Falling particle receivers are an emerging technology for use in concentrating solar power systems. In this study, quartz half-shells are investigated for use as full or partial aperture covers to reduce receiver thermal losses. Quartz half-shell aperture covers offer the ability to minimally interfere with incoming solar radiation from the heliostat field while obstructing thermal radiation and advection from leaving the receiver cavity. The fluid dynamics and heat transfer of a receiver subdomain and surrounding air are modeled using ANSYS® FLUENT. We compare the percentage of total incident solar power lost due to conduction through the receiver walls, advective losses through the aperture, and radiation exiting the aperture. Contrary to expected outcomes, results show that quartz aperture covers can increase radiative losses and result in modest to nonexistent reductions in advective losses. The increased radiative losses are driven by elevated quartz half-shell temperatures and have the potential to be mitigated by active cooling and/or material selection. Quartz half-shell total transmissivity was measured experimentally using a radiometer and the National Solar Thermal Test Facility heliostat field with values up to 0.97 ± 0.01. Quartz half-shell aperture covers did not yield expected efficiency gains in numerical results due to increased radiative losses, but efficiency improvement in some numerical results and the performance of quartz half-shells subject to concentrated solar radiation suggest that quartz half-shell aperture covers should be investigated further.
This paper evaluates the on-sun performance of a 1 MW falling particle receiver. Two particle receiver designs were investigated: obstructed flow particle receiver versus free-falling particle receiver. The intent of the tests was to investigate the impact of particle mass flow rate, irradiance, and particle temperature on the particle temperature rise and thermal efficiency of the receiver for each design. Results indicate that the obstructed flow design increased the residence time of the particles in the concentrated flux, thereby increasing the particle temperature and thermal efficiency for a given mass flow rate. The obstructions, a staggered array of chevron-shaped mesh structures, also provided more stability to the falling particles, which were prone to instabilities caused by convective currents in the free-fall design. Challenges encountered during the tests included nonuniform mass flow rates, wind impacts, and oxidation/deterioration of the mesh structures. Alternative materials, designs, and methods are presented to overcome these challenges.
Solid particle receivers provide an opportunity to run concentrating solar tower receivers at higher temperatures and increased overall system efficiencies. The design of the bins used for storing and managing the flow of particles creates engineering challenges in minimizing thermomechanical stress and heat loss. An optimization study of mechanical stress and heat loss was performed at the National Solar Thermal Test Facility at Sandia National Laboratories to determine the geometry of the hot particle storage hopper for a 1 MWt pilot plant facility. Modeling of heat loss was performed on hopper designs with a range of geometric parameters with the goal of providing uniform mass flow of bulk solids with no clogging, minimizing heat loss, and reducing thermomechanical stresses. The heat loss calculation included an analysis of the particle temperatures using a thermal resistance network that included the insulation and hopper. A plot of the total heat loss as a function of geometry and required thicknesses to accommodate thermomechanical stresses revealed suitable designs. In addition to the geometries related to flow type and mechanical stress, this study characterized flow related properties of CARBO HSP 40/70 and Accucast ID50-K in contact with refractory insulation. This insulation internally lines the hopper to prevent heat loss and allow for low cost structural materials to be used for bin construction. The wall friction angle, effective angle of friction, and cohesive strength of the bulk solid were variables that were determined from empirical analysis of the particles at temperatures up to 600°C.
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.
The thermal environment generated during an intense radiation event like a nuclear weapon airburst, lightning strike, or directed energy weaponry has a devastating effect on many exposed materials. Natural and engineered materials can be damaged and ignite from the intense thermal radiation, potentially resulting in sustained fires. Understanding material behavior in such an event is essential for mitigating the damage to a variety of defense systems, such as aircraft and weaponry. Flammability and ignition studies in this regime (very high heat flux, short duration) are less plentiful than in the heat flux regimes representative of typical fires. The flammability and ignition behavior of a material may differ at extreme heat flux due to the balance of the heat conduction into the material compared to other processes. Length scale effects may also be important in flammability and ignition behavior, especially in the high heat flux regime. A variety of materials have recently been subjected to intense thermal loads (~100–1000 kW/m2) in testing at both the Solar Furnace and the Solar Tower at the National Solar Thermal Test Facility at Sandia National Laboratories. The Solar Furnace, operating at a smaller scale (≈30 cm2 area), provides the ability to test a wide range of materials under controlled radiative flux conditions. The Solar Tower exposes objects and materials to the same flux on a much larger scale (≈4 m2 area), integrating complex geometry and scale effects. Results for a variety of materials tested in both facilities are presented and compared. Material response often differs depending on scale, suggesting a significant scale effect. Mass loss per unit energy tends to go down as scale increases, and ignition probability tends to increase with scale.
