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Perspectives

J. Sol. Energy Eng. 2019;141(2):020301-020301-1. doi:10.1115/1.4042269.
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In the roughly 15 years that I have been active as a concentrating solar technologies (CST—power and fuels) researcher, I have seen the field develop substantially. When I started my career in CST working at Sandia National Labs in 2004, our group of researchers was very small, the Central Receiver Test Facility was being used mostly for astronomy and re-entry vehicle testing, and utility-scale solar thermal power was essentially limited to the SEGS plants. A lot has changed since then, and today there is roughly 5 GW of global CSP capacity including molten salt power towers producing electricity at a price that is competitive with conventional generation while demonstrating the feasibility of utility scale thermal energy storage. In a sense, the successes of the CSP industry over the past decade or so have fulfilled many of the technical and economic goals of the industry. In that same time period other renewable energy technologies have enjoyed similar success; wind power capacity has grown from less than 50 GW to more than 500 GW, and the cost of utility scale solar photovoltaic power will soon drop below $0.04/kWh, if it has not already. There have also been advances in the area of electrochemical energy storage, electric vehicles are becoming mainstream, and power transmission and utilization strategies that look fundamentally different than the utility grid of the past are being proposed and pursued. One could easily argue that we are now seeing the beginning of a rapid transformation in the way that energy is produced and consumed globally. What is less easy to see clearly is specifically where CST can best contribute to this transformation. Where should researchers in our field focus their efforts to have the greatest impact on a rapidly changing energy landscape?

Commentary by Dr. Valentin Fuster

Research Papers

J. Sol. Energy Eng. 2019;141(2):021008-021008-9. doi:10.1115/1.4042241.

The most advanced solar thermochemical cycles in terms of demonstrated reactor efficiencies are based on temperature swing operated receiver-reactors with open porous ceria foams as a redox material. The demonstrated efficiencies are encouraging but especially for cycles based on ceria as the redox material, studies have pointed out the importance of high solid heat recovery rates to reach competitive process efficiencies. Different concepts for solid heat recovery have been proposed mainly for other types of reactors, and demonstration campaigns have shown first advances. Still, solid heat recovery remains an unsolved challenge. In this study, chances and limitations for solid heat recovery using a thermal storage unit with gas as heat transfer fluid are assessed. A numerical model for the reactor is presented and used to analyze the performance of a storage unit coupled to the reactor. The results show that such a concept could decrease the solar energy demand by up to 40% and should be further investigated.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(2):021009-021009-9. doi:10.1115/1.4042069.

Three crucial aspects still to be overcome to achieve commercial competitiveness of the solar thermochemical production of hydrogen and carbon monoxide are recuperating the heat from the solid phase, achieving continuous or on-demand production beyond the hours of sunshine, and scaling to commercial plant sizes. To tackle all three aspects, we propose a moving brick receiver–reactor (MBR2) design with a solid–solid heat exchanger. The MBR2 consists of porous bricks that are reversibly mounted on a high temperature transport mechanism, a receiver–reactor where the bricks are reduced by passing through the concentrated solar radiation, a solid–solid heat exchanger under partial vacuum in which the reduced bricks transfer heat to the oxidized bricks, a first storage for the reduced bricks, an oxidation reactor, and a second storage for the oxidized bricks. The bricks may be made of any nonvolatile redox material suitable for a thermochemical two-step (TS) water splitting (WS) or carbon dioxide splitting (CDS) cycle. A first thermodynamic analysis shows that the MBR2 may be able to achieve solar-to-chemical conversion efficiencies of approximately 0.25. Additionally, we identify the desired operating conditions and show that the heat exchanger efficiency has to be higher than the fraction of recombination in order to increase the conversion efficiency.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(2):021010-021010-11. doi:10.1115/1.4042226.

Ca-Mn-based perovskites doped in their A- and B-site were synthesized and comparatively tested versus the Co3O4/CoO and (Mn,Fe)2O3/(Mn,Fe)3O4 redox pairs with respect to thermochemical storage and oxygen pumping capability, as a function of the kind and extent of dopant. The perovskites' induced heat effects measured via differential scanning calorimetry are substantially lower: the highest reaction enthalpy recorded by the CaMnO3–δ composition was only 14.84 kJ/kg compared to 461.1 kJ/kg for Co3O4/CoO and 161.0 kJ/kg for (Mn,Fe)2O3/(Mn,Fe)3O4. Doping of Ca with increasing content of Sr decreased these heat effects; more than 20 at % Sr eventually eliminated them. Perovskites with Sr instead of Ca in the A-site exhibited also negligible heat effects, irrespective of the kind of B site cation. On the contrary, perovskite compositions characterized by high oxygen release/uptake can operate as thermochemical oxygen pumps enhancing the performance of water/carbon dioxide splitting materials. Oxygen pumping via Ca0.9Sr0.1MnO3–δ and SrFeO3–δ doubled and tripled, respectively, the total oxygen absorbed by ceria during its re-oxidation versus that absorbed without their presence. Such effective pumping compositions exhibited practically no shrinkage during one heat-up/cool-down cycle. However, they demonstrated an increase of the coefficient of linear expansion due to the superposition of “chemical expansion” to thermal-only one, the effect of which on the long-term dimensional stability has to be further quantified through extended cyclic operation.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(2):021011-021011-13. doi:10.1115/1.4042227.

