Shading and Mismatch effect on the Performance of PV Module

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ICAER-2015
Shading and Mismatch Effect on the Performance of PV Module
Sukanya Buragohain a, Sadhan Mahapatra a,1, Nabin Sarmah a, Sanjai Kumar b
aDepartment of Energy, Tezpur University, Tezpur 784028, Assam, India
bSolar Photovoltaic Group, Central Electronics Limited, Sahibabad 201010, India
Abstract
The installation of a PV system at a place for optimum yield is influenced by various factors; geographic location, system
design and various atmospheric parameters of the location. This study presents the effect of shading and mismatch in the
module inter-connection configuration and mismatch in the characteristic of the individual cells of a module. Different
percentage area of a single cell of a module is shaded and current generation and output power of the module at varying solar
radiation is measured. Non-uniformity in the behavior of the module power output is also observed when single cell of a
100Wp module is shaded individually at a time at constant radiation. This behavior of the cells or module leads to mismatch
effect on the module performance which affects the overall output of a plant. Mismatch effect is also observed by connecting
a number of different capacity modules together in different connection configurations. The array consisting of the different
module configuration showed different behavior when exposed to constant radiation thus leading to a decrease in the current
generation of the system by 80%, thereby decreasing the total energy generation capacity of the entire system.
Keywords: Shading, mismatch, performance, photovoltaic
1. Introduction
The installation of a PV system for optimum yield is primarily influenced by geographic location (latitude,
longitude and available solar radiation), installation design (tilt angle, orientation and altitude) to maximize solar
exposure, environmental factors like ambient temperature, wind, humidity, dust, rain etc. and the type of the
module technology used i.e. mono-crystalline, poly-crystalline or thin film [1]. Among the different factors,
shading and mismatch effect on a PV plant leads to a considerable loss in the total output power of the plant
depending on the intensity and area of shading and the mismatch in the connection configuration of the
modules. Shading can be due to a tree branch, bird dropping, building or module dust. When the cells of the
array are shaded, the photo current generated reduces thus reducing the total system performance. The shaded
cell acts as a resistive load and absorbs the power from the un-shaded cells. The large dissipation of power from
the shaded cell results in local overheating or hotspots. During installation, specific measurement of the area
must be done so that there is no front row shading phenomenon [2]. The effect of mismatch is observed when
solar cells or modules having different properties are connected to form a module or an array. This causes
serious problems as under worst case condition the output of the PV module is determined by the solar cell with
the lowest output which reduces the overall efficiency of the system. These factors for mismatch include
manufacturing mismatch of the cells, thermal gradients within an array which directly affects the power output,
uneven soiling, cloud shading and refraction, failed bypass diodes which may persist for the full lifetime of the
system, voltage drop in the additional components, variable degradation of the solar cells and accumulated wear
and tear of mechanical and electrical components [3].
Effect of shadow influence on the module is difficult to model as it depends on various parameters such as
the configuration of the PV array, the relative rate of shadow, and the shaded area of the module. For the same
percentage of the shading on a PV array, the impact can vary from 0% to 100% depending on where the shadow
forms and also the topology of the module circuit in the PV array [4]. Mohammedi et al. experimentally
observed the effect of shading on a PV pumping system with an array of 990W with 55W module connected to
each other into three parallel strings and each string consisting of six series of module for a period of 33 days in
the month of June and July, 2013. Different configurations of shading was studied and found that significant
shading can reduce the PV output to about 50%. Thus in order to reduce shading, proper location for installation
must be chosen and recommended the use of electronic controller to reconfigure the system according to the
shaded part [5]. Taha and Salih evaluated the effect of shading on a solar PV module by covering 25%, 50%,
75% and 100% of the module and compared it with an un-shaded module at constant radiation and temperature.
1 Corresponding author: Tel: (+91) 3712 275306
E-mail: sadhan.mahapatra@gmail.com

