The heliostat collector field is the front-end of large solar power tower plants. Any negative performance impacts on the collector field will propagate down the stream of subsystems, which can negatively impact energy production and financial revenues. An underperforming collector field will provide insufficient solar flux to the receiver resulting in the receiver running at below capacity and not producing the thermal energy required for thermal storage and to run the power block at optimum efficiency. It is prudent to have an optimally operating collector field especially for future Gen3+ plants. The performance of a deployed collector field can be impacted by mirror quality (surface and shape), mirror canting errors, tracking errors, and soiling. Any of these error sources can exist during installation and further degrade over time and, if left unattended, can drastically reduce the overall performance of the plant. Concentrating solar power (CSP) plant operators require information about the collector field performance to quickly respond with corrections, if needed, and maintain optimum plant performance. This type of fast response is especially critical for future Gen3+ plants, which require high collector field performance consistently. However, power tower operators have struggled with finding or developing the right tools to assess and subsequently fix canting errors on in-field heliostats efficiently and accurately. Sandia National Laboratories National Solar Thermal Test Facility (NSTTF) is developing an aerial imaging system to evaluate facet canting quality on in-situ and offline heliostats. The imaging system is mounted on an unmanned aerial system (UAS) to collect images of targets structures in reflection. Image processing on the collected images is then performed to get estimates of the heliostat canting errors. The initial work is to develop the system definition that achieves the required measurement sensitivities, which is on the order of 0.25-0.5 mrad for canting errors. The goal of the system is to measure heliostat canting errors to <0.5 mrad accuracy and provide data on multiple heliostats within a day. In this paper, the development of the system, a sensitivity analysis, and initial measurement results on two NSTTF heliostats are provided.
High heat flux (>500 kW/m2) ignitions occur in scenarios involving metal fires, propellants, lightning strikes, above ground nuclear weapon use, etc. Data for material response in such environments is primarily limited to experimental programs in the 1950s and 1960s. We have recently obtained new data in this environment using concentrated solar energy. A portion of the experimental data were taken with the objective that the data be useful for model validation. To maximize the utility of the data for validation of predictive codes, additional focus is placed on repeatability of the data, reduction of uncertainties, and characterization of the environment. We illustrate here a portion of the data and methods used to assess environmental and response parameters. The data we present are novel in the flux range and materials tested, and these data constitute progress in the ability to characterize fires from high flux events.
In order for Concentrating Solar Power plants (CSP) to achieve the desired cost breakpoint, significant improvement in performance is required resulting in the need to increase temperatures of fluid systems. A US DOE Small Business Voucher project was established at Sandia to explore the performance characteristics of Ceramic Tubular Products (CTP) silicon carbide TRIPLEX tubes in key categories relating to its performance as a solar receiver in next generation CSP plants. Along these lines, the following research tasks were completed : (1) Solar Spectrum Testing, (2) Corrosion Testing in Molten Chloride Salt, (3) Mechanical Shock Testing, and (4) Thermal Shock Testing. Through the completion of these four tasks, it has been found that the performance of CTP's material across all of these categories is promising, and merits further investigation beyond this initial investigation. Through 50 solar aging cycles, the CTP material exhibited excellent stability to high temperatures in air, exhibited at or above 0.95 absorptance, and had measured emittances within the range of 0.88-0.90. Through molten salt corrosion testing at 750degC it was found that SiC exhibits significantly lower mass change (-- 90 times lower) than Haynes 230 during 108 hours of salt exposure. The CTP TRIPLEX material performed significantly better than the SiC monolithic tube material in mechanical shock testing, breaking at an average