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.

Two-Phase Cooling of Power 

Electronics 

Principal investigator.: Gilbert Moreno

National Renewable Energy Laboratory

2014 Annual Merit Review

Washington, D.C.

June 17, 2014

NREL/PR-5400-61359 Project ID: APE037

This presentation does not contain any proprietary, confidential, or otherwise restricted information.
2

Overview

• Project Start Date: FY11

• Project End Date: FY14

• Percent Complete: 90%

• Barriers addressed: 

– weight

– cost

– efficiency

• Total Project Funding: 

$1,960K

– DOE Share: $1,960K

• Funding Received in FY13: 

$400K

• Funding for FY14: $460K

Timeline

Budget

Barriers

• Delphi 

• 3M 

• DuPont 

• University of Colorado-Boulder

• Iowa State University 

• University of Illinois-Chicago

• Project lead: National Renewable Energy 

Laboratory

Partners
3

Relevance

Objective

• Significantly improve thermal management of automotive power electronics 

by utilizing the high heat transfer rates of two-phase cooling.

o Design and build a passive two-phase cooling system for automotive power 

modules (cool six Delphi discrete power switches). Demonstrate that the 

system dissipates automotive heat loads and provides superior thermal 

performance.

o Quantify key system metrics (thermal resistance, coefficient of 

performance, volume, weight) and compare with conventional cooling 

systems.

Motivation

• Accelerate the adoption of electric-drive vehicles through improved thermal 

management.

o Reduce power electronics cost and increase power density, specific power, 

and efficiency.
4

Milestones

Month / Year Milestone or 

Go/No-Go Decision

September 2013 Go/No-Go Decision: Demonstrated that the passive two-phase cooling system can 

dissipate at least 3.5 kW of heat with only 250 mL of refrigerant.

• 3.5 kW of heat is an estimate of the maximum power electronics heat output for a 

55 kW inverter.

November 2013 Milestone: Experimentally measured the condenser thermal resistance and evaluated

methods to enhance condensation heat transfer.

• Conducted analysis to estimate the required condenser size at various refrigerant 

temperatures.

January 2014 Milestone: Conducted analysis on an advanced evaporator design. Modeling predicted 

advanced evaporator can reduce thermal resistance by 58% to 65% compared with the 

state-of-the-art automotive cooling systems.

April 2014 Milestone: Experimentally characterized the thermal resistance of the advanced 

evaporator design. 

• Bonded a Delphi power module to the evaporator using a thermoplastic interface 

and measured the junction-to-liquid thermal resistance. 

June 2014 Milestone: Evaluate the effect of inclination/orientation on the two-phase cooling 

system thermal performance.
5

Approach/Strategy

Fundamental 

Research

Module-Level 

Research

Inverter-Scale 

Demonstration

Characterized performance of 

HFO-1234yf and HFC-245fa.

Achieved heat transfer rates of up 

to ~200,000 W/m2-K.

Reduced thermal resistance by 

over 60% using immersion twophase cooling of a power module.

Quantified refrigerant volume 

requirements.

Dissipated 3.5 kW of heat with only 

250 mL of refrigerant.

Predicted 58%-65% reduction in 

thermal resistance via indirect and 

passive two-phase cooling.

Photo Credit: Gilbert Moreno, NREL Photo Credit: Gilbert Moreno, NREL

Photo Credit: Bobby To, NREL
6

Approach/Strategy

Impacts

The high heat transfer rates and isothermal 

characteristics of two-phase cooling can:

• Enable cost reductions (decrease 

semiconductor device size/count) 

through increased power density.

• Increase efficiency through a passive (no 

pumping requirement) two-phase 

cooling approach.

Uniqueness

• Two-phase cooling (although used to cool vehicle cabin) is not used to cool power 

electronics in vehicles.

