Numerical comparison of thermoacoustic couples with modified stack plate edges

It has been demonstrated that the bulk of time-averaged heat transfer between the oscillating fluid and a thermoacoustic couple is concentrated towards the edges of the stack plate. Previous numerical studies which have considered thermoacoustic couples of finite thickness have used a rectangular form for the plate edge. In practice however, current manufacturing practices allow for a variety of stack edges which may improve the efficiency of heat transfer and/or reduce entropic losses. In this numerical study, the performance of a thermoacoustic couple is investigated at selected drive ratios and using a variety of stack plate edge profiles. Results indicate that stack profiles with enlarged and blunter shapes improve the rate of heat transfer at low drive ratios but retard the rate of heat transfer at higher drive ratios due to increased residence time of the fluid in contact with the stack plate. The improvement in COP through minimisation of acoustic streaming on the inside face of the stack, and increased effective cooling power by greater retention of stack thickness at the plate extremities, leads to recommendation of the Rounded edge shape profile for thermoacoustic stack plates in practical devices.     

Transient temperature profile inside thermoacoustic refrigerators

The linear theory used to calculate the thermal quantities inside the stack in the classical thermoacoustic refrigerators always overestimates those measured. The causes of these discrepancies have to be found in the complex processes of thermal exchanges. The analytical study of the transient response should provide an interpretation of these complex processes. This present paper provides such analytical modelling. This modelling remains within the framework of the classical linear theory. It includes the effects of the thermoacoustic heat flux carried along the stack, the conductive heat flux returning in the solid walls of the stack and through the fluid inside the stack, the transverse heat conduction in the stack and the heat leakages through the duct walls, the heat generated by viscous losses in the stack, the heat generated by vorticity at the ends of the stack, and the heat transfer through both ends of the stack. A modal analytical solution for the temperature profile is proposed, assuming the usual approximations in such thermal problems to avoid intricate calculations and expressions. The theoretical transient response of a thermoacoustic refrigerator is compared with experimental data. A good qualitative agreement is obtained between analytical and experimental results after fitting empirical coefficients.     

Effect of humidified water vapor on heat balance management in a proton exchange membrane fuel cell stack

Thermal management has been considered as one of the most important issues for the operation of proton exchange membrane fuel cells (PEMFCs). Phase change affects the performance and even the heat balance of the stack during operation. A 46 single cell PEM stack with anode and cathode humidification is developed to investigate, both theoretically and experimentally, the effect of phase change on the heat generation and removal characteristics of the stack. The results show that the heat removed by the coolant water is greater than that generated by the electrochemistry reaction, and heat released due to the phase change of water vapor cannot be neglected. Heat generated in the stack can be removed completely by the coolant water, which need to be forced cooling for recycling use when the current density reaches 1000 mA·cm^?2. The arithmetic product of the specific heat capacity and mass of the stack can be used as a novel criterion to evaluate the validity of the heat balance in the system. The exothermic reaction is very fast in the stack, which consequently requires bipolar plates with high heat conductivity coefficient to improve the temperature uniformity at the elevated operational current density. Copyright © 2014 John Wiley & Sons, Ltd.     

Integration of Heat Pipe into Fuel Cell Technology

This paper describes recent applications of heat pipe technology in fuel cell systems, which include new stack designs with heat pipes to improve heat transfer as well as work on fuel cell system level design and engineering with adopting the heat pipe concept. In one design, micro-heat pipes are inserted and bonded in bipolar plates for thermal control in the fuel cell stack. In another design, flat heat pipes are integrated with a carbon bipolar plate for improving thermal control in the fuel cell stack. Finally, based on the heat pipe concept, we specifically developed a series of direct methanol fuel cell (DMFC) systems characterized as passive technology for methanol fuel delivery, water recirculation, and air and thermal management. Long-term durability and stability of the passive DMFC systems have been proved experimentally.     