Intense, dynamic radiant heat loads damage and ignite many common materials, but are outside the scope of typical fire studies. Explosive, directed-energy, and nuclear-weapon environments subject materials to this regime of extreme heating. The Solar Furnace at the National Solar Test Facility simulated this environment for an extensive experimental study on the response of many natural and engineered materials. Solar energy was focused onto a spot (∼10 cm2 area) in the center of the tested materials, generating an intense radiant load (∼100 kW m−2 –1000 kW m−2) for approximately 3 seconds. Using video photography, the response of the material to the extreme heat flux was carefully monitored. The initiation time of various events was monitored, including charring, pyrolysis, ignition, and melting. These ignition and damage thresholds are compared to historical ignition results predominantly for black, α-cellulose papers. Reexamination of the historical data indicates ignition behavior is predicted from simplified empirical models based on thermal diffusion. When normalized by the thickness and the thermal properties, ignition and damage thresholds exhibit comparable trends across a wide range of materials. This technique substantially reduces the complexity of the ignition problem, improving ignition models and experimental validation.
Nuclear weapon airbursts can create extreme radiative heat fluxes for a short duration. The radiative heat transfer from the fireball can damage and ignite materials in a region that extends beyond the zone damaged by the blast wave itself. Directed energy weapons also create extreme radiative heat fluxes. These scenarios involve radiative fluxes much greater than the environments typically studied in flammability and ignition tests. Furthermore, the vast majority of controlled experiments designed to obtain material response and flammability data at high radiative fluxes have been performed at relatively small scales (order 10 cm2 area). A recent series of tests performed on the Solar Tower at the National Solar Thermal Test Facility exposed objects and materials to fluxes of 100 – 2,400 kW/m2 at a much larger scale (≈1 m2 area). This paper provides an overview of testing performed at the Solar Tower for a variety of materials including aluminum, fabric, and two types of plastics. Tests with meter-scale objects such as tires and chairs are also reported, highlighting some potential effects of geometry that are difficult to capture in small-scale tests. The aluminum sheet melted at the highest heat flux tested. At the same flux, the tire ignited but the flames were not sustained when the external heat flux was removed; the damage appeared to be limited to the outer portion of the tire, and internal pressure was maintained.
Intense, dynamic radiant heat loads damage and ignite many common materials, but are outside the scope of typical fire studies. Explosive, directed-energy, and nuclear-weapon environments subject materials to this regime of extreme heating. The Solar Furnace at the National Solar Test Facility simulated this environment for an extensive experimental study on the response of many natural and engineered materials. Solar energy was focused onto a spot (∼10 cm2 area) in the center of the tested materials, generating an intense radiant load (∼100 kW m−2 –1000 kW m−2) for approximately 3 seconds. Using video photography, the response of the material to the extreme heat flux was carefully monitored. The initiation time of various events was monitored, including charring, pyrolysis, ignition, and melting. These ignition and damage thresholds are compared to historical ignition results predominantly for black, α-cellulose papers. Reexamination of the historical data indicates ignition behavior is predicted from simplified empirical models based on thermal diffusion. When normalized by the thickness and the thermal properties, ignition and damage thresholds exhibit comparable trends across a wide range of materials. This technique substantially reduces the complexity of the ignition problem, improving ignition models and experimental validation.
