Desikan Bharathan

Two-Phase Spray Cooling of Hybrid Vehicle Electronics

As part of the U.S. Department of Energy (DOE) Advanced Power Electronics (APE) program, DOEs National Renewable Energy Laboratory (NREL) is currently leading a national effort to develop next-generation cooling technologies for hybrid vehicle electronics. Spray cooling has been identified as a potential solution that can dissipate 150- 200 W/cm2 while maintaining the chip temperature below 125degC. This study explores the viability and implementation of this cooling scheme. First, commercial coolants are assessed for their suitability to this application in terms of thermal, environmental, and safety concerns and material compatibility. In this assessment, HFE-7100 is identified as the optimum coolant in all performance categories. Next, spray models are used to determine the HFE-7100 spray conditions that meet such stringent heat dissipation requirements. These findings are verified experimentally, demonstrating that spray cooling is a viable thermal management solution for hybrid vehicle electronics.

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.

Use of a Thermal Manikin to Evaluate Human Thermoregulatory Responses in Transient, Non-Uniform, Thermal Environments

People who wear protective uniforms that inhibit evaporation of sweat can experience reduced productivity and even health risks when their bodies cannot cool themselves. This paper describes a new sweating manikin and a numerical model of the human thermoregulatory system that evaluates the thermal response of an individual to transient, non-uniform thermal environments. The physiological model of the human thermoregulatory system controls a thermal manikin, resulting in surface temperature distributions representative of the human body. For example, surface temperatures of the extremities are cooler than those of the torso and head. The manikin contains batteries, a water reservoir, and wireless communications and controls that enable it to operate as long as 2 hours without external connections. The manikin has 120 separately controlled heating and sweating zones that result in high resolution for surface temperature, heat flux, and sweating control. The physiological finite element model uses approximately 40,000 solid thermal and blood network elements to represent the human body. The manikin and physiological model demonstrate their value in evaluating the thermoregulatory response of a person in a protective uniform. They can also be used to evaluate the effectiveness of personal cooling systems.

Two-phase spray cooling of hybrid vehicle electronics

As part of the U.S. Department of Energys (DOEs) Power Electronics and Electric Machines Program area, the DOEs National Renewable Energy Laboratory (NREL) is currently leading a national effort to develop next-generation cooling technologies for hybrid vehicle electronics. Spray cooling has been identified as a potential solution that can dissipate 150-200 W/cm2 while maintaining the chip temperature below 125degC. This paper explores the viability and implementation of this cooling scheme. First, commercial coolants are assessed for their suitability to this application in terms of thermal, environmental, and safety concerns and material compatibility. In this assessment, HFE-7100 is identified as the optimum coolant in all performance categories. Next, spray models are used to determine the HFE-7100 spray conditions that meet such stringent heat dissipation requirements. These findings are verified experimentally, demonstrating that spray cooling is a viable thermal management solution for hybrid vehicle electronics.

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.

Using a Sweating Manikin, Controlled by a Human Physiological Model, to Evaluate Liquid Cooling Garments

This paper discusses the use of NRELs Advanced Automotive Manikin (ADAM) for evaluating NASAs liquid cooling garments for space suits.

Efficiency and Loss Models for Key Electronic Components of Hybrid and Plug-in Hybrid Electric Vehicles' Electrical Propulsion Systems

Isolated gate bipolar transistors (IGBTs) are widely used in power electronic applications including electric, hybrid electric, and plug-in hybrid electric vehicles (EVs, HEVs, and PHEVs). The trend towards more electric vehicles (MEVs) has demanded the need for power electronic devices capable of handling power in the range of 10-100 kW. However, the converter losses in this power range are of critical importance. Therefore, thermal management of the power electronic devices/converters is crucial for the reliability and longevity of the advanced vehicles. To aid the design of heat exchangers for the IGBT modules used in propulsion motor drives, a loss model for the IGBTs is necessary. The loss model of the IGBTs will help in the process of developing new heat exchangers and advanced thermal interface materials by reducing cost and time. This paper deals with the detailed loss modeling of IGBTs for advanced electrical propulsion systems. An experimental based loss model is proposed. The proposed loss calculation method utilizes the experimental data to reconstruct the loss surface of the power electronic devices by means of curve fitting and linear extrapolating. This enables the calculation of thermal losses in different voltage, current, and temperature conditions of operation. To verify the calculation method, an experimental test set-up was designed and built. The experimental set-up is an IGBT based bi-directional DC/DC converter. In addition, simulation results are presented to verify the proposed calculation method.

An assessment of air cooling for use with automotive power electronics

In this research, we evaluate the potential for air cooling to meet the stringent cooling requirements of advanced automotive power electronics. We assess air cooling of power electronic components using laminar airflow micro-channel heat exchangers. Comparisons are made with ethylene glycol systems commonly used in tandem with engine cooling. Our analysis shows that despite a lower coefficient of performance and higher parasitic losses, air cooling compares quite favorably, offering lower mass, fewer components, and a lower projected cost. Air cooling also has many significant, less obvious advantages such as simpler design and greater reliability. Micro-channel heat exchangers appear to offer the most promise and can be further enhanced by simple design changes, such as reducing passage lengths. Direct air cooling appears to be a viable option for the current generation of silicon-based power switches and will be more attractive for anticipated future electronic components made of materials that operate at higher temperatures. Continuing work includes experimentation and data validation. Recommendations for future research include fabricating and testing air-cooled inverters. A micro-channel performance estimator program we developed was found to over-project heat flux in comparison to a more detailed computational fluid dynamics model. However, the program provides an initial estimate that can be used as a quick, convenient means of estimating micro-channel heat transfer with a variety of configurations and fluids.

Numerical Simulations of Boiling Jet Impingement Cooling in Power Electronics

Boiling jet impingement cooling is currently being explored to cool power electronics components. In hybrid vehicles, inverters are used for DC-AC conversion. These inverters involve a number of insulated gate bipolar transistors (IGBTs), which are used as on/off switches. The heat dissipated in these transistors can result in heat fluxes of up to 200 W/cm2, which makes the thermal management problem quite important. In this paper, turbulent jet impingement involving nucleate boiling is explored numerically. The framework for these computations is the CFD code FLUENT. For nucleate boiling, the Eulerian multiphase model is used. A mechanistic model of nucleate boiling is implemented in a user-defined function (UDF) in FLUENT. The numerical results for boiling water jets (submerged) are validated against existing experimental data in the literature. Some representative IGBT package simulations that use R134a as the cooling fluid are also presented

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