––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Direct Energy Conversion

Gang Chen

Mechanical Engineering DepartmentMassachusetts Institute of Technology

Office: Room 3-260Tel: 617-253-0006

Email: gchen2@mit.eduURL: http://web.mit.edu/nanoengineering

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Direct Thermal-to-Electric Energy Conversion Technologies

Thermoelectric Converter

Thermionic Converter

ThermophotovoltaicConverter

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Thermionic Power Generation

• Electron Distribution isf(E) ~ exp(-E/kBT)

• Ec, Ea are working functionsat cathode and anode

• Only electrons with energylarger than working function or barrier height can flow from one electrode to another

EXTERNAL LOAD

CATHODE ANODE

E E

Ec Ea

Tc Tae

EXTERNAL LOAD

CATHODE ANODE

E E

Ec Ea

Tc Tae

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Performance of Thermionic Converters

USSR TOPAZHatsopoulos and Kaye, JAP, 1958

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Challenges and Opportunities

• Space charge effects• Reliability• Low work function materials• Small gap devices• Field-emission enhancement

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

THERMOPHOTOVOLTAICSH

eat S

ourc

e

Phot

ovol

taic

Ce l

ls

Filte

r

• Frequency Selective Emitter• Frequency Selective Filters• Photon Recycling Structures• Evanescent Wave Structures• High Efficiency PV Cells

10-1

100

101

102

103

104

0 2 4 6 8 10

EMIS

SIVE

PO

WER

(W/c

m2 µm

)WAVELENGTH (µm)

5600 K

2800 K

1500 K

800 K

EG

UselessUseful

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Potential Performance

Badalsaro et al., JAP, 89, 3319 (2001)Experimentally Demonstrated ~ 18%

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Challenges and Opportunities

• Spectral control– Selective emitters– Selective reflectors– Selective filters

• High efficiency cells• Thermal management• Near-field devices

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Photonic Crystal Selective Emitter

Si substrate

Alternating layers of tungsten and alumina

A. Narayanaswamy and G. Chen, PRB 70,125101, 2004

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Near Field Energy Conversion

10-1

100

101

102

103

0 100 200 300

Pow

er a

bsor

bed

(Wcm

-2)

Vacuum gap (nm)

Power absorbed

Blackbody

0

4 107

8 107

1.2 108

0.14 0.145 0.15 0.155 0.16

88.258.58.759

Flux

(Wm

-2eV

-1)

Frequency (eV)

Wavelength (µm)

d = 0 nmd = 1 nm

d = 5 nm

d = 10 nmSource (BN, SiC) PV material

SiC

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Near-Field Effect on Efficiency

Laroche et al., JAP, 100, 063704 (2006)

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Radioisotope Powered Thermoelectric Generators

Voyager 2(1977)

Voyager 1(1977)

Radioisotope Missions

Pioneer 11(1973)

Cassini(1997)

Pioneer 10(1972)

Galileo(1989)

Viking 1 & 2 (1975)Mars Pathfinder (1996)

(RHU’s only)

Ulysses(1990)

Transit 4 A(1961)

Transit 4 B(1961)

Transit 5BN-1(1963)

Transit 5BN-2(1961)

Nimbus 3(1969)

TransitTriad-01-0X

(1972)

LES 8(1976)

LES 9(1975)

Apollo 11 (1969)Apollo ALSEP (1969-1972)

10 Earth orbit (Transit, Nimbus, LES)7 planetary (Pioneer, Voyager, Galileo, Ulysses, Cassini)6 on lunar surface (Apollo ALESEP)4 on Mars surface (Viking 1& 2) 3 RHUs on Mars Pathfinder

Voyager 2(1977)

Voyager 1(1977)

Radioisotope Missions

Pioneer 11(1973)

Cassini(1997)

Pioneer 10(1972)

Galileo(1989)

Viking 1 & 2 (1975)Mars Pathfinder (1996)

(RHU’s only)

Ulysses(1990)

Transit 4 A(1961)

Transit 4 B(1961)

Transit 5BN-1(1963)

Transit 5BN-2(1961)

Nimbus 3(1969)

TransitTriad-01-0X

(1972)

LES 8(1976)

LES 9(1975)