ASME 2018 12th International Conference on Energy Sustainability, ES 2018, collocated with the ASME 2018 Power Conference and the ASME 2018 Nuclear Forum
Prior research at Sandia National Laboratories showed the potential advantages of using light-trapping features which are not currently used in direct tubular receivers. A horizontal bladed receiver arrangement showed the best potential for increasing the effective solar absorptance by increasing the ratio of effective surface area to the aperture footprint. Previous test results and models of the bladed receiver showed a receiver efficiency increase over a flat receiver panel of ~ 5-7% over a range of average irradiances, while showing that the receiver tubes can withstand temperatures > 800 °C with no issues. The bladed receiver is being tested at various peak heat fluxes ranging 75-150 kW/m2 under transient conditions using Air as a heat transfer fluid at inlet pressure ~250 kPa (~36 psi) using a regulating flow loop. The flow loop was designed and tested to maintain a steady mass flow rate for ~15 minutes using pressurized bottles as gas supply. Due to the limited flow-time available, a novel transient methodology to evaluate the thermal efficiencies is presented in this work. Computational fluid dynamics (CFD) models are used to predict the temperature distribution and the resulting transient receiver efficiencies. The CFD simulations results using air as heat transfer fluid have been validated experimentally at the National Solar Thermal Test Facility in Sandia National Labs.
ASME 2017 11th International Conference on Energy Sustainability, ES 2017, collocated with the ASME 2017 Power Conference Joint with ICOPE 2017, the ASME 2017 15th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2017 Nuclear Forum
Previous research at Sandia National Laboratories showed the potential advantages of using light-trapping features which are not currently used in direct tubular receivers. A horizontal bladed receiver arrangement showed the best potential for increasing the effective solar absorptance by increasing the ratio of effective surface area to the aperture footprint. Ray-tracing analyses using SolTrace were performed to understand the light-trapping effects of the bladed receivers, which enable re-reflections between the fins that enhance the effective solar absorptance. A parametric optimization study was performed to determine the best possible configuration with a fixed intrinsic absorptivity of 0.9 and exposed surface area of 1 m2. The resulting design consisted of three vertical panels 0.584 m long and 0.508 m wide and 3 blades 0.508 m long and 0.229 m wide with a downward tilt of 50 degrees from the horizontal. Each blade consisted of two panels which were placed in front of the three vertical panels. The receiver was tested at the National Solar Thermal Test Facility using pressurized air. However, the receiver was designed to operate using supercritical carbon dioxide (sCO2) at 650 °C and 15 MPa for 100,000 hours following the ASME Boiler and Pressure Vessel Code Section VIII Division 1. The air flowed through the leading panel of the blade first, and then recirculated toward the back panel of the blade before flowing through one of the vertical back panels. The test results of the bladed receiver design showed a receiver efficiency increase over a flat receiver panel of ∼5 - 7% (from ∼80% to ∼86%) over a range of average irradiances, while showing that the receiver tubes can withstand temperatures > 800 °C with no issues. Computational fluid dynamics (CFD) modeling using the Discrete Ordinates (DO) radiation model was used to predict the temperature distribution and the resulting receiver efficiencies. The predicted thermal efficiency and surface temperature values correspond to the measured efficiencies and surface temperatures within one standard deviation. In the near future, an sCO2 flow system will be built to expose the receiver to higher pressure and fluid temperatures which could yield higher efficiencies.
When exposed to a strong radiant heat source (>1,000 kW/m2), combustible materials pyrolyze and ignite under certain conditions. Studies of this nature are scarce, yet important for some applications. Pyrolysis models derived at lower flux conditions do not necessarily extrapolate well to high-heat-flux conditions. The material response is determined by a complex interplay of thermal and chemical transport phenomena, which are often difficult to model. To obtain model validation data at high-heat-flux conditions (up to 2500 kW/m2), experiments on a variety of organic and engineered materials were performed at the National Solar Thermal Test Facility at Sandia National Laboratories. Mass loss during the short duration (2-4 sec) heat pulse was determined using the pre- and post-test weight. The mass-loss data were fairly linear in the fluence range of 200-6000 kJ/m2. When divided into subsets based on material types, the mass loss was similar at the peak flux/fluence condition for engineered polymers (≈1 g) and organic materials (≈2.5 g), although some exceptions exist (PMMA, dry pine needles). Statistical correlations were generated and used to evaluate the significance of the observed trends. These results contribute to the validation data for simulating fires and ignition resulting from very high incident heat flux.
