NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

High Temperature Air-Cooled 

Power Electronics Thermal Design

Scot K. Waye

Principal Investigator

National Renewable Energy Laboratory

Annual Merit Review and Peer Evaluation Meeting

June 18, 2014

Washington, D.C.

PR-5400-61355 Project ID: APE019

This presentation does not contain any proprietary, confidential, or otherwise restricted information.
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Overview

Phase II Start Date: FY10

Project End Date: FY14

Phase II Complete: 80%

Total Project Phase II Funding:

DOE Share: $1,700K

Funding Received in FY13: $450K

Funding for FY14: $450K

Timeline

Budget

Barriers

• Sapa Extrusions North America 

(aluminum extrusion/manufacturing)

• General Electric Global Research 

(previous air jet work)

• Oak Ridge National Laboratory (ORNL)

– Madhu Chinthavali

Partners

• Cost – Eliminate need for secondary 

liquid coolant loop and associated cost 

and complexity

• Weight – Reduce unnecessary coolant, 

coolant lines, pump, and heat 

exchangers for lower system-level 

weight

• Performance – Maintain acceptable 

device temperatures while reducing 

complexity and system-level parasitic 

power
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Relevance

All vehicles are air cooled 

Air cooling a goal 

for batteries:

• Simple

• Direct

Just indirectly

Project Perspective

Jim Snyder, NREL Image Gallery 15165
4

State of the Art

LS 600H 2008

Camry 2007

Indirect Air Cooling via

Liquid Cooling

Intermediate liquid cooling loop 

rejects heat to air

Direct Air Cooling

Has been done before

Honda Insight (12 kW)

AC Propulsion (150 kW)

Mitsubishi SiC Inverter 

(50 kVA/L, 400 V output, 156 kVA)

SiC = Silicon Carbide

Relevance

Low Power

Low Power Density

Prototype:

not in vehicle

NREL: John Rugh

Courtesy ORNL

DOE Advanced Vehicle Testing Activity 

(Idaho National Laboratory)
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• Build and demonstrate module-level thermal (NREL) and 

electrical (ORNL) design (10 kW)

• Conduct detailed analysis and proof of concept at system level 

to show progress relative to DOE’s technical targets

Project Objective

Develop air-cooled thermal management system solution that 

contributes to:

• Accelerating electric drive vehicle adoption

• Eliminating intermediate cooling loops

• Meeting DOE’s APEEM 2015 technical targets (12 kW/kg, 

12 kW/L)

• Supporting EV Everywhere

FY14 Project-Specific Goals

Relevance
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Project Overview

• Air is a poor heat-transfer fluid 

o Low specific heat

o Low density

o Low conductivity

• Potential parasitic power of fan

• Not yet used in high power production vehicles

• Filtering, fouling, fan noise

Challenges

Advantages

• Rejecting heat directly to air can eliminate intermediate liquid-air loops

• Attractive for high-temperature device (wide bandgap) applications

• Air is benign, is not carried, and does not need to be replaced

• Air is a dielectric and can contact the chip directly

• No global warming potential

Relevance
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Project Pathway

This collaborative research with ORNL is on track to demonstrate an aircooled inverter for 30-kW continuous, 55-kW peak power 

Opens path to adoption by industry as an alternative to standard 

liquid-cooled system that lowers weight, volume, and cost

Air-cooled inverter technology can meet 2015 DOE targets (12 kW/kg, 12 kW/L) 

and provides pathways to meet 2020 targets with acceptable parasitic loads

Relevance

FY10 FY11 FY12/FY13 FY14
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Project Summary: FY11 – FY14

FY12

Q1 Q2 Q3 Q4

FY13

Q1 Q2 Q3 Q4

FY14

Q1 Q2 Q3 Q4

Heat Transfer Feasibility

Thermal and System 

Design and Prototyping

Module and Thermal 

System Demonstration

Deliverable/Milestone Go/No-Go

M1 (NREL): Heat transfer feasibility study

M1 (ORNL): Device-level evaluation

Heat transfer accomplished with reasonable flow 

and pressure loss?