This study observed that the current of the module decreases with the increase in shading from zero to 100% [6].
Ramaprabha and Mathur studied the harmful effects of partial shading on the performance of a PV module with
a PSPICE simulation model representing 36 cells PV module. The study was conducted under partial shaded
conditions to test several shading profiles. It was concluded that a substantial power loss occurs due to non-
uniform illumination of a series string with the power generated by the highly illuminated cells wasted as heat in
the poorely illuminated cells emphasizing the need to have uniform illumination under different shading patterns
[7]. Yunyun et al. determined the effect of frame shadow on a Photovoltaic/Thermal system using a
mathematical model which showed that the performance of a partly shaded cell can be calculated as it operates
below the average radiation intensity of the shaded and un-shaded parts with a worst case scenario of 39.3%
decrease in the efficiency of the system [8]. Sun et al. analyzed the effect of shading on a grid connected PV
system in Northwest China. This study concluded that shading affects the electrical properties of a PV module
and observed different impact on the output of a PV module when same shading area was applied on different
position of the module [9]. Fialho et al. adopted a heuristic method to obtain the I-V and P-V curves of a series
connected mono-crystalline PV system and presented a simulation for partial shading of the system to illustrate
a feasible assessment for designing a PV system. The partial shading simulation was carried out with two PV
modules connected in series with bypass diode connected across the modules. Due to non-uniform illumination
of the two series modules two local maximum power peaks were obtained thus posing a difficulty to the
maximum power point tracking [10]. Patel and Agarwal with the help of MATLAB based modeling and
simulation studied the effect of partial shading on the I-V and P-V characteristics of a PV array and observed
that the array configuration of the modules significantly affected the maximum power available under partial
shading condition [11]. Alsayid et al. analyzed the impact of different shading particularly partial shading in a
PV array first by simulation using Matlab/Simulink and then illustrated experimentally on a series connected
two 140 W PV module for verifying the validity of the simulation results [12].
Lu et al. investigated the performance of various module layouts when one or more cells in the modules
were partially shaded. A SPICE software Micro-Cap was used for simulation where a shadow with zero
transmittance was simulated to pass over an area of one 6-inch solar cell in the corner of a module from two
orthogonal directions and the maximum power was calculated for each configuration. The results showed that a
series-parallel hybrid connection of cells in a module has a significant improvement on the output power under
partial shading conditions [13]. Kanters and Davidsson examined the effect of mutual shading on the technical
and economic aspect of a PV system installed on a flat roof [14]. A comparison was performed between an un-
shaded and shaded module with different row distances and inclination angles. This study found that a row
distance less than 1 meter affected the output of the modules due to mutual shading. A system with 0 degree
inclination angle and 0 meter row distance was found to be the most favorable configuration in a small roof
whereas a small row distance between sections was found to be mandatory in case of a larger roof . This study
showed that the row distance and inclination angle are an important factor to be kept in mind while designing a
big system on a flat roof. Thakkar et al. emphasized the nonlinear impact of partial shading on the PV
performance by incorporating a model calibrated with data from Tucson Electric Power solar test yard. Shading
de rating factors of the model were compared with the daily data for a year [15]. It was observed that the system
generated 22% more kWh for the month of December, when the modules were separated by twice the distance
and predicted the generation of 1.5 times as many kWh/yr per square-meter of land in a system with 12 degree
inclination angle compared to a system with modules at 32-degrees inclination angle thus showing a nonlinear
impact on the PV system performance. Picault et al. proposed a method to simulate PV arrays with adaptable
module interconnection to reduce the mismatch losses [16]. This study experimentally validated the model by
taking a 2.2 kWp plant with three different interconnection schemes. The experimental work on alternative array
configuration showed a rise in 4 % of the output power in partially shaded condition with respect to traditional
module interconnection schemes. Wurster and Schubert analyzed the effect of current mismatch and shading on
the power out of PV module [17]. This study found that for most configuration of PV system, strings of different
length in parallel to several others having an equal module count have a mismatch loss below 1 %. The
mismatch losses were found to be below 0.5% when one string is shorter. Guerrero et al. measured the
performance of a photovoltaic array consisting of different technologies under ambient condition with an
AMPROBE Solar Analyzer [18]. The array curves obtained were validated with a model developed in
MATLAB Simulink and found a loss of 20 % in the output of the array due to module mismatch which lead to
scenarios for conceivable modifications in the planning of PV field configurations. Wang and Hsu studied five
different configurations of PV cells i.e. the simple series (SS) configuration, series parallel (SP), total cross tied
(TCT), bridge linked (BL) and honey comb (HC) configuration to compare their performance under partial
shading condition [19]. By using Newton Raphson algorithm the comparison of the maximum power and fill
factor of the five connection configuration was carried out and found that in most cases the TCT configuration
had superior performance over the other four connection configuration. Belhachat and Larbes also analyzed the
performance of different PV array configuration by modeling the array using model of Bishop and

