4. on several factors; some of which are external or environmental, others
are internal to the PV system itself and other miscellaneous factors.
Since renewable energy, especially solar energy, is one of the most
widely-used sources, many researchers have contributed in different
studies resulting in diverse outcomes concerning the factors affecting
PV panels as illustrated in Table 1. The contribution of this study is
compared to the previous articles and is also shown in the mentioned
table. The previous studies performed in that field, since 1991 till 2016,
didn’t focus on integrating all the factors affecting the PV panels,
rather, each study focused only on certain factors related to the PV
panels. Moreover, these studies didn’t reveal the direct quantitative
effect of each factor on the performance of the panel, they rather
focused on showing the overall increase or decrease effect of some
factors whether related to environment or internal factors. Thus, this
study not only integrates all the factors that can have major and minor
effects on the performance of the panel from several previous studies
but it also shows the quantitative effect of each factor on the
performance of the PV panel by showing a certain range of PV panel
power loses that these factors can cause. Also, this paper considers
some of the factors that were overlooked in most of the previous studies
such as cost of the system, glass breakage, characteristic resistance of
the PV panel, shunt resistance and sizing of the system. In addition,
this paper divides the factors under categories and subcategorized that
were not introduced earlier in other studies which are: environmental
factors (external), PV system factors (internal), PV system installation
factors (operational), PV system cost factors (economic) and other
miscellaneous factors. This inclusive and categorized study can help
both practitioners and researchers by removing the burden of having to
search several studies for obtaining an overall idea about the factors
affecting the performance of PV panels. Researchers can further
investigate and quantify the percentage losses due to many environ-
mental or system factors and this can lead to further research in how to
improve the negative effect that those factors have on the performance
of PV panels. On the other hand, practitioners can build experimental
models that can reduce the effect of the negative factors on the
performance of PV panels such as cleaning systems for PV panels,
cost-reduction plans, maintenance plans, improving the efficiency of
the components such as inverters and batteries and other practices that
take into consideration the mentioned factors in this study.
Table 1, summarizes findings concerning the key factors affecting
the performance of PV systems as reported in the studied research
papers. It can be concluded from Table 1 that previous research
studies, to our knowledge, have not integrated and categorized all the
mentioned factors using the proposed technique in this paper.
Moreover, this paper focuses on as quantifying the percentage losses
of each factor to be able to understand the impact of those factors on
the PV panel's performance. In addition, previous research studies
neglected some important factors such as cost of the system, glass
breakage, characteristic resistance of PV, shunt resistance and sizing
that have an impact on the performance of PV panels. Consequently,
the objective of this research is to present an integrated review about all
the mentioned factors and study the influence of each factor on the
performance and operation of PV systems whether those factors are
environmental, system-oriented, installation-related, cost considera-
tions or other miscellaneous factors. The structure of this paper is as
follows: Section 2 includes the methodology used for selection of
factors and for the papers analyzed while Section 3 includes the
environmental factors. Section 4 includes the PV system factors,
Section 5 has the PV installation factors, Section 6 shows the PV
system cost factors while Section 7 shows the miscellaneous factors and
finally Section 8 includes the conclusion obtained from the paper.
2. Methodology
Extensive research has been done in Scopus, Google Scholar, IEEE
Explore, and Springer databases concerning articles that discussed
factors affecting the performance of PV panels. The articles chosen for
analysis are the ones that are most cited when discussing the topic of
factors affecting PV panels especially in the period from 1991 to 2016
which covers a considerable time interval and this gives more
credibility to the factors under investigation. Moreover, according to
the literature, those articles concentrate on the factors that have major
influence on PV panels. They discuss and present problems that are
commonly faced by practitioners and researchers concerning PV
panels. This has helped in creating a review that can include effective
factors as well as commonly-faced problems. Also, some of the articles
used in this study are review papers which provide a wider view and
more inclusive scope of the factors affecting the performance of PV
panels. Accordingly, a new way of classification has been proposed for
the factors under 5 main categories which are: environmental influ-
ences, PV system influences, PV installation factors, PV system cost
issues and finally the miscellaneous factors category. Those categories,
which are obtained from the chosen articles, are proposed due to their
relevance to the current research trends besides their coverage of
environmental, economic, technical and sustainable areas of interest
for researchers and practitioners. Approximately 50% of the articles
studied have focused on discussing factors that fall under the environ-
mental category, 20% of them focused on PV system factors, 35% of
which focused on installation factors, 5% on cost factors, 18% on
degradations of PV systems and a small percentage of the articles
focused on other different factors. The articles under study are chosen
since they are the most relevant to the integration of the most effective
and most common factors from the total articles that have been read.
However, some deficiencies in those articles are that they overlooked
some of the factors that can affect the performance of PV panels
whether a minor or major effect. Moreover, the previous studies didn’t
quantify some of the factors and didn’t give an explanation why those
factors were not quantified before. To sum up, enclosed papers were
selected based upon the criteria demonstrated below in Fig. 1.
Thus, the objective of this research is to present an integrated
review about all the mentioned factors and study the influence of each
factor on the performance and operation of PV systems since there was
no previous research, to our knowledge, that integrated all those factors
in one study and that classified the factors according to the proposed
classification in this paper. In addition, the paper proposes some
suggestions about why some variables cannot be quantified concerning
their percentage losses.
List final selected papers that provides a ground on which practitioners and researchers can build
further work. A total of 110 papers were of interest and were narrowed down specifically to
approximately 100 papers
Select most cited papers and those that shared common and most influential factors on the
performance of the PV panels . A total of 200 papers were selected
Perform manual and automated citation-based search and select papers that cover the period from
1991 to 2016 which represents a wide research period for more credibility. A total of 500 papers
were identified
Identify available publications and start search based on topics identified using keywords as:
factors, PV panels and performance
Selection criteria starts
Fig. 1. Search and selection criteria.
