Abstract

A multi-tubular solar thermochemical cavity reactor is proposed and the tubular array optimized. The optimized reactor design aims at operating under different temperatures and carrying out different kinds of thermochemical reactions. The radiation entering the receptacle comes from a solar concentrating system and the reactor consists of a cubic receptacle made of woven graphite, housing nine 2.54 cm diameter tungsten tubes. A model is developed and implemented considering high-temperature radiative transfer at steady state. The temperature distribution within the cavity surfaces is determined by employing a hybrid Monte Carlo-Finite Volume approach. Optimal tube distributions are explored by using a custom-made stochastic, multi-parameter, optimization algorithm. In this way, multiple global maxima are determined. Patterns among all possible optimal tube distributions within the cavity are obtained for different scenarios, by maximizing average tube temperature. From this study, practical guidelines are obtained for future application in the design of solar cavity reactors and more specifically, on the layout of multi tubular arrays to optimize radiative heat transfer.

Abstract

A three-dimensional transient heat transfer model is presented to predict the start-up operation of a multi-tubular cavity reactor under concentrated irradiation in a solar furnace. The reactor consists of a cavity containing nine absorber tubes, through which a suspension of CeO2 in Argon flows. An iterative splitting scheme coupling a Continuous Random Walk, a Finite Volume, and a Ray-Tracing Monte Carlo methods, is implemented to estimate the temperature gradients in the tubes and gas-particle media. The radiation heat transfer among the tubes and cavity walls is considered, as well as conduction and convection in the tubes and the particle suspension. During the initial heating stage, gradients are mainly angular, while in steady-state they are primarily axial. The former may cause tube bending or cracking, and strategies to reduce them are examined. In particular, different heating ramps were simulated, which was found to reduce these initial thermal gradients.

Abstract

Solar technology operating at elevated temperature conditions demands accurate knowledge of the optical and thermal properties of the materials involved in the construction and operation of solar collectors, reactors, and energy storages, among many others. Thermal energy storage (TES) devices involve successive melting and crystallization processes, which result in high complexity materials where the morphology, composition, and porosity could be highly non-homogeneous. In these cases, contact techniques for determining the thermal properties are highly susceptible and do not provide reliable measurements. It is under these conditions that non-contact photothermal techniques can provide superior performance, because in this case, the heat inducing source is a laser beam and the detector is usually a photodiode or a thermographic camera which are in non-contact with samples.

The materials applied as storage medium in a TES unit can be divided into four groups: metals and alloys, ceramics and glasses, polymers and elastomers, and composites that include natural materials. Soda lime silicate glass recyclable waste is a very promising material for storage medium due to its inexpensive and wide availability. In this paper, we examined soda lime silicate glass-graphite composites for use as storage medium in a TES unit. A simple one-dimensional model for thermal conductivity was developed based on equivalent thermal circuits for series-parallel composite walls, and we found that thermal conductivity values depend on the amount of graphite dispersed into the samples, the porous media, and their structure.

Abstract

Agave angustifolia leaves and tomato pruning biomasses were processed into carbon materials by solar pyrolysis. The influence of temperature and heating rate was studied in the physicochemical properties of the obtained biochars. The characterization techniques include elemental analyses (CHONS), physisorption to determine surface area by BET and DFT models, and capacitance values were determined by cyclic voltammetry as the electrochemical technique. It was found that for both biomasses, temperatures below 900°C are beneficial because more homogeneous and porous structures are obtained. The highest values of capacitance and surface area were obtained for tomato carbons at 450 and 600 °C, respectively. REFEREN

Abstract

The various lignin isolation methods and pretreatments are continuously developing and thermochemical conversion of lignocellulosic biomass and tuning of the activation parameters are vital to obtaining high energy density materials. In this review, different lignin extraction methods, pretreatments, and influence of the extraction conditions on the yield and properties are presented. The thermochemical conversion of lignin-based biomass and application in supercapacitors and hydrogen storage were investigated.

The study revealed that chemical extraction via the organosolv process presents higher purity and partly preserved lignin structure compared to sulfur processes. Different parameters such as the method of extraction, the temperature, pH, resident time, and pressure greatly influences the Kappa value and yield of lignin. The potassium hydroxide (KOH) dosage as an activating agent and the activating temperature is vital to obtaining high surface area and microporosity, which enhances the lignin-based activated carbon performance towards high hydrogen storage and capacitance. Metals doping on activated carbon marginally enhance the hydrogen storage capacity and capacitance, however, reversible desorption of the adsorbed hydrogen requires a higher temperature for hydrogen storage. Besides, high metal doping reduces available surface area, collapses the cage-like structures of fullerenes, and results in lower hydrogen storage capacity of activated carbon.

