energies

Review
A Comprehensive Review on Bypass Diode
Application on Photovoltaic Modules
Romênia G. Vieira 1 , Fábio M. U. de Araújo 2 , Mahmoud Dhimish 3, * and Maria I. S. Guerra 1
1 Department of Engineering and Technology, Semi-Arid Federal University, Francisco Mota Av.,
Mossoro 59625-900, Brazil; romenia.vieira@ufersa.edu.br (R.G.V.); maria.guerra@ufersa.edu.br (M.I.S.G.)
2 Department of Computer and Automation Engineering, Federal University of Rio Grande do Norte,
Natal 59078-970, Brazil; fabio.u.m@ufersa.edu.br
3 Department of Engineering and Technology, University of Huddersfield, Huddersfield HD1 3DH, UK
* Correspondence: M.A.Dhimish@hud.ac.uk




Received: 22 March 2020; Accepted: 6 May 2020; Published: 14 May 2020


Abstract: Solar photovoltaic (PV) energy has shown significant expansion on the installed capacity
over the last years. Most of its power systems are installed on rooftops, integrated into buildings.
Considering the fast development of PV plants, it has becoming even more critical to understand the
performance and reliability of such systems. One of the most common problems faced in PV plants
occurs when solar cells receive non-uniform irradiance or partially shaded. The consequences of
shading generally are prevented by bypass diodes. A significant number of studies and technical
reports have been published as of today, based on extensive experience from research and field
feedbacks. However, such material has not been cataloged or analyzed from a perspective of the
technological evolution of bypass diodes devices. This paper presents a comprehensive review and
highlights recent advances, ongoing research, and prospects, as reported in the literature, on bypass
diode application on photovoltaic modules. First, it outlines the shading effect and hotspot problem
on PV modules. Following, it explains bypass diodes’ working principle, as well as discusses how
such devices can impact power output and PV modules’ reliability. Then, it gives a thorough review
of recently published research, as well as the state of the art in the field. In conclusion, it makes a
discussion on the overview and challenges to bypass diode as a mitigation technique.

Keywords: bypass diode; partial shading; hotspot; mismatch; photovoltaic module;

mitigation technique

1. Introduction
Electric energy is considered essential to economic development, as well as the population’s
well-being. There is a strong relationship between energy resources and economic and populational
development in the world. During the 1970s, oil crises showed the world’s dependence on fossil fuels,
and, over the past two decades, the concerns were about increasing demand and declining production.
Several alternative energy sources can be used instead of fossil fuels. The decision on what type
of energy source should be utilized must consider some essential aspects such as ecological, safety,
and economics. Therefore, solar energy is frequently regarded as a promising alternative source,
considering the possibility of offering sustainability with the least damage to the environment [1].
Solar photovoltaic energy has been showing worldwide expansion over the last decades. In 2018,
the total global installed capacity reached 500 GW (Giga Watt) [2]. Most of its power systems are
installed on rooftops, incorporated into construction. The plants mounted into industrial, commercial,
and domestic buildings are known as building integrated photovoltaics (BIPVs), and they perform as
primary or supplementary source of electric power [3].

Energies 2020, 13, 2472; doi:10.3390/en13102472 www.mdpi.com/journal/energies

Energies 2020, 13, 2472 2 of 21

It is even more critical to understand the performance and reliability of photovoltaics(PV) plants,
considering the fast expansion of such systems. Power generation and payback time of PV installations
depend on the electrical performance of the PV module and also on its operational lifetime [4].
Faults arising under real operations’ conditions may lead to drastically affecting the reliability and
performance of PV modules over time. Besides, the electrical performance of PV modules is limited by
some aspects, such as cells’ low efficiency, discontinuity of solar source, the unpredictability of weather
conditions, and, finally, not efficient working conditions due to electrical mismatch [5].
Electrical mismatch conditions on PV module can occur when solar cells receive non-uniform
irradiance or partially shaded, or even if there are differences between solar cells intrinsic to the
manufacturing process. Shadowing conditions is a widespread situation, especially on BIPV. 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, 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 [6].
There are different solutions presented for addressing this issue. The most common is to place a
diode in antiparallel to the PV cells or a substring of cells. If the current generated by one cell becomes
smaller than other cells, the current flow will find the bypass diode path [7]. Therefore, the topology of
bypass diodes, as well as the PV array arrangement, can affect the possibility of hotspot failure [8].
The reduction in the output power of PV modules due to shading and consequent reliability
issues such as hotspots have been extensively studied. Nevertheless, not much information is offered
on the reliability of bypass diodes themselves.
This paper constitutes a survey of literature and research conducted on the use of bypass diode
on PV modules over the years. The primary objective of this review study was to help understand
the shading effect and the hotspot problem, as well as the bypass diode as a mitigation technique to
the hotspot problem and power losses. A significant number of studies and technical reports have
been published to date, based on extensive experience from research and field feedbacks. However,
such material has not been cataloged or analyzed from a perspective of the technological evolution of
bypass diodes’ devices.
Thus, this paper brings an overview since the first published research about the shading effect,
the hotspot problem, and mostly about the bypass diode as a protection device to PV modules.
The work discusses the bypass diode evolution over the years and briefly discuss new mitigation
techniques as well as the used of bypass diodes on new PV modules technologies.
The paper is briefly structured as follows. Section 2 outlines the shading effect and hotspot
problem on PV modules. Then, Section 3 explains the bypass diodes’ working principle, as well as
discussing how such devices can impact power output and PV modules’ reliability. Section 4 presents
the state of the art about bypass diodes, outlining the accumulated experience within this subject.
Finally, in Section 5, the overall conclusions and future challenges are briefly discussed.

2. Shading Effect and Hotspot Problem

Since early applications of photovoltaic technology, shading conditions have been reported.
The first solar array that successfully operated was launched in 1958, onboard Earth satellite
Vanguard I [9]. Partial shading problems caused by parts of the spacecraft were first reported
in 1961 by Luft [10]. Ever since, many studies about the shading effect on PV modules have been
developed [11–14].
Shading of solar PV 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 [15].
Energies 2020, 13, x FOR PEER REVIEW 3 of 22

Shading of solar PV modules is an essential consideration in the design. The primary

Energies 2020, 13, 2472 3 of 21
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 [15].
A
A PV
PV module
module is is aaseries-connected
series-connected string
string of of cells,
cells, and
and all
all the
the cells
cells must
must conduct
conduct thethe same
same amount
amount
of
of current.
current. On
On aa shading
shading event,
event, even
even ifif just
just aa few
few cells
cells are
are shaded,
shaded, these
these cells
cells are
are also
also forced
forced to
to carry
carry
the same current as other cells that are receiving sunlight. Therefore, the shaded
the same current as other cells that are receiving sunlight. Therefore, the shaded cells may become cells may become
reverse-biased.
reverse-biased. InIn this
thissituation,
situation,the
the shaded
shaded cellscells behave
behave as loads
as loads and dissipate
and dissipate powerpower from
from the the
entirely
entirely lighted
lighted cells [6].cells [6].shading
Partial Partial shading causes distortion
causes distortion on the I-V on(Current
the I-V (Current versus Voltage)
versus Voltage) and P-
and P-V (Power
V (Power versus Voltage) curves of PV modules, as Figure 1 illustrates,
versus Voltage) curves of PV modules, as Figure 1 illustrates, not considering bypass diodes. not considering bypass
diodes.

