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Journal of Electrostatics 73 (2015) 65e70

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Journal of Electrostatics

journal homepage: www.elsevier .com/locate/elstat

Electrostatic cleaning system for removal of sand from solar panels

Hiroyuki Kawamoto*, Takuya Shibata Department of Applied Mechanics and Aerospace Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan

a r t i c l e i n f o

Article history: Received 20 May 2014 Received in revised form 16 September 2014 Accepted 27 October 2014 Available online 7 November 2014

Keywords: Cleaner Electrostatic force Mega solar Sand Solar panel

* Corresponding author. Tel./fax: þ81 3 5286 3914. E-mail address: [email protected] (H. Kawamoto).

http://dx.doi.org/10.1016/j.elstat.2014.10.011 0304-3886/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

An improved cleaning system has been developed that uses electrostatic force to remove sand from the surface of solar panels. A single-phase high voltage is applied to parallel wire electrodes embedded in the cover glass plate of a solar panel. It has been demonstrated that more than 90% of the adhering sand is repelled from the surface of the slightly inclined panel after the cleaning operation. The performance of the systemwas further improved by improving the electrode configuration and introducing natural wind on the surface of the panel, even when the deposition of sand on the panel is extremely high. The power consumption of this system is virtually zero. This technology is expected to increase the effective effi- ciency of mega solar power plants constructed in deserts at low latitudes.

© 2014 Elsevier B.V. All rights reserved.

Introduction

Solar power generation has grown drastically in recent years owing to increasing energy demand as well as the environmental and economical concerns associated with fossil fuel consumption [1], and many mega solar power generation plants are being planned and constructed, especially in deserts at low altitudes, where the sun shines the brightest. However, sand storms occur frequently in deserts, and solar panels can become covered by stirred-up sand, causing a drastic decrease in the output power of a photovoltaic power generation plant [2e7]. Because sand on solar panels is not cleaned by rain over a long period of time in an arid region, the capacity utilization of the power plant is reduced if the panels are not cleaned frequently.

To mitigate this problem, we have developed an automatic cleaning system that does not need scarce cleaning water but instead utilizes an alternating electrostatic force [8]. Transporting particles using electrostatic force was first developed and imple- mented by Masuda et al. [9], and many investigations of this technology have subsequently been conducted mainly as a toner supplier in electrophotography [10e21]. Numerous other applica- tions for the electrostatic particle transport have been proposed, including control of bubbles in dielectric liquid [22], removal of radioactive dust in a fusion reactor [23], transport of liquid droplet

[24], movement of blood cells in liquid [25], classification of particle size [26], separation of seed by-products derived from agricultural processes [27], and dust removal from solar panels and solar hydrogen generators [28]. Theoretical and numerical studies of electrostatic particle transport have been conducted to clarify the mechanism and to support the development by many researchers [10,12,13,21,29]. Cleaning of lunar or Martian dust on solar panels for space exploration is another potential application of this tech- nology [30e33]. It has been demonstrated that more than 98% of the dust on a glass plate can be removed using electrostatic trav- eling waves generated by a four-phase rectangular voltage applied to a transparent conveyer consisting of transparent indium tin oxide (ITO) electrodes printed on a glass substrate [33].

However, this technology is not suitable for use in commercial mega solar systems because it requires prohibitively expensive ITO electrodes, the ends of the electrodes must be three-dimensional to prevent the intersection of phases, and the power supply and in- terconnections required are relatively complicated and expensive for large-scale commercial plants. To mitigate these issues we have developed an improved system that consists of a sand-repelling glass plate with parallel wire electrodes embedded in a cover glass plate of a solar panel and a high-voltage power supply that generates a single-phase rectangular voltage. The alternating electrostatic field generates a standing wave that causes a flipeflop motion of the sand particles on the device, and when airborne, the sand particles are transported downward by gravity [8]. This report describes a basic principle and performance of the system, and how the performance of the systemwas further improved by improving

Fig. 1. Schematic diagram of the electrostatic cleaning system that uses a standing wave and gravity to remove sand from a solar panel.

Table 1 Specification of sand used for experiments.

Item Unit A B C D E F

Area Namib Japan Eurasia Oceania North America Africa Relative

permittivity 4.2 2.2 3.2 4.3 4.0 5.3

Elongation 0.72 0.53 0.76 0.83 0.81 0.71 Angle of

repose deg 36 38 39 31 34 35

Bulk density g/cm2 1.5 1.4 1.4 3.0 1.7 3.0

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the electrode configuration and introducing natural wind on the surface of the panel, evenwhen the deposition of sand on the panel is extremely high. The power consumption of this system is extremely low compared to the output power of the solar panel. This technology is expected to increase the effective efficiency of mega solar power generation plants constructed in deserts at low latitudes.

