Search

The direct measurement of particle temperatures and advective losses from solid particle receiver has been a topic of necessary interest to de-risk the technology and improve its performance. Due to the flow's transient and stochastic nature, it has presented a challenge to traditional thermometry and metrology methods. In this work, a simplified quantitative non-intrusive imaging methodology previously developed is applied to Sandia's falling particle receiver during on-sun tests to collect image sets, which could help estimate the average particle temperature and particle egress rate from the system. Further additions to the technique are presented to permit the estimation of the plume (air and particles) egress rate and total advective heat losses during on-sun operation. The falling particle receiver testing campaign in 2020 and 2021 allowed the team to capture multiple flow configurations and environmental conditions used to build a large database of data captures with different characteristics. Finally, the results captured were used to complete a regression study to gain further insight into the factors which impact the particle egress and the receiver's efficiency. Particle temperature, receiver flow configuration, and wind speed were found to be the most impactful factors in the study. [ABSTRACT FROM AUTHOR]

This paper presents a detailed analysis of the optical performance of circular trough concentrators with tubular receivers. First, a simple analytical formula for the achievable geometric concentration ratio as a function of the rim angle and acceptance angle is developed. Notably, the development reveals the existence of three distinct concentration ratio regimes: a first regime where the receiver is sized based on the reflection of the edge rays from the rim alone, a second regime where the receiver is sized based on the rim and the edge ray caustics, and a third regime where two reflections from the mirror are permitted. Several exemplary designs are proposed and further analyzed using Monte Carlo ray tracing to obtain transmission angle curves and receiver flux distributions. For an acceptance angle of 1 deg, the circular trough concentrator with circular receiver is found to achieve a maximum geometric concentration ratio of 7.695× with a peak flux of 30 suns. For large acceptance angles (10 deg), the circular trough achieves a geometric concentration ratio as high as 82% of that of a parabolic trough. This noteworthy performance, along with the fact that a circular mirror is amenable to an inflated polymer construction, makes this configuration promising for low-cost, low-concentration solar thermal applications. [ABSTRACT FROM AUTHOR]

The direct measurement of particle temperatures in particle-laden flows presents a unique challenge to thermometry due to the flow's transient and stochastic nature. Previous attempts to measure the bulk particle temperature of a dilute particle plume or particle curtain using intrusive and non-intrusive methods have been mildly successful. In this work, a non-intrusive method using a high-speed infrared (IR) camera and a visible-light camera to yield an indirect particle temperature measurement technique is developed and tested. The image sequences obtained from the IR camera allow for the calculation of the apparent particle temperature, while the visible-light image sets allow for the calculation of the plume opacity as a function of flow discharge position. To extract the true particle temperature, a post-processing algorithm based on Planck's radiation theory was developed. The results were validated through a series of lab-scale tests at the University of New Mexico using a test rig capable of generating particle curtains at various temperatures. The temperature profiles extracted from the methodology presented were compared to the temperature data measured during experimental measurements yielding agreement of the bulk particle temperature of the plume within 10% error. The methods described here will be developed further to estimate the heat losses from the falling particle receiver at Sandia National Laboratories. [ABSTRACT FROM AUTHOR]

The solar flux incident on a volumetric receiver is inherently unsteady, resulting in high thermal stresses, fatigue failures, and reduced component life. The knowledge of transient response characteristics of a porous volumetric receiver used in concentrating solar technologies is cardinal for its reliable and safe working. The dynamic controlling of the solar-to-thermal conversion process is also possible with the prior prediction of the output variations. The present study aims to investigate the transient behavior of a porous volumetric receiver subjected to flux variations approximations occurring in real working scenarios with the help of a coupled transient model. The solid and fluid temperature fields, output fluid temperature, and pressure drop variations are determined for transient flux conditions during start-up, shut-down, clear sky, and cloud passage. The results are used to analyze the thermal response of the receiver during various operating conditions. In addition, the effects of structural parameters of the porous absorber are also investigated. The results indicate that the receiver transient performance is comparatively more affected by the variation in porosity than in pore size for all conditions. Smaller porosities and pore sizes show slower thermal response to transient fluctuations and less temperature changes during cloud passage. Conversely, higher values help in the faster restoration of the steady-state output conditions without dynamic control. [ABSTRACT FROM AUTHOR]

This paper reports on a parametric investigation of the thermal enhancement of a double-reflector parabolic trough collector when employing an in-line mixed V-shape (IMVS) ribbed absorber tube. Three heat transfer fluids (HTFs) are investigated, and a wide range of fluid inlet temperatures are studied. Various geometric parameters of the V-shape rib are analyzed to determine the optimum design of such a modification to the wall of the absorber tube. Results show that the HTF thermal oil Syltherm 800 is superior to the other HTFs that were studied. Results also show that a lower inlet temperature of the HTF leads to better thermo-hydraulic performance. The study provides a set of values for designing a V-shape ribbed absorber tube that produces optimum thermo-hydraulic performance. The optimum ribbed tube design shows a performance enhancement of about 64% compared to a smooth tube. [ABSTRACT FROM AUTHOR]

