Jeremy Stark

Characterization of Electrodynamic Screen Performance for Dust Removal from Solar Panels and Solar Hydrogen Generators

The direct solar energy conversion in gigawatt scales by photovoltaic, photothermal, and photoelectrochemical processes is of national and global importance in meeting energy needs. Dust depositions on solar panels and solar concentrators cause efficiency loss from 10% to 30% depending upon the surface mass concentration of dust requiring manual cleaning with water. Such a cleaning process is expensive for large-scale installations where water is scarce. Transparent electrodynamic screens, consisting of rows of transparent parallel electrodes embedded within a transparent dielectric film, can be used for dust removal for their application as self-cleaning solar collectors. When the electrodes are activated by phased voltage, the dust particles on the surface of the film become electrostatically charged and are removed by the traveling wave generated by applied electric field. Over 90% of deposited dust is removed within 2 min, using a very small fraction of the energy produced by the panels. No water or mechanical movement is involved. An analysis of the electrodynamic removal mechanisms based on electrostatic and dielectrophoretic forces opposed by the adhesion forces due to van der Waals and image forces is presented.

Characterization of electrodynamic screen performance for dust removal from solar panels and solar hydrogen generators

The direct solar energy conversion in gigawatt scales by photovoltaic, photothermal, and photoelectrochemical processes is of national and global importance in meeting energy needs. Dust depositions on solar panels and solar concentrators cause efficiency loss from 10% to 30% depending upon the surface mass concentration of dust requiring manual cleaning with water. Such a cleaning process is expensive for large-scale installations where water is scarce. Transparent electrodynamic screens, consisting of rows of transparent parallel electrodes embedded within a transparent dielectric film, can be used for dust removal for their application as self-cleaning solar collectors. When the electrodes are activated by phased voltage, the dust particles on the surface of the film become electrostatically charged and are removed by the traveling wave generated by applied electric field. Over 90% of deposited dust is removed within 2 min, using a very small fraction of the energy produced by the panels. No water or mechanical movement is involved. An analysis of the electrodynamic removal mechanisms based on electrostatic and dielectrophoretic forces opposed by the adhesion forces due to van der Waals and image forces is presented.

Development of Self-Cleaning Solar Collectors for Minimizing Energy Yield Loss Caused by Dust Deposition

Concentrated Solar Power (CSP) systems used for photothermal conversion of solar energy to electricity are capable of meeting a large fraction of the global energy requirements. CSP plants are inherently robust with respect to the availability of materials, technology, and energy storage. However, dust depositions on solar collectors cause energy yield loss annually, ranging from 10 to 50% depending upon their location in the semi-arid and desert lands. Mitigation of energy loss requires manual cleaning of solar mirrors with water. A brief review of the soiling related losses in energy yield of the CSP plants is presented, which shows that cleaning of the CSP mirrors and receivers using water and detergent is an expensive and time-consuming process at best and is often impractical for large-scale installations where water is scarce. We report here our research effort in developing an electrodynamic dust removal technology that can be used for keeping the solar collectors clean continuously without requiring water and manual labor. Transparent electrodynamic screens (EDS), consisting of rows of transparent parallel electrodes embedded within a transparent dielectric film can be integrated on the front surface of the mirrors and on the receivers for dust removal for their application as self-cleaning solar collectors. When the electrodes are activated, over 90% of the deposited dust is removed. A summary of the current state of prototype development and evaluation of EDS integrated solar mirrors and experimental data on the removal of desert dust samples are presented. A brief analysis of cost-to-benefit ratio of EDS implementation for automated dust removal from large-scale solar collectors is included.Copyright

Modeling of Trajectories in an Electrodynamic Screen for Obtaining Maximum Particle Removal Efficiency

An electrostatic self-cleaning panel for solar collectors is described. An electrodynamic screen (EDS) is formed by interdigitated transparent surface electrodes energized by three-phase low-frequency ac voltages in the range of 5-200 Hz and 500-1000 V. The resulting electrostatic field wave exerts force on the particles and sweeps them laterally across the panel. Particle trajectories are simulated to help ascertain parameters for maximum dust-removal efficiency. The electric field of the EDS is found by a Fourier expansion of Laplaces equation solutions for a surface potential that is periodic in space and time. Trajectories are found for particles of various sizes and charges and for different electrode spacings and excitations. Computed trajectories are compared qualitatively to experimental observations. One unexpected result is the chaotic behavior of larger particles which jump sporadically back and forth and only slowly migrate in the direction of the imposed electrostatic surface wave.

