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Solar mirrors are exposed, during their operation in Concentrated Solar Power plants –CSP-, to climatic stress factors that cause their degradation and therefore a decrease in the global efficiency of the plant. Sandstorms are among parameters that cause a decrease of mirrors optical performances by generating surface erosion. The intensity and the gravity of this erosion phenomenon is function of climatic, geological parameters and mirrors surface nature. To evaluate the effect of these parameters on the optical performance degradation, two approaches were adopted, namely the natural aging tests in two different sites in Morocco, and the aging tests in controlled environment in a sandblasting chamber. The objectives are, by monitoring the stress factors in natural aging sites, to define aging tests under controlled environment that reproduce similar degradation phenomenon that those observed on mirrors exposed in natural aging sites. Degradations observed in both natural and controlled aging tests are compared and correlated to validate the methodologies and the hypotheses on the analysis of the degradation phenomenon. The aging tests in controlled environment permits the evaluation of the effect of each influencing parameter separately from the others on the mirrors surface erosion, and eventually accelerate the apparition of surface erosion on mirrors. Under controlled environment, tests show that glass mirrors present maximum surface erosion at normal impact angle and that the loss in specular reflectivity is directly related to the wind speed. Exposed mirrors in natural aging sites present low loss in reflectivity which doesn't exceed 0.4% after 240 days of outdoor exposure. Concerning the effect of sand properties on erosion phenomenon, it was found that the sand hardness affect the roughness parameters, while the sharp forms influence on the impacts properties (roughness parameters, impacts number, impacted area, impacts size diameter). By increasing the sand particle's size, the impacted area increase and the losses in relative specular reflectivity increase.
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ScienceDirect
Energy Procedia 00 (2015) 000–000
www.elsevier.com/locate/procedia
1876-6102 © 2015 The Authors. Published by Elsevier Ltd.
Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG.
International Conference on Concentrating Solar Power and Chemical Energy Systems,
SolarPACES 2014
Study of the surface damage of glass reflectors used in Concentrated
Solar Power Plants
M. Karima,c, S. Naamanea*, C. Delordb*, A. Bennounac
a Moroccan foundation for Advanced Science Innovation and Research MAScIR- Rabat Morocco
b CEA LITEN, National Institute of Solar Energy, Thermal Energy, Biomass and Hydrogen Department, High Temperature Solar
Systems Laboratory, 73375, Le Bourget-du-lac, France
c Cadi Ayad University, Faculty of Science, Department of Physics, BP: 2390, Marrakech, Morocco.
Corresponding authors: s.naamane@mascir.com; christine.delord@cea.fr
Abstract
Solar mirrors are exposed, during their operation in Concentrated Solar Power plants –CSP-, to climatic stress factors that
cause their degradation and therefore a decrease in the global efficiency of the plant. Sandstorms are among parameters that
cause a decrease of mirrors optical performances by generating surface erosion. The intensity and the gravity of this erosion
phenomenon is function of climatic, geological parameters and mirrors surface nature.
To evaluate the effect of these parameters on the optical performance degradation, two approaches were adopted, namely the
natural aging tests in two different sites in Morocco, and the aging tests in controlled environment in a sandblasting chamber. The
objectives are, by monitoring the stress factors in natural aging sites, to define aging tests under controlled environment that
reproduce similar degradation phenomenon that those observed on mirrors exposed in natural aging sites. Degradations observed
in both natural and controlled aging tests are compared and correlated to validate the methodologies and the hypotheses on the
analysis of the degradation phenomenon. The aging tests in controlled environment permits the evaluation of the effect of each
influencing parameter separately from the others on the mirrors surface erosion, and eventually accelerate the apparition of
surface erosion on mirrors.
Under controlled environment, tests show that glass mirrors present maximum surface erosion at normal impact angle and that
the loss in specular reflectivity is directly related to the wind speed. Exposed mirrors in natural aging sites present low loss in
reflectivity which doesn’t exceed 0.4% after 240 days of outdoor exposure. Concerning the effect of sand properties on erosion
phenomenon, it was found that the sand hardness affect the roughness parameters, while the sharp forms influence on the impacts
properties (roughness parameters, impacts number, impacted area, impacts size diameter). By increasing the sand particle’s size,
the impacted area increase and the losses in relative specular reflectivity increase.
© 2015 The Authors. Published by Elsevier Ltd.
Peer review by the scientific conference committee of SolarPACES 2014 under responsibility of PSE AG.
M.KARIM/ Energy Procedia 00 (2015) 000–000
Keywords: Solar mirrors; Surface erosion; Natural aging; Outdoor aging
1. Introduction
In a Concentrated Solar Power plant -CSP-, more than 30% of the global investment is reserved for solar field
that includes mirrors costs (installation, operation and maintenance, replacement of mirrors breakage…) [1-2]. To
ensure high global yield of CSP plant, solar specular reflectance of mirrors are among the key-parameters that
should be kept at its highest level during the entire service lifetime of mirrors [3-4]. This latter is directly related to
the location of the power plant since several climatic stress factors like temperature, humidity, dew, irradiation and
windstorms can cause various mirrors degradations such as surface erosion, corrosion of reflective layer,
delamination, photo-degradation of polymeric protective layer, etc [5].
