2. and therefore they should be considered together. Mani and Pillai [13]
reviewed the dust effect on PV module performance during two time
periods, 1940–1990 and 1990–2010. They developed and used an ap-
propriate cleaning/maintenance cycle for their PV systems that con-
sidered the prevalent climatic and environmental conditions. However,
such studies focused only on changing parameters that significantly
affect PV module performance without investigating effective ways of
mitigating their impact.
This study presents a review of recently published studies on factors
influencing PV module performance, particularly dust fouling, and
different methods of mitigation. The first part focuses on how perfor-
mance is impaired due to a variety of environmental conditions. An
overview of studies describing the mechanisms that cause these effects
and how they affect each other is also given. In the second part we look
at some of the past, current, and promising future approaches for mi-
tigating dust-fouling effects and summarize how the effectiveness of
these mitigation techniques is affected by other environmental factors.
2. Effect of climatic and environmental conditions on PV module
performance
The variation of climatic conditions from location to location
around the world has a corresponding effect on PV module performance
in different regions [7]. The parameters that may impact PV module
performance include solar radiation, wind velocity, rainfall, tempera-
ture, humidity and the probability of dust presence. The following sub-
sections summarize the effects of each of these conditions on PV module
performance, as reported in the literature.
2.1. Effect of wind velocity
Wind velocity can have both positive and negative effects on PV
module performance. The impact of wind velocity on PV module per-
formance is mainly a function of wind speed and direction, PV module
surface structure, and dust deposition. In an outdoor environment, wind
velocity, ambient temperature, surface structure, and sun irradiance
influence module temperature.
2.1.1. Dust deposition impact
Wind blows away dust particles from the PV module surface, which
can reduce dust deposition [14]. In Egypt, it is observed, a decrease in
the rate of dust deposition occurs on a module at a particular tilt angle
due to wind blowing after 2 weeks of exposure to weather conditions
[15].
However, the wind can also add to the negative effect of dust by
transferring and spreading dust and sand particles within the atmo-
sphere, which may lead to increased deposition layers. As the wind
speed increases more dust deposition will occur, which results in de-
terioration of solar cell's fill factor [12]. In the Greatest Desert-Libya,
Clarke et al. [16] reported that increase in dust deposition generally
coincides with increase in mean monthly wind speed. For example wind
velocities of 6.5 m/s represent the minimum threshold velocity for dust
entraining winds in Libya [17]. To simulate the effect of wind speed and
direction on dust deposition and accumulation and resulting PV module
performance, Goossens et al. [18] performed a wind tunnel simulation
and field experiments. Their results indicated that wind direction af-
fects dust deposition and distribution more than wind speed does.
However, it is concluded that the reflected effect of wind on dust
deposition depends on the wind speed. The wind speed was tested in
range between 0.63 m/s and 2.59 m/s. High wind speeds (2.59 m/s)
lead to high dust accumulation on a cell, resulting in sharp performance
drops. In cases of low wind, dust accumulation is smaller, and the drop
in cell performance is less [19]. Additionally, it is observed that the
effects of Martian dust particle size on PV performance vary with wind
velocity. When the wind velocity was very high (89–116 m/s) there was
no significant difference between the degradation caused by large
particles (> 75 µm) and that caused by small particles (30 µm) [20].
That results was confirmed by who observed that the surface mass
density of the dust deposition was negligible at a high wind velocity
which is greater than 24 m/s [21].
2.1.2. Wind speed and direction impact
The wind enhances dissipative convective heat transfer from the
module, thereby reducing its temperature, which helps maintain its
conversion efficiency [5,22–25]. For example in Dhahran, KSA in-
creasing the wind speed from 2.8 to 5.3 m/s resulted in reduction in
module temperature within a range of 10 °C [26]. In Slovenia, the
difference between the module temperature and the ambient tem-
perature for any non-zero conversion efficiency (regardless of the
mounting conditions) reduce to half the value, if the wind speed in-
creases from zero to 12 m/s [27]. In Greece, the findings of an outdoor
measurement clearly indicated that the difference between modular
and ambient temperature (Tc-Ta) decreases with increasing wind speed,
as shown in Fig. 1 [28]. It is also observed that the difference between
the cell and ambient temperatures is between 10 and 20 °C during calm
spells, then demonstrating a gradual shift to zero for high wind speed
cases [28]. However, under series of artificial tests for wind speed and
direction effects on PV performance, it was found that the PV cell
temperature rise over the ambient one is extremely sensitive to wind
speed, less so to wind direction [29].
2.1.3. PV surface structure impact
It has been reported that the surface structure has a significant
cooling effect, increasingly pronounced at higher wind speed [30]. At
higher wind speeds, the module temperature decreased due to the in-
creased convective module cooling. Experiments carried out in a wind
tunnel in USA showed that at high wind speed, structured glass modules
operate at lower temperature than flat glass modules. Fig. 2 shows that
at a wind speed of 10 m/s, a module fabricated with grooved glass
Fig. 1. Impact of wind speed on temperature difference between PV modules and the
environment-case study in Greece [28].
Fig. 2. Effect of wind speed on module temperature (temperature difference compared
with flat glass module) in USA [30].
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744
3. cover was found to be 3.5 °C cooler than a flat glass cover [30].
Duell et al. [30] indicates that at low wind speed (i.e. when the wind
speed is less than 3 m/s) the structured glass modules temperature was
relatively higher than that of flat glass modules. This behavior is at-
tributed to increasing in light transmission to the cells. Interestingly, as
the wind speed increased, the structured glass modules were observed
to be progressively cooler than the flat glass modules. Up to a 2.5 °C
temperature difference was observed at 12 m/s winds speed for the
grooved glass module. This is attributed to increase in convective
cooling of the structured glass [30]. Whereas in Dhahran, KSA, four
glass surface structures were tested at outdoor conditions, with the
wind ranging between 0 and 10.5 m/s during the testing period. The
modules included a flat surface with anti-reflection coating, a flat glass
surface with no anti-reflection coating, a micro-textured surface coated
with anti-reflection coating, a micro-textured surface with no anti-re-
flection coating, and a mm-scale textured surface. All tested modules
except mm-scale texture showed an increase in their temperature re-
gardless of the wind speed. The average ranges of increase in modules
temperature was 1–2 °C as shown in Figs. 3 and 4 [26].
2.2. Effect of temperature
The PV modules transform only a small portion of solar radiation
into electricity and the rest into heat. Therefore, the modules tem-
perature increased and their electrical efficiency is reduced [31]. The
PV cell performance is sharply sensitive to cell temperature which is
function of weather variables (ambient temperature) solar irradiance
[32–35] cell material and module encapsulation absorption [12].
However, the expected temperature coefficient and PV performance
given by the manufacturers, did not always fit the real modules per-
formance [36]. As module temperature increases, the band gap of the
cells usually decreases, resulting in the absorption of longer wavelength
photons, and the minority carrier lifetime generally increases. These
factors slightly increase the light-generated current (Isc) but lead to a
reduction in the cell's open circuit voltage (Voc), which results in the
cell's fill factor overall reduction. The fill factor determines the module's
overall power and hence its efficiency [37]. An obvious drop in PV
performance has been observed during high-temperature periods (as
shown in Fig. 5) [38–42].
Katkar et al. [43] used an environmental chamber to test the effect
of temperature on the performance of industrial solar cells. The effi-
ciency of the solar cells was seen to increase from 9.7% at 31 °C to
12.0% at 36 °C, beyond which the efficiency started to decrease, as
shown in Fig. 6.
Hence, the output power reduction mainly depends on panel
mounting and weather conditions [1,44]. Some studies showed that
power output of a PV module decreases by about 0.5% per degree (°C)
of cell temperature rise [28]. Preliminary results with respect to module
efficiency demonstrated that the single crystal silicon solar cell effi-
ciency highly depends on cell operating temperature. It was observed
that at 64 °C operating temperature, the efficiency of the solar cell
decreased by 69% compared with that measured at STC [45]. At out-
door conditions of 1000 W/m2
irradiation level without cooling, the
cell temperature increased to 56 °C this increase lead to 3.13% decrease
in module electrical efficiency [46]. Another study indicated that for an
increase in module temperature from 43 to 47 °C, the module efficiency
decreased by around 5% which indicated the effect of wind speed
variation on the rate of temperature increase [40]. Literature reports
the maximum PV efficiency at 25 °C ambient temperature. In contrast,
Garg [47] reported that the DC output obtained at 40 °C ambient
temperature in Western Rajasthan, India.
On the other hand, it was noticed that mounting the module on a
tracker system makes the hourly backside temperatures of the solar PV
module higher than the temperature of modules mounted on fixed
stand. This is due to the fact that modules with tracking systems receive
more solar radiation and hence their back temperature is higher as
shown in Fig. 7 [40]. Hence, the simplified proposed linear correlations
between both module electrical efficiency and its power output with its
operating temperature is considered to be doubtable; as the relations
Fig. 3. Average temperature increases of anti-reflective coated and textured modules with
respect to flat glass modules – Dhahran, KSA [26].
Fig. 4. Effect of wind speed on modules temperature – KSA [26].
Fig. 5. Solar panel temperature and Efficiency [38].
Fig. 6. Solar cell efficiency variation with cell temperature [43].
