Kenneth Kelly

Thermal interface materials for power electronics applications

In a typical power electronics package, a grease layer forms the interface between the direct bond copper (DBC) layer or a baseplate and the heat sink. This grease layer has the highest thermal resistance of any layer in the package. Reducing the thermal resistance of this thermal interface material (TIM) can help achieve the FreedomCAR program goals of using a glycol water mixture at 105degC or even air cooling. It is desirable to keep the maximum temperature of the conventional silicon die below 125degC, trench insulated gate bipolar transistors (IGBTs) below 150degC, and silicon carbide-based devices below 200degC. Using improved thermal interface materials enables the realization of these goals and the dissipation of high heat fluxes. The ability to dissipate high heat fluxes in turn enables a reduction in die size, cost, weight, and volume. This paper describes our progress in characterizing the thermal performance of some conventional and novel thermal interface materials. We acquired, modified, and improved an apparatus based on the ASTM D5470 test method and measured the thermal resistance of various conventional greases. We also measured the performance of select phase-change materials and thermoplastics through the ASTM steady-state and the transient laser flash approaches, and compared the two methodologies. These experimental results for thermal resistance are cast in the context of automotive power electronics cooling. Results from numerical finite element modeling indicate that the thermal resistance of the TIM layer has a dramatic effect on the maximum temperature in the IGBT package.

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.

FEDERAL TEST PROCEDURE EMISSIONS TEST RESULTS FROM ETHANOL VARIABLE-FUEL VEHICLE CHEVROLET LUMINAS

The first round of Federal Test Procedure (FTP) emissions testing of variable-fuel ethanol vehicles from the U.S. Federal fleet was recently completed. The vehicles tested include 21 variable-fuel E85 1992 and 1993 Chevrolet Lumina sedans and an equal number of standard gasoline Luminas. Results presented include a comparison of regulated exhaust and evaporative emissions and a discussion of the levels of air toxics, as well as the calculated ozone-forming potential of the measured emissions. Two private emissions laboratories tested vehicles taken from the general population of Federal fleet vehicles in the Washington, D.C., and Chicago metropolitan regions. Testing followed the standard U.S. Environmental Protection Agency’s FTP and detailed fuel changeover procedures as developed in the Auto/Oil Air Quality Improvement Research Program. Variable-fuel vehicles were tested on single respective batches of E85, E50, and California Phase 2 reformulated gasoline (RFG) blended specifically for this test program.

Battery usage and thermal performance of the Toyota Prius and Honda Insight during chassis dynamometer testing

This study describes the results from the National Renewable Energy Laboratorys (NREL) chassis dynamometer testing of a 2000 model year Honda Insight and 2001 model year Toyota Prius. The tests were conducted for the purpose of evaluating the battery thermal performance, assessing the impact of air conditioning on fuel economy and emissions, and providing information for NRELs Advanced Vehicle Simulator (ADVISOR). A comparative study of the battery usage and thermal performance of the battery packs used in these two vehicles during chassis dynamometer testing is presented. Specially designed charge and discharge chassis dynamometer test cycles revealed that the Insight limited battery usage to 60% of rated capacity, while the Prius limited battery usage to 40% of the rated capacity. The Prius uses substantially more pack energy over a given driving cycle but at the same time maintains the pack within a tight target state of charge (SOC) of 54% to 56%. The Insight does not appear to force the battery to a specific target SOC. The Prius battery contributes a higher percentage of the power needed for propulsion. The study also found that while both vehicles have adequate battery thermal management system for mild driving conditions, the Prius thermal management is more robust, and the Insight thermal management limits pack performance in certain conditions.

Duty Cycle Characterization and Evaluation Towards Heavy Hybrid Vehicle Applications

Four metrics related to vehicle duty cycle are derived from the energy equation of vehicle motion. Three key application areas are introduced. The first is the ability to quantify the sameness between vehicle duty cycles and the ability to asses a duty cycle’s suitability for hybrid vehicle usage. The second area of application allows for the estimation of fuel consumption for a given vehicle over a target duty cycle. The third area of application allows us to predict how non-propulsion fuel use will affect energy use. The paper ends with real-world examples involving actual heavy-duty hybrids.

Development of Refuse Vehicle Driving and Duty Cycles

Research has been conducted to develop a methodology for the generation of driving and duty cycles for refuse vehicles in conjunction with a larger effort in the design of a hybrid-electric refuse vehicle. This methodology includes the definition of real-world data that was collected, as well as a data analysis procedure based on sequencing of the collected data into micro-trips and hydraulic cycles. The methodology then applies multi-variate statistical analysis techniques to the sequences for classification. Finally, driving and duty cycles are generated based on matching the statistical metrics and distributions of the generated cycles to the collected database. Simulated vehicle fuel economy for these cycles is also compared to measured values.

Topology, design, analysis and thermal management of power electronics for hybrid electric vehicle applications

Power electronics circuits play an important role in the success of electric, hybrid and fuel cell vehicles. Typical power electronics circuits in hybrid vehicles include electric motor drive circuits and DC/DC converter circuits. Conventional circuit topologies, such as buck converters, voltage source inverters and bidirectional boost converters are challenged by system cost, efficiency, controllability, thermal management, voltage and current capability, and packaging issues. Novel topologies, such as isolated bidirectional DC/DC converters, multilevel converters, and Z-source inverters, offer potential improvement to hybrid vehicle system performance, extended controllability and power capabilities. This paper gives an overview of the topologies, design, and thermal management, and control of power electronics circuits in hybrid vehicle applications.

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.

Test Results and Modeling of the Honda Insight using ADVISOR

Paper describing a series of chassis dynamometer and road tests that NREL conducted on the 2000 model-year Honda Insight.

Rapid modeling of power electronics thermal management technologies

A methodology was developed to rapidly evaluate trade-offs associated with alternative packaging configurations and thermal management technologies for power electronics packaging. The methodology includes the integration of available experimental correlations, computational fluid dynamics results, parametric 3D finite element analysis (FEA) thermal models, and established heat exchanger analysis techniques. The parametric 3D FEA model enables sensitivity studies related to the power module package configuration and cooling technologies. This paper focuses on the study of alternative cooling technologies as they are applied to a fixed power module package. The methodology is applied to a double-sided power module package for several alternative cooling technologies.