The distributions of heat flux over the circumference of the receiver tubes have an immense influence on the performance and reliability of the parabolic trough solar thermal collectors. The location of the receiver tube and the secondary reflector configuration may largely influence the performance of the system. Therefore, in this study, the effect of receiver tube position and parabolic secondary reflector configuration has been analyzed, and the non-uniformity of solar flux distribution, heat gradient, and power output has been compared. The results of the Monte Carlo ray-tracing analysis to homogenize the receiver tube flux distribution and maximize the output power, making the use of the cutting edge solar optical simulation tool Tonatiuh, has been presented. A parabolic trough collector with a rim angle of 80 deg and aperture area of 40 m2 have been used for the analysis. It has been confirmed that the circumferential heat flux gradient and the local hot spot could be greatly diminished, while the power output tended to reduce slightly due to the shading effect of the secondary reflector. Under the conditions investigated in this work, although the output power decreased by 4.83%, flux gradient reduced significantly, and the non-uniformity of flux distribution has reduced from 0.9757 to 0.5176. A simple design procedure for receiver tube position and secondary reflector configurations to homogenize the receiver tube temperature distribution has also been proposed. [ABSTRACT FROM AUTHOR]

The objectives of this work carried out in the Central African Republic are to propose new correlations between the components of solar radiation and the sunshine duration on a horizontal surface on the ground, and then to make an evaluation of the solar potential in the cities of Bambari, Birao, and Bangui. Polynomial regression models were used and their parameters were estimated by the ordinary least-squares method. A statistical evaluation allowed us to compare the performance of the models. The best correlations are then used to estimate the global and diffuse radiation. In the city of Birao, the estimated global radiation is around 6 kW h/m²j and the diffuse radiation is around 2 kW h/m² j; in Bambari, the global radiation is around 5.4 kW h/m² j and the diffuse is around 2.3 kW h/m² j; in Bangui, the global radiation is around 5 kW h/m² j and the diffuse radiation is around 2.3 kW h/m2 j. The potential solar in all these regions is very favorable for small- and large-scale solar photovoltaic applications. [ABSTRACT FROM AUTHOR]

This study presents a sensitivity analysis of the levelized cost of electricity (LCOE) for a particle-based system with the costs of the most current components. New models for the primary heat exchanger, thermal energy storage, and tower are presented and used to establish lower and upper bounds for these three components. The rest of component costs such as particle cost, cavity cost, and lift cost are set to lower and upper bounds estimating an uncertainty between 25% and 50%. Other relevant parameters related to lift and storage performance are also included in the analysis with the same uncertainty. This study also includes an upgrade to the receiver model by including the wind effect in the efficiency, which was not included in previous publications and may have a big impact in the system design. A parametric analysis shows the optimum values of solar multiple, storage hours, tower height, and concentration ratio, and a probabilistic analysis provides a cumulative distribution function for a range of LCOE values. The results show that the LCOE could be below $0.06/kWh with a probability of between 80% and 90%, where the costs of primary heat exchanger, particles, and lifts have largest contribution to the variance of the LCOE. [ABSTRACT FROM AUTHOR]

Particle solar receivers promise economical and operational advantages compared to the molten salt-based solar receivers. In this study, an experiment is designed to observe the particle flow characteristics in the Centrifugal Particle Solar Receiver. A set of experiments for various receiver rotational speeds and particle mass flowrates is conducted, and experimentally obtained raw results are post-processed by means of an Image Processing Routine based on 4BestEstimate algorithm. The axial advance of the particles in one receiver rotation, the particle film thickness, and the ratio of the length of the stationary zone to the receiver circumference is measured to be later used in the validation study of the discrete element method-based numerical model. [ABSTRACT FROM AUTHOR]

Spinning-elevation (SE) tracking system produces a decent image on the receiver surface; however, it is subjected to large variations in tracking speed. In this research, a graphical ray tracing (GRT) model for center-oriented spinning-elevation (COSE) tracking method is developed to evaluate tracking angles. Instead of a target, a heliostat is pointed toward the on-field center point of the tower. Therefore, a spinning-axis of rotation is a line joining a heliostat, and a center of the tower and elevation-axis is perpendicular to it. This aiming strategy has shown a substantial reduction in rotations of spinning-motor. In contrast, the elevation-motor runs at slightly higher rotations than the target-oriented SE method for the same application. Also, COSE tracking method obtains better shape of the reflected image with less aberration on the receiver surface as compared with SE and the traditional Azimuth-elevation (AE) method. [ABSTRACT FROM AUTHOR]