Quantitative analysis on cold start process of a PEMFC stack with intake manifold

To systematically explore the low-temperature operating characteristics of polymer electrolyte membrane fuel cell (PEMFC) stack, a three-dimensional PEMFC stack model with intake manifold is developed in this study. The characteristics of different cold start modes in the stack are compared and analyzed. The distribution and transmission characteristics of water, ice, and heat in each cell of the stack are analyzed in detail. The location of water accumulation in each cell of the stack is also explored. Finally, finite difference sensitivity is calculated for the cumulated charge transfer density to quantify the effects of operating parameters on the cold start process at low temperature. And how these parameters affect the operation of the PEMFC stack at low temperature is investigated. The results show that inconsistency exists in stack operation due to the position particularity of the intermediate cell. Irreversible heat is the main heat source for the cold start of the stack, and the cathode catalyst layer is the main heat-generating component. The heat production proportion of cathode catalyst layer can reach 90%, which decreases with the increment of current density and the running time, especially for the edge cell. The initial ionomer water content is most sensitive to the cold start process of the stack, followed by the porosity of cathode catalyst layer. These parameters are sensitive to the cold start process mainly because of the change in volumetric exchange current density and oxygen concentration.     

The development of a small PEMFC combined heat and power system

A proton exchange membrane fuel cell combined heat and power system has been chosen as a platform on which key components and system integration technologies have been developed to advance the applications of fuel-cell technology. The prototype system consists mainly of a fuel-cell stack, a natural gas reformer to supply hydrogen-rich reformate gas to the stack, a power conditioner to convert and invert the electricity generated by the stack, and a water and heat management network to provide the oxidizer (air) to and necessary cooling and humidification for the stack. The design of the stack and its components, catalysts development, design and testing of the natural gas reformer, were studied. In addition, the durability of a CO tolerant membrane-electrode-assembly has been studied with the assistance of air bleeding. Preliminary testing of the prototype combined heat and power system was carried out. It was found that the maintenance of a uniform output voltage across all single cells was more difficult with reformatted gas rather than using pure hydrogen. It was also found that the design of water and heat management network played an important role in the overall efficiency of the system.     

A computational fluid dynamics and finite element analysis design of a microtubular solid oxide fuel cell stack for fixed wing mini unmanned aerial vehicles

Computational fluid dynamics (CFD) and finite element analysis (FEA) are important modelling and simulation techniques to design and develop fuel cell stacks and their balance of plant (BoP) systems.The aim of this work is to design a microtubular solid oxide fuel cell (SOFC) stack by coupling CFD and FEA models to capture the multiphysics nature of the system. The focus is to study the distribution of fluids inside the fuel cell stack, the dissipation of heat from the fuel cell bundle, and any deformation of the fuel cells and the stack canister due to thermal stresses, which is important to address during the design process. The stack is part of an innovative all-in-one SOFC generator with an integrated BoP system to power a fixed wing mini unmanned aerial vehicle. Including the computational optimisation at an early stage of the development process is hence a prerequisite in developing a reliable and robust all-in-one SOFC generator system. The presented computational model considers the bundle of fuel cells as the heat source. This could be improved in the future by replacing the heat source with electrochemical reactions to accurately predict the influence of heat on the stack design.     

Computation of the flow and thermal fields in a thermoacoustic refrigerator

The hydro- and thermodynamic processes near and within two-dimensional stack plates are simulated by numerical solution of the unsteady compressible Navier–Stokes, continuity, energy equations, and the equation of state (for air as the working fluid). The stack is assumed to consist of flat plates of equal thickness. The second order mean velocity field is computed in the neighborhood of the stack plates. In the stack plate extremities the vortical mean flow is observed which is due to the abrupt change of a slip condition to a no-slip velocity boundary condition. The temperature of the stack is governed by the energy equation; therefore the entire problem is treated as a conjugate heat transfer problem. The temperature fields in the neighborhood of the solid stack plate are also observed. From the location of the heat exchangers in Fig. 1(a), it is obvious that knowledge of the flow and thermal fields at the edges of the stack plates is the key for the development of a systematic design methodology for heat exchangers in thermoacoustic devices.     