S.A.M. Said et al. Renewable and Sustainable Energy Reviews 82 (2018) 743–760
745
4. were extracted at a specific mounting geometry or building integration
level. Thus, one must be careful in applying a particular relation for a
certain conditions [48–50].
2.2.1. Solar based material impact
Temperature coefficients of modules are specified by heating the PV
module to a predetermined temperature (commonly 80 °C). The I-V
characteristics measured under a sun simulator as the module was al-
lowed to cool down uniformly to ambient temperature (25 °C). Indeed,
the effect of module temperature on module performance varies with its
cell material based. For example, At 1000 W/m2
irradiance, The max-
imum powers of all three modules have relatively low temperature
dependence as compared to mono-crystalline and multi-crystalline
modules [41]. Also, it was found that the a-Si:H/aSiGe:H tandem solar
cell maintained a higher output power than the others even after
longtime operation during which a temperature range of 25 °C to 80 °C
[51]. Monocrystalline maximum efficiency (78%) was achieved at high
temperature 49.9 °C at 12:45 p.m. But the maximum efficiency for
amorphous PV is 61.6% corresponding to lowest temperature 40.9 °C at
15:45 p.m., where Δ Efficiency/1 °C for monocrystalline is −0.010 and
for amorphous equals −0.030) [30]. Hashim [52] compared the PV
performance (mono-crystalline Silicon (mc-Si), poly-crystalline Silicon
(pc-Si), amorphous Silicon (a-Si) and Cupper Indium Gallium di-sele-
nide (CIGS) under climate temperature of Baghdad city. The results
revealed that the output power dropped with temperature by −0.0114,
−0.0915, and −0.0276 W/°C for a-Si, pc-Si and CIGS, respectively
while the maximum reported drop was −0.1353 W/°C for mc-Si. Based
on field studies, Table 1 compares the effect of different solar cell
materials on the overall efficiency.
The hourly backside temperature profile of the polycrystalline
module (HDS130P), mounted on the fixed stand, is compared in Fig. 8.
The modules were mounted in sun tracker that focused directly to solar
radiation. Therefore, the PV modules stores heat and the module
backside temperature is greater than the ambient temperature. The
results indicated that the polycrystalline module generally stores more
heat than monocrystalline modules subjected to similar outdoor con-
dition [40].
2.2.2. Material encapsulating impact
Temperature effects on PV performance strongly depend on the
material used to encapsulate the PV module. The thermal dissipation
and absorption properties of the materials encapsulating the module
dictate the effects of temperature. Highly dissipative materials can
mitigate the heating effects of climate factors. On the other hand, highly
absorptive properties can exacerbate heating effects from climate fac-
tors [54,55]. The correlations for the PV operating temperature are
either explicit in form, thus giving Tc directly, or they are implicit. The
temperature of the cells within a PV module, i.e. Tc, may be higher than
the back-side temperature, Tb, by a few degrees, their difference de-
pends on the module substrate materials and on the solar radiation flux
levels [49].
Historically, the encapsulant copolymer ethylene vinyl acetate
(EVA) which is shown in Fig. 9 was commonly used in nearly all PV
modules [56]. Over the years, in addition to ethylene-vinyl-acetate
encapsulant (EVA), different encapsulant materials have been used in
PV modules such as Polyvinyl Butyral (PVB) and silicone rubber. A
number of studies have shown the relative advantage of silicone en-
capsulant over EVA [57]. It has been demonstrated that silicone en-
capsulated modules outperform EVA encapsulated modules at levels
higher than 2% kWhr/kWp relative efficiency gain after exposing
modules to all seasons, experiencing temperatures from approximately
95°F to − 10°F, and sun, rain and snow conditions [58]. Walwil et al.
[59] Compared the temperature impact on commercial glass cover and
three different commercial PV module encapsulates (EVA, silicone and
Ionomere encapsulate). It was found that the temperature in all coated
and textured modules increased by 0.5–3 °C compared to flat surface
Fig. 7. Efficiency vs. temperature [40].
Table 1
Shows the overall efficiency drop of different solar cells with their testing temperature
reported from different fields.
Study & location Solar cell based Testing
temperature
Efficiency
Drop
Greece [28] mono-crystalline 20 °C 0.5% /C
Saudi Arabia [1] multi-crystalline 25 °C 16.5%
Saudi Arabia [1] thin film 25 °C 0.48%/C
Japan [53] amorphous 20 °C 15%
Japan [51] hydrogenated
amorphous silicon
25 °C 8.5%
Japan [51] hydrogenated
amorphous silicon
germanium
25 °C 8.0%
Fig. 8. Hourly backside temperature of PV modules [40].
Fig. 9. Schematic view of a PV module with ethylene vinyl acetate (EVA) encapsulant
[60].
S.A.M. Said et al. Renewable and Sustainable Energy Reviews 82 (2018) 743–760
746
5. cover. However, the performance of encapsulation will depend strongly
on the location of the modules and the weather temperature to which
they are subjected [57].
2.3. Effect of humidity
An increase in solar panel efficiency is observed at low relative
humidity. Suspended water vapor droplets in the atmosphere during
humid days, particularly those that have a size larger than the wave-
length of the solar beam, can scatter, refract, or diffract incident solar
light. Hence, the humid conditions can reduce the power produced by
the solar module [12,61]. Conversely, Omubo-Pepple [38] studied the
effect of relative humidity and solar flux on the PV module efficiency
using a B-K Precision Model 615. The conversion efficiency of solar
panel was calculated using the following formula:
=
×
×
Power of solar panel
Area of solar panel
Efficiency
100%
100
W
m2
The result indicate that a direct proportionality between, output
current, solar flux and PV module efficiency. Also the authors reported
that a current of 1.842 A produced 82% conversion efficiency.
Therefore, it has been reported that at low relative humidity the solar
flux increases, and thus, the solar panel efficiency becomes higher (as
seen by Fig. 10) [35,38,39] and Fig. 11 [62].
The effect of relative humidity on efficiency of solar panel is also
reported by Ettah et al. [35] (Fig. 12). The efficiency of solar cell in-
creases from 9.7% at 60% humidity to 12.04% at 48% humidity [43]. In
terms of the output power, it has been reported that the power de-
creased by about 3.16 W with a 20% increase in relative humidity [36].
Also, in MENA region, Ramli et al. [63] studied the reduction of PV
output power under rainy and cloudy conditions. The experimental
results revealed that, under rainy conditions, the PV output power de-
creased by 40% at relative humidity of 76.3%, and a decrease in output
power during cloudy conditions by 45% at 60.5% relative humidity. On
the other hand, in India, Nair et al. [64] elucidated that humidity might
have a positive impact on PV output power. This can be attributed to
the other environmental factors influencing the dust accumulation. It
was observed that, near saturation humidity (above 60%), the PV
output power increased by 6–12%.
2.3.1. Humidity and dust adhesion
It is observed that dust sticks to modules glass covers due to hu-
midity which thus requires vigorous but careful cleaning action to re-
store modules to their initial power outputs [40]. For example, the
countries close to the Mediterranean Sea such as Spain, the registered
values of humidity are high, as a result the adherence of dust particles
on the surface of the modules is also high [65]. Quantitatively, it has
been found that an increase in relative humidity from 40% to 80% lead
to increase in the adhesion of around 80% as shown in Fig. 13 [66]. In
addition to promote the adhesion of dust, high relative humidity (RH)
create formation of sticky and cementing dust layers on PV surfaces
Fig. 10. Efficency vs. relative humidity [38].
Fig. 11. Correlation between humidity and PV output [62].
Fig. 12. Efficency vs. relative humidity [35].
Fig. 13. Relation between humidity and adhesion [66].
Fig. 14. Optical performance vs. relative humidity, during exposure to dust, for uncoated
glass and glass with an anti-soiling hydrophilic coating [67].
S.A.M. Said et al. Renewable and Sustainable Energy Reviews 82 (2018) 743–760
747
6. [21,62].
Fig. 14 compared the optical performance (transmittance percen-
tage) of dusty coated glass samples with respect to clean glass substrate.
The optical performance decreased with increasing the dust amount on
the surface. However, the humidity effect on dusty glass cover surface is
significant measured as below 50%RH as shown in Fig. 14. It is also
observed that applying a hydrophilic anti-soiling coating to the glass
lead to reduction in the amount of soiling on the surface, although there
is still a measureable effect of %RH as shown in Fig. 14 [67].
2.3.2. Moisture ingression impact and encapsulation
Exposing the PV module to humidity for long time causes de-
gradation in the module performance due humidity ingression to solar
cell enclosure [12]. Studies of PV module performance in a humid en-
vironment (produced by a humidity chamber) demonstrated an adverse
effect due to water vapor, moisture and oxygen ingress into the solar
cell enclosure. The presence of these substances lead to corrosion and
power degradation, as shown in Fig. 15 [68–70]. Water condensing at
the interface between the encapsulant and the solar cell materials
create areas of increased corrosion rate and the risk of encapsulant
delamination [54,70]. Significant reduction of the moisture ingress
requires a true hermetic seal, the use of an encapsulant loaded with a
desiccant, or the use of an encapsulant with a very low diffusivity [70].