Figure 1.
Figure I-Vand
1. I-V andP-V
P-Vcurve
curveof
ofaaPV
PVmodule
module when
when cells
cells are
are shaded
shaded without
without bypass
bypass diodes.
diodes.

Because of
Because ofthethenature
natureof electrical characteristics
of electrical of solarofcells,
characteristics solarthecells,
powerthe losses are not
power proportional
losses are not
to the shaded areas, but greater [16]. Therefore, just a few parts of shading
proportional to the shaded areas, but greater [16]. Therefore, just a few parts of shading can drastically decrease
can
the performance of the entire PV system. Considering small PV plants, where
drastically decrease the performance of the entire PV system. Considering small PV plants, where there are a few or no
parallel connections, a small portion of shading can result in substantial power
there are a few or no parallel connections, a small portion of shading can result in substantial power losses or the entire
systemorfailure
losses [17].system failure [17].
the entire
In addition
In addition to to power
power losses,
losses, reverse-biased
reverse-biased behavior
behavior maymay leadlead to
to overheating
overheating the the solar
solar cell.
cell. Thus,
Thus,
and if the PV module does not have protection, the hotspot failure can arise
and if the PV module does not have protection, the hotspot failure can arise and, in extreme cases, and, in extreme cases,
the panel
the panel can
can get
get permanently
permanently damaged.
damaged.
A hotspot failure is a condition characterizedwhen
A hotspot failure is a condition characterized whena apart partofofthe
thecell
cellororPVPVmodule
module indicates
indicates a
a higher temperature than its surroundings. Typically, a hotspot arises
higher temperature than its surroundings. Typically, a hotspot arises as a consequence of poweras a consequence of power
dissipation in
dissipation in aa reverse-biased
reverse-biased PV PV cell
cell [18].
[18].
Permanent damage can
Permanent damage can occur when occur when a PVacell
PVreaches breakdown
cell reaches voltage,voltage,
breakdown defined as the maximum
defined as the
allowed reverse
maximum allowed voltage
reverseforvoltage
the safeforoperation
the safe of the p-n of
operation junction.
the p-nReaching
junction. this voltage
Reaching thisleads to a
voltage
massive increase of the reverse current and eventually to device destruction [19].
leads to a massive increase of the reverse current and eventually to device destruction [19]. Figure 2 Figure 2 illustrates
the I-V curve
illustrates the of
I-Va curve
photovoltaic cell when operating
of a photovoltaic cell when as fully illuminated
operating and under and
as fully illuminated shading
under conditions
shading
until reaching
conditions untila reaching
breakdown voltage. voltage.
a breakdown
The hotspot problem was identified early, also on the first PV module application [20,21].
Numerous studies have investigated the hotspot phenomenon and mitigations’ techniques to avoid
power losses and prevent its adverse consequences [22–26]. Based on these experiences, a hotspot
endurance test became part of the approval for types of crystalline silicon modules, according to IEC
(International Electrotechnical Commission) 61215, which verifies the hotspot tolerance by recreating
the worst expected operating conditions [27].
Energies 2020, 13, 2472 4 of 21
Energies 2020, 13, x FOR PEER REVIEW 4 of 22

Figure 2. I-V
Figure 2. I-V curve
curve of
of aa PV
PV module
module in
in aa reverse
reverse bias
bias region.
region.

Despite all efforts

The hotspot to avoid
problem wastheidentified
hotspot failure,
early, italso
frequently
on the occurs, leading
first PV moduleto aapplication
decrease in [20,21].
the PV
modules’
Numerouslifetime
studies[28].
haveToinvestigated
prevent the hotspot failure,
the hotspot usually a passive
phenomenon bypass diode
and mitigations’ on PV modules
techniques is
to avoid
mounted. Further details on the essential theoretical background about bypass diodes
power losses and prevent its adverse consequences [22–26]. Based on these experiences, a hotspotas a mitigation
technique
endurancewill
testbe brieflypart
became given
of in
theSection 3. for types of crystalline silicon modules, according to IEC
approval
(International Electrotechnical Commission) 61215, which verifies the hotspot tolerance by recreating
3. Bypass Diode: Working Principle
the worst expected operating conditions [27].
Aiming
Despite to allprevent the
efforts to shading
avoid consequences,
the hotspot failure, manufacturers included
it frequently occurs, one to
leading or amore diodes
decrease on
in the
commercial
PV modules' PVlifetime
panels.[28].
Bypass
To (BP) diodes
prevent the are connected
hotspot in antiparallel
failure, between
usually a passive a solardiode
bypass cell strings’
on PV
positive
modulesand negative output
is mounted. Furtherterminal,
details onand
thegenerally
essential is used for abackground
theoretical small groupabout
of series cellsdiodes
bypass [29]. as
Most modules
a mitigation techniqueintended forbriefly
will be grid connection consists
given in Section 3. of 72–60 cells, arranged in three submodules
of 24–20 cells, allowing placement of BP diodes to each of the submodules. Therefore, in conventional
3. Bypasspart
modules, Diode:of itsWorking
cells getsPrinciple
bypassed when a BP diode is activated. Such devices are accommodated
in the junction box on the module’s rear side [30].
Aiming to prevent the shading consequences, manufacturers included one or more diodes on
If one cell becomes reversed biased, it will directly bias the BP diode. Bypass diodes perform the
commercial PV panels. Bypass (BP) diodes are connected in antiparallel between a solar cell strings’
function of providing a bypass path for the current flow. As the shading is removed, the cell typically
positive and negative output terminal, and generally is used for a small group of series cells [29].
returns to the for bias state, and the diode returns to the reversed bias state. Thus, the power generated
Most modules intended for grid connection consists of 72–60 cells, arranged in three submodules
Energies
by 2020, 13, x cells
illuminated FOR PEER
will REVIEW
not affected by the shading of other cells. Figure 3 exemplifies a PV module 5 of 22
of 24–20 cells, allowing placement of BP diodes to each of the submodules. Therefore, in conventional
with 60 cells and subdivided with 20 cells per BP diode, working with one shaded cell.
modules, part of its cells gets bypassed when a BP diode is activated. Such devices are accommodated
in the junction box on the module’s rear side [30].
If one cell becomes reversed biased, it will directly bias the BP diode. Bypass diodes perform the
function of providing a bypass path for the current flow. As the shading is removed, the cell typically
returns to the for bias state, and the diode returns to the reversed bias state. Thus, the power
generated by illuminated cells will not affected by the shading of other cells. Figure 3 exemplifies a
PV module with 60 cells and subdivided with 20 cells per BP diode, working with one shaded cell.

Figure
Figure 3. PV module
3. PV module with
with one
one shaded
shaded cell.
cell.

