Solar Thermochemical Applications Research Articles

Cobalt-Based Perovskites as Efficient Oxygen Exchange Redox Materials for Solar Thermochemical Application

Solar thermochemical CO2 splitting (STCS) cycles provide the promising routes to generate renewable fuel, fuel precursors, or chemical feedstocks by the utilization of renewable resources like solar thermal energy and CO2 relating to current energy as well as climate change. Perovskites have been proposed as the potential oxygen exchange redox materials for the two-step STCS process due to their greater oxygen release and redox stability. Herein, the oxygen release and redox stability were investigated for the cobalt-based perovskites (SrCoO3-δ and SrCo0.9Zr0.1O3-δ) as the potential STCS candidates. Consequently, the XRD, FT-IR, and Raman confirmed the dual-phase perovskite constitutions of SrCo0.9Zr0.1O3-δ (i.e. SrCoO2.29 and SrZrO3) compared to SrCoO3-δ. The thermogravimetric analysis indicated the incorporation of Zr could facilitate the oxygen release and enhance the phase stability. The near-equilibrium lattice oxygen release and uptake at high temperatures could be observed through the redox performance of SrCo0.9Zr0.1O3-δ at different heating and cooling rates. The results are noteworthy and demonstrate the potential of Zr-enhanced cobalt-based perovskites as the oxygen exchange redox materials for practical solar thermochemical applications.

Characterization and Deviation Analysis of a High-Flux Solar Simulator With Metal-Halide Lamps

Abstract Utilizing highly concentrated solar power for thermochemical processing as one of the extraterrestrial in situ resource utilization (ISRU) applications has been highlighted as an essential technique to support deep-space exploration in the future. Multi-source high-flux solar simulators (HFSSs) are widely employed to provide stable irradiance for indoor solar thermal experiments. Meanwhile, numerical modeling that can characterize the radiation transport mechanisms within the solar thermal system has been developed for performance evaluation before field trials. However, significant differences between simulated and measured flux distributions were shown for existing models developed based on the Monte-Carlo ray-tracing (MCRT) method, which has been attributed to only one or two specific reasons. In this paper, we proposed a comprehensive analysis of the concentration characteristics of a 42 kW metal-halide lamp HFSS, developed at Swinburne University of Technology, considering the effect of five aspects. The flux distribution, uniformity, and vector distribution under different configurations were compared to quantify the influence of these factors on receiving irradiance. The suitable arc size, reflector shape, and reflector surface properties of the existing HFSS have also been numerically determined to improve the model and reduce the root mean square error (RMSE) for the lamp array from 38.2% to 8.3%. This research provides a potential pathway to numerically predict the radiation transfer performance of HFSSs and determine the suitable configuration for desired solar thermochemical applications.

Exploring Ca–Ce–M–O (M = 3d Transition Metal) Oxide Perovskites for Solar Thermochemical Applications

Solar thermochemical (STC) processes hold promise as efficient ways to generate renewable fuels, fuel precursors, or chemical feedstocks using concentrated sunlight. Specifically, one actively rese...

A First‐Principles‐Based Sub‐Lattice Formalism for Predicting Off‐Stoichiometry in Materials for Solar Thermochemical Applications: The Example of Ceria

AbstractTheoretical models that reliably can predict off‐stoichiometry in materials via accurate descriptions of underlying thermodynamics are crucial for energy applications. For example, transition‐metal and rare‐earth oxides that can tolerate a large number of oxygen vacancies, such as CeO2 and doped CeO2, can split water and carbon dioxide via a two‐step, oxide‐based solar thermochemical (STC) cycle. The search for new STC materials with a performance superior to that of state‐of‐the‐art CeO2 can benefit from predictions accurately describing the thermodynamics of oxygen vacancies. The sub‐lattice formalism, a common tool used to fit experimental data and build temperature‐composition phase diagrams, can be useful in this context. Here, sub‐lattice models are derived solely from zero‐temperature quantum mechanics calculations to estimate fairly accurate temperature‐ and oxygen‐partial‐pressure‐dependent off‐stoichiometries in CeO2 and Zr‐doped CeO2. Physical motivations for deriving some of the “excess” sub‐lattice model parameters directly from quantum mechanical calculations, instead of fitting to minimize deviations from experimental and/or theoretical data, are identified. Important limitations and approximations of the approach used are specified and extensions to multi‐cation oxides are also suggested to help identify novel candidates for water and carbon dioxide splitting and related applications.