• Use of HFO-1234yf (new, environmentally friendly refrigerant) to cool electronic 

devices.

q′′ (W/cm2)

∆T (°C)

1 10 1,000 100

100

1

0.01

0.1

1,000

10

10,000

Adapted from: I. Mudawar, et al., 2008, "Two-Phase Spray Cooling of Hybrid Vehicle Electronics,"
7

Approach/Strategy

Passive Two-Phase Cooling Concept/Strategy

• Ease of implementation via an indirect-cooling 

approach (devices not in contact with refrigerant).

• Increased efficiency through a passive (no pump or 

compressor) two-phase cooling approach. 

• Use of automotive air-conditioning refrigerant (HFO1234yf) and a lower pressure alternative (HFC245fa).

• Evaluate liquid-cooled condenser to decrease 

condenser size. Liquid-cooled condenser may allow 

use of waste heat for cabin heating (integrated 

thermal management).
8

Technical Accomplishments and Progress

Designed and Fabricated a Proof-of-Concept Two-Phase Cooling System

• Surface area: air-side: ~2.9 m2, 

condensation-side: 0.12 m2

• 17.8-cm (7-in.) diameter fan: 

total parasitic power: 38 W, 

~250 cfm

• Interchangeable cold plate design

• Designed to cool one half bridge per cold plate

finned-tube heat exchanger

evaporator crosssectional view

Condenser

Evaporator

• Designed to cool three half bridges 

(inverter-scale). Can accommodate six 

Delphi power modules.

• Refrigerants: HFO-1234yf and HFC245fa 

• Refrigerant charge: 250 mL

• Maximum operating pressure: 1 MPa 

(150 psig).

25.4 cm (10 in.)

20.5 cm 

(8.1 in.)

4.5 cm 

(1.75 in.)

25.4 cm (10 in.)

 Ø 6.98 cm 

(2.75 in.) Tl

T v,

P v

Ta, inlet

Ta, outlet 

Thtr

Photo Credit: Gilbert Moreno, NREL
9

Technical Accomplishments and Progress

RRtt = 

T�heeeeee− Tllllll

HHH dddddd d

× TTTTT he ee

25 mm

15 mm

Condenser thermal resistance:

Evaporator thermal resistance:

•Initial test conducted using six 

ceramic heaters that were attached 

to evaporator using thermal grease.

• Six heaters represent six power 

switches (inverter-scale).

• Maximum power output from the 

six heaters is 3.5 kW.

T air, outlet

T vapor

P vapor

T air, inlet

T liquid

T heaters Heater 

power

Fan 

power

RRtt = 

Tv vv −T� i

HHH dddddd d

× C CC CC f

Testing Procedures

Photo Credit: Gilbert Moreno, NREL
10

0.00

0.25

0.50

0.75

1.00

0 1,000 2,000 3,000 4,000

R"th (cm

2-K / W)

Heat Dissipated (W)

 HFC-245fa

Technical Accomplishments and Progress

Evaporator Thermal Resistance 

HFC-245fa (250 mL, 330 grams)

• Demonstrated that the passive twophase cooling system can dissipate at 

least 3.5 kW of heat (steady-state) 

with only 250 mL HFC-245fa.

o Significance: 3.5-kW estimate of the 

maximum heat dissipated by a 55-kW 

inverter.

• Heat dissipation can be increased 

beyond 3.5 kW using microporous 

(boiling enhancement) coatings. 

Previous fundamental research 

demonstrated that 3M microporous 

coating can increase critical heat flux 

by 50%.

HFC-245fa: Tsat = 75°C

Psat = 0.7 MPa (106 psia)

Noncoated/plain evaporator
11

0.00

0.25

0.50

0.75

1.00

0 1,000 2,000 3,000 4,000

R"th (cm

2-K / W)

Heat Dissipated (W)

 HFC-245fa

 HFO-1234yf

Evaporator Thermal Resistance 

HFO-1234yf (250 mL, 280 grams)

• Prototype limited heat dissipation to 

1.3 kW with HFO-1234yf because of 

higher pressures. Expect we could 

dissipate more heat by increasing 

system pressure capacity.

