Impact of Partial Shading and bypass diode on PV panel Output Power

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Laboratory of Production engineering, Energy and Sustainable
Development, Smart Energy Systems and Information Processing
Research Team, higher school of technology, USMBA University,
Fez, Morocco
Impact of Partial Shading and bypass
diode on PV panel Output Power
Authors :
Saad MOTAHHIR
Abdelaziz El Ghzizal
Souad Sebti
Aziz Derouich
WoISA'2015
Second Workshop On Imagery, Systems and Applications
31/10/2015
saad.motahhir@usmba.ac.ma

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Plan
1. Introduction
2. Modeling of photovoltaic panel
3. Shading and bypass diode impacts
4. Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power31-10-2015

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Introduction
33
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power31-10-2015

4. 4Impact of Partial Shading and bypass diode on PV panel Output Power
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Modeling
Simulation and
validation of the
model
Datasheet
Model
Simulate the
effect of
Simulate the effect of shading
Modeling of
shading effect
Simulate the impact
of bypass diode
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Modeling of photovoltaic panel
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power31-10-2015

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Cell model – Panel model - Effect of solar radiation variation
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power
𝐼,𝑐 = Iph,c −𝐼0, 𝑐 𝑒𝑥𝑝
𝑞 𝑉,𝑐 + 𝑅 𝑠, 𝑐 𝐼,𝑐
𝑎𝐾𝑇
− 1 −
(𝑉,𝑐 + 𝑅 𝑠, 𝑐 𝐼,𝑐)
𝑅 𝑠ℎ, 𝑐
Modeling Iph,c
(1)
Id,c
I,c
Ish,c
+
-
V,c
❑ Iph,c : The cell photocurrent;
❑ I0,c : The cell reverse saturation current of the diode;
❑ q : The electron charge;
❑ a : The ideality factor of the diode;
❑ K : The Boltzmann's constant;
❑ T :The junction temperature;
❑ V,c : The voltage across the cell;
❑ I,c : Current generated by the cell;
❑ Rs,c : The series resistors of the cell;
❑ Rsh,c : The shunt resistors of the cell.
Rsh,c
Rs,c
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Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Cell model - Panel model - Effect of solar radiation variation
෍
𝑘=1
𝑛
Cell Panel
The IV characteristic of this panel is represented by the following equations [1, 2]:
𝐼 = 𝐼 𝑝ℎ − 𝐼 𝑜 𝑒𝑥𝑝
𝑞 𝑉 + 𝑅 𝑠 𝐼
𝑎𝐾𝑇𝑁𝑠
− 1 −
(𝑉 + 𝑅 𝑠 𝐼)
𝑅 𝑠ℎ
Where:
𝐼 𝑝ℎ = (𝐼𝑠𝑐+𝐾𝑖 𝑇 − 298.15 )
𝐺
1000
𝐼0 =
𝐼𝑠𝑐 + 𝐾𝑖(𝑇 − 298.15)
exp
𝑞 𝑉𝑜𝑐 + 𝐾𝑣 𝑇 − 298.15
𝑎𝐾𝑇𝑁𝑠
− 1
(2)
(3)
(4)
Impact of Partial Shading and bypass diode on PV panel Output Power
❑ Ns : The number of cells connected in series;
❑ G : Sun irradiation in w/m2.
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Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Cell model - Panel model - Effect of solar radiation variation
" As a result, the complete physical behavior of the PV panel is
related to I0, Rs and Rsh on the one hand and with two
environmental parameters as the solar radiation and the
temperature on the other hand. "
Solarex MSX-60 is composed by 36 cells in series
Maximum power, Pmax 60W
Voltage at Pmax ,Vmp 17.1V
Current at Pmax , Imp 3.5A
Short-circuit current, Isc 3.8A
Open-circuit voltage, Vco 21.1V
Temperature coefficient of
open-circuit voltage Voc, Kv
-80mV/°C
Temperature coefficient of
short-circuit current Isc, Ki
2.4mA/°C
The number of cells 36
Table 1 : Typical Electrical Characteristics of Solarex MSX-60
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Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Cell model - Panel model - Effect of solar radiation variation
Impact of Partial Shading and bypass diode on PV panel Output Power
I-V and P-V curves for different solar radiation
“The PV panel current is highly dependent on
the irradiation. However, the voltage increases
by 1V when the irradiation increased from 400
W/m² to 1000 W/m².”
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Shading and bypass diode impact
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power31-10-2015

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Effect of shading - Mathematical modeling of the shading effect - Simulation
Introduction Modeling a photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power31-10-2015
Shading is when The PV panel is not completely exposed to light due to to trees, passing of clouds, neighboring
buildings and any other means.
❑ Shading reduces energy production either by minimizing the energy supply of the shaded cells or by increasing the
energy loss in the shaded cells.

