Julius Yellowhair

Analysis of a scanning pentaprism system for measurements of large flat mirrors

The optical surface of a large optical flat can be measured using an autocollimator and scanning pentaprism system. The autocollimator measures the slope difference between a point on the mirror and a reference point. Such a system was built and previously operated at the University of Arizona. We discuss refinements that were made to the hardware, the alignment procedure, and the error analysis. The improved system was demonstrated with a 1.6 m flat mirror, which was measured to be flat to 12 nm rms. The uncertainty in the measurement is only 9 nm rms.

On-sun testing of an advanced falling particle receiver system

A 1 MWth high-temperature falling particle receiver was constructed and tested at the National Solar Thermal Test Facility at Sandia National Laboratories. The continuously recirculating system included a particle elevator, top and bottom hoppers, and a cavity receiver that comprised a staggered array of porous chevron-shaped mesh structures that slowed the particle flow through the concentrated solar flux. Initial tests were performed with a peak irradiance of ~300 kW/m2 and a particle mass flow rate of 3.3 kg/s. Peak particle temperatures reached over 700 °C near the center of the receiver, but the particle temperature increase near the sides was lower due to a non-uniform irradiance distribution. At a particle inlet temperature of ~440 °C, the particle temperature increase was 27 °C per meter of drop length, and the thermal efficiency was ~60% for an average irradiance of 110 kW/m2. At an average irradiance of 211 kW/m2, the particle temperature increase was 57.1 °C per meter of drop length, and the th...

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.

Measurement of optical flatness using electronic levels

Conventional measurement methods for large flat mirrors are generally difficult and expensive. In most cases, comparison with a master or a reference flat similar in size is required. Using gravity, as in modern pendulum-type electronic levels, takes advantage of a free reference to precisely measure inclination or surface slopes. We describe using two electronic levels to measure flatness of large mirrors. We provide measurement results on a 1.6-m-diameter flat mirror to an accuracy of 50 nm rms of low-order Zernike aberrations.

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

Reduction of radiative heat losses for solar thermal receivers

Solar thermal receivers absorb concentrated sunlight and can operate at high temperatures exceeding 600°C for production of heat and electricity. New fractal-like designs employing light-trapping structures and geometries at multiple length scales are proposed to increase the effective solar absorptance and efficiency of these receivers. Radial and linear structures at the micro (surface coatings and depositions), meso (tube shape and geometry), and macro (total receiver geometry and configuration) scales redirect reflected solar radiation toward the interior of the receiver for increased absorptance. Hotter regions within the interior of the receiver also reduce thermal emittance due to reduced local view factors in the interior regions, and higher concentration ratios can be employed with similar surface irradiances to reduce the effective optical aperture and thermal losses. Coupled optical/fluid/thermal models have been developed to evaluate the performance of these designs relative to conventional designs. Results show that fractal-like structures and geometries can reduce total radiative losses by up to 50% and increase the thermal efficiency by up to 10%. The impact was more pronounced for materials with lower inherent solar absorptances (< 0.9). Meso-scale tests were conducted and confirmed model results that showed increased light-trapping from corrugated surfaces relative to flat surfaces.

COUPLED OPTICAL-THERMAL-FLUID MODELING OF A DIRECTLY HEATED TUBULAR SOLAR RECEIVER FOR SUPERCRITICAL CO2 BRAYTON CYCLE

Recent studies have evaluated closed-loop supercritical carbon dioxide (s-CO2) Brayton cycles to be a higher energy density system in comparison to conventional superheated steam Rankine systems. At turbine inlet conditions of 923K and 25 MPa, high thermal efficiency (similar to 50%) can be achieved. Achieving these high efficiencies will make concentrating solar power (CSP) technologies a competitive alternative to current power generation methods. To incorporate a s-CO2 Brayton power cycle in a solar power tower system, the development of a solar receiver capable of providing an outlet temperature of 923 K (at 25 MPa) is necessary. The s-CO2 will need to increase in temperature by similar to 200 K as it passes through the solar receiver to satisfy the temperature requirements of a s-CO2 Brayton cycle with recuperation and recompression. In this study, an optical-thermal-fluid model was developed to design and evaluate a tubular receiver that will receive a heat input similar to 2 MWth from a heliostat field. The ray-tracing tool SolTrace was used to obtain the heat-flux distribution on the surfaces of the receiver. Computational fluid dynamics (CFD) modeling using the Discrete Ordinates (DO) radiation model was used to predict the temperature distribution and the resulting receiver efficiency. The effect of flow parameters, receiver geometry and radiation absorption by s-CO2 were studied. The receiver surface temperatures were found to be within the safe operational limit while exhibiting a receiver efficiency of similar to 85%.

Testing and optical modeling of novel concentrating solar receiver geometries to increase light trapping and effective solar absorptance

Concentrating solar power receivers are comprised of panels of tubes arranged in a cylindrical or cubical shape on top of a tower. The tubes contain heat-transfer fluid that absorbs energy from the concentrated sunlight incident on the tubes. To increase the solar absorptance, black paint or a solar selective coating is applied to the surface of the tubes. However, these coatings degrade over time and must be reapplied, which reduces the system performance and increases costs. This paper presents an evaluation of novel receiver shapes and geometries that create a light-trapping effect, thereby increasing the effective solar absorptance and efficiency of the solar receiver. Several prototype shapes were fabricated from Inconel 718 and tested in Sandia’s solar furnace at an irradiance of ~30 W/cm2. Photographic methods were used to capture the irradiance distribution on the receiver surfaces. The irradiance profiles were compared to results from raytracing models. The effective solar absorptance was also evaluated using the ray-tracing models. Results showed that relative to a flat plate, the new geometries could increase the effective solar absorptance from 86% to 92% for an intrinsic material absorptance of 86%, and from 60% to 73% for an intrinsic material absorptance of 60%.

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