M.M. Fouad et al. Renewable and Sustainable Energy Reviews 80 (2017) 1499–1511
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5. 3. Influential factors for the performance of PV systems
Several factors affect the operation of PV systems and the power
generated from them. These factors are classified in this research as:
Environmental factors, PV system factors, installation factors, cost
factors or miscellaneous factors and each of which will be discussed
separately along with its sub-factors.
4. Environmental factors
Various environmental factors affect the performance of PV systems
such as: Solar irradiance, temperature, dust and shades. Each of these
factors is discussed separately in this section.
4.1. Solar irradiance/radiation
The quantity of power coming from solar source per unit area is
known as irradiance [4]. The energy produced by a photovoltaic
module is directly related to the availability of solar energy, and as a
result is site dependent [6,7]. Irradiance usually fluctuates according to
the weather and the sun's location in the sky. The location of the sun
changes throughout the day due to changes in the sun's altitude, which
is the angle between the sun's rays and the horizontal plane, and
azimuth angle, which is the angle between true north and the
projection of the sun rays onto the horizontal, in the sky [4].
Solar irradiation impinging on a surface consists of direct, diffused,
and reflected radiations. The largest fraction of the solar irradiation is
the direct component; however, both diffuse and reflected radiation
must be taken into account during the analysis of the operation of the
system. Solar irradiation on PV modules varies with the module's
position; the maximum solar irradiation takes place when the modules
are perpendicular to the direct radiation [1]. Some studies showed that
0.08% loss occurs for each degree of deviation from the south especially
in the azimuth orientation [8]. As the solar irradiance increases, the
electrical power output from the PV panel increases too [9]. The
relationship between the module current and the solar irradiance is
approximately linear where, as the solar irradiance increases, the
module current increases too [10,11]. Thus, based on the literature,
the solar irradiance effect on the performance of the PV panel cannot
be quantified by a certain value of percentage increase because the
relationship is approximately linear between the module current and
the irradiance value.
4.2. Module temperature
A PV cell converts a small portion, approximately less than 20%, of
the irradiance into electrical energy while the remaining is converted
into heat. Overheating of the module mainly occurs due to excessive
solar radiation and high ambient temperatures [12,13]. Module
temperature is a parameter that has great influence on the behavior
of a PV system, as it greatly affects the system efficiency and energy
output. The main effect of the increase in cell temperature is on the
open circuit voltage which decreases linearly with the cell temperature
increase. The Cell voltage decreases by approximately 2.2 mV per 1 °C
rise in operating temperature and thus the efficiency drops by about
0.5% for crystalline PV cells [14,15,9]. For crystalline modules, a
typical temperature reduction factor is 89% so if we take a 100-watt
module as an example, it will typically operate at about 89 Watts (100
× 0.89 = 89 W) [16]. There are some proposed correlations in the
literature that express the module temperature as a function of
variables such as the weather variables (depends on the location)
especially the ambient temperature, the local wind speed, as well as the
solar or irradiance incidence on the plane of the array. Not only this but
also the temperature depends on the material and system-dependent
properties such as the transmittance of the glazing-cover, absorbance
of the plate and other factors. The effect of the temperature on the
module performance is mainly reflected in both the open circuit voltage
and the fill factor (measure of how much series resistance and how little
shunt resistance there is in a solar cell and its circuit) which decrease
substantially with temperature [17,18]. The operating temperature
effect on the electrical power produced by a PV module can be
attributed to the temperature's influence upon the current, I, and the
voltage, V, as the maximum power is shown in Eq. (1) below [19–21]:
P V I FF V I
= × = ( )
m m m oc sc (1)
Nominal Operating Specific Temperature or NOST, is defined as the
site specific module temperature operating at the maximum power
point, under 800 W/m2
irradiance and 20 °C ambient temperature.
The value of NOST is established from the complete measured data set
by plotting the module and ambient temperature differences against
irradiance. A sensitivity analysis showed an energy prediction variation
of −1.5% per 5 °C increase in the NOST [19,22–25]. Not only this, but
also an important parameter that determines the loss in power due to
temperature increase is the power temperature coefficient (the rate of
change in power with respect to temperature) [26]. For example,
typical power temperature coefficients for mono-crystalline PV panels
are between 0.38% and 0.45%/°C [27–30] which means that for every
1 °C temperature rise, there is from 0.38% to 0.45% of power lost.
From this, it can be concluded that there is no exact range of power
percentage losses due to temperature increase of the PV modules. The
effect of temperature on the module efficiency is shown in Fig. 2 [14].
4.3. Dust accumulation
Some of the sunlight can be blocked from the PV module due to
presence of dirt or dust which thus causes considerable amount of losses in
the generated power since the solar irradiance is scattered on the surface of
the solar panel [14,31,46,32]. A typical annual dust reduction factor is 93%
or 0.93 so if we take a 100-watt module as an example, it will typically
operate at about 93 W (100 × 0.93 = 93 W) due to dirt accumulation
[14,33]. El-Shobokshy and Hussein [34,35] showed that a 73 g/m2
deposition of cement dust resulted in an 80% drop in PV short circuit
voltage. Moreover, atmospheric dust with mean diameter 80 µm at 250 g/
m2
was tested and found to reduce the short circuit current by 82%.