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Effect of shading - Mathematical modeling of the shading effect - Simulation
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power
Equivalent circuit of a PV panel consists of 36 cells in series under partial shading [3] :
❑ A set of illuminated cells is connected in series with another set of shaded cells;
❑ The photon current βIph of shaded cells is lower than the photon current Iph of illuminated cells;
❑ If the module current Ims is low compared with βIph then, the diode Ds is forward biased and there is no risk;
❑ If Ims is greater than βIph , the diode current IDs = Iphs-Ims -IRshs <0. As a consequence, the diode is in the reverse biased;
Reverse biased diode Ds offers high resistance will consume power and will significantly reduce the load current I itself.
This causes a drop in the output voltage. If the difference in illumination levels is high, Ds may get damaged due to
overheating;
❑ This phenomenon called HOT SPOT and it can cause permanent damage to the panel.
❑ The subscript ‘i’ and ‘s’ indicate the parameters of fully
illuminated cells and shaded illuminated cells respectively;
❑ Iph: the photon current of illuminated cells;
❑ βIph : The photon current of shaded cells ;
❑ β : The ratio of the photo current generated by the shaded
cells and the current generated by the illuminated cells.
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Effect of shading - Mathematical modeling of the shading effect - Simulation
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power31-10-2015
HOT SPOT phenomenon
❑ Hot-spot heating occurs in a module when its operating current exceeds the reduced short-circuit current of a
shadowed cell or group of cells within it;
❑ These hot spot mechanisms are the result of reverse biasing of cells which can lead to localized p-n junction
breakdown.

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Effect of shading - Mathematical modeling of the shading effect - Simulation
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power
Since the diode Ds is in the reverse biased so, VDs tends to negative infinity. As a result, 𝑒𝑥𝑝(
𝑉 𝐷𝑠
𝑉𝑡𝑠
) tends to zero. So, the equation
can be simplified as:
𝐼 𝑚𝑠 = 𝛽𝐼 𝑝ℎ + 𝐼0 −
𝑉𝑠 + 𝐼 𝑚𝑠 𝑅 𝑠𝑠
𝑅 𝑠ℎ𝑠
(7)
𝐼 𝑚𝑠 = 𝐼𝑖= 𝐼 𝑝ℎ − 𝐼0 𝑒𝑥𝑝
𝑉𝐷𝑖
𝑉𝑡𝑖
− 1 −
𝑉𝐷𝑖
𝑅 𝑠ℎ𝑖
(5)
(6)
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𝐼 𝑚𝑠 = 𝐼 𝑆
= 𝛽𝐼 𝑝ℎ − 𝐼0 𝑒𝑥𝑝
𝑉𝐷𝑠
𝑉𝑡𝑠
− 1 −
𝑉𝐷𝑠
𝑅 𝑠ℎ𝑠

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Effect of shading - Mathematical modeling of the shading effect - Simulation
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power
In order to prove the theory presented by (7), the simulation of the PV panel under shading is made and presented as follows :
“As is shown in simulation, once Ims is greater than Iphs, the curve I-V loses its
exponential characteristic like it is presented in the equation (7).”
𝐼 𝑚𝑠 = 𝛽𝐼 𝑝ℎ + 𝐼0 −
𝑉𝑠 + 𝐼 𝑚𝑠 𝑅 𝑠𝑠
𝑅 𝑠ℎ𝑠
Impact of Shading
(7)
“The power loss in the shaded cells is converted into heat which can lead to HOT SPOT
phenomenon causing permanent damage to the panel.”
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Effect of shading - Mathematical modeling of the shading effect - Simulation
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power
Bypass diode
“Under uniform insolation, the bypass diodes are
reverse biased and have no impact. However under
shades, the bypass diode is forward biased and the
current passes through the diode instead of the
shaded cells. Only those cells that are not influenced
by shadow generate power.”
“One method to protect the module against
hot-spot problem is when the bypass diodes
are connected in parallel with PV cells.”
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In order to simulate the effect of partial shading with bypass diodes, the string1 (18 cells) is exposed by 700 W/m²
and string2 (18 cells) by 1000 W/m² :

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Effect of shading - Mathematical modeling of the shading effect - Simulation
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power
Bypass diode
Illustrate the effect of the bypass diode on the characteristics of the PV panel :
When Ims>Iphs then, only the illuminated cells are functional. However, once
Ims<Iphs then, all cells are functional. As a consequence there are two
maximum power points (MPP), point A (global) and point B (local).
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Conclusion and Perspectives
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power31-10-2015

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Conclusion :
❑ The work provides an idea about the effect of partial shading and bypass diode
on PV panel;
❑ The conventional maximum power point tracking algorithms (i.e. P&O, IncCond
etc.) can fail to track the real MPP.
Perspectives :
❑ How it can identify the global maximum and local maxima, will be studied in
the future.
❑ There are still some optimizations and experiments needed to be done as the
future work. e.g. the problems how it can track the MPP instantly when the
weather condition is changing;
Introduction Modeling of photovoltaic panel Shading and bypass diode impact Conclusion
Impact of Partial Shading and bypass diode on PV panel Output Power31-10-2015

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References :
1. Saad MOTAHHIR, Abdelaziz El Ghzizal, Aziz Derouich, " Modélisation et commande d’un panneau photovoltaïque
dans l’environnement PSIM ",Congrès International de Génie Industriel et Management des Systèmes, 2015.
2. Saad MOTAHHIR, Abdelaziz El Ghzizal, Souad Sebti, Aziz Derouich, Une ressource pédagogique pour
l'enseignement par simulation : cas des panneaux photovoltaïques, Proceedings of International Workshop on
Pedagogic Approaches & E-Learning (APEL 2015).
3. Saad MOTAHHIR, Abdelaziz El Ghzizal, Souad Sebti, Aziz Derouich, " Shading effect to energy withdrawn from the
photovoltaic panel and implementation of DMPPT using C language ", International review of automatic control,
Vol. 9, No. 2, 2016, pp. 88-94.
Appendix :
Please refer to the link below to download the data of this paper:
https://github.com/motahhir/PV-panel-under-shading-and-implementation-of-DMPPT

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Questions & Answers
saad.motahhir@usmba.ac.ma