ASME 2017 11th International Conference on Energy Sustainability Es 2017 Collocated with the ASME 2017 Power Conference Joint with Icope 2017 the ASME 2017 15th International Conference on Fuel Cell Science Engineering and Technology and the ASME 2017 Nuclear Forum
Previous research at Sandia National Laboratories showed the potential advantages of using light-trapping features which are not currently used in direct tubular receivers. A horizontal bladed receiver arrangement showed the best potential for increasing the effective solar absorptance by increasing the ratio of effective surface area to the aperture footprint. Ray-tracing analyses using SolTrace were performed to understand the light-trapping effects of the bladed receivers, which enable re-reflections between the fins that enhance the effective solar absorptance. A parametric optimization study was performed to determine the best possible configuration with a fixed intrinsic absorptivity of 0.9 and exposed surface area of 1 m2. The resulting design consisted of three vertical panels 0.584 m long and 0.508 m wide and 3 blades 0.508 m long and 0.229 m wide with a downward tilt of 50 degrees from the horizontal. Each blade consisted of two panels which were placed in front of the three vertical panels. The receiver was tested at the National Solar Thermal Test Facility using pressurized air. However, the receiver was designed to operate using supercritical carbon dioxide (sCO2) at 650 °C and 15 MPa for 100,000 hours following the ASME Boiler and Pressure Vessel Code Section VIII Division 1. The air flowed through the leading panel of the blade first, and then recirculated toward the back panel of the blade before flowing through one of the vertical back panels. The test results of the bladed receiver design showed a receiver efficiency increase over a flat receiver panel of ∼5 - 7% (from ∼80% to ∼86%) over a range of average irradiances, while showing that the receiver tubes can withstand temperatures > 800 °C with no issues. Computational fluid dynamics (CFD) modeling using the Discrete Ordinates (DO) radiation model was used to predict the temperature distribution and the resulting receiver efficiencies. The predicted thermal efficiency and surface temperature values correspond to the measured efficiencies and surface temperatures within one standard deviation. In the near future, an sCO2 flow system will be built to expose the receiver to higher pressure and fluid temperatures which could yield higher efficiencies.
A supercritical carbon dioxide (sCO2) Brayton cycle is an emerging high energy-density cycle undergoing extensive research due to the appealing thermo-physical properties of sCO2 and single phase operation. Development of a solar receiver capable of delivering sCO2 at 20 MPa and 700 °C is required for implementation of the high efficiency (∼50%) solar powered sCO2 Brayton cycle. In this work, extensive candidate materials are review along with tube size optimization using the ASME Boiler and Pressure Vessel Code. Temperature and pressure distribution obtained from the thermal-fluid modeling (presented in a complementary publication) are used to evaluate the thermal and mechanical stresses along with detailed creep-fatigue analysis of the tubes. The resulting body stresses were used to approximate the lifetime performance of the receiver tubes. A cyclic loading analysis is performed by coupling the Strain-Life approach and the Larson-Miller creep model. The structural integrity of the receiver was examined and it was found that the stresses can be withstood by specific tubes, determined by a parametric geometric analysis. The creep-fatigue analysis displayed the damage accumulation due to cycling and the permanent deformation on the tubes showed that the tubes can operate for the full lifetime of the receiver.
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.
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.
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.
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
This paper evaluates novel particle release patterns for high-temperature falling particle receivers. Spatial release patterns resembling triangular and square waves are investigated and compared to the conventional straight-line particle release. A design of experiments was developed, and a simulation matrix was developed that investigated three twolevel factors: amplitude, wavelength, and wave type. Results show that the wave-like patterns increased both the particle temperature rise and thermal efficiency of the receiver relative to the straight-line particle release. Larger amplitudes and smaller wavelengths increased the performance by creating a volumetric heating effect that increased light absorption and reduced heat loss. Experiments are also being designed to investigate the hydraulic and thermal performance of these new particle release configurations.
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
This paper evaluates the on-sun performance of a 1 MW falling particle receiver. Two particle receiver designs were investigated: obstructed flow particle receiver vs.free-falling particle receiver. The intent of the tests was to investigate the impact of particle mass flow rate, irradiance, and particle temperature on the particle temperature rise and thermal efficiency of the receiver for each design. Results indicate that the obstructed flow design increased the residence time of the particles in the concentrated flux, thereby increasing the particle temperature and thermal efficiency for a given mass flow rate. The obstructions, a staggered array of chevronshaped mesh structures, also provided more stability to the falling particles, which were prone to instabilities caused by convective currents in the free-fall design. Challenges encountered during the tests included non-uniform mass flow rates, wind impacts, and oxidation/deterioration of the mesh structures. Alternative materials, designs, and methods are presented to overcome these challenges.