Mastekbayeva and Kumar [36] showed in another study that an 11%
reduction in transmittance was estimated for a 5 g/m2
dust deposition over
a month. Hassan et al. [37] investigated the effect of airborne dust
concentration on PV performance and a decrease in efficiency from
33.5% to 65.8% for an exposure of 1–6 months was achieved, respectively
[38]. In a study by Zaihidee et al.,[32] a 20 g/m2
of dust accumulation on
PV panel reduced the short circuit current by 15–21%, and the open circuit
voltage by 2–6% and the efficiency by 15–35%. Ζώγου [8] showed that in
Greece, a typical annual dust reduction factor is 93%. Thus it can be
concluded that the dust deposition is site climate-specific and thus its
quantity depends on the place, type of dust, and several other factors.
Fig. 2. PV efficiency versus module temperature [14].
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6. 4.4. Shading
The output power from PV panels is lowered due to the shadowing
effect [39,46]. Not only do the shades affect the current flow in the
shaded cells, but they also affect the current flow in the whole panel
since normally the cells are connected and wired in series. Shadows can
be caused by poles, trees and buildings and may also be caused by the
module mounting structures on other structures. Leaves, birds and bird
droppings that may fall directly on the modules can also cause
shadowing. Several interconnection schemes are proposed to reduce
the losses caused by partial shading [40,41]. In one of the studies,
Quaschning & Hanitsch [42] showed that the performance loss was
70% although only 2% of the module area is shaded. Viitanen [43]
showed that if 5–10% of the array is shaded, the power output can be
reduced over 80%. In a study conducted by Alonso-Garcia et al. [44],
different power losses have been noticed, for the same amount of
shading, by only varying the characteristics of the cell that is shaded.
The losses varied from 59% to 73%. Thus, quantifying the losses due to
shading depends on the percentage of shaded cells as well as the
material of the cell and the connection between the panels. Plus, the
shades on the panel depend on the height of the nearby buildings and
the presence of trees or cross-shading from other panels.
4.5. Soiling of PV panels
Dust accumulation can cause soiling of solar panels. In most cases,
panel surfaces are washed off by rainfall; however, dirt may sometimes
stay even after heavy rains. The most critical part of a module is the
lower edge especially with low inclinations where the soiling at the edge
of the frame occurs. This accumulated dirt causes shading of the cells
and thus reduces the available power from a module generally in the
range of 1%; however the power is restored if the modules are cleaned
[33,41,46]. However, Kimber [45] conducted a controlled study of
soiling losses on three identical systems at a commercial office park in
Los Angeles and showed that annual system losses from soiling effects
are nearly 5% and thus the effect of soiling varies according to the
location and the cleaning rate of the panel. Shading due to soiling is
divided in two categories which are soft shading such as air pollution,
and hard shading when dust accumulates and blocks the sunlight [46].
The relationship between the losses from PV power and the soiling
mass has been deeply investigated where some studies [47–49] showed
a linear proportional relationship between the two variables. On the
other hand, other studies [50,51] showed that when new dust particles
settle on the existing ones, the soil mass increases and thus the surface
becomes heavily soiled but doesn’t cause further obstruction of light.
The relationship between the soiling mass and the losses from PV
power is affected by the geographical location since different light
transmission is affected by the different dusts [52]. Since the large
particles have smaller cross-sectional area to volume ratio compared to
the fine particles, they obstruct less light. Moreover, the composition
and shape of the dust particle affect the absorption and scattering
characteristics of the particle [50].
5. PV system factors
Several factors related to the PV system components can affect the
power output from the overall system. These factors can be:
5.1. I-V characteristics of the PV panel
The panel's rated current IMPP, rated voltage VMPP, short circuit
current Isc, open circuit voltage Voc and rated power PMPP are all
characteristics of the PV cell itself that affect the power generated from
it [3,2,53].
5.2. Inverter efficiency
Inverter is a device that changes the direct power (DC) from the PV
array to alternating power (AC) used in the electrical grid or AC loads at
home [41,54,53].. The inverter affects the overall performance of the
photovoltaic (PV) systems [54,55]. In other words, if the power
conversion efficiency (a measure of the losses experienced during the
conversion from DC to AC) of the inverter in a grid-connected PV
system is too small, the power generated by the PV array cannot be
output to the AC utility system effectively. These losses are due to
multiple factors, some of which are: the presence of a transformer and
the associated magnetic and copper losses, inverter self-consumption,
and losses in the power electronics. It is thus necessary to increase the
conversion efficiency to be as maximum as possible [41]. It is
important to note that the inverter efficiency declines with a small
rate after peaking with incident energy levels around 400–700 W/m2
due to the temperature increase inside the inverter when it handles
loads with more power [54]. Actual field conditions usually result in
overall DC-to-AC conversion efficiencies of about 88–92%, with 90%
reduction factor as an average value [8,16]. So following the example of
100-watt module, it will operate at about 90 W (100 × 0.9 = 90 W).
Gonzalez et al. [56] presented some studies that proposed transformer-
less inverters which achieved higher efficiencies that can reach 97%
and that reduced the losses to about 3% only. Thus, it can be concluded
that the quantity of losses due to inverters depend on the type of
inverter used as well as the presence or un-presence of transformers.
5.3. Battery efficiency
In standalone PV systems, batteries are mainly used to store the
excess electrical energy produced when the PV system covers the load
completely or when there is no load required. Additionally, batteries
are required because of the fluctuating nature of the PV system output.
When batteries are used in a PV system, they must be located in an area
without extreme temperatures and with adequate ventilation [53,57].
Moreover, the major difficulty with this form of storage is the relatively
high cost of the batteries and the large amount required for large-scale
application [58].
