Joshua M. Christian

CFD Simulation and Performance Analysis of Alternative Designs for High-Temperature Solid Particle Receivers

Direct-absorption solid particle receivers are theoretically capable of yielding temperatures in excess of 1000°C, which enables higher efficiency power cycles and lower thermal storage costs. This paper presents rigorous CFD simulations of alternative solid particle receiver designs with recirculation to help identify optimal configurations that maximize the receiver thermal efficiency. The alternative receiver designs considered are a north-facing cavity receiver and a face-down surround-field cavity receiver. The CFD simulations model incident solar radiation from a heliostat field as a boundary condition on the model domain. The CFD simulations also couple convective flow with the thermal and discrete-phase (particle) solutions, which in turn affects absorption of incident solar radiation and thermal re-radiation within the receiver. The receivers are optimized to yield comparable particle temperatures at the outlets of 750–850°C, heated from an injection temperature of 300°C, and are compared on the basis of thermal efficiency. The CFD simulations yielded thermal efficiencies of the north-facing receiver at 72.3% (losses were 6.5% radiative and 20.9% convective) and the face-down receiver at 78.9% (losses were 11.4% radiative and 9.6% convective) at solar noon on March 22. Ongoing efforts are focused on reducing convective and radiative losses from both receiver configurations.Copyright

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...

CFD SIMULATION AND HEAT LOSS ANALYSIS OF THE SOLAR TWO POWER TOWER RECEIVER.

Solar Two was a demonstration of the viability of molten salt power towers. The power tower was designed to produce enough thermal power to run a 10-MWe conventional Rankine cycle turbine. A critical component of this process was the solar tower receiver. The receiver was designed for an applied average heat flux of 430 kW/m2 with an outlet temperature of 565°C (838.15 K). The mass flow rate could be varied in the system to control the outlet temperature of the heat transfer fluid, which was high temperature molten salt. The heat loss in the actual system was calculated by using the power-on method which compares how much power is absorbed by the molten salt when using half of the heliostat field and then the full heliostat field. However, the total heat loss in the system was lumped into a single value comprised of radiation, convection, and conduction heat transfer losses. In this study, ANSYS FLUENT was used to evaluate and characterize the radiative and convective heat losses from this receiver system assuming two boundary conditions: (1) a uniform heat flux on the receiver and (2) a distributed heat flux generated from the code DELSOL. The results show that the distributed-flux models resulted in radiative heat losses that were ∼14% higher than the uniform-flux models, and convective losses that were ∼5–10% higher due to the resulting non-uniform temperature distributions. Comparing the simulations to known convective heat loss correlations demonstrated that surface roughness should be accounted for in the simulations. This study provides a model which can be used for further receiver design and demonstrates whether current convective correlations are appropriate for analytical evaluation of external solar tower receivers.Copyright

Novel Tubular Receiver Panel Configurations for Increased Efficiency of High-Temperature Solar Receivers

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 high-temperature 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.Copyright

High-Temperature Receiver Designs for Supercritical CO2 Closed-Loop Brayton Cycles

High-temperature receiver designs for solar powered supercritical CO2 Brayton cycles that can produce ∼1 MW of electricity are being investigated. Advantages of a supercritical CO2 closed-loop Brayton cycle with recuperation include high efficiency (∼50%) and a small footprint relative to equivalent systems employing steam Rankine power cycles. Heating for the supercritical CO2 system occurs in a high-temperature solar receiver that can produce temperatures of at least 700 °C. Depending on whether the CO2 is heated directly or indirectly, the receiver may need to withstand pressures up to 20 MPa (200 bar). This paper reviews several high-temperature receiver designs that have been investigated as part of the SERIIUS program. Designs for direct heating of CO2 include volumetric receivers and tubular receivers, while designs for indirect heating include volumetric air receivers, molten-salt and liquid-metal tubular receivers, and falling particle receivers. Indirect receiver designs also allow storage of thermal energy for dispatchable electricity generation. Advantages and disadvantages of alternative designs are presented. Current results show that the most viable options include tubular receiver designs for direct and indirect heating of CO2 and falling particle receiver designs for indirect heating and storage.Copyright

Finite element modeling of concentrating solar collectors for evaluation of gravity loads, bending, and optical characterization.

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° position (mirrors facing upward) and the 0° 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° and 0° positions. The slope errors resulting from comparison between the deformed vs. undeformed shape were as high as ∼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° position to the 0° position with gravity loading were as high as ∼3 mrad, depending on the location of the facet.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.

Evaluation of Air Recirculation for Falling Particle Receivers.

This paper presents simulations and designs of a prototype falling particle solar receiver with air recirculation as a means of mitigating heat loss and impacts of external wind on particle flow. The flow and dispersion of different sizes of ceramic proppant (CARBO HSP) particles (1 mm, 100 μm, and 10 μm) were simulated in recirculating air flows without heating effects. Particles on the order of 0.1–1 mm yielded desirable simulated flow patterns when falling through the cavity receiver with an air-injection velocity of 3 m/s. Simulations of smaller particles on the order of 10 microns yielded unstable flow patterns that may lead to large losses of particles through the aperture. A prototype cavity receiver with air recirculation was designed and fabricated to validate the unheated simulations. The blower nozzles and suction plenum were engineered to yield the most uniform flow pattern along the entire width of the aperture. Observed and simulated air velocity distributions around the aperture and particle flow patterns using CARBO HSP particles with an average diameter of ∼0.7 mm were found to be qualitatively similar.Copyright

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%.