• HFO-1234yf’s higher heat transfer 

coefficients resulted in lower 

evaporator thermal resistance.

Noncoated/plain evaporator

HFO-1234yf: Tsat = 42°C

Psat = 1.1 MPa (154 psia)

Technical Accomplishments and Progress
12

Technical Accomplishments and Progress

Identified techniques to increase performance 

and reduce the size of the evaporator (record 

of invention submitted):

• Designed to cool six Delphi power modules.

• Increased evaporation surface area to 

improve thermal performance. 

• Fabricated from low-cost materials 

(aluminum) and low-cost manufacturing 

techniques.

• Reduced refrigerant requirements to 180 mL, 

(HFO-1234yf = 200 g, HFC-245fa = 240 g).

o Comparison: 2010 Toyota Camry air conditioning 

system uses 510 grams of R-134a (source: 

autozone.com).

17.8 cm (7 in.)

7 cm (2.75 in.)

2.54 cm 

(1 in.)

Delphi power module: 

Image courtesy of Delphi

Advanced Evaporator Design
13

Technical Accomplishments and Progress

R”th

(mm2- K/W)

% R”th

Reduction

Lexus Hybrid (2008) double-side

cooled modules 33.2* -

Two-phase: aluminum evaporator 

(finite element analysis results) 13.9 58%

Two-phase: copper evaporator

(finite element analysis results) 11.5 65%

based on die 

footprint

Indirect twophase cooling 

(aluminum 

evaporator)

* Calculated using one side of the die (12.78 mm × 12.78 mm)

* Performance from: Sakai, Y., and Ishiyama, H., 2007, “Power Control Unit for High Power Hybrid System,” SAE International

Double-side 

cooled inverter

Indirect twophase cooling 

(copper 

evaporator)

Conducted Finite Element Analysis to Estimate Performance of 

Advanced Evaporator Design 

• Reduced thermal resistance by 58% to 65% 

with the advanced evaporator design compared 

with state-of-the-art cooling system.

• Enhancements enable increased device power 

densities.

Image courtesy of Oak Ridge National Lab

2.54 cm 

(1 in.)

7 cm (2.75 in.)

Length: 

17.8 cm (7 in.) 

Power 

devices

2.54 cm 

(1 in.)

7 cm (2.75 in.)

Length: 

17.8 cm (7 in.) 

Power 

devices
14

Technical Accomplishments and Progress

Refrigerant 

R”th (cm2- K/W)

Plain Tubes Rifled Tubes

Reduced R”th

with Rifled 

Tubes

HFC-245fa 9.30 7.58 18%

HFO-1234yf 8.12 6.06 25%

Rifled Tube

• Evaluated two condenser designs: plain tube condenser and rifled tube condenser.

• Increased condensation heat transfer coefficients using rifled tubes. Rifled tube 

enhancements decreased thermal resistance by 18% for HFC-245fa and by 25% for HFO1234yf. Decreasing the condenser thermal resistance allows for a more compact 

condenser.

• HFO-1234yf provided higher condensation heat transfer coefficients compared with

HFC-245fa. HFO-1234yf’s higher heat transfer resulted in 13% to 20% lower condenser 

thermal resistance compared with HFC-245fa.

RRtt = 

Tv vv −T� i

HHH dddddd d

× C CC CC f

Characterized and Enhanced Condenser Thermal Performance

Photo Credit: Gilbert Moreno, NREL
15

0

500

1,000

1,500

2,000

50 60 70 80 90 100

Condenser Frontal Area (cm

2)

System Liquid/Vapor Temperature (°C)

 HFC-245fa

 HFO-1234yf

Technical Accomplishments and Progress

• Estimated condenser frontal area/size 

requirements at various refrigerant 

temperatures for the following operating 

condition:

o 3.3 kW of total heat 

(estimated heat dissipation from a 95% 

efficient, 55-kW inverter), 43°C inlet air.

•Higher refrigerant temperatures enable 

a more compact condenser.