The efficiency of a battery is the ratio of the charge extracted (Ah;
Ampere hours) during discharge divided by the amount of charge (Ah)
that is needed to restore the initial state of charge (SOC). Therefore, the
efficiency depends on the state of charge, which is the present capacity
of the battery divided by the nominal capacity, and the charging and
discharging current [14,53]. Some studies investigated the issue of
battery charging and discharging efficiencies which proved to be not
constant and depends on the battery type as well as the charger. The
lower the SOC of the battery, the higher the charging efficiency is. For
example, it was found in a study by Viitanen [43] that the charging
efficiency was 91% between 0–84% SOC, whereas the charging
efficiency was only 55% between 79–84% SOC and the efficiency was
lower than 50% at SOC levels above 90%. This means that a large
portion of the energy produced and stored inside batteries can go as
losses when the SOC of the battery is high during sunny conditions. If
the batteries are operating on low SOC levels for extended times, their
capacity may be reduced permanently. Therefore, the battery efficiency
is dependent on several factors and the amount of losses can be highly
dependent on the SOC of the battery [43]. The charging/ discharging
cycles of the battery as well as the temperature and other parameters
are major contributors affecting the lifetime span of the battery which
ranges from 3 to 5 years. The major disadvantages of the PV batteries
are their high cost compared to normal car batteries; however, they
have longer lifetime and lower discharging rates which makes their
maintenance cost lower [14].
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7. 5.4. PV panel structure
The structure of the PV panel greatly affects the power output. This
structure may include the material from which the panel is constructed,
its atomic structure as well as the band gap energy of the material used.
5.4.1. PV material
The choice of the PV material can have important effects on system
design and performance. PV materials include silicon, gallium arsenide
(GaAs), copper indium diselenide (CuInSe2), cadmium telluride
(CdTe), indium phosphide, and many others. Each of these materials
generates different cell efficiencies as well as different panel cost
[14,59,60]. The conversion efficiency of mono crystalline cells are
generally higher than that of poly crystalline cells; that is, 16–22% for
mono crystalline and 14–18% for poly crystalline ones [61]. Crystalline
silicon cells are the most efficient among the current commercially-
available solar cell technologies, but they require a relatively large
amount of raw materials in comparison to other technologies and this
increases their costs. However, the efficiencies for other materials,
under the thin film technology cells category, are approximately 7–9%
for amorphous silicon, 10–15% for CdTe and 7–12% for copper-
indiumgallium-selenide cells [61]. Due to the thinness of TF cells
compared to the thicker crystalline silicon cells, the heat can be
conducted away from the solar module more easily. Another recent
material is the dye- sensitized solar cell which currently achieved a
highest laboratory efficiency of 12.3% for glass substrate and 8.6% for
flexible stainless steel substrate [43]. Polman et al. [62] presented some
studies that reviewed the electrical characteristics of some several
geometries of photovoltaic materials with efficiencies ranging from
10% to 29% and compared those materials in terms of efficient light
management and charge carrier collection.
5.4.2. PV atomic structure
The atomic structure of a PV cell can be mono-crystalline, poly-
crystalline, amorphous or nano. The main advantage of mono-crystal-
line cells is their high efficiency, which is typically around 15% but it
requires complex manufacturing processes and higher costs. Poly-
crystalline cells are less expensive but have lower average efficiencies of
around 12%. Amorphous can be deposited on various substrates but
have efficiencies of 6%. Thus the atomic structure of the PV panel
affects its efficiency as well as its cost [14,43,60, 63].
5.4.3. Band-gap energy
When photons of sunlight strike the surface of the PV panel, only
the photons with a certain minimum level of energy are able to free
electrons from their atomic bonds to produce an electric current. The
band-gap energy, which is the amount of energy required to move an
outer-shell electron from the valence band to the conduction band,
frees those electrons. The band-gap energy is different for each
material and for each atomic structure of the same material. For
crystalline silicon, the band-gap energy is 1.1 electron-volts (eV). Other
PV cell materials have band-gap energies ranging from 1 to 3.3 eV
[64,65]. Moreover, in recent studies, researchers are trying to find the
optimum use of band gaps in a multi-junction device using several
techniques [66].
5.5. PV panel efficiency
A PV panel's energy conversion efficiency is the percentage of power
collected and converted (from absorbed light to electrical energy) when
a PV cell is connected to an electrical circuit.Thus the efficiency is
dependent on the rated power of the PV panel, the surface area of the
panel and the solar irradiance [14]. Not only this, but also the efficiency
of the PV panel depends on its material [61], its band-gap energy as
well as its atomic structure as discussed in Section 4.4.
6. PV system installation factors
Some of the most important factors that can affect the outcome
from the PV systems are dependent on the installation of the system
and the losses associated with these installed components. Some of
which are related to the cables, the orientation of the panel, the
mismatch, the tracking and the MPPT.
6.1. Cable characteristics
The cables used for wiring the grid-connected PV system need to be
carefully selected to ensure that they can withstand the extreme
conditions of the environment, the voltage and current conditions,
under which they may be expected to operate [46,67]. Moreover, there
is power dissipated in the wiring connections between the array, the
converter, the batteries and other components [68]. These losses
should be kept as minimum as possible but it is difficult to keep these
losses below 3% for the system. An average reduction factor for these
losses is 95% [16]. Several equations can be used to calculate the power
losses in the cables which depends on the internal of the wires, the
irradiance on the array, the array temperature, the frequency distribu-
tion of irradiance, the power output from the PV array as well as the
array voltage [68]. Also, some studies showed that losses show
quadratic relation with respect to the current. One of the proposed
equations to calculate power losses in the cables is shown in Eq. (2)
below [43]:
P
ρlp
V S cosφ
=
2
.
loss
ο
2
2 (2)
Where:
P loss is the power loss occurring in the cable,
ρ is the specific resistance of the conductor material,
lis the distance between the source and the load,
p is the power consumption of the load,
Vο is the voltage of the source,
S is the cross-section of the conductor and
cos φ is the power factor of the load.