To evaluate the effect of degradation phenomena on the mirrors optical performance, many studies are conducted
on natural aging sites [6-10] where glass sheet or mirrors samples are tested during an exposure time. Natural aging
tests are useful and necessary to observe and analyze degradation mechanisms under natural climatic conditions.
However, it requires a long time before obtaining remarkable degradations and it is complex, in this case, to
evaluate and understand the degradation linked to only one stress factor since samples are exposed to a mix of
climatic parameters leading to several simultaneous degradation mechanisms. In fact, to accelerate the apparition of
degradation phenomena and to dissociate the effect of each influencing climatic parameter, aging tests in controlled
environment are often adopted [2, 11-16]. They permit to test, on specific chambers, the effect of some climatic
parameters such as UV radiation, humidity, temperature, windstorms.
In this work, we are interested on windstorms effect on the surface erosion of mirrors. This phenomenon is directly
influenced by the climatic and the geological parameters of the mirrors exposure site and their surface’s nature
parameters [5, 11-12]. The aim of the present study is to evaluate the effect of these influencing parameters on
surface erosion phenomenon during natural aging and by conducting representative and accelerated tests in
controlled environment. For this purpose, natural aging tests are made in two representative sites in Morocco.
Conditions of representative tests in sandblasting chamber are defined based on the monitoring of influencing
climatic parameters and on the geological analysis of both natural aging sites. These tests used both sand particles
representatively extracted from the natural aging sites to approach the real aging conditions. To confirm our results,
normalized sand particles are used in order to compare the obtained tendencies with literature.
2. Experimental procedure
Silvered mirrors of 0.95 mm thickness are used to evaluate the effect of surface erosion on the mirrors
performances. In this conception, the glass substrate is coated with a silver reflective layer on the backside to create
a second-surface mirror. To protect this reflective layer from any eventual degradation, mirror backing system is
covered by several protective paints. For the present study, mirrors of 7 x 7 cm² are used for both natural and
controlled environment tests. These mirrors are cut and supplied by the manufacturer without edge protection. In
order to avoid the edge effects, samples of 20 x 20 cm² are added for the natural aging tests.
2.1. Natural aging tests
Two different sites in Morocco, distant with more than 500 km, are chosen to conduct the natural aging tests. The
first site is a seaside site located in Temara near to the Moroccan capital Rabat. The site is aboard the Atlantic
Ocean, see.Fig.1, and it is characterized by its high degree of humidity and wind speed. The second site is a desert
site located in Skoura near to Ouarzazate city in the south of Morocco; see.Fig.1 which presents a high ambient
M.KARIM/ Energy Procedia 00 (2015) 000–000
temperature. In both natural aging sites, mirrors are exposed in racks with an inclination equal to the sites latitude,
and oriented towards south.
Ouarzazate
Marrakec h
Casablanca
Rabat
Tanger
Seaside
site
Desert
site
Fig.1. (a): Temara seaside site; (b): Skoura desert site; (c): Map of direct solar bearing in Morocco DNI (Source: solar-med-atlas.org).
Climatic parameters of both sites are monitored with meteorological stations. The main measured parameters are
wind speed and direction, ambient temperature, relative humidity, global irradiation and rainfall.
Among climatic parameters, wind speed and direction are known as the most influencing parameters on surface
erosion phenomenon [16].
The recorded data from September 2012 to June 2014 show that for the maximum wind speed values, the seaside
site present more than 25% of wind values higher than 5 m/s, see.Table1. This later value is considered as threshold
value of wind speed able to transport sand particles with diameter lower than 0.2 mm [17-18]. In desert site, wind
values above 5 m/s represent only 18% of the global recorded data. The high wind speed values in the seaside site
which reach a value of 22 m/s against 17 m/s for the desert site can be explained by the proximity of the site of the
Ocean, see. Fig.1above. This wind speed analysis is used to determine the wind velocities used for aging tests in
controlled environment.
Table.1. Occurrence frequency of maximum wind speeds recorded by a mesh of 10 minutes in both exposure sites between
September 2012 and June 2014.
Wind speed (m/s) Seaside site (%) Desert site (%)
V < 5 74.74 81
5 < V < 10 22.82 17.83
10 < V < 15 2.17 1.15
V > 15 0.28 0.01
The wind direction is interesting to estimate the main impact angle of sand particles on mirrors surface. The
Mirror’s orienta tion
0%
5%
10%
15%
20%
N
NE
E
SE
S
SW
W
NW
0%
5%
10%
15%
20%
N
NE
E
SE
S
SW
W
NW
(a) (b)
M.KARIM/ Energy Procedia 00 (2015) 000–000
(a) (b)
obtained results from the wind direction analysis in both sites will be used to choose the impact angles used for
aging tests in controlled environment. For the wind rose recorded from September 2012 to May 2014, see. Fig.2, it
was shown that the wind direction in the seaside site is characterized by a dominance of the south and south-west
orientations, while in the desert site the wind direction is dominated by the east-north-east and west-south-west
directions. Assuming that mirrors are exposed in both sites according to the south and with inclination equal to the
site latitude, the potential incidence angle in seaside site will be more close to the normal impact angle that in the
desert site.
Fig.2: Wind rose from the recorded data between September 2012 and June 2014, (a):Seaside exposure site, (b): Desert exposure site.