Therefore, some proposed techniques were used to assess and monitor
the moisture ingression rate in PV modules for long term out door PV
system. This monitoring techniques provide the manufactures with
detailed information about encapsulate diffusion coefficient and water
vapor transmission rate [71]
2.4. Orientation effect
A perfect tilt angle of the PV module affecting significantly the
amount of solar radiation that falls on the PV panel surface. The max-
imum PV generation is greatly affected by optimal tilt angle which
depend on several condition such as the geographic latitude, utilization
period, surroundings, climate, dust, pollution and other atmospheric
factors. It was reported that the optimum tilt angle with respect to local
latitude can be considered as (latitude ± 15°) [72]. Based on a series of
experiments, the authors proposed a more accurate formula that is
(latitude ± 8°), while others suggested that latitude ± 2.8 according to
specific coastal radiation data. However, the accuracy of the proposed
formulas is not always guaranteed because of the geographical condi-
tion limitations [72].
Navid et al. [73] proposed a method to identify the optimum or-
ientation and location of the PV system based on the generated output
data. Their model provide an accurate estimation for azimuth, tilt and
PV panel location. The result conclude that, for typical PV system, la-
titude, longitude, azimuth and tilt can be estimated as 4.08°, 0.2°, 5.85°
and 2.75° respectively. Tarek et al. [74] investigated experimentally the
optimum tilt angles of PV panels installed in Saudi Arabia. MATLAB
software was used in the simulation part to optimize the appropriate
orientation. It was found that the tilt angles should be adjusted six times
yearly in order to harvest 99.5% of the incoming solar radiation in the
tested region.
Rhythm and Rangan [75] compared the performance of large scale
rooftop solar PV scenario under different panel orientations in Mumbai.
Different orientations were compared and analysed (fixed tilt orienta-
tion, horizontal E-W axis N-S tracking and two-point system orienta-
tion). The analyses revealed that the optimum tilt angle for the best
performance is same as the region latitude (19°). Moreover, the hor-
izontal E-W axis N-S tracking orientation provided 10.2% higher plane
of array while the two-point system orientation produced 2.2% higher
plane of array as compared with fixed tilt.
As discussed earlier, the environmental parameters (e.g. tempera-
ture) affecting significantly the PV module performance. It was re-
ported that overheating because of high ambient temperature and ex-
cessive solar radiation will significantly reduce the PV panel efficiency.
Sharaf et al. [76] study the feasibility of using PV solar tracking system
in cold and hot regions. They developed a mathematical model that
validated experimentally under different operating conditions (hot and
cold). The result revealed that 8% gain in electrical energy in the hot
city (Aswan, Egypt) while the gain can reach up to 39% in the cold
region (Berlin, Germany). The variation of electrical energy gain can be
attributed to PV overheating effect. Therefore, if the required energy to
run the tracking system reached 10% then using tracking system in the
got region will not be feasible. It can conclude that the energy cost
produced from PV fixed system is lower than the energy cost generated
from tracking PV system due to the initial and running cost of the
tracking system.
Rustu and Ali [77] studied and compared the performance of wo
double axis sun tracking PV system after one year of operation. The
analysis were carried out using both, experimental measurement and
simulation with difference less than 5%. It was found that PV electricity
obtained in the double axis sun-tracking system was greater by 30.79%
compared with the latitude tilt fixed system. The performance in-
vestigation done by Bashar et al. [77] revealed that the annual pro-
duction of the tracking system was 31.3% which higher than that of the
fixed system. In addition, the annual conversion efficiency of the
tracking system was 13.85%, while it was 13.83% for the fixed system.
It can be clearly observed that using of solar tracking system is better
than the fixed system from efficiency and electricity generation point of
view. However, the economic feasibility of using tracking system will
be analysed in the following section.
2.4.1. Economic feasibility of PV tracking systems
Few countries around the world produce solar trackers (e.g. Spain,
US, Germany and China). Various prices of solar trackers were observed
based on and the manufacturer and the implemented method. Bahrami
et al. [78] studied the techno-economic feasibility for using single and
dual-axis solar tracking PV panels in 21 low latitude countries (from 0
to 15° N) located at Sub-Sahara Africa, Latin America and Southeast
Asia. They used Perez and Koronakis sky diffuse radiation models to
predict the electrical output energy from PV panels. The observed
maximum annual electrical energy from 1 kWP PV panel was 2024 kWh
which contribute significantly to alleviating the chronic energy
Fig. 15. Humidity effect on PV module power output [68].
Table 2
Prices of different solar tracing system.
Axis type Solar tracking system for 1 Kwp Cost US$ Reference
Dual-axis Dual-axis tracking system 600–1900 [79,80]
Single axis V axis tracking system 350–930
IEW tracking system 205–840
EW/NS tracking system 135–700
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7. shortage. Table 2 compares the installation cost (minimum and max-
imum prices) of different tracking technology based on the levelized
cost of electricity (LCOE) for 1Kwp. It was concluded that for the single
axis tracker, EW/NS tracker was the most cost effective tracker system
due to the simple rotation formulation of the PV panel around the single
axis while the vertical axis tracker (V) was the most expensive option
that must follow the sun azimuth angle that varies during the year.
However, the dual-axis tracker is more expensive option than the
single-axis trackers.
Fewer studies compared tracking to fixed PV systems from economic
point of view. Generally, the economic related study was based on some
parameter such as Payback Period, Net Present Value, Levelized Cost of
Energy, Net Present Benefits and Internal Rate of Return. Bianchini
et al. [81] reported that the feasibility of tracking systems is greatly
dependent on tariff policies, tax and system initial cost.
Bashar et al. [82] compared the economic parameters of fixed and
double-axis tracking PV grid-connected systems installed in Jordan over
20 year. The economic analysis was conducted based on the electricity
cost, payback period and internal rate of return. The payback period
revealed that the fixed system was 7 months shorter than the 35 months
for the tracking system. In addition, internal rate of return for the
tracking system was 8.7% lower than that of fixed system. Finally, the
electricity cost that calculated as the annual installments divided by the
annual energy production (US$/kWh). The results have been sum-
marized in Fig. 16. It was observed that the electricity cost increased
with the time due to the annual degradation modules. for the tracking
system was 20% more expensive than that of the fixed system (0.08 US
$/kWh) for the first year and the difference grew over years.
Although the energy prduced by fixed system was lower than the
energy prduced by tracking system, the economic analysis did not
support the installation of double–axis tracking system in Jordan or in
countries of similar geographical location.
2.5. Shading effect
The PV module output power is greatly affected by partial or
complete shading that depend on module position, array configuration
and shading scenario [83]. Shadowing scenario plays a major role in
the PV module performance that affects the current flow in the shaded
cells. The cells can be shaded by trees, poles, bird droppings, buildings
and dust accumulation [84,85].
Another type of partial shading is called self-shading that caused by
the preceding row of PV modules. A careful planning will minimize the
effect of self-shading but it is impossible to avoid. Self-shading in
mainly depend on solar elevation angle, minimal distance (d) and in-
clination angle (Ɓ), as shown in Fig. 17 [86]. Etal [86] developed an
empirical formula that relate only the self-shading losses with spacing
factor (F). The relative annual energy losses (RAEL) can be represented
by:
= + < <
−
e
RAEL A. –0.00 1 F 0.01; 1.5 F 5
F
2.3
Where A is an energy loss parameter and F is the spacing factor =
d
b
.
This formula is derived only for 30° inclination angle in Slovenia and it
can be valid for all other places with some modification in terms of
diffuse part fractions.
Fatih et al. [87] studied the electrical performance of PV panels
under partial shading conditions. A thermodynamics analysis and ex-
perimental tests were carried out for three different shading scenario
(cell, vertical and horizontal shading) at several shading ratio percen-
tage. The result revealed that shading has a significant effect on exergy
and energy efficiencies. The maximum power loss was 69.9% in cel-
lular, 99.9% in horizontal and 66.9% in vertical shading.
2.5.1. Modelling of partial shading
The multiple peaks on the power–voltage characteristics curve were
due to the Partial shading PV arrays. The common global searching
techniques (e.g. perturb and observe) might fail in global maximum
searching in terms of implementation complexity and speed. Therefore,
a fast global tracking method should be proposed. Tsang and Chan [88]
proposed current based method instead of the conventional voltage
based method. This technique provided a good estimation of the global
maximum power point under partial shading conditions.
Advanced models are highly important in order to track and eval-
uate the PV system performance under shading condition. Many models
reported in the literature to investigate partial shading condition such
as by-pass diode, Lambert W function, voltage band, two stages
Fig. 16. The electricity cost for tracking and fixed PV system in
US$/kWh [82].
Fig. 17. Self-shading caused by the preceding row of PV modules [86].
S.A.M. Said et al. Renewable and Sustainable Energy Reviews 82 (2018) 743–760
749
8. successive estimation, predictive Model and Bishop model etc. [89].
Belhachat and Larbes [90] studied and analysed the performance of
different PV array configuration (series, parallel, series–parallel, total-
cross-tied, bridge-linked and honey-comb) under different partial
shading conditions. Bishop model was used and implemented by Si-
mulink Power software. The analyses of these configurations revealed
that the optimal configuration strongly depended on shading type
(uniform or not), shading pattern and location and the intensity of
shading. It was reported that according to their analysis, the total-cross-
tied configuration provide the best performances under all partial
shading conditions. Similarly, Mahmoud et al. [91] compared and
analysed the same multiple PV array configurations under faulty PV
conditions and different partial shading conditions. MATLAB/Simulink
software was used to study different indicators such as open circuit
voltage, short circuit current, current-voltage at maximum power point,
thermal voltage and fill factor. This study provide a useful data to
predict the partial shading conditions and detect the faulty PV module
in the tested PV array configurations.