6.2. Angle of inclination or orientation of PV panels
The daily and monthly energy production from PV panels is
strongly influenced by the module orientation. There are some optical
losses that vary with the angle-of-incidence (AOI) of sunlight striking
the module. The direct component of solar irradiance is affected by the
AOI whereas the diffuse component is nearly independent of module
orientation [6,7]. A PV panel utilizes solar irradiance most efficiently
when its surface is perpendicular to the sun [8]. If the panels are
installed at a fixed tilt angle, then the rule of thumb for annual
optimum tilt angle states the tilt angle should be the same as the
latitude of the installation location. The tilt angle deviates approxi-
mately +15° from the latitude angle in winter and about −15° of the
latitude angle in summer. However, this rule of thumb does not work
very well in the latitudes above 45° [43]. In a study by Kaddoura et al.
[69], a the optimum tilt angle was calculated using MATLAB software
by maximizing solar radiation. Results showed that 99.5% of the solar
radiation is harvested when the tilt angles are adjusted six times per
year. Therefore, the angle of inclination of the PV panels depends on
the location and thus is site-specific.
6.3. Mismatch effects
Mismatch losses are caused by the connection of non-matching
solar modules in series and parallel especially those which don’t have
common properties or which are experienced to different conditions
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8. from one another. One of the most serious problems associated with
the mismatch is that the output of the entire PV array is determined by
the solar module with the lowest output [41,46]. Array performances
depend strongly on the degree of variation of the modules that
comprise the array, the solar cells forming the PV modules and also
on the kind of series and parallel connections of the PV modules in the
network. Module mismatch amounts to at least 2% loss in system
power and can sometimes reach 10% (average 6% losses) [70,16].
6.4. Fixed or tracking mechanism
The energy output from a PV module, made from crystalline silicon
cells, is maximum if the sunlight is incident with a perpendicular angle
on it [46,71]. Thus, if the surface of the module is made to track the sun
so that the light is also perpendicular to the panel, the energy yield
increases [41]. The tracking system can be either single axis or dual
axis type [72] as shown in Fig. 3 below. Viitanen [43] presented in one
the studies that panels with a 2-axis tracking system produced 25–45%
more power than panels installed on fixed optimum tilt angle.
However, in cloudy conditions the efficiency of the tracking system is
reduced, as the solar radiation is iso-tropically distributed. In one of
the studies presented by Sungur [73], 50% more power was generated
from a horizontal PV module compared to a 2-axis solar tracking
system during cloudy conditions. Some trackers in certain countries
showed 42.6% more energy obtained from the PV panels which had
dual-axis tracking of the sun when compared to the PV panels at fixed
positions. Mousazadeh et al. [74] noted that a power generation
increase of 24.5% was obtained using one-axis tracking mechanism
as compared to that of a fixed PV module. In another study by Abdallah
[75], a 43.87%, 37.53%, 34.43%,and 15.69% power generation increase
for the two axes,east–west,vertical and north-South tracking respec-
tively was obtained compared to the fixed surface which was inclined
by 32° south in Amman, Jordan. It was found in a study by Abu-
Khader et al. [76] that the north-south axes tracking system achieved a
30–45% increase in the power compared to the fixed one. Moreover,
Abdallah and Nijmeh [77] found a 41.34% increase in the energy
output for a two axes tracking PV panel compared to a fixed one. From
this, it can be concluded that using tracking mechanism depends on the
available initial investment since tracking increases the cost of the
system. Moreover, it depends on the location and weather conditions
since tracking in some locations can cause excess heating and thus
power loss from the panel while it causes power increase in locations
where the sun is not abundant enough.
6.5. Maximum power point tracker (MPPT)
Change in the direction of sun, the solar irradiance level and
variations in the temperature cause changes in the power output of a
solar PV module. The P-V (Power vs. Voltage) curve of the module has
single maxima which exists at a peak point of the power corresponding
to a certain voltage and current. It is desirable to operate the panel at
the maximum point of power so that the load acquires the maximum
power that can operate it. MPPT is used to obtain the maximum power
from the PV module and transfer that power to the load by ensuring
that the panel output is always at the maximum power point which
significantly increases the output from the solar system [13,41]. Several
factors influence the MPPT behavior which include the power which is
dependent on irradiance level, voltage which depends on the tempera-
ture, fluctuations due to clouds, I-V curve of the PV panel and state of
the battery [78]. A charge current increase of 42% can be achieved in
100% efficient cases by turning the entire harvested module power into
useable charge current. But, since nothing is 100% efficient, the actual
charge current increase is somehow lower as some power is lost in
wiring, fuses, circuit breakers and other losses [79]. With typical
battery conditions, a charge current increase of between 10 – 25%
can be achieved [79]. However, the benefit of using an MPPT should be
weighed against its additional cost and reliability risks [58].
7. PV system costs
Another crucial factor is the cost related to the PV panel system the
connection cables, the panels themselves or the other components of
the system.
7.1. Increased cost of cables
Using large cables between array and point of connection to the
battery or inverter reduces array power losses, but increases the costs
of the cables which mainly depend on the cable characteristics and
sizes. Equations can be used to calculate the cost of the cables and the
financially optimum cable size [68]. Fig. 4 shows the estimated and
actual cable costs versus resistance for a standard copper cable [68].
Fig. 3. Fixed, single-axis and double-axis tracking mechanisms.
Fig. 4. Cost and resistance for a standard copper cable [68].