System configuration

Parallel wire electrodes embedded in the cover glass plate of the solar panel was employed instead of ITO electrodes to reduce the manufacturing cost of the cleaning plate. Although the wire elec- trodes create a shadow and disturb the absorption of light, this is minimized by using a fine wire and a wide pitch configuration. The diameter chosen for the wire electrodes was 0.3 mm, and the size chosen for the pitch between the electrodes was 7 mm.

To mitigate the complexity of the electrode wiring, power supply, and interconnections, we adopted a standing wave instead of a traveling wave [34e36]. That is, a single-phase rectangular voltage was applied to parallel wire electrodes. Because a traveling wave is not generated by the application of a single-phase voltage, particles are not transported in one direction but rather repelled from the plate, and when airborne the sand particles are trans- ported downward by gravity. We generated a single-phase rect- angular voltage by using a set of positive and negative amplifiers switched by semiconductor relays that were controlled by a microcomputer. Because a high slew-rate is not required for this system, we employed conventional low-capacity onboard ampli- fiers (HUR30-6, Matsusada Precision, Tokyo).

Fig. 2. Six types of sand u

Fig. 1 shows a schematic diagram of the system. If the system is operated intermittently, the sand that has adhered to the cover glass of the solar panels is repelled. On the other hand, if the system is operated continuously, the sand that approaches the cover glass is also repelled, and thus, the system can protect solar panels against the adhesion of sand.

Six types of sand, collected from desert areas around the world, were used for evaluation. Photographs of the sand particles are shown in Fig. 2, and these specifications are summarized in Table 1. Sand A was commonly used in the experiments unless otherwise specified.

Results and discussion

Effect of plate inclination

We manufactured a small device for use in the basic investiga- tion of the system. The dimensions of the substrate glass plate were 100 � 100 � 3 mm. After 0.3-mm-diameter copper wires were arranged on the plate, a thin glass plate, 0.1 mm in thickness, was adhered using transparent adhesive to make the surface smooth and to prevent insulation breakdown. Cross-sectional drawing of the device is shown in Fig. 3.

The device was inclined, and sand was uniformly scattered on the cover glass. A single-phase rectangular voltage was then applied to the parallel electrodes. The experimentwas conducted in an air-conditioned laboratory (20e25 �C, 40e60 RH). As shown in Fig. 4, the sand particles on the glass plate were repelled and transported downward, as confirmed by direct observation of particle motions using a high-speed microscope camera (Fastcam- max 120 K model 1, Photoron, Tokyo) [21,33,35] and numerical

sed for experiments.

Fig. 3. Cross-sectional drawing of cleaning device.

Fig. 5. Relationship between the inclination of the panel and the cleaning efficiency (100-g/m2 surface loading, 0.86 kVp-p/mm, 1 Hz).

H. Kawamoto, T. Shibata / Journal of Electrostatics 73 (2015) 65e70 67

calculations. The numerical calculations were on the basis of a three-dimensional hard-sphere model of the distinct element method (DEM). Details of the numerical method are reported in the literature [21,33,35]. The electrostatic field that determines the Coulomb force and the dielectrophoresis force applied to the sand particles is calculated by a two-dimensional differential element method in a cyclic domain. Although the dynamic motion of the particles cannot be conveyed by still images as shown in Fig. 4, we confirmed the calculated and observed motions are in qualitative agreement by comparing calculated and measured movies. As described later, the calculated performance agrees well with the measured results, not only qualitatively but also quantitatively.

Fig. 5 shows the cleaning efficiency, i.e., the ratio between the weight of the sand fed onto the panel for 30 s and that after the cleaning operation of the system, versus the inclination of the plate. A cleaning experiment using a four-phase traveling wave was also conducted for comparison with single-phase cleaning. High per- formance was achieved even when the plate was only slightly in- clined, and the performance achieved by standing wave cleaning was almost the same as that for traveling-wave cleaning when the inclination was greater than 20�. This suggests that the system would even be effective at low latitudes at which solar panels are installed at low inclinations.