Modeling of trajectories in an electrodynamic screen for obtaining maximum particle removal efficiency

An electrostatic self-cleaning panel for solar collectors is described. An electrodynamic screen (EDS) is formed by interdigitated transparent surface electrodes energized by three-phase low-frequency ac voltages in the range of 5-200 Hz and 500-1000 V. The resulting electrostatic field wave exerts force on the particles and sweeps them laterally across the panel. Particle trajectories are simulated to help ascertain parameters for maximum dust-removal efficiency. The electric field of the EDS is found by a Fourier expansion of Laplaces equation solutions for a surface potential that is periodic in space and time. Trajectories are found for particles of various sizes and charges and for different electrode spacings and excitations. Computed trajectories are compared qualitatively to experimental observations. One unexpected result is the chaotic behavior of larger particles which jump sporadically back and forth and only slowly migrate in the direction of the imposed electrostatic surface wave.

Electrodynamic removal of dust from solar mirrors and its applications in concentrated solar power (CSP) plants

Concentrating Solar Power (CSP) systems based on parabolic trough and power tower technologies provide inherent advantage of energy storage and high efficiency for utility-scale solar plants. The specular reflectance efficiency of the solar mirrors plays a critical role in the efficiency of electric power generation. The deposition of atmospheric dust on the surface of the mirrors reduces its reflectance efficiency and requires frequent cleaning to avoid energy-yield loss. Electrodynamic screen (EDS) can provide an efficient method for maintaining the specular reflectivity above 90% by removing the deposited dust particles. In this paper, we briefly review (1) electrostatic charging mechanisms involved in EDS, (2) optimization of EDS for high dust removal efficiency, and (3) minimization of cleaning cost and water consumption. Prototype EDS-integrated solar mirrors were produced and tested in an environmental test chambers simulating desert atmospheres. The test results show that frequent removal of dust layer can maintain the specular reflectivity of the mirrors above 90% subjected to dust deposition ranging from 0 to 10 g/m2.

Environmental degradation of the optical surface of PV modules and solar mirrors by soiling and high RH and mitigation methods for minimizing energy yield losses

Utility-sale solar plants are mostly installed in semi-arid and desert lands and are subjected to high dust deposition rate. Dust layer build up on solar collectors causes major energy-yield loss. Maintaining designed plant capacities requires more than 90% efficiency of light transmission or specular reflection for PV modules and CSP mirrors, respectively. The combinations of high relative humidity (RH), high surface temperature, and long residence time of the dust on the optical surface degrades the solar collectors over time. A tenacious mud like coating is formed, which strongly adheres to the PV modules and concentrating mirrors and requires scrub cleaning. If the global solar-power output is to increase from current GW levels to the TW level, as is envisioned, the water cleaning process would result in an unsustainable demand for water. This paper provide a brief review of the application of an emerging technology of transparent electrodynamic screen (EDS) for removing dust, as frequently as needed, from the solar collectors without water. Power output efficiency is maintained greater than 90% compared to that of the panel under clean conditions. Dust removal efficiency (DRE) is more than 90% with test dust samples obtained from different arid zones and energy consumption for EDS operation is less than 0.03 Wh/m2/cleaning cycle. The method is water-free and provides easy retrofitting onto existing panels and has a high potential for a cost-effective large-scale roll-to-roll production, commercial application, and a significant reduction of operation and maintenance costs.

Self-Cleaning Solar Mirrors Using Electrodynamic Dust Shield: Prospects and Progress