height of 3 times that for the monolithic tubes. Through simulated rain thermal shock testing of CTP composite tubes at 800degC it was found that CTP's SiC composite tubes were able to survive thermal shock, while the SiC monolithic tubes did not. ACKNOWLEDGEMENTS * US Department of Energy Office of EERE for sponsorship of this project * Andrew Dawson of the DOE Office of EERE for Project Management, including the excellent technical insights that he provided throughout the project * Ken Armijo lead the Thermal Shock Testing activities * Cliff Ho and Julius Yellowhair led the Solar Spectrum Testing activities * Jeff Halfinger prepared the CTP specimens for each of the research tasks * Herb Feinroth provided guidance and input into the preparation for the test specimens and the associated research tasks * Alan Kruizenga collaborated with CTP to apply for and be awarded this project from DOE EERE. The scope for the project was developed by Alan together with CTP. * Rio Hatton and Jesus Ortega (student interns) helped with portions of the solar simulator testing, reflectance/emittance data collection, and image (including microscope) collection. * Kent Smith helped design and fabricate the high temperature molten salt corrosion setup * Jeff Chames and Javier Cebrian completed the microscopy for the molten salt corrosion test specimens * Amy Bohinsky (student intern) and Kevin Nelson helped complete the mechanical shock testing for the monolithic and composite tubes, including organizing the results for the final report. * Josh Christian and Daniel Ray helped with portions of the Thermal Shock Testing * Mark Stavig completed the polyethylene plug testing associated with the Thermal Shock Testing
A 1 MWt falling particle receiver prototype was designed, built and is being evaluated at Sandia National Laboratories, National Solar Thermal Test Facility (NSTTF). The current prototype has a 1 m2 aperture facing the north field. The current aperture configuration is susceptible to heat and particle losses through the receiver aperture. Several options are being considered for the next design iteration to reduce the risk of heat and particle losses, in addition to improving the receiver efficiency to target levels of ~90%. One option is to cover the receiver aperture with a highly durable and transmissive material such as quartz glass. Quartz glass has high transmittance for wavelengths less than 2.5 microns and low transmittance for wavelengths greater than 2.5 microns to help trap the heat inside the receiver. To evaluate the receiver optical performance, ray-tracing models were set up for several different aperture cover configurations. The falling particle receiver is modeled as a box with a 1 m2 aperture on the north side wall. The box dimensions are 1.57 m wide x 1.77 m tall x 1.67 m deep. The walls are composed of RSLE material modeled as Lambertian surfaces with reflectance of either 0.9 for the pristine condition or 0.5 for soiled walls. The quartz half-shell tubes are 1.46 m long with 105 mm and 110 mm inner and outer diameters, respectively. The half-shell tubes are arranged vertically and slant forward at the top by 30 degrees. Four configurations were considered: concave side of the half-shells facing away from the receiver aperture with (1) no spacing and (2) high spacing between the tubes, and concave side of the half-shells facing the aperture with (3) no spacing and (4) high spacing between the tubes. The particle curtain, in the first modeling approach, is modeled as a diffuse surface with transmittance, reflectance, and absorptance values, which are based on estimates from previous experiments for varying particle flow rates. The incident radiation is from the full NSTTF heliostat field with a single aimpoint at the center of the receiver aperture. The direct incident rays and reflected and scattered rays off the internal receiver surfaces are recorded on the internal walls and particle curtain surfaces as net incident irradiance. The net incident irradiances on the internal walls and particle curtain for the different aperture cover configuration are compared to the baseline configuration. In all cases, just from optical performance alone, the net incident irradiance is reduced from the baseline. However, it is expected that the quartz half-shells will reduce the convective and thermal radiation losses through the aperture. These ray-tracing results will be used as boundary conditions in computational fluid dynamics (CFD) analyses to determine the net receiver efficiency and optimal configuration for the quartz half-shells that minimize heat losses and maximize thermal efficiency.