M.M. Fouad et al. Renewable and Sustainable Energy Reviews 80 (2017) 1499–1511
1506
9. 7.2. Cost of the system
The use of PV systems requires a big initial capital investment, but
has low running costs. The initial cost mainly consists of: cost of PV
panels, batteries, inverters, charge controllers, cables and accessories,
transportation and installation cost of project management and design
and engineering costs; however, there are no running costs (except for
maintenance) once the payback period is reached [58]. The initial cost
of batteries is about 15% of the total initial investment in typical PV-
diesel hybrid, however, their share of the costs can rise up to 35–50%
over the 25 years of the system lifetime due to the short lifetime of
batteries. On the other hand, inverters present about 8–12% of the
total lifetime cost of PV systems [43]. Hardware is an important driver
of PV system cost structures, constituting between 45% and 65% of
modeled total system cash purchase prices [80]. For small residential
PV systems, the world's average capital cost caries from 3000$ to
3500$ per Kilowatt, while the operations and maintenance costs are
estimated to be 1.5% of the total initial investment cost of the PV
system [81]. A summary for the system cost as well as the operation
and maintenance cost for typical systems in different developing
countries in 2013 is shown in Table 2 below [81]. The typical capital
costs of PV systems in the Sub-Saharan African region have decreased
since 2002 till 2013. The cost is in the range of 6000$ to 12,000$ in
2013 for off-grid systems, compared to an average of $18,000 in 2002
[82], $14,000 in 2006 [83], and $12,000 in 2010 [84].
8. Miscellaneous factors
Other important but miscellaneous factors also affect the output
from the PV system which should also be taken in consideration such
as:
8.1. Degradation in PV panels
Manufacturers consider a PV module degraded when its power
reaches a level below 80% of its initial power as per Wohlgemuth et al.
Several degradations can affect the performance of PV panels on the
short term as well as long term such as: degradation of packaging
materials, adhesion loss, and degradation of interconnects, degradation
due to moisture intrusion and semiconductor device degradation
[6,85]. Thomas et al. [86] showed that losses of 1–2% per year in
module performance were found in systems tested over a ten-year
period from the mid-eighties through the mid-nineties. Moreover, King
et al. [87] presented data from a poly crystalline module that was
continuously exposed to outdoor conditions in open circuit configura-
tion for eight years at Sandia which showed about 0.5% performance
loss per year. Osterwald et al. [88] showed in a recent study that Isc
losses were caused by UV absorption at the top silicon layer for both
single and multi-crystalline field-aged modules and this degraded the
performance about 0.7% per year. Other reports showed that power
degradation rates on c-Si modules ranged from 0.7% to 9.6% in the first
year of exposure and 0.7–4.9% in the second year of exposure [85]. For
example, Boron doped Cz-Si resulted in a 1–1.5% loss in the absolute
cell efficiency due to Light Induced Degradation (LID) [89]. This LID
effect causes defects in the wafer itself formed by prolonged exposure to
light [89]. Another example of degradation is the Potential-Induced
degradation (PID) which is characterized by the power loss of solar
modules under high voltage stress between framing/glass surface and
solar cells [90]. Another type of degradation is the micro-cracks of cells
[91]. Fig. 5 shows the degradation power losses due to number of
cracked cells.
Some causes of degradation are shown in Table 3 below and are
subdivided in the following section with some examples of degradation
[92].
8.1.1. Glass breakage
One of the famous failures is the glass breakage of frameless PV
modules caused by the clamps or tightening of screws in mounting.
Glass breakage leads to loss of performance on the long term due to
electrical circuit corrosion which is caused by the penetration of oxygen
and water vapour into the PV module. Glass breakage can also cause
hot spots, which lead to overheating of the module [93].
8.1.2. Hotspots
Hot spot heating occurs when a cell is shaded, damaged, or simply
generates less current than the series-connected cells in the module.
This causes the cell in a string of series connected cells to dissipate
power in the form of heat instead of producing electrical power and
also causes deformations of the p–n junction [94]. The amount of
power dissipation for any given faulty cell depends on the series-
Table 2
Typical system costs in 2013 for developing countries [81].
Off-grid
Country Typical system
size (Wp)
System cost
($/kWp)
O & M cost (% of the
initial investment cost)
Kenya 25–30 12000
Malawi 40–65 12500
Zambia 20–100 6000–10000a
Bangladesh 50 8000
Africa 2.5b
Developing world 40 8750
a
Authors’ estimate based on system costs and sizes given in the source.
b
O & M is given as a percentage of the initial investment cost of solar PV system
Fig. 5. Power loss due to cracked cells [91].
Table 3
Degradation of solar modules [92].
Component Degradations
Crystalline silicon PV module Interconnections broken
Broken cells
Corrosion
Delamination of the encapsulant
Discoloration of the encapsulant
Broken glass
Failure of the bypass diode
Failure of the weld ribbons
M.M. Fouad et al. Renewable and Sustainable Energy Reviews 80 (2017) 1499–1511
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10. parallel configuration of cells in the PV module. Generally, increasing
the number of cells in series increases the power dissipation while
increasing the number of parallel-connected cells decreases the power
dissipation of the faulty cell. The worst possible condition of hot spot
heating is to completely shade a single cell [95]. Muñoz et al. [96]
showed in one of the studies that there was an annual decrease in
energy production of 2% in one type of PV panels and a decrease of 1%
in another type due to presence of hotspots.
8.2. Characteristic resistance of PV panels
At the panel's maximum power point, there is an output resistance
which is the characteristic resistance of a solar cell. The maximum
power is translated to the load and the panel operates at its maximum
power only if the resistance of the load is equal to the characteristic
resistance of the solar cell [97].
8.3. Shunt resistance
Manufacturing defects such as impurities can cause significant
power losses which are caused by the presence of a shunt resistance.
