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Content uploaded by Hamdi Hichem
Author content
All content in this area was uploaded by Hamdi Hichem on Jan 29, 2022
Content may be subject to copyright.
1
Shading effect on the performance of a
photovoltaic panel
Gharbi Abdelaziz
University of Tunis, Higher National Engineering
School Of Tunis (ENSIT) Engineering Laboratory of
Industrial Systems and Renewable Energies (LISIER)
Tunis, Tunisia.
abdelaziz.gharbi@sidibouzid.r-iset.tn
of Industrial Systems and Renewable Energies
(LISIER) Tunis, Tunisia.
Gharbi Rached
University of Tunis, Higher National Engineering
School Of Tunis (ENSIT) Engineering Laboratory of
Industrial Systems and Renewable Energies (LISIER)
Tunis, Tunisia.
Rached.gharbi@ensit.rnu.tn
Abstract--Photovoltaic modules are very sensitive to the
reduction of solar irradiation due to shading. Shading can be
caused by a fixed obstacle (wall, tree or even a simple pillar) or in
case of circumstantial events (cloudy sky or covered with heavy
smoke or dust). In order to illustrate the influence of shading on
the behaviour of a photovoltaic device, a study using MatLab
Simulink was carried out on a polycrystalline silicon module
YL250P29. The degradation of the incident solar irradiation on a
single cell of the photovoltaic panel leads to a considerable
decrease in the power produced by the system (about 1/3 in the
case of a fully shaded cell). We have monitored the behaviour of
the bypass diode, and the pivotal role it plays in preserving the
series of cells adjacent to the affected one and ensuring a
reasonable rate of output.
Keywords-- Photovoltaic Panel, diode bypass, Shading, MPP
I. INTRODUCTION
A photovoltaic panel produces electricity by reacting to
sunlight. More a panel is irradiated more electricity it
produces. Any shadow is therefore considered as an unwanted
disturbance. The photovoltaic effect is a physical phenomenon
unique to semiconductor materials that generate electricity
when exposed to light [1, 2].
Crystalline silicon is one of those semiconductor materials,
widely used today in photovoltaic panels. Light is made up of
photons, which generate the circulation of electrons in silicon
cells by photovoltaic effect. A continuous electric current is
created. These electrons then travel from cell to cell through
metallic connections. The continuous electricity is finally
collected at the junction box of the photovoltaic panel [3, 4].
A shadow means that a mask prevents the arrival of photons
on the photovoltaic cells. If a cell is in the shade, its current
production is degraded. The photovoltaic cells of a solar panel
are connected in series. The cell that produces the least
imposes its current on all the cells in the panel. Therefore,
bypass diodes are fitted to photovoltaic panels to minimize the
production degradation of one or more neighbouring cells.
However, it is important to understand and predict them in
order to extract as much power as possible. For this reason,
over the years, researchers have been studying the
characteristics of PV modules and the meteorological factors
affecting them [5, 6].
This paper is organized as follows: In section 2, we have
presented the characteristics of our solar panel and the adopted
model. The third section deals with the effect of shading on
the output quantities of a solar panel. Improvements of our
solar panel are given in section 4. Finally, section 5 presents
some concluding remarks.
II. PHOTOVOLTAIC CELL MODEL
The simplest equivalent circuit of a solar cell is a current
source connected in parallel with a diode as shown in Fig. 1.
The output of the current source is directly proportional to the
light falling on the cell. During darkness, the solar cell is not
an active device, and it works as a diode. It produces neither
current nor voltage.
However, if light falls on the solar cell, it generates a diode
current. The diode , determines the I–V characteristics of the
cell. A series resistance , represents the resistance inside
each cell, while the shunt resistance , is neglected because
it has a large resistance value [7].
Fig. 1. Solar cell circuit diagram.
In an ideal solar cell, it is assumed that and
. The net current of the cell is the difference between the
photocurrent, and the normal diode current, which given
by:
(1)
The photocurrent , depends on reference, temperatures
and respectively, and it is given by [8]:
(2)
where is the present solar radiation and is the solar
radiation at the reference test and is the short circuit
temperature coefficient.
The saturation current of the diode, , is given by:
(3)
Where
hamdi hichem
University of Tunis, Higher National Engineering
School Of Tunis (ENSIT) Engineering Laboratory
hichem.hamdi@isetsbz.rnu.tn
2
A GPV generator is obtained by combining several
identical panels in a S*P configuration, where S is the number
Ns of panels connected in series and P is the number Np of
panels connected in parallel. This association of a number
N=Np×Ns of modules allows the generation of the required
power. The equivalent diagram of such a system is translated
by the following expression: [8]:
(4)
The parameters mentioned in the previous equation are
grouped in the following table:
TABLE 1: EQUIVALENT MODEL PARAMETERS
The current of solar cells (Ampere)
The photocurrent (Ampere)
The saturation current of solar cells (Ampere)
The output voltage of solar cells (Volt)
the temp. coef. of the short circuit current (
)
The Silicon bandgap energy (=1.12 eV)
The temperature of the solar cell (Kelvin),
The reference temperature of Solar Cells (Kelvin)
&
The irradiance and the irradiance ref
Boltzmann’s constant
electron charge
The ideality factor of PV technology
, , and represent the short circuit current, the
open circuit voltage, the maximum power point current and
voltage which are shown in Table 1.