The reconstruction of the angular and spatial intensity distribution from radiative flux maps measured in high flux solar simulators (HFSS) or optical concentrators is an ill-posed inverse problem requiring special solution strategies. We aimed at providing a solution strategy for the determination of intensity distributions of arbitrarily complicated concentrating facilities. The approach consists of the inverse reconstruction of the intensities from multiple radiative flux maps recorded at various positions around the focal plane. The approach was validated by three test cases including uniform spatial, Gaussian spatial, and uniform angular distributions for which we successfully predicted the intensity for a square-shaped target with edge length of 0.5 m and for a displacement range spanning ±1.5 m at a resolution of 3.2 × 106 elements, yielding relative errors between 19.8–26.4% and 15.7–25.6% when using Tikhonov regularization and the conjugate gradient least square (CGLS) method, respectively. The latter method showed superior performance and was used at a resolution of 2.35 × 107 elements to analyze EPFL's HFSS comprising 18 lamps. The inverse solution for a single lamp retrieved from experimentally measured and simulated radiative flux maps showed peak intensities of 13.7 MW/m2/sr and 16.0 MW/m2/sr, respectively, with a relative error of 81.1%. The inverse reconstruction of the entire simulator by superimposing the single lamp intensities retrieved from simulated flux maps resulted in a maximum intensity of 18.8 MW/m2/sr with a relative error of 80%. The inverse method proved to provide reasonable intensity predictions with limited resolution of details imposed by the high gradients in the radiative flux maps.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(2):021012-021012-10. doi:10.1115/1.4042228.

A thermodynamic model of an isothermal ceria-based membrane reactor system is developed for fuel production via solar-driven simultaneous reduction and oxidation reactions. Inert sweep gas is applied on the reduction side of the membrane. The model is based on conservation of mass, species, and energy along with the Gibbs criterion. The maximum thermodynamic solar-to-fuel efficiencies are determined by simultaneous multivariable optimization of operational parameters. The effects of gas heat recovery and reactor flow configurations are investigated. The results show that maximum efficiencies of 1.3% (3.2%) and 0.73% (2.0%) are attainable for water splitting (carbon dioxide splitting) under counter- and parallel-flow configurations, respectively, at an operating temperature of 1900 K and 95% gas heat recovery effectiveness. In addition, insights on potential efficiency improvement for the membrane reactor system are further suggested. The efficiencies reported are found to be much lower than those reported in literature. We demonstrate that the thermodynamic models reported elsewhere can violate the Gibbs criterion and, as a result, lead to unrealistically high efficiencies. The present work offers enhanced understanding of the counter-flow membrane reactor and provides more accurate upper efficiency limits for membrane reactor systems.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(2):021013-021013-7. doi:10.1115/1.4042229.

A 200 W CO2 laser-based heating system coupled with in operando Raman spectroscopy has been developed. The system delivers highly concentrated radiation capable of driving thermochemical reactions and simulates heat fluxes expected by 3D solar concentrating systems. 10 mol% Gd-doped and pure ceria pellets were prepared and used to characterize the system because of their well-established thermodynamic and kinetic properties, as well as their strong Raman peak due to F2 g symmetrical mode at 460 cm−1. Reduction in an H2 atmosphere has been carried out to investigate the behavior of the full width at half maximum (FWHM) of the F2 g Raman peak resulting from changes in temperature and oxidation state. For both samples, an increase in temperature during heating in air (i.e., fully oxidized) resulted in a peak shift toward low wavenumber and an increase of FWHM. The FWHM versus temperature curves were then measured for controlled reduction extents ranging between sample averaged nonstoichiometries of δ = 0–0.209 as a function of temperature. At a fixed temperature, Gd-doped ceria exhibited an increase in FWHM with increasing reduction extent until δ = 0.056. At greater reduction extents, the FWHM decreased with increasing reduction extents. We attribute this to changes in the lattice parameter caused by the eventual formation of intermediate cubic Ce2O3 at the radiated surface. This study demonstrates the promise of utilizing Raman spectroscopy to probe thermochemical reactions in operando. Going forward, we expect that this will be an especially promising tool for characterizing emerging thermochemical materials with complex phase equilibria, especially for nonequilibrium processes.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(2):021014-021014-11. doi:10.1115/1.4042059.

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.

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(2):021015-021015-9. doi:10.1115/1.4042127.
OPEN ACCESS

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).

Commentary by Dr. Valentin Fuster
J. Sol. Energy Eng. 2019;141(2):021016-021016-14. doi:10.1115/1.4042128.

Oxide particles have potential as robust heat transfer and thermal energy storage (TES) media for concentrating solar power (CSP). Particles of low-cost, inert oxides such as alumina and/or silica offer an effective, noncorrosive means of storing sensible energy at temperatures above 1000 °C. However, for TES subsystems coupled to high-efficiency, supercritical-CO2 cycles with low temperature differences for heat addition, the limited specific TES (in kJ kg−1) of inert oxides requires large mass flow rates for capture and total mass for storage. Alternatively, reactive oxides may provide higher specific energy storage (approaching 2 or more times the inert oxides) through adding endothermic reduction. Chemical energy storage through reduction can benefit from low oxygen partial pressures (PO2) sweep-gas flows that add complexity, cost, and balance of plant loads to the TES subsystem. This paper compares reactive oxides, with a focus on Sr-doped CaMnO3–δ perovskites, to low-cost alumina-silica particles for energy capture and storage media in CSP applications. For solar energy capture, an indirect particle receiver based on a narrow-channel, counterflow fluidized bed provides a framework for comparing the inert and reactive particles as a heat transfer media. Low-PO2 sweep gas flows for promoting reduction impact the techno-economic viability of TES subsystems based on reactive perovskites relative to those using inert oxide particles. This paper provides insights as to when reactive perovskites may be advantageous for TES subsystems in next-generation CSP plants.

Commentary by Dr. Valentin Fuster

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