implementing using Simulink/ Simpower software [20]. The result confirmed the superiority of TCT
configuration under partial shading condition. In addition the results confirmed the equivalent power production
of all the configurations under uniform illumination and pointed out that for determining the optimal and
appropriate configuration the shading, its pattern, location of shading and type of shading of a place must be
analyzed before installing a system. In this study the effect of shading and mismatch in the module inter-
connection configuration and mismatch in the characteristic of the individual cells of a module are
experimentally studied. The mismatch and shading is also analysed using Simulink/MATLB simulation tool.
Nomenclature
Voc open circuit voltage
Isc short circuit current
Pmax maximum power
Vpm voltage at maximum power
Ipm current at maximum power
Vld load voltage
Ild load current
Pvld load power
FF fill factor
 efficiency
2. Modelling and experimental method
To understand the effect of shading on the different solar cell configuration and modules an indoor
experimental set-up is used and solar simulator is used for constant solar radiation. More detailed analysis on the
mismatch effect is carried out by MATLAB simulation followed by experimental validation using the same set-
up.
2.1 Experimental method for shading analysis
The effect of shading is observed in a 35 Wp module inclined at 250 angle in an outdoor environment by
randomly choosing cell and artificially shaded different percentage of area of the cell with a black chart paper.
The open circuit voltage (Voc) and short circuit current (Isc) of the module is measured at different shaded area of
the cell of the module (25%, 50%, 75% and 100%) as shown in Fig.1. A variable load in the form of a rheostat is
connected to the module and the voltage and current generated from the module at varying solar radiation at
different shaded condition is recorded. A similar kind of experiment is also performed considering a 100Wp
module under a solar simulator, (1000W/m2 radiation) Spire 350i to study the shading effect. In this module,
individual cells are shaded at a time with a black chart paper, where other cells are exposed to a constant
radiation of the solar simulator. Fig. 2 presents the cell number of the 100Wp module. I-V characteristic of the
module is measured each time, considering different cell shaded. Similarly, different combinations of cells are
shaded to observe the impact of shading on the performance characteristic of the module.
Fig.1. Different shading patters used for studying the shading effect (shown with black colour on a solar cell)
17 39 29 7
26 21 35 18

19 3 25 24
12 33 16 5
1 40 11 20
13 38 30 22
10 37 23 8
9 32 34 31
36 2 6 28
Fig.2. Solar cell arrangement in a 100Wp module
2.2 Simulation and experimental set-up for mismatch analysis
The mismatch effect on the voltage, current and output power of an array is studied by connecting five
modules of different ratings in different array configurations (series, parallel and various combinations of series
and parallel, denote as A, B, C, D, E, F, G, H, I, J, and K) and exposing the configurations to a constant radiation
of 1000 W/m2 under a solar simulator. The Voc, Isc and maximum power profile of the various configurations
are obtained and analyzed for the different connection configurations. Table 1 presents the performance
characteristics of the five modules considered. A model to understand the mismatch effect of the PV modules in
an array, MATLAB Simulink model is developed. The block diagram of the model developed for the simulation
study is shown in Fig.3. The PV module is modelled based on the standard solar cell model or equivalent circuit.
The block diagram shows one representative configuration. Modules of different specification (as given in Table
1) and configurations (as shown in Fig.4) are used in this study. For experimental analysis and validation of the
developed model, an indoor characterization set-up with a solar simulation is used. The Spire Sun Simulator
350i is used to test the photovoltaic modules under simulated air mass of 1.5. Pulsed Xenon light is used as the
light source which matches the solar spectrum. A standard radiation of 1000W/m2 is used for this study. The
same configurations as shown in figure 3 are used in mismatch simulation studies for experimental validation.
Table 1 Specification and performance of the module used for studying the effect of mismatch
Module number PM5 PM6 PM12 PM18 PM24
Parameters 1 2 3 4 5
Voc(V) 21.71 21.76 21.93 22.04 21.47
Isc(A) 0.38 0.45 0.87 1.27 1.65
Pmax 6.05 7.17 14.64 21.77 26.81
Vpm 17.11 17.54 18.13 18.4 18.2
Ipm 0.35 0.41 0.81 1.18 1.47
Vld 16.4 16.4 16.4 16.4 16.4
Ild 0.36 0.42 0.83 1.22 1.53
Pvld 5.95 6.93 13.61 19.9 25.14
FF 0.73 0.73 0.76 0.78 0.76
η (%) 6.96 8.25 16.86 8.51 10.49
Fig.3. Block diagram of the MATLAB Simulink model for simulation of PV module mismatch analysis