Systematic approach for design optimization of a 3 kW solar cavity receiver via multiphysics analysis

Use of Computational Fluid Dynamics (CFD) modeling for solar reactor design optimization is a powerful tool in developing efficient reactors for solar thermochemical applications. CFD models can capture complex multiphase flow dynamics to explain the physics linking the heat transfer and flow dynamics on chemical conversion efficiency and continuous operation of a solar reactor. Flux map and temperature data can be numerically obtained via CFD inside the solar reactor, which is a harsh environment for penetrating measurement tools. In this paper, a CFD analysis of a solar cavity receiver operated with air flow and is directly irradiated by a 10 kWe High Flux Solar Simulator (HFSS) is presented. Radiative heat transfer from the HFSS is modeled by implementing Monte Carlo Ray Tracing (MCRT) for tracking the ray paths from the radiative source to the aperture plane of the receiver. Flux densities were computed at the aperture and validated by experimental heat flux characterization of the solar simulator. The validated MCRT model provided directional information on the radiative flux and was sequentially coupled to the computational fluid domain. Through experimental testing, receiver temperatures up to 1200 K were measured with solar input at the entrance of the cavity of around 3 kW radiative power. Experimentally measured non-uniform temperature distribution in the axial direction was recorded, which developed a hot spot at the backplate of the cavity. Accuracy of the numerical model was validated by comparing the temperature prediction inside the cavity receiver to experimental results. Validated model was then used to conduct a parametric study on the flow configuration to identify run conditions yielding enhanced fluid mixing and improved heat transfer characteristics. The results showed that altering the angle of the main inlet ports proved to be effective in reducing the local hot spot in the back of the receiver. Moreover, results of the parametric study identified the increase of main flow rate as a positive contribution towards temperature uniformity throughout the fluid domain.

Monte Carlo Ray Tracing-Coupled Computational Fluid Dynamic Modeling and Experimental Testing of a 1-kW Solar Cavity Receiver Radiated via 7-kW HFSS

Abstract 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.

Effects of multilayer porous ceramics on thermochemical energy conversion and storage efficiency in solar dry reforming of methane reactor

In solar thermochemical systems, the utilization of porous ceramics plays an important role in the enhancement of heat transfer and optimization of reaction conditions, thereby effectively improving the energy conversion and storage efficiency. Compared with the common filling pattern of one-layer porous ceramic (1-LPC), novel changes in the thermal and chemical characteristics can be induced using multilayer porous ceramics (MPCs). To determine whether MPCs have advantages over 1-LPC in solar thermochemical applications, a numerical model was established in this study by combining computational fluid dynamics with dry reforming of methane reaction kinetics. The local thermal non-equilibrium model coupled with the P1 approximation was adopted to solve the solar radiation diffusion and convective heat transfer problems, while the non-Darcy flow effect was considered to predict the momentum dissipation resulting from the porous ceramics. Based on this, the effects of layer number, gap position, porosity, and cell size were investigated to find the optimal application strategies for MPCs. The simulation results indicate that a large temperature gradient in the first gap between two layers of MPCs can usually reduce the wall heat loss and improve the thermal efficiency, but has no universal effect on improving the solar-to-fuel efficiency. Under the current operational conditions, although improvement of the solar-to-fuel efficiency by approximately 0.03%–2.43% can be obtained using a 4-LPC in the cases of high porosities (ϕ⩾0.86) and large mean cell sizes (dp⩾7mm), 1-LPC remains the most reliable filling pattern with a wider range of applications and stable performance.

Design of a compound parabolic concentrator for a multi-source high-flux solar simulator

A three-dimensional compound parabolic concentrator (CPC) is designed for high-temperature solar thermochemical applications driven by radiation from a multi-source high-flux solar simulator. The basic geometrical parameters of the CPC including the acceptance angle and the entry aperture radius are determined using optical simulations. A cooling system for overheating protection is designed using engineering heat transfer correlations. A prototype CPC is manufactured using additive manufacturing and single point diamond turning techniques. The optical simulations show that the CPC increases the concentration ratio by a factor of 4.1 at an optical efficiency of 85.4%, reduces spillage loss from 78.9% to 32.1%, and reduces the flux non-uniformity on the target surface.

Characterization of a New 10 kWe High Flux Solar Simulator Via Indirect Radiation Mapping Technique

This paper presents characterization of a new high flux solar simulator consisting of a 10 kW Xenon arc via indirect heat flux mapping technique for solar thermochemical applications. The method incorporates the use of a heat flux gauge (HFG), single Lambertian target, complementary metal oxide semiconductor (CMOS) camera, and three-axis optical alignment assembly. The grayscale values are correlated to heat flux values for faster optimization and characterization of the radiation source. Unlike previous work in heat flux characterization that rely on two Lambertian targets, this study implements the use of a single target to eliminate possible errors due to interchanging the targets. The current supplied to the simulator was varied within the range of 120–200 A to change the total power and to mimic the fluctuation in sun's irradiance. Several characteristic parameters of the simulator were studied, including the temporal instability and radial nonuniformity (RNU). In addition, a sensitivity analysis was performed on the number of images captured, which showed a threshold value of at least 30 images for essentially accurate results. The results showed that the flux distribution obtained on a 10 × 10 cm2 target had a peak flux of 6990 kWm−2, total power of 3.49 kW, and half width of 6.25 mm. The study concludes with the illustration and use of a new technique, the merging method, that allows characterization of heat flux distributions on larger areas, which is a promising addition to the present heat flux characterization techniques.

