photovoltaic (PV) panels, this study focuses primarily on the impact of dust accumulation on concentrated photovoltaic (CPV) and concentrated solar power ...
Mitigation of Soiling Losses in Concentrating Solar Collectors Arash Sayyah, Mark N. Horenstein, and Malay K. Mazumder Department of Electrical and Computer Engineering, Boston University, Boston, MA 02215 Abstract—The adverse impact of soiling (dust deposition) on solar collectors, and the mitigation of the related energy yield losses, are the main scopes of this paper. While soiling related losses have been studied more extensively for flat-plate photovoltaic (PV) panels, this study focuses primarily on the impact of dust accumulation on concentrated photovoltaic (CPV) and concentrated solar power (CSP) systems. We report on different methods used for cleaning solar collectors: (i) natural cleaning by rain and snowfall, (ii) manual cleaning by water and detergent, and (iii) an emerging method of dust removal by electrodynamic screens (EDS). Development of EDS technology as an automated, low-cost dust removal method which does not require any water or manual labor is presented. Index Terms—dust accumulation, soiling, photovoltaic, concentrated photovoltaic, concentrated photovoltaic, surface cleaning, electrodynamic screen.
upon geographical location, atmospheric dust loading, particle size, electrostatic particle charge, particle distribution, wind velocity and turbulence, time, collector surface material, and meteorological conditions. Soiling losses recorded during outdoor exposure of various CSP and CPV systems in different parts of the world are presented to highlight the deteriorating effect of dust deposition. Different manual cleaning methods used in previous studies are discussed, and the advantages and drawbacks of each method are also elucidated. The development of the electrodynamic screen (EDS) as an automatic dust removal mechanism is presented and its advantages over other cleaning methods are emphasized. II. I NFLUENTIAL PARAMETERS IN S OILING R ATE
I. I NTRODUCTION Solar power generation by photovoltaic (PV), concentrated photovoltaic (CPV), and concentrated solar power (CSP) systems has grown steadily in recent years due to ever-increasing energy demand as well as the environmental and economical concerns associated with fossil fuel consumption. According to the US Energy Information Administration, PV, CPV, and CSP systems have shown the most increase in annual growth rate (11.7%) among the renewable energy resources [1]. As is the case with any other developing technology, the harvesting of solar energy presents significant challenges. Mitigation of energy-yield loss caused by dust depositing on the solar collector surfaces, often called “soiling” is one of the more critical of these challenges. Soiling adversely impacts the performance of solar energy conversion systems by preventing the solar radiation from reaching the energy conversion devices. Sunlight is obscured by absorption and scattering of incident light as it passes through the dust layer. Analyses show that while a portion of the forward scattered light caused by dust particles on a flat-plate PV surface would be absorbed for energy production by light trapping, the corresponding soiling losses by dust layers on concentrating solar collectors are more severe. Experiments performed by Sandia National Laboratories and Jet Propulsion Laboratory show that concentrating collectors in the southwestern region of the US experience performance decreases of about 15% over one year under natural outdoor conditions [2]. Dust related episodes in arid areas, such as dust storms, can affect systems severely, but the predominant dust collection is via suspended atmospheric particulates that are deposited on the solar-collector surfaces due to gravity, electrostatic attraction, wind-induced turbulent deposition, and diffusion [3]. Such dust buildup is a complicated process that depends
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Soiling has more destructive effect on concentrated solar systems than on flat-plate PV panels. A portion of sunlight forward scattered by dust particles deposited on a PV surface would be absorbed by light trapping and produce energy, but a dust layer on a concentrating solar collector, such as a mirror and Fresnel lenses, will prevent the light from reaching from the energy conversion device since (1) CPV and CSP systems work mainly from beam radiation, (2) a direct beam must travel twice through the dust layer during reflection, and (3) in general, concentrating plants are designed in large-scales, and hence are mainly installed in the semi-arid and desert regions where the direct normal irradiance (DNI) is highest, with concomitant high dust concentrations, dust storms, and scarcity of water. Therefore, removing deposited dust from the surfaces of solar collectors surfaces is of utmost importance in order to maximize their efficiency. A. Orientation of Solar Collector The angle of inclination of a solar collector, and more generally its orientation, has been shown to be a decisive factor in soiling losses. Inclination angle also plays a pivotal role in the efficiency of natural cleaning agents such as rainfall and snow, as discussed later in this paper. Figure 1 illustrates the mean and standard deviation of specular reflectance losses of mirror specimens having five different orientations exposed to the outdoor environment in Albuquerque, NM between July 1976 and November 1977 without washing. The orientations of the samples, as denoted in Fig. 1, are as follows: (a) permanently face-up, (b) face-up/face-down with astronomical timer, (c) near vertical stow position with astronomical timer, (d) sensor face-up/face-down, and (e) permanently face-down. As is evident in the figure, the reflectance losses decreased significantly for the sample equipped with a tracking system.
