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As the temperature range of the cooling machine affects the cooling performance, parametric optimization of moderate temperature Stirling refrigerator has valuable concept. In this paper, a nonideal modified simple numerical model was used to investigate the effects of parameters and to propose optimized Stirling refrigerator design for moderate temperature applications. The analysis was conducted through simulating the numerical model using MATLAB code for beta configuration FEMTO-60 prototype operating as a refrigerator. The effects of shuttle heat loss and mass leakage on the cooling performance of domestic Stirling refrigerator have been studied. Moreover, the effects of operating parameters (operating frequency and charging pressure) and design parameters (phase angle, regenerator length and porosity, displacer height, swept volume ratio, and piston diameter to stroke ratio) have been investigated. A cooling power of 625 W and COP of 1.3 have been found at a charging pressure of 22.5 bar and an operating frequency of 6 Hz using the optimized design parameters. The optimized parameters (operating and specified design parameters) could deliver from 48% to 60% more cold production and from 30% to 40% more COP than the existing design with a safe working condition.

A key issue in the optimal designing of a Stirling machine is to develop a precise thermodynamic numerical model that could predict the performances and provide means for further optimization. In this paper, a non-ideal second-order numerical model called modified simple analysis model has been presented for the Stirling cycle refrigerating machine. The model incorporates effects of shuttle heat loss to the expansion space and mass leakage to the crankcase in the differential equations of pressure change, rate of change of mass of gas in compression and expansion spaces, and mass flow rates across these working spaces. The model was simulated using MATLAB code for Beta configuration FEMTO 60 Stirling engine operating as a refrigerator and validated with an experiment. The validation of the numerical model with experimental work showed that the results of the simulation are consistent with the results of the experiment. Hence, the numerical model could be used to design a Stirling cycle refrigerating machine for moderate temperature applications with reasonable accuracy especially if optimization is performed further. The effects of incorporating shuttle heat loss in the differential equations on the temperature of working gas and the overall performance of the Stirling refrigerator have been analyzed. Lastly, parametric investigations have also been performed to evaluate the effect of operating parameters (temperature, pressure, and frequency) on the performances of the refrigerating machine. Better performance could be achieved at relatively lower frequency or higher pressure.

The Stirling cycle machines have many applications as prime mover and cooling purpose. A review is presented for the development of regenerative Stirling cycle machines in the refrigeration area. The Stirling cycle refrigerators of gas cycle machines, which are the counterparts of the Stirling engines. They can operate from low temperature to moderate temperature applications. The present paper describes an overview of the Stirling refrigerating machine and its associated researches carried out in this area.Initially, the review explains the general working principle, the configuration and the drive mechanisms as well as the research findings within the range from low-temperature cryocoolers to moderate temperature cooling applications. Furthermore, this review points out and discusses the various models and methods of optimization to improve the performance of the Stirling cycle refrigerator.

International audience; As the concern for CO2 emissions and global warming raises, the Stirling machine appears as a candidate alternative technology to vapor compression cycles that use refrigerant fluids. In this paper, we present the design, manufacturing and testing of a Beta Stirling machine for refrigeration applications. The first part of the paper presents the principle of the Stirling cycle, the mechanical configuration of the designed machine, and the key design parameters. The second part describes the experimental test bench and instrumentation of the Stirling machine. The last part of the paper presents experimental results. The prototype typical performance is a 450 W heat lift at 4°C and a COP of 0.9 when Nitrogen is used as the working gas. Cold end temperature variation as a function of heat lifts and charge pressure are presented. Coefficient of Performance is also discussed. Mechanical power, instantaneous torque and velocities curves are reported.