2.6. Effect of dust
Efficiency of PV modules is governed by alterable and unalterable
factors, thus dust is an unalterable factor which falls under the classi-
fication location-dependent environmental factors [92]. Dust has two
possible effects on the energy output of PV modules. First, suspended
dust particles in the atmosphere that have a size larger than the wa-
velength of the incoming solar beam can scatter the sunlight, reducing
the amount of solar radiation that reaches the surface of the module.
This effect becomes worse when combined with the effect of air pol-
lutants [93]. A reduction in PV module energy output of over 60% has
been attributed to the presence of dust and air pollutants (i.e. toxic
gases and suspended particles) in the atmosphere [94]. The second
effect relates to the formation of a thick layer of dust on the PV module
surface. This layer can change the surface optical properties to promote
light reflection and absorption and reduce surface transmissivity, and
hence PV module output. Table 3 summarizes some of published lit-
erature in relation to the effect of dust on PV performance.
There are variation in dust accumulation effects from one region to
another. These can be attributed to site conditions such as wind velo-
city, humidity and rainfall, PV module technology used, source of dust
particles, particle type, and the material of PV module surface cover.
Significant reductions in power output were reported in regions that
suffer from a lack of rainfall, and are thereby prone to high levels of
dust, such as countries in the Middle East and North Africa (hot and
humid environment), as shown in Table 4. In Dhahran, KSA, where
rainfall is very scarce, after six months of outdoor exposure, a 50%
reduction in PV power output has been reported by [26,40,66,95], and
a 2.78% reduction in short-circuit current per day has been reported in
Arar, KSA [96], where rainfall conditions are similar. In Egypt, where
rainfall is again also scarce, a 60–70% power reduction has been re-
ported over a period of six months with a corresponding glass trans-
missivity reduction of approximately 20% after one month of outdoor
exposure [15,97].
The magnitude of the effect of dust on PV cover transmissivity and
module performance are primarily affected by dust deposition rate, as
shown in Table 5. Generally, an increase in dust deposition leads to a
drop in PV module performance [104–108], due to a decrease in its
cover's spectral and overall transmissivity [109,110]. This relationship
is not exactly linear due to the nature of the distribution of dust par-
ticles on the PV cover [111,112]. Different studies have quantified the
magnitude of PV performance reduction for various dust deposition
rates, as shown in Fig. 18. Reductions in conversion efficiency of 10%,
16% and 20% were observed for dust densities of 12.5 g/m2
, 25 g/m2
and 37.5 g/m2
, respectively [113]. Moreover, another study observed
that PV module efficiency was reduced by up to 26% when dust was
deposited at a density of 22 g/m2
[69]. However, many models have
been developed to simulate the PV overall transmissivity drop due to
dust deposition rate. Based on NASA model, Wang and Gong [114]
developed an improved tilt and incident angle model to provide com-
plete analysis. These models can be applied to evaluate the effect of
dust deposition rate in the parametric PV systems design.
Dust density does not increase linearly with exposure time, but
strongly depends on climatic conditions during the exposure period
[120]. For instance, the transmittance amount reduction is not uniform
and depends on climatic conditions and sand storms frequency. Hence,
the relation between dust deposition rates and glass transmittance re-
duction is nonlinear, as shown in Fig. 19. In some regions, dust density
may drop due to rainfall and wind [121]. Greater exposure times
generally lead to greater dust accumulation, and hence more significant
PV degradation [15,97,122,123]. For this reason, it is recommended to
employ an appropriate cleaning cycle to recover maximum PV module
output [13,40,98], and counter the adverse impact of environmental
conditions. The optimal cleaning frequency depends mainly on the re-
gion environmental conditions.
Table 3
Observed dust effects on PV performance in different countries.
Study & location Duration Dust effect
KSA [95] One Month The average degradation rate of the efficiency was 7% per month
KSA [66] Five weeks The average power reduction without any cleaning action was around 6%
KSA [26] Six weeks The average power reduction was 13% for a plain glass module
KSA [40] Six months The output power decrease by as much as 50% without cleaning
Qatar [42] 100 days The efficiency decreased by around 10% because of dust accumulation
Palestine [98] One week 5 to 6% decrease in the solar panel efficiency
Egypt [99] 21 days 5% reduction of the output for 15° tilt angle
Egypt [97] One month The output power decreased about 17.4% per month
UAE [100] Five weeks The output power reduction after 5 weeks was about 10%
Spain [101] Two months After 15 days without rain the losses were greater than the 4%. The losses reach up to 15% after 2 months without raining
USA [102] Two months With soiling rates of less than 1.0% per month in the low desert and peak rates of 11.5% per month in heavy agricultural regions of the Central
Valley, California.
Athens [103] Eight weeks 6.5% power output reduction
Table 4
Observed onsite module power output degradation for locations known to suffer a lack of
rainfall in MENA region (hot and humid environment).
Location Yearly rainfall
(ml)
Decrease in power
output on site
Period of
exposure
KSA [40,95] 6–10 50% 26 weeks
UAE [100] 80–90 10% 5 weeks
Qatar [39] 70–75 10% 14 weeks
Palestine [98] 30–40 5–6% 1 week
Egypt [15,97] 18–50 60–70% 26 weeks
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9. 2.6.1. Module glass covers impact
Dust accumulation varies from surface to surface as shown in
Table 6. Garg [124] studied the effect of dust accumulation on the
transmittance of light through glass and plastic plates for two months in
India, with the plastic plate showing a greater reduction in transmit-
tance. Similarly, Nahar and Gupta [125] observed that polyvinyl
chloride (PVC) plates showed a greater reduction in transmittance than
acrylic, which, in turn, suffered a greater reduction than glass. Simi-
larly, Said et al. [26] indicated that texturing a module's surface and
adding an anti-reflective coating boosts the power output of a clean PV
module by an average of 4–8%. It is also shown that texturing and anti-
reflective coating of PV modules glass cover can relatively reduce
power reduction due to dust coverage [101,126,127].
2.6.2. Field dust particles characterization
Dust particles are characterized to better understand the cause of
the effect of dust deposition on PV module performance and to come up
with effective mitigation techniques. Dust particles characterization
methods include particle sizing, morphology analysis and chemical
composition analysis.
2.6.2.1. Particle sizing analysis. The particle sizes and morphology have
been determined using both optical techniques (including commercial
particle analyzers), scanning electron microscopy (SEM), and recently,
scanning probe microscopies. These important studies involved
evaluating particle-size distributions (Fig. 20) using various
microscopy techniques. Table 7 shows the dust particle sizing and
distribution morphology characteristics for dust collected from
different cities in different countries.
The literature indicates that the deposition of finer particles has a
more significant effect on PV module performance than that of coarser
particles. El-Shobokshy and Hussein [111] studied carbon, cement, and
three types of limestone particles having median diameters of 5, 10, 50,
60, and 80 m. The dust particles were deposited on a PV surface at a
controlled surface-mass density, and the power output was measured.
Depositing equal densities (surface mass density of 25 g/m2
) of lime-
stone particles of different sizes on a PV module surface showed that the
finer particles caused a greater power reduction. Their results also
showed that normalized power output in the case of cement and carbon
particles dropped by 40% and 90%, respectively. This was attributed to
the greater uniformity of distribution of the fine particles on the surface
causing more scattering losses [111]. Moreover, high wind speeds re-
move coarser particles more effectively than fine ones [112]. On the
other hand, gravity influenced significantly the dust deposition rate.
The deposition rate due to gravity for small dust particles (Dp < 5 µm)
was 5% and increased to 75% for large dust particles (Dp > 5 µm)
[132].
2.6.2.2. Particle chemical analysis. The nature of the dust particles,
including chemical composition and color, plays a major role in the
degree of reduction in glass cover transmittance and hence PV
performance. In Greece under cloudless sky, Kaldellis et al. [108]
have deposited various specific red soils, limestone's and ash's
deposition densities on the surface of PV-panels implying the
deterioration of their performance when dust particles are deposited
on their surface. The mean power reduction between the clean and the
polluted PV pair, vary from approximately 3 W to 5 W for red soil
particles’ accumulation ranging from 0.12 to 0.35 g/m2
, 4 W to 7 W for
red soil particles’ accumulation ranging from 0.28 to 1.51 g/m2
and
1–8 W for red soil particles’ accumulation ranging from 0.63 to 3.71 g/
m2
respectively. These results indicated that red soil deposition on PVs’
surfaces causes the most considerable impact on PVs’ performance and
thus the highest reduction in generated energy, followed by the
limestone and secondly by the carbon-based ash. An amount of
0.35 g/m2
of red soil deposition on PV-panels’ surfaces may reduce
the generated energy by almost 7.5% while approximately the same
deposition density for limestone (0.33 g/m2
) causes almost 4% energy
reduction.
On the other hand, an indoor experimental study was conducted at
Saudi Arabia to examine the degree of degradation due to different
deposits type [111]. The PV panel, the sun simulator and the experi-
mental dust (carbon, limestone and cement) were the main components
used in the experimental setup. It was concluded that the fine dust
Table 5
Observed dust effect on PV glass cover transmissivity.