Concerning the geological influencing parameters, size distribution, hardness and shape of sand particles are the
most influencing parameters on surface erosion phenomenon. A complete geological study was made on
representative sand extracted from both exposure sites to determine the main sand properties. Results in details were
reported in a previous work [19]. The main sand properties of both sands are presented in the followed section. This
geological analysis permits the identification of the sand particles properties which are helpful to highlight the effect
of each sand particles property.
In addition to the geological analysis and in order to determine the sand particles properties that reach different
heights from the ground, sand trap systems have been installed in desert site, see.Fig.3.a. The trapped sand particles
are regularly analyzed. To evaluate the combined effect of height and impact angle on surface erosion, an erosion
rack has been installed in the desert site, see. Fig.3.b. The exposed mirrors are oriented trough four different
orientations according to the predominant wind orientations, e.g. east-north-east, and four different inclination.
Fig.3: (a): Sand trap systems installed in Skoura desert site, (b): Erosion rack exposed in desert site in order to evaluate the effect of impact angle
on surface erosion during natural aging tests.
To focus on the effect of surface erosion on mirrors optical performance and in order to simulate and accelerate
the effect of each influencing parameter, several tests were made in controlled environment. The used chamber is
presented in what follow.
2.2. Erosion tests in sandblasting chamber
Horizontal sandblasting chamber is used for tests in controlled environment, see.Fig.4. The nozzle diameter is
about 1.5 cm and the homogeneity of the flow is ensured by using an air homogenization tube. The sample holder
can rotate from 0° to 90° to vary the impact angle. The wind velocity varies with the distance L, between the air
homogenization tube and the sample holder.
M.KARIM/ Energy Procedia 00 (2015) 000–000
Compr essed airflow
Sand
Particles
Container
L
Sample
Airhomogenization
tube
Nozzle
Sample
holder
Fig. 4: Sandblasting chamber used for simulating the surface erosion phenomenon.
To approach the real conditions of natural aging in controlled environment, four wind velocities are chosen
according to the recorded data from both natural aging sites, namely 3.8 m/s; 6.3 m/s; 9.2 m/s and 15.1 m/s.
In order to evaluate the effect of the impact angle as parameter on surface erosion rate, mirrors are tested under
four different impact angles, namely 20°, 40°, 70° and 90°. These chosen impact angles cover all the potential
angles identified from the wind direction analysis in the natural aging sites. The normal impact angle was added in
order to evaluate the effect of this impact angle on surface erosion and to confirm our obtained trend with literature.
To evaluate the effect of sand properties on the surface erosion, sand particles are extracted from both natural
aging sites. In addition to these natural sand particles, normalized sand MIL-STD 810-F is used in order to compare
the obtained results with the literature. The main sand properties of the three used sand particles are regrouped in the
table below.
Table 2: Sand properties of both extracted sand samples from the exposure sites.
Sand properties Seaside sand Desert sand Normalized sand
Dominant size
fraction 70% between 0.1 and
0.4 mm 50% bigger than 1 mm 100% bigger than 150
µm
Composition
6%: Quartz
22%: Rock
fragments
50%: Carbonate
20%: Shell
fragments
2%: Other
40%: Quartz
34%: Rock
fragments
23%: Carbonate
3%: Other
98%: Quartz
2%: Other
Hardness 3-4 5-6 7
Shape Sharp Sharp to round Sharp
The sand hardness is defined according to the mineralogical composition of the sand aggregate. Mohs scale
permits the classification of the mineral hardness from 1 for talc mineral to 10 for diamond. Concerning the sand
particle shape, a visual chart is used to determine the sand particle’s shape from binocular loupe observations, see.
Fig.5 [20]. This parameter is considered as quite affecting the surface erosion rate since round particle present less
damage than a sharp one [21].
M.KARIM/ Energy Procedia 00 (2015) 000–000
200µm
(a
)
200µm 200µm
(b) (c )
Fig.5: (a): Seaside sand particles; (b): Desert sand particles; (c): Normalized sand particles.
To evaluate the impact of the sand particles granularity, two different fractions of natural sand particles are used,
namely sand particles with diameter φsand higher than 150 µm to simulate the sand storm and sand particles with
diameter lower than 150 µm to simulate the dust storm. This classification was done according to the military
standard MIL-STD-810 which classify the windstorms into sand storms (150 µm < φsand < 800 µm) and dust storm
(φsand < 150 µm) [10]. For the normalized sand particles, only the coarse sand fraction was used for tests.
It is worth to note that during natural sand transportation, bigger sand particles need higher wind speed to be
transported in comparison with lower particles. In order to simulate the real conditions of exposure and simplify the
tests matrix, it was decided to simulate the surface erosion by the fine sand particles at wind velocities of 3.8; 6.3
and 9.2 m/s, and by the coarse sand at wind velocities of 6.3; 9.2 and 15.1 m/s, see. Fig.6.
Fig.6: Sand fraction used with each wind velocity during aging tests in controlled environment.
2.3. Characterization techniques
For the characterization techniques of mirrors tested in both natural aging sites and in controlled environment, the
optical performances are followed by measuring the specular reflectivity at 660 nm under an incidence angle of 15°
and an acceptance angle of 25 mrad, ρs(SW, 15°, 25 mrad). This measure is ensured using a portable reflectometer
15R-USB from Devices & Services.