M2: Build and test thermal module and initial balance of system

M2: Module electrical design

Demonstrated design on track to meet targets?

M3: Demonstrate operating module and thermal system

M3: Electrical inverter design and module build

Met targets for module level (10 kW)?

M2

M3

M1

FY11 M0: Proof of principle 

and synthetic jet vs. steady jet

Milestones
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FY14 Tasks to Achieve Key Deliverable

2013

Oct Nov Dec

2014

Jan Feb Mar Apr May Jun Jul Aug Sep

Go/No-Go 1: Down selection of optimized design meets targets?

Go/No-Go 2: Module thermal/electrical design on track to meet targets?

Go/No-Go 3: Balance-of-system design on track to meet air flow requirements? 

Key Deliverable, M3: Demonstrate operating 10-kW module and deliver system 

metrics (inverter power density, specific power, parasitic power)

Design, model, and test feasible ducting design and fans

Build and test module electrical components

Demonstrate balanceof-system design

Build and test 10 kW module thermal 

and electrical design

Go/ 

No-Go 

3

NREL ORNL

Test optimized thermal design with 

simulated heat loads

Go/ 

No-Go 

2

Key 

Deliverable

M3

Model 

validation

Go/ 

No-Go 

1

Milestones
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System Level Analysis Approach/Strategy
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• Electrical Design: Device type 

and location; electrical duty 

cycles; temperature-dependent 

losses, efficiency

• Thermal Constraints: Maximum 

junction temperature; heat 

generation; coolant temperature

• Feasibility / Trade-Off: Modeling; 

extrapolate to inverter scale

• Thermal Design: Sub-module 

testing and model validation; 

fan/ducting testing; optimization

• Thermal System Design: Balanceof-system analysis; full system 

models

• Hardware: Module prototype, 

improve design; balance-ofsystem testing

Thermal Constraints

Thermal System Design

Thermal Design

Feasibility / Trade-Off

Hardware

Electrical Design (ORNL) Design

Prototype

Development

Validation

High-Temperature Air-Cooled Inverter Approach/Strategy
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• †Casing volume adjusted for fin geometry

• †Capacitor ~1.13 L*, ~1.62 kg*

• †Gate driver + control board ~0.88 L*, ~0.42 kg**

* Assumption provided by ORNL; ** NREL assumption based on similar device measurement

Module Optimization to Inverter Accomplishments

FY13 - previously presented

Heat dissipation for 55-kW peak power

From conservative analytical analysis ~ 2.7 kW heat (95% efficient)

• Fixed device heat generation and temperature (175oC)

• Parametrically optimized geometry by varying air flow 

rate using computational fluid dynamics (CFD) – ANSYS 

Fluent

• Extrapolated sub-module modeling and 

testing results to module level

• Added in balance-of-inverter components†

Baseline

Optimized

Capacitor Capacitor

2 devices

9 modules 6 modules
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Parametric Channel Geometry Optimization

9 Modules, DBA, 175°C Junction Temperature

Baseline

2015 

Target

2020 

Target

Low parasitic power, but only most aggressive 

geometries barely meet 2015 targets

Accomplishments

12.0 kW/kg 14.1 kW/kg

DBA = direct-bond-aluminum

FY14

AC blower or condenser 

fan power draw

Each geometry represents one CFD simulation
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Parametric Channel Geometry Optimization

6 Modules, DBA, 175°C Junction Temperature

2015 

Target

2020 

Target

Accomplishments

12.0 kW/kg 14.1 kW/kg

Selection of geometries meets targets 

with acceptable parasitic power

FY14

Go/ 

No-Go 

2

Each geometry represents one CFD simulation

AC blower or condenser 

fan power draw
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Parametric Channel Geometry Optimization

6 Modules, DBA, 175°C Junction Temperature

2015 

Target

2020 

Target

Accomplishments

12.0 kW/L 13.4 kW/L

Selection of geometries meets targets 

with acceptable parasitic power

FY14

Go/ 

No-Go 

2

AC blower or condenser 

fan power draw

Each geometry represents one CFD simulation
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Heat Exchanger Experiments

