Andreas Vlahinos

Predicting human thermal comfort in a transient nonuniform thermal environment

The National Renewable Energy Laboratory has developed a suite of thermal comfort tools to assist in the development of smaller and more efficient climate control systems in automobiles. These tools, which include a 126-segment sweating manikin, a finite element physiological model of the human body, and a psychological model based on human testing, are designed to predict human thermal comfort in transient, nonuniform thermal environments, such as automobiles. The manikin measures the heat loss from the human body in the vehicle environment and sends the heat flux from each segment to the physiological model. The physiological model predicts the body’s response to the environment, determines 126-segment skin temperatures, sweat rate, and breathing rate, and transmits the data to the manikin. The psychological model uses temperature data from the physiological model to predict the local and global thermal comfort as a function of local skin and core temperatures and their rates of change. Results of initial integration testing show the thermal response of a manikin segment to transient environmental conditions.

Energy Efficient Battery Heating in Cold Climates

In cold climates batteries in electric and hybrid vehicles need to be preheated to achieve desired performance and life cycle of the energy storage system and the vehicle. Several approaches are available: internal core heating; external electric heating of a module; internal electric heating in the module around each cell, internal fluid heating around each cell; and external fluid heating around each module. To identify the most energy efficient approach, we built and analyzed several transient thermal finite element models of a typical battery. The thermal transient response of the battery core was computed for the first four heating techniques, which were compared based on the energy required to bring the battery to the desired temperature in a given time. Battery core heating was the most effective method to warm battery quickly with the least amount of energy. Heating the core by applying high frequency alternating currents through battery terminals is briefly discussed.

Improving battery design with electro-thermal modeling

Operating temperature greatly affects the performance and life of batteries in electric and hybrid vehicles. Increased attention is necessary to battery thermal management. Electrochemical models and finite element analysis tools are available for predicting the thermal performance of batteries, but each has limitations. In this study we describe an electro-thermal finite element approach that predicts the thermal performance of a cell or module with realistic geometry. To illustrate the process, we simulated the thermal performance of two generations of Panasonic prismatic nickel-metal-hydride modules used in the Toyota Prius. The model showed why the new generation of Panasonic modules had better thermal performance. Thermal images from two battery modules under constant current discharge indicate that the model predicts the experimental trend reasonably well.

Single-Phase Self-Oscillating Jets for Enhanced Heat Transfer

In hybrid electric vehicles (HEVs), the inverter is a critical component in the power module, which conditions the flow of electric power between the AC motor and the DC battery pack. The inverter includes a number of insulated gate bipolar transistors (IGBTs), which are high frequency switches used in bi-directional DC-AC conversion. The heat generated in the IGBTs can result in degraded performance, reduced lifetime, and component failures. Heat fluxes as high as 250 W/cm2 may occur, which makes the thermal management problem quite important. In this paper, the potential of self-oscillating jets to cool IGBTs in HEV power modules is investigated experimentally. A full factorial design of experiments was used to explore the impact of nozzle design, oscillation frequency, jet flow rate, nozzle-to-target distance, and jet configuration (free-surface or submerged) on heat transfer from a simulated electronic chip surface. In the free-surface configuration, self-oscillating jets yielded up to 18% enhancement in heat transfer over a steady jet with the same parasitic power consumption. An enhancement of up to 30% for the same flow rate (and velocity since all nozzles have the same exit area) was measured. However, in the submerged configuration, amongst the nozzle designs tested, the self- oscillating jets did not yield any enhancements in heat transfer over comparable steady jets. Results also suggest that oscillating jets provide a more uniform surface temperature distribution than steady jets.

Impacts of cooling technology on solder fatigue for power modules in electric traction drive vehicles

This paper presents three power module cooling topologies that are being considered for use in electric traction drive vehicles such as a hybrid electric, plug-in hybrid electric, or electric vehicle. The impact on the fatigue life of solder joints for each cooling option is investigated along with the thermal performance. Considering solder joint reliability and thermal performance, topologies using indirect jet impingement look attractive.

Effect of Material and Manufacturing Variations on Membrane Electrode Assembly Pressure Distribution

A design is robust when it is not sensitive to variations in noise parameters such as manufacturing tolerances, material properties, environmental temperature, humidity, etc. In recent years several robust design concepts have been introduced in an effort to obtain optimum designs and minimize the variation in the product characteristics. Increasing the pressure on a PEM (Proton Exchange Membrane) fuel cell’s MEA (Membrane Electrode Assembly) leads to increasing the electric conductivity and reducing the permeability of the assembly. In this study, a probabilistic FEA analysis was performed on a simplified fuel cell stack in order to identify the effect of material and manufacturing variations on the MEA’s pressure distribution. The bi-polar flow plate thickness, the modulus of elasticity and the end plate bolt loading were considered as randomly varying parameters with given mean and standard deviation. The normal stress uniformity of the MEA was determined in terms of the probabilistic input variables. The methodology for implementing robust design used in this research effort is summarized in a reusable workflow diagram.Copyright