The surface roughness was characterized by a stylus profiling system Dektak XT provided by Brucker. The
obtained results from the surface scanning are analyzed by ImageJ software in order to determine the impacts
properties (impacts number, impacted area, impacts size diameter). An optical microscope Leica is also used to
observe the different shapes of generated impacts upon mirrors surfaces.
3. Results and Discussion
Mirrors samples are exposed in the seaside site from August 2013 and from May 2013 in the desert site. During
this period, no degradation other than surface erosion was identified on the exposed mirrors. Therefore, the decrease
Fine fraction
(φsand<150 µm)
Coarsefraction
(150µm < φSand)
3.8 6.3 9.2 15.1
Win d s pee d (m/s)
M.KARIM/ Energy Procedia 00 (2015) 000–000
measured in specular reflectance can be directly related to the effect of surface erosion. The difference between all
measured mirrors is represented in the graph by bars. After 240 days of natural aging, the measured loss in relative
specular reflectivity, Rr, is lower than 0.4%, see. Fig.7:
(Eq.1)
With:
Rr: Relative loss is specular reflectivity
Ra: Specular reflectivity after aging
Ri: Initial specular reflectivity
0,00%
0,20%
0,40%
0,60%
0,80%
1,00%
0 50 100 150 200 250
Lossesinrelative specularreflectiv ity(%)
Exposure time(Days)
Temara
Skoura
Fig.7 . Irreversible losses in relative specular reflectivity Rr of exposed mirrors samples in both natural aging sites.
Regarding the effect of the climatic parameters, no conclusion can be deduced from the graph since the loss in
relative specular reflectivity is still low in both sites.
According to the results of aging in controlled environment, tests show that mirror’s tested at 90° present the
maximum loss in specular reflectivity which fits with the literature, see. Fig.8 [13]. The surface characterization of
the tested mirrors at 20° and 90° shows that the samples tested at high impact angle, 90°, presents higher impacts
properties (roughness parameters, impacts number, impacted area, impacts size diameter) than those observed on
samples tested at low impact angle, 20°. This trend is confirmed in the literature [13]. Results also show that the
optical performance decreases by increasing the wind velocity in the sandblasting chamber, see. Fig.8.
M.KARIM/ Energy Procedia 00 (2015) 000–000
Fig.8: Effect of impact angle and wind velocities on the surface wear rate of glass mirrors tested at 2 g of sand particles. Logarithmic trend curve.
The effect of wind speed observed during testing in controlled environment can be compared to those observed
on natural aging tests. Two shapes of impacts are distinguished and can be directly related to the wind speed in the
region. In the desert site, where wind speed is low with approximately 80% of wind values below 5 m/s, the
generated impacts are presented as ring cracks, see Fig. 9.a. In seaside site, where the wind speeds are higher and up
to 22 m/s, the generated impacts are normal cracks with direct removal material, see Fig. 9.b. This was confirmed by
the results obtained in controlled environment when the observed impacts on mirror tested at high wind speed
present impacts with removal material, see. Fig. 9.c.
30µm 30µm
(a) (b)
50µm
(c)
Fig.9. Observed impact upon exposed solar mirrors; (a): Mirror exposed in desert site, (b): Mirror exposed in seaside site, (c) Mirror tested in
controlled environment at 90° and V=9.21 m/s.
In desert site, the wind speed is not high enough to create normal impacts with direct material removal such as
those observed on surface of seaside’s site mirror. In this case, the generated impacts are presented as a ring crack
where the material removal caused during the shock between sand particle and the mirror’s surface remains on the
middle of the impact as showing below see. Fig.10.a. For the mirror exposed on the seaside site, the observed impact
presents material removal which is presented as a hole on the middle of the impact during the surface scanning by
the stylus profiling system, see. Fig.10.b.
0,0
0,5
1,0
1,5
2,0
2,5
3,0
20° 40° 60° 80° 100°
Lossinspecularreflectiv ity(%)
Impactangle(°)
6.33m/s 9.21m/s
12.88m/s
M.KARIM/ Energy Procedia 00 (2015) 000–000
Fig.10: Observed impact upon exposed solar mirror in both sites analyzed using stylus profiling system, (a): Impact on desert’s site mirror, (b):
Impact on the seaside’s site mirror. Resolution X=1µm and Y= 0.1 µm.
The effect of sand properties on the surface erosion phenomenon is evaluated by making a comparison of the
roughness parameters and impact properties of two mirrors exposed in both natural aging sites. The main obtained
results are reported in the table below.
Table 3: Surface roughness properties of two mirrors exposed in different sites for 240 days of natural aging.
Roughness parameters Impacts parameters
Exposure site Ra (nm) Rv (nm) Rp (nm) Impacts number Impacted surface
(%)
Seaside site 1.17 -1900 2630 3298 1.6
Desert site 1.22 -2284 2925 1358 1.1
The roughness parameters compared in this study, namely Ra, Rv and Rp are defined as:
Ra: The average arithmetic gap of the roughness profile
Rv: The maximum height of the projection in the analysed profile
Rp: The maximum depth of hollow in the analysed profile
For the same aging time, desert’s site mirror presents higher roughness parameters than seaside’s site mirror.
