The product of current and

voltage will increase in some part of the PV module due

to the shading, and then temperature of these areas will

rise. After longtime use, some areas will become darker

on the surface of PV module and this phenomenon is

called “hot spot” [5]. Certainly, defects of solar cell itself

could also produce heat and bring “hot spot” for PV

module even in the normal work [6]. Such cases can re-

sult in reduction of the output power of PV module, as

shadows on partial PV module, uneven irradiance on PV

array or mismatching power of PV module in a PV array

[7].

2. Classification of Shading

According to field research, it can be found that shading

phenomena on PV module differ somewhat depending on

the specific circumstances such as different regions or

PV systems [8]. On the contrary, shades have something

in common; they can be the caused by power distribution

rooms nearby, the surrounding vegetation, or sometimes

the front row arrays [9]. During the construction process,

the surface structure and ground condition may not

match exactly with design drawings. So PV arrays some-

times can not be at the same height so that the front row

shadows would cover the rear row [10].

There are many types of shading phenomena in PV

system, and they can be divided into the following cate-

gories after longtime surveys and observation on one

large ground-based grid-connected PV system.

1) The front row shading phenomenon (Figures 1, 2)

2) The surrounding plant and guano shading phe-

nomenon (Figures 3, 4)

3) The nearby power distribution room and wire pole

shading phe n o menon (Figures 5, 6)

3. The Effect of PV Array Shading on

Electrical Properties of PV Module

*Corresponding author. A PV system is constituted of many PV modules that are

Y. L. SUN ET AL.

216

connected by series and in the parallel way. If the arrays

layout is unreasonable or there are shades around the

arrays, shading will appear on some modules. In practical

situations, a modu le with shad ing in a lon g series will not

produce photovoltage, while other modules in the same

series still produce voltage normally. So there is current

going through this shaded module [11, 12]. However,

even current runs through the module, it can not produce

a normal output power without photovoltage inside. Be-

cause of this reason, the shaded module becomes a load

in the string and dissipates power because of the gener-

ated heat. So other normal modules need to work at a

higher voltage in order to maintain the total voltage of

whole series and reduce voltage loss of the shaded mod-

ule. To the normal modules, the higher voltage will cre-

ate less current and more output power loss.

Actually, the method usually used to reduce the power

loss from partial shading is to divide a string into several

parts with some bypass diodes [13]. The part with shaded

module will be isolated by the bypass diode from other

parts, so the voltage and cu rrent of this string will lose in

proportion to the shading areas. And the output power of

other not shaded parts without being protected by the

diode will not lose [14, 15].

Figure 1. Front row shadows on the rear row in tracking

PV system.

Figure 2. Front row shadows on the rear row in fixed PV

system.

Figure 3. Plant shadows on the PV module.

Figure 4. Guano shadows on the PV module.

Figure 5. Power distribution room shadows on the PV

module.

Y. L. SUN ET AL. 217

Figure 6. Wire pole shadows on the PV module.

4. The Effect of Wire Pole Shading on

Electrical Properties of PV Module

In a large grid-connected PV system in Northwest China,

aiming at wire pole shading phenomenon, some experi-

ments have been done to analyze the effect of nearby

building shading on electrical properties of PV module.

This experiment uses some poly-Si modules produced

by a China manufacturer. The module is made up by 60

pieces of solar cells (156 mm × 156 mm) connected in

series. There are three bypass diodes in one module and

each diode can protect 20 pieces of solar cells. The no-

minal parameters of the module are: Pmax = 220 W,

Vmpp = 29.0 V, Impp = 7.95 A, Voc = 36 V, Isc = 8.19

As the Figure 7 shows, wire pole shadows on PV

modules in on-the-spot investigation, and each module is

numbered. Figure 8 shows six I-V curves corresponding

to six numbered modules.

Combining these two figures, it is shown that there are

no shadows on NO.2 and NO.3 module and the electrical

properties of them can be maintained. The electrical

properties of NO.1, NO.4, NO.5 and NO.6 modules with

wire pole shadows on them are affected obviously, and

their I-V curves change; as a result, their performances

have degraded. However, modules with shadows are not

in complete failure because of protections by bypass di-

odes, and their performances are different along with

different shadow position and area. Thus, it can be seen

that the properties of PV module not only could be in-

fluenced by shades like wire poles, but also change along

with the changeable sh adow position and area.

5. Summery

According to the field investigation results on a large

ground-based grid-connected PV system, this paper clas-

sifies the common shading phenomena on the grid-con-

Figure 7. Wire pole shadows on the PV modules.

Current (A)

Voltage (V)

Figure 8. Six I-V curves corresponding to six numbered

modules.

nected PV system in Northwest China. They are mainly

front row shading, plant and guano shading and nearby

building shading. Then, aiming at nearby building shad-

ing phenomenon, some experiments have been done.