Parabolic trough and power tower technologies provide inherent advantage of thermal energy storage and high efficiency of the Concentrating Solar Power (CSP) systems for utility scale solar plants. High efficiency CSP power generation with minimal water use is one of the SunShot goals of the US Department of Energy. The specular reflectance efficiency of the solar mirrors plays a critical role in the efficiency of power generation. The optical surface of the mirrors and the receiver must be kept clean for efficient operation of the plant. Some environmental challenges in operating the large-scale CSP plants at high reflectance efficiency arise from high concentration of atmospheric dust, wind speed and variation of relative humidity (RH) over a wide range. Deposited dust and other contaminant particles, such as soot, salt, and organic particulate matters attenuate solar radiation by scattering and absorption. Adhesion of these particles on the mirror surface depends strongly by their composition and the moisture content in the atmosphere. Presence of soluble inorganic and organic salts cause corrosion of the mirror unless the contaminants are cleaned frequently.In this paper, we briefly review (1) source of atmospheric dust and mechanisms involved in degradation of mirrors caused by salt particles, (2) loss of specular reflection efficiency as a function of particle size distribution and composition, and (3) an emerging technology for removing dust layer by using thin transparent electrodynamic screen (EDS). Feasibility of integration of EDS on the front surface of the solar collectors has been established to provide active self-cleaning properties for parabolic trough and heliostat reflectors.Prototype EDS-integrated solar collectors including second-surface glass mirrors, metallized acrylic film mirrors, and dielectric mirrors, were produced and tested in an environmental test chambers simulating desert atmospheres. The test results show that frequent removal of dust layer can maintain the specular reflectivity of the mirrors above 90% under dust deposition at a rate ranging from 0 to 10 g/m2, with particle size varying from 1 to 50 μm in diameter. The energy required for removing the dust layer from the solar was less than 10 Wh/m2 per cleaning cycle. EDS based cleaning could therefore be automated and performed as frequently as needed to maintain reflection efficiency above 90% and thus reducing water usage for cleaning mirrors in the solar field. A comparative cost analysis was performed between EDS and deluge water based cleaning that shows the EDS method is commercially viable and would meet water conservation needs.Copyright

Optical Modeling of Reflectivity Loss Caused by Dust Deposition on CSP Mirrors and Restoration of Energy Yield by Electrodynamic Dust Removal

For large scale CSP power plants, vast areas of land are needed in deserts and semi-arid climates where uninterrupted solar irradiance is most abundant. These power facilities use large arrays of mirrors to reflect and concentrate sunlight onto collectors, however, dust deposition on the optical surfaces causes obscuration of sunlight, resulting in large energy-yield losses in solar plants. This problem is compounded by the lack of natural clean water resources for conventional cleaning of solar mirrors, often with reflective surface areas of large installations exceeding a million square meters. To investigate the application of transparent electrodynamic screens (EDS) for efficient and cost effective dust removal from solar mirrors, both optical modeling and experimental verifications were performed. Prototype EDS-integrated mirrors were constructed by depositing a set of parallel transparent electrodes into the sun-facing surface of solar mirrors and coating electrodes with thin transparent dielectric film. Activation of the electrodes with a three-phase voltage creates an electrodynamic field that charges and repels dust electrostatically by Coulomb force and sweeps away particles by a traveling electrodynamic wave. We report here brief discussions on (1) rate of deposition and the properties of dust with respect to their size distribution and chemical composition in semi-arid areas of the southwest US and Mojave Desert and their adhesion to solar mirrors, (2) optical models of: (a) specular reflection losses caused by scattering and absorption by dust particles deposited on the surface based on Mie scattering theory, and (b) reflection loss by the integration of EDS on the mirror surface, computed by FRED ray-tracing model. The objective is to maintain specular reflectivity of 90% or higher by frequent removal of dust by EDS. Our studies show that the incorporation of transparent EDS would cause an initial loss of 3% but would be able to maintain specular reflectivity more than 90% to meet the industrial requirement for CSP plants. Specular reflection measurements taken inside a climate controlled environmental chamber show that EDS integration can restore specular reflectivity and would be able to prevent major degradation of the optical surface caused by the deposition of dust.© 2014 ASME

Mitigation of dust impacts on solar collectors by water-free cleaning with transparent electrodynamic films: Progress and challenges

Energy-yield loss caused by soiling of photovoltaic modules and concentrated solar power (CSP) mirrors in utility-scale power plants installed in semiarid lands and deserts would result in unsustainable demands for fresh water needed for cleaning. This paper reviews the progress of the electrodynamic screen (EDS) film technology for frequent water-free cleaning with low-energy requirements. Results presented here, based on laboratory-scale EDS-film-laminated solar panel cleaning, show that the output power can be restored higher than 95% of the initial power under clean conditions. For solar mirrors, the specular reflection efficiency can be maintained over 90% ensuring high efficiency of the CSP plants. Operation of the EDS film for maintaining high optical efficiency of solar collectors requires less than 0.2 Wh/m