Designs of conventional heliostats have been varied to reduce cost, improve optical performance or both. In one case, reflective mirror area on heliostats has been increased with the goal of reducing the number of pedestals and drives and consequently reducing the cost on those components. The larger reflective areas, however, increase torques due to larger mirror weights and wind loads. Higher cost heavy-duty motors and drives must be used, which negatively impact any economic gains. To improve on optical performance, the opposite may be true where the mirror reflective areas are reduced for better control of the heliostat pointing and tracking. For smaller heliostats, gravity and wind loads are reduced, but many more heliostats must be added to provide sufficient solar flux to the receiver. For conventional heliostats, there seems to be no clear cost advantage of one heliostat design over other designs. The advantage of ganged heliostats is the pedestal and tracking motors are shared between multiple heliostats, thus can significantly reduce the cost on those components. In this paper, a new concept of cable-suspended tensile ganged heliostats is introduced, preliminary analysis is performed for optical performance and incorporated into a 10 MW conceptual power tower plant where it was compared to the performance of a baseline plant with a conventional radially staggered heliostat field. The baseline plant uses conventional heliostats and the layout optimized in System Advisor Model (SAM) tool. The ganged heliostats are suspended on two guide cables. The cables are attached to rotations arms which are anchored to end posts. The layout was optimized offline and then transferred to SAM for performance evaluation. In the initial modeling of the tensile ganged heliostats for a 10 MW power tower plant, equal heliostat spacing along the guide cables was assumed, which as suspected leads to high shading and blocking losses. The goal was then to optimize the heliostat spacing such that annual shading and blocking losses are minimized. After adjusting the spacing on tensile ganged heliostats for minimal blocking losses, the annual block/shading efficiency was greater than 90% and annual optical efficiency of the field became comparable to the conventional field at slightly above 60%.
Various ganged heliostat concepts have been proposed in the past. The attractive aspect of ganged heliostat concepts is multiple heliostats are grouped so that pedestals, tracking drives, and other components can be shared, thus reducing the number of components. The reduction in the number of components is thought to significantly reduce cost. However, since the drives and tracking mechanisms are shared, accurate on-sun tracking of grouped heliostats becomes challenging because the angular degrees-of-freedom are now limited for the multiple number of combined heliostats. In this paper, the preliminary evaluation of the on-sun tracking of a novel tensile-based cable suspended ganged heliostat concept is provided. In this concept, multiple heliostats are attached to two guide cables. The cables are attached to rotation spreader arms which are anchored to end posts on two ends. The guide cables form a catenary which makes tracking on-sun interesting and challenging. Tracking is performed by rotating the end plates that the two cables are attached to and rotating the individual heliostats in one axis. An additional degree-of-freedom can be added by differentially tensioning the two cables, but this may be challenging to do in practice. Manual on-sun tracking was demonstrated on small-scale prototypes. The rotation arms were coarsely controlled with linear actuators, and the individual heliostats were hand-adjusted in local pitch angle and locked in place with set screws. The coarse angle adjustments showed the tracking accuracy was 3-4 milli-radians. However, with better angle control mechanisms the tracking accuracy can be drastically improved. In this paper, we provide tracking data that was collected for a day, which showed feasibility for automated on-sun tracking. The next steps are to implement better angle control mechanisms and develop tracking algorithms so that the ganged heliostats can automatically track.
Solar glare reflections and avian solar-flux hazards are an important concern for concentrating solar installations. Reflected sunlight from "standby" heliostats has been noted by pilots as potentially hazardous, and reports of birds being singed by concentrated sunlight has created concern. This paper presents the Tower Illuminance Model ("TIM"), a software application developed to investigate glare and avian-flux hazards at concentrating solar power towers in a convenient and interactive manner. TIM simulates a field of heliostats in standby mode, wherein sunlight is not reflected toward the central receiver but at some location in the airspace around the receiver. The user can select a range of aiming strategies and field configurations and can navigate the simulated airspace above the heliostat field in real-time using an interactive 3D interface. As the user "flies" through the airspace, TIM calculates the irradiance, glare hazard, and potential avian flux hazard. TIM is currently undergoing validation and industry testing.
