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.

Light-Weight, Low-Cost, Single-Phase, 

Liquid-Cooled Cold Plate

This presentation does not contain any proprietary or confidential information

Sreekant Narumanchi

National Renewable Energy Laboratory

Email: sreekant.narumanchi@nrel.gov

Phone: 303-275-4062

Team Members: Mark Mihalic, Gilbert Moreno, 

and Kevin Bennion (NREL)

UQM, Wolverine Tube Inc.

Project Duration: FY 11 to FY 12

DOE Vehicle Technologies Program 

Advanced Power Electronics and Electric Motors R&D 

FY12 Kickoff Meeting

Oak Ridge National Laboratory

Oak Ridge, Tennessee

November 3, 2011

NREL/PR-5400-52808
 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

The Problem

• Directly addresses program goals 

of increased power density, specific 

power, and lower cost of power 

electronics components through 

improved thermal management.

• Enabler to using high-temperature 

water-ethylene glycol (WEG) 

mixture as coolant.

2

Courtesy: Electrical and Electronics Technical Team Roadmap, 2010

http://www1.eere.energy.gov/vehiclesandfuels/pdfs/program/eett_roadmap_12-7-10.pdf
 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

Project Overview

Objective

• Demonstrate prototype inverter heat exchanger of lighter weight and higher 

thermal performance than the baseline heat exchanger; demonstrate 

potential for reducing cost.

Research Focus Area: Thermal Management

 Heat Transfer Technologies, Thermal Systems Integration

Addresses Targets

• Increased power density, specific power, and lower cost.

• Use of higher-temperature coolant.

Uniqueness and Impacts

• Combination of microfinned surfaces, single-phase liquid jets, light-weight 

and low-cost plastic manifold demonstrated on a commercial inverter (UQM) 

platform. 

• Reliability assessment also performed.

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 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

Description of Technology/Approach

Heat exchanger 

MicroCool Surface 

(Wolverine)

• Reduce thermal resistance, increase heat transfer rates through impingement on 

enhanced surfaces, and use light-weight, low-cost plastic fluid manifold.

– Demonstrate increased power density, specific power, and lower cost.

• Characterize thermal performance based on steady-state and transient/realistic 

loading conditions (dynamometer testing).

Glycol-water jets

Credit: Sreekant Narumanchi, NREL

Credit: Mark Mihalic, NREL

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 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

Heat Exchanger Design

• A jet plate designed using computational fluid 

dynamics (CFD) with WEG coolant (50–50% 

mixture by volume).

• A glass-fiber reinforced nylon plastic material 

envisioned for the bulk of the fluid manifold 

(replacing aluminum).

• Microfinned structures on the copper base plate on 

which the jets impinge.

Credit: Sreekant Narumanchi, NREL

5
 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

Validation of CFD Modeling for Baseline Heat 

Exchanger

• 105.3 W dissipated in four diodes in the center 

power module.

• WEG mixture (50–50% by volume) at 70°C 

used as coolant. 

• Junction temperature in the four diodes 

measured using the transient thermal tester 

during the steady-state heating.

• Reasonable match between modeling and 

experimental results. 

Flow rate

(m3/s) 

Pressure drop (Pa) Average 

temperature in 

diodes (oC) 

Experiment CFD Experiment CFD

0.336e-04 1,076 1,079 84.9 84.0 

1.667e-04 19,374 16,988 82.8 82.9 

Credit: Sreekant Narumanchi, NREL

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 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

CFD Modeling Results for New Heat Exchanger

• Design case with 2.5-kW heat dissipation spread over 24 insulated gate 

bipolar transistor (IGBTs) (72 W in each IGBT) and 24 diodes (33 W in 

each diode).

• WEG at 70°C as coolant, thermal resistance based on 4 diodes in the 

center module: Rth, ja = (T4diodes, avg – Tcoolant)/q4diodes.

• For similar parasitic power as the baseline

- Jets on plain surface yield 34% lower thermal resistance

- Jets on enhanced surface yield 44% lower thermal resistance compared to 

baseline.

Parameter Baseline 

(channel flow)

Jets impinging

on plain surface

Jets impinging on 

microfinned 

surface 

∆P (Pa) 16,858 17,458 17,458 

Rth, ja (K/W) 0.294 0.191 0.166 

Isothermality (0C) 2.7 0.3 0.15 

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 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

Impacts on Coefficient of Performance (COP), 

Power Density, Specific Power and Cost

• Up to 71% increase in COP.

• Up to 79% increase in power density. 

• Using plastic results in approximately 2.9 kg (6.3 lb) or 50% weight 

reduction of the heat exchanger

- Results in up to 118% increase in specific power.

• Cost likely to be competitive with respect to aluminum baseline heat 

exchanger.

