Selective emitters design and optimization for thermophotovoltaic applications

Among several solutions to exploit solar energy, thermophotovoltaics (TPV) have been popularized and have known great breakthroughs during the past two decades. Yet, existing systems still have low efficiencies since the wavelength range of optimal photovoltaic (PV) conversion is very small compared to the emitter spectral range. Selective emitters are a very promising solution to this problem. We developed numerical tools to design and optimize such emitters. Some of the resulting structures composed of two or four layers of metals and semiconductors are presented in this paper. We also show that the usual PV devices efficiency limits (30% for crystalline silicon under solar radiation, according to Shockley-Queisser model) can be easily overcome thanks to these structures.Comment: 12 pages, 10 figure

Sensitivity optimization of micro-machined thermo-resistive flow-rate sensors on silicon substrates

We report on an optimized micro-machined thermal flow-rate sensor as part of an autonomous multi-parameter sensing device for water network monitoring. The sensor has been optimized under the following constraints: low power consumption and high sensitivity, while employing a large thermal conductivity substrate, namely silicon. The resulting device consists of a platinum resistive heater deposited on a thin silicon pillar ~ 100 $\mu$m high and 5 $\mu$m wide in the middle of a nearly 100 $\mu$m wide cavity. Operated under the anemometric scheme, the reported sensor shows a larger sensitivity in the velocity range up to 1 m/s compared to different sensors based on similar high conductivity substrates such as bulk silicon or silicon membrane with a power consumption of 44 mW. Obtained performances are assessed with both CFD simulation and experimental characterization

Does public awareness increase support for invasive species management?:Promising evidence across taxa and landscape types

Management of invasive species often raises substantial conflicts of interest. Since such conflicts can hamper proposed management actions, managers, decision makers and researchers increasingly recognize the need to consider the social dimensions of invasive species management. In this exploratory study, we aimed (1) to explore whether species’ taxonomic position (i.e. animals vs. plants) and type of invaded landscape (i.e. urban vs. nonurban) might influence public perception about the management of invasive species, and (2) to assess the potential of public awareness to increase public support for invasive species management. We reviewed the scientific literature on the conflicts of interest around the management of alien species and administered two-phased questionnaires (before and after providing information on the target species and its management) to members of the public in South Africa and the UK (n = 240). Our review suggests that lack of public support for the management of invasive animals in both urban and non-urban areas derives mainly from moralistic value disagreements, while the management of invasive plants in non-urban areas mostly causes conflicts based on utilitarian value disagreements. Despite these general trends, conflicts are context dependent and can originate from a wide variety of different views. Notably, informing the public about the invasive status and negative impacts of the species targeted for management appeared to increase public support for the management actions. Therefore, our results align with the view that increased public awareness might increase the public support for the management of invasive species, independent of taxonomic position and type of landscape

Conception et optimisation de sources thermiques cohérentes pour applications thermo-photovoltaïques

Thermo-photovoltaic conversion of thermal radiation is a concept that has been thoroughly investigated during the two last decades because of its high efficiency when compared to classical photovoltaics (PV). These high performances are mainly due to the good-matching between the thermal source radiation spectrum and the PV cell conversion spectrum. Maximal efficiencies are obtained with coherent sources that emit in narrow spectral bands, just above the band gap energy of the cell. In this report, a stochastic method to design and optimize such sources, the particle swarm optimization in this case, is firstly presented. This method leads to simple one-dimensional structures, composed of thin films of dielectrics, metals and semiconductors. The radiative properties of these sources are easily tunable with control parameters as simple as films thicknesses and doping concentrations. They are stable at high temperatures up to 1000 K. Second, a parametric optimization study of usual materials optical properties models (Drude and Lorentz) is presented in order to maximize radiative heat transfer between semi-infinite planes separated by nanometric gaps. This leads to a simple tool in the form of abacuses which would guide the choice of relevant materials to maximize the output power of nano thermo-photovoltaic devices.Le thermo-photovoltaïque (TPV), conversion du rayonnement thermique par des cellules photovoltaïques (PV), est un dispositif qui a suscité un intérêt croissant depuis deux décennies, notamment pour son efficacité supérieure à celle de la conversion photovoltaïque classique. Ceci est essentiellement dû à l'accord entre le spectre du rayonnement de la source thermique et le spectre de conversion de la cellule PV. Les rendements maximaux sont obtenus pour des sources thermiques cohérentes, émettant dans une gamme spectrale étroite, énergétiquement au-dessus de l'énergie de la bande interdite de la cellule PV. On propose dans ce travail d'appliquer une méthode d'optimisation stochastique, en l'occurrence l'optimisation par essaims de particules, pour concevoir et optimiser de telles sources. On aboutit alors à des structures unidimensionnelles simples, à base de films minces de diélectriques, métaux et de semi-conducteurs. Les propriétés radiatives de ces sources, stables pour des températures allant jusqu'à 1000 K, sont aisément contrôlables à l'aide de paramètres simples comme les épaisseurs des films ou la concentration de dopage. Finalement, on propose une étude d'optimisation paramétrique des propriétés optiques des matériaux susceptibles de maximiser l'échange radiatif en champ proche entre deux milieux plans semi-infinis. Cette étude aboutit à un outil pratique, sous forme d'abaques, permettant de guider le choix des matériaux pertinents afin de maximiser les puissances au même temps que l'efficacité des systèmes TPV nanométriques