This report describes software tools that can be used to evaluate and mitigate potential glare and avian-flux hazards from photovoltaic and concentrating solar power (CSP) plants. Enhancements to the Solar Glare Hazard Analysis Tool (SGHAT) include new block-space receptor models, integration of PVWatts for energy prediction, and a 3D daily glare visualization feature. Tools and methods to evaluate avian-flux hazards at CSP plants with large heliostat fields are also discussed. Alternative heliostat standby aiming strategies were investigated to reduce the avian-flux hazard and minimize impacts to operational performance. Finally, helicopter flyovers were conducted at the National Solar Thermal Test Facility and at the Ivanpah Solar Electric Generating System to evaluate the alternative heliostat aiming strategies and to provide a basis for model validation. Results showed that the models generally overpredicted the measured results, but they were able to simulate the trends in irradiance values with distance. A heliostat up-aiming strategy is recommended to alleviate both glare and avian-flux hazards, but operational schemes are required to reduce the impact on heliostat slew times and plant performance. Future studies should consider the trade-offs and collective impacts on these three factors of glare, avian-flux hazards, and plant operations and performance.
This report summarizes the results of a two-year project funded by the U.S. Department of Energy's Solar Energy Technologies Office (SuNLaMP 1506) to evaluate the performance of high-temperature (>700 °C) particle receivers for concentrating solar power (see Appendix A for project information). In the first year, novel particle release patterns were designed and tested to increase the effective solar absorptance of the particle curtain. Modeling results showed that increasing the magnitude and frequency of different wave-like patterns increased the effective absorptance and thermal efficiency by several percentage points, depending on the mass flow rate. Tests showed that triangular-wave, square-wave, and parallel-curtain particle release patterns could be implemented and maintained at flow rates of ~10 kg/s/m. The second year of the project focused on the development and testing of particle mass-flow control and measurement methods. An automated slide gate controlled by the outlet temperature of the particles was designed and tested. Testing demonstrated that the resolution accuracy of the slide-gate positioning was less than ~1 mm, and the speed of the slide gate enabled rapid adjustments to accommodate changes in the irradiance to maintain a desired outlet temperature range. Different in-situ particle mass-flow measurement techniques were investigated, and two were tested. The in-situ microwave sensor was found to be unreliable and sensitive to variations in particle flow patterns. However, the in-situ weigh hopper using load cells was found to provide reliable and repeatable measurements of real-time in-situ particle mass flow. On-sun tests were performed to determine the thermal efficiency of the receiver as a function of mass flow rate, particle temperature, and irradiance. Models of the tests were also developed and compared to the tests.
Ho, C.K.; Christian, Joshua M.; Yellowhair, Julius; Jeter, S.; Golob, M.; Nguyen, C.; Repole, K.; Abdel-Khalik, S.; Siegel, N.; Al-Ansary, H.; El-Leathy, A.; Gobereit, B.
A 1 MWt continuously recirculating falling particle receiver has been demonstrated at Sandia National Laboratories. Free-fall and obstructed-flow receiver designs were tested with particle mass flow rates of ∼1 - 7 kg/s and average irradiances up to 1,000 suns. Average particle outlet temperatures exceeded 700 °C for the free-fall tests and reached nearly 800 °C for the obstructed-flow tests, with peak particle temperatures exceeding 900 °C. High particle heating rates of ∼50 to 200 °C per meter of illuminated drop length were achieved for the free-fall tests with mass flow rates ranging from 1 - 7 kg/s and for average irradiances up to ∼ 700 kW/m2. Higher temperatures were achieved at the lower particle mass flow rates due to less shading. The obstructed-flow design yielded particle heating rates over 300 °C per meter of illuminated drop length for mass flow rates of 1 - 3 kg/s for irradiances up to ∼1,000 kW/m2. The thermal efficiency was determined to be ∼60 - 70% for the free-falling particle tests and up to ∼80% for the obstructed-flow tests. Challenges encountered during the tests include particle attrition and particle loss through the aperture, reduced particle mass flow rates at high temperatures due to slot aperture narrowing and increased friction, and deterioration of the obstructed-flow structures due to wear and oxidation. Computational models were validated using the test data and will be used in future studies to design receiver configurations that can increase the thermal efficiency.
A method to evaluate avian flux hazards at concentrating solar power plants (CSP) has been developed. A heat-transfer model has been coupled to simulations of the irradiance in the airspace above a CSP plant to determine the feather temperature along prescribed bird flight paths. Probabilistic modeling results show that the irradiance and assumed feather properties (thickness, absorptance, heat capacity) have the most significant impact on the simulated feather temperature, which can increase rapidly (hundreds of degrees Celsius in seconds) depending on the parameter values. The avian flux hazard model is being combined with a plant performance model to identify alternative heliostat standby aiming strategies that minimize both avian flux hazards and negative impacts on plant performance.