The presence of a low shunt resistance provides an easier path for the
light-generated current. This reduces the amount of current flowing
through the solar cell and also reduces the voltage from the solar cell
[70,98]. As the shunt resistance increases, the current moving in the
load increases too as shown in Fig. 7 [99] [100], as governed by Eq. (3)
shown below:
R
nV
V IR
R
I = I − I [exp(
V + I
) − 1] − (
+
)
ph o
s
t
s
sh
(3)
where I and V are the current and voltage, Rs is the series resistance,
Rsh is the shunt resistance, Iph is the photo-generated current, I0 is the
saturation current, n is the ideality factor, and Vt is the thermal voltage
[70,101]. Shunt current can lead to cell heating and hotspots appearing
in the module's material [102]. A simple method for estimating the
shunt resistance from a solar cell is to find the slope of the IV curve at
the short circuit current point as shown in Fig. 6 below [102].
8.4. Performance ratio (PR)
It shows the relationship between the actual and theoretical energy
outputs of the PV plant which represents the proportion of the energy
that is actually available for being transported to the grid after
deducing the energy consumed and energy losses. It is observed that
PR depends on the irradiation, the tilt angle, the temperature of the
ambient, some design parameters, the quality of the modules, the
efficiency of the electric components, environmental conditions,
shades, etc. [41]. The performance ratio for mono-crystalline cells
was measured in the years 1994, 1997 and 2010 as a function of annual
irradiance on module plane and is shown in Fig. 8 below [103]. In a
study made by Reich [103] in Germany, the PR for 100 panels was
measured and was found to be between approximately 70% and 90%
with a median PR of approximately 84%.
8.5. Maintenance and cleaning of panels
A thin film of dust can degrade the performance of modules
considerably. Thus, ensuring the cleaning of panels on a periodic basis
is crucial to keep the front surface clean and ensure good performance
of the modules. Preventive maintenance of all components must be
carried out as per planned by the manual to keep the performance of
the plant considerable [41]. Moharram et al. [104] showed that the
efficiency of the PV panels has decreased by 50% after 45 days of
cleaning using non-pressurized water; however, the efficiency re-
mained constant when a mixture of anionic and cationic surfactants
was used for cleaning. Another study by Mavroidis et al. [105] used
water sprayed by a robotic device for cleaning photovoltaic panel arrays
which cools the panels while cleaning them. This further increased the
efficiency of the array by up to 15%. On the other hand, a study showed
the effect of low mass dust wiper technology with a robotic dust wiper
technology which improved the efficiency of solar panels by 7% due to
dust removal.
8.6. Sizing of PV system installed
To determine how much energy is required to run the system, the
number of PV modules needed, the power of each module and the
connections of the modules are all related to the sizing of the system. A
PV system has to generate enough energy to cover the energy
consumption of the loads and the energy used by the system itself
Fig. 6. Shunt losses in I-V characteristic curve for PV panels [102].
Fig. 7. PV performance versus shunt resistance [100].
Annual Irradiation in module plane [ kWh m-2
yr-1
]
Specific
Yield
[
kWh/kW
p
]
1994
1997
2010
Fig. 8. PR for mono-crystalline panels as function of annual irradiance [103].
M.M. Fouad et al. Renewable and Sustainable Energy Reviews 80 (2017) 1499–1511
1508
11. [58]. Several methods have been developed for the purpose of system
sizing. These methods differ in terms of simplicity or reliability. Some
analytical methods were developed to seek functional relationships
between variables of interest to the sizing problem [106].
8.7. Surface area of PV panel
The solar cell's energy conversion efficiency is calculated by dividing
the maximum power point, PMPP, by the input solar irradiance (G)
under standard test conditions multiplied by the surface area (A) of the
PV panel as shown in Eq. (4) below [14]. Thus, the amount of current
generated by a PV cell depends on its size [4].
η
P
G A
=
×
MPP
(4)
A chart showing the percentage losses, of some of the above
mentioned factors in the paper, on the performance of PV panels is
shown in Fig. 9. The effect of the other factors is still under study and
opens the door for further research in that area.
9. Conclusions and recommendations
It can be concluded that there are numerous influences that affect
the performance of the overall PV systems; some of which have positive
effects and others have negative effects on the output. It can be seen
also that many researchers have contributed in studying the factors
affecting PV panels but they didn’t focus on integrating all those factors
and each studied some selected factors only. Moreover, these studies
didn’t display a range of losses that can be obtained due to these
factors. Consequently, this study integrates all the factors that can have
key effects on the performance of the panel, it also shows the direct and
quantitative effect of each factor on the performance of the PV panel by
showing a certain range of PV panel power loses that these factors can
cause. Plus, it divides the factors into categories and subcategorizes
that were not introduced earlier in other studies which are: environ-
mental factors (external), PV system factors (internal), PV system
installation factors (operational), PV system cost factors (economic)
and other miscellaneous factors. Each of those factors has a consider-
able contribution on the system performance.
This study results can help both practitioners and researchers by
removing the encumbrance of having to search several studies for
obtaining an overall idea about the factors affecting the performance of
PV panels as this paper assimilates the environmental factors, internal
factors, installation and operational factors, economic factors and
miscellaneous factors affecting the performance of PV panels.
Researchers can further make in depth studies concerning those factors
and how to improve some of them and how to solve problems
associated with factors that negatively affect the performance of PV
panels. Also, practitioners can build models and systems that can be
used experimentally to reduce the effect of the negative factors on the
performance of PV panels such as cleaning systems for PV panels, cost-
reduction plans, maintenance plans, improving the efficiency of the
components such as inverters and batteries and other practices that
take into consideration the mentioned factors in this study.