Optimization of cable preloading on cable-stayed bridges

Generally, geometric nonlinearities of cable-stayed bridges depend on the behaviors of cables, pylons, the bridge deck and their interactions. These are geometry change, cable sag, and the interactions of axial forces, the bending moment and their deformations in the pylons and bridge deck. Therefore, a large cable-stayed bridges system having a large number of cables can be analyzed under different load conditions. In investigating nonlinear behaviors of cable- stayed bridges, the nonlinear behavior of cables needs to be considered because it may cause the nonlinear behavior of whole bridge system. The nonlinear behavior of a cable gained from its sag. With an increasing axial load, the elongation of the cable is increased but the total cable sag is decreased. Cable-stayed bridge uses cables instead of the internal piers to support the bridge deck. Usually, cable- stayed bridge decks are straight with a little camber compared to the total length of the bridge. Keeping the bridge deck in the position where is the designer desired is not only for bridge aesthetics but also for people on the bridge in terms of psychological effect of improving confidence in structure and engineering considerations. To achieve the serviceability and engineering requirements, preloading of the cable is necessary. In this paper, one such a bridge with geometry similarly to an existing cable- stayed bridge. Quincy Bayview Bridge, located in Illinois, USA, has been considered. Quincy Bayview Bridge has 58 cables in the two planes. Four methods have been considered in this paper to make the optimum selection of cable preloading. The objective is to select appropriate method to determine cable prestrains in order to minimize the deformations and stresses due to dead load of the bridge. Thus, it is not a trivial problem since a change in the prestress of a cable influence the deformation every where in the structure. The best method would be determined by comparing the calculated bending and vertical displacements of the bridge deck.

Robust Design Techniques for Evaluating Fuel Cell Thermal Performance

The National Renewable Energy Laboratory (NREL) and Plug Power Inc. have been working together to develop fuel cell modeling processes to rapidly assess critical design parameters and evaluate the effects of variation on performance. This paper describes a methodology for investigating key design parameters affecting the thermal performance of a high temperature, polybenzimidazole (PBI)-based fuel cell stack. Nonuniform temperature distributions within the fuel cell stack may cause degraded performance, induce thermo-mechanical stresses, and be a source of reduced stack durability. The three-dimensional (3-D) model developed for this project includes coupled thermal/flow finite element analysis (FEA) of a multi-cell stack integrated with an electrochemical model to determine internal heat generation rates. Sensitivity and optimization algorithms were used to examine the design and derive the best choice of the design parameters. Initial results showed how classic design-of-experiment (DOE) techniques integrated with the model were used to define a response surface and perform sensitivity studies on heat generation rates, fluid flow, bipolar plate channel geometry, fluid properties, and plate thermal material properties. Probabilistic design methods were used to assess the robustness of the design in response to variations in load conditions. The thermal model was also used to develop an alternative coolant flow-path design that yields improved thermal performance. Results from this analysis were recently incorporated into the latest Plug Power coolant flow-path design. This paper presents an evaluation of the effect of variation on key design parameters such as coolant and gas flow rates and addresses uncertainty in material thermal properties.Copyright

Innovative Thermal Management of Fuel Cell Power Electronics

Deep at the heart of any fuel cell system lays a crucial component, the power inverter. The design of this crucial component is a challenge for fuel cell systems due to packaging, thermal and electrical constraints. Unless the inverter is adequately and uniformly cooled it will suffer material degradation and premature failure. The search for a thermally viable inverter design is one of many challenges facing the fuel cell industry today. In this research effort several cooling techniques were considered such as pin-finned design, “cook-top” serpentine flow field, a “fish bone” fin design, high thermal conductivity graphite foam, heat pipes and aluminum extrusion with expanded metal turbulator. The pin-finned design techniques were evaluated using computational fluid dynamics. In order to enable design engineers to rapidly generate optimum designs two simplified techniques were introduced using the CFD results. 1) Formulas for computing the film coefficient based on spacing, side and configuration are provided for thermal finite element analysis that includes conduction and convection. This technique is an order of magnitude faster than the CFD analysis. 2) Behavioral modeling, an optimization technique imbedded within a feature based parametric CAD system is utilized to automatically size and build the solid model of the pin-finned design. The designer input is the heat that needs to be rejected and the available space. Behavioral modeling generates the design and plots the temperature distribution.Copyright

Shape Optimization of Fuel Cell Molded-on Gaskets for Robust Sealing

In typical Proton Exchange Membrane fuel cells, a compressed gasket provides a sealing barrier between cell and cooler bipolar plate interfaces. The gasket initially bears the entire bolt load, and its resisting reaction load depends on the cross-sectional shape of the gasket, bipolar plate’s groove depth, and the hyperelastic properties of the gasket material. A nonlinear, finite element analysis (FEA) model with various hyperelastic material models, large deformations, and contact was used to evaluate the load-gap curves. The deformed shapes and the distributions of stress, strain, and deflections are presented. Mooney-Rivlin and Arruda-Boyce hyperelastic material models were used, and a comparison of load-gap curves is shown. A process is presented that couples the computer-aided design geometry with the nonlinear FEA model that was used to determine the gasket’s cross-sectional shape, which achieves the desired reaction load for a given gap.Copyright