Study & location Period of exposure Transmittance reduction
KSA [26] 45 days The overall transmittance of glass cover was reduced by 20%
KSA [66] 40 days The spectral transmittance reduction was 37%
Belgium [115] 5 weeks The transmittance decrease between 3% and 4% after 5 weeks of exposure
Egypt [15] 30 days The glass loss 20% of its transmittance for an inclination of 30°
Algeria [116] Indoor (2 h) The optical transmission drop is 16%
Thailand [117] 30 days The global transmittance reduced from about 87.9–75.8%
UK [62] Not reported The transmittance reduced by 5% with particle sizes typically in the range 1–500 µm,
USA [67] 4 months 25% reduction of glass transmittance
South Africa [118] one week Transmittance reduced by 1–2%
China [119] 8 days The relative PV module transmittance declined by 20%
Fig. 18. Module efficiency reduction vs. dust density reported by different studies.
[69,113].
Fig. 19. Dust deposition with tilt angle for different exposure periods in days [66].
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10. particles deteriorated the PV performance more than the coarser par-
ticles due to the ability of the fine particles to spread and cover the PV
surface. In addition, 28 g/m2
of carbon deposit caused a greater re-
duction in PV module power output than 73 g/m2
of cement or 250 g/
m2
of limestone. They attributed the reason for effective solar radiation
absorption by carbon and hence adversely affects PV performance. El-
minir et al. [97] conducted extensive mineralogical analysis using X-ray
diffraction (XRD) to identify the chemical composition of the deposited
layers of dust particles on PV module in Egypt. The dust particles were
mostly composed of quartz and calcite, with smaller amounts of dolo-
mite and clay minerals. Table 8 shows the observed dust chemical
composition collected from different locations. Fig. 21 presents the
results of the XRF analysis. The major constituents were silicon from
desert sand (quartz, or silicon dioxide, SiO2) calcium from the mineral
calcite (calcium carbonate, CaCO3). The minor constituents consisted
of aluminum, iron, magnesium, potassium, and sodium. Similarly, Said
and Walwil [66] reported that oxygen had the highest chemical con-
centration followed by calcium, silicon, sulfur, and iron as shown in
Fig. 22. Also, it is found that quartz and calcite compounds occupied
more than 60% of dust particle content (Fig. 23).
Figs. 24 and 25 shows some SEM micrographs of dust particles. The
Figures indicate that dust particles are composed of different shapes
and sizes. Large dust particles attach to the small dust particles, because
of the electrostatic charges of small dust particles [136]. Small dust
particles exposed to the solar irradiation for long durations and at-
taching of ionic compounds to the dust particles caused static charging
of the particles [137].
2.6.3. Dust particle chemistry impact
Dust particles absorb water vapor in humid air environments and
form mud at the surfaces. Once the mud becomes dried at high tem-
perature conditions under the solar radiation, it becomes difficult to
remove from the glass cover surfaces [138]. Water interaction with dust
particles took place due to effective adsorption of water molecules by
dust particles surfaces [139]. The interaction between water molecules,
dissolved ions and soil particles occurred because of the unbalanced
force field which depended on the particle size. It should be noted that
the formed mud solution had chemically active characteristics
[140,141]. The dissolution of dust particles such as calcite (CaCO3) and
halite (NaCl), are expressed by Eqs. (1), (2) and (3) show below. In the
first example, halite reactions take place with molecules of polarized
water which separate chloride and sodium into single ions which be-
come surrounded by hydration sheaths and dissolved in the water
Table 6
Observed impact of cover surfaces on PV performance.
Study Glass surface Region Field Effect
Said et al. [18], Anti-reflective coating and textured surfaces Dhahran, KSA The power output losses mitigated by 5% using anti-reflecting coating and
texturing
Said and Walwil
[66]
Piliougine et al.
[65]
Thin film coated with Self-cleaning properties and
antireflective coating
Malaga, Spain The losses in output power due to uncoated modules was 3.3% while for
coated was 2.5%
Rocha et al. [81] Textured glasses are composed of little pyramids on
the surface
Malaga, Spain There are no significant power loss of the flat glass modules and textured
cover glass modules.
Appels et al. [95] Coated glass samples (self-cleaning coating (SC), an
anti-reflection coating and a multilayer coating)
Heverlee,
Belgium
The transmittance decreased by 1.75%, 1.3% and 0.85% for Anti-reflection
coating, Self-cleaning and Multilayer coating, respectively.
Duell et al. [22] Structured glass (pyramids, grooves, inverted
pyramids and a very light structured type and flat)
Golden CO., USA The Isc enhanced by 3% at normal incidence for structured glass cover.
Garg [104] Glass and plastic plates India Solar transmittance at normal incidence for plastic plates was 94% and for
glass was 90%
Nahar and Gupta
[125]
PVC plates, acrylic, glass India The maximum annual transmittance reduction was 7.15%, 5.16%, 2.35% for
PVC plates, 5.27%, 3.98%, 1.78% for acrylic and 4.26%, 2.94%, 1.36% for
glass with 0°, 45° and 90° tilt angle from the horizontal, respectively.
Brown et al. [67] Coated glass Minnesota, USA 20% reduction of glass transmittance after 4 months compared with 25% for
traditional glass
Brown et al. [67] Coated acrylate polymer and acrylate polymer Minnesota, USA 26% reduction of surface transmittance compared with 34% for uncoated
acrylate
Kazmerski et al.
[128]
superhydrophobic and superhydrophilic coatings Lab work The dust particles-glass cover adhesion force decreased from 90 to 12 nN due
to SHP coating
Fig. 20. The fractions of the number, area, and volume of the particles distribution [66].
Table 7
Observed dust particle sizing and distribution morphology characteristics.
Study Area of Dust collection Particle sizing Additional
Qasem et al. [129] Kuwait 4–8 μm The major grain size was silt. The small silt grains were of slate, whereas the bigger
grains were quartz
Said and Walwil [66] Dhahran, KSA 0.5 −176 µm Different and irregular shapes, but generally, tend to be spherical shape.
Appels et al. [95] KU Leuven, Belgium 2 – 10 μm Other tested samples are: cement 10 µm, clay 68 µm and white sand 250 µm
Bouaouadja et al. [116] Algeria 95 − 780 µm The shape of the grains is irregular but approximately spherical.
(Mastekbayeva and Kumar [117] Bangkok, Thailand 53 − 75 µm Soft Bangkok clay used to prepare the artificial dust
Clarke et al. [25], Mohamed and Hasan [130] Libya (Sahara desert) 0.5 − 1000 µm dust size variable from month to other
Hussein A. Kazem and Chaichan [131] Oman 2 − 63 μm dust deposition on PV was found to vary from one location to another
Wang and Gong [114] Qatar Average 2 µm Some non-uniform particles of few tens of micron
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752
11. molecules. The carbonation process takes place in calcite (CaCO3) dis-
solution, where the carbon dioxide attracted and dissolved in water
producing carbonic acid [142].
NaCl + H2O→Na+1
+ Cl−1
+H2O (1)
H2O + CO2→H2CO3 (2)
H2CO3 + CaCO3→Ca+2
+ 2(HCO3)−
(3)
During drying, the dissolved ions (Na+
, K+
, Ca+2
, Cl-
, SiO−
) attract
mud structure due to electrostatic and ionic bonding force. These ions
become dissolved in the mud solution particles holding them together
and forming crystals in between. The ions in the mud structure during
water evaporation increasing the adhesion force. Yilbas et al. [143,144]
demonstrated that dust particles consist of neutral and ionic
compounds. The alkaline and alkaline earth metallic compounds of dust
particles dissolve in the water condensate on surfaces in humid en-
vironments, which gives rise to the formation of a chemically active
mud solution that flows around dust particles under the effect of gravity
and reaches the solid surface where the dust particles have settled. This
chemically active mud solution layer has a major effect on increasing
the adhesion force between the formed mud and surface.
Qasem et al. [133] investigated the air pollution impact on PVs’
performance for several amounts of carbon-based ash deposited on PV
panel surfaces. They compared the power output decrease of the arti-
ficially polluted panel with the clean one control for the different
quantity of ash deposited on the PV panel surface. The results showed
that for ash mass depositions of 0.63 g/m2
, 1.89 g/m2
, 3.15 g/m2
,
3.78 g/m2
; the panel power output dropped by 2.3%, 7.5%, 17%, and
27% respectively.
2.6.4. Dust particle adhesion
Another issue is the impact of humidity in dust fouling. Vapor
condensation on the PV module surface forms capillary bridges in gaps
between the particles and the surface, generating large meniscus forces
that enhance adhesion between the particle and surface, which en-
courages dust build-up [145–147]. Table 9 reports the effect of surface
on the dust deposition amount. Generally, an increase in absolute hu-
midity causes an increase in dust accumulation [68,100]. Mekhilef et al.
[12] reported that adhesion force between dust particles and surfaces
was highly influenced by the atmosphere humidity. As the relative
humidity decreased, the efficiency of solar panel increased due to less
dust particles adhered to the surface.