This study developed a methodology for determining the intercept factor (γ) and reports the effects of varying the design parameters, within realistic ranges, on the optical performance of parabolic trough collectors (PTC). The design equations account for the aperture width of the concentrator, the geometric concentration ratio, rim-angle, and misalignment of the receiver, characterized by the radial and angular offsets. The Latin Hypercube Sampling (LHS)-based ray-tracing method was used. Validation of the method was performed by comparing the results obtained by tracing a single ray with measurements obtained in a two-dimensional (2D) drafting program. Then, parametric analyses were performed to determine the effects of changing the radial and angular offset, geometric concentration ratio, and rim-angle on the percentage change in intercept factor (γΔ) with respect to the γ of a PTC with a perfectly aligned receiver. Finally, contour plots depicting the γ of various standard troughs over the range of radial and angular offsets were plotted, compared, and discussed. An online tool is also presented to help designers and researchers analyze these effects quickly by providing the design parameters. [ABSTRACT FROM AUTHOR]

In this paper, the energy collection performance of north/south- and east-west-facing collector pairs, installed at optimized fixed and seasonally adjusted tilt angles, considering both portrait and landscape alignments, and setups with and without reflectors, were numerically compared with each other. For the purpose, a method based on a hybrid ray-tracing model was employed, as developed in this work. A case study, considering an installation site in Auckland (New Zealand), is presented. For this, the fixed and seasonally adjusted optimum tilt angles for the layouts were first obtained. Then, the yearly energy collection for the layouts was evaluated at fixed tilt angles. An hourly analysis was also performed to understand the behavior of these layouts in the morning, and at noon and evening. Then, the effects of seasonal adjustments were investigated for each layout. Finally, the layouts were ranked according to their performance. The north-facing landscape layout with reflectors showed the best performance, followed by its portrait alignment version. The east-west-facing portrait layout with reflectors ranked third. The east-west-facing layouts without reflectors were found to give the poorest performance. [ABSTRACT FROM AUTHOR]

Carbon particles can be used as catalyst in solar reactors where they serve as radiant absorbent and nucleation sites for the heterogeneous decomposition reaction. Unlike commonly used metal catalysts, carbon catalyst does not have durability problem and high cost. However, in order to achieve sustainable catalytic decomposition of feedstock over carbon catalysts at elevated temperatures, the surface area of the carbon particles must be maintained. A subsequent treatment of deactivated carbon samples with CO2 at about 1000 °C would increase the surface and would recover the original activity as catalyst. In a windowed solar reactor, carbon particles are directly exposed to the high flux irradiation providing efficient radiation heat transfer directly to the reaction site. Therefore, one of the key parameters to achieve higher conversion efficiencies in a solar reactor is the presence and transport of carbon particles. In this paper, a transient one-dimensional model is presented to describe the effect of carbon particle feeding on energy transport and temperature profile of a cavity-type solar receiver. The model was developed by dividing the receiver into several control volumes and formulating energy balance equations for gas phase, particles, and cavity walls within each control volume. Monte Carlo ray tracing method was used to determine the solar heat absorbed by particles and cavity walls, as well as the radiative exchange between particles and cavity walls. Model accuracy was verified by experimental work using a solar receiver where carbon particles were injected uniformly. Comparison of simulation results with the experimentally measured temperatures at three different locations on cavity receiver wall showed an average deviation of 3.81%. The model was then used to study the effect of carbon particle size and feeding rate on the heat transfer, temperature profile, and energy absorption of the solar receiver. Based on the simulation results, it was found that injection of carbon particles with a size bigger than 500 µm has no significant influence on heat transfer of the system. However, by reducing the particle size lower than 500 µm, temperature uniformity and energy absorption can be enhanced. [ABSTRACT FROM AUTHOR]

Concentrated solar power (CSP) is a promising technology in transitioning to renewable energy because of its abundance in nature and thermal energy storage (TES) capability. Among the four types of available CSP technology, including parabolic trough, linear Fresnel, power tower, and parabolic dishes, a power tower using a central receiver has more potential to generate high-temperature heat in a scale supporting power cycles efficiency and achieve low levelized cost of energy (LCOE). Other than the conventional type of receiver design, the high-absorptive receiver concept developed and presented in this paper is novel in its design approach. The novel receiver design originated from National Renewable Energy Laboratory (NREL) consists of an array of solar flux absorb tubes. The solar absorb tubes require uniform flux distribution and in-depth flux penetration through the three different reflective sections of tubes in a hexagonal shape. To evaluate this unique receiver design and thermal performance, the flux distribution, flux uniformity, and intensity were numerically simulated using ansys fluent and SolTrace modeling program. On-sun testing has been done at NREL high flux solar testing facility, based on the computational analysis. [ABSTRACT FROM AUTHOR]

This paper describes the design methodology for a novel Fresnel lens. The original Fresnel lens is obtained from a plano-convex lens, whose spherical surface is split into a number of divisions (called facets), collapsed onto the flat base. Thus, all the facets of the original Fresnel lens have the same radius as that of the plano-convex lens. The proposed design aims to achieve better ray concentration and reduced spherical aberration than the original Fresnel lens by constructing spherical facets with unequal radii. The centers and radii of facets are constrained so that the ray refracted from the bottom vertex of each facet on one side of the optical axis and the ray refracted from the outer vertex of the corresponding facet on the other side of the optical axis must intersect at the focal plane. The proposed lens design has resulted in a 275% gain in the concentration ratio and a 72.5% reduction in the spherical aberration compared to the original lens of the same aperture diameter and number of facets. The performance of both novel and original Fresnel lenses when used as solar concentrators with a conical coil receiver is evaluated. The novel Fresnel lens led to increased heat gain and resulted in a compact solar collector design. [ABSTRACT FROM AUTHOR]