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40 Monthly Average of Transmission Loss [%]
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a
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Fig. 1. The specular reflection losses for different orientations of mirror specimens in Albuquerque, NM [4]. The orientations of the mirror specimens are: (a) permanently face-up, (b) face-up/face-down with astronomical timer, (c) near vertical stow position with astronomical timer, (d) sensor face-up/facedown, and (e) permanently face-down. The losses for the permanent face-up specimen (a) is approximately 5 times that of permanent face-down specimen (e).
As a further example that shows the significance of inclination angle, sample sheets of glass, acrylic, and polyvinyl chloride (PVC) were exposed to the outdoors in the Thar desert, India [5]. The monthly average of transmission losses of glass samples cleaned daily are shown in Fig. 2. The annual average of transmission reduction percentages are 4.26%, 2.94%, and 1.36% for glass samples having tilt angles of 0◦ , 45◦ , and 90◦ , respectively. B. Soiling Rate Most of the studies that deal with the soiling losses of solar energy harvesting systems consider the exposure period as a determining factor in quantification of soiling rate. Specifically, the longer exposure period, the more the soiling losses, assuming that no natural or human-initiated cleaning event has occurred. El-Shobokshy et al. [6] believe, however, that the amount of dust deposited on a collector should be correlated to its performance degradation, rather than the exposure time of the solar collector to its natural environment. They have investigated the impact of dust accumulation on a CPV-cell system in Riyadh, Saudi Arabia, where the mean dust deposition rate during the test period was recorded as 0.387 g/m2 /day. Figure 3 shows the decrease in short-circuit current vs. dust concentration density. As can be observed in Fig. 3, a saturating behavior is observed in the losses in short-circuit current. Similar saturating performance degradation has also been observed in other studies like [7]. In [8], sample mirrors
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θ = 45◦
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θ = 90◦
5 4 3 2 1 0 Jan. Feb. Mar. May June Sept. Oct. Nov. Dec. Month
e
Fig. 2. Average of transmission loss for daily cleaned glass samples tilted at 0◦ , 45◦ , and 90◦ in Thar Desert, India [5].
tested over 2-day, 6-day, and 12-day cleaning cycles were exposed to the outdoor environment in Albuquerque, NM. As both a general observation and as measured quantitatively, specular reflectance of the mirror samples decreased considerably the day after the cleaning procedure. Confirming the nonlinearity of dust accumulation, it is concluded that the dust accumulation rate decreases as the deposited dust increases. 1
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Fig. 3. The impact of dust concentration density on the normalized shortcircuit current for solar insolation intensity of 370 W/m2 [6].
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Dust does not necessarily accumulate uniformly on the surfaces of solar collectors. Rather, deposition is more a function of the collector’s position and environment than its surface type. Wind direction plays a pivotal role in this nonuniform pattern, whereby it deposits a dirty water film in the case of light rain towards one particular spot, where settles dust as the water evaporates [4]. Hence, for large surfaces such as heliostat mirrors, specular reflectance is measured at different spots, while heavily-soiled areas like the edges are avoided; the mean value is then reported. C. Wavelength Soiling losses are not identical at all wavelengths in the spectrum of solar insolation. If the size of accumulated dust particles is comparable with the wavelength of solar insolation, the investigation of dependency of specular reflectance on wavelength for dusty solar collectors can provide us with significant insight into the dust accumulation problem. This has been investigated a study by [9] in which silvered glass mirrors were exposed for five weeks to the outdoor environment of a 5 MW solar thermal test facility located in Albuquerque, NM. Figure 4 shows the specular reflectance vs. wavelength for both clean and dusty samples. Dust concentration density increases from (a) to (d). The specular reflectance losses associated with the dusty samples (a), (b), (c), and (d) were 6.5%, 10%, 16%, and 24%, respectively, relative to the clean sample at 500 nm. Further, the reflectance loss decreases with an increase in wavelength, such that the reflectance loss values range from 3.8% to 14% at 900 nm. 95
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Fig. 4. The variation of specular reflectance vs. wavelength for clean and dusty mirror samples exposed in Albuquerque, NM [9]. Dust concentration density increases from (a) to (d).