TABLE 2: CHARACTERISTICS OF THE YL250P29B
The following Table 2 summarising the five parameters of the
adopted equivalent model for the block formed by a parallel
photovoltaic cells.
TABLE 3: PARAMETERS OF CONSIDERED PHOTOVOLTAIC BLOCK
Parameterize by:
By s/c current and o/c voltage 5 parameters
8.79
[A]
0.64
[V]
Irradiance
1000
[W/m^2]
Quality factor
0.998
Serie resistance
0.007
Ω
III. PHOTOVOLTAIC PANEL
The elementary photovoltaic cell is a very low-power
electrical generator compared to the needs of most domestic
or industrial applications. Indeed, an elementary cell of a few
tens of square centimeters delivers, at most, a few watts under
a very low voltage, since it is a junction voltage. Photovoltaic
generators are therefore made by combining many elementary
cells.
A circuit diagram of a 60-cell module, 60 solar cells are
split into 3 strings, 20 cells on each string, each two strings are
connected in series with an ideal diode. The power produced
by a photovoltaic field is the sum of the powers of all the cells
composing it, but it is lower there, and that depends on the
meteorological conditions [8, 9]
Fig. 2: A circuit diagram of a 60-cell PV module.
Simulations were carried out in the Matlab Simulink
environment for a photovoltaic module consisting of 60 cells
identical to the one studied previously.
Fig. 3 and 4 show the and characteristics for
different solar irradiation.
It should be noted that:
(5)
(6)
(7)
In nature, the uniform irradiation is not always satisfied
because of the shadows of buildings or trees, the fluctuation
of the atmosphere, clouds and variations of the solar angle
and the loss of power occurs precisely by the effect of
shading.
Maximum power generated from the
module (W)
250.496 W
Open-circuit voltage (
38.4 V
The voltage at MPP (
30.4 V
Temperature coeff of Voc
-0.33701 (%/deg.C)
Light-generated current IL
8.8002 A
Diode saturation current I0
1.2723e-10
Series resistance
0.436 Ω
Number of Cells
60
Short-circuit current (
8.79 A
The current at MPP ((
8.24 A
Temperature coeff of
0.0438 (%/deg.C)
Diode ideality factor
0.9985
Shunt resistance
364.6382 Ω
3
The impact of this effect is reduced by several factors
including the bypass diode. In practice, a module contains
bypass diodes to prevent damage due to reverse bias on
shaded cells. These diodes are placed in antiparallel with a
group of 20 cells (). The bypass diode will start
driving as soon as a shaded cell is reverse biased. It allows
the current from the non-shaded modules to shunt around the
shaded cells. This diode protects this cell group and limits the
shading effect to only the neighbouring group of cells
protected by the same bypass diode.
The effect of shading on the output of a module is non-
linear because a small amount of shadow on part or the entire
module can result in a significant reduction in output power.
A simulation in the Matlab/Simulink environment at different
irradiance gave the following array of and
characteristics.
Fig. 4. Changing of MPP with Temperature for: (a)
single photovoltaic solar cell and (b) Solar panel.
Similarly, the degradation of solar irradiation reduces the
power produced by the cell. The maximum power point MPP
is shifted downwards. The relations deduced from the series
association of the photovoltaic cells concerning the short-
circuit current, the open circuit voltage and the maximum
power are well verified.
IV. IMPACT OF SHADING
Disturbances affecting a photovoltaic array under real
operating conditions lead to a degradation of the reliability
and performance of photovoltaic modules over time.
Furthermore, the instability of the GPV output quantities (V-
pv, I-pv and P-pv) due to unpredictable variations in weather
conditions affects the operation of the upstream components
of the electricity production chain. [10].
Electrical mismatch conditions on PV module can occur
when solar cells receive non-uniform irradiance or partially
shaded, or even if there are dierences between solar cells
intrinsic to the manufacturing process.
Shadowing conditions is a widespread situation, especially
on BIPV (Building Integrated Photovoltaics). Managing the
shadow possibility is a challenge for designers, once the
partial shadowing problem can appear from several sources,
such as surrounding buildings, trees, antennas, poles, and dirt,
for instance. In a series-connected string of cells, all the cells
carry the same current. When one or more cells are shaded,
(a)
(b)
(a)
(b)
Fig. 3. Changing of MPP with solar irradiation for: (a)
single photovoltaic solar cell and (b) Solar panel.
From the graph we find that as the irradiation
decreases is manifested by a decrease in the current
supplied by the cell with a small variation in the open
circuit voltage .
the maximum permitted current is reduced, consequently
decreasing the output power. Moreover, the shaded cells can
reach high temperatures, leading to the hotspot phenomenon
and permanent damage to the PV module [11].