Novel designs to increase light trapping and thermal efficiency of concentrating solar receivers at multiple length scales have been conceived, designed, and tested. The fractal-like geometries and features are introduced at both macro (meters) and meso (millimeters to centimeters) scales. Advantages include increased solar absorptance, reduced thermal emittance, and increased thermal efficiency. Radial and linear structures at the 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 can reduce thermal emittance due to reduced local view factors to the environment, and higher concentration ratios can be employed with similar surface irradiances to reduce the effective optical aperture, footprint, and thermal losses. Coupled optical/fluid/thermal models have been developed to evaluate the performance of these designs relative to conventional designs. Modeling results showed that fractal-like structures and geometries can increase the effective solar absorptance by 5 – 20% and the thermal efficiency by several percentage points at both the meso and macro scales, depending on factors such as intrinsic absorptance. Meso-scale prototypes were fabricated using additive manufacturing techniques, and a macro-scale bladed receiver design was fabricated using Inconel 625 tubes. On-sun tests were performed using the solar furnace and solar tower at the National Solar Thermal Test facility. The test results demonstrated enhanced solar absorptance and thermal efficiency of the fractal-like designs.
Operation of concentrated solar power receivers at higher temperatures (>700°C) would enable supercritical carbon dioxide (sCO2) power cycles for improved power cycle efficiencies (>50%) and cost-effective solar thermal power. Unfortunately, radiative losses at higher temperatures in conventional receivers can negatively impact the system efficiency gains. One approach to improve receiver thermal efficiency is to utilize selective coatings that enhance absorption across the visible solar spectrum while minimizing emission in the infrared to reduce radiative losses. Existing coatings, however, tend to degrade rapidly at elevated temperatures. In this report, we report on the initial designs and fabrication of spectrally selective metamaterial-based absorbers for high-temperature, high-thermal flux environments important for solarized sCO2 power cycles. Metamaterials are structured media whose optical properties are determined by sub-wavelength structural features instead of bulk material properties, providing unique solutions by decoupling the optical absorption spectrum from thermal stability requirements. The key enabling innovative concept proposed is the use of structured surfaces with spectral responses that can be tailored to optimize the absorption and retention of solar energy for a given temperature range. In this initial study through the Academic Alliance partnership with University of Texas at Austin, we use Tungsten for its stability in expected harsh environments, compatibility with microfabrication techniques, and required optical performance. Our goal is to tailor the optical properties for high (near unity) absorptivity across the majority of the solar spectrum and over a broad range of incidence angles, and at the same time achieve negligible absorptivity in the near infrared to optimize the energy absorbed and retained. To this goal, we apply the recently developed concept of plasmonic Brewster angle to suitably designed nanostructured Tungsten surfaces. We predict that this will improve the receiver thermal efficiencies by at least 10% over current solar receivers.
Novel designs to increase light trapping and thermal efficiency of concentrating solar receivers at multiple length scales have been conceived and tested. The fractal-like geometries and features are introduced at both macro (meters) and meso (millimeters to centimeters) scales. Advantages include increased solar absorptance, reduced thermal emittance, and increased thermal efficiency. Radial and linear structures at the 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 can reduce thermal emittance due to reduced local view factors to the environment, and higher concentration ratios can be employed with similar surface irradiances to reduce the effective optical aperture, footprint, and thermal losses. Coupled optical/fluid/thermal models have been developed to evaluate the performance of these designs relative to conventional designs, and meso-scale tests have been performed. Results show that fractal-like structures and geometries can increase the thermal efficiency by several percentage points at both the meso and macro scales, depending on factors such as intrinsic absorptance. The impact was more pronounced for materials with lower intrinsic solar absorptances (<0.9). The goal of this work is to increase the effective solar absorptance of oxidized substrate materials from ∼0.9 to 0.95 or greater using these fractal-like geometries without the need for coatings.