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III. NATURAL S OILING L OSSES In contrast to the plethora of research activities that focus on the impact of dust accumulation on flat-plate PV modules, soiling studies on CSP and CPV systems are very limited. A summary of natural soiling studies is provided in Table I. For each study, the table indicates (a) test location, (b) general climate at the installation site based on K¨oppen climate classification system, (c) collector type, (d) orientation of the collector, (e) outdoor exposure period, (e) affected output(s), and (f) maximum recorded loss. In cases of multiple data points in the study, only a few results are tabulated in this paper for the sake of brevity. In the table, R, Isc , Pout , and η denote specular reflection, short-circuit current, output power, and efficiency, respectively. Chronologically ordering the studies does not necessarily indicate when a study was actually performed, because the range of exposure periods among the experiments varied between one day and a few months. To obtain the amount of loss due to soiling, two approaches were followed in the studies: 1) one module was cleaned on a regular basis as the reference , while the other were left unattended to collect dust over the predetermined exposure period. The output(s) of the clean and dusty modules were compared continuously to attain the soiling losses, and 2) the desired output(s) were measured before and just after each cleaning cycle to obtain the soiling losses. IV. C LEANING AGENTS A. Natural Cleaning Processes Rain is the predominant natural agent that restores the reflectivity of solar collectors. A secondary agent is the tilting of glass mirror specimens to the horizontal while exposing them to the outdoor environment. Such a study in Dalton, GA resulted in specular reflection losses of less than 8% over a one-month exposure period, while identical samples succumbed to losses of up to 62% over a similar period in Henderson, NV [12]. This result is attributable to the significant amount of rainfall received in Dalton. In contrast, Henderson has a very arid climate. Rain does not necessarily have a cleaning effect on solar collectors. This has been clearly shown in [8], in which a second surface mirror sample from a heliostat was exposed to the outdoor environment of Albuquerque, NM continuously for 200 days without cleaning. As can be seen in Fig. 5, a noticeable drop was observed in the course of the experiment on day 154, when the reflectance dropped from 0.846 to 0.720. This significant drop can be attributed to a light rain followed by dusty and windy conditions that decreased the reflectance of the mirror. After two days, however, rain increased the reflectance by 0.121 reflectance units (from 0.702 to 0.823). In general, light rain often leaves a spotty appearance on collector surfaces, particularly on those having a low inclination angle. After a period of time, the spotty surface decreases the specular reflectance of the exposed mirror [3]. As stated previously,
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Maximum loss 17.7% 25% 12.5% 23% 52% 24% 9% 28.6% 30.6% 10% 6.5% 16.8% 23.2% 12.3% 15.4%
the day after either manual or natural cleaning, the specular reflectance drops significantly.
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b Alzak:
a trade name for aluminized acrylic sheet with a 0.003 cm layer of adhesive backing. sheet of highly polished aluminum made by Alcoa Corp. c X: Geometric concentration ratio. d PMMA: polymethylmethacrylate.
Fig. 5. Specular reflectance vs. exposure days for a second surface silvered mirror sample, tilted at 45◦ with respect to horizontal and faced south. Based on a weekly measurement of specular reflectance, losses up to 24.5% was observed for this mirror in the exposure period [8].
a FEK:
Oceanic Canberra, Australia
[13] [14] 1988 2010
desert Mediterranean
[6] 1985
Kuwait Madrid, Spain
Paraboloid dishes CPV with ‘V’ mirror 2X Cylindrical parabolic trough 10X Cylindrical parabolic trough 20X PMMAd Fresnel lens 300X CSP system: silvered 38X mirror
tracking system
45◦ , south 45◦ , south [12] 1980
hot desert desert humid subtropical desert
[11] 1979
Henderson, NV Lovington, NM San Antonio, TX Riyadh, Saudi Arabia
second surface glass mirror FEK-244a aluminized acrylic mirror Alzakb mirror CPV 56Xc
permanent face-up horizontal, face-up 45◦ , south second surface mirror heliostat with acrylic coating second-surface silvered mirror semi-arid arid semi-arid [10] 1978
Albuquerque, NM China Lake, CA Albuquerque, NM
Collector Type Climate Location Reference Year
TABLE I S OILING L OSSES IN CSP AND CPV S YSTEMS .
