International audienceThe Monte Carlo method is partially reviewed with the objective of illustrating how some of the most recent methodological advances can benefit to concentrated solar research. This review puts forward the practical consequences of writing down and handling the integral formulation associated to each Monte Carlo algorithm. Starting with simple examples and up to the most complex multiple reflection, multiple scattering configurations, we try to argue that these formulations are very much accessible to the non specialist and that they allow a straightforward entry to sensitivity computations (for assistance in design optimization processes) and to convergence enhancement techniques involving subtle concepts such as control variate and zero variance. All illustration examples makePROMES - UPR CNRS 8521 - 7, rue du Four Solaire, 66120 Font Romeu Odeillo, France use of the public domain development environment EDStar (including advanced parallelized computer graphics libraries) and are meant to serve as start basis either for the upgrading of existing Monte Carlo codes, or for fast implementation of ad hoc codes when specific needs cannot be answered with standard concentrated solar codes (in particular as far as the new generation of solar receivers is concerned). (C) 2013 Elsevier Ltd. All rights reserved

Smoldering is involved in a variety of natural situations such as forest fires and also in man-controlled processes such as oil recovery and gas production from oil shale. A general feature of these situations is that a heat wave is propagating through the solid porous medium, powered by the flameless combustion of a solid. In numerous applications, this heat supply results from the partial oxidation of the carbon left after the devolatilization of the medium as the hot wave is approaching. The process of carbon oxidation in the complex geometry of a porous medium with forced air flow is not yet fully understood. In particular, the amounts of CO and CO 2 produced, that strongly impact the velocity and the temperature of the front, remain unpredictable to date. In this work, a new model porous medium has been produced by adding pyrolytic carbon into inert porous particles, and it has been characterized in detail, aiming at experiments in situations as simple as possible: the oxidation of carbon deposited at the surface of an inert solid matrix. Two of the main parameters that influence the frontthe carbon content and the fed air velocitywere varied over wide ranges. During experiments, carbon was always totally oxidized whereas oxygen was not totally consumed at low carbon content and high air velocities. Depending on the situations, the fraction of carbon oxidized to CO (and not to CO 2) varied between 23 and 37%. It was clearly established that the combustion of a particle is limited by internal mass transfer. The thickness of the combustion front is clearly observable, and it is shown to vary drastically depending on the operating parameters. These results are intended to provide a benchmark for the validation of a numerical model in a future work.

Modeling the propagation of smoldering fronts with forced air feeding in a porous medium remains a challenge to science. One of the main difficulties is to describe the carbon oxidation reaction that supports this self-sustained process. Pore scale approaches are required to tackle this complex coupled heat and mass transfer problem with chemistry. They, nevertheless, require high computation effort and still miss experimental validation. Furthermore, the heat loss at the walls of the cells inherent to every laboratory scale system adds another level of complexity in the understanding of the coupling between the phenomena at stake. Indeed, it induces a nonhomogeneous temperature field throughout the system. In this article, a 2D Darcy scale model is developed and validated by confrontation with experimental results from the literature, covering wide ranges of carbon content of the medium and forced air velocity. A reasonable description of the front temperature, velocity, and non-consumption oxygen amount is reached. The model finally enables understanding of the impact of heat loss, which controls the front shape and stability near the system walls.

According to recent studies in the solar energy community, a promising ways of solar energy conversion seems to be the beam down concentration technology associated to a fluidized bed receiver. The advantage of this system is its ability to heat air at temperature reaching 1000 K in a receiver directly exposed to a concentrated solar beam and integrated in a thermodynamic cycle. This paper focus on the modelling of radiative heat transfer from the optical concentrator to the receiver by taking into account absorption and multiple scattering of light in the particles bed. To achieve this objective, we develop in this work an efficient tool based on an algorithm solving the integral formulation of the Radiative Transfer Equation by the Monte Carlo Method. This algorithm is implemented in EDStaR environment where computer graphics libraries, parallel computing and specific functionalities to produce statistical quantities and their associated derivatives are available. One of the main advantage of the proposed radiative transfer modelling is the determination of the sensitivities (derivatives) of the physical quantities to any physical or geometrical parameter without significant additional CPU time.

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In order to evaluate the potential of solar bubbling fluidized bed receivers compared to other methods for the solar heating of gases at high temperature, a thorough knowledge of the heat transfer of the receiver is necessary. Since the external energy source of the system is radiative and because of high working temperatures, it is particularly important to model the radiative heat transfer to later predict the temperature field in the solar receiver. The aim of this study is to model the radiative flux distribution in a fluidized bed by taking into account the time-dependent absorption and scattering of light in the particulate medium. For this purpose, we propose a model using the Monte Carlo Method as well as a time-dependent field of optical properties that was predicted using a Computational Fluid Dynamics tool implemented with an Eulerian model. A statistical treatment using the k-distribution method was later applied to the time-dependency of the radiative properties of the solar fluidized bed receiver. This method has proven to be useful to reduce computational time while keeping a good accuracy. An experimental set-up was designed to validate the numerical predictions of the particle volume fraction and the penetration of radiation into the fluidized bed. The good agreement of the current model with the experimental data confirms its suitability.