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![Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
An integrated review of factors influencing the performance of photovoltaic
panels
M.M. Fouada,⁎
, Lamia A. Shihatab,c
, ElSayed I. Morgana
a
Mechatronics Engineering Department, German University in Cairo, Egypt
b
Design and Production Engineering Department, Ain Shams University, Cairo, Egypt
c
Design and Production Engineering Department, German University in Cairo, Egypt
A R T I C L E I N F O
Keywords:
Photovoltaic
Factors
PV panels’ performance
A B S T R A C T
The objective of this paper is to introduce the integration of the diverse factors that affect the performance of
Photovoltaic panels and how those factors affect the performance of the system. Those factors include:
environmental, PV system, installation, cost factors as well as other miscellaneous factors. Each of these factors
is further classified into novel subcategories along with the reduction/increase effects of some factors on the
system output. Solar irradiance, module temperature, dust accumulation, shading and soiling factors are some
of the environmental concerns that have major effects. Panel's I-V characteristics, inverter, battery and panel
efficiencies, panel material, atomic structure and band-gap energy are some of the system factors. As for the
installation factors, cable characteristics, angle of inclination, mismatch effects, fixed/tracking PV mechanisms
as well as MPPT are crucial to consider. Concerning the cost factors, the cables and the overall system costs are
to be taken into consideration especially during payback period calculations. Other miscellaneous factors have a
role in affecting the performance of the system such as degradations in the PV panel, panel resistances,
performance ratio, maintenance and cleaning, sizing of the system as well as the surface area of the panel. The
study results shall help both practitioners and researchers in having an inclusive overview about the several
factors that can have an effect on the performance of the PV panels by removing the burden of having to search
several studies under each category of factors. Moreover, researchers can build on this study by researching new
ways to decrease the effect of certain factors which can further enhance the performance of the system. On the
other hand, practitioners can introduce new systems that can be physically implemented in our daily lives to
hinder the effect of factors negatively affecting PV panels’ performance.
1. Introduction
Global energy consumption has been steadily growing for the past
few decades. Fossil fuel resources are decreasing while the world
energy consumption is increasing considerably. Not only that, but also,
the consumption of fossil fuels causes pollution. A considerate solution
for energy problem is utilization of renewable energies.
[1]. One of the forms of sustainable and alternative energy is the
renewable energy which has become in considerable use in today's
world due to several factors including: depletion of fossil fuels as well
as the emissions associated with the use of those fossil fuels [2,3].
There are several renewable energy sources such as: solar, wind, hydro,
geothermal, biomass energies and others. One of the most important
renewable energies is the solar energy. Energy from the sun has been
used to provide electricity for many years [4]. Solar energy is available
in abundance in most parts of the world. Solar energy can be
transformed into a useful form of energy, which is electricity, using
photovoltaic (PV) panels. A typical silicon PV cell is a thin wafer,
usually square or rectangular wafers with dimensions 10 cm × 10 cm ×
0.3 mm, consisting of a very thin layer of phosphorous-doped (N-type)
silicon on top of a thicker layer of boron-doped (p-type) silicon. The
most commonly used materials from which PV panels are manufac-
tured are mono-crystalline and poly-crystalline silicon. There is a built-
in electric field in PV panels as they contain junctions between different
materials. These junctions follow a certain behavior once the panel
absorbs photons from the sun that have an energy that is greater than
the band gap energy of the panels’ semiconductor material. Once those
photons are absorbed, electrons are freed and mobilized from the
valence band to the conduction band which creates electron-hole pairs.
Due to these pairs, DC power is generated by the flow of the electron-
hole pairs in opposite directions across the junction [5].
The amount of current and power generated by a PV cell depends
http://dx.doi.org/10.1016/j.rser.2017.05.141
Received 26 July 2016; Received in revised form 8 May 2017; Accepted 19 May 2017
⁎
Corresponding author.
E-mail address: menna-m7md@hotmail.com (M.M. Fouad).
Renewable and Sustainable Energy Reviews 80 (2017) 1499–1511
Available online 20 June 2017
1364-0321/ © 2017 Elsevier Ltd. All rights reserved.
MARK](https://image.slidesharecdn.com/fouad2017-220527172521-5c1914c1/85/fouad2017-pdf-1-320.jpg)
![Table
1
Factors
affecting
PV
panels
as
shown
in
previous
research.
Paper
Solar
irradiance
Temperature
Dust
Shading
Soiling
I-V
Inverter
η
Battery
η
Panel
material
Panel
atomic
structure
Panel
band
gap
energy
Panel
η
Cable
Char.
AOI
Mismatch
[1,4]
√
[6]
√
√
[7]
√
√
√
[8]
√
√
√
[9]
√
√
[10]
√
[11]
√
[12]
√
[13]
√
[14]
√
√
√
√
√
√
√
√
√
√
[15]
√
[17–25]
√
[31,32]
√
[33]
√
√
[34–38]
√
[39]
√
[40]
√
√
[41]
√
√
√
√
√
√
√
√
[42]
√
[43]
√
√
√
√
√
√
[44]
√
[45]
√
[46]
√
√
√
√
√
[47–52]
√
[53]
√
√
√
[54]
√
[55]
√
[56]
√
[57]
√
[59–61]
√
[62]
√
[63]
√
[65]
√
[66]
√
[68]
√
[69]
√
[70]
√
[71–106]
This
paper
√
√
√
√
√
√
√
√
√
√
√
√
√
√
√
Paper
Fixed/
Tracking
MPPT
Cost
of
cables
Cost
of
system
Degradation
PID
degradation
LID
degradation
Glass
breakage
Hotspots
Characteristic
resistance
of
PV
Shunt
resistance
Performance
ratio
Maintenance
and
cleaning
Sizing
Surface
area
[1,4,6–
12]
[13]
√
√
[14]
√
[15,39]
[41]
√
√
√
√
[43]
√
√
(continued
on
next
page)
M.M. Fouad et al. Renewable and Sustainable Energy Reviews 80 (2017) 1499–1511
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