Apollo 11 (1969)Apollo ALSEP (1969-1972)

Voyager 2(1977)

Voyager 1(1977)

Radioisotope Missions

Pioneer 11(1973)

Cassini(1997)

Pioneer 10(1972)

Galileo(1989)

Viking 1 & 2 (1975)Mars Pathfinder (1996)

(RHU’s only)

Ulysses(1990)

Transit 4 A(1961)

Transit 4 B(1961)

Transit 5BN-1(1963)

Transit 5BN-2(1961)

Nimbus 3(1969)

TransitTriad-01-0X

(1972)

LES 8(1976)

LES 9(1975)

Apollo 11 (1969)Apollo ALSEP (1969-1972)

10 Earth orbit (Transit, Nimbus, LES)7 planetary (Pioneer, Voyager, Galileo, Ulysses, Cassini)6 on lunar surface (Apollo ALESEP)4 on Mars surface (Viking 1& 2) 3 RHUs on Mars Pathfinder

GPHS Radioisotope Thermoelectric Generator

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Thermoelectric Power Generation

- +

I N P

I I

COLD SIDE

HOT SIDE

Figure of Merit:

kTSZT

2σ=

Thermal Conductivity

ElectricalConductivity

SeebeckCoefficient

COLD SIDE

HOT SIDE

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

ZT DILEMMA

S

k

σZT

INSULATOR

SEMICONDUCTOR

SEMIMETAL

METAL

kTSZT

2σ=

Wanted:Phonon Glass / Electron Crystal

Methods of Reducing kIn Bulk Materials:

• Alloy, 1950s (Ioffe)• Rattlers, 1990 (Slack)

T vacant

square array of Pn(not to scale)

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

State-of-the-Art in Thermoelectrics

0.0

0.5

1.0

1.5

2.0

2.5

3.0

1940 1960 1980 2000 2020

FIG

UR

E O

F M

ERIT

(ZT)

max

YEAR

Bi2Te3 alloy

PbTe alloy

Si0.8Ge0.2 alloy

Skutterudites(Fleurial)

PbSeTe/PbTeQuantum-dotSuperlattices(Lincoln Lab)

Bi2Te3/Se2Te3Superlattices(RTI)

AgPbmSbTe2+m(Kanatzadis)

Dresselhaus

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Nanocomposites Approach

– Increase interfacial scattering by mixing nano-sized particles.

– Enable low-cost, large scale application.

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Nanocomposite Synthesis

50 nm

Si Ge

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Electron Transport Over Potential Barriers

5 nm

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Thermal Conductivity of Si0.8Ge0.2

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Challenges and Opportunities

• Further improving ZT• System and device developments

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Comparison of Technologies

THERMALPOWERPLANT

AUTOMOTIVE ENGINES

0

0.1

0.2

0.3

0.4

0.5

0.6

1 10

POW

ER G

ENER

ATI

ON

EFF

ICIE

NC

Y

TEMPERATURE RATIO (T hot /T cold )2 3 4 5 6 7 8 9

CARNOT CYCLE 10

7

4

2

1

0.5

ZTm

THERMOELECTRICPOWER GENERATORS

STIRLINGGENERATOR

THERMIONICGENERATORS

THERMALPOWERPLANT

AUTOMOTIVE ENGINES

THERMALPOWERPLANT

0

0.1

0.2

0.3

0.4

0.5

0.6

1 100

0.1

0.2

0.3

0.4

0.5

0.6

0

0.1

0.2

0.3

0.4

0.5

0.6

1 101 10TEMPERATURE RATIO (T hot /T cold )

2 3 4 5 6 7 8 9

CARNOT CYCLE 10

7

4

2

1

0.5

ZTm

STIRLINGGENERATOR

ThermoelectricConverter

IC Engine

ThermionicConverter

DieselPlant

PowerPlant

TPV

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

Potential Applications in Nuclear Power Generation

• In combination with mechanical power generation

• Combinations of direct conversion technologies for high efficiency

––WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MITWARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT

ACKNOWLEDGMENTS• Current Members

H. Asegun (Molecular Dynamics)V. Berube (hydrogen storage)J.W. Gao (nanofluids)S. Goh (nanowires and polymers)T. Harris (Thermoelectrics&Nanomaterials)