Biomass conversion processes can be energy intensive. There is a need to seek renewable sources to meet such energy demand. In this study, a partial heat of pyrolysis from a concentrated solar power (CSP)-based solar heat collector is modeled. A numerical investigation of the flow in the heat collector element (HCE) of the CSP is performed. By using the finite volume formulation, the fluid flow behavior within the pipe of the HCE is simulated to determine the thermal absorption, heat losses, and efficiency obtainable from the solar irradiation on the receiver tube. Monte-Carlo Ray-tracing based software, soltrace, is used to estimate the incident heat flux on the LS-2 receiver with an average direct normal irradiance (DNI) value, based on a comparative performance study of three types of molten salts (i.e., the heat transfer fluids (HTFs)). The performance of three kinds of the LS-2 HCE (i.e., vacuum, air-filled, and evacuated types) is also analyzed in terms of heat loss and efficiency measurements respectively. HITEC provides the best performance in terms of stability in delivering the partial heat of pyrolysis for the gasification of food waste on moderate velocities of 0.75-2 m/s. Also, the pyrolysis of the feedstock is proposed with an operating feedstock rate of 0.19 kg/s and 0.17 kg/s for lower heating value (LHV) and higher heating value (HHV), respectively. It is anticipated that the combined heat of pyrolysis from the proposed dual heat cycle, comprising both the CSP and a biogas plant, will provide a solution to the demand for alternative power from biomass energy conversion. [ABSTRACT FROM AUTHOR]

A novel method for pairing surface irradiation and volumetric absorption from Monte Carlo ray tracing to computational heat transfer models is presented. The method is well-suited to directionally and spatially complex concentrated radiative inputs (e.g., solar receivers and reactors). The method employs a generalized algorithm for directly mapping absorbed rays from a Monte Carlo ray tracing model to boundary or volumetric source terms in the computational mesh. The algorithm is compatible with unstructured, two and three-dimensional meshes with varying element shapes. Four case studies were performed on a directly irradiated, windowed solar thermochemical reactor model to validate the method. The method was shown to conserve energy and preserve spatial variation when mapping rays from a Monte Carlo ray tracing model to a computational heat transfer model in ANSYS FLUENT. [ABSTRACT FROM AUTHOR]

A performance assessment was conducted for a solar-biogas hybrid micro gas turbine integrated with a solar power tower technology. The considered system is a solar central receiver integrated with a micro gas turbine hybrid with biogas fuel as a backup. The Brayton cycle is designed to receive a dual integrated heat source input that works alternatively to keep the heat input to the system continuous. The study considered several key performance parameters including meteorological condition effects, recuperator existence and effectiveness, solar share, and gas turbine components performance. This study shows a significant reduction in CO2 emissions due to the utilization and hybridization of the renewable energies, solar, and biogas. The study reveals that the solar-biogas hybrid micro gas turbine for 100-kW power production has a CO2 emission less than a conventional fossil fuel gas turbine. Finally, the study shows that the method of power generation hybridization for solar and biogas gas turbines is a promising technique that leads to fuel-savings and lower CO2 emissions. [ABSTRACT FROM AUTHOR]

A linear receiver able to achieve temperatures up to 800 °C is presented. The high-temperature resistance is achieved by avoiding critical aspects (vacuum, glass-metal joints, surface films) that limit the temperature in usual receivers; the thermal insulation is obtained by enclosing the receiver tube in an elliptic reflecting cavity. The tube is placed near a focus of the cavity, and the primary collector concentrates the radiation on the other focus, where the cavity has a small opening: the ellipse reflects the radiation toward the tube and largely contains the reflected radiation and thermal emission, thus acting both as a secondary reflector and as a cavity receiver. Optical and thermal simulations show that temperatures up to 800 °C can be achieved, with optical efficiency above 70% and thermal efficiency in the range 45-85% for temperatures in the range 500-800 °C; the local overall efficiency ranges from about 40% to 66%, depending on the receiver tube emissivity and fluid temperature. In this way, the field of applicability of the linear collector technology can be significantly extended to include a vast amount of processes such as thermochemical cycles for hydrogen production, and solar fuel production processes, which require temperatures above 700 °C. [ABSTRACT FROM AUTHOR]