•HFO-1234yf’s higher performance allows 

for a smaller condenser.

C CC CC f

= RRtt ×

H d dd

Tv vv − T� ii

Air-Cooled Condenser Size Requirements (Finned-Tube Condenser)

Condenser depth = 4.4 cm (1.75 in.)
16

Technical Accomplishments and Progress

• Estimated the size requirements for a 

brazed folded-fin condenser  matched 

air-side surface area (assumed air-side was 

the dominant thermal resistance).

• Greater air-side surface area of the foldedfin design allows for a more compact 

condenser.

• Condenser results are consistent with those 

reported by Barnes and Tuma (2009).

Conducted Analysis to Estimate Size Requirements for Folded-Fin Condenser

2.54 cm 2.54 cm

~36% more air-side surface 

area for folded-fin 

condenser

Finned-tube Folded-fin 

0

500

1,000

1,500

2,000

50 60 70 80 90 100

Condenser Frontal Area (cm

2)

System Liquid/Vapor Temperature (°C)

finned tube

folded-fin

HFC-245fa

HFO-1234yf

Barnes and Tuma (2009)

Condenser depth = 4.4 cm (1.75 in.)

Conditions: 3.3 kW 

of total heat, 43°C 

inlet air

Photo Credit: Gilbert Moreno, NREL
17

0

25

50

75

100

125

150

175

200

0

500

1,000

1,500

2,000

50 60 70 80 90 100

Tjunction (°C)

Condenser Frontal Area (cm

2)

System Liquid/Vapor Temperature (°C)

Tjunction

folded-fin

HFC-245fa

HFO-1234yf

Technical Accomplishments and Progress

• For lower system temperature 

applications typical of 

automotive power electronic 

coolant temperatures (70°C), 

HFO-1234yf is recommended 

because its greater performance 

allows for a smaller condenser.

• With HFO-1234yf at 70°C, the 

required condenser frontal area 

is ~470 cm2.

• At 70°C, HFO-1234yf pressure is 

~300 psia.

• Estimated junction temperature 

using advanced evaporator 

design and six Delphi power 

module configuration is 125°C.

Air-Cooled Condenser Size Requirements (Folded-Fin Condenser)

Conditions: 3.3 kW 

of total heat, 43°C 

inlet air

Condenser depth = 4.4 cm (1.75 in.)
18

0

25

50

75

100

125

150

175

200

0

500

1,000

1,500

2,000

50 60 70 80 90 100

Tjunction (°C)

Condenser Frontal Area (cm

2)

System Liquid/Vapor Temperature (°C)

Tjunction

folded-fin

HFC-245fa

HFO-1234yf

Technical Accomplishments and Progress

Air-Cooled Condenser Size Requirements (Folded-Fin Condenser)

Conditions: 3.3 kW 

of total heat, 43°C 

inlet air

• If higher temperature operation 

is allowed or required (i.e., wideband-gap devices), HFC-245fa 

may be a better choice because 

of its higher critical temperature 

and lower pressure.

• With HFC-245fa at 95°C, the 

required condenser frontal area 

is reduced to ~310 cm2.

• At 95°C, HFC-245fa pressure is 

~164 psia.

• Estimated junction temperature 

using advanced evaporator 

design and six Delphi power 

modules configuration is 150°C.

Condenser depth = 4.4 cm (1.75 in.)
19

Responses to Previous Year Reviewers’ Comments

Reviewer Comments/Questions: 

“The reviewer pointed out that this concept has some special requirements, such as condenser 

location above the evaporator and in a location with adequate air flow for the fan. The reviewer 

cautioned that these physical requirements could cause interference with other components and 

increase the space required.”

“The reviewer opined that the approach is novel and the results do indicate that it is technically 

capable of cooling the switches. The reviewer recommended that more involvement with the overall 

packaging of the inverter and the cooling system is needed to provide a solution.”

“The reviewer said that resources have been sufficient to date as the results are impressive, but that 

vehicle level integration support is needed to move the project to the next level.”