Orientation
Exposed period 30 days 6 months 2 months 160 days 1 month 1 month 2 months 12 days
Affected output(s) R R R R R R R Isc Pout η Isc Isc Isc Isc Isc
0.92
The efficacy of rain in removing accumulated dust particles is dependent upon orientation of the collectors’ surface. Specifically, deposited dust can be washed off from surfaces having a more vertical alignment. In order to maximize the cleaning effect of rain, stowing heliostats and specimens in a near-vertical position is recommended during heavy precipitation [4]. One of the other factors that can significantly affect the role of rain in removing accumulated dust is the glazing material used for the solar-collector surface. For heliostats exposed in Indian Wells Valley, CA [4], having two types of reflective surfaces: (1) second surface silvered laminated glass and (2) first surface silvered glass with an experimental acrylic protective coating, it was observed that water was not able to wet the acrylic surface as well as did glass, and dust particles adhered more strongly to the acrylic than to the glass. This fact also made the cleaning effect of frost less significant, as a water/ice mixture does not flow easily on an acrylic surface. B. Manual Cleaning Various non-contact and contact manual cleaning methods have been used in small- and large-scale concentrated solar fields. Although the applicability of some of the methods examined is limited to laboratory environments, they are mentioned herein for the sake of completeness. In a heliostat installation at the Naval Weapons Center (NWC), China Lake, CA [4], the cleaning procedure consisted of the following steps:
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1) The reflector was positioned at 70 degrees, faced Northeast, North, and again Northeast for morning, midday, and afternoon washing, respectively, to minimize spotting of the surface due to fast drying of washing solution. 2) The reflector surface was sprayed with a washing solution. 3) The reflector surface was rinsed completely with deionized water after approximately one-minute soak time. The washing solution was used instead of tap water, because tap water usually leaves a spotty surface dependent upon its minerals. Without using deionized water, sheeting agents like the ones used in household dishwashers can avoid such spots. High-pressure water spray, as shown in Fig. 6, is considered to be one of the most effective cleaning methods among the existing routines, because it is not detrimental to the surface, is economical, and is environmentally friendly.
ultrasonically for 3 minutes. The samples were then removed from the beaker and gently wiped with a soft tissue. Subsequent measurements showed 100% reflectivity restoration. As mentioned previously, the practicality of this method and similar routines is considerably limited in large-scale solar facilities. In addition, some of the cleaning results reported in the literature are not fully compatible with commercial plants, even for field tests, as special care is taken in those plants to use a minimal amount of water and solutions. In contrast, in research studies, the goal is to maximize the reflectance as much as possible and to restore initial reflectivity. Although several advantages are associated with the normal routine of using high-pressure spray, as currently practiced in solar fields, this cleaning mechanism requires a significant amount of water, which is scarce at the most solar sites situated in arid environments. Furthermore, manual cleaning methods require teams of experienced technicians to perform the operations. Labor cost is a further prohibitive factor. According to the detailed cost analysis conducted in [17], labor cost comprises 45.7% of cost among different governing parameters for the cleaning reflective surfaces using the high-pressure spray method. Table II summarizes these results. Although cost is a function of many parameters, including inflation and geographical location, Table II provides a good representation of the percentage of each sector toward the total cost. C OST A NALYSIS FOR
Fig. 6. The normal routine for cleaning reflective solar surfaces using highpressure spray in a plant which has been used since early days of developments of solar energy harvesting fields [15].