Impact of a liquid drop on a granular medium: Inertia, viscosity and surface tension effects on the drop deformation

International audienceAn experimental study of liquid drop impacts on a granular medium (glass beads) is proposed. Four fluids were used to vary physical properties: pure distilled water, water with glycerol at two concentrations 1:1 and 1:2 v/v and water with surfactant (Tween 20) at the concentration of 0.1 g l À1 (%1 CMC). The drop free fall height was varied to obtain a Weber number (We) between 10 and 2000 and a Reynolds number (Re) between 100 and 13000. Different behaviors during the drop spreading, receding and absorption are highlighted as function of the fluids viscosity (l) and surface tension (c). The role plays by the surfactant on the drop behavior after impact was found significant especially when the kinetic energy was high. On the other hand, the final diameter of the drop was found insensitive to the viscosity of the fluid and the Weber number. The maximal spreading factor b max = D max /D 0 was found to scale as We 1 4 for water + surfactant drops (c % 36 mJ m À2 , l = 1 mPa s) whereas for the other fluids (c % 70 mJ m À2 but l = 1 À 19 mPa s) b max $ We 1 5. The boundary between splashing and non-splashing was determined using the splash parameter K d = We 1/2 Re 1/4. The threshold value K ds was found higher that for impact of liquid drops on solid dry substrates. Experimental absorption times are also commented and compared with a simplified theoretical model. Finally, the morphology of the craters is discussed and a relationship between the crater diameter and the drop kinetic energy at impact is presented

Coévolutions des systèmes techniques et des comportements : le cas de la rénovation énergétique

International audienc

Coévolutions des systèmes techniques et des comportements : le cas de la rénovation énergétique

International audienc

Coévolutions des systèmes techniques et des comportements : le cas de la rénovation énergétique

International audienc

Ballistic effects on thermal conductivity in 1D and 2D configurations from single and multiple localized sub mean free path heat sources: a numerical investigation

International audienceThe effective thermal conductivity of materials decreases when their dimensions are comparable to orlower than the mean free path of heat carriers (phonons in dielectrics) and when the influence of the thermal boundaryconductance increases. Distinguishing the two phenomena is not experimentally straightforward, while the phononBoltzmann transport equation (BTE) in the Relaxation Time Approximation (RTA) can be used to investigate thisnumerically. We solve a particular form of the BTE under the RTA, the Equation of Phonon Radiative Transfer, withthe Discrete Ordinates Method (DOM) in one-dimensional and two-dimensional configurations for gray and non-graymedia. The simulations give access to local effective temperature and conductive heat flux fields. We consider inparticular the cross-plane and the in-plane thermal conductivities through 1D and 2D calculations, respectively.Diffuse, specular and diffuse-specular reflections of the heat carriers on the boundaries are implemented. The casesof extended and localized thermal sources are studied in 2D. Reduction factors associated to the effective thermalconductivities are obtained for the different cases and compared to the predictions based on the Fuchs-Sondheimer-Casimir-Ziman theory [1–4]. Constant temperature and adiabatic boundary conditions are also distinguished and theireffects on the thermal transport are highlighted. Collective effects due to multiple localized heat sources are alsoconsidered. The competition between different characteristic length-scales (heat source size, distance between heatsources, sample size) is investigated. Finally, we discuss the obtained results in light of the recent thermo-optical andelectrical experiments involving 2D heat spreading from sub-mean free path heat sources [5,6].References:[1] Fuchs K.., The conductivity of thin metallic films according to the electron theory of metals, Math. Proc. Camb.Philos. Soc. 34, 100 (1938)[2] Sondheimer E.H., The mean free path of electrons in metals, Adv. Phys. 1, 1 (1952)[3] Casimir H.B.G., Note on the conduction of heat in crystals, Physica 5, 495 (1938)[4] Ziman J.M., Electrons and Phonons (Clarendon Press), 1960[5] Hoogeboom-Pot K., et al., A new regime of nanoscale thermal transport: Collective diffusion increases dissipationefficiency, Proc. Natl. Acad. Sci. 112, 4846 (2015)[6] Hu Y., et al., Spectral mapping of thermal conductivity through nanoscale ballistic transport, Nat. Nanotechnol., 10 ,701 (2015