However, the impacts parameters (impacts number, impacted area and impacts size) calculated upon surface of
seaside’s site mirror are higher than those calculated on surface of desert’s site mirror. This difference should be due
to the variety of the sand particles properties present in both sites or to the difference in the wind speed. From the
geological analyses, it was shown that desert’s site sand particles are harder and smoother than seaside’s site
particles which are softer and sharper. By correlating this information with the obtained results, we can make the
hypotheses that the hardness impacts on the roughness parameters and the sharpness influence the impacts
parameters or that the wind speed is influencing on the impacts properties.
In order to confirm these hypotheses, two mirrors were tested at the same wind velocity, the same impact angle
and the same size particles range and with sands extracted from both natural aging sites. Results show that the
roughness parameters of mirror’s surface tested by desert sand particles are higher than those calculated upon
mirror’s tested by seaside sand particles, see Table.4. However, the impacts parameters (impacted area, impacts size,
impacts number) are higher for mirror’s tested by seaside sand particles. This confirmed the hypothesis emitted for
the obtained results from the natural aging tests that assumed that this difference is directly related to the sand
particles properties.
(a) (b)
M.KARIM/ Energy Procedia 00 (2015) 000–000
Table.4: Surface roughness properties of two mirrors tested by fine sand particles extracted from studied
sites; V= 6.33 m/s and at impact angle of 90°.
Roughness parameters Impacts parameters
Used sand
particles Loss in specular
reflectivity (%) Ra (nm) Rv (nm) Rp(nm) Impacts
number Impacted
surface (%)
Seaside
site 1.7 2.3 -460 270 2860 2.6
Desert site 1.5 2.4 -780 620 1880 1.3
To evaluate the effect of size particles on surface erosion, both sand fractions, fine and coarse, extracted from
the natural aging sites are used for tests. Results show that for the same sand mass and the same wind velocity,
mirror tested by the coarse sand fraction present higher decrease in specular reflectivity in comparison with mirror’s
tested by the fine sand fraction, see. Fig.11.
0
2
4
6
8
10
0 5 10 15 20
Loss in specu lar reflectivity (%)
Wind speed (m/s)
Fine frac tion
Coarse fr action
Fig. 11. Effect of particle size on surface wear phenomenon at an impact angle of 90° and at recived mass of 1g : Example of mirrors tested by
desert sand particles.
4. Conclusion
Surface erosion is among phenomena responsible of optical performance decrease of mirrors optical
performances and then on decrease in global yield of CSP plant. In this paper, the influencing parameters, climatic
(wind speed and direction), geological (sand properties), on surface erosion were monitored and analyzed in two
different sites in Morocco. The objective is on one hand to evaluate the effect of these parameters on the observed
degradation phenomena during the natural aging tests and on the other hand to define the conditions for the aging
tests in controlled environment in sandblasting chamber.
The correlation of results from natural aging tests and tests made in sandblasting chamber indicates that the loss
of specular reflectivity is increasing with the wind velocity. Two different impacts shape can be distinguish as
function of wind velocity, namely ring cracks formed at low wind speed and normal cracks that generate direct
material removal in case of high wind speed.
Regarding the impact angle, the aging tests under controlled environment show that at normal impact angle, the
loss in specular reflectivity presents a maximum rate.
In order to evaluate the effect of sand properties on surface erosion, tested mirrors in both natural and controlled
environment are analyzed by stylus profiling system. It was found that the sand hardness affects the roughness
parameters, while the sharp forms influence on the impacts properties (roughness parameters, impacts number,
impacted area, impacts size diameter).
Other aging tests in controlled environment are planning in order to compare between surface erosion caused by
sand particles extracted from studied sites and those caused by normalized sand particles.
M.KARIM/ Energy Procedia 00 (2015) 000–000
Acknowledgements
The authors wish to acknowledge Professeur I. El Amrani from the Scientific institute, Departement of earth
Science for his collaboration. They want also to thank their colleagues at the two institutions MAScIR (Morocco)
and CEA (France) for their contributions. The present work is founded by both institutions.
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[21]: ‘Abrasive, Erosive and Cavitation Wear: Engineering Tribology’, Engeniering Tribology, Book.
... The soiling of the horizontal mirrors was twice the +45° ones and the mean reflectivity loss was 0.5% per day. Other similar studies on the effect of reflectors' material, tilt and direction on the intensity of soiling can be found in table II [98][99][100][101]. ...
... To assess the risk of soiling, monitoring climatic parameters at CSP sites is crucial, namely: AOD [46], precipitations [36,108], wind speed and direction [108], and relative humidity [53]. Tilted reflectors are less soiled, and glass mirrors -compared to other materials-lose their reflectivity faster and restore it better after cleaning, these properties are helpful in sites where frequent cleaning is possible [36,46,98]. Erosion experiments should include dust particles larger than 250 μm, since these particles can be lifted and deposited on the surfaces of reflectors or cause their erosion [43,56]. ...