In relation with particles size, Corn [148] studied the adhesion force
between solid particles and demonstrated that their adhesion force in-
creased significantly with the particle size. Furthermore, the contact
area between a particle and rough surface was found to have a major
role in the adhesion between the particles and surface. The adhesion of
dust and contact potentials were investigated by Penney [149]. It was
reported that the adhesive forces of electrostatically deposited dust
were much greater than a similar dust deposited mechanically. Each
Table 8
The dust chemical composition collected from different locations.
Study Area of dust collection Dust composition
Qasem et al. [129,133] Kuwait The dominating component of dust was quartz followed by calcite and albite
Said and Walwil [66] Dhahran, KSA 60% of dust particles were calcite and quartz.
Elminir et al. [97] Helwan, Cairo, Egypt Quartz and calcite predominated with smaller amounts of dolomite and clay minerals.
Adinoyi and Said [40] Dhahran, KSA The dust was composed of oxygen (58%), calcium (13%), carbon (10%) and sulfur (6%).
Clarke et al. [25] 24 sites across western Libya Quartz with lesser amounts of calcite, illite, and halite
Modaihsh [134] Riyadh, KSA The dominant minerals were quartz, calcite, and heavy metals (Pb, Zn, Cd, Ni and Co)
Javed et al. [135] Doha, Qatar The dominant minerals were dolomite, calcite, quartz and gypsum.
Fig. 21. Organic components of the polluting material using XRF analysis [97].
Fig. 22. XRF and EDS chemical element analysis [66].
Fig. 23. XRD qualitative and quantitative analysis [66].
Fig. 24. SEM micrograph of dust [40].
S.A.M. Said et al. Renewable and Sustainable Energy Reviews 82 (2018) 743–760
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12. particle in an electric field takes specific orientation by the dipole
moment which is produced by the contact potential differences. The
coulomb forces between the particles layers producing high adhesive
forces by the dipoles orient electrostatically. In general, dust particle
charge should be influenced by the adhesion force between the dust
particles and PV glass cover and decrease the PV voltage output [137].
Mclean [150] presented the cohesion of dust layer and the cohesive
force in electrostatic precipitators on the sediment layers of dust in
particular. An electrostatic precipitator has a significant cohesive force
that influences the sediment layers because of the electric field of
particles in air influenced by the corona current across the layers. It was
found that the electric field which flows through the layer had a linear
proportional to the cohesive force approximately. Somasundaran et al.
[151] reported using a cohesive force apparatus. It was improved to
measure the various shapes, various sizes, and nature of the particles
chemical structure under various conditions for very low cohesive force
of around 1 nN. The cohesive force between glass surfaces and dust
particles increased with the decreasing of PH
and an additional amount
of salt resulted in a significant increase of the cohesive force. Also, the
cohesion between dust particles and surfaces was reduced by the in-
teraction of anionic surfactant with polyethylene oxide layer.
Various models for adhesion force measurements are reported
[152,153]. These models are the JRK model, Rabinovich model and
Derjaguin, Muller and Toporov (DMT) models, all of which are used to
characterize the adhesion force for surfaces and adhering dust particles.
For similar particles, the adhesion force varied due to different values of
substrate roughness and the actual contact area which has an important
role influencing particles adhesion. Kazmerski et al. [128] evaluated
the basic interactions of dust particles adhesion to the PV module sur-
faces. Their main interest was to investigate morphology, and chemical
mechanisms of dust particles soiling. The measured adhesion forces of
dust particles with surface chemistry were correlated using Cuddihy
principles. The results revealed that relatively high adhesive forces are
due to dust particles chemical bonding to the glass surface. One of the
chemical solutions was suggested by Brown et.al [67] in 2012. They
applied an anti-soiling hydrophilic coating to the glass cover which
reduced the dust soiling amount on the surface. However, it is indicated
that beads with large size do experience large adhesion force due to
increase in the contact area between the bead and the surface [66]. In
relation with adhesion forces measurements, Kazmerski et al. [128,154]
reported that the inter-particle adhesion force is higher than particle-
module glass cover adhesion. On the other hand, Hassan et al. [136]
measured the adhesion force of the dry mud formed from environ-
mental dust particles on the PV glass cover. It was reported that the
adhesion force increased due to formation of dry mud solution film at
the interface of the dry mud- glass surface. Fig. 26 shows variation of
adhesion force due to dust particles size.
3. Methods of mitigating negative impacts of dust deposition
Reviews of dust-impact mitigation approaches, consider the tech-
niques that address the effect of dust, as dust is one of the most sig-
nificant environmental factors affecting PV module performance. The
methods are important in the Middle East and North Africa, two regions
where solar power may become a particularly viable alternative to
fossil fuels considering the high level of sunshine. To recover the PV
module performance that is impaired because of dust, it is important to
carry out periodic cleaning. However, the required frequency of
cleaning depends on environmental conditions. There are a variety of
methods that have been used or developed to mitigate the dust effect on
Fig. 25. SEM micrograph small dust particles adhering to the surface of large dust particles [136].
Table 9
Effect of cover surface on PV performance.
Study & location Surface Testing
KSA [66] Coated and uncoated The transmittance reduce by 30% for coated glass and after 37% for uncoated glass
after 40 days of exposure.
KSA [26] Anti-reflecting coating The power output enhanced by 8% using anti-reflective coatings on PV glass cover
surface.
(NREL) [30] Grooved, pyramid structured, lightly textured and flat glasses Module Voc was higher by 50 mV, 80 mV and 10 mV for grooved, pyramid structured
and lightly textured glass module as compared to the reference module
Belgium [115] Three different coated glass samples Transmittance decreased by (%): Multilayer (ml) (0.85), Self-cleaning (SC) (1.30),
Anti-reflection (AR) ( 1.75) and Regular glass (2.63)
KSA [26] Texturing and anti-reflecting coating Texturing and anti-reflecting coating reduced the dust accumulation by 5%.
Málaga [65] Three different types of glasses (textured with little pyramids follow
an angle pattern and pyramids show an orthogonal pattern)
Energy losses due to soiling for coated modules was 10% and 12% for uncoated
modules during summer months.
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13. PV module performance. Some also contribute to mitigating other cli-
mate factors as well, such as temperature and humidity. The reported
dust mitigation methods can be divided into four categories: sponta-
neous, mechanical and electromechanical, electrostatic shields, and
micro- and nanoscale surface functionalization. These are discussed in
the following sections.
3.1. Spontaneous dust-removal processes
The natural means of cleaning dust from the surface of PV module
covers are rainfall, wind, and gravity. Water is the most effective
cleaning agent and high rates of rainfall reduce the effect of dust. In wet
regions, such as Singapore, the effect of dust accumulation is only a
minor problem [121]. Although rainfall improves the power output of
dusty solar modules, it cannot be relied upon for cleaning as it occurs
occasionally and minimally in dry regions.
Wind may partially improve PV performance as it works to pull dust
particles off the PV surface. Mekhilef et al. [12] reported that increased
wind velocity leads to greater heat dissipation from the PV cell surface
and a reduction in the relative humidity of the surroundings that in
turn, also lead to better efficiency. While wind can have a positive ef-
fect, it also cannot be depended upon completely. As well as being
unreliable in less windy regions, wind may also increase dust accu-
mulation. Wind speed and direction have a strong effect on PV module
performance [19,121,155].
The angle of inclination affects dust deposition on the modules
cover. Tilting the surface of PV modules can reduce dust deposition
through the effect of gravity, but it can also decrease the captured solar
energy. It is important to determine the optimal tilt angle that will
reduce dust deposition and at the same time maximize the captured
amount of solar radiation. The effect of inclination is dependent on
climatic conditions. For example, in Singapore there was no obvious
effect on dust accumulation due to changing the angle of inclination of
PV modules [121]. Elsewhere, in contrast, decreases in dust accumu-
lation were observed as a result of increasing the angle of inclination
[15,97,110,124,125]. However, PV efficiency is affected by the mod-
ule's tilt angle due to the change in the amount of solar energy received
at the module surface. Such angle will primarily depend on climatic
conditions during the year, and the type of application [94,156]. High
module temperature reduces significantly the energy conversion.
However, a high temperature variation between ambient air and the
surface of the PV module is reported as contributing to in decreasing the
dust deposition on glass cover of PV modules [157].
3.2. Mechanical and electromechanical dust-removal methods
Mechanical methods to remove dust from the surface of PV module
covers include mechanical wiping, blowing, and the use of removable
covers. Electromechanical methods encompass shaking or vibrating the
PV module array and using subsonic or ultrasonic waves to break dust
adhesion. Al Shehri et al. [158] reviewed different dry cleaning me-
chanisms that use robotic systems. It was reported that dry cleaning
methods using Nylon brushes did not affect the optical characteristics of
the PV glass surface after equivalent simulation of 20 years. Williams
et al. [159] reported that the use of mechanical vibration to remove
dust resulted in a restoration of 95% of the power-generating capacity
of the photovoltaic module. Another electromechanical method con-
sisting of a microprocessor and a programmable logic controller (PLC)
was used to operate a mechanical wiper or brushing system, as shown
in Fig. 27. Fernandez et al. [160] designed a robotic dust wiper sup-
ported by a high performance brush to clean surfaces from deposited
Martian dust particles. The results showed that the cleaning efficiency
was above 93%. Moreover, Lamont and El Chaar [144] presented
cleaning approaches based on PLC and peripheral interface controllers
(PIC) that are effective in reducing the accumulation of dust and bird
droppings on PV modules. PIC- and PLC-based combination cleaning
systems have demonstrated promising results, with better cleaning ef-
ficiencies for the PLC-based systems. In relation to dust removal process
from PV cover, Rifai et al. [162] studied the dynamic response of ac-
cumulated dust particles on rotating polycarbonate disk. Their result
revealed that centrifugal force was higher than the friction, lift, drag
and adhesion forces which affect the dust particles removal. The cost of
system oversizing or manual cleaning is approximately 1.5 times that of
the PIC or PLC-based systems [161].