A high-flux solar simulator is essential for evaluating solar thermal components under controlled and adjustable flux input conditions. This study presents a newly built high-flux solar simulator composed of 19 individual units. Each unit includes a xenon short-arc lamp (each consuming up to 6 kW electricity power) coupled with a truncated ellipsoidal reflector, a cooling blower, and a power module. The power module yields a current in the range of 50-160 A. The number of lamps in use is flexible, which allows for a wide range of radiation flux (10%-100%) on the focal plane. The radiation power, peak value, flux distribution on the circular target plane, and conversion efficiency are evaluated based on a flux mapping method. The results indicate that the proposed solar simulator is capable of achieving thermal power of 23.3 kW, peak flux in excess of 1.78 MW/m², a stagnation temperature exceeding 2360 °C, and average irradiance of 773.4 kW/m² on the focal plane (diameter of 260 mm). The electro-thermal conversion efficiency of the simulator is 35.7%. A ray-tracing method was employed, and the simulation results were found to be in good agreement with those in the experiments. An experimental test of a volumetric ceramic receiver was conducted, and the results indicate the availability and applicability of the high-flux solar simulator when carrying out studies about solar receivers. [ABSTRACT FROM AUTHOR]

This research details the design, fabrication, and partial testing of a concentrated solar receiver and an air-cooled heat exchanger. The solar receiver and heat exchanger have been fabricated for use in an experimental system that uses the supercritical carbon dioxide Brayton cycle. They are coupled with a Science Applications International Corporation (SAIC) solar dish 250× concentrator located on the University of Nevada, Las Vegas campus. The purpose of this solar-powered supercritical CO2 system is to function as a testbed for testing the cycle, system components, and alternate system configurations. Photographic flux mapping of the dish showed peak solar flux just above 200× and is used to appropriately size the receiver. Sun tests of the tubing, receiver, and air-cooled heat exchanger were performed achieving fluid temperatures in the range of 973 K (700 °C) using nitrogen in an open loop at low mass flowrates, and above 1173-K (900 °C) receiver wall temperatures in a no-flow case. [ABSTRACT FROM AUTHOR]

Current state-of-the-art development of concentrated solar power (CSP) applications targets cost-effective and highly efficient processes in order to establish commercialization of these technologies. The design of solar receivers/reactors and their respective flow configuration have a direct impact on the operational performance of the solar thermochemical processes. Thermal efficiencies, reaction kinetics, and other key output metrics are the intrinsic result of the chosen configuration. Therefore, reactor design optimization plays a crucial role in the development of solar thermochemical applications. In this study, a computational fluid dynamics (CFD) model of a directly-irradiated cavity receiver has been developed. The CFD-domain is coupled with incoming radiation that is obtained by using Monte Carlo ray tracing (MCRT). Experimental campaigns of the cavity receiver were carried out using a 7-kW high flux solar simulator (HFSS) as a radiative source. Temperature readings were obtained at different locations inside the cavity receiver for both wall and gas temperatures. In order to mimic naturally changing insolation conditions, the HFSS was run at different power levels. Heat flux at the aperture of the solar receiver was experimentally characterized. The acquired heat flux maps validated the intermediate results obtained with the MCRT method. The coupled computational model was validated against the measured temperatures at different locations inside the receiver. Computed temperature contours inside the receiver confirmed the experimentally observed non-uniformity of the axial temperature distribution. The validated analysis presented in this paper was then used as a baseline case for a parametric study. Design optimization efforts were undertaken toward obtaining temperature uniformity and achieving efficient heat transfer within the fluid domain. Enhanced flow circulation was achieved which yielded temperature uniformity of the receiver at steady-state conditions. The outcome of this parametric analysis provided valuable insights into the development of thermal efficient solar cavity receivers. Hence, findings of this study will serve as a starting point for the future solar reactor design. For example, it was found that reversing flow direction has an adverse effect on the temperature uniformity inside the receiver. Similarly, increasing the inlet angle does not positively affect the temperature distribution and hence should be chosen carefully when designing a solar reactor. [ABSTRACT FROM AUTHOR]

Solar energy is critical because it does not only sustain global environment but also produce ample power to use. This study addresses solar concentrator layout to maximize the profit for the firm manufacturing the concentrator, while helping an energy user receives as much sunlight as possible. We consider several key design factors of a solar concentrator layout, such as light transmission loss, to make the model more accurate. As for the economic scale of production, some key constraints are considered, such as concentrator thickness and the number of exits, where a sunbeam is delivered to an optical cable for energy transmission. To obtain a high brightness from a good solar concentrator for the user, the concentrator manufacturer requires to increase the complexity of concentrator layout, but suffers from the high manufacturing cost. In this study, we simultaneously address the issues of the optimal solar concentrator layout and trade-off of conflict objectives between the energy user and the concentrator manufacturer. This study proposes a nondominated sorting genetic algorithm for multiple-objective optimization solution. To summarize, the result from this study presents a promising solution both for the light efficiency to supply users and for the profit of the firm on delivering the solar concentrator layout. [ABSTRACT FROM AUTHOR]