Once bypass diodes conduct, they introduce inevitable voltage drop, may heat up significantly,
and consume power generated. Hence, it brings impact to the maximum power delivered by the
photovoltaic modules [31]. The configuration of bypass diodes on the PV modules forming part of
the array has an important influence on the possibility of hotspot presence [8].
Energies 2020, 13, 2472 5 of 21

Once bypass diodes conduct, they introduce inevitable voltage drop, may heat up significantly,
and consume power generated. Hence, it brings impact to the maximum power delivered by the
photovoltaic modules [31]. The configuration of bypass diodes on the PV modules forming part of the
array has an important influence on the possibility of hotspot presence [8].
Even with the diode bypassing a complete substring and only one failing cell, the maximum
power point (MPP) can be higher with activated bypasses. The difficulty with this approach is that the
BP diodes create multiple local MPPs, which makes it hard for the maximum power point tracking
(MPPT) to find the global MPP [7]. Therefore, distortion of the shaded I-V curve may lead to an error
in the determination of the global MPP. Figure 4 illustrates the I-V and P-V curves of a shaded PV
module with one BP diode activated (see Figure 3).

Figure 4. I-V and P-V curves of a shaded PV module with one bypass diode activated.

Comparing Figures 1 and 3, it is clear that under shading conditions the MPP is higher using BP
diodes. Nevertheless, when BP diodes are activated, the PV module presents multiple local maxima
(LM), but only one of them relates to the global maximum (GM) [32].
Overlapped and no-overlapped are two standard topologies of bypass diodes, as illustrated in
Figure 5. It is worthy of mentioning that analyzing the PV modules with overlapped configuration
is more complicated than no-overlapped because there will be a few different paths for the current
flow [33].
Moreover, both configurations should be analyzed under a shading condition, as illustrated in
Figure 6. The overlapped configuration may lead to an overcurrent situation. Increasing the output
current of a PV module is not always attractive to the power plant, once all the project is developed to
a current standard rate, and this situation may lead to negative consequences to the PV plant. Thus,
the most used BP diode configuration by the PV module industry is the no-overlapped.
Energies 2020, 13, 2472 6 of 21
Energies 2020, 13, x FOR PEER REVIEW 6 of 22

Figure 5. Standard BP diode configurations.

Moreover, both configurations should be analyzed under a shading condition, as illustrated in

Figure 6. The overlapped configuration may lead to an overcurrent situation. Increasing the output
current of a PV module is not always attractive to the power plant, once all the project is developed
to a current standard rate, and this situation may lead to negative consequences to the PV plant. Thus,
the most used BP diode configuration by the PV module industry is the no-overlapped.
Figure 5. Standard BP diode configurations.
Figure 5. Standard BP diode configurations.

Moreover, both configurations should be analyzed under a shading condition, as illustrated in

Figure 6. The overlapped configuration may lead to an overcurrent situation. Increasing the output
current of a PV module is not always attractive to the power plant, once all the project is developed
to a current standard rate, and this situation may lead to negative consequences to the PV plant. Thus,
the most used BP diode configuration by the PV module industry is the no-overlapped.

Figure 6. BP diodes’ configuration under shading condition.

Various
Various BP
BP diode
diode topologies inside aa PV
topologies inside PV module
module can can create
create aa different
different path
path to
to the
the current
currentflow.
flow.
Thus, the BP diode arrangements on the PV panel may impact the voltage,
Thus, the BP diode arrangements on the PV panel may impact the voltage, current, and power current, and power
characteristics
characteristics of
ofshaded
shadedand
andunshaded
unshadedcells,
cells,asaswell
wellasas
thethe
maximum
maximum power
powerextraction of the
extraction entire
of the PV
entire
system. Hence, several studies have been done on the whole of the impact of
PV system. Hence, several studies have been done on the whole of the impact of BP diode BP diode configurations
on the maximum
configurations onpower of the PVpower
the maximum module.
of the PV module.
Figure 6. BP diodes’ configuration under shading condition.
How to Choose a Bypass Diode
3.1. How to Choose a Bypass Diode
Various
ConsideringBP diode topologies
the bypass diodeinside
working a PV modulediscussed
principle can createina Section
different
3, path
the BP todiode
the current
needs flow.
to be
Thus, Considering thearrangements
bypass diode on working principle discussed inthe
Section 3, the BP diode needs to
activated even when a single cell is shaded. Ideally, the device should have forward voltage as power
the BP diode the PV panel may impact voltage, current, and low as
be activated even
characteristics of thewhen aand
shaded single cell is shaded.
unshaded Ideally,
cells,isasthe
well as thethemaximum
device should
power have forward
extraction ofvoltage as
thevoltage.
entire
possible. Hence, most widely used diode Schottky diode because of its low forward
low
PV as possible.
system. Hence, the most widely used diode is the Schottky diode because of its low forward
Since the firstHence,
industrial several studiesofhave
application the BP been donetheon
diodes, theofwhole
kind of the impact
diode mounted on theof BP diode
junction box
voltage. Since the
configurations on firstmaximum
the industrialpower
application
of the of the
PV BP diodes, the kind of diode mounted on the
module.
was the Schottky diode [34].
junction box was the Schottky diode [34].
However, a lower forward voltage means a higher leakage current, raising the risk of thermal
However,
3.1. How to Choosea lower
a Bypassforward
Diode voltage means a higher leakage current, raising the risk of thermal
runaway [35]. Thermal runaway occurs when BP diodes are operating on a quick switching mode,
runaway [35]. Thermal runaway occurs when BP diodes are operating on a quick switching mode,
like when the shading
Considering on the diode
the bypass PV module
working is suddenly
principleremoved.
discussedTheThe power dissipation
in Section 3, the BP diodein theneeds
reverseto
like when the shading on the PV module is suddenly removed. power dissipation in the reverse
biasactivated
be is greatereven
than whenthe cooling
a capacity
single cell ofshaded.
is the junction box.the
Ideally, Therefore,
device the diode
should haveis subject
forward to permanent
voltage as
bias is greater than the cooling capacity of the junction box. Therefore, the diode is subject to
damage,
low as is consequently,
as possible. Hence, the the PV
most module
widely used[36].
permanent damage, as is consequently, the PVdiode
moduleis the Schottky diode because of its low forward
[36].
To avoid
voltage. Since thethe risks of permanent
first industrial damageofand
application thepower lossesthe
BP diodes, associated with the
kind of diode BP diode,
mounted on the
To avoid the risks of permanent damage and power losses associated with the BP diode, the
correct device
junction box sizing
was the is essential.
Schottky The [34].
diode first criterion should be the PV module short-circuit current (Isc );
correct device sizing is essential. The first criterion should be the PV module short-circuit current (Isc);
once However,
it is the current
a lower rateforward
the diode will conduct.
voltage means aThen, highertheleakage
maximum repetitive
current, reverse
raising voltage
the risk (VRRM )
of thermal
needs to be
runaway considered.
[35]. Thermal runaway occurs when BP diodes are operating on a quick switching mode,
The Vthe
like when RRM is related
shading ontothe
thePVnumber
module of is
cells protected
suddenly by the diode.
removed. For safe
The power and efficient
dissipation in theoperation,
reverse
the BP
bias is diode
greatermustthanconduct whencapacity
the cooling one cell of is shadowed,
the junctionand theTherefore,
box. shadowedthe celldiode
voltage must stay
is subject to
under its breakdown
permanent damage, as voltage. Thus, the maximum
is consequently, the PV module number [36].of cells bridged by the one device (nmax ) is
To avoid the risks of permanent damage and power losses associated with the BP diode, the
correct device sizing is essential. The first criterion should be the PV module short-circuit current (Isc);
Energies 2020, 13, 2472 7 of 21

limited by the cell’s breakdown voltage (Vcell ), and the forward diode voltage (Vdiode ) is calculated by
Equation (1) [37].
V − Vdiode
nmax < cell +1 (1)
0.5
Knowing the nmax and the panel open-circuit voltage (Voc ), the maximum repetitive reverse
voltage (VRRM ) can be calculated by Equation (2).