Model based design and test of cooling plates for an air-cooled polymer electrolyte fuel cell stack

Design of an effective cooling system in polymer electrolyte membrane fuel cells (PEMFCs) is vital for the heat management and overall performance of stacks. Depending on the stack size and application, either air or water-cooling can be used to extract excess heat and maintain the desired temperature distribution throughout the stack.A computational model previously assembled by the authors has been used to design cooling plates for a typical air-cooled stack configuration. The aim of these designs was to minimise temperature differences between cells, and dissipate heat from the stack. Three different cooling plate designs are analysed both computationally and experimentally within stacks containing electrically heated pads in place of active MEAs.Good agreement was achieved between the model and experiment, and results showed that implementing a cooling plate is an effective way to balance temperature variation within a stack and minimise thermal issues. It was found that the temperature variation may be minimised by implementing plates with wider cooling channels. As a result, more air may be forced through the channels with less resistance, which minimises the power required by the air blower, and hence the parasitic load on the system.     

Computation of the time-averaged temperature fields and energy fluxes in a thermally isolated thermoacoustic stack at low acoustic Mach numbers

A simplified calculus model to investigate on the transverse heat transport near the edges of a thermally isolated thermoacoustic stack in the low acoustic Mach number regime is presented. The proposed methodology relies on the well-known results of the classical linear thermoacoustic theory which are implemented into an energy balance calculus-scheme through a finite difference technique. Details of the time-averaged temperature and heat flux density distributions along a pore cross-section of the stack are given. It is shown that a net heat exchange between the fluid and the solid walls takes place only near the edges of the stack plates, at distances from the ends not exceeding the peak-to-peak particle displacement amplitude. The structure of the mean temperature field within a stack plate is also investigated; this last results not uniform near its terminations giving rise to a smaller temperature difference between the plate extremities than that predicted by the standard linear theory. This result, when compared with experimental measurements available in literature, suggests that thermal effects localized at the stack edges may play an important role as sources of the deviations found between linear theory predictions and experiments at low and moderate Mach numbers.     

Analysis of thermal balance in high-temperature proton exchange membrane fuel cells with short stacks via in situ monitoring with a flexible micro sensor

The heat produced from the electrochemical reaction in a fuel cell is worth studying, the heat recycled make the fuel cell more efficiency, especially in a high-temperature proton exchange membrane fuel cell (HTPEMFC). In low temperature PEMFC system, the heat is removed by cooling system avoid the membrane degradation exceed 100 °C. But in HTPEMFC system, the membrane can afford higher temperature (T_g 420 °C), means the cooling system could be removing and through changing the inside flow field to uniform the unit cell temperature in stack. In this study, a 50–100 W HTPEMFC stack is demonstrated and a micro sensor was integrated with the HTPEMFC stack for in situ measurements during the experiments. The results show that when the stack is operated at low and high current loads, the heat generation from the fuel cell causes noticeable changes in the cell temperature, especially in the middle of the stack. In the middle cell of the stack, the temperature exceeds the operating temperature (160 °C) by 10–30 °C when the current increases. Moreover, changing the flow field to counter-flow or co-flow with U- or Z-type flow fields causes changes to the thermal balance in the stack. The performance, however, remains almost the same for each type of flow field when there is no water affecting the HTPEMFC, even though the change in thermal balance in the stack still occurs. The results of the micro sensor in situ monitoring for each type of flow field displayed higher temperatures on the middle cells. If the waste heat is appropriately used, the high-temperature fuel cell will then be more efficient than the low-temperature fuel cell. The results also show that, in the HTPEMFC stack, the heat generated from the fuel cell can be reused in other ways.     