In this paper, a continuous tracking strategy for the heliostat in the James S. Markiewicz Concentrated Solar Energy Research Facility at Valparaiso University is developed. A model of the nonlinear dynamics of the heliostat motion is developed and the open-loop control strategy is presented. Asymptotic stability of the heliostat control using the Lyapunov and LaSalle's theorems was proven. Simulations using the nonlinear dynamic model are presented and interpreted to identify the feedback gain that maximizes the time response of the heliostat without introducing oscillations in its motion. Finally, the control strategy is put to the test during summertime operation. The data presented show that the tracking strategy has an root mean square (RMS) tracking error of 0.058 mrad, where the error is defined as the difference between the desired and actual heliostat positions. Images of the aperture of a high-temperature solar receiver over 8 h of testing are also presented to qualitatively demonstrate the success of the tracking strategy. [ABSTRACT FROM AUTHOR]

This work reports on the development of a transient heat transfer model of a solar receiver-reactor designed for thermochemical redox cycling by temperature and pressure swing of pure cerium dioxide in the form of a reticulated porous ceramic (RPC). In the first, endothermal step, the cerium dioxide RPC is directly heated with concentrated solar radiation to 1500 °C while under vacuum pressure of less than 10 mbar, thereby releasing oxygen from its crystal lattice. In the subsequent, exothermic step, the reactor is repressurized with carbon dioxide as it cools, and at temperatures below 1000 °C, the partially reduced cerium dioxide is re-oxidized with a flow of carbon dioxide. To analyze the performance of the solar reactor and to gain insight into improved design and operational conditions, a transient heat transfer model of the solar reactor for a solar radiative input power of 50 kW during the reduction step was developed and implemented in ANSYS cfx. The numerical model couples the incoming concentrated solar radiation using Monte Carlo ray tracing, incorporates the reduction chemistry by assuming thermodynamic equilibrium, and accounts for internal radiation heat transfer inside the porous ceria by applying effective heat transfer properties. The model was experimentally validated using data acquired in a high-flux solar simulator (HFSS), where temperature evolution and oxygen production results from model and experiment agreed well. The numerical results indicate the prominent influence of solar radiative input power, where increasing it substantially reduces reduction time of the cerium dioxide structure. Consequently, the model predicts a solar-to-fuel energy conversion efficiency of >6% at a solar radiative power input of 50 kW; efficiency >10% can be obtained provided the RPC macroporosity is substantially increased, and better volumetric absorption and uniform heating is achieved. Managing the ceria surface temperature during reduction to avoid sublimation is a critical design consideration for direct absorption solar receiver-reactors. [ABSTRACT FROM AUTHOR]

This paper presents FluxTracer, an advanced open source computer tool to assist in the analysis, design, and optimization of solar concentrators and receivers. FluxTracer is a postprocessor for Monte Carlo ray tracers used to simulate the optical behavior of solar concentrating systems. By postprocessing the rays generated by the ray tracer, FluxTracer can partition into volumetric pixels (voxels) a region of interest in three-dimensional (3D) space defined by the user and compute for each voxel the radiant power density of the concentrated solar radiation. Depending upon the set of rays provided by the ray tracer, it may be able to integrate the radiant power density in every voxel over time. The radiant energy density analysis described is just one of the analyses that FluxTracer can carry out on the set of rays generated by the ray tracer. This paper presents the main analyses that FluxTracer can provide. It also presents examples of how the information provided by FluxTracer can be used to assist in the analysis, design, and optimization of solar concentrators and receivers. FluxTracer is the first of a series of components of an open-source computational framework for the analysis, design, and optimization of solar concentrators and receiver, being developed by The Cyprus Institute (CyI) and the Australian National University (ANU). [ABSTRACT FROM AUTHOR]

This paper introduces a chemical-looping configuration integrated with a concentrating solar thermal (CST) system. The CST system uses an array of mirrors to focus sunlight, and the concentrated solar flux is applied to a solar receiver to collect and convert solar energy into thermal energy. The thermal energy then drives a thermal power cycle for electricity generation or provides an energy source to chemical processes for material or fuel production. Considerable interest in CST energy systems has been driven by power generation, with its capability to store thermal energy for continuous electricity supply or peak shaving. However, CST systems have other potential to convert solar energy into fuel or to support thermochemical processes. Thus, we introduce the concept of a chemical-looping configuration integrated with the CST system that has potential applications for thermochemical energy storage or solar thermochemical hydrogen production. The chemical-looping configuration integrated with a CST system consists of the following: a solar-receiver reactor for solar-energy collection and conversion, thermochemical energy storage, a reverse reactor for energy release, and system circulation. We describe a high-temperature reactor receiver that is a key component in the chemical-looping system. We also show the solar-receiver design and its performance analyzed by solar-tracing and thermal-modeling methods for integration within a CST system. [ABSTRACT FROM AUTHOR]