Vocmax
VRRM > (2)
nmax

Frequently, the junction box manufacturer is different from that of the PV module. The junction
box is not designed to one specific panel but could be used by a range of solar PV modules. Thus,
most of the used BP diodes show a VRRM = 45 V [37].
Considering the working principle and all the BP diode characteristics discussed, Section 4 will
present a comprehensive literature review of BP diode studies, summarizing the collected experience
and the state of the art within this subject.

4. Bypass Diode: State of the Art

A significant amount of published research studies have been reported in the literature about
the possibility of hotspot appearance since early applications of PV systems and the importance of
configuration of bypass diodes on the PV modules to minimize this problem.
In early steps of such studies, Baron and Virobik [38], in 1963, based on the shading problem, first
successfully incorporated a shunt diode in the Pioneer Spacecraft solar arrays. Since then, some authors
contributed to optimizing the PV module design using BP diodes as a mitigation technique for power
losses and the hotspot problem.
In 1969, Blake and Hanson [20] investigated the hotspot failure mode on space solar arrays and
suggested the use of BP diodes to avoid the hotspot phenomenon. Their conclusions were reinforced
by other authors, affirming that adding diodes increases the fault tolerance and reliability of solar
arrays [39,40]. The use of BP diodes was so far considered the best circuit design tool to reduce power
loss and hotspot problems [41].
Meanwhile, in the 1980s, a few circuit-design strategies were investigated in order to study BP
diodes’ application and reliability [42,43]. Ross [44] affirmed that it was mostly not viable to mount a
bypass diode around every cell inside the module. Therefore, a single diode should protect a series of
cells or entire panels.
An alternative approach to BP diodes was studied by Green [45], suggesting a new technique of a
BP diode integrated into the cell in its manufacturing process. In this case, each cell would have its
own integrated BP diode, and Green observed a reduction of power losses.
Other protection methods against hotspot phenomena were investigated. Larue and Trieu [21]
tried to mitigate the heating problem by increasing the number of cells in parallel but concluded that
BP diodes were better than the proposed method. Feldman et al. [13] also investigated an alternative
protection method using a “pseudo-random” interconnection scheme. They concluded that for regular
shadows in a terrestrial application, the pseudo-random interconnection scheme gave substantially
higher output than the more conventional shunt-diode methodology.
Still in the 1980s, two other studies tested alternative techniques to the shading problem. In 1982,
Cox et al. [46] explored mounting a peripheral bypass diode, aiming to reduce the PV cell reverse
breakdown. Swaleh and Green [47] incorporated a relatively low shunt resistance in the solar cell.
They concluded that bypass diodes across individual cells provide a more effective tolerance to the
effects of shadows.
Forman and Ross [48,49] performed experimental tests on terrestrial PV modules. They observed
numerous cell failures on a test PV system, and some of the PV modules did not use BP diodes, leading
them to conclude that the severity of this condition would have been limited by using protective diodes.
Energies 2020, 13, 2472 8 of 21