Most of the aforementioned non-contact cleaning mechanisms were able to restore 98% of the original reflectivity. The 2% loss is attributed to not scrubbing the collector’s surface or not using hydrofluoric acid, which is harmful for the environment [12]. Furthermore, non-contact cleaning methods were unable to remove the tenacious layer of soil on the mirror surfaces, which developed due to presence of moisture and specific types of soil [16]. In a few experiments performed in Lovington, NM, it was observed that increasing the cleaning frequency significantly decreased the rate of soil layer formation but it did not completely stop it. Increasing the cleaning frequency inevitably adds to the maintenance cost. Some of the cleaning methods pursued in laboratory environments were able to restore the initial specular reflectance of tested samples. In an experiment conducted in [8], the samples were placed in a distilled water beaker and excited
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THE
TABLE II H IGH - PRESSURE S PRAY M ETHOD [17].
I TEM Materials Water (at 300 psi) Sheeting Agent (at 200 ppm) Fuel Labor Equipment Maintenance Capital Expenditures Total
P ERCENT OF C OST 2.4 1.4 20.5 45.7 4.8 25.2 100.0
V. EDS D EVELOPMENT FOR AUTOMATIC D UST R EMOVAL The transparent electrodynamic screen (EDS) [18]–[21], consisting of a series of transparent interdigitated electrodes embedded in a transparent dielectric film, can be used as a viable dust mitigation system for removing dust particles from solar-collector surfaces. Unlike the aforementioned methods of actively cleaning for maintaining a clean surface, the EDS method requires no mechanical movement or expensive and interruptive manual cleaning. EDS is a dust shield that can be integrated on the surface of the front glass cover plate of PV modules, Fresenel lens or at the top of the second surface glass mirrors used in heliostat mirrors and parabolic troughs. An EDS is comprised of thin, parallel, transparent conducting electrodes made from materials such as indiumtin oxide (ITO), aluminum doped zinc oxide oxide (Al:ZnO
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or AZO), nanowire silver (AgNW), or a conducting polymer (PEDOT:PSS). These electrodes are deposited over the glass covering of PV modules, Fresnel lenses or reflectors of CSP systems. Using a transparent dielectric film of thickness approximately 50 µm, the electrodes are covered, which protects them from environmental degradation. The cross-section of a typical EDS is shown in Fig. 7. When energized by low power, three-phase pulsed voltages at frequencies of 5 to 20 Hz, these electrodes produce a traveling electric-field wave that moves charged particles laterally over the surface via Coulomb and dielectrophoretic forces. Dust particles are charged electrostatically and are lifted up by the Coulomb force and are transported to the edges by the traveling wave, thereby clearing the screen. Dust particles can become charged in several ways. Primary charging mechanisms are contact charging against the dielectric film and charge injection from the electrodes. Some particles are naturally charged when they become airborne. Experiments for numerous trials have shown more than 90% reflectivity restoration is possible after EDS activation on the order of a few minutes [19].
Fig. 7.
The cross section of the EDS panel [21].
VI. C ONCLUSION Deposition of dust particles on solar collectors’ surfaces significantly reduces the power output of solar systems, particularly for concentrated solar systems, as they are more sensitive to direct light absorption. Soiling losses in CPV and CSP systems due to dust accumulation in different geographical regions of the world have also been considered. Various contact and noncontact cleaning methods were discussed, and their advantages and disadvantages were highlighted. The development of the electrodynamic screen as an alternative method for removing dust particles from solar surfaces without water or moving parts was discussed. Since the electrodynamic screen cleaning method is easily automated, it can cope with the shortcomings associated with current manual cleaning methods used widely in practice. ACKNOWLEDGMENT