Conjugate laminar natural convection heat transfer and air flow with radiation of tube solar receiver with glass window were numerically investigated. The discrete ordinate method was used to solve the radiative transfer equation. And the three-dimensional steady-state continuity, Navier-Stokes, and energy equations were solved. The temperature difference based on environment and high temperature surface of receiver is varied from 100 K to 1000 K. The influence of the surface emissivity, heating temperature, convective coefficient, and convective temperature of environment on the heat transfer from the receiver with glass window has also been investigated. The numerical results indicated that the highest temperature of glass window increases and the high temperature area becomes wide, with the temperature of heating wall and surface emissivity increasing. Adopting higher convective coefficient of glass window can reduce the peak magnitude of temperature distribution on glass window of tube receiver up to 45%. [ABSTRACT FROM AUTHOR]

Maintaining receiver's thermal stresses and corrosion below the material limits are issues that need careful attention in solar thermal towers. Both depend on heliostats' aiming points over the central receiver and available direct solar radiation at any instant. Since this technology relies on an unavoidable time-changing resource, aiming points need to be properly manipulated to avoid excessive hot spots. This paper proposes a new aiming point strategy based on a multivariable model predictive control (MPC) approach. It shows an alternative approach by introducing an agent-based group behavior over heliostats' subsets, which makes possible either concentrating or dispersing solar radiation as required by the MPC algorithm. Simulated results indicate that it is feasible to develop a closed-loop control procedure that distributes solar irradiance over the central receiver according to the predefined heat flux limits. The performance of the proposed approach is also compared with the results found in the available literature that uses a different methodology. [ABSTRACT FROM AUTHOR]

The thermal performance of an array of pressurized-air solar receiver modules integrated to a gas turbine power cycle is analyzed for a simple Brayton cycle (BC), recuperated Brayton cycle (RC), and combined Brayton-Rankine cycle (CC). While the solar receiver's solar-to-heat efficiency decreases at higher operating temperatures and pressures, the opposite is true for the power cycle's heat-to-work efficiency. The optimal operating conditions are achieved with a preheat stage for a solar receiver outlet air temperature of 1300 °C and an air cycle pressure ratio of 9, yielding a peak solar-to-electricity efficiency--defined as the ratio of the net cycle work output divided by the solar radiative power input through the receiver's aperture--of 39.3% for the combined cycle configuration. [ABSTRACT FROM AUTHOR]

The working principle of particle-based solar receivers is to utilize the absorptivity of a dispersed particle phase in an otherwise optically transparent carrier fluid. In comparison to their traditional counterparts, which use a solid surface for radiation absorption, particle-based receivers offer a number of opportunities for improved efficiency and heat transfer uniformity. The physical phenomena at the core of such receivers involve coupling between particle transport, fluid turbulence, and radiative heat transfer. Previous analyses of particle-based solar receivers ignored delicate aspects associated with this three-way coupling. Namely, these investigations considered the flow fields only in the mean sense and ignored turbulent fluctuations and the consequent particle preferential concentration. In the present work, we have performed three-dimensional direct numerical simulations of turbulent flows coupled with radiative heating and particle transport over a range of particle Stokes numbers. Our study demonstrates that the particle preferential concentration has strong implications on the heat transfer statistics. We demonstrate that "for a typical setting" the preferential concentration of particles reduces the effective heat transfer between particles and the gas by as much as 25%. Therefore, we conclude that a regime with Stokes number of order unity is the least preferred for heat transfer to the carrier fluid. We also provide a 1D model to capture the effect of particle spatial distribution in heat transfer. [ABSTRACT FROM AUTHOR]

A volumetric solar receiver for superheating evaporated sulfuric acid is developed as part of a 100 kW pilot plant for the hybrid sulfur (HyS) cycle. The receiver, which uses silicon carbide foam as a heat transfer medium, heats evaporated sulfuric acid using concentrated solar energy to temperatures of 1000 °C or greater, which are required for the downstream catalytic reaction to split sulfur trioxide into oxygen and sulfur dioxide. Multiple parallel approaches for modeling and analysis of the receiver are used to design the prototype. Focused numerical modeling and thermodynamic analysis are applied to answer individual design and performance questions. Numerical simulations focused on fluid flow are used to determine the best arrangement of inlets, while thermodynamic analysis is used to evaluate the optimal dimensions and operating parameters. Finally, a numerical fluid mechanics and heat transfer model is used to predict the temperature field within the receiver. Important lessons from the modeling efforts are given, and their impacts on the design of a prototype are discussed. [ABSTRACT FROM AUTHOR]

The problem of particle deposition on the glass window of a solar receiver has restricted its continuous operation by reducing solar radiation transmission. A rigorous attempt has been made in this analysis by exploring the understanding of particle deposition mechanisms and their mitigating strategies. A simplified form of a fluidized bed solar receiver (FBSR) having the same flow phenomena of FBSR is chosen for the numerical analysis. In the numerical analysis, the turbulent flow in the receiver is investigated by renormalized group (RNG) theory based k-ε models. The validation of the numerical model is carried out by measuring the turbulent flow properties using a turbulent flow instrumentation (TFI) Cobra probe. The results of this analysis revealed that mass flow into the secondary concentrator of the receiver was reduced significantly when the ratio between the outlets and inlet areas was 0.5, and the ratio between the aperture and receiver diameter was 0.41. When using window shielding jets, only 5% of the inlet mass as a window jet was sufficient to prevent any particle deposition on the glass window, however, the number of jets was found to be an important factor. At a constant mass flow rate, increasing the number of window shielding jets reduced the suction pressure from the core to the aperture, which helped to restrict the inlet flow in the secondary concentrator. [ABSTRACT FROM AUTHOR]