Furthermore, in 1984, Shepard and Sigmura [50] discussed some BP diode configuration on a
PV module, suggesting the integration of such device to the array mounting and external to the
array configuration. They observed that the more BP diodes connected, the smaller the power losses.
Gonzalez et al. [51] compared a PV module with and without the BP diode and concluded the use of
BP diodes as a strategy for controlling the hotspot problem.
At the same time, General Electric (GE) developed a research under Jet Propulsion Laboratories’
(JPL) contract to study the design and processing techniques necessary to incorporate bypass diode
to a PV module. The study analyzed some mounting topologies for BP diodes and compared their
performance on mechanical, electrical, and thermal behaviors [34].
The report concluded that the use of BP diodes is essential to terrestrial PV arrays’ applications.
The number of BP diodes should be proportional to the PV array area, not exceeding 15 series-connected
solar cells within a bypassed group. The reliability of PV systems could reach a 20-year lifetime if
correctly designed and installed with BP diodes [52].
This study indicated the use of the junction box to accommodate the BP diodes on the PV module,
as manufacturers practice to date. The study also indicated that the cost to integrate such diodes could
raise PV module costs [53].
Since GE’s research, bypass diodes incorporated into PV modules have been manufactured and
placed in antiparallel to groups of series-connected solar cells and accommodated on the back of the
panel on a designed site to mount these devices, the junction box. Although it established a standard
technique to protect against the destructive effects of the hotspot, if shading any of these cells, BP
diode would prevent the remainder of the group from contributing to the module output power.
Likewise, the number of diodes, as well as their reliability, continued to be investigated in subsequent
years [54,55].
In 1988, Bishop [56] conducted initial experiments about the performance of PV cells under
an uneven irradiance and using BP diodes. His work described a software developed to simulate
the effects on PV module due to mismatching conditions, such as manufacturers’ mismatching or
shadowing conditions. Using the software developed, Bishop concluded that the drop in PV cell
performance due to inadequate conditions was inevitable, so the use of BP diode is essential. Further,
Abete et al. [57] employed Bishop’s method on a mismatched PV array with a BP diode to evaluate its
performance. The conclusions helped to understand when the BP diode goes into operation.
In the same year, Lashway [58] raised a discussion about specific testing techniques for PV
modules. One of these procedures was about measuring BP diodes’ current to identify a failure module.
Besides, the author pointed out the same failure types that could occur on PV modules, and the BP
diodes were responsible for 2% of the most common failures on a PV array.
During the period 1990–2005, there was no extensive research that emphasized the development
of new concepts or, at least, a new investigation on the impact of hotspots on the PV modules’ reliability.
Molenbroek et al. [22] tested PV modules’ hotspot susceptibility and concluded that the worst condition
of hotspot heating was completely shading a single cell. However, the hotspot condition can be
minimized using the BP diodes to limit the reverse bias voltage across a module to less than 1 V.
Soon after, Nabeel et al. [59,60] tested PV modules’ reliability by theoretical and experimental results.
The experiment about BP diode led them to conclude that the reliability of the module increased as the
number of shunted diodes increased.
Afterward, Herrmann et al. [25] investigated the module design regarding bypass diodes.
According to their conclusions, to avoid the overheating caused by partial shading, one single bypass
diode should protect 20 cells maximum. Quaschning and Hanitscht [61] developed a simulation
method to reproduce the solar cell I-V curve. Through this method, it was possible to study the electric
performance of a PV system under a shading condition using BP diodes. They observed that the use of
a BP diode reduced the power that would be lost with shading.
Still in the 1990s, studies comparing the application of integral diodes and conventional BP diodes
to PV modules were performed. Analyzing how many cells of a PV module protected by one single
Energies 2020, 13, x FOR PEER REVIEW 9 of 22
Energies 2020, 13, 2472 9 of 21
Still in the 1990s, studies comparing the application of integral diodes and conventional BP
diodes to PV modules were performed. Analyzing how many cells of a PV module protected by one
BP diode could be affected by shading, Bhattacharya and Neogy [23] compared the usage of integral
single BP diode could be affected by shading, Bhattacharya and Neogy [23] compared the usage of
diodes or single diode for a group of cells. They pointed out that diodes integral with the solar cell
integral diodes or single diode for a group of cells. They pointed out that diodes integral with the
would be the best solution in respect to power production.
solar cell would be the best solution in respect to power production.
Furthermore, Roche et al. [62] compared three methods to reduce mismatch losses: The use
Furthermore, Roche et al. [62] compared three methods to reduce mismatch losses: The use of
of integral BP diodes, series/parallel connections on the PV module, and reduced shunt resistance.
integral BP diodes, series/parallel connections on the PV module, and reduced shunt resistance. The
The authors settled that integral BP diodes are a viable technique to prevent hotspot and power losses
authors settled that integral BP diodes are a viable technique to prevent hotspot and power losses on
on PV systems. Furthermore, Yoshioka et al. [24] tested two PV modules, one using integral BP diodes,
PV systems. Furthermore, Yoshioka et al. [24] tested two PV modules, one using integral BP diodes,
and the other with conventional external bypass diode. Their results indicated that modules with
and the other with conventional external bypass diode. Their results indicated that modules with
bypass diode function shown more power output and lower temperature increase when operating
bypass diode function shown more power output and lower temperature increase when operating
under shading conditions. Also, the conclusions suggested the use of PV modules with BP diodes to
under shading conditions. Also, the conclusions suggested the use of PV modules with BP diodes to
home installations.
home installations.
Lastly, Danner and Bucher [26] analyzed reverse I-V characteristics of commercial cells that were
Lastly, Danner and Bucher [26] analyzed reverse I-V characteristics of commercial cells that were
measured and the impact on power dissipation. According to the study, the number of BP diodes
measured and the impact on power dissipation. According to the study, the number of BP diodes in
in a PV panel should not be defined by the number of cells, but by the power capacity of the string
a PV panel should not be defined by the number of cells, but by the power capacity of the string cells
cells if they become bypassed. This work once more emphasized the importance of incorporating
if they become bypassed. This work once more emphasized the importance of incorporating bypass
bypass diodes into modules. Figure 7 summarizes in a chronological sequence the primary references
diodes into modules. Figure 7 summarizes in a chronological sequence the primary references
aforementioned through the 1990s.
aforementioned through the 1990s.

Figure 7. Main references through the 1990s.

Figure 7. Main references through the 1990s.
Other studies [63–67] show different ways to understand the impact of shading and hotspotting
Other
as well studies [63–67] of
as non-uniformity show different
solar ways
irradiance ontoPV understand
output powerthe impact of shading
generation, whileand hotspotting
using infrared
as well as non-uniformity of solar irradiance on PV output power
imaging technique, which was firstly used in the field of PV installations. generation, while using infrared
imaging
The technique, which was firstly
interest in understanding theused in the
impact field of PV
of hotspots andinstallations.
shading increased as the production of
The interest in understanding the impact of
PV modules increased across the globe. In 2005, Meyer and Vanhotspots and Dyk
shading increased as the
[67] investigated therelationship
production
of PV modules
between increased across
the increase/decrease in the globe. resistance
the shunt In 2005, Meyerof PV andcells Van
and Dyk [67] investigated
the existence the
of hotspots.
relationship between the increase/decrease in the shunt resistance of PV cells
It confirmed along with other research, such as [68–70], that shunt resistance causes hotspots in and the existence of
hotspots. It confirmed along with other research, such as [68–70], that shunt
PV modules and the mitigation of this problem would not be merely possible by having a separate resistance causes
hotspots in
resistance loadPVintegrated
modules and
withthe
themitigation
bypass diodeof this problem
junction box. would not be merely
The researchers possible
affirmed it wasby having
necessary
a separate resistance load integrated
for the use of thermography, too. with the bypass diode junction box. The researchers affirmed it
was necessary for the use of thermography, too.
Early 2010, Simon and Meyer [71] developed a suitable method to detect and analyze the hotspots
usingEarly
a unique2010,approach.
Simon and Meyer
The [71] developed
proposed a suitableofmethod
technique consisted to detect
using infrared and
(IR) analyze the
thermography
hotspots using a unique approach. The proposed technique consisted of using infrared (IR)
Energies 2020, 13, 2472 10 of 21

to assess the heat distribution on the PV modules’ surface. Moreover, using scanning electron
microscopy, it was observed that the structural materials of the hotspotted solar cells were affected by
irreversible destruction.
Additionally, Kernahan [72] proposed a new detection and prevention method for hotspots’
mitigation in solar cells. In simple, he proposed to connect subcells that are affected by a hotspot to an
external BP diode or connect the whole substring to an external bypass diode interconnection, which
would inherit the reversed current passing thought the hotspotted string, increasing the output current
and, consequently, increasing the PV module power generation.
Until the early 2000s there was a relatively slow evolution on published research about the
BP diodes. Since then, solar PV power has emerged as a possible alternative energy source, so the
reliability of such systems has been extensively explored. Therefore, the shading problem and the
hotspot, as wells as the BP diodes and other mitigation techniques, are widely studied and will be
discussed in Section 4.1.

4.1. Recent State of the Art on Hotspot Mitigation Techniques

So far, BP diodes are consolidated as a protective device against power losses and the hotspot
problem, although some failures and losses still are observed on PV modules. Thus, research about
BP diodes continues to be developed. Recent works will be categorized into five types of research
groups: Partial Shading, Bypass Diode Topology, Field Tests, Artificial Intelligence, and New Mitigation
Techniques. Such studies are summarized in Table 1.

Table 1. Classification of reference by specific subjects.