R EFERENCES [1] (2012) Annual energy outlook 2012 with projections to 2035. [Online]. Available: http://www.eia.gov/forecasts/aeo/pdf/0383(2012).pdf [2] P. J. Call, “Summary of solar experience with the soiling of optical surfaces,” Tech. Rep. SERI/TP-334-478, 1980. [3] R. M. Bethea, M. T. Barriger, P. F. Williams, and S. Chin, “Environmental effects on solar concentrator mirrors,” Solar Energy, vol. 27, no. 6, pp. 497–511, 1981. [4] J. B. Blackmon and H. H. Dixon, “Dust buildup tests of heliostats and mirror specimens,” McDonnell Douglas Astronautics Co., Huntington Beach, CA, Tech. Rep. MDC-G-7543, 1978. [5] N. M. Nahar and J. P. Gupta, “Effect of dust on transmittance of glazing materials for solar collectors under arid zone conditions of India,” Solar & Wind Technology, vol. 7, no. 2, pp. 237–243, 1990. [6] M. S. El-Shobokshy, A. Mujahid, and A. K. M. Zakzouk, “Effects of dust on the performance of concentrator photovoltaic cells,” Proc. IEE, vol. 132, no. 1, pp. 5–8, Feb. 1985. [7] H. K. Elminir, A. E. Ghitas, R. Hamid, F. El-Hussainy, M. Beheary, and K. M. Abdel-Moneim, “Effect of dust on the transparent cover of solar collectors,” Energy Conversion and Management, vol. 47, no. 1819, pp. 3192 – 3203, 2006. [8] J. M. Freese, “Effects of outdoor exposure on the solar reflectance properties of silvered glass mirrors,” Sandia Labs., Albuquerque, NM, Tech. Rep. 78-1649, 1978. [9] R. B. Pettit, J. M. Freese, and D. E. Arvizu, “Specular reflectance loss of solar mirrors due to dust accumulation,” in Proc. Testing Solar Energy Materials and Systems, 1978, pp. 164–168. [10] J. Blackmon and M. Curcija, “Heliostat reflectivity variations due to dust buildup under desert conditions,” in Proc. Inst. Environ. Sci., 1978, pp. 169–183. [11] J. M. Freese, “Effects of outdoor exposure on the solar reflectance properties of silvered glass mirrors,” in Proc. Int. Solar Energy Meeting, May 1979, pp. 1340–1344. [12] V. L. Morris, “Cleaning agents and techniques for concentrating solar collectors,” Solar Energy Materials, vol. 3, no. 1-2, pp. 35–55, 1980. [13] A. Al-Kandari, A. M. R. Al-Marafie, R. K. Suri, and G. P. Maheshwari, “Performance assessment of a paraboloid dish collector field,” Solar Energy, vol. 41, no. 2, pp. 163 – 167, 1988. [14] M. Vivar, R. Herrero, I. Ant´on, F. Mart´ınez-Moreno, R. Moreton, G. Sala, A. Blakers, and J. Smeltink, “Effect of soiling in CPV systems,” Solar Energy, vol. 84, no. 7, pp. 1327–1335, 2010. [15] G. E. Cohen, D. W. Kearney, and G. J. Kolb, “Final report on the operation and maintenance improvement for concentrating solar power plants,” Sandia National Labs., Tech. Rep. SAND99-1290, 1999. [16] D. E. Randall and V. L. Morris, “Initial experience and preliminary results: solar collector materials exposure to the IPH site environment,” Sandia National Labs., Albuquerque, NM., Tech. Rep. SAND-81-0290, 1981. [17] M. B. Sheratte, “Cleaning agents and technologies for concentrating solar collectors,” Sandia Labs., Albuquerque, NM, Tech. Rep. 79-7052, 1979. [18] R. Sharma, C. A. Wyatt, J. Zhang, C. I. Calle, N. Mardesich, and M. K. Mazumder, “Experimental evaluation and analysis of electrodynamic screen as dust mitigation technology for future Mars missions,” IEEE Trans. Ind. Appl., vol. 45, no. 2, pp. 591–596, 2009. [19] M. K. Mazumder, M. N. Horenstein, J. Stark, D. Erickson, A. Sayyah, S. Jung, and F. Hao, “Development of self-cleaning solar collectors for minimizing energy yield loss caused by dust deposition,” in Proc. of the ASME 7th International Conference on Energy Sustainability, Minneapolis, MN, 2013. [20] M. K. Mazumder, R. Sharma, A. S. Biris, J. Zhang, C. Calle, and M. Zahn, “Self-cleaning transparent dust shields for protecting solar panels and other devices,” Particulate Sci. and Technology, vol. 25, no. 1, pp. 5–20, 2007. [21] M. K. Mazumder, M. N. Horenstein, J. Stark, P. Girouard, R. Sumner, B. Henderson, O. Sadder, I. Hidetaka, A. Biris, and R. Sharma, “Characterization of electrodynamic screen performance for dust removal from solar panels and solar hydrogen generators,” in Industry Applications Society Annual Meeting (IAS), 2011 IEEE, 2011, pp. 1–8.
The studies reported here are being supported by a grant from US DOE EERE SETP CSP Research and Development Office (Grant No. DOE DE-EE0005794).
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