Abstract

Reticulate porous ceramic reactors use foam-type absorbers in their operation which must fulfill two essential functions: favoring the volumetric effect and increasing the mass and heat transfer by acting as a support for the reactive materials. Heating these absorbers with highly inhomogeneous concentrate irradiation induces high thermal gradients that affect their thermal performance. Owing to the critical function of these component in the reactor, it is necessary to define a selection criterion for the foam-type absorbers. In this work, we performed an experimental and numerical thermal analysis of three partially stabilized zirconia (PSZ) foam-type absorbers with pore density of 10, 20, and 30 PPI (pores per inch) used as a volumetric absorber. A numerical model and an analytical approximation were developed to reproduce experimental results, and calculate the thermal conductivity, as well as volumetric heat transfer coefficient.

The results show that an increase in pore density leads to an increase in the temperature difference between the irradiated face and the rear face of the absorber, this occurs because when pore density increases the concentrated energy no longer penetrates in the deepest space of the absorber and energy is absorbed in areas close to the surface; therefore, temperature gradients are created within the porous medium. The opposite effect occurs when the airflow rate increases; the temperature gradient between the irradiated face and the rear face is reduced. This behavior is more noticeable at low pore densities, but at high pore densities, the effect is less relevant because the internal structure of porous absorbers with high pore density is more complex, which offers obstructions or physical barriers to airflow and thermal barriers to heat transfer.

When the steady state is reached, the temperature difference between the two faces of the absorber remains constant if the concentrate irradiation changes slightly, even changing the airflow rate. The results obtained in this work allow us to establish a selection criterion for porous absorbers that operate within solar reactors; this criterion is based on knowledge of the physical properties of the porous absorber, the environment, the working conditions, and the results expected.

Abstract

The synthesis gas or hydrogen‐rich gas, at longer residence times and higher temperatures, is frequently the target of pyrolysis biomass. Biochars obtained at such conditions contain less functional groups but they are more effective for longer‐term soil C storage. The aim of this study was to elucidate the adsorption mechanisms and the effect of the ageing process on high‐temperature biochar towards lead adsorption. The biochars under study were produced from the pyrolysis of almond wood and olive tree pruning.

The almond hardwood biochar was the most susceptible to the oxidation process. This biochar exhibited higher carbon loss than the olive biochar, 22.07% and 11.76%, respectively. The adsorption process was better described by the Langmuir model and pseudo‐second order kinetics. The maximum adsorption capacity decreased from 40.3 to 24.8 mg g–1 after the oxidation process for almond‐derived biochar and remained unchanged for the olive‐biochar. Analyses demonstrated that 67% of the adsorbed Pb2+ was removed through complexation by oxygen functionalities and phosphorous compounds. After the oxidation process, this fraction accounted for only 45% of the adsorbed Pb2+, ion exchange and cation‐π being the most prominent mechanisms for the adsorption.

Abstract

We have found the flux homogenization technique described in the paper published in volume 93, pages 115–124 (august 2016), does not provide homogenized fluxes in all out-of-focus distances of the solar receiver, but only above a certain transition distance, and that some of the numerical analysis procedures used in that document should be revised or complemented to achieve better reproducibility of the results. In addition, we also note that some statements and conclusions in that document are mistaken or formulated incorrectly, so it is necessary their correction.

Abstract

Tantalum carbide (TaC) nanoparticles were synthesized using the IER-UNAM (HoSIER) solar furnace, which reduces polluting gas emissions and dependence on fossil fuels through the use of concentrated solar energy. TaC synthesis was performed through a carbothermal reduction method from Ta/O/C complex, using tantalum pentachloride (TaCl5) and synthesized phenolic resin as sources of tantalum and carbon, respectively, at a temperature of 1200 °C, in a reaction time of 30 min, under argon atmosphere. A solar reactor equipped with a quartz window was used, designed to work in controlled atmospheres. Complex Ta/O/C bonds and thermal decomposition were analyzed by FT-IR and TG/DSC, respectively, while the structure and morphology of TaC were analyzed by XRD, TEM, and SEM techniques. Results showed a TaC with a cubic crystalline structure, a low amount of Ta2O5 and a near-spherical …

Abstract

Rotary kilns are worldwide used for industrial processes that involve thermal treatments of particulate materials. However, a great amount of fossil fuels is employed in such processes. As alternative, solar rotary kilns are considered for this application due to their versatility and potential to substitute traditional fossil-fuel driven devices. In order to boost the development of this technology, efforts have to be focused on the control of the particle temperature during the treatment. In this context, a lab-scale rotary kiln was built and tested using a 7-kWe high-flux solar simulator at University of Antofagasta. It was conceived to treat particulate materials of different nature and it is able to reach temperatures higher than 800 °C under different operation strategies. Silicon carbide was selected for initial tests because it is inert, endures high temperatures (up to 1600 °C) and it has been proposed as thermal storage vector in several researches on concentrated solar power. In a first stage, the empty kiln was preheated up to about 800 °C, reaching a steady state in less than three hours and with a power of approximately 370 W entering the kiln cavity. Afterwards, 43 g of silicon carbide were introduced in the furnace and the system was heated again up to a second steady state above 800 °C. In this stage, particles showed a fast increment of their temperature and exceeded 700 °C in less than three minutes after loading. A one-dimensional transient numerical model was also developed to perform the thermal analysis of the kiln and the estimation of both the particle temperature and the system efficiency. Numerical results showed good agreement with experimental data and thermal losses could be quantified in detail. Therefore, the model was also used to predict the thermal behavior of a solar rotary kiln working in batch mode..

