Implementation of ‘chaotic’ advection for viscous fluids in heat exchanger/reactors

When viscous fluids are involved, laminar hydraulic conditions and heat and mass transfer intensification are conflicting phenomena. A channel geometry based on Split-And-Recombine (SAR) patterns is experimentally investigated. The principle implements the Baker’s transformation and ‘chaotic’ structures are generated to promote heat and mass transfer. This work assesses the energy efficiency of different heat exchanger/reactors integrating these SAR patterns. The heat transfer capacity is assessed and compared with the energy consumption of each mock-up. It is sensitive to the cooling mode and to the number of SAR patterns per length unit as well. The continuous oxidation of sodium thiosulfate with hydrogen peroxide has been implemented. Conversions up to 99% are reached according to the utility fluid temperature and the residence time. Finally, the whole performances of the SAR geometries are compared to a plate-type heat exchanger/reactor with a corrugated pattern. The more viscous the fluid, the more the energy efficiency of the SAR design increases compared to the corrugated design because of the balance between advection and diffusion mechanisms. The interest in terms of energy efficiency in working with SAR heat exchanger/reactor appears from Reynolds numbers below 50

DAMP HEAT STABILITY OF TRANSPARENT CONDUCTIVE ZINC OXIDES: ROLE OF ENCAPSULANTS AND PROTECTIVE LAYERS

The mechanisms and environmental influences that cause photovoltaic modules performance degradation are poorly understood, but it is well known that water vapour is deeply implicated in the degradation process. Indeed, some layers and interfaces of thin film modules can be moisture sensitive and depending on the processing conditions, they degrade after exposure to damp heat conditions (85°C, 85% relative humidity) [1]. Transparent conductive oxides (TCO), as used in CIGS or thin silicon film cells play a particular role linked to reliability issues. We showed recently that low-pressure chemical vapour deposition zinc oxide (LPCVD ZnO) can withstand damp heat test even without encapsulant providing doping of the ZnO is high enough, though this is unfavourable for free carrier absorption (reduction of spectral response in the infrared part) [2]. Reduction of doping leads to improved optical properties but needs therefore an optimized encapsulation strategy to avoid the deterioration of the TCO conductivity. In previous work, the degradation of LPCVD ZnO used in thin-film silicon solar cells was investigated [3]. It was shown that the decrease of the ZnO conductivity was essentially due to the humidity increasing inside the encapsulant. However other effects take part in the degradation process and remained yet unexplained. In this paper we will report on several other possible sources of degradation, which have been identified. In order to demonstrate and quantify these effects, we used various encapsulants, but without back protection (foil or glass), and we exposed the samples to different type of atmospheres. The resistivity of the ZnO was monitored using an inductive contactless and a four points probe methods. Finally, schemes to perform highly reliable laminates when using lightly doped ZnO are proposed

Towards in-line determination of EVA Gel Content during PV modules Lamination Processes

Poly (ethylene-co-vinyl acetate) (EVA) is the major polymer used for photovoltaic (PV) modules encapsulation. Its degree of cross-linking (related to its gel content) is taken as a major quality reference. Differential Scanning Calorimetry (DSC) has been proven to be fast and effective but is to determine the gel content, however, destructive for the PV module. With the aim to develop a non-destructive quality assessment tool, a detailed discussion on the DSC thermogram of EVA PV encapsulant is presented here. A possible path towards a fast and non-destructive method for determing EVA gel content is proposed based on the DSC analysis

Sécurisation passive du rendez-vous orbital par des polynômes non négatifs

International audienceLe sujet de cet article porte sur l'évitement de col- lision lors de manoeuvres de proximité pour le rendez-vous orbital. Il s'agit d'assurer la sécurité passive de la mission, en imposant au véhicule chasseur, en cas de panne, de res- ter sur une trajectoire périodique comprise dans un cˆone de visibilité de la cible. En s'appuyant sur les expressions ra- tionnelles du mouvement autonome périodique, nous trans- formons les contraintes de visibilité en contraintes de non- négativité de polynômes. Nous formulons ainsi le problème d'évitement de collision comme un problème d'optimisation polynomiale dont la solution garantit une satisfaction des contraintes continue dans le temps. Nous utilisons la pro- grammation semi-définie pour résoudre ce problème poly- nomial

Designing Continuously Constrained Spacecraft Relative Trajectories for Proximity Operations

International audienceThis paper presents a new method for designing an optimal plan of impulsive maneuvers for spacecraft proximity operations. The proposed method accounts for the presence of linear continuous constraints on the spacecraft relative trajectory. Impulsive control and continuous constraints are brought together through the parameterization of the spacecraft relative trajectory between two consecutive maneuvers. This parameterization is used in order to develop a finite convex description of all the admissible trajectories. It enables the transformation of the continuous constraints on the spacecraft relative trajectory into a finite number of polynomial nonnegativity constraints. The resulting optimal control problem can be solved using semidefinite programming