Research Group Purpose Reference

Estimate the shading effect to power
Partial Shading output on PV modules using different [6,29,32,73–85]
connections of BP diode
To investigate how different BP diodes
arrangements within a PV module
Bypass Diode Topology [5,8,17,19,31,86–91]
influences the voltage, current, and
power characteristics
Evaluate PV systems performance
Filed Tests [27,92–96]
under real conditions
Applying artificial intelligence
Artificial Intelligence techniques for modelling and prediction [97–108]
of the performance of PV systems
Search for new strategies to address the
New Mitigation Techniques [7,18,109–111], [112]
hotspot and shading problem

The Partial Shading category revisits the effect of the shading on the PV module, as well as the
entire PV system, evaluating the power losses and the possibility for the hotspot rising. Partial shading
is a critical discussion, especially when the urban PV systems became popular. These papers raised
the demand for those studies developed in the Bypass Diode Topology category. In these studies,
the number of BP diodes mounted on a PV module, along with the arrangement inside the panel,
was extensively discussed.
Regarding the Field Tests’ category, on this research the performance of commercial PV modules
was analyzed under real conditions, and by an extended period. In this kind of research, it was possible
to identify the requirements of the PV plants, especially those related to the PV module reliability.
The Artificial Intelligence (AI) category on Table 1 refers to research that applies an AI technique
for modelling [97,98,101] PV systems and predicting their performance [102–104], especially the ones
related to analyzing shading effect [105], the hotspot problem, and the bypass diodes [106]. Using
AI techniques on this concern enables reducing experimental costs once the results can be close to
Energies 2020, 13, 2472 11 of 21

reality. On the other hand, there will be some unpredictable situations that were not considered in
model training.
One new trend related to the BP diodes is the fault diagnosis on these devices using AI, developed
by [99,100,107,108]. This is an excellent tool for identifying defects on the BP diodes, as well as detecting
the hotspot problem. It is essential to early identify this kind of fault in order to avoid power losses,
possibly permanent damage to the PV modules, and, consequently, the PV system reliability.
Even though bypass diodes are used for protection and qualification and tests are used to reduce
cell mismatch, a few studies have reported these strategies to be insufficient for hotspot prevention.
Some authors have recently asserted the need for new mitigation techniques.
Bauwens and Doutreloigne [7] developed a smart bypass switching device, using a NDMOS
(Doubled-Diffused MOSFET). Simulations using the smart bypass, a typical BP diode, and an ideal
diode were made. The authors concluded that smart bypass was more efficient once it did not produce
heat or power losses, such as conventional BP diodes.
Kim and Krein [18] re-examined BP diodes, showing their inadequacy for hotspot protection.
The authors concluded hotspotting could lead to a second breakdown or cell encapsulant damage
and permanently degrade the PV panel or create safety concerns. The paper detailed the hotspot
conditions and how they could occur. Moreover, the research explored some hotspot preventing
methods, namely: BP diodes, active bypass switches, low reverse-breakdown voltage PV cells, and
active hotspot detection and protection. These findings demonstrated that BP diodes are more effective
at mitigating hotspots for short PV string lengths, but this is not conventionally implemented in
modern panel construction.
One of the hotspots-preventing methods studied by [18] was the low reverse-breakdown voltage
PV cells. In this regard, the SunPower® company manufactures a more shading-tolerant PV cell.
This kind of cell has a low breakdown voltage, which occurs uniformly across the cell. In a shading
situation or any other reverse-bias condition, thermal runaway is mitigated, lowering the PV cell
temperature and, consequently, avoiding the hotspot, regardless of whether or not BP diodes are
presented. However, when enough cells are in reverse bias, the cumulative power loss from the shaded
cells can exceed the power produced from the cells in forwarding bias, so SunPower includes diode
protection to enhance energy yield [113].
The lower heating on this kind of cell happens because the cell architecture places a heavily doped
positive region immediately adjacent to a heavily doped negative region evenly across the back of the
cell, resulting in a lower voltage potential for a cell in reverse bias. Additionally, SunPower® cells are
connected with a strain-relieved copper bar and six solder pads per cell. Solder joints are small, so the
mismatch from thermal expansion results in less stress in the joint [113].
Pannebakker et al. [109] proposed simulation and experiments using smart bypass diodes. They
simulated and tested various patterns of shading, using 60, 20, 12, 6, and 3 BP diodes, in order
to evaluate the decreasing power losses and heating by the BP diodes. The study concluded the
more BP diodes were used, the higher the maximum power under shading condition, creating a
more shade-resilient module. Although the proposed topology showed excellent results, the cost of
integrating this new technique was considered high.
Even though previous studies showed high costs and unfeasibility to place one diode to each cell
within the PV panel, there are some manufacturers producing modules with this kind of BP diode
topology. For instance, AE Solar® produces a specific kind of PV module, named “smart hotspot-free”,
which places one bypass diode for each cell in the module. The BP diodes are not placed on the junction
box, as traditionally installed, but on the cell interconnection, as illustrate in Figure 8.
Energies 2020, 13, 2472 12 of 21
Energies 2020, 13, x FOR PEER REVIEW 12 of 22

Figure 8. BP diodes place on each cell [114].

The manufacturers’ test showed that the cell temperature under a shading situation could fall
from 160 °C on conventional modules to 85 °C on this kind of BP diode topology. Moreover, it may
increase the module lifetime and PV installation reliability [114].
In 2017, Dhimish et al. [115] and [110] proposed two hotspots’ mitigation techniques, where both
consist of the integration of two MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistor)
connected to the affected PV module, as seen in Figure 9 (a). This arrangement was tested under
Figure
Figure 8. BP diodes
8. BP diodes place
place on
on each
each cell [114].
cellboth
[114].techniques would increase the
several environmental conditions, where it was observed that
output
Thepower of the PV module by 1.44 W the and 3.97temperature
W, respectively. a shading situation could fall
The manufacturers’
manufacturers’ test test showed
showed thatthat the cell
cell temperature underunder a shading situation could fall
from By contrast
◦ with Dhimish et al. [115] ◦observations,
kindtwo BPother researchers, [116] and it[117],
from 160
160 °C C on
on conventional
conventional modulesmodules to to 85
85 °C C on
on this
this kind of BP
of diode
diode topology. Moreover,
topology. Moreover, it may
may
observed
increase the
the impact
module of the
lifetime hotspots
and PV on the output
installation power
reliability of the
[114]. PV modules. It was concluded that
increase the module lifetime and PV installation reliability [114].
the loss
In in the output power could be as high as 15% when the hotspots are affecting various cells in
In 2017,
2017, Dhimish
Dhimish et et al.
al. [110,115]
[115] andproposed two hotspots’
[110] proposed mitigation
two hotspots’ techniques,
mitigation where both
techniques, whereconsist
both
the
of PV module.
the integration of two MOSFETs
consist of the integration of two (Metal-Oxide-Semiconductor
MOSFETs (Metal-Oxide-Semiconductor Field Effect Transistor)
Field Effectconnected
Transistor)to
the Dhimish
affectedto et
PVthe al.
module, [111] conducted
as seen another
in Figureas 9a.seen observation
Thisinarrangement of the
was impact
tested of the second
under several mitigation
connected affected PV module, Figure 9 (a). This arrangement wasenvironmental
tested under
technique (shown
conditions, where in
it Figure
was 9 (b) onthat
observed the temperatures
both techniquesof the hotspotted
would increase solar
the cells. It power
output was noticed
of thethat
PV
several environmental conditions, where it was observed that both techniques would increase the
the temperature
module by 1.44 W ofand
the 3.97
hotspotted
W, cells could decrease in the range of 2.7–5.7 °F, as shown in Figure
respectively.
output power of the PV module by 1.44 W and 3.97 W, respectively.
10.
By contrast with Dhimish et al. [115] observations, two other researchers, [116] and [117],
observed the impact of the hotspots on the output power of the PV modules. It was concluded that
the loss in the output power could be as high as 15% when the hotspots are affecting various cells in
the PV module.
Dhimish et al. [111] conducted another observation of the impact of the second mitigation
technique (shown in Figure 9 (b) on the temperatures of the hotspotted solar cells. It was noticed that
the temperature of the hotspotted cells could decrease in the range of 2.7–5.7 °F, as shown in Figure
10.