Article
Many sites with high solar radiation face high dust loads that reduce energy generation by concentrated solar power plants. This review presents the attenuative impacts of atmospheric aerosols, as well as reflectivity losses due to soiling of solar reflectors, by covering both experimental investigations and numerical studies; along with presenting the theoretical background. The chemical nature of aerosols, and the physics of soiling and atmospheric extinction phenomena (scattering and absorption) are also reviewed. Suspended particles like aerosols result in atmospheric extinction of the solar radiation that reaches the concentrators, and the deposition of these particles on the solar reflectors provokes decreases up to 80% in their reflectivity, and thus enhances the cumulus of optical losses and the reduction of energy production. Even though dust affects both CSP and photovoltaics, CSP technologies suffer more losses. The impact of dust should be particularly considered during the planning phase of solar thermal plants, since its consequent reduction in energy output can be severe. While there have been multiple papers to review dust-related problems for PV, the present paper is the first literature review dedicated to the impact of soiling on concentrated solar power.
... Figure 1. Concentrating solar power reflector testbeds: (a) Sandia national laboratories, Albuquerque, NM [1]; (b) Plataforma Solar de Almería (PSA) Tabernas, Almería, Spain [6]; (c) the semi-arid southern Kalahari region in Northern Cape, South Africa [27]; (d) left is Temara Oceanside site and right is the Skoura desert site in Morocco [28]; (e) Southwest Morocco [29]; (f) University of Oujda in Eastern Morocco [10]; (g) Gardens Point campus, Brisbane, Australia [30]; (h) PSA in Tabernas, Almería, Spain [23]; (i) Benguerir, Morocco [14]; (j) Portugal on an experimental site (Valverde) of the University of Évora [26]; (k) in a rural region of Évora, Alentejo, Portugal [31]. ...
... Therefore, frequent reflector washing is required, and the effectiveness of the washing varies with location and time of year. The solar specular reflectance should be kept at its highest level to ensure high global yield, but economically keeping the solar collectors clean is the biggest maintenance challenge for CSP technology [28,32]. An economic analysis was performed at the Kramer Junction solar power park located in Boron, California, and it indicated that maintaining an average field reflectivity above 90% is cost effective [33]. ...
Article
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Soiling effects influence the output of solar thermal plants, significantly causing unwanted transmittance, reflectance, and absorbance losses. Research is needed to identify what type of reflective surfaces are best suited for semitropical climates, such as the southeastern United States. This paper initially presents a review of several concentrating solar power (CSP) reflector testbeds used to analyze the soiling effects of various reflective materials. A soiling testbed is developed for this study that comprised six sets of reflective surfaces mounted at a fixed tilt of 30 degrees: three sets of thin-film surfaces and three sets of glass types. Two generations of 3M solar mirror film (SMF), 3M SMF 1100 and 3M SMF 2020, were used along with Konica Minolta SMF, silvered Corning Willow Glass, a dichroic cold mirror, and a standard mirror. Results show that the 3M SMF 2020 and Konica Minolta SMF performed the best during regular cleaning intervals, whereas the silvered Corning Willow Glass gave the best performance if only cleaned naturally. The other glass types showed the least average loss due to soiling throughout this study but gave the lowest performance for specular reflection.
... These barriers act on the wind to deflect and slow the speed of the dust and sand particles, thus preventing them from reaching the mirror surfaces. This will have an additional benefit of protecting the outermost mirror rows from erosion due to direct sand impact, thus reducing this performance loss over time [4] To evaluate the performance of different designs of dust barriers, full-scale CFD modelling of multiple mirrors rows protected by two different candidate shapes was performed. The mass of particles landing on the surfaces of the mirrors was compared with the case of no barrier, taking into account the different tracking angles of the mirrors during the day. ...
Conference Paper
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This paper describes the development and validation of computational fluid dynamics modelling of dust barriers for parabolic trough plants. Two differently shaped porous barriers were simulated with a parabolic trough field of up to 25 rows. Sand and dust were introduced into the simulation and the amount that collects on each mirror row was calculated. This was done for different mirror tracking angles and also for the case of no barrier present. The effectiveness of the barriers was evaluated by comparing the reduction in the amount of soiling present on the mirror surfaces. An additional simulation of the Cranfield University wind tunnel was done, with the results compared with experimental data for validation. It has been found that the dust barriers can lead to up to 45% reduction in the soiling in the outer row, with some effect seen as far as 25 rows into the plant.
... A realistic laboratory simulation of SDS degradation crucially needs to be developed in accordance with observations made on exposed materials at the respective SPP sites. Current research is therefore consequently tailoring artificial aging tests in agreement with detected failures from outdoor exposure [13,23,24]. However, the observed degradation effects and associated reflectance losses on outdoor exposed samples depend on a variety of parameters. ...
Article
A large share of future solar energy plants is going to be located in desert environments where the involved aging effects of the installed components still exhibit a lack of knowledge. The acquired data within this project is considered of high interest for plant developers because for the first time representative samples were exposed at three heights above ground (1.2, 2.4 and 3.6 m) and varying orientations at a Moroccan site which is known to exhibit significant sandstorm activity for 12 months. Additionally, the present wind velocities and directions were measured. It could be concluded, that the strongest reflectance losses are detected on the samples which are orientated towards the directions from which the maximum wind velocities were measured, even though their frequency was three orders of magnitude smaller than the frequency of intermediate winds. Furthermore, an explicit height dependence of the erosion effects with increasing distance from the ground is demonstrated. Analysis of the horizontal inclination angles showed that the reflectors exposed at 90° experienced twice the damage that was measured at the reflectors exposed at 45°. This is in line with theoretical assumptions and results from laboratory experiments dealing with the dependence of erosion effects on the impact angle.