A two-impulse method for stabilizing the spacecraft relative motion with respect to a periodic trajectory

International audienceThe article presents an analytical method for computing a two-impulse control law that stabilizes the spacecraft relative motion with respect to an invariant set. The invariant set contains the states belonging to a desired periodic relative trajectory and is described using linear equations. The two impulses are computed analytically based on the prediction of the evolution of the relative trajectory. Closed-loop tests are conducted using the non-linear relative dynamics, for different eccentricities of the reference orbit and for different levels of navigation uncertainty. Encouraging results are obtained using a control strategy that requires very few computational effort

Minimizing the Effects of Navigation Uncertainties on the Spacecraft Rendezvous Precision

International audienceThe ability to robustly and precisely control the spacecraftrelative motion will play an important role in future on-orbitinspection and on-orbit servicing missions. Model predictivecontrol (MPC) is considered to be an effective control strategy forthese types of spacecraft operations, which can easily handle missionspecificconstraintswhileexplicitlyminimizingthefuelconsumption. The maneuvers plan is obtained by solving a finite horizon open-loop optimal control problem starting from the spacecraft relativestate, and the optimal solution consists of a series of control actions out of which only the first one is executed.Some ideas from tube-basedMPCare used in this Note to solve therobust fixed-time spacecraft rendezvous problem for eccentricreference orbits. The purpose is to obtain a sequence of feedbackpolicies that steers the spacecraft from an initial relative state towardan ellipsoidal set centered around a desired final state, in the presenceof navigation uncertainties. This must be done while respecting theactuators saturation constraints and while pursuing a doubleobjective: minimize the fuel cost of the mission and minimize the sizeof the arrival set to guarantee a good rendezvous precision. Thecontrol policies are restricted to affine disturbance feedback policiesto ensure a convex formulation of the control synthesis problem. Theobtained sequence of feedback policies drives the system to theguaranteed arrival set without any need for recurrent optimization

Constrained periodic spacecraft relative motion using non-negative polynomials

International audienceA new method for obtaining constrained periodic relative motion between spacecraft on Keplerian orbits is presented. The periodic relative trajectory is required to evolve autonomously inside a tolerance box centered in a specified position. Unlike the classical time-sampling approaches, our method guarantees continuous satisfaction of the constraints on infinite horizon. This is done by reformulating the tolerance box constraints on the relative trajectory as conditions of non-negativity of some polynomials. The resulting problem is solved using semi-definite programming

Periodic H2 synthesis for spacecraft in elliptical orbits with atmospheric drag and J2 perturbations

International audienceThe problem of the stationkeeping for a small spacecraft is studied and a solution based on periodic feedback control laws is considered. Linearized equations of the relative motion of the satellite near an eccentric reference orbit are derived in the presence of the second zonal gravitational harmonic J2 and atmospheric drag perturbations. The obtained linear continuous-time model of the relative motion is T-periodic where T is the orbital period. After a discretization of the model, a state-feedback control law with performance requirement defined by the generalized H2 operator norm may be computed by a linear matrix inequality-based algorithm. Illustrative non linear simulations show the efficiency of the approach based on the use of linearized spacecraft relative motion dynamics associated to systematic H2 synthesis of stabilizing memoryless N-periodic state-feedback control laws

AN HYBRID LED/HALOGEN LARGE-AREA SOLAR SIMULATOR ALLOWING FOR VARIABLE SPECTRUM AND VARIABLE ILLUMINATION PULSE SHAPE

Commercial large-area solar simulators are principally in the form of Xenon flashers, which intrinsically have a short flash duration time, typically 100 ms at low spectrum quality and 10 ms at high spectrum quality. This implies measurement time constraints not adapted to the characterization of new generations of photovoltaic modules such as high-efficiency crystalline silicon modules, which can exhibit transient effects leading to measurement artifacts [1]. Moreover, thin film technologies require a high spectral match and possibly a variable spectrum of the illumination source for the accurate power-rating of multi-junctions based solar modules. An alternative large-area solar simulator solution is proposed for the power rating and the diagnostic of such generations of solar modules. The developed simulator makes use of a combination of different power LEDs and halogen lamps in a particular matrix configuration, integrated into a 1 m × 1 m table with mirrors. The constructed prototype demonstrates an AAA classification according to the IEC norms [2], allows for long illumination (200 ms to continuous) and permits controlled variation of the spectrum and of the intensity of the illumination