8

0

20

40

60

80

100

120

140

Jets on plain

surface

Jets on

enhanced

surface

COP

Power density

Specific power

Percentage 

increase 
 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

Experimental Reliability Assessment of Jet 

Impingement 

Credit: Gilbert Moreno, 

NREL (all photos)

• Tests initiated with WEG impinging on 

microfinned surface (on 12.5-mmdiameter copper target surface).

• Evidence of oxidation on surfaces (no 

initial coating/plating on the surface).

t=0 t= 30 days t= 60 days

Sample W1 

(7 m/s jet 

velocity)

Sample W2 

(2 m/s jet 

velocity)

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t= 90 days
 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

Experimental Reliability Assessment of Jet Impingement

• Change in thermal performance of the surfaces 

over time (90 days of impingement) within the 

95% confidence intervals (CI).

Error bars: ± t s / √n

t = student t-distribution (95% CI)

s = sample standard deviation

n = number of tests (3-4 typical)

 

10

0

40,000

80,000

120,000

160,000

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0

h (W/m2-K)

U (m/s)

Sample W1

t=0

t=30 days

t=60 days

t=90 days

0

40,000

80,000

120,000

160,000

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0

h (W/m2-K)

U (m/s)

Sample W2

t=0

t=30 days

t=60 days

t=90 days
 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

28.4146

29.0489

28.4115

29.0447

28.4101

29.0437

28.0

28.3

28.5

28.8

29.0

29.3

29.5

1 2

Mass (grams)

MicroCool Samples

t=0

t=30 days

t=60 days

Sample Mass

Experimental Reliability Assessment of Jet Impingement

• Negligible change in mass of the enhanced 

surface samples and the nozzles after 60 days 

of nearly continuous impingement.

• Negligible change in jet nozzle diameter after 

90 days of nearly continuous impingement.

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1.41 1.37 1.40 1.38 1.39 1.36 1.38 1.39

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

1 2

Diameter (mm)

Nozzle 

Nozzle inside orifice diameter

t=0

t=30 days

t=60 days

t= 90 days

7.2790

7.2720

7.2791

7.2716

7.2791

7.2716

7.25

7.26

7.27

7.28

7.29

7.30

1 2

Mass (grams)

Nozzle 

Nozzle mass

t=0 

t=30 days

t=60 days
 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting 12

FY12 Approach and Challenges

2011

Oct Nov Dec

2012

Jan Feb Mar Apr May Jun Jul Aug Sep

Complete 

design

Fabrication of first 

prototype (no surface 

enhancements).

Go/No Go Decision: Assess whether performance improvements from the first prototype (jets on 

plain surface) merits fabrication of second prototype (jets in conjunction with 

enhanced surfaces).

Challenges/Barriers: 

• Various aspects of reliability of the system have to be proven.

• Performance improvements specific to this inverter configuration.

• Inferring cost benefits for a large number of units is a challenge.

Annual/

milestone 

Report

Continue reliability assessment of impinging jets 

• Impact of impingement on surface enhancement.

• Impact of impingement on DBC/DBA and on coatings/metallization.

Thermal 

performance 

characterization 

(T3ster and 

dynamometer) Fabrication of second 

prototype (with surface 

enhancement)

Thermal performance 

characterization (T3ster and 

dynamometer) 

First 

prototype 

heat 

exchanger

Second 

prototype 

heat 

exchanger

Go / 

No Go 

Decision 

Point
 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

FY12 Approach Highlights

• Two prototype heat exchangers will be fabricated and 

performance demonstrated. 

• Modeling results indicate up to 71% increase in COP, 79% 

increase in inverter power density and 118% increase in 

specific power as compared to the baseline

- Novel impingement configuration which also eliminates thermal 

grease from the system.

• Cost likely to be competitive with respect to the baseline 

heat exchanger. 

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 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

FY12 Approach Highlights

• Results for reliability of the system will be obtained

- Initial results suggest thermal performance of the enhanced 

surfaces do not degrade after three months of continuous 

impingement.

• Dynamometer testing of the inverter with the baseline and 

enhanced heat exchangers.

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 NATIONAL RENEWABLE ENERGY LABORATORY DOE APEEM FY12 Kickoff Meeting

Beyond FY12 

• Project in current form ends in FY12.

• FY13-FY15

– Low-cost, light-weight, reliable, high-thermal performance concepts 

could be proposed with single-phase liquid flow configurations for 

alternative power electronics packaging configurations.

– With the constant trend towards reducing the footprint of 

components, concepts related to miniaturization of cold plates will 

be investigated.

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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. 16

For more information contact:

Principal Investigator

Sreekant Narumanchi

sreekant.narumanchi@nrel.gov

Phone: (303)-275-4062

APEEM Task Leader

F. Patrick McCluskey

Patrick.McCluskey@nrel.gov

Phone: (303)-275-3653

Acknowledgments: 

Susan Rogers and Steven Boyd, 

U.S. Department of Energy

Team Members:

Mark Mihalic

Gilbert Moreno

Kevin Bennion (NREL)

UQM

Wolverine Tube Inc.