International audience; Abstract

International audience; In this paper, we present a new model design and parametric studies of a miniature Stirling cooler machine for on-site refrigeration. The MEMS (Microelectromechanical systems) technology is investigated to design this machine. The concept could be used to provide cooling at chip scale and mitigate hot spots in electronic devices. Whereas numerous works deal with Stirling engines at a macroscopic scale, only a few works concern miniaturized Stirling engines. Therefore, a model analysis giving insights of the impact of the technological choices and downsizing of the machine is needed. A base design model is presented. The model results lead to a cooling power of 10 mW and a Coefficient Of Performance of 1.45. A parametric study is conducted for operational and design parameters. Compared to macro-scale design, the same trend is observed for the influence of the thermal performance regenerator. Different trends from macroscopic engines were observed for hysteresis losses importance, and the choice of the working gas. The raise in power due to the raise in frequency expected for micro-scale devices is counterbalanced by the degradation of the COP due to the increase in thermofluidic losses. Squeeze film damping and finite speed losses can be neglected at this scale.

The design of a Stirling machine requires the understanding of the processes that govern their operation to predict the optimum operating parameters of the machine. The modified simple model using the geometrical parameters of the FEMTO-ST engine model is applied in this paper to investigate the trend of losses with respect to operating frequency and charging pressure. The variation of pressure drops across the regenerator and heat exchangers have been investigated as functions of operating frequency and charging pressure in order to predict the extent of fluid friction losses. The share of different power and heat losses as well as their effects on cooling performance have been evaluated at the different operating frequencies and charging pressures. The share of fluid friction loss and loss due to regenerator imperfection increase with rotational speed and charging pressure.

The Stirling cycle refrigerators, which are the counterparts of the Stirling engines are of gas cycle machines. In the present paper, experimental investigation and numerical analysis of Beta-type Stirling refrigerator for domestic applications are conducted. The refrigeration performances such as input power requirement, cooling power, and coefficient of performance for moderate temperature applications have been analyzed using different working fluids (air, nitrogen, helium, and hydrogen). The numerical analysis is conducted to evaluate the performance of a machine with respect to different operating frequencies and charging pressures. The result of the analysis showed that air and nitrogen have better cooling power than helium and hydrogen in the operating ranges (15–25 bar and 6–12 Hz) of the cooling machine. On the other hand, the coefficient of performances in the case of helium shows a higher rate of increase with charging pressure than that of air and nitrogen.

A regenerator of a Stirling machine alternately absorbs and releases heat from and to the working fluid which allows to recycle rejected heat during theoretical isochoric processes. This work focuses on a milli-regenerator fabricated with a multiple jet molding process. The regenerator is a porous medium filled with a dense pillar matrix. The pillars have a geometrical lens shape. Two metallic layers (chromium and copper) are deposited on the polymer pillars to increase heat transfer inside the regenerator. We performed experiments on different milli-regenerators corresponding to three porosities (ε = 0.80, 0.85 and 0.90) under nitrogen steady and oscillating compressible flows (oscillating Reynolds number in the range 0 < Reω < 60 and Reynolds number based on the hydraulic diameter ReDh,maxT < 100°C). Temperature, velocity and pressure experimental measurements are performed with microthermocouples (type K with 7,6 µm diameter), hotwires and miniature pressure sensors, respectively. We identified a threeterm composite correlation equation for the friction factor based on a Darcy-Forchheimer flow model that best-fit the experimental data. In steady and oscillating flows permeabilities and inertial coefficients are of the same magnitude order. Inertial coefficients decrease when the porosities increase.

International audience; In this paper, we present a new model and design of a MEMS Stirling machine. The concept could be used to provide electricity to low level power systems. A modified adiabatic model including losses of a Stirling engine such as regenerator thermal efficiency, wall conduction, pressure drop, hysteresis, membrane finite speed and squeeze film damping losses is presented. A base design machine is proposed. A parametric study is conducted and used to derive guidelines for miniature design. Compared to macro-scale design, the same trend was observed for the influence of the thermal performance regenerator. However, different trends from macroscopic engines were observed: conduction losses are of major importance due to the low power of the miniature engines, hysteresis losses in the bounce space are also of high amplitude due to the small size if this volume, the choice of the working gas leads to hydrogen or air and not helium. We computed that membrane finite speed losses and squeeze film damping losses can be neglected in the investigated machine size. The first part of the paper presents the micro-machine specific features, the second part describes the model, the last part presents the base design results and design guidelines.