Baseline

Optimized

Accomplishments

Prototypes provided by Sapa

FY14

devices

heaters
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Heat dissipation: 2.7 kW target

Accomplishments

~55 CFM

4 W Fluid Power

Specific Power: 9.4 kW/kg

Power Density: 10.1 kW/L

Heat Dissipation for Baseline Case (9 modules)

Fan sized to satisfy flow 

rate and pressure drop

FY14

Tout ~ 120°C 100°C 70°C
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Heat Dissipation for Optimized Case (6 modules) Accomplishments

Specific Power: 14.6 kW/kg

Power Density: 14.5 kW/L

~200 CFM

110 W Fluid Power

FY14

Go/ 

No-Go 

2

Heat dissipation: 2.7 kW target

Tout ~ 80°C 60°C

Specific power and power 

density projected to meet 

2020 targets, with tradeoff 

of higher parasitic power
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Feedback: The reviewer indicated that this is a good team, but felt that a vehicle manufacturer should be added to the 

team to get industry input. This reviewer suggested that this would help the project to become easier to 

manufacture and get industry acceptance.

Response: We have a strong collaboration with ORNL. NREL is addressing thermal issues, and ORNL is focused on the 

electrical design. On the thermal work, NREL is collaborating with Sapa Extrusions North America to aid in 

manufacturing concerns regarding extruded aluminum, a potential method for simple and inexpensive 

cooling channels. ORNL is working with device manufacturers and other partners on the electrical design. We 

have met with OEMs and first-tier suppliers through on site visit presentations to get feedback on the 

direction of the research and to address the advantages, challenges, and potential applications for vehicle 

platform adoption. We are addressing the barriers and constraints that they have brought up in these 

meetings.

Feedback: The reviewer suggested that future work should produce a proof-of-concept of a working prototype and 

address practical application issues.

Response: This fiscal year, the focus has been shifted from modeling and analysis to direct experimentation and proofof-concept. Baseline and optimized sub-modules have been tested with data extrapolated to module and 

inverter level. An optimized module has also been manufactured and will be tested this fiscal year. One 

module heat sink will be tested for thermal performance at NREL, while another was sent to ORNL so they 

could mount the electrical topology and test the electrical design.

Feedback: The reviewer observed that this project is very relevant to future wide-band applications because cooling 

requires significant energy that reduces the EV drive efficiency.

Response: We agree that air cooling is better used on high-temperature application, where the large temperature 

difference helps overcome the shortcomings of air properties. Wide-band-gap devices provide an emerging 

opportunity for air cooling as they can be run at higher temperatures. They are more efficient, but with 

smaller areas, the heat flux is still a challenge. We believe air cooling will be a potential effective alternative 

to traditional cooling strategies.

Responses to AMR FY13 Reviewers’ Comments
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Collaboration and Coordination

Collaborations

• Industry: Sapa Extrusions North America

o Provided aluminum channel heat exchanger prototypes

o Offered guidance on manufacturability for various designs and 

direction on reducing material and manufacturing costs

• Federal Laboratory: Oak Ridge National Laboratory (Madhu Chinthavali)

o Building and testing electrical topology

o Provided device information, estimated heat loads, balance-ofinverter component sizing, and design constraints

• University/Industry: WBG Institute (outside Vehicle Technology Office)

o Developing relationships with industry and university partners for 

potential application of technology

Coordination

• Industry: Domestic OEMs and inverter suppliers

o Contributed feedback on research and potential applications through 

direct one-on-one meetings

OEM = original equipment manufacturer
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Remaining Challenges and Barriers

• Demonstrating prototype that meets technical 

targets

Cooling 

Technology

• Showing how balance-of-inverter components 

complete efficient inverter packaging

• Under-hood location considerations for vehicle 

platforms

Packaging

• Parasitic power requirements

• Comparison of weight, volume, and cost of 

necessary components (i.e. ducting, fans) 

compared to liquid-cooled loop

Balance of 

System
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Electrical and Thermal Module (10 kW)