3.3. Electrostatic dust-removal methods
NASA has proposed electrostatic approaches for mitigating the ne-
gative effects of dust on lunar solar panels. An electrodynamic screen
can be attached to the PV module surface. The screen is made of
transparent plastic sheets, such as polyethylene terephthalate (PET)
(which is UV- radiation resistant), and a parallel or spiral configuration
of conducting electrodes made of transparent indium tin oxide that are
Fig. 26. Effect of particle size on adhesion force from different studies [67,108,136].
Fig. 27. Mechanical and electromechanical dust-removal methods. Left: PIC-based cleaning approach. Right: PLC-based cleaning [161].
S.A.M. Said et al. Renewable and Sustainable Energy Reviews 82 (2018) 743–760
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14. embedded beneath a thin transparent film. A single- or multiple-phase
AC voltage supply connects to the electrodes to produce an electro-
magnetic field on the surface, which repels the dust particles, as illu-
strated in Fig. 28 [163–166]. This approach, known as the “electric
curtain method” is the best strategy for dust removal [167].
Electrodynamics screens showed approximately 80% dust removal
efficiency from a PET surface [168] and an increase in solar cell per-
formance of up to 90% [163]. However, the efficiency of the screen is
dependent on many parameters, including dust deposition rate, type of
dust particle, method of operation and the applied voltage.
The dust-clearing ability of such an electric curtain depends on the
type of accumulated particles. A Mars dust simulant was shown to be
the easiest powder to be cleaned from the shields, with more than 90%
being removed using high voltage, compared with lower values for
acrylic polymer powder and lactose [169]. This is because the Mars
dust simulant particles are smaller and the electric curtain
[164,169,170] has overcome the van der Waal forces that are a function
of particle size [170].
Under normal operating conditions there is the possibility of inter-
mittent operation of electrodynamic screens. A comparative study be-
tween continuous and intermittent operation was conducted, and it was
found that the average dust removal efficiency during continuous op-
eration was over 95%, whereas it was approximately 90% when the
screen was activated intermittently [165]. One concern that must be
addressed is the power needed to operate the electrodynamic screen.
The power required is dependent on dust deposition. The average
power consumption of the screen varied between 1.0 and 2.9 mW when
dust deposition varied from 0.4 mg/cm2
to 0.6 mg/cm2
[165]. For a
dust load of 0.6 mg/cm2
, the total power needed to operate the screen
was approximately 10 W/m2
, that is, 5 W/m2
for the supply operation
and 5 W/m2
for removing the dust [166,168]. To reduce the power
necessary for the supply operation, a low-power microcontroller can be
used instead of a digital signal controller system [171].
The dust-clearing ability of a transparent electrodynamic shield is
sensitive to changes in the amplitude, pulse (wave shape) and fre-
quency of the applied voltage. Higher voltages lead to better dust re-
moval efficiency, as do pulsed and triangular signals. Low frequencies
improve removal efficiency by increasing the velocity across the shield
surface [169,172]. Recently, a comparative study of various dust
cleaning methods (manual cleaning, vacuum suction cleaning, auto-
matic wiper based cleaning and electrostatic precipitator) was con-
ducted by Hudedmani et al. [156]. The results recommended that using
of Arduino-controller electro static precipitator (shown in Fig. 29) uti-
lized the solar energy effectively and enhanced the solar panel effi-
ciency.
3.4. Self-cleaning surface approaches
3.4.1. Dry hydrophobic surfaces
The purpose of micro- and nanoscale surface fabrication is to
develop self-cleaning surfaces with optimal optical properties. This
method enables the creation of a super-hydrophobic surface that has
low wettability and high water droplet mobility. Such a surface can
enhance the cleaning efficiency and thereby reduce the necessary
cleaning frequency. Super-hydrophobic surfaces include micro- or
nano-structure surfaces coated by a thin film of low surface energy
material or vice versa [174]. A number of recent efforts have attempted
to mitigate the effect of dust in this manner. Park et al. [175] developed
a super-hydrophobic surface with a contact angle greater than 150° and
a hysteresis of less than 20°. However, developing a novel surface with
the necessary properties of low adhesion, low wettability, high trans-
missivity, and high resistivity to aggressive environments (such as high
humidity and temperature) has posed a difficult challenge. Verma et al.
[146] created a super-hydrophilic nano-structured glass with a contact
angle less than 5°. This material caused an improvement in solar cell
performance because of an increase in net optical transmission and the
self-cleaning nature of the surface. Hee et al. [121] deposited different
thicknesses of a titanium dioxide film on the glass surfaces and ob-
served reduction in dust accumulation due to deposition, with the effect
becoming more significant with increasing film thickness. However, the
accumulated dust particles increased the panel's temperature which
influences the voltage losses. Therefore, using Nano-coated PV modules
with self-cleaning nanomaterial will not only reduce the dust deposition
rate but also have better temperature mitigation, especially in the hot
climate of the MENA region [176].
3.4.2. Wet hydrophobic surfaces
Texturing hydrophobic surfaces enhances their non-wetting prop-
erties by increasing the surface area through changes in its geometry.
Fig. 28. Electric curtain: (a) Three-phase electric curtain [163]. (b) The electrical field distribution between the electrodes of the shield [169].
Fig. 29. Arduino Interface to Electro-Static Precipitator [173].
S.A.M. Said et al. Renewable and Sustainable Energy Reviews 82 (2018) 743–760
756
15. Trapped air between the surface encourages super hydrophobic beha-
vior, as the water drop sits partially on air [177]. One challenge is to
keep these air pockets stable as, under humid conditions such as those
in the KSA, the air pockets may collapse due to the condensation of dew
or the evaporation of a water drop. These processes may occur at the
nanoscale, rendering the textured surface highly wetting [177]. At-
tempts to produce super-hydrophobic surfaces that are effective in
humid conditions have led to the development of non-wetting surfaces
with self-healing properties. The properties of the texture are main-
tained by creating pockets of liquid instead of air by impregnating the
surface with lubricant, which is stabilized by capillary wicking which is
resultant from the micro- or nanoscale texture [178]. Lubricant-im-
pregnated surfaces show extremely low-contact-angle hysteresis within
the droplet, which enhances the slippery nature of these surfaces,
preventing droplets from being pinned and thereby offering self-
cleaning properties [179–181]. It has also been observed that lubricant-
impregnated surfaces enhance condensation and maintains their non-
wetting properties under conditions of high humidity [182]. Table 10
summarizes glass transmittance improvement due to coating methods.
4. Conclusions
PV module performance can be significantly affected by both en-
vironmental conditions, such as outside temperature, wind velocity,
and humidity, as well as the accumulation of dust on the module sur-
faces. Climatic factors can also influence the amount of dust accumu-
lation directly. This manuscript reviews the latest published literature
on the subject of dust accumulation on PV module glass covers and
different mitigation techniques. Hence, it serves as a quick reference
and guide for researchers and engineers interested in this subject.
Reducing the amount of dust accumulation on PV glass cover sur-
faces is an important consideration in arid regions such as the Middle
East and North Africa where dust build-up can significantly affect the
performance of PV modules. The loss of performance due to dust ac-
cumulation depends on the chemical composition, size and density of
the deposited dust particles. A number of approaches to reduce the
effect of dust on PV module performance have been explored, but there
is no one clear optimum approach. The most effective dust-removal
method depends on climate conditions at the site of interest. There is no
fixed recommended frequency of module cleaning, as this strongly
depends on the frequency of local dust storms.
The published literature on this subject indicates a lack of studies on
the effect of dust type and module cover characteristics on dust de-
position. There is also a lack of research on the adhesion forces between
the dust particles and the surface. Further work is needed to devise
glass covers for PV modules that offer low dust adhesion properties for
use in dry, dusty regions. The economic feasibility of the discussed
mitigation methods also require further consideration.
Microscale and nanoscale textured PV module surfaces impregnated
with lubricants demonstrate superior non-wetting properties at all le-
vels of ambient humidity. Electrostatic and microscale/nanoscale
surface functionalization methods show great promise in mitigating
dust deposit on PV modules and combining the two methods could
prove fruitful in yielding PV module surfaces with low dust adhesion,
particularly for use in dry regions of the world.
Acknowledgments
The authors would like to acknowledge the support of King
Abdulaziz City for Science and Technology (KACST /NSTIP Project 11-
ADV2134-04) in conducting this study.