Response: We agree with the reviewer comments/suggestions and are addressing them in the following 

manner:

• Developing alternative variations to the condenser to reduce cooling system size and packaging 

constraints. 

• Decreased the size and increased the performance of the evaporator.

• Meeting with industry OEMs and inverter suppliers to better understand packaging requirements 

and to develop collaborations to build a two-phase cooled inverter.
20

Collaboration and Coordination with Other Institutions

Industry Partners

• Delphi (supplied power modules)

• 3M Electronics Markets Materials Division (supplied boiling 

enhancement coating)

• DuPont (supplied HFO-1234yf refrigerant)

University Partners

• University of Colorado–Boulder (graduate student)

• Iowa State University (provided enhanced surfaces)

• University of Illinois–Chicago (provided enhanced surfaces)
21

Remaining Challenges and Barriers

Challenges and Barriers:

• Two-phase is not used to cool automotive power electronics. There are reliability and 

cost concerns associated with the technology.

• Packaging of the cooling system presents a challenge.

Strategy: 

• Propose a simple two-phase cooling solutions (indirect cooling, passive, low-cost 

materials, minimize refrigerant requirements).

• Propose solution with pressures equivalent or lower than those used in vehicle airconditioning systems.

• We are working to develop industry partnerships to demonstrate a two-phase cooled 

inverter system.

• Two-phase cooling technology is used in the vehicle air-conditioning systems. 

Technology is available to develop reliable refrigerant-based cooling systems. 
22

Proposed Future Work

FY14 

• Bond a Delphi power module to the advanced evaporator using a 

thermoplastic bonded interface. 

• Experimentally characterize the junction-to-liquid thermal 

resistance. 

• Evaluate the effect of cooling system inclination/orientation on 

thermal performance.

• Experimentally quantify key system metrics (thermal resistance, 

coefficient of performance, volume, weight) and compare against 

conventional cooling systems.

• Develop industry partnerships to demonstrate a two-phase cooled 

inverter system.
23

Summary

DOE Mission Support

• Accelerate the adoption of electric-drive vehicles through improved thermal 

management. 

• Support the President’s EV Everywhere Challenge.

Approach

• Utilize the high heat transfer rates of two-phase cooling to improve performance.

• Demonstrate a passive, two-phase cooling solution for automotive power electronics.

Accomplishments

• Demonstrated that an inverter-scale passive two-phase cooling system can dissipate at 

least 3.5 kW of heat (steady-state) with only 250 mL HFC-245fa.

• Improved evaporator design (advanced evaporator concept) to increase performance, 

reduce size, and reduce refrigerant requirements.

• Reduced junction-to-fluid resistance by 58% to 65% with an advanced evaporator design. 

Enhancements are relative to state-of-the-art cooling system.

• Conducted an analysis to estimate required condenser size at various refrigerant 

temperatures.
24

Summary

Future work

• Bond a Delphi power module to the advanced evaporator using a thermoplastic 

bonded interface. 

• Experimentally characterize the junction-to-liquid thermal resistance. 

• Evaluate the effect of cooling system inclination/orientation on thermal performance.

• Experimentally quantify key system metrics (thermal resistance, coefficient of 

performance, volume, weight) and compare against conventional cooling systems.

• Develop industry partnerships to demonstrate a two-phase cooled inverter system.

Collaborations

• Delphi 

• 3M 

• DuPont

• University of Colorado–Boulder

• Iowa State University

• University of Illinois–Chicago
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.

For more information contact:

Principal Investigator

Gilbert Moreno

Gilbert.Moreno@nrel.gov

Phone: (303) 275-4450

APEEM Task Leader

Sreekant Narumanchi

Sreekant.Narumanchi@nrel.gov

Phone: (303) 275-4062

Acknowledgment:

Susan Rogers and Steven Boyd, 

U.S. Department of Energy

Team Members:

Jana Jeffers

Charlie King