Configuration A Configuration B Configuration C
Configuration D Configuration E Configuration F
Configuration G Configuration H Configuration I
Configuration J Configuration K
Fig.4. Different module connection configuration
3. Results and discussion
3.1 Shading effect
I-V and P-V curve depicts the drop in the current and power output due to shading of different percentage
area of a single cell (Fig.5). It is observed that the power output of the module decreases with the increase in the
shaded area of the module, which is obvious in photovoltaics. A small amount of shading leads to a large
deviation in the I-V and P-V profiles from the actual module characteristic. Due to the partial shading, the cell is
subjected to non-uniform illumination and therefore the current generated by the cell decreases. Since the cells
are in series in a module, shading of a single cell causes the current in the string to drop to the level of the
shaded cell. It is found that the output of the module decreases proportionally to the shaded area of the cell.
Thus with the increase in shading, the current generated by the cell decreases which results in a substantial
power loss due to non-uniform illumination in a series string of the module. A non- uniformity in the module I-
V characteristic is observed due to shading of individual cells of the 100Wp module. The Isc and Voc of the
module exhibit a variation in their drop on shading the different cells connected at different positions in the
module (Fig.6). Isc decreases at a higher rate than the Voc of a cell which leads to unexpected difference in the
cell behaviour resulting in changing of the module I-V curve dynamically due to different shading patterns. It
can be incurred that, on shading cell number 11, module has the highest percentage of drop in Isc, which reduces
the module output as the current generated by the weakest cell is the maximum that can flow through the
module circuit. In case of cell number 33, a minimum impact on the module performance is observed as shading

this cell have a minimum impact on the module Isc. The percentage drop in Isc for each cell is shown in Fig.7.
Thus a difference is observed in the behaviour of the cells on shading. This difference in cell behaviour results
may be due to different intrinsic and extrinsic material defects of the cell. The rate of recombination of carrier,
minority carrier lifetime, diffusion length etc. forms some of the intrinsic defects. Micro cracks, broken fingers
and poor cell contacts form some of the extrinsic defects which results in different performance of the cells in a
module.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 5 10 15 20
Current (A)
Voltage (V)
0% shading
25% s hading
50% s hading
75% s hading
0
5
10
15
20
25
30
0 5 10 15 20
Power (W)
Voltage (V)
0% s hading
25% s hading
50% s hading
75% s hading
(a) (b)
Fig.5. (a) I-V and (b)P-V profiles of 35 Wp module under different shading
(a) (b)
Fig.6. (a) Variation of Isc and (b) Variation of Voc when different cells are shaded in a 100 Wp module
Fig.7. Percentage drop in Isc when individual cells are shaded
3.2 Mismatch effect
The study on the mismatch effect on the connection of different rated modules in number of configuration
shows that there can be significant loses in certain configurations. It is observed that the Voc for configuration A
(modules are connected in series) is maximum whereas Isc is minimum. The overall Voc is the summation of the
open circuit voltage of the modules in the configuration and overall Isc is that of the lowest rated module. In
configuration B, Isc is the maximum as the modules are in parallel combination the current gets summed up.
Similarly when five modules are connected in number of combinations, Isc and Voc varies as the modules
configuration varies; accordingly the maximum power output of the module also varies. Fig.8 shows the
experimental values of Isc, Voc and Pmax, which also validates the simulation results for the mismatch study.

The validation analysis shows a good agreement of simulation and experimental results except the
configurations C, D, E. Further investigation for these configurations is being carried out for validation of the
model.
0
20
40
60
80
100
120
A
B
C
D
E
F
G
H
I
J
K
Voc (V)
Configurations
Voc (Simulation)
Voc (Experiment)
(a)
0
1
2
3
4
5
6
A
B
C
D
E
F
G
H
I
J
K
Isc (A)
Configurations
Isc (Simulation)
Isc (Exper iment)
(b)
0
10
20
30
40
50
60
70
80
90
A
B
C
D
E
F
G
H
I
J
K
Power output (W)
Con figurations
Pmax (Simulation)
Pmax (Experiment)
(c)
Fig.8. Variation of (a) Isc, (b) Voc and (c) Pmax of the different module configuration
4. Conclusions
It is observed from the shading analysis that on shading a single cell the output power of a module decreases
and this drop in the performance increases with the increase in shading area. It is also observed a non-
uniformity in the behavior of the cells when single cell of a module is shaded individually. This non-uniformity
results due to various extrinsic and intrinsic differences of similar types of the cells of the module. This behavior
of cells as well shading of a cell or module leads to mismatch effect on the module performance which affects
the overall module output. The mismatch effect is also observed when a number of different capacity modules
are connected together. These effects can lead to a decrease in the current generation up to 80%, thereby
decrease in power output of the module. It is also noted that this study cannot be generalised for all the modules
available in the market for this specific configuration and mismatch. However, the study has given a clear
indication both quantitative and qualitative manner about the losses that arises due to the partial shading and
mismatch of the module specifications in a particular configurations.

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