NUMERICAL STUDY OF THERMOACOUSTIC HEAT EXCHANGERS IN THE THIN PLATE LIMIT

The velocity and temperature fields in an idealized thermoacoustic refrigerator are analyzed computationally. The numerical model simulates the unsteady mass, momentum, and energy equations in the thin-plate, low-Mach-number limits. Two-dimensional unsteady calculations of the flow field in the neighborhood of the stack and heat exchangers are performed using a vorticity-based scheme for stratified flow. The computations are applied to analyze the effects of heat-exchanger length and position on the performance of the device. The results indicate that the cooling load peaks at a well-defined heat-exchanger length, stack gap, and distance between the heat exchangers and the stack plates.     

Numerical study of heat pipe application in heat recovery systems

Heat pipes are two-phase heat transfer devices with extremely high effective thermal conductivity. They can be cylindrical or planar in structure. Heat pipes can be embedded in a metal cooling plate, which is attached to the heat source, and can also be assembled with a fin stack for fluid heat transfer. Due to the high heat transport capacity, heat exchangers with heat pipes have become much smaller than traditional heat exchangers in handling high heat fluxes. With the working fluid in a heat pipe, heat can be absorbed on the evaporator region and transported to the condenser region where the vapour condenses releasing the heat to the cooling media. Heat pipe technology has found increasing applications in enhancing the thermal performance of heat exchangers in microelectronics, energy and other industrial sectors.Utilisation of a heat pipe fin stack in the drying cycle of domestic appliances for heat recovery may lead to a significant energy saving in the domestic sector. However, the design of the heat pipe heat exchanger will meet a number of challenges. This paper presents a design method by using CFD simulation of the dehumidification process with heat pipe heat exchangers. The strategies of simulating the process with heat pipes are presented. The calculated results show that the method can be further used to optimise the design of the heat pipe fin stack. The study suggests that CFD modelling is able to predict thermal performance of the dehumidification solution with heat pipe heat exchangers.     

Techno-economic modelling of a solid oxide fuel cell stack for micro combined heat and power

Solid oxide fuel cell combined heat and power (CHP) is a promising technology to serve electricity and heat demands. In order to analyse the potential of the technology, a detailed techno-economic energy-cost minimisation model of a micro-CHP system is developed drawing on steady-state and dynamic SOFC stack models and power converter design. This model is applied it to identify minimum costs and optimum stack capacities under various current density change constraints. Firstly, a characterisation of the system electrical efficiency is developed through the combination of stack efficiency profiles and power converter efficiency profiles. Optimisation model constraints are then developed, including a limitation in the change of current density (A cm⁻²) per minute in the stack. The optimisation model is then presented and further expanded to account for the inability of a stack to respond instantaneously to load changes, resulting in a penalty function being applied to the objective function proportional to the size of load changes being serviced by the stack. Finally, the optimisation model is applied to examine the relative importance, in terms of minimum cost and optimum stack maximum electrical power output capacity, of the limitation on rate of current density change for a UK residential micro-CHP application. It is found that constraints on the rate of change in current density are not an important design parameter from an economic perspective.     

Dynamic modeling and analysis of a 20-cell PEM fuel cell stack considering temperature and two-phase effects

Dynamic characteristics and performance of a PEM fuel cell stack are crucial factors to ensure safe, effective and efficient operation. In particular, water and heat at varying loads are important factors that directly influence the stack performance and reliability. Herein, we present a new dynamic model that considers temperature and two-phase effects and analyze these effects on the characteristics of a stack.     

On the significance of developing boundary layers in integrated water cooled 3D chip stacks

We present in this paper a fundamental hydrothermal investigation of the next generation interlayer integrated water cooled three-dimensional (3D) chip stacks, with high volumetric heat generation. Such investigation of flow through microcavities with embedded heat transfer structures such as micro pin-fin arrays and microchannels is crucial for the successful realization of 3D chip stacks. We focus mainly on the complex physics of the entrance region of the cooling microcavities in order to assess its importance. The flow and heat transfer in the entrance region is strongly influenced by developing boundary layers and, as we show herein, the development lengths can occupy a significant portion of the microcavity due to the size restrictions of the 3D chip stack. These effects make a fundamental understanding of conjugate heat transfer in microcavities with heat transfer structures a necessity. The flow field and heat transfer in the entrance region are characterized by means of correlations determining the effective coolant permeability as well as the heat transfer coefficient as a function of the streamwise coordinate x, the flow Reynolds number (Re) and the Prandtl number. Based on a thermal non-equilibrium porous medium model relying on these results, a substantially improved estimation of pressure drop and temperature distribution inside the chip stack is realized. The modeling results are validated against measurements on a 3D chip stack simulator. The range of flow rates and thermal loads in the hot spots of the chip stack, over which it is crucial to consider the developing hydrothermal effects, are analyzed and discussed in detail. Moreover, microchannel and micro pin-fin structures are compared, showing more than 20% increased performance of the latter for all operating conditions investigated.     