High-flux optical systems for solar thermochemistry

High-flux optical systems (HFOSs) are optical concentrators used to increase the radiative flux of the natural terrestrial solar irradiation. High radiative flux concentration leads to high energy density in solar receivers which allows to obtain high temperatures. In solar thermochemical applications, the high-temperature heat drives endothermic thermochemical reactions. HFOSs have been deployed for research and development of solar thermochemical devices and systems, from solar reacting media to solar reactors. Here, we review the designs and characteristics of HFOSs as well as challenges and opportunities in the area of high-flux optical systems for solar thermochemical applications.

Spatial and angular finite element method for radiative transfer in participating media

A computational approach for the modeling of multi-dimensional radiative transfer in participating media, including scattering, is presented. The approach is based on the sequential use of angular and spatial Finite Element Methods for the discretization of the Radiative Transfer Equation (RTE). The angular discretization is developed with an Angular Finite Element Method (AFEM) based on the Galerkin approach. The AFEM leads to a counterpart of the RTE consisting of a coupled set of transient-advective–reactive equations that are continuously dependent on space and time. The AFEM is ideally suited for so-called h- and/or p-refinement for the discretization of the angular domain: h-refinement is obtained by increasing the number of angular elements and p-refinement by increasing the order of the angular interpolating functions. The spatial discretization of the system of equations obtained after the angular discretization is based on a Variational Multi-Scale Finite Element Method (VMS-FEM) suitable for the solution of generic transport problems. The angularly and spatially discretized system is solved with a second-order accurate implicit predictor multi-corrector time stepper together with a globalized inexact Newton–Krylov nonlinear solver. The overall approach is designed and implemented to allow the seamless inclusion of other governing equations necessary to solve coupled fluid–radiative systems, such as those in combustion, high-temperature chemically reactive, and plasma flow models. The combined AFEM and VMS-FEM for the solution of the RTE is validated with two- and three-dimensional benchmark problems, each solved for 3 levels of angular partitioning (h-refinement) and for 2 orders of angular basis functions (p-refinement), i.e. piecewise constant (P0) and piecewise linear (P1) basis over spherical triangles. The overall approach is also applied to the simulation of radiative transfer in a parabolic concentrator with participating media, as encountered in solar thermochemical applications, for different values of absorption and scattering coefficients, and for different angles of inclination of the incident radiation.

Experimental and numerical investigation of the aperture size effect on the efficient solar energy harvesting for solar thermochemical applications

In this paper, experimental and numerical work have been undertaken to investigate the steady state temperatures throughout the day of a cylindrical solar receiver when using fixed and variable size apertures. A high flux solar simulator, consisting of a 7kW xenon short arc lamp, is employed as a light source. The sunlight intensity variations at early morning (06:30), morning (07:15) and noon (12:00) time of a reference day are imitated by changing the input current to the lamp. Experiments have been performed with different aperture diameters across selected irradiance levels to imitate sunlight variations. An optical model is developed to simulate incident flux distribution and the output is compared with the experimental measurements for validation. A finite volume algorithm is developed, based on a coupled Monte Carlo heat transfer model, to calculate the steady state temperatures in the receiver. Experimental and numerical temperatures are compared and an excellent agreement with an average temperature difference of ±0.2%, is observed. The optimum aperture size varies with the change in irradiance intensity and therefore the time of day. Simulations for a 30kW light source show that the daily steady state temperature differential for fixed apertures of 8–10cm is 170–190K. Variable apertures reduce power consumption by half when compared to fixed apertures. Variable apertures maintain steady state temperatures of 1000K, 1100K and 1200K by consuming 26.8kWday, 33.2kWday and 26.9kWday, respectively.

Review of Heat Transfer Research for Solar Thermochemical Applications

This article reviews the progress, challenges and opportunities in heat transfer research as applied to high-temperature thermochemical systems that use high-flux solar irradiation as the source of process heat. Selected pertinent areas such as radiative spectroscopy and tomography-based heat and mass characterization of heterogeneous media, kinetics of high-temperature heterogeneous reactions, heat and mass transfer modeling of solar thermochemical systems, and thermal measurements in high-temperature systems are presented, with brief discussions of their methods and example results from selected applications.