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
Multiple receiver designs have been evaluated for improved optics and efficiency gains including flat panel, vertical-finned flat panel, horizontal-finned flat panel, and radially finned. Ray tracing using SolTrace was performed to understand the light-trapping effects of the finned receivers. Re-reflections of the fins to other fins on the receiver were captured to give an overall effective solar absorptance. The ray tracing, finite element analysis, and previous computational fluid dynamics showed that the horizontalfinned flat panel produced the most efficient receiver with increased light-trapping and lower overall heat loss. The effective solar absorptance was shown to increase from an intrinsic absorptance of 0.86 to 0.96 with ray trace models. The predicted thermal efficiency was shown in CFD models to be over 95%. The horizontal panels produce a re-circulating hot zone between the panel fins reducing convective loss resulting in a more efficient receiver. The analysis and design of these panels are described with additional engineering details on testing a flat panel receiver and the horizontal-finned receiver at the National Solar Thermal Test Facility. Design considerations include the structure for receiver testing, tube sizing, surrounding heat shielding, and machinery for cooling the receiver tubes.
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.
Cavity receivers used in solar power towers and dish concentrators may lose considerable energy by natural convection, which reduces the overall system efficiency. A validated numerical receiver model is desired to better understand convection processes and aid in heat loss minimization efforts. The purpose ofthis investigation was to evaluate heat loss predictions using the commercial computational fluid dynamics (CFD) software packages fluent 13.0 and solidworks flow simulation 2011 against experimentally measured heat losses for a heated cubical cavity receiver model (Kraabel, 1983, "An Experimental Investigation of the Natural Convection From a Side-Facing Cubical Cavity," Proceedings of the ASME JSME Thermal Engineering Joint Conference, Vol. 1, pp. 299-306) and a cylindrical dish receiver model (Taumoefolau et al., 2004, "Experimental Investigation of Natural Convection Heat Loss From a Model Solar Concentrator Cavity Receiver," ASME J. Sol. Energy Eng., 126(2), pp. 801-807). Simulated convective heat loss was underpredicted by 45% for the cubical cavity when experimental wall temperatures were implemented as isothermal boundary conditions and 32% when the experimental power was applied as a uniform heat flux from the cavity walls. Agreement between software packages was generally within 10%. Convective heat loss from the cylindrical dish receiver model was accurately predicted within experimental uncertainties by both simulation codes using both isothermal and constant heat flux wall boundary conditions except when the cavity was inclined at angles below 15 deg and above 75 deg, where losses were under- and overpredicted by fluent and solidworks, respectively. Comparison with empirical correlations for convective heat loss from heated cavities showed that correlations by Kraabel (1983, "An Experimental Investigation of the Natural Convection From a Side-Facing Cubical Cavity," Proceedings ofthe ASME JSME Thermal Engineering Joint Conference, Vol. 1, pp. 299-306) and for individual heated flat plates oriented to the cavity geometry (Pitts and Sissom, 1998, Schaum's Outline of Heat Transfer, 2nd ed., McGraw Hill, New York, p. 227) predicted heat losses from the cubical cavity to within experimental uncertainties. Correlations by Clausing (1987, "Natural Convection From Isothermal Cubical Cavities With a Variety of Side-Facing Apertures," ASME J. Heat Transfer, 109(2), pp. 407-412) and Paitoonsurikarn et al. (2011, "Numerical Investigation of Natural Convection Loss From Cavity Receivers in Solar Dish Applications," ASME J. Sol. Energy Eng. 133(2), p. 021004) were able to do the same for the cylindrical dish receiver. No single correlation was valid for both experimental receivers. The effect ofdifferent turbulence and air-property models within fluent were also investigated and compared in this study. However, no model parameter was found to produce a change large enough to account for the deficient convective heat loss simulated for the cubical cavity receiver case.