International audience; In this paper, a parametric investigation of domestic regenerative Stirling refrigeratingmachines is conducted using a modified simple model. The model is adapted from an ideal adiabaticengine model and different losses have been included at different stage based on their effect. Themodel is validated experimentally using the Beta type FEMTO 60 Stirling engine as a case study.The simulation and experimental results show that the coefficient of performance of the Stirling cyclerefrigerator at a charging pressure of 17.5 bar, frequency of 12.1 Hz, cold end temperature of −22oCand using nitrogen as a working fluid are found as 38.2% and 35% respectively. Furthermore, theeffects of phase angle, working fluid type, operating pressure, and operating frequency of a machine onthe performance of a regenerating Stirling cycle refrigerator are investigated.

International audience; In this paper, we study the influence of annealing on the performance of Resistive Temperature Detectors made from Nickel thin films. The aimed application is heat flux sensing. The substrate is made of Borofloat glass with a Chromium adhesive layer. Several annealing temperatures between 150°C and 300°C are applied to this assembly. The thin films as deposited and after annealing are analyzed through SEM images. The evolution of the resistance and the temperature coefficient of the sensor are discussed. An annealing temperature is selected that ensures the repeatability of measurements.

International audience; Résumé - Cette étude se situe dans le cadre du développement de fluxmètres thermiques en microtechnologie. Ces fluxmètres consistent en deux résistances de platine déposées sur un substrat en verre. Dans une première partie, nous présentons le principe du fluxmètre et de la mesure de température. Dans une deuxième partie, nous présentons également les étapes de fabrication en salle blanche et les différentes températures de recuits étudiées. Nous présentons des résultats expérimentaux sur la variation de résistance observée selon les différentes températures de recuit et une analyse par Diffraction de Rayons X complétée par des images au microscope à balayage. Nous proposons comme hypothèse que la baisse de résistivité observée pour les faibles températures de recuit est due à l’amélioration du réseau cristallin et que la hausse observée pour les températures les plus élevées est due à la diffusion de la couche d’accroche en chrome dans le platine.

International audience; In this paper, we present an experimental study on the pressure, pressure drop, velocity and friction factor in a regenerator made from an array of pillars in reciprocating flow conditions. The fluid used in the experiment is air. We show that the pressure drop depends on the porosity, frequency and stroke. We observe a phase shift between the crank angle sinusoidal variation of the piston and the pressure variation that increases with frequency. We also observe that the instantaneous friction factor is higher in the discharge phase than in the suction phase, and higher in the acceleration than in the deceleration phase. Friction factor in oscillating flow appears to be higher than friction factor in steady unidirectional flow for the highest porosity. For the lower porosities, we observe that the pressure drop and friction factors are similar for maximum Reynolds numbers lower than 5000. An experimental correlation for the friction factor of the regenerator is derived in order to design a millimetric Stirling engine. The friction factor is compared with friction factors reported for woven screens, metal felts and involute foil regenerators.

International audience; This paper describes a Stirling Engine used as an Auxiliary Power Unit in a Series Hybrid Vehicle. The engine modeling is presented, then its integration in the vehicle power train is described. Fuel consumption, power train efficiency are compared between Stirling Engines and Spark ignition Engines. The results show that Stirling engines present good performances compared to internal combustion engines in the same configuration. Finally, the prototype design and first results are presented.

International audience; Study of dynamic response of silicone elastomer microfabricated hybrid membranes versus temperatures and aging time

The implementation of Fuel Cell (FC) systems in transportation systems, as aircrafts, requires some better understanding and mastering of the new generator behaviours in low temperature environments. To this end, a PEMFC stack is tested and characterised in a climatic chamber. The impacts of the low temperatures over different FC operation and start-up conditions are estimated using a specific test bench developed in-lab. Some descriptions concerning the test facilities and the experimental set-up are given in the paper, as well as some information about the test procedures applied. Some examples of test results are shown and analysed. The experiments are derived from aircraft requirements and are related with different scenarios of airplane operation. Finally, some assessments concerning the FC system behaviour in low temperature conditions are made, especially with regard to the constraints to be encountered by the next embedded FC generators.