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![Contents lists available at ScienceDirect
Renewable and Sustainable Energy Reviews
journal homepage: www.elsevier.com/locate/rser
The effect of environmental factors and dust accumulation on photovoltaic
modules and dust-accumulation mitigation strategies
Syed A.M. Saida,b,⁎
, Ghassan Hassana,b
, Husam M. Walwila
, N. Al-Aqeelia
a
Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia
b
Center of Research Excellence in Renewable Energy (CoRE-RE), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia
A R T I C L E I N F O
Keywords:
PV performance
Dust accumulation
Dust mitigation
A B S T R A C T
The mitigation of environmental effects on clean-energy technology is an area of increasing interest.
Photovoltaic (PV) modules have been widely used in small and large-scale applications for many years.
However, they are not yet competitive with other electrical energy-generation technologies, especially in en-
vironments that suffer from dust, airborne particles, humidity and high ambient temperatures. This paper
presents a review of the effect of climatic conditions on PV module performance, in particular, the effect of dust
fouling. Research to date indicates that dust deposition has a considerable effect on PV module performance as it
reduces the light transmissivity of the PV module surface cover. Studies on the ways in which dust is deposited
on PV module surfaces are reviewed, as understanding this process is essential to develop effective mitigation
approaches. Module performance is also adversely affected by high ambient temperature, humidity and lack of
rainfall. The current review summarizes the past, current and promising future approaches towards mitigating
environmental effects, in particular dust fouling. Electrostatic cleaning methods and micro/nanoscale surface
functionalization methods both have the potential to counteract the negative effects of dust deposition, with the
combination of the two methods showing special efficacy, particularly in arid regions.
1. Introduction
The use of, and interest in, renewable sources of energy is growing
rapidly. This trend has been driven by increases in the price of fossil
fuels, concerns about air quality and human health and the effect of
fossil-fuel dependence on the environment. Photovoltaic technology
could offer a promising and clean alternative to fossil-fuel-based energy
generation. The Kingdom of Saudi Arabia (KSA) planned to install PV
systems capable of producing 1481 MW of power over a four year
period as it scales up production of renewable energy [1]. Renewable
energy reliability had always been a huge question mark, as we have no
control on natural parameters like solar radiation, Wind etc. Also, the
maintenance required for a renewable energy is more than most of the
conventional forms of energy, Advantages of the renewable energy is
that they are eco-friendly which makes them "not harmful" to our en-
vironment. It is abundant and is a natural form of resource. Despite the
development of new higher-performance materials, the greatest dis-
advantage of photovoltaic systems remains their limited efficiency in
converting solar to electrical energy, which ranges between 12% and
20% [2].
The degradation of PV system performance is mainly due to several
factors such as climate and environmental conditions, mishandling of
PV modules which happens in the transport and installation, the site at
which is installed and the maintenance. Although significant advances
have been made in PV systems within the last few decades, designers
still face challenges related to the effects of climate conditions on PV
system performance. In practice, PV module performance is much lower
when operating outdoors than under controlled laboratory conditions
[3,4]. Key climatic conditions that strongly affect PV module perfor-
mance include solar radiation, temperature, wind speed and direction,
relative humidity, and dust [5–8]. Ordinarily, solar irradiation and air
temperature have more significant impact on PV output power than
wind speed and humidity [9,10]. Dust is a unique problem that sig-
nificantly affects the performance of PV modules in the Gulf Region.
Studies have been conducted on the effects of different climatic
conditions on PV system performance. In general, the environmental
factors have different impact on the PV systems performance. For ex-
ample, the PV power outcomes reduced with elevated relative hu-
midity, air temperature and dust deposition while it increased with
increasing in wind speed [11]. Mekhilef et al. [12] reviewed the effects
of dust accumulation, humidity level and air velocity surrounding the
PV system. They concluded that each condition influences the other,
http://dx.doi.org/10.1016/j.rser.2017.09.042
Received 4 May 2017; Received in revised form 4 September 2017; Accepted 14 September 2017
⁎
Corresponding author at: Department of Mechanical Engineering, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia.
E-mail address: samsaid@kfupm.edu.sa (S.A.M. Said).
Renewable and Sustainable Energy Reviews 82 (2018) 743–760
1364-0321/ © 2017 Elsevier Ltd. All rights reserved.
MARK](https://image.slidesharecdn.com/said2018-220527172526-4c7a9b02/85/said2018-pdf-1-320.jpg)
![and therefore they should be considered together. Mani and Pillai [13]
reviewed the dust effect on PV module performance during two time
periods, 1940–1990 and 1990–2010. They developed and used an ap-
propriate cleaning/maintenance cycle for their PV systems that con-
sidered the prevalent climatic and environmental conditions. However,
such studies focused only on changing parameters that significantly
affect PV module performance without investigating effective ways of
mitigating their impact.
This study presents a review of recently published studies on factors
influencing PV module performance, particularly dust fouling, and
different methods of mitigation. The first part focuses on how perfor-
mance is impaired due to a variety of environmental conditions. An
overview of studies describing the mechanisms that cause these effects
and how they affect each other is also given. In the second part we look
at some of the past, current, and promising future approaches for mi-
tigating dust-fouling effects and summarize how the effectiveness of
these mitigation techniques is affected by other environmental factors.
2. Effect of climatic and environmental conditions on PV module
performance
The variation of climatic conditions from location to location
around the world has a corresponding effect on PV module performance
in different regions [7]. The parameters that may impact PV module
performance include solar radiation, wind velocity, rainfall, tempera-
ture, humidity and the probability of dust presence. The following sub-
sections summarize the effects of each of these conditions on PV module
performance, as reported in the literature.
2.1. Effect of wind velocity
Wind velocity can have both positive and negative effects on PV
module performance. The impact of wind velocity on PV module per-
formance is mainly a function of wind speed and direction, PV module
surface structure, and dust deposition. In an outdoor environment, wind
velocity, ambient temperature, surface structure, and sun irradiance
influence module temperature.
2.1.1. Dust deposition impact
Wind blows away dust particles from the PV module surface, which
can reduce dust deposition [14]. In Egypt, it is observed, a decrease in
the rate of dust deposition occurs on a module at a particular tilt angle
due to wind blowing after 2 weeks of exposure to weather conditions
[15].
However, the wind can also add to the negative effect of dust by
transferring and spreading dust and sand particles within the atmo-
sphere, which may lead to increased deposition layers. As the wind
speed increases more dust deposition will occur, which results in de-
terioration of solar cell's fill factor [12]. In the Greatest Desert-Libya,
Clarke et al. [16] reported that increase in dust deposition generally
coincides with increase in mean monthly wind speed. For example wind
velocities of 6.5 m/s represent the minimum threshold velocity for dust
entraining winds in Libya [17]. To simulate the effect of wind speed and
direction on dust deposition and accumulation and resulting PV module
performance, Goossens et al. [18] performed a wind tunnel simulation
and field experiments. Their results indicated that wind direction af-
fects dust deposition and distribution more than wind speed does.
However, it is concluded that the reflected effect of wind on dust
deposition depends on the wind speed. The wind speed was tested in
range between 0.63 m/s and 2.59 m/s. High wind speeds (2.59 m/s)
lead to high dust accumulation on a cell, resulting in sharp performance
drops. In cases of low wind, dust accumulation is smaller, and the drop
in cell performance is less [19]. Additionally, it is observed that the
effects of Martian dust particle size on PV performance vary with wind
velocity. When the wind velocity was very high (89–116 m/s) there was
no significant difference between the degradation caused by large
particles (> 75 µm) and that caused by small particles (30 µm) [20].
That results was confirmed by who observed that the surface mass
density of the dust deposition was negligible at a high wind velocity
which is greater than 24 m/s [21].
2.1.2. Wind speed and direction impact
The wind enhances dissipative convective heat transfer from the
module, thereby reducing its temperature, which helps maintain its
conversion efficiency [5,22–25]. For example in Dhahran, KSA in-
creasing the wind speed from 2.8 to 5.3 m/s resulted in reduction in
module temperature within a range of 10 °C [26]. In Slovenia, the
difference between the module temperature and the ambient tem-
perature for any non-zero conversion efficiency (regardless of the
mounting conditions) reduce to half the value, if the wind speed in-
creases from zero to 12 m/s [27]. In Greece, the findings of an outdoor
measurement clearly indicated that the difference between modular
and ambient temperature (Tc-Ta) decreases with increasing wind speed,
as shown in Fig. 1 [28]. It is also observed that the difference between
the cell and ambient temperatures is between 10 and 20 °C during calm
spells, then demonstrating a gradual shift to zero for high wind speed
cases [28]. However, under series of artificial tests for wind speed and
direction effects on PV performance, it was found that the PV cell
temperature rise over the ambient one is extremely sensitive to wind
speed, less so to wind direction [29].
2.1.3. PV surface structure impact
It has been reported that the surface structure has a significant
cooling effect, increasingly pronounced at higher wind speed [30]. At
higher wind speeds, the module temperature decreased due to the in-
creased convective module cooling. Experiments carried out in a wind
tunnel in USA showed that at high wind speed, structured glass modules
operate at lower temperature than flat glass modules. Fig. 2 shows that
at a wind speed of 10 m/s, a module fabricated with grooved glass
Fig. 1. Impact of wind speed on temperature difference between PV modules and the
environment-case study in Greece [28].
Fig. 2. Effect of wind speed on module temperature (temperature difference compared
with flat glass module) in USA [30].
S.A.M. Said et al. Renewable and Sustainable Energy Reviews 82 (2018) 743–760
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