... The study carried out by Karim et al. [84] focused on the effects of sandstorms on mirror surfaces. Figure 4 shows some effects. ...
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The global energy production model is changing from fossil fuels to renewable and nuclear energies. Concentrated solar power is one of the growing technologies that is leading this process. This growth implies the sophistication and size of the systems and, therefore, it requires an increase in maintenance tasks to ensure reliability, availability, maintainability and safety. The aim of this paper is to describe the current context of concentrated solar power, to summarise and analyse the main degradation mechanisms and the main techniques to detect, prevent and mitigate these faults. An exhaustive literature study is presented, considering the most advanced techniques and approaches. A novel qualitative and quantitative analysis of the literature is provided. Finally, the current trends and the future challenges in this field are gathered from this study.
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This review provides a comprehensive, detailed description and contextualization of soiling research evolution in the solar energy field throughout time. The analysis consists of past soiling research, including important notes on notable works and main researches. The current state of the art is presented, followed by an extended literature survey covering from 1942 to 2019, facilitating the finding of primordial research concerning each of the available technologies, and enriching knowledge regarding the existing extensive research database. Moreover, soiling analysis and comments are made for several specific topics, such as cleaning techniques and environmental effects on soiling deposition. Finally, future prospects and research directions on the soiling effect are given.
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Sand/dust storms (SDSs) are a common natural phenomenon occurring in the spring in arid and semi-arid areas. To investigate the influence of sand/dust on the performance of concentrating solar power systems, the effects of four SDS events and four natural dust deposition (NDD) periods were tested from March 7 to June 11, 2021, and were compared with the performance of the clean state. The results reveal that the sand/dust concentration on the mirror increased significantly after the occurrence of SDS events. In particular, after a severe sandstorm, the dust density was found to have reached 6.177 g/m ² , which was 2.09 times that under the NDD exposure period of 40 days. With the increase in the dust density by 1 g/m ² after the occurrence of the four SDS events, the average reflectivity, energy flux density, and thermal efficiency were found to be decreased by 12.1%, 11.9%, and 12.5%, respectively, whereas they are decreased by 13.7%, 10.5%, and 13.7% under the four NDD periods.
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Modeling radiative transfer on a dusty photovoltaic (PV) module is a complicated problem. In this work, an improved optical light pathway model was established based on a three-layer system (dust particles–cover glass–solar cell); this system models radiative transfer by considering absorption, reflection, and transmission. Transmittance and reflectance formulae based on the light transmission and reflection pathway were obtained. Optical experiments were conducted for incident light with a spectral range of 0.30–0.7 μm to verify the improved model. The results show that when the thickness of the particle layer was larger than the sum of the minimum and maximum diameters, the theoretical spectral transmittance and reflectance agree well with the experimental data. The relative errors of spectrum transmittance and reflectance were 1.17–16.79% and 4.17–11.68%, respectively. Spectral transmittance through the composite layers was very low, and the dust particle composition and layer thickness had strong inhibiting effects on the transmission of incident light.
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During the life-time of Concentrated Solar Power (CSP) plants, optical performance of solar mirrors are affected by soiling phenomenon and surface degradations, especially in desert and oceanic environments such as those prevailing in North Africa. This optical loss results from the deposition of dust particles, salts, organic materials or any other contaminants present in the solar field or around the CSP installation sites. This phenomenon depends on the different exposure sites with every location having its own meteorological and geological characteristics. Moroccan CSP sites are planned in many locations with different environmental conditions: arid, semi-arid and Saharan, with or without oceanic influence. These environmental factors can have an aggressive impact on the CSP mirrors. Indeed, large particles will tend to degrade the mirror by scratching or breaking the glass surface, while small particles have more chance to deposit on solar mirrors and thus create a soiling layer. These particles are generated from many sources (sand storm, pollution, vehicular movements, etc.) and are transported through the air. This mode of transport is determined as a function of mineral, size, shape and hardness. In this paper, we present a simple methodology for analyzing the chemical and physical characteristics of the sand particles, characterization techniques, and their appropriate laboratory equipment. All the factors previously mentioned could be critical for the CSP mirrors. That is why analyzing these data may be a key point for the industry to understand the effects of soiling and degradation on the CSP mirrors in order to increase the global economic profitability of their solar plants.
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This paper describes work to compare the optical properties and surface texture of glass and polymer film collectors. We also present the results of experiments designed to simulate collector cleaning processes (both contact and non-contact), and the degradation of glass and polymer reflecting surfaces owing to sand and dust abrasion. Finally we present initial results on the applicability of anti-soiling and self-cleaning coatings on glass and polymer film collector surfaces. Measurements, which include specular and hemispherical reflectance, surface roughness, and electron microscopy, indicate the excellent performance of currently available polymer film in terms of its optical performance and robustness in comparison with traditional glass collectors in CSP applications.
