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Dhimish, Mahmoud, Holmes, Violeta, Mehrdadi, Bruce, Dales, Mark and Mather, Peter
Output Power Enhancement for Hot Spotted Polycrystalline Photovoltaic Solar Cells
Original Citation
Dhimish, Mahmoud, Holmes, Violeta, Mehrdadi, Bruce, Dales, Mark and Mather, Peter (2017)
Output Power Enhancement for Hot Spotted Polycrystalline Photovoltaic Solar Cells. IEEE
Transactions on Device and Materials Reliability. ISSN 1558-2574
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1530-4388 (c) 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TDMR.2017.2780224, IEEE
Transactions on Device and Materials Reliability
Abstract—Hot spotting is a reliability problem in photovoltaic
(PV) panels where a mismatched cell heats up significantly and
degrades PV panel output power performance. High PV cell
temperature due to hot spotting can damage the cell encapsulate
and lead to second breakdown, where both cause permanent
damage to the PV panel. Therefore, the development of two hot
spot mitigation techniques are proposed using a simple and
reliable method. PV hot spots in the examined PV system was
inspected using FLIR i5 thermal imaging camera.
Multiple experiments have been tested during various
environmental conditions, where the PV module I-V curve was
evaluated in each observed test to analyze the output power
performance before and after the activation of the proposed hot
spot mitigation techniques. One PV module affected by hot spot
was tested. The output power during high irradiance levels is
increased by approximate to 1.26 W after the activation of the first
hot spot mitigation technique. However, the second mitigation
technique guarantee an increase in the power up to 3.97 W.
Additional test has been examined during partial shading
condition. Both proposed techniques ensure a decrease in the
shaded PV cell temperature, thus an increase in the PV output
power.
Index Terms— Hot spot mitigation; photovoltaic (PV) hot
spotting analysis; solar cells; thermal imaging.
I. INTRODUCTION
hotovoltaic (PV) hot spots are a well-known phenomenon,
described as early as in 1969 [1 and 2] and still present in PV
modules [3 and 4]. PV hot spots occur when a cell, or group of
cells, operates at reverse-bias, dissipating power instead of delivering
it and, therefore, operating at abnormally high temperatures. This
increase in the cells temperature will gradually degrade the output
power generated by the PV module as explained by M. Simon & L.
Meyer [5].
Hot spots are relatively frequent in current PV modules and this
situation will likely persist as the PV module technology is evolving to
thinner wafers, which are prone to developing micro-cracks during the
manipulation process such as manufacturing, transportation, and
installation [6-8].
Other reliability issues in PV modules such as PV micro cracks [9],
PV module disconnection [10], maximum power point tracking
(MPPT) efficiency [11], and PV wind speed and humidity variations
[12]. These factors can affect the PV modules output power
performance, thus decrease its annual yield energy. However, in this
paper hot spots in PV modules will be investigated.
Mahmoud Dhimish, Violeta Holmes, Bruce Mehrdadi, Mark Dales
and Peter Mather are with the Department of Engineering and
Technology, University of Huddersfield, HD1 3DH, UK (email:
Mahmoud.dhimish2@hud.ac.uk; V.Holmes@hud.ac.uk;
PV hot spots can be easily detected using IR inspection, which has
become a common practice in current PV applications as shown in [13
and 14]. However, the impact of hot spots on operational efficiency
and PV lifetime have been narrowly addressed, which helps to explain
why there is lack of widely accepted procedures which deals with hot
spots in practice as well as specific criteria referring to acceptance or
rejection of affected PV modules in commercial frameworks as
described by R. Moretón et al [15].
In the past, the increase in the number of bypass diodes (up to one
diode for each cell) was proposed as a possible solution [16 and 17].
However, this approach has not encountered the favor of crystalline
PV modules producers since it requires a not negligible technological
cost and can be even detrimental in terms of power production when
many diodes are activated because of their power consumption as
discussed by S. Daliento et al [18].
In addition, the main prevention method for hot spotting is a passive
bypass diode that is placed in parallel with a string of PV cells. The
use of bypass diodes across PV strings is standard practice that is
required in crystalline silicon PV panels [19]. Their purpose is to
prevent hot spot damage that can occur in series-connected PV cells
[20-22]. Bypass diodes turn “on” to provide an alternative current path
and attempt to prevent extreme reverse voltage bias on PV strings. The
general misunderstanding is that bypassing a string protects cells
against hot spotting.