International audience; In this paper, experimental investigations on the influence of operational parameters on PEM fuel cell cold start are presented. The effect of current density, stack impedance at 1 kHz prior to start, as well as gas flow rate, gas pressure, coolant flow rate and surrounding subfreezing temperature are studied. The experimental apparatus is briefly described. It includes a main unit at room temperature and a smaller separate unit in a climatic chamber. Low current density, high impedance prior to start, moderate subfreezing temperature (–5 °C), high gas flow rate, low gas pressure and low coolant flow rate are found to have a positive impact on the cold start performance. Combining these parameters, self start-up of the fuel cell without additional energy is achieved at –5 °C in 30 min. The whole set of observations leads to the following hypotheses on freeze mechanism: in the first phase, dry membranes and low current lead to a transient phase of membrane humidification. Then, in the second phase, ice clogging of the active layers occurs. In the third phase, a variable quantity of the produced water reaches the gas diffusion layers and channels.

International audience; The article presents the design, fabrication and characterization of platinum thin-film Resistance Temperature Detectors (RTDs)for heat flux sensing applications. The microfabrication process is described and the thin films are analysed by several methodsincluding SEM, XRD and profilometry. Estimation of experimental uncertainties is performed. The influence of annealing, selfheating,2-wire and 4-wire connections are studied. The analysis and experimental results show that the RTDs have a goodrepeatability when an annealing process is performed and a maximal injected current are used. A measurement uncertainty of0.14°C is achieved. The observed Temperature Coefficient Ratio is sufficient to ensure variation of the resistance in thetemperature range. The heat flux measurements made with the sensor are validated in a dedicated test bench and present agood repeatability.

This paper deals with the solution of inverse problems in magnetostatics. The cases the authors have broached are finding the current density or positions of air coils on the basis of magnetic field measurements. These problems are ill-posed, so regularization techniques are needed. The authors discuss the main iterative regularization principles, and the stability issues of these inverse problems. This method is applied to MRI magnets. Numerical results are presented.

This paper presents the use of D-optimal experimental design to improve the accuracy of the solutions of inverse problems in magnetostatics. This method is applied to the following inverse problem: finding the current density on the basis of magnetic field measurements. The basic idea is to choose the measurement point positions to build a D-optimal experiment, then to solve the inverse problem by iterative regularization. The algorithms used are discussed. Numerical results that show the large improvement in the accuracy of the solutions are presented.

This paper deals with the solution of linear inverse problems in magnetostatics. The case the authors have broached is finding the current density on the basis of magnetic field values. Solving this kind of equation is an ill-posed problem. Exact magnetic field values and measured values lead to different cases, each of which is presented. To solve them, the authors use the conjugate gradient method with iterative regularization. They present numerical results for the design of magnets, gradient and shim coils, and numerical results for the problem of recovering current density values from measured field values.

International audience; Fuel cell self start at negative temperature conditions is not an easy task due to the water produced by the reduction of oxygen at the cathode. This amount of water can turn into ice and block the reaction before the temperature inside the fuel cell reaches positive values. The mechanism of the physical process which leads to oxidant starvation phenomena when ice appears is not yet well identified. In order to obtain a deeper understanding of this problem, the article presents some experimental investigations conducted on a short fuel cell stack. These experiments simulate vehicle technology operated in cold start conditions not with the primary objective to reach a successful and rapid start-up but much rather to characterise and understand the cold start phenomena until starvation occurs. A number of polarisation curves, electrochemical spectroscopy and cyclic voltammetry measurements are done on the stack before, after and also during the cold starts experiments. It is observed that the process of drying and cooling down prior to cold start have a great impact on the electrochemical cathode area. The results obtained show the evolution of the stack behaviour during the low temperature operation with a slow production of frost. The consequence on the individual cells in terms of inhomogeneous degradation is highlighted.

