Cyril Caliot

SOLFAST, a Ray-Tracing Monte-Carlo software for solar concentrating facilities

In this communication, the software SOLFAST is presented. It is a simulation tool based on the Monte-Carlo method and accelerated Ray-Tracing techniques to evaluate efficiently the energy flux in concentrated solar installations.

A hybrid transport-diffusion model for radiative transfer in absorbing and scattering media

A new multi-scale hybrid transport-diffusion model for radiative transfer is proposed in order to improve the efficiency of the calculations close to the diffusive regime, in absorbing and strongly scattering media. In this model, the radiative intensity is decomposed into a macroscopic component calculated by the diffusion equation, and a mesoscopic component. The transport equation for the mesoscopic component allows to correct the estimation of the diffusion equation, and then to obtain the solution of the linear radiative transfer equation. In this work, results are presented for stationary and transient radiative transfer cases, in examples which concern solar concentrated and optical tomography applications. The Monte Carlo and the discrete-ordinate methods are used to solve the mesoscopic equation. It is shown that the multi-scale model allows to improve the efficiency of the calculations when the medium is close to the diffusive regime. The proposed model is a good alternative for radiative transfer at the intermediate regime where the macroscopic diffusion equation is not accurate enough and the radiative transfer equation requires too much computational effort.

Numerical simulation of convective heat transfer for inline and stagger stacked plain-weave wire mesh screens and comparison with a local thermal non-equilibrium model

Volumetric receivers made of metallic wire meshes are a promising technology that allows different designs and configurations. In this paper the local volumetric heat transfer coefficient for a volumetric absorber with two extreme arrangements, inline and stagger, is presented. The results show that the heat transfer rates for the stagger stack is nearly two times higher than the inline stack. Moreover, the local volumetric heat transfer coefficient is implemented in a Homogeneous Equivalent Model using a Local Thermal Non-Equilibrium model to compare the results. The stagger stack performs better than the inline stack despite their higher frontal losses and pressure drop. Finally, a validation of the Homogeneous Equivalent Model is carried out with experimental data showing a good agreement.

Backward-gazing method for heliostats shape errors measurement and calibration

The pointing and canting accuracies and the surface shape of the heliostats have a great influence on the solar tower power plant efficiency. At the industrial scale, one of the issues to solve is the time and the efforts devoted to adjust the different mirrors of the faceted heliostats, which could take several months if the current methods were used. Accurate control of heliostat tracking requires complicated and onerous devices. Thus, methods used to adjust quickly the whole field of a plant are essential for the rise of solar tower technology with a huge number of heliostats. Wavefront detection is widely use in adaptive optics and shape error reconstruction. Such systems can be sources of inspiration for the measurement of solar facets misalignment and tracking errors. We propose a new method of heliostat characterization inspired by adaptive optics devices. This method aims at observing the brightness distributions on heliostat’s surface, from different points of view close to the receiver of the po...

Tracking and shape errors measurement of concentrating heliostats

In solar tower power plants, factors such as tracking accuracy, facets misalignment and surface shape errors of concentrating heliostats are of prime importance on the efficiency of the system. At industrial scale, one critical issue is the time and effort required to adjust the different mirrors of the faceted heliostats, which could take several months using current techniques. Thus, methods enabling quick adjustment of a field with a huge number of heliostats are essential for the rise of solar tower technology. In this communication is described a new method for heliostat characterization that makes use of four cameras located near the solar receiver and simultaneously recording images of the sun reflected by the optical surfaces. From knowledge of a measured sun profile, data processing of the acquired images allows reconstructing the slope and shape errors of the heliostats, including tracking and canting errors. The mathematical basis of this shape reconstruction process is explained comprehensively. Numerical simulations demonstrate that the measurement accuracy of this “backward-gazing method” is compliant with the requirements of solar concentrating optics. Finally, we present our first experimental results obtained at the THEMIS experimental solar tower plant in Targasonne, France.

Assessment of the Single-Mixture Gas Assumption for the Correlated K-Distribution Fictitious Gas Method in H2O–CO2–CO Mixture at High Temperature

This paper deals with the comparison of spectral narrow band models based on the correlated-K (CK) approach in the specific area of remote sensing of plume signatures. The CK models chosen may or may not include the fictitious gas (FG) idea and the single-mixture-gas assumption (SMG). The accuracy of the CK and the CK-SMG as well as the CKFG and CKFG-SMG models are compared, and the influence of the SMG assumption is inferred. The errors induced by each model are compared in a sensitivity study involving the plume thickness and the atmospheric path length as parameters. This study is conducted in two remote-sensing situations with different absolute pressures at sea level (105Pa) and at high altitude (16.6km, 104Pa). The comparisons are done on the basis of the error obtained for the integrated intensity while leaving a line of sight that is computed in three common spectral bands: 2000–2500cm−1, 3450–3850cm−1, and 3850–4150cm−1. In most situations, the SMG assumption induces negligible differences. Furthermore, compared to the CKFG model, the CKFG-SMG model results in a reduction of the computational time by a factor of 2.

Addressing nonlinearities in Monte Carlo

Monte Carlo is famous for accepting model extensions and model refinements up to infinite dimension. However, this powerful incremental design is based on a premise which has severely limited its application so far: a state-variable can only be recursively defined as a function of underlying state-variables if this function is linear. Here we show that this premise can be alleviated by projecting nonlinearities onto a polynomial basis and increasing the configuration space dimension. Considering phytoplankton growth in light-limited environments, radiative transfer in planetary atmospheres, electromagnetic scattering by particles, and concentrated solar power plant production, we prove the real-world usability of this advance in four test cases which were previously regarded as impracticable using Monte Carlo approaches. We also illustrate an outstanding feature of our method when applied to acute problems with interacting particles: handling rare events is now straightforward. Overall, our extension preserves the features that made the method popular: addressing nonlinearities does not compromise on model refinement or system complexity, and convergence rates remain independent of dimension.

Improvement of Radiative Performances of High Temperature Solar Particle Receivers Using Coated Particles and Mixtures

The use of H fB2, ZrB2, H fC, ZrC, W and SiC particles in a high temperature Solar Particle Receiver (SPR) is analyzed. The SPR is modeled as a 1D slab of spherical particles dispersion, submitted to a concentrated and collimated solar flux (q0 = 1500kW=m2). The temperature inside the SPR is taken constant (T = 1300K), as for a well-stirred receiver. For the W and SiC, the refractive indexes reported in the literature are retained while the real and imaginary parts of the refractive indexes of the others materials are obtained from available reflectance data, using the Kramers-Kronig relationships. Three SPR configurations are considered: a homogeneous medium with only one kind of particles, a medium with a mixture of two materials and, a medium with coated particles. The three configuration results are compared with those obtained using particles made with an ideal material. For the first configuration, the lowest radiative losses are found using small particles of sizes close to d = 2μm. For the second configuration, non noticeable improvements are found by the use of mixtures of the studied materials. For the third configuration, when the SiC is used as mantle, the radiative losses decrease to approach the ideal minimum. The best combination corresponds to a particle with a core of W coated by SiC. Improvements of 2.6% and 2.8% may be achieved using coating thickness of 50nm with particles of d = 2μm and d = 100μm, respectively. The use of coated particles may thus lead to significant improvements in theradiative performances of a SPR working at high temperature.