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 thermal efficiency was ∼65%. Tests with higher irradiances are being performed and are expected to yield greater particle temperature increases and efficiencies.
ASME 2016 10th International Conference on Energy Sustainability, ES 2016, collocated with the ASME 2016 Power Conference and the ASME 2016 14th International Conference on Fuel Cell Science, Engineering and Technology
Direct solar power receivers consist of tubular arrays, or panels, which are typically tubes arranged side by side and connected to an inlet and outlet manifold. The tubes absorb the heat incident on the surface and transfer it to the fluid contained inside them. To increase the solar absorptance, high temperature black paint or a solar selective coating is applied to the surface of the tubes. However, current solar selective coatings degrade over the lifetime of the receiver and must be reapplied, which reduces the receiver thermal efficiency and increases the maintenance costs. This work presents an evaluation of several novel receiver shapes which have been denominated as fractal like geometries (FLGs). The FLGs are geometries that create a light-trapping effect, thus, increasing the effective solar absorptance and potentially increasing the thermal efficiency of the receiver. Five FLG prototypes were fabricated out of Inconel 718 and tested in Sandia's solar furnace at two irradiance levels of ∼15 and 30 W/cm2 and two fluid flow rates. Photographic methods were used to capture the irradiance distribution on the receiver surfaces and compared to results from ray-tracing models. This methods provided the irradiance distribution and the thermal input on the FLGs. Air at nearly atmospheric pressure was used as heat transfer fluid. The air inlet and outlet temperatures were recorded, using a data acquisition system, until steady state was achieved. Computational fluid dynamics (CFD) models, using the Discrete Ordinates (DO) radiation and the k-? Shear Stress Transport (SST) equations, were developed and calibrated, using the test data, to predict the performance of the five FLGs at different air flow rates and irradiance levels. The results showed that relative to a flat plate (base case), the new FLGs exhibited an increase in the effective solar absorptance from 0.86 to 0.92 for an intrinsic material absorptance of 0.86. Peak surface temperatures of ∼1000°C and maximum air temperature increases of ∼200°C were observed. Compared to the base case, the new FLGs showed a clear air outlet temperature increase. Thermal efficiency increases of ∼15%, with respect to the base case, were observed. Several tests, in different days, were performed to assess the repeatability of the results. The results obtained, so far, are very encouraging and display a very strong potential for incorporation in future solar power receivers.
ASME 2016 10th International Conference on Energy Sustainability, ES 2016, collocated with the ASME 2016 Power Conference and the ASME 2016 14th International Conference on Fuel Cell Science, Engineering and Technology
Flux distributions from solar field collectors are typically evaluated using a beam characterization system, which consists of a digital camera with neutral density filters, flux gauge or calorimeter, and water-cooled Lambertian target panel. The pixels in camera image of the flux distribution are scaled by the flux peak value measured with the flux gauge or the total power value measured with the calorimeter. An alternative method, called PHLUX developed at Sandia National Laboratories, can serve the same purpose using a digital camera but without auxiliary instrumentation. The only additional information required besides the digital images recorded from the camera are the direct normal irradiance, an image of the sun using the same camera, and the reflectivity of the receiver or target panel surface. The PHLUX method was evaluated using two digital cameras (Nikon D90 and D3300) at different flux levels on a target panel. The performances of the two cameras were compared to each other and to measurements from a Kendall radiometer. For consistency in comparison of the two cameras, the same focal length lenses and same number of neutral density filters were used. Other camera settings (e.g., shutter speed, f-stop, etc.) were set based on the aperture size and performance of the cameras. The Nikon D3300 has twice the number of pixels as the D90. D3300 provided higher resolution, however, due to the smaller pixel sizes the images were noisier, and the D90 with larger pixels had better response to low light levels. The noise in the D3300, if not corrected, could result in gross overestimation of the irradiance calculations. After corrections to the D3300 flux images, the PHLUX results from the two cameras showed they agreed to within 8% for a peak flux level of 1000 suns on the target, and less than 10% error in the peak flux when compared to the Kendall radiometer.