ASME 2015 9th International Conference on Energy Sustainability, ES 2015, collocated with the ASME 2015 Power Conference, the ASME 2015 13th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2015 Nuclear Forum
Falling particle receivers are being evaluated as an alternative to conventional fluid-based solar receivers to enable higher temperatures and higher efficiency power cycles with direct storage for concentrating solar power applications. This paper presents studies of the particle mass flow rate, velocity, particle-curtain opacity and density, and other characteristics of free-falling ceramic particles as a function of different discharge slot apertures. The methods to characterize the particle flow are described, and results are compared to theoretical and numerical models for unheated conditions.
ASME 2015 9th International Conference on Energy Sustainability, ES 2015, collocated with the ASME 2015 Power Conference, the ASME 2015 13th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2015 Nuclear Forum
Typical Concentrated Solar Power (CSP) central receiver power plants require the use of either an external or cavity receiver. Previous and current external receivers consist of a series of tubes connected to manifolds that form a cylindrical or rectangular shape such as in the cases of Solar One, Solar Two, and most recently the Ivanpah solar plant. These receivers operate at high surface temperatures (>600°C) at which point thermal re-radiation is significant. However, the geometric arrangement of these heat transfer tubes results in heat losses directly to the environment. This work focused on how to fundamentally reduce this heat loss through the manipulation of heat transfer tube configurations. Four receiver configurations are studied: flat receiver (base case study), a radial receiver with finned structures (fins arranged in a circular pattern on a cylinder), a louvered finned structure (horizontal and angled fins on a flat plate), and a vertical finned structure (fins oriented vertically along a flat plate). The thermal efficiency, convective heat loss patterns, and air flow around each receiver design is found using the computational fluid dynamics (CFD) code ANSYS FLUENT. Results presented in this paper show that alternative tubular configurations increase thermal efficiency by increasing the effective solar absorptance of these hightemperature receivers by increasing the light trapping effects of the receiver, reducing thermal emittance to the environment, and reducing the overall size of the receiver. Each receiver configuration has finned structures that take advantage of the directional dependence of the heliostat field resulting in a light trapping effect on the receiver. The finned configurations tend to lead to "hot" regions on the receiver, but the new configurations can take advantage of high local view factors (each surface can "see" another receiver surface) in these regions through the use of heat transfer fluid (HTF) flow patterns. The HTF reduces the temperatures in these regions increasing the efficiency of heat transfer to the fluid. Finally, the new receiver configurations have a lower overall optical intercept region resulting in a higher geometric concentration ratio for the receiver. Compared to the base case analysis (flat plate receiver), the novel tubular geometries results showed an increase in thermal efficiency.
ASME 2015 9th International Conference on Energy Sustainability, ES 2015, collocated with the ASME 2015 Power Conference, the ASME 2015 13th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2015 Nuclear Forum
Recent studies have evaluated closed-loop supercritical carbon dioxide (s-CO2) Brayton cycles to be a higher energydensity system in comparison to conventional superheated steam Rankine systems. At turbine inlet conditions of 923K and 25 MPa, high thermal efficiency (∼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 ∼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 ∼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 ∼85%.
ASME 2015 9th International Conference on Energy Sustainability, ES 2015, collocated with the ASME 2015 Power Conference, the ASME 2015 13th International Conference on Fuel Cell Science, Engineering and Technology, and the ASME 2015 Nuclear Forum
Closed-loop super-critical carbon dioxide (sCO2) Brayton cycles are being evaluated in combination with concentrating solar power to provide higher thermal-to-electric conversion efficiencies relative to conventional steam Rankine cycles. However, high temperatures (650-700°C) and pressures (20-25 MPa) are required in the solar receiver. In this study an extensive material review was performed along with a tube size optimization following the ASME Boiler and Pressure Vessel Code and B31.1 and B313.3 codes respectively. Subsequently a thermal-structural model was developed using ANSYS Fluent and Structural to design and analyze the tubular receiver that could provide the heat input for a ∼2 MWth plant. The receiver will be required to provide an outlet temperature of 650°C (at 25 MPa) or 700°C (at 20 MPa). The induced thermal stresses were applied using a temperature gradient throughout the tube while a constant pressure load was applied on the inner wall. The resulting stresses have been validated analytically using constant surface temperatures. The cyclic loading analysis was performed using the Larson-Miller creep model in nCode Design Life to define the structural integrity of the receiver over the desired lifetime of ∼10,000 cycles. The results have shown that the stresses induced by the thermal and pressure load can be withstood by the tubes selected. The creep-fatigue analysis displayed the damage accumulation due to the cycling and the permanent deformation of the tubes. Nonetheless, they are able to support the required lifetime. As a result, a complete model to verify the structural integrity and thermal performance of a high temperature and pressure receiver has been developed. This work will serve as reference for future design and evaluation of future direct and indirect tubular receivers.