(a) (b)

Figure 9. Hotspot mitigation techniques proposed by [115]. (a) First mitigation technique, where the
connected across
MOSFETs are connected across the
the PV
PV strings
strings of
of the
the PV
PV module.
module. (b) Second mitigation technique,
the MOSFETs
where the MOSFETs are
areintegrated
integratedacross
acrossthe
thewhole
wholeterminals
terminalsof
ofthe
thePV
PVmodule.
module.

By contrast with Dhimish et al. [115] observations, two other researchers, [116,117], observed the
impact of the hotspots on the output power of the PV modules. It was concluded that the loss in the
output power could be as high as 15% when the hotspots are affecting various cells in the PV module.
(a) (b)
Dhimish et al. [111] conducted another observation of the impact of the second mitigation
Figure(shown
technique 9. Hotspot mitigation
in Figure 9b ontechniques proposed of
the temperatures bythe
[115]. (a) First mitigation
hotspotted technique,
solar cells. where the
It was noticed that the
MOSFETs are connected across the PV strings of the PV module. (b) ◦
Second mitigation
temperature of the hotspotted cells could decrease in the range of 2.7–5.7 F, as shown in Figure 10. technique,
where
In the MOSFETs
contrast are integrated
with [111] results, itacross
wasthe whole terminals
observed of the PV
by [118–121] module.
that the hotspots’ temperature
distribution was strongly dependent on various conditions including (1) the integration of the MPPT,
(2) day-to-night temperature variations, (3) day-to-night irradiance variations, and, most importantly,
(4) the amount of partial shading affecting the hotspotted PV modules.
The up-to-date research [122–124] discussed the fact that hotspots in PV modules are likely to
occur when the PV modules are affected by cracks/microcracks. It was observed during the last three
years that many old PV modules affected by hotspots, most frequently, are affected by microcracks.
Energies 2020, 13, 2472 13 of 21
Energies 2020, 13, x FOR PEER REVIEW 13 of 22

Figure
Figure 10. Impact of
10. Impact of hotspot
hotspot mitigation
mitigation technique
technique on
on the
the hotspot
hotspot temperature
temperature distribution
distribution [115].
[115].

In
In 2020,
contraststate-of-the-art research
with [111] results, it on
wasthe mitigation
observed by of PV hotspots
[118–121] using
that the a new temperature
hotspots’ principle of
integrating
distribution was strongly dependent on various conditions including (1) the integration of is
a current-limited circuit was proposed by [112]. The integration of this circuit thenot only
MPPT,
providing a suitabletemperature
(2) day-to-night solution to mitigate the problem
variations, of hotspotting,
(3) day-to-night but also eliminating
irradiance variations, theand,increase
most
Energies
of the 2020,
PV 13, x temperature
cells’ FOR PEER REVIEW
during partial shading scenarios. Figure 11 shows the thermal 14 of
image of22
a
importantly, (4) the amount of partial shading affecting the hotspotted PV modules.
PV module, where two cells are under shading conditions.
The up-to-date research [122–124] discussed the fact that hotspots in PV modules are likely to
occur when the PV modules are affected by cracks/microcracks. It was observed during the last three
years that many old PV modules affected by hotspots, most frequently, are affected by microcracks.
In 2020, state-of-the-art research on the mitigation of PV hotspots using a new principle of
integrating a current-limited circuit was proposed by [112]. The integration of this circuit is not only
providing a suitable solution to mitigate the problem of hotspotting, but also eliminating the increase
of the PV cells’ temperature during partial shading scenarios. Figure 11 shows the thermal image of
a PV module, where two cells are under shading conditions.

Figure
Figure 11. Impact of
11. Impact of hotspot
hotspot mitigation
mitigation technique
technique developed
developed by
by [112] on aa PV
[112] on PV module
module affected
affected by
by two
two
shaded solar cells.
shaded solar cells.

It was presented that after using the proposed mitigation technique, the temperature of the shaded
It was presented that after using the proposed mitigation technique, the temperature of the
cells decreased equally to adjacent cells.
shaded cells decreased equally to adjacent cells.

4.2. Emerging Modules’ Technologies and Bypass Diode Protection Device

Up to now, the paper discussed in detail about BP diodes and shading mitigation techniques on
conventional PV modules. However, there are some emerging modules’ technologies, such as
shingled modules and half-cells modules. These devices are also subjected to shading situations and
 Figure 11. Impact of hotspot mitigation technique developed by [112] on a PV module affected by two
shaded solar cells.
Energies 2020, 13, 2472 14 of 21

It was presented that after using the proposed mitigation technique, the temperature of the
shaded cells decreased equally to adjacent cells.
4.2. Emerging Modules’ Technologies and Bypass Diode Protection Device
4.2.
UpEmerging Modules’
to now, the paperTechnologies
discussed and Bypassabout
in detail Diode BP
Protection
diodesDevice
and shading mitigation techniques on
conventional
Up toPV modules.
now, the paperHowever,
discussedthere are some
in detail aboutemerging
BP diodesmodules’
and shading technologies, such as shingled
mitigation techniques on
modules and half-cells
conventional modules.
PV modules. These there
However, devices
areare alsoemerging
some subjected to shading
modules’ situationssuch
technologies, andas need
protection
shingledagainst
modulestheand
hotspot problem.
half-cells modules. These devices are also subjected to shading situations and
need
One protection
of the ways against the hotspot
to increase power problem.
density on a PV module is to improve the interconnection of
the cells, making them closer, so the areadensity
One of the ways to increase power on a PV
producing module
power is to improve
is more the This
extensive. interconnection of
logic is applied
the cells, making them closer, so the area producing power is more extensive.
to shingled modules technology, where the cells are made from long strings of series-connected This logic is applied to cell
shingled modules technology, where the cells are made from long strings of series-connected
strips [125]. The shingled PV modules are specially designed to connect by a bus bar, using no space cell
strips [125]. The shingled PV modules are specially designed to connect by a bus bar, using no space
between each other, as illustrated in Figure 12.
between each other, as illustrated in Figure 12.

Figure
Figure 12.Shingle
12. ShinglePV
PV module
module interconnection.
interconnection.