A high-temperature pressurized-air solar receiver, designed for driving a Brayton cycle, consists o f a cylindrical SiC cavity and a concentric annular reticulated porous ceramic (RPC) foam enclosed hy a steel pressure vessel. Concentrated solar energy is absorbed by the cavity and transferred to the pressurized air flowing across the RPC by combined conduction, convection, and radiation. The governing mass, momentum, and energy conservation equations are numerically solved by coupled Monte Carlo (MC) and finite volume (FV) techniques. Model validation was accomplished with experimental data obtained with a 50 kW,h modular solar receiver prototype. The mode! is applied to elucidate the major heat loss mechanisms and to study the impact on the solar receiver performance caused by changes in process conditions, material properties, and geometry. For an outlet air temperature range 700-1000 °C and pressure range 4-15 bar, the thermal efficiency--defined as the ratio o f the enthalpy change of the airflow divided by the solar radiative power input through the aperture--exceeds 63% and can he further improved via geometiy optimization. Reradiation is the dominant heat loss. [ABSTRACT FROM AUTHOR]

The optical design and thermal performance o f a solar power tower system using an array of higli-temperature pressurized air-based solar receivers is analyzed for Brayton, recuperated, and combined Brayton-Rankine cycles. A 50 MWpower tower system comprising a cluster of 500 solar receiver modules, each attached to a hexagon-shaped secondary concentrator and arranged side-by-side in a lioneycomb-type structure following a spherical fiv-eye optical configuration, can yield a peak solar-to-electricity efficiency of 37%. [ABSTRACT FROM AUTHOR]

Supercritical carbon dioxide (s-CO2) can be used both as a heat transfer and working fluid in solar power tower plants. The main concern in the design of a direct s-CO2 receiver is the high operating pressures, i.e., close to 20 MPa. At such high pressures, conventional receivers do not exhibit the necessary mechanical strength or thermal performance. In this paper, a receiver based on compact heat exchanger technology is developed. The receiver consists of a group of plates with square-shaped channels which are diffusion bonded together to tolerate the high operating pressure. A computational model is developed and validated against data in the literature. Inconel 625 is used as the base material because of its superior resistance against corrosion in the presence of s-CO2. The receiver heats s-CO2 with mass flow rate of / kgls from 530 °C to 700 °C under a solar flux density of 500kW/m². The influence of different parameters on the performance of the receiver is evaluated by a parametric analysis. Subsequently, a multi-objective optimization is performed to determine the optimal geometry of the heat exchanger considering the tradeoff between objective functions, such as unit thermal resistance and pressure drop. The design variables are hydraulic diameter, number of layers, and distance between the channels. The mechanical strength of the system is the constraint to the problem, which is evaluated using an ASME code for the pressure vessels. Finally, the temperature profiles inside the channels and the surface of the receiver are presented. It is shown that the fluid reaches the desired temperature while the maximum temperature of the surface remains well below the material limit. [ABSTRACT FROM AUTHOR]

The operational conditions of the solar-thermal receiver for a Bray ton cycle engine are challenging, and lack a large body of operational data unlike steam plants. We explore the receiver's fundamental element, a pressurized tube in time varying solar flux for a se-ries of 30 yr service missions based on hypothetical power plant designs. We developed and compared two estimation methods to predict the receiver tube lifetime based on available creep life and fatigue data for alloy 617. We show that the choice of inelastic strain model and the level of conservatism applied through design rules will vary the life-time predictions by orders of magnitude. Based on current data and methods, a turbine inlet temperature of 1120 K is a necessary 30-yr-life-design condition for our receiver. We also showed that even though the time at operating temperature is about three times longer for fossil fuel powered (steady) operation, the damage is always lower than cyclic operation using solar power. [ABSTRACT FROM AUTHOR]

Pyromark 2500 is a silicone-based high-temperature paint that has been used on central receivers to increase solar absorptance. The radiative properties, aging, and selective absorber efficiency of Pyromark 2500 are presented in this paper for use as a baseline for comparison to high-temperature solar selective absorber coatings currently being developed. The solar absorptance ranged from s0.97 at near-normal incidence angles to s0.8 at glancing (80°) incidence angles, and the thermal emittance ranged from s0.8 at 100 °C to 0.9 at 1000 °C. After thermal aging at temperatures of s750°C or higher, the solar absorptance decreased by several percentage points within a few days. It was postulated that the substrate may have contributed to a change in the crystal structure of the original coating at elevated temperatures. [ABSTRACT FROM AUTHOR]