This paper is the second paper dealing with the evaluation of different power sources and hybridization strategies in a heavy-duty vehicle. It focuses on the physical integration of a fuel cell system (FCS) into the vehicle as well as the preliminary experimental results obtained with the FCS coupled to batteries. The vehicle considered here is a mobile research platform (ECCE) dedicated to the integration and characterization of various hybridization technologies and architectures in real operating conditions. The installation of the FCS into ECCE required some heavy modifications of the vehicle structure and architecture. Prior to moving toward extensive development and testing of energy management algorithms, a validation stage was necessary. The objective of the present paper is to describe this first stage. The first step was the functionality validation of the embedded FCS in term of communication with ECCE's controller and electrical performance in static vehicle operation. The second step of the validation process was to proceed to a slow displacement of the vehicle powered by the FCS and batteries. This research activity was initiated within the framework of the SPACT-80 program and is now continued under a new project called ECCE 2, supported by the DGA (Technology & Procurement Agency of the French Ministry of Defense).

This paper presents the implementation of an 80kW-fuel cell system in a heavy duty vehicle and the development of its control strategy with real-time simulation. The considered hybrid vehicle is here a heavy-duty mobile test bench nicknamed ECCE. The paper is organized in the following manner. First, the SPACT-80 program (80kW fuel cell systems for transportation applications) supported by the French ANR agency and the ECCE test bench are presented. Then the paper will present the integration of this Fuel Cell System (FCS) in ECCE. The paper will finally report the development of the control management program of the vehicle with real time simulations.

International audience; In this paper, a method to monitor PEM fuel cells internal temperature from surface measurements is presented. The aim of this work is to monitor fuel cells to prevent damages due to internal overheating. The measurements are taken at the side of the bipolar plate, and heat flux and temperature at the border of the active zone are estimated. The method is based on sensitivity analysis and inverse problem algorithms. The mathematical formulation and algorithm are described. The model is a transient heat conduction model in two dimensions, the inverse problem is solved with an optimization method using adjoint equation. Numerical test cases are presented for graphite and steel bipolar plates. The results show that internal temperature can be correctly estimated. The response time of the method is limited by the heat transfer rate in the material. Therefore, the method is particularly appropriate to fuel cells made of steel bipolar plates.

International audience; This paper presents the design and validation of a 2 kW fuel cell test bench for subfreezing studies. The test bench is designed to study the influence of different operational parameters on the cold start of fuel cell. Thus, the effects of ambient temperature, gas and coolant flow rates, current density and fuel cell impedance can be investigated. For cold start experiments, the apparatus is designed to reproduce the environment of a fuel cell in a vehicle parked in a subzero environment. Therefore, the test bench is divided into two parts: the fuel cell, its coolant circuit and main sensors are located in a climatic chamber while the main part of the test bench is at room temperature. The different parts of the test bench are described and validation results are presented. The heat exchange between the fuel cell coolant circuit and the environment is computed using parameters estimation techniques. The power requested to heat the coolant circuit to a given temperature is computed.

This paper presents a stability analysis of a free piston Stirling engine. The model and the detailed calculation of pressures losses are exposed. Stability of the machine is studied by the observation of the eigenvalues of the model matrix. Model validation based on the comparison with NASA experimental results is described. The influence of operational and construction parameters on performance and stability issues is exposed. The results show that most parameters that are beneficial for machine power seem to induce irregular mechanical characteristics with load, suggesting that self-sustained oscillations could be difficult to maintain and control.

International audience; This paper reports the design, fabrication and CFD modeling of a microregenerator in order to be integrated in a multiphase piezoelectric smart membrane Stirling engine. The application aims to recover waste heat at low temperatures and to convert it into electricity via the piezoelectric element. The suggested geometry for the microregenerator is a microchannel integrated with staggered DRIE etched silicon micropillars, the whole is encapsulated between two glass wafers. The porosity ranges from 0.8 to 0.9. A 2D numerical study of the microregenerator thermofluidic performances was investigated. Three gases (air, helium andhydrogen) were investigated. The gas flow is set to be incompressible and viscous under laminar unidirectional steady flow conditions for low Reynolds number (

International audience; Design and microfabrication of a Microscale-Stirling Engine (M-SE) for low temperature heat recovery

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