Effects of pitch, applied voltage, and frequency

Figs. 6 and 7 show the cleaning efficiency versus the averaged electrostatic field strength determined by the applied voltage divided by the pitch of the parallel electrodes and the frequency of the applied voltage, respectively. The solid curves in Figs. 6 and 7 show the calculated results, which agree well with the measured results. We observed that producing a high field strength achieved high performance; however, saturation occurred at a high value. Because the applied voltage is limited by the insulation breakdown, which is determined by the electrostatic field, the system perfor- mance is almost independent of the electrode pitch at the threshold

Fig. 4. Observed and calculated motions of dust particles during operation of the system (20� inclination, 100-g/m2 surface loading, 0.8 kVp-p/mm, 1 Hz).

voltage. The threshold voltage was 9.8 kVp-p for the 10-mm-pitch device and 8.4 kVp-p for the 7-mm-pitch device.

The maximum cleaning efficiency was approximately 80% and was achieved at a low frequency (less than 20 Hz). The cleaning performance decreased at higher frequencies because particle motion cannot follow the high-speed change of polarity [21,33,35,36]. However, low-frequency operation is not an issue because high-speed cleaning is not necessary for this system.

Improved device

The cleaning performance of the system was further improved by adopting a V-shaped configuration for the wire electrodes, as shown in Fig. 8. An angle of 0� corresponds to the horizontal (original) configuration and an angle of 90� corresponds to the vertical configuration. It is clear that a V-shaped configuration (with angles between 45� and 75�) for the electrodes improved the cleaning performance of the system. Careful observation of particle motion made using the high-speed microscope camera and nu- merical calculations suggest that when the V-shaped configuration is used, some particles on the panel are repelled not only down- ward but also toward the lateral sides of the panel, and this phe- nomenon increases the cleaning efficiency.

Effect of surface loading of sand

El-Shobokshy et al. [37] reported a mean deposition rate of sand on solar panels of 0.387 g/m2/day and a cumulative dust deposition in one month of approximately 10 g/m2 in Riyadh, Saudi Arabia (latitude 24.9�). A much higher level of cumulative sand deposition, more than 400 g/m2 in one month, has been recorded at Kuwait international airport [38]. Because the amount of sand deposition on the panel depends on the geographic and meteorological con- dition of the location where the solar panels are installed, the

Fig. 6. Measured and calculated relationships between the applied voltage (electro- static field) and cleaning efficiency (20� inclination, 100-g/m2 surface loading, 1 Hz).

Fig. 7. Measured and calculated relationships between the frequency of the applied voltage and the cleaning efficiency (20� inclination, 100-g/m2 surface loading, 0.86 kVp-p/mm).

Fig. 9. Calculated and measured relationships between the surface loading of sand and the cleaning efficiency (20� inclination, 0.86 kVp-p/mm, 1 Hz). The photograph shows aggregated sand on the panel after operation. The sand bridges adjacent electrodes and locks on the plate.

H. Kawamoto, T. Shibata / Journal of Electrostatics 73 (2015) 65e7068

appropriate design criterion for sand deposition is not clear; however, it is reasonable to assume that a cleaning system must be able to remove more than 100 g/m2 of sand. Thus, we conducted experiments and calculations to examine the performance of the cleaning system in cases of high loadings.

Fig. 9 shows the effect of the surface loading of the sand. If an amount of sand greater than 300 g/m2 accumulated on the cover glass, the performance of the cleaning system declined owing to the aggregation of sand that bridges the adjacent electrodes as shown in the photograph in Fig. 9 [24]. However, high performance was achieved when the surface loading was less than 300 g/m2, which corresponds to a sand layer thickness of approximately 0.3 mm.

Although the performance of the cleaning systemwas worse for high surface loading conditions, it was experimentally confirmed that the cleaning performance improved when a weak wind, with the velocity greater than 1e2m/s, flowed parallel to the plate in the inclined direction while the electrostatic cleaner was in operation. It is reported that high speed wind increases the deposition of dust [39]; however, stirring dust particles by the alternating electrostatic force in the presence of wind enhances the cleaning.

Effect of particle diameter

To determine the sizes of the particles that can be cleaned by this system, the sand particles were classified into five groups ac- cording to their particle sizes, determined using sieves, and the cleaning experiment was conducted using each classified sand size. Fig. 10 shows the cleaning efficiency versus the particle size. Par- ticles smaller than 25 mm in diameter and those larger than 300 mm in diameter were not cleaned efficiently. The reasons for the problems cleaning solar panels with small and large particles are

Fig. 8. Relationship between the angle of the V-shaped configuration of the wire electrodes and the cleaning efficiency (20� inclination, 100-g/m2 surface loading, 0.86 kVp-p/mm, 1 Hz).

different. For small particles, the electrostatic image force and adhesion force are relatively large compared to the Coulomb and dielectrophoretic driving forces. As a result, these particles adhere to the surface of the glass plate, which reduces the performance of the cleaning system [33]. For large particles, the large gravitational force hinders their bouncing and transport. Because finer particles have a greater impact than coarser particles on the performance of a solar panel [2], the cleaning system and its operational scheme must be improved and optimized to enhance its performance in cleaning small particles from solar panels.