Abstract

This study investigates the phase transformations that can occur in an austenitic stainless steel (AISI 316) by demonstrating the appearance of δ-ferrite that is obtained in the initial heating cycles using Concentrated Solar Irradiation (CSI) at magnitudes needed to obtain the operational temperatures of central tower systems. Four AISI 316 stainless steel specimens cut from one single initial piece, were exposed to CSI in the Horno Solar de Alto Flujo Radiativo at the Universidad Nacional Autónoma de México to perform the thermal cycles. AISI 316 stainless steel is fully austenitic and is selected because it is reportedly one of the cheaper material used in CSI receivers. Monotonic tensile strength tests were performed, and it is assumed that there is no relevant effect on the mechanical behavior for the reported experiment. Phase transformations were characterized using optical microscopy, X-ray diffraction and by scanning electron microscopy analysis with an energy-dispersive X-ray spectroscopy. The appearance of δ-ferrite phase was the principal difference between CSI treated specimens, a non-treated specimen and one specimen heated by conventional method. Concentrated UV irradiation from the solar spectrum on Earth surface demonstrated to have the potential to obtain the phase transformation at a temperature near 630 °C.

Abstract

A method to achieve a uniform flux distribution with a multi-faceted point focus concentrator for laboratory tests is proposed in this work. The method can be applied to different types of receiver - thermal or photovoltaic - and no additional device is required to homogenize the flux. The technique consists in moving the receiver from the focal plane and enlarging the solar spot impinging on it. At the same time, each mirror aim-point is adjusted in order to superimpose the images that have been generated by every facet. To evaluate the method, a real multi-faceted concentrator composed of eighteen spherical mirrors was modeled in a ray-tracing software. The procedure was validated through the comparison of an image of the real solar spot on the receiver generated by three mirrors, and the simulated flux obtained the same way. This way a mean concentrator global optical error of 2.8 mrad was estimated. This value was used then for further analyses. Results show that the concentration factor can be varied in a range of 150–900 suns over a receiver diameter of up to 7 cm. Hence, according to the receiver requirements, it is possible to expand the distribution and to alter the intensity of the flux. Finally, optical parametrical analyses were carried out, from which it is inferred that good quality optics give rise to a more homogeneous solar flux on the receiver.

Abstract

Radiative heat transfer in a solar thermochemical reactor for the thermal reduction of cerium oxide is simulated with the Monte Carlo method. The directional characteristics and the power distribution of the concentrated solar radiation that enters the cavity is obtained by carrying out a Monte Carlo ray tracing of a paraboloidal concentrator. It is considered that the reactor contains a gas/particle suspension directly exposed to concentrated solar radiation. The suspension is treated as a non-isothermal, non-gray, absorbing, emitting, and anisotropically scattering medium. The transport coefficients of the particles are obtained from Mie-scattering theory by using the optical properties of cerium oxide. From the simulations, the aperture radius and the particle concentration were optimized to match the characteristics of the considered concentrator.

Abstract

Radiative heat transfer in a solar thermochemical reactor for the thermal reduction of cerium oxide is simulated with the Monte Carlo method. The directional characteristics and the power distribution of the concentrated solar radiation that enters the cavity is obtained by carrying out a Monte Carlo ray tracing of a paraboloidal concentrator. It is considered that the reactor contains a gas/particle suspension directly exposed to concentrated solar radiation. The suspension is treated as a non-isothermal, non-gray, absorbing, emitting, and anisotropically scattering medium. The transport coefficients of the particles are obtained from Mie-scattering theory by using the optical properties of cerium oxide. From the simulations, the aperture radius and the particle concentration were optimized to match the characteristics of the considered concentrator.

Abstract

It is a common practice in the development of point focus solar concentrators to use multiple identical reflecting facets, as a practical and economic alternative for the design and construction of large systems. This kind of systems behaves in a different manner than continuous paraboloidal concentrators. A theoretical study is carried out to understand the effect of the size of facets and of their optical errors in multiple facet point focus solar concentrating systems. For this purpose, a ray tracing program was developed based on the convolution technique, in which the brightness distribution of the sun and the optical errors of the reflecting surfaces are considered. The study shows that both the peak of concentration and the optimal focal distance of the system strongly depend on the size of the facets, and on their optical errors. These results are useful to help concentrator developers to have a better understanding of the relationship between manufacturing design restrictions and final optical behavior.