Pierre Neveu

Thermochemical Solar Reactor: Simplified Method for the Geometrical Optimization at a Given Incident Flux

This paper aims to describe a simplified method to optimize the geometry of a solar thermochemical reactor. As a first step, this paper focuses on a purely thermal analysis. The chemical reaction is represented by a uniform heat sink inside the material. The heat transfer modes are radiation in the empty part (cavity) and conduction inside the reactive material. The aim is to find the optimal geometry of the reactor, by maximizing its efficiency, for a fixed value of the incident solar flux and of the total volume of the reactor. An analytical solution can be found thanks to some simplifying hypothesis. The influence of different operational parameters on the maximal efficiency and on the optimal shape is studied. A comparison between different reactor designs (cylindrical and cavity reactors) is shown. A 2D study, based on CFD software using a finite element method, allows for quantifying the effects of the simplifying assumptions. The constructal theory aims to optimize the internal structure of a system in order to provide easier access to its internal currents and increase the system efficiency. Thus, this study can be seen as the optimization of the elemental volume of the constructal approach. In a next step this optimization method will be used to optimize more complex reactor design, as for example, a honeycomb reactor obtained by the assembling of several cavities, in order to optimize a thermochemical reactor for hydrogen production or high temperature heat storage.

Operating results of a thermocline thermal energy storage included in a parabolic trough mini power plant

A thermocline thermal energy storage tank consists in using one single tank to store sensible heat. At almost any time, three zones coexist in the tank: a hot fluid zone at the top, a cold fluid zone at the bottom, and an intermediate zone called thermocline. Filling the tank with solid materials enables to reduce cost and to maintain the thermal stratification during stand-by periods. The present paper deals with a 230 kWh experimental thermocline tank that is included into a 150 kWth parabolic trough mini power plant. The heat transfer fluid is a non-pressurized synthetic oil (dibenzyltoluene) that flows through alumina spheres in the storage tank. The solid materials are contained into baskets in order to facilitate their removing and replacement. Thermocouples measure temperature along the center of the cylinder and along its radius. It is therefore possible to study the thermocline behavior thanks to the measured temperature profiles. A typical charge, a typical discharge and a stand-by process are p...

Convective Heat Transfer Enhancement in Solar Receivers Using Minimum Entropy Generation Optimization

Convective heat transfer enhancement can significantly improve the thermal efficiency in the conversion, utilization, recovery and storage of energy (in particular solar thermal). Modifying velocity field is the most direct approach to enhance convective heat transfer. However, in most cases the optimal velocity field is unknown and difficult to find even for an experienced researcher. In this paper, a predictive optimization methodology in convective heat transfer enhancement based on minimum entropy generation (MEG) principle was developed. A set of Euler’s equations were derived by the variation calculus to the Lagrange function established by governing equations, specific constraints and objective functional—total entropy generation rate. The solution of these equations resulted in the optimal velocity fields, leading to the minimum entropy generation. To validate and demonstrate the future application of this methodology to solar absorbers used to convert concentrated solar energy, the steady laminar convection heat transfer process in a two-dimensional channel with fixed heat flux boundaries was optimized for given total viscous dissipations. The numerical simulation results showed that lower value of maximum wall temperature was obtained by MEG optimization, which means cheaper and safer materials. The present work indicated that the new methodology could be a good guide in convective heat transfer enhancement design work, especially in CSP receivers.© 2011 ASME

Dynamic modeling and adapted design of a low cost linear Fresnel power plant for rural areas in West Africa

An optimized and low cost linear Fresnel collector, adapted to the West Africa context, is being designed using local mankind and locally available materials. There is an already built collector. The concentrator has 7.5u2005m2 of mirrors segmented in equal width of 0.1u2005m. The receiver has a trapezoidal cavity with seven copper absorber tubes. The sun tracking system operates in open loop and the sun position is calculated using an algorithm. The components of the collector were selected purposely to reduce production cost while optimizing its performances. The cost of collector is €275 / m2 of mirrors and €255 / kWth for solar field and receiver respectively. The material and method for the collector characterization are outlined. An organic Rankine cycle will be coupled with the collector for electricity generation. Dynamic modelling, of an organic Rankine cycle, is being investigated using Gibbs equivalents systems method.

Thermal modeling of a pressurized air cavity receiver for solar dish Stirling system

A solar cavity receiver model for the dish collector system is designed in response to growing demand of renewable energy. In the present research field, no investigations into the geometric parameters of a cavity receiver have been performed. The cylindrical receiver in this study is composed of an enclosed bottom at the back, an aperture at the front, a helical pipe inside the cavity and an insulation layer on the external surface of the cavity. The influence of several critical receiver parameters on the thermal efficiency is analyzed in this paper: cavity inner diameter and cavity length. The thermal model in this paper is solved considering the cavity dimensions as variables. Implementing the model into EES, each parameter influence is separately investigated, and a preliminary optimization method is proposed.

Compatibility tests between Jarytherm® DBT synthetic oil and solid materials from wastes

Direct thermocline thermal energy storage is the cheapest sensible thermal energy storage configuration. Indeed, a thermocline tank consists in one tank instead of two and reduces costs. Thermocline thermal energy storages are often filled with cheap solid materials which could react with the heat transfer fluid in the case of incompatibility. PROMES laboratory is building a pilot-scale parabolic trough solar loop including a direct thermocline thermal energy storage system. The working fluid will be a synthetic oil, the Jarytherm® DBT, and the thermal energy storage tank will be filled with stabilized solid materials elaborated from vitrified wastes. Compatibility tests have been conducted in order to check on one hand if the thermo-mechanical properties and life time of the energy storage medium are not affected by the contact with oil and, on the other hand, if the thermal oil performances are not degraded by the solid filler. These experiments consisted in putting in contact the oil and the solid materials in small tanks. In order to discriminate the solid materials tested in the shortest time, accelerating aging conditions at 330 °C for 500 hours were used. The measurements consisted in X-Ray Diffraction and Scanning Electron Microscopy for the solids, and thermo-physical and chemical properties measurements for the oil. Regarding the solid samples, their crystalline structure did not change during the test, but it is difficult to conclude about their elementary composition and they seem to absorb oil. While thermal properties still makes Jarytherm® DBT a good heat transfer fluid after the accelerated aging tests, this study results in differentiating most compatible materials. Thus according to our study, Jarytherm® DBT can be used in direct thermocline thermal energy storage applications when compatibility of the solid material has been demonstrated.