From the experimental results, it can be seen that shad ing

can affect the electrical properties of PV modules.

Meanwhile, same shading area on different shading posi-

tions can cause different impacts on the identical PV

module.

REFERENCES

[1] K. Morita, et al., “Correlation between the Change of the

Appearance and Reduction in the Output Power of Crys-

talline Si Solar Cell Modules by Outdoor Exposure,”

Proceedings of JSES/JWEA Joint Conference, 2000, pp.

287-290.

[2] D. Uchida, K. Otani and K. Kurokawa, “Evaluation of

Effective Shading Factor by Fitting a Clear-day Pattern

Obtained from Hourly Maximum Irradiance Data,” Solar

Energy Materials and Solar Cells, Vol. 67, No. 1-4, 2001,

pp. 519-528.

doi:10.1016/S0927-0248(00)00321-4

Y. L. SUN ET AL.

218

[3] J. Appelbaum and J. Bany, “Shadow Effect of Adjacent

Solar Collectors in Large Scale Systems,” Solar Energy,

Vol. 23, No. 6, 1979, pp. 497-507.

doi:10.1016/0038-092X(79)90073-2

[4] H. Kawamura, K. Naka, N. Yonekura, S. Yamanaka, H.

Kawamura, H. Ohno and K. Naito, “Simulation of I-V

Characteristics of a PV Module with Shaded PV Cells,”

Solar Energy Materials & Solar Cells, Vol. 75, No.

3-4,2003, pp. 613-621.

doi:10.1016/S0927-0248(02)00134-4

[5] A. Woyte, J. Nijs and R. Belmans, “Partial Shadowing of

Photovoltaic Arrays with Different System Configura-

tions: Literature Review and Field Test Results,” Solar

Energy, Vol. 74, No. 3, 2003, pp. 217-233.

doi:10.1016/S0038-092X(03)00155-5

[6] J. C. Arnett and C. C. Gonzales, “Photovoltaic Module

Hot spot Durability Design and Test Methods,” Proceed-

ings of the 15th IEEE Photovoltaic Specialists Confer-

ence, 1981, pp. 1099-1105.

[7] E. Karatepe, M. Boztepe and M. Colak, “Development of

a Suitable Model for Characterizing Photovoltaic Arrays

with Shaded Solar Cells,” Solar Energy, Vol. 81, No. 8,

2007, pp. 977-992. doi:10.1016/j.solener.2006.12.001

[8] M. Meinhardt and G. Crame, “Multi-string-converter- the

Next Step in Evolution of String-converter Technology,”

Proceedings of Evolution of the European Power Elec-

tronics Conference, 2001.

[9] M. Drif, P. J. Perez, J. Aguilera, et al., “A New Estima-

tion Method of Irradiance on a Partially Shaded P-V

Generator in Grid-connected Photovoltaic Systems,”

Renewable Energy, Vol. 33, No. 9, 2008, pp. 2048-2056.

doi:10.1016/j.renene.2007.12.010

[10] J. Bany and J. Appelbaum, “The Effect of Shading on the

Design of a Field of Solar Collectors. Solar Cells,” Vol.

20, No. 3, 1987, pp. 201-228.

doi:10.1016/0379-6787(87)90029-9

[11] S. S. A. Chouder, “Effects of Shadowing on Photovoltaic

Module Performance,” Progress in Photovoltaics, Vol. 16,

No. 2, 2008, pp. 141-149. doi:10.1002/pip.780

[12] P. Spirito and V. Albergamo, “Reverse Bias Power Dis-

sipation of Shadowed or Faulty Cells in Different Array

Configurations,” Proceeding of the 4th EC PV Solar En-

ergy Conference, 1982, pp. 296-300.

[13] M. C. Alonso-Garcia, J. M. Ruiz and F. Chenlo, “Ex-

perimental Study of Mismatch and Shading Effects in the

I-V Characteristic of a Photovoltaic Module,” Solar En-

ergy Materials & SolarCells, Vol. 90, No. 3, 2006, pp.

329-340.

doi:10.1016/j.solmat.2005.04.022

[14] A. K. Sharma, R. Dwivedi and S. K. Srivastava, “Per-

formance Analysis of a Solar Array under Shadow Con-

dition,” IEE Proceedings G Circuits, Devices and Sys-

tems, Vol. 138, No. 3, 1991, pp. 301-306.

doi:10.1049/ip-g-2.1991.0051

[15] B. Hu, Y. Nonaka and R. Yokoyama, “Influence of

Large-scale Grid-connected Photovoltaic System on Dis-

tribution Networks,” Automation of Electric Power Sys-

tems, Vol. 36, No. 3, 2012, pp. 34-36.