Shading on a single photovoltaic cell can disrupt the
operation of the entire photovoltaic module.
The diagram below shows the impact of partial shading (grey
cells) on the total production of a panel consisting of 3
parallel rows, each of which is composed of 20 cells in series.
To avoid excessive losses on the crystalline panels, they are
internally subdivided into several cell series (often three)
series).
4
These series are equipped with a bypass diode that allows
each series to operate separately. Thus, a series affected by a
shadow (in whole or in part) does not prevent the electrical
production of the other series not affected by the shadow. In
addition, manufacturers use various devices, both in the
manufacture of the collectors and in the way are connected to
each other and to the inverter, to reduce losses due to shading
and to minimise the effect of cells in series. These
improvements also reduce overheating and therefore the risk
of damage [12].
Fig. 5. PV module with 1 shaded cell.
Ever since, many studies about the shading eect on
modules have been developed. Shading of solar
modules is an essential consideration in the design.
The primary consequence of shading is a reduction of power
generated from the solar array. The amount of power losses
depends on the size of the shade and how it falls across the
PV modules.
We applied different irradiation levels (from 1000 to 0 W/m2)
to the solar panel under study. The following two figures
and show the important role of the bypass diode.
Fig. 6. Changing of MPP with solar irradiation for single cell shaded: (a)
and (b) .
Fig. 6.a shows the effect of the decrease in irradiance due to
the presence of shading affecting one or more cells mounted
with the same bypass diode. There is a decrease in the current
supplied. In case of zero irradiance the decrease is remarkable
for the open circuit voltage (voltage drop of one third).
Fig. 6.b shows the effect of partial shading on the maximum
power point. In the case of zero irradiation there can be a
significant reduction in power due to the drop in both output
voltage and current.
Fig. 7. The variations in the current and the voltage absorbed by the loads.
icell
idbp
(a)
(b)
(a)
(b)
5
For this test, a decreasing irradiation (from 1000 to 0 W/m2)
was applied with a resistive load absorbing a current close to
the nominal current.
The bypass diode enters conduction for an irradiation of
about 500W/m2. As the current supplied by the panel is that
of the least irradiated branch, a decrease in the load current is
observed.
It is from this moment onwards that the charging current is
kept almost constant even when the irradiation reaches its
minimum value.
Referring to Figs. 7.a and 7.b the variation of the voltage and
the current absorbed by the load when the irradiation
decreases from 1000 W/m² to 0 W/m² are expressed by:
- (8)
- (9)
V. IMPROVING THE PERFORMANCE OF THE PHOTOVOLTAIC
PANEL:
We have shown the important role of bypass diodes in cases
of mismatch due to the imbalance between the photovoltaic
cells mainly introduced by shading.
We therefore propose to increase the number of bypass
diodes in order to reduce the effect of shading on one or more
cells of the solar module.
Fig. 8 shows the improved diagram without making any
changes to the cell arrangement. We have kept the
dimensions of the solar panel (6 rows of 10 cells each) with
90° rotation of the cells, in order to place the bypass diodes
as mentioned in the previous Fig. 8.
Fig. 8. Circuit diagram of a 60-cell PV module (5 bypass diode).
Fig. 9. influence of irradiation case of an improved panel :
(a) and (b) .
Fig. 10Variation of the electrical quantities of a cell in the case of an
improved panel:
a) current characteristics b) Voltage characteristics
Bypass diode
grouping of 12 cells in a row
(a)
(b)
(b)
(a)
6
For this test, a decreasing irradiation (from 1000 to 0 W/m2).
The bypass diode enters conduction for an irradiation of
about 800W/m2.
The I-V and P-V curves in Figure 9 show a significant
improvement in the open circuit voltage and the maximum
power point.
It is from this moment (Irradiation = 800 W/m²) that the
charging current is kept almost constant.
Referring to the two figures 10.a and 10.b the variation of the
voltage and the current absorbed by the load when the
irradiation decreases from 1000 W/m² to 0 W/m² are
expressed by:
- (10)
- (11)
VI. CONCLUSION
In this work, we studied the effect of different shading
conditions on the output power of PV modules. We used
Matlab / Simulink to perform our simulations and used the
Yingli YL250P29 module specifications from SAM (2014).
First, we simulated the panel studied with the manufacturer's
design (presence of three bypass diodes at the junction box).
We noticed that the reduction of the illumination acts on the
output values of the panel while protecting the cells against
overheating (hotspot).
Secondly, we proposed an improvement of the panel by
modifying the cell diagram with a different arrangement to
introduce five bypass diodes. This solution allowed us to
mitigate the effect of partial shading on the I-V and P-V
characteristics.
Furthermore, we have proposed a practical improvement
solution that the manufacturer can implement by simply
changing the cell diagram in order to increase the number of
bypass diodes.
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