Future Work

Build 10-kW module

Demonstrate 10-kW 

module (thermal 

performance)

Build 10-kW module

Demonstrate 10-kW 

module (electrical 

performance)

M3

Photo courtesy of ORNL 

(Madhu Chintavali)
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System Test Platform

Build and test 

sample system 

to measure and 

characterize 

parasitic load

Future Work

M3

Jim Snyder, NREL Image Gallery 15165
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Summary

DOE Mission Support

• Overcome barriers to accelerate adoption of low-cost air-cooled heat 

exchangers for power electronics, especially for high-temperature (i.e. WBG) 

devices; air remains the ultimate sink

Approach

• Create system-level understanding and designs addressing advanced cooling 

technology, packaging, and balance of system

• Develop solutions from fundamental heat transfer, then system-level design, to 

application − culminating in demonstration of concept

Collaborations

• ORNL developed electrical topology and is testing with NREL air-cooled heat 

exchanger and provided balance-of-inverter component sizing and 

characterization and device loss (heat dissipation) direction

• Sapa Extrusions North America provided aluminum heat exchanger prototypes 

and guidance on manufacturing constraints to control cost

• Met with OEMs and 1st tier suppliers to obtain feedback on air-cooling 

applications and overcoming barriers and constraints for adoption
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Summary

Technical Accomplishments

• Optimized thermal design showing candidate designs that 

meet DOE power density and specific power technical 

targets with acceptable parasitic loads

• Demonstrated baseline and optimized module design 

thermal performance and, using balance-of-system 

assumptions, projected to meet 2015 DOE technical targets

• Built module-level (10-kW) heat exchanger that will be 

tested in FY14

Maximum Junction 

Temperature – 175°C

2015 

Target

2020 

Target

Baseline 

(9 modules)

Baseline 

(6 modules)

Optimized 

(9 modules)

Optimized 

(6 modules)

Power Density [kW/L] 12.0 13.4 10.1 12.8 11.8 14.5

Specific Power [kW/kg] 12.0 14.1 9.4 12.7 11.1 14.6

Fluid Power [W] N/A N/A 4 34 70 110
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For more information contact:

Principal Investigator

Scot K. Waye

Scot.Waye@nrel.gov

Phone: (303) 275-4454

APEEM Task Leader

Sreekant Narumanchi

Sreekant.Narumanchi@nrel.gov

Phone: (303)-275-4062

Acknowledgments:

• Susan Rogers and Steven Boyd 

U.S. Department of Energy

• Andrew Wereszczak

Oak Ridge National Laboratory 

Team Members:

Matt Musselman 

Charlie King

Kevin Bennion

Madhu Chinthavali (ORNL)

Acknowledgments and Contact

Dennis Schroeder, NREL Image Gallery 19050
Technical Back-Up Slides
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Optimized Geometries Improve Specific Power

On track to meet DOE 2015 technical target

9 modules, DBC

Baseline

Optimized

Changes to channel 

height and length

FY13 – Go/No-Go 1
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Component Contributions to Inverter Weight

Other 

components 

need 

optimization
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• Meets Fan Test Standard

o ANSI/AMCA 210-07

o ANSI/ASHRAE 51-07

• Range: 5–500 CFM

• U95 Flow: ±1.5 CFM

• U95 Pressure: ±1.6 Pa

System Level Test Bench

Fan pressure curves Fan efficiency curves

Fan or duct 

test subject

Blower
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Air Outlet Temperature for Baseline Case Accomplishments

~100°C

~55 CFM

FY14
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Heat Dissipation for Optimized Case (9 modules) Accomplishments

Specific Power: 11.1 kW/kg

Power Density: 11.8 kW/L

Heat dissipation: 2.7 kW target

FY14

~70 CFM

70 W Fluid Power

Tout ~ 105°C 90°C 70°C

Similar thermal 

performance to baseline, 

but higher specific power 

and power density, and 

parasitic power