Effect of sand characteristics

The six types of sand shown in Fig. 2 and summarized in Table 1 were evaluated to confirm the effectiveness of the cleaning system for a range of sand characteristics. Fig.11 shows a comparison of the cleaning performance for the six different types of sand considered. Because many factors affect the performance of the cleaning sys- tem, it is difficult to clarify the reasons for the differences observed in the cleaning performance; however, the electrostatic cleaning systemwas shown to be effective for a variety of sands. The system and its operational scheme must be modified and optimized to fit the environmental conditions of the site where the mega solar plant is located.

Power consumption

The power consumption of the cleaning system is shown in Fig. 12. The ordinates of the figures represent the power con- sumption (the input power to the device) per unit area of the

Fig. 10. Cleaning efficiencies for the classified particle sizes (100-g/m2 surface loading, 20� inclination, 0.7 kVp-p/mm, 0.2 Hz).

Fig. 11. Cleaning efficiencies for six types of sand (300-g/m2 surface loading, 20�

inclination, 0.86 kVp-p/mm, 0.2 Hz).

Fig. 13. Demonstration of the effectiveness of the electrostatic cleaning system on a large solar panel (150-g/m2 surface loading, 20� inclination, 0.7 kVp-p/mm, 0.2 Hz).

H. Kawamoto, T. Shibata / Journal of Electrostatics 73 (2015) 65e70 69

cleaning plate assuming that the power consumption is propor- tional to the area of the plate. Because the transient current flows immediately after the polarity change, the power consumption is proportional to the frequency. On the other hand, the power con- sumption is proportional to the square of the applied voltage if insulation breakdown does not occur [33]. Because the voltage limit for insulation breakdown is 8.4 kVp-p for the 7-mm-pitch elec- trodes, and the optimal frequency is less than 10 Hz, the power consumption is only 0.2 W/m2 under operational conditions of 7 kVp-p and 1 Hz. An important factor that influences the energy consumption is the operational time of the system, which depends on the operational scheme, i.e., the number of operational cycles and the operational period. For example, if the system is operated for 30 min a day, the energy consumption is only 0.1 Wh/m2 a day. The energy consumption of this system is extremely low compared to the typical energy output by the solar cell.

Demonstration

The performance of this system was demonstrated using an actual large solar panel (560 mm � 320 mm). The left-hand side of

Fig. 12. Power consumption of the electrostatic cleaning system.

Fig. 13 shows the sand accumulated on the panel, and the right- hand side shows the panel after the cleaning operation was applied to the left half of the solar panel for 3 min. Fig. 13 clearly shows that the cleaning system is effective in removing accumu- lated sand from a solar panel. The output power of the panel with the cleaner plate (without dust) was 97% compared to that without the cleaner plate and dust. The power was reduced to 60%when the dust covered the plate, and it was recovered to 90% after operation. Another experiment was conducted to demonstrate that the sand that approaches the cover glass is repelled if the system is operated continuously. The performance of the cleaning system was better under continuous operation than under intermittent operation. Field experiment must be conducted under desert conditions to determine the optimal operational scheme and to demonstrate the effectiveness of the system for the specific conditions of interest.

If the surface of the plate gets wet owing to rainfall or dewfall, or if a sandstorm and rainfall occur simultaneously, the accumulated sand will adhere strongly to the plate owing to liquid bridging force. Cleaning experiments conducted under these conditions confirmed that high performance was achieved after the plate dried.

Concluding remarks

An improved cleaning system for removal of the sand that ac- cumulates on solar panels using electrostatic force has been developed. This system is suitable for use in mega solar power plants constructed in deserts at low latitudes because it is poten- tially inexpensive, requires virtually no power, and operates auto- matically without water and other consumables.

Acknowledgment

The author would like to express his gratitude to Haruna Takahashi and Shogo Shibata (Waseda University) who helped to carry out the experiments. This work was supported by JSPS KAKENHI Grant Number 23360116.

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