More recently, it has been shown that the distributed MPPT
approach suggested by M. Coppola [23] is beneficial for mitigating the
hot spot in partially shaded modules with a temperature reduction up
to 20
0
C for small shadows. On the other hand, authors in [24 and 25]
show the “inadequateness” of the standard bypass diodes, the insertion
of a series-connected switch are suited to interrupt the current flow
during bypass activation process. However, this solution requires a
quite complex electronic board design that needs devised power supply
and appropriate control logic for activating the hot spot protection
device.
This paper presents a simple solution for mitigating the impact of
hot spots in PV solar cells. Two techniques are proposed, where both
hot spot mitigation techniques consists of two MOSFETs connected to
the PV panel which is affected by a hot spot. Several experiments have
been examined during various environmental conditions. The PV
module I-V curve was evaluated in each observed test to analyze the
output power performance before and after the activation of the
proposed hot spot mitigation techniques.
One PV module affected by a hot spot was tested. After activating
the first technique the output power of the PV module increased by
1.26 W in high irradiance levels, 1.44 W in medium irradiance levels
and 0.48 W in low irradiance levels. Same experiments were carried
out using the 2
nd
proposed hot spot mitigation technique, while the
B.R.Mehrdadi@hud.ac.uk, m.r.dales@hud.ac.uk,
p.mather@hud.ac.uk).
Output Power Enhancement for Hot Spotted
Polycrystalline Photovoltaic Solar Cells
Mahmoud Dhimish, Violeta Holmes, Bruce Mehrdadi, Mark Dales, and Peter Mather
P
1530-4388 (c) 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TDMR.2017.2780224, IEEE
Transactions on Device and Materials Reliability
output power increased by 3.97 W for high irradiance levels, 3.51 W
in medium irradiance levels and 1.31 W in low irradiance levels.
This paper is organized as follows: section II presents the examined
PV module electrical characteristics, while section III describes the
proposed hot spot mitigation techniques. Section IV shows the
validation process of the proposed hot spot protection method using
two case studies. Lastly, section V demonstrates the conclusion and
the future work.
II. EXAMINED PHOTOVOLTAIC MODULE CHARACTERISTICS
In this work, the PV system used comprises a PV plant containing
9 polycrystalline silicon PV modules each with a nominal power of
220 W
p
. The photovoltaic modules are organized in 3 strings and each
string is made up of 3 series-connected PV modules. Using a
photovoltaic connection unit which is used to enable or disable the
connection for any PV module from the entire PV plant. Each
photovoltaic string is connected to a Maximum Power Point Tracker
(MPPT) unit which has an output efficiency not less than 98.5% [26].
The existing PV system is shown in Fig. 1.
The SMT6 (60) P solar module manufactured by Romag has been
used. The tilt angle of the PV installation is 42
o
. The electrical
characteristics of the solar modules are shown in Table 1. Additionally,
the standard test condition (STC) for these solar panels are: solar
irradiance (G): 1000 W/m
2
and module temperature (T): 25 °C.
III. HOT SPOT DETECTION AND PROTECTION SYSTEM
The investigation of the hot spots in the tested PV system was
captured using i5 FLIR thermal camera as shown in Fig. 2, where its
specification is shown in Table 2 [27]. After examining the hot spots
in the PV modules, the hot spot mitigation techniques will be activated.
The first proposed hot spot protection system is connected to each
PV string in the PV module. As can be seen in Fig. 3(a), the examined
PV module used in this work contains three sub strings connected
throw bypass diodes. In order to apply the proposed hot spot mitigation
technique, two MOSFETs were connected to each PV string as shown
in Fig. 3(b). Switch 1 is in series with the PV string and is normally
“on”; it opens when a hot spot condition is detected to prevent further
hot spotting. While, switch 2 is in parallel with the PV string and it is
normally in “open” mode, it turns “on” to allow a bypass current path
when the PV string is open circuited.
Another hot spot mitigation technique was used with the PV module
instead of the connection for each MOSFET to the PV string as shown
in Fig. 3(c). The same concept has been applied, where switch 1 is in
series with the PV module is normally “on”; it opens when a hot spot
condition is detected to prevent further hot spotting. Switch 2 is in
parallel with the PV module and is normally “open”; it turn “on” to
allow a bypass current path when the PV string is open circuited. The
two switch PV protection device has been implemented and connected
to the PV panel which contains the hot spot.
As can be noticed, the proposed techniques are simple to implement,
where the connection steps is also within the PV module limit, since it
requires only to add additional MOSFETs to the hot spotted PV
module. In the next section, the validation and comparison between the
developed hot spot mitigation techniques will be presented.
