The objective of this research team is the optimization of the heat transfers within concentration solar systems and the development of knowledge related to the material ageing subjected to high heat flux.
The recent developments of solar receivers for the thermodynamic conversion of concentrated solar energy require the development of high temperature solar receivers producing hot air under pressure.
The optimization of these solar receivers goes through the improvement of heat exchange with the working liquid (in particular of the air) and through the reduction of the pressure losses at the time of its passage in the receiver. The performance of the receivers is strongly related to their internal architecture, which is the location of heat exchanges between solid and gas.
In addition, the solar receivers are subjected to violent and cyclic thermal aggressions. Although they are mainly composed of high resistant materials like metal alloys or ceramics, their mechanical and thermal performances are prematurely impaired due to the extreme conditions in which these receivers are operating. More particularly, their ability to efficiently absorb the solar energy in order to transmit it to the working fluid can really be damaged.
The work of this team goes and gets reinforced through exchanges between experiments and numerical simulations. It stands in the priority axis n°3 of the National Strategy of Research and Innovation. On the one hand, we explore the possibilities of the new means for high efficiencies calculation (material accelerators, calculation grids, cloud) and the possibilities of the paradigms of adapted simulations (DNS, LES and cellular automats) in order to provide physical simulations more and more precise. On the other hand, resorting to the standard experiments and methods of numerical simulation makes it possible to study the quality of the calculations by making sure that they are sufficiently precise and useful compared to the other means of exploration of the physical properties of the fluids and materials considered for research.
Optimization of solar receivers
With regard to the optimization of the receivers, our approach consists in studying these flows from an element of internal architecture (elementary channel) to the complete receiver.
The challenges of the optimization of the solar receivers are numerous. We can mention:
- to better understand how the high thermal gradients modify the structure of the flow (turbulent and strongly anisothermal),
- to envisage the geometry of the receiver and the materials to use in order to obtain best energy efficiencies.
To each of these challenges corresponds a physical level of description, a numerical strategy and specific devices for experimental diagnostics.
Thus, a better comprehension of the complex couplings between the turbulence and the high thermal gradients requires fine simulations where all the scales of the flow are explicitly solved. Direct numerical simulations (DNS) or Large Eddy Simulation (LES) are then used.
On the other hand, the development for an optimized geometry of a receiver requires a parametric study, i.e. many simulations. Statistical models are then used (RANS for Reynolds Average Navier-Stokes). These models provide less information, but are numerically much more economic.
Lastly, on the real scale of the receiver, one uses correlations in order to evaluate the output temperatures of the fluid, as well as the total pressure losses. The finest simulations (DNS and LES) inform and are used as a validation to the more complex models (RANS) as well as to the correlations.
Figure 1, where is represented the instantaneous temperature for a strong parietal temperature gradient in the case of a simplified geometry of a solar receiver, illustrates various levels of physical description (LES, under-solved DNS, DNS) and the interest of the approach. The computer code used for the DNS and the LES is the parallel software Trio_U, developed by the CEA of Grenoble. For the RANS, one rather uses commercial softwares such as FLUENT and COMSOL.
The validation of numerical simulations requires high-standard and powerful experimentation and diagnostic means.
An experimental platform in Odeillo is operational. It consists of an open wind tunnel “MEETIC” (Test Facility of the Turbulent Flows for the Intensification of Heat Transfers). This wind tunel, developed by the laboratory in collaboration with the Laboratory of Mechanics of Lille (LML) and the national platform of Optical Metrology of Lille (MéOL), is instrumented with multiple pressure points and speed sensors, and is associated to an optical diagnostic tool by laser layer “S-PIV” (Stereo Particle Image Velocimetry).
In order to validate the results of the simulations on a large-scale, pilot receivers from several tens to several hundreds of kilowatts are tested on the 1000 kW solar furnace of the laboratory at Odeillo.
Study of the material ageing
The study of the ageing of the components and materials in the “concentrated solar energy end-use applications” (CSP) is a set of themes whose interest appears in a very increasing way. In the case of the industrial production of electricity by thermodynamic solar way, the knowledge of the lifetime of the solar receivers subjected to high thermal flux and irregular cycles is an essential parameter for the development and the competitiveness of these key components.
The lifetime of the components has a direct effect mainly on the cost, the reliability and the maintenance of these systems. Current works in this field are rare, even almost non-existent.
The research team set itself the goals to:
- Determine the materials (metals, ceramics and/or deposits) that are the most sensitive to ageing in the CSP for the electrical production,
- Define theoretical and experimental methodologies to identify and quantify the principal mechanisms which control their degradation,
- Develop and validate test conditions of acceleration of the phenomena of ageing.
On the fundamental level and thanks to the simulation code (2D and 3D), we are searching for the main parameters responsible for the heat gradients within the material and their influence on its degradation.
Moreover, associated experimental studies using solar furnaces, allow the simulation of real and accelerated conditions of the ageing of the materials
The solar facilities of PROMES-CNRS at Odeillo are used to simulate ageing under high flux. Shutters make it possible to reproduce the cyclic, real or intensified and accelerated thermal aggressions, to which the materials are subjected.
Techniques of temperature measurement by optical means (pyroreflectometry, imagery IR) are implemented in order to evaluate and to analyze the heat gradients in the materials, tested within the focal volume of the solar installations.
The evaluation of the properties is done using non destructive photothermal methods, coupled with the inverse methods.
The photothermal methods consist in exciting the material by using a luminous source and in recording its response. Two methods in particular can be used, the modulated photoreflection and the impulse photothermal method, whose experimental devices were developed by the laboratory within the framework of various conventions/collaborations with the DGA (French Directorate-General for Army).
The inverse methods consist in minimizing a comparison criterion representing the variation/interval between experimental values and theoretical values during the response of the material.