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PV modules that are sold on the worldwide market today have to pass the relevant IEC tests for certification. These tests are only a mark for a certain quality level, not a reliability test. Nevertheless, manufacturers of PV modules give performance guaranties of 20 to 25 years on their products, some even more. Therefore the question to be asked is: ‘How to survive 25+ years between the pole and the equator?’ or, seen from the other side: ‘Why do PV modules fail?’ To answer this important question we will show a global approach, starting with the presentation of general failure reasons. On the one hand, extrinsic PV module failures can be caused by different climatic stress factors and by defective installations. In the following, we will show a classification of those factors, which provides the basis for weathering analyses in the lab. With this data background some major failures and related test methods will be presented. On the other hand, intrinsic failure reasons are to be taken into account. We will exemplify some intrinsic failure reasons on material level. Also, an overview of major failures and their frequency detected during certification will be presented in order to arrive at an assumption as to why modules fail. Finally we will explain adequate test methods to detect and measure the relevant failure factors and to test new materials, components and modules for future photovoltaic systems.
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Surface abrasion of engineering materials by blown sand bombardment is an extensive near-surface phenomenon in desert areas. In this study, experiments conducted in a straight-line, blow-type wind tunnel provided useful information for glass abrasion profiles, the intensity of abrasion with time and vertical distribution of blown sand energy. Micro-morphology of the flaws on abraded glass surface was observed with SEM. The micro-morphology of the surface flaws was observed as conchae, fan, semicircle, and petal shapes, which were generated by lateral cracks or radial cracks. Average axes length of flaws increased exponentially with time. The intensity of abrasion was good exponentially relation to the time. On the abrade profile, the height of maximum abrasion (Hmax) occurred at 15-17.5cm above the sand surface. Under the Hmax, the abrade intensity increased with the height. On the contrary, abrade intensity decreased with the height above the Hmax. There was a good exponentially relation between the ratio of sand transport and the height. The abrasion capacity of sand particles increased power with the height. The curve of abrade intensity obtained in the experiments were consistent with the results of Sharp and Liu et al, but the Hmax was different.
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During their operation in Concentrated Solar Power plants (CSP), solar reflectors are exposed to harsh climatic conditions which cause their degradation. Surface damage caused by direct impact or by mechanical contact cleaning is among phenomena that decrease the optical performance of solar mirrors. This degradation is directly related to the exposure site properties. Up to now, no standardized mechanical durability standard exists related to the site exposure conditions. The originality of the present study is to find out which geological and climatic parameters are influencing the surface wear rate of the solar mirrors and to define an approach to determine these parameters on each potential implantation site. This approach was applied on two sites in Morocco with more than 500 km between them: a desert site and a seaside site. The aim of this work is to identify the influencing parameters on the surface damage phenomenon. Additional studies are ongoing in this direction to define a criterion to evaluate how external conditions of an implantation site can affect the surface damage of solar mirrors and to define the weighting of the parameters to get a “level of surface damage” of each potential site. By analyzing the exposure conditions impact on mirror’s surface damage, the sustainability of the solar reflectors can be determined. The present study can also help the investors to optimize the performance of the solar power plant by choosing the appropriate technology and materials for each implementation site.
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The performance of solar systems (thermal or photovoltaic) is influenced by the ability of the glazing to transmit solar radiation to the collection surface, besides other factors, such as, incident radiation, tilt of collector, properties of materials, operating strategy, surroundings, etc. This paper discusses the influence of dust on the transmittance of a 0.2-mm-thick low-density polyethylene (LDPE) glazing used commonly in solar air heaters. The reduction in transmittance due to various dust deposition densities of Bangkok clay (size 53–75 μm) has been measured and a correlation relating the dust deposition density and the transmittance given. Experimental observations of natural dust accumulation on an inclined (15°) LDPE glazing at a tropical climatic condition during a 30-day period indicates a dust accumulation of 3.72 g/m2 and is found to reduce the global transmittance of the glazing from about 87.9% to 75.8%.
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Research and development in photovoltaic (PV) systems has usually been concentrated in studies on radiation availabil-ity, efficient operating strategies, design and sizing of these systems. On the other hand, the influence of dust on the per-formance of PV systems has not been given much attention. The work described here contributes considerably to overcome this deficit. To validate that concept, we have developed an experimental set up involving 100 glass samples with different tilt (b) and azimuth angles (c). The transmittance of the glass was evaluated at regular intervals over a period of about seven months and after every thunderstorm in the surrounding area. Although it is only a short term view, the preliminary results indicate that the reduction in glass normal transmittance depends strongly on the dust deposition density in conjunction with plate tilt angle, as well as on the orientation of the surface with respect to the dominant wind direction. With this consideration, one sees that as the dust deposition density goes from 15.84 g/m 2 (for glass sample installed at a tilt angle of 0°) to 4.48 g/m 2 (for glass sample installed at a tilt angle of 90° and oriented with 135° deviation from north), the corresponding transmittance diminishes by approximately 52.54– 12.38%, respectively. The evolutions of the output power variation with increasing cell pollution were also examined. It has been found that the slope of the best straight line passing through the data points of the solar cell installed at a 45° angle facing south suggests a decrease in the output power of about 17.4% per month.
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A computer system for surface studies was developed in order to measure particle size distributions on solar collector surfaces. In this study, an attempt was made to apply computerized optical microscopy for comparative analysis of dry deposition and scavenging of dust by raindrops during starting rain. For this purpose, dust load of evaporated raindrops on the glass slides was examined. In this study, the main input dust load of raindrops was found to belong to particles, captured by falling drops in atmosphere.