IV. VALIDATION OF THE PROPOSED HOT SPOT MITIGATION
TECHNIQUES
In this section the validation for both proposed hot spot
mitigation techniques are illustrated and compared in brief. The
output power are compared using the I-V curve analysis, and the
detection of the hot spots have been captured using i5 FLIR camera.
Fig. 1. Examined PV system installed at the University of Huddersfield,
United Kingdom
TABLE I
PV MODULE ELECTRICAL CHARACTERISTICS
PV module parameter
Value
PV peak power
220 W
One PV cell peak power
3.6 W
Voltage at maximum power
point (V
mpp
)
28.7 V
Current at maximum power
point (Impp)
7.67 A
Open Circuit Voltage (V
oc
)
36.74 V
Short Circuit Current (I
sc
)
8.24 A
Number of cells connected in
series
60
Number of cells connected in
parallel
1
TABLE 2
FLIR I5 CAMERA SPECIFICATION
Comparison
Value
Thermal image quality
100x100 pixels
Field of view
21
0
(H) x 21
0
(V)
Thermal sensitivity
32.18 F
Fig. 2. Hot Spot detection using FLIR thermal camera
1530-4388 (c) 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TDMR.2017.2780224, IEEE
Transactions on Device and Materials Reliability
A. PV Hot Spot and I-V Curve Analysis
The proposed hot spotting techniques were tested in an
experimental setup with a resistive load powered by the PV module
which contains the hot spot previously shown in Fig. 2. The
MOSFETs are placed in the examined PV module as shown in Fig.
3(b) and Fig. 3(c).
There are several stages that have been carried out during the
operation of the proposed hot spotting mitigation techniques, these
stages are describes as follows:
i. Hot spot mitigation technique 1:
The results obtained by the first mitigation technique is shown in
Fig. 4(a), the results can be described as the following:
a) Before the activation: the temperature of the hot spotted
PV solar cell is equal to 70
0
F, while the adjacent
(reference) solar cells temperature is equal to 61.5
0
F.
b) 1 minute after the activation: the temperature of the hot
spotted PV solar cell reduced to 68.7
0
F, the difference
between the hot spotted PV solar cell with the reference
solar cell temperature is equal to 7.2
0
F.
c) 2 minutes after the activation: the maximum
enhancement of the temperature for the hot spotted PV
solar cell is reduced to 67.1
0
F, comparing to 70
0
F
before the activation of the mitigation technique.
ii. Hot spot mitigation technique 2:
The results obtained by the first mitigation technique is shown in
Fig. 4(b), the results can be described as the following:
a) Before the activation: the temperature of the hot
spotted PV solar cell is equal to 70.6
0
F, while the
adjacent (reference) solar cells temperature is
equal to 61.8
0
F.
b) 1 minute after the activation: the temperature of
the hot spotted PV solar cell reduced to 66.3
0
F,
the difference between the hot spotted PV solar
cell with the reference solar cell temperature is
equal to 4.5
0
F.
c) 2 minutes after the activation: the maximum
enhancement of the temperature for the hot spotted
PV solar cell is reduced to 64.9
0
F, comparing to
70.6
0
F before the activation of the mitigation
technique.
As can be noticed, the obtained results from the hot spot
mitigation technique 2 has better performance comparing to
technique 1, where the where the maximum difference between the
hot spotted PV solar cell and the adjacent solar cells is equal to 3.1
0
F.
(a) (b)
(c)
Fig. 3. (a) Structure of the PV string in the examined PV module, (b) First hot spot mitigation technique, (c) Second proposed hot spot mitigation technique
1530-4388 (c) 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.
This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TDMR.2017.2780224, IEEE
Transactions on Device and Materials Reliability
The main reason for the proposed hot spotting mitigation technique
is to improve the output power efficiency for the examined hot spotted
PV modules.
The value of the power before and after the activation for each
proposed technique was monitored in three different irradiance levels:
High irradiance level: 840 W/m
2
, medium irradiance level: 507 W/m
2
and low irradiance level: 177 W/m
2
, while in all tested scenarios, the
PV temperature is estimated at a fixed value approximately equals to
16.2
o
C.
Fig. 5(a) shows the output I-V curves of the PV module at high
irradiance level. The measured output power after the activation of
the proposed 1
st
technique has a power loss equals to 3.95 W
comparing to 5.21 W with no mitigation technique deployed in the
PV module. However, the minimum loss in the output power is
(a)
(b)
Fig. 4. (a) Output hot spot mitigation using technique 1, (b) Output hot spot mitigation using technique 2