Thermal analysis and management of proton exchange membrane fuel cell stacks for automotive vehicle

The thermal management of a proton exchange membrane fuel cell (PEMFC) is crucial for fuel cell vehicles. This paper presents a new simulation model for the water-cooled PEMFC stacks for automotive vehicles and cooling systems. The cooling system model considers both the cooling of the stack and cooling of the compressed air through the intercooler. Theoretical analysis was carried out to calculate the heat dissipation requirements for the cooling system. The case study results show that more than 99.0% of heat dissipation requirement is for thermal management of the PEMFC stack; more than 98.5% of cooling water will be distributed to the stack cooling loop. It is also demonstrated that controlling cooling water flow rate and stack inlet cooling water temperature could effectively satisfy thermal management constraints. These thermal management constraints are differences in stack inlet and outlet cooling water temperature, stack temperature, fan power consumption, and pump power consumption.     

Performance evaluation of an energy recovery system for fuel reforming of PEM fuel cell power plants

This paper describes an energy recovery system that recovers waste thermal energy from a fuel cell stack and uses it for fuel reforming purposes. The energy recovery system includes a throttling valve, a heat exchanger, and a compressor, and is coupled with a coolant loop of the fuel cell stack. The feed stock of a fuel reformer, which is primarily a mixture of water and fuel, is vaporized in the heat exchanger and is compressed to a sufficiently high pressure before it is ducted into the fuel reformer. The performance of a fuel cell power plant equipped with the energy recovery system is evaluated. The results indicate that the power plant efficiency can be increased by more than 40% compared to that of a fuel cell power plant without the energy recovery system. Additionally, up to 90% of the waste heat generated in the fuel cell stack is recovered. As a result, the required heat dissipation capacity of the radiator that is used for cooling the fuel cell stack can be drastically reduced.     

A simple equation for temperature gradient in a planar SOFC stack

Our recent model of heat transport in a planar SOFC stack is extended to take into account finite hydrogen utilization. The extended model includes the heat balance equations in the interconnect and air flow, and the hydrogen mass balance equation in the anode channel. An approximate analytical expression for the gradient of stack temperature along the air channel is derived. The analytical result is in excellent agreement with the exact numerical solution. The resulting expression can be used for rapid estimate of the temperature gradient in a planar SOFC stack under real operating conditions.     

Compact Air-to-Air Thermosyphon Heat Exchanger

Outdoor cabinets containing power electronics components need to be cooled effectively and at the same time protected from outside air, which may contain moisture and various kinds of dirt that would reduce the reliability of the electronics. Air-to-air heat exchangers are widely used in the industry as they are cost-competitive and easy to install and maintain. On the other hand, they are inefficient and bulky. ABB holds a patent on a cost-effective modular compact thermosyphon-based air-to-air heat exchanger for power electronics cabinets. This technology uses numerous multiport extruded tubes with capillary-sized channels disposed in parallel to achieve the desired compactness. The heat exchanger is made of a stack of thermosyphon units to cope with the required heat loads. The experimental performance of this novel power electronics cooling system with R134a was measured for a single unit and a stack of thermosyphons. The influence of different parameters such as the heat load, fluid filling ratio, air temperature, and flow rate were investigated. A numerical model was developed in order to predict the performance of the thermosyphon unit and stack for various and changing operating conditions. Prediction shows good agreement with the experimental results.