The presence of heteroatoms on activated carbons enhances the performance towards high hydrogen storage and capacitance. Moreover, techno-economic and exergy-based sustainability analysis of the different lignin isolation techniques must be explored to provide valuable insights on energy and associated operational costs.

Abstract

Hydrothermal processes are attractive options for the transformation of mixtures of biomass with large amounts of water, i.e. above 20wt. At hydrothermal conditions, the special properties of water makes it an attractive reaction medium to obtain several bio-based platform chemicals or fuel gases, such as hydroxymethilfurfural or fufurals, syngas, hydrogen, methane, etc. However, one of the main challenges is that a large amount of energy is required to heat reactants (mixture of water and biomass), which is usually achieved by combustion of a fraction of the bio-oil product. Therefore, to reduce this consumption, their integration with an external renewable energy source, such as concentrated solar radiation has been proposed. This approach has been recently analyzed by several research groups as an option to have sustainable and economically attractive processes. This work provides an overview of the different experimental and theoretical strategies to incorporate concentrated solar technologies into hydrothermal processing of biomass, including the main challenges of such integration for process technical feasibility.

Abstract

Solar reactors designed and constructed for thermochemical applications present different configurations and general performance. The selection of a solar reactor that optimizes a particular process is always a difficult challenge. This work studies two types of reactor configuration by means of a comparative experimental analysis. It was employed a solar device, which is able to operate as fixed reactor with packed bed samples and as rotary kiln. The reduction of CuO into Cu2O was tested under both operation modes, due to its proved potential and interest as thermochemical storage material. It was found that heat transfer was hindered in static experiments limiting the fraction of reactive sample. Thermal gradients of about 200 °C were found in the packed bed through thermocouple and IR camera measurement. Heating rates and total fed energy must be restricted at the risk of front of the sample to melt, resulting in several operation drawbacks. In contrast, mixing conditions in rotary kilns allowed for higher heating rates and led to homogenous sample temperature. Maximum reaction yields in stationary mode did not overpass 14% while it was achieved more than 80% in rotary mode at temperatures about 860 °C. Thermal efficiencies were very limited in both operation modes due to the high thermal inertia of the solar reactor. Because rotary mode admitted much more energy, its thermal efficiency was even lower than static. A solution to increase rotary kilns thermal efficiency is working in continuous mode.

Abstract

In this paper the simulation of the thermal reduction for hydrogen production through the decomposition of cerium oxide is presented. The thermochemical cycle for hydrogen production consists of the endothermic reduction of CeO2 at high temperature, where concentrated solar energy is used as a source of heat; and of the subsequent steam hydrolysis of the resulting cerium oxide to produce hydrogen. For the thermochemical process, a solar reactor prototype is proposed; consisting of a cubic receptacle made of graphite fiber thermally insulated. Inside the reactor a pyramidal arrangement with nine tungsten pipes is housed. The pyramidal arrangement is made respect to the focal point where the reflected energy is concentrated. The solar energy is concentrated through the solar furnace of high radiative flux. The endothermic step is the reduction of the cerium oxide to lower-valence cerium oxide, at very high temperature. The exothermic step is the hydrolysis of the cerium oxide (III) to form H2 and the corresponding initial cerium oxide made at lower temperature inside the solar reactor. For the modeling, three sections of the pipe where the reaction occurs were considered; the carrier gas inlet, the porous medium and the reaction products outlet. The mathematical model describes the fluid mechanics; mass and energy transfer occurring therein inside the tungsten pipe. Thermochemical process model was simulated in CFD. The results show a temperature distribution in the solar reaction pipe and allow obtaining the fluid dynamics and the heat transfer within the pipe. This work is part of the project “Solar Fuels and Industrial Processes” from the Mexican Center for Innovation in Solar Energy (CEMIE-Sol).

Abstract

Hydrogen is a promising energy carrier for transportation, domestic and industrial applications. Nowadays hydrogen is consumed basically by the chemical industry, but in long term its demand is expected to grow significantly due to emerging markets. Hence production of hydrogen with sustainable methods is a relevant issue. This work presents a review of the different CSP- aided thermochemical processes for hydrogen and syngas production. For each process, some relevant solar-tested reactor prototypes are described. In a second part, the developed solar furnaces for investigation of thermochemical process are also discussed. In addition, relevant research on hydrogen or syngas production in solar tower installations is presented. Finally the current challenges of the technology and the process for its future commercialization are also analyzed.