Despite
Despitethethehigher power
higher power capacity,
capacity,this
thiskind
kind ofof module, whenunder
module, when undera ashading
shading condition,
condition, willwill
face face
the same
the same consequences
consequences asasthetheconventional
conventional ones. Thus,like
ones. Thus, likeother
otherPVPV modules’
modules’ technologies,
technologies,
bypass
bypass diodesdiodesareare placed
placed in in antiparalleland
antiparallel andmounted
mounted on on the
thejunction
junctionboxboxlocated
locatedonon
thethe
rearrear
sideside
of of
Energies 2020, 13, x FOR PEER REVIEW 15 of 22
the module. The number of BP diodes must follow the same principles applied to conventional PV
modules [125].
the module. The number of BP diodes must follow the same principles applied to conventional PV
Another[125].
modules rising technology is the half-cell PV modules. On this kind of PV module, the PV cells
are cut in Anotherchanging
half, the configuration
rising technology of the
is the half-cell module. On
PV modules. The current
this kind ofgenerated
PV module, bythe
a solar
PV cellscell is
proportional to the area exposed to solar irradiance. Thus, half-cells will produce half
are cut in half, changing the configuration of the module. The current generated by a solar cell is current. However,
losses due to the to
proportional series’ resistance
the area exposed of the cell connector
to solar irradiance.inThus,
a PV half-cells
module are willproportional
produce halftocurrent.
the square
However,
of the current losses due to theSo,
they conduct. series’ resistance
half-cells of the
reduce cell losses
these connector in a PV module
substantially [126].are proportional to
the square modules
Half-cell of the current
havethey conduct.
twice So, half-cells
the number of cellsreduce
than these losses substantially
conventional modules. [126].
So, they require a
Half-cell modules have twice the number of cells than conventional
rearranging of the PV module layout, especially on the bypass diodes’ connections. modules. So, theyBPrequire
diodes a are
rearranging of the PV module layout, especially on the bypass diodes’ connections.
used as hotspot protection devices and are mounted on the junction box, as in conventional modules. BP diodes are
used as hotspot protection devices and are mounted on the junction box, as in conventional modules.
Nevertheless, the number of cells protected by one single diode is going to change, as illustrated in
Nevertheless, the number of cells protected by one single diode is going to change, as illustrated in
Figure 13.
Figure 13.

Figure 13. Half-cell module BP diode connections.

Figure 13. Half-cell module BP diode connections.

Given the valuable experiences and ongoing feedback from the presented literature, the last part
of this review paper, Section 5, provides the overall conclusions and discusses the future issues about
the BP diodes and hotspot mitigation techniques.
Energies 2020, 13, 2472 15 of 21

Given the valuable experiences and ongoing feedback from the presented literature, the last part
of this review paper, Section 5, provides the overall conclusions and discusses the future issues about
the BP diodes and hotspot mitigation techniques.

5. Conclusions
This review paper was structured to give an overview of the bypass diode application on PV
modules. First, the partial shading effect was discussed, aiming to understand how it can lead to the
hotspot problem, as well as how it can result in power losses and I-V and P-V curves’ distortions.
Moreover, hotspots may cause permanent cell damage, and are still one of the most frequently reported
phenomena to limit module lifetime.
The primary prevention method for hotspotting is a bypass diode, wired in antiparallel to solar
cells’ submodule, and mounted on the junction box on the back of PV modules. Bypass diodes perform
the function of providing a bypass path for the current flow in case some cells are partially shaded,
avoiding the hotspot problem and increasing the MPP. However, once bypass diodes are activated,
the MPPT becomes disoriented because of the multiple peaks on the P-V curve. Moreover, when bypass
diodes conduct, they consume part of generated power, impacting the maximum power delivered by
PV modules.
Lastly, the study brings an extensive overview of all research progress on bypass diodes since its
first uses, allowing us to understand how the technology developed and advanced over recent years.
Recent research was organized into five categories, as shown in Table 1.
Partial shading concerns studies on the impact of shading on PV systems. It is an essential field of
study because it helps to clearly understand the power losses and the possible permanent damage to
the PV modules associated with shading situations. It is becoming increasingly important with the
recent growth of PV systems’ installations in urban areas. On the other hand, extensive studies have
shown that there always will be some unpredictable situations involving the shading of PV modules.
Regarding the Bypass Diode Topology category, it was subject to extensive research over the past
decades. These studies stated that the PV systems’ reliability is related to the number of BP diodes
mounted on the PV modules. The more diodes installed, the fewer power losses associated with
shading, as well as the decrease of the hotspot rising. However, increasing the number of diodes is
costly and makes the PV module arrangement complex. Thus, changing the BP diode topology has
not shown a significant increase in the PV systems’ yields. Up to now, the Schottky diode was the
most applied kind of device on the PV modules’ industry. Therefore, it is possible to conclude that this
research led us to investigate new mitigation techniques to the shading and hotspot problem.
New strategies for the shading and hotspot phenomenon, as well as the BP diodes, have been
investigated in recent studies. For instance, the studies developed by [109,111] used a smart BP diode
and a MOSFET, respectively, substituting the conventional BP diode. Both studies have proven to be
efficient in reducing the hotspot and power losses. Though, just like any new technology, most of these
studies have shown to be costly
Field Tests is an indispensable category because it analyzes the PV systems exposed to real
conditions, so its results can bring support to other researchers and adapt other studies to what issues
need to be addressed on the PV systems. However, it can be costly compared to simulation studies
and can take a longer time. The field test analyzed in the article reinforced that the BP diodes are
essential to the PV modules’ reliability, although they are devices susceptible to fails, which may lead
to consequences of power losses and the hotspot phenomenon.
Some recent researchers are applying artificial intelligence techniques on the PV systems field.
It can be used on modelling and predicting PV systems’ performance. Specifically about shading,
hotspots, and BP diodes, these studies improved detecting device faults, supports visual inspections,
and detecting shading on PV systems. It can be a helpful tool, because it enables detecting earlier
failures that can cause losses or damage to the systems. Nevertheless, there will be some situations not
predicted on the AI training.
Energies 2020, 13, 2472 16 of 21

All these categories analyzed in this review article aim to mitigate the shading effect and the
hotspot problem. The primary strategy is using the BP diodes, as discussed before, but all studies cited
are correlated with the purpose of minimizing power losses, possibly permanent damage, and the
overall PV module reliability regarding the shading situation.
Even though bypass diodes are a consolidated mitigation technique to the shading problem, a few
studies have reported these strategies to be insufficient for hotspot prevention, especially concerning
power consumption, diodes faults, and introducing multiple peaks on P-V curves.

Author Contributions: R.G.V. conceived the methodology, developed theory, and performed simulations.
F.M.U.d.A. conceived the idea, performed the supervision, and contributed to the revision of the manuscript. M.D.
developed theory, provided critical feedback, and helped shaping the research. M.I.S.G. provided critical feedback
and contributed to the revision of the manuscript. All authors have read and agreed to the published version of
the manuscript.
Funding: This research received no external funding.
Acknowledgments: The authors would like to acknowledge the support provided by the Semi-Arid Federal
University, the University of Rio Grande do Norte and the University of Huddersfield in the framework of an
international contribution.
Conflicts of Interest: The authors declare no conflict of interest.

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