Traditional tubular receivers used in concentrating solar power are formed using tubes connected to manifolds to form panels; which in turn are arranged in cylindrical or rectangular shapes. Previous and current tubular receivers, such as the ones used in Solar One, Solar Two, and most recently the Ivanpah solar plants, have used a black paint coating to increase the solar absorptance of the receiver. However, these coatings degrade over time and must be reapplied, increasing the receiver maintenance cost. This paper presents the thermal efficiency evaluation of novel receiver tubular panels that have a higher effective solar absorptance due to a light-trapping effect created by arranging the tubes in each panel into unique geometric configurations. Similarly, the impact of the incidence angle on the effective solar absorptance and thermal efficiency is evaluated. The overarching goal of this work is to achieve effective solar absorptances of ∼90% and thermal efficiencies above 85% without using an absorptance coating. Several panel geometries were initially proposed and were down-selected based on structural analyses considering the thermal and pressure loading requirements of molten salt and supercritical carbon-dioxide receivers. The effective solar absorptance of the chosen tube geometries and panel configurations were evaluated using the ray-tracing modeling capabilities of SolTrace. The thermal efficiency was then evaluated by coupling computational fluid dynamics with the ray-tracing results using ANSYS Fluent. Compared to the base case analysis (flat tubular panel), the novel tubular panels have shown an increase in effective solar absorptance and thermal efficiency by several percentage points.
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 Sandiaas 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%.
The Ivanpah Solar Electric Generating System (ISEGS), located on I - 15 about 40 miles (60 km) south of Las Vegas, NV, consists of three power towers 459 ft (140 m) tall and over 170,000 reflective heliostats with a rated capacity of 390 MW. Reports of glare from the plant have been submitted by pilots and air traffic controllers and recorded by the Aviation Safety Reporting System and the California Energy Commission since 2013. Aerial and ground - based surveys of the glare were conducted in April, 2014, to identify the cause and to quantify the irradiance and potential ocular impact s of the glare . Results showed that the intense glare viewed from the airspace above ISEGS was caused by he liostats in standby mode that were aimed to the side of the receiver. Evaluation of the glare showed that the retinal irradiance and subtended source angle of the glare from the heliostats in standby were sufficient to cause significant ocular impact (pot ential for after - image) up to a distance of %7E6 miles (10 km), but the values were below the threshold for permanent eye damage . Glare from the receivers had a low potential for after - image at all ground - based monitoring locations outside of the site bound aries. A Letter to Airmen has been issued by the Federal Aviation Administration to notify pilots of the potential glare hazards. Additional measures to mitigate the potential impacts of glare from ISGES are also presented and discussed. This page intentionally left blank
ASME 2014 8th International Conference on Energy Sustainability, ES 2014 Collocated with the ASME 2014 12th International Conference on Fuel Cell Science, Engineering and Technology
Solar optical modeling tools are valuable for modeling and predicting the performance of solar technology systems. Four optical modeling tools were evaluated using the National Solar Thermal Test Facility heliostat field combined with flat plate receiver geometry as a benchmark. The four optical modeling tools evaluated were DELSOL, HELIOS, SolTrace, and Tonatiuh. All are available for free from their respective developers. DELSOL and HELIOS both use a convolution of the sunshape and optical errors for rapid calculation of flux profiles on the receiver surfaces. SolTrace and Tonatiuh use ray-tracing methods to determine reflected solar rays on the receiver surfaces and construct flux profiles. We found the raytracing tools, although slower in computation speed, to be more flexible for modeling complex receiver geometries, whereas DELSOL and HELIOS were limited to standard receiver geometries. We provide an example of using SolTrace for modeling non-conventional receiver geometries. We also list the strengths and deficiencies of the tools to show tool preference depending on the modeling and design needs.
ASME 2014 8th International Conference on Energy Sustainability, ES 2014 Collocated with the ASME 2014 12th International Conference on Fuel Cell Science, Engineering and Technology
Cavity receivers have been an integral part of Concentrated Solar Power (CSP) plants for many years. However, falling solid particle receivers (SPR) which employ a cavity design are only in the beginning stages of on-sun testing and evaluation. A prototype SPR has been developed which will be fully integrated into a complete system to demonstrate the effectiveness of this technology in the CSP sector. The receiver is a rectangular cavity with an aperture on the north side, open bottom (for particle collection), and a slot in the top (particle curtain injection). The solid particles fall from the top of the cavity through the solar flux and are collected after leaving the receiver. There are inherent design challenges with this type of receiver including particle curtain opacity, high wall fluxes, high wall temperatures, and high heat losses. CFD calculations using ANSYS FLUENT were performed to evaluate the effectiveness of the current receiver design. The particle curtain mass flow rate needed to be carefully regulated such that the curtain opacity is high (to intercept as much solar radiation as possible), but also low enough to increase the average particle temperature by 200°C. Wall temperatures were shown to be less than 1200°C when the particle curtain mass flow rate is 2.7 kg/s/m which is critical for the receiver insulation. The size of the cavity was shown to decrease the incident flux on the cavity walls and also reduced the wall temperatures. A thermal efficiency of 92% was achieved, but was obtained with a higher particle mass flow rate resulting in a lower average particle temperature rise. A final prototype receiver design has been completed utilizing the computational evaluation and past CSP project experiences.
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.
Solar optical modeling tools are valuable for modeling and predicting the performance of solar technology systems. Four optical modeling tools were evaluated using the National Solar Thermal Test Facility heliostat field combined with flat plate receiver geometry as a benchmark. The four optical modeling tools evaluated were DELSOL, HELIOS, SolTrace, and Tonatiuh. All are available for free from their respective developers. DELSOL and HELIOS both use a convolution of the sunshape and optical errors for rapid calculation of the incident irradiance profiles on the receiver surfaces. SolTrace and Tonatiuh use ray-tracing methods to intersect the reflected solar rays with the receiver surfaces and construct irradiance profiles. We found the ray-tracing tools, although slower in computation speed, to be more flexible for modeling complex receiver geometries, whereas DELSOL and HELIOS were limited to standard receiver geometries such as flat plate, cylinder, and cavity receivers. We also list the strengths and deficiencies of the tools to show tool preference depending on the modeling and design needs. We provide an example of using SolTrace for modeling nonconventional receiver geometries. The goal is to transfer the irradiance profiles on the receiver surfaces calculated in an optical code to a computational fluid dynamics code such as ANSYS Fluent. This approach eliminates the need for using discrete ordinance or discrete radiation transfer models, which are computationally intensive, within the CFD code. The irradiance profiles on the receiver surfaces then allows for thermal and fluid analysis on the receiver.
ASME 2012 6th International Conference on Energy Sustainability, ES 2012, Collocated with the ASME 2012 10th International Conference on Fuel Cell Science, Engineering and Technology
Understanding the effects of gravity and wind loads on concentrating solar power (CSP) collectors is critical for performance calculations and developing more accurate alignment procedures and techniques. This paper presents a rigorous finite-element model of a parabolic trough collector that is used to determine the impact of gravity loads on bending and displacements of the mirror facets and support structure. The geometry of the LUZ LS-2 parabolic trough collector was modeled using SolidWorks, and gravity-induced loading and displacements were simulated in SolidWorks Simulation. The model of the trough collector was evaluated in two positions: the 90{sup o} position (mirrors facing upward) and the 0{sup o} position (mirrors facing horizontally). The slope errors of the mirror facet reflective surfaces were found by evaluating simulated angular displacements of node-connected segments along the mirror surface. The ideal (undeformed) shape of the mirror was compared to the shape of the deformed mirror after gravity loading. Also, slope errors were obtained by comparing the deformed shapes between the 90{sup o} and 0{sup o} positions. The slope errors resulting from comparison between the deformed vs. undeformed shape were as high as {approx}2 mrad, depending on the location of the mirror facet on the collector. The slope errors resulting from a change in orientation of the trough from the 90{sup o} position to the 0{sup o} position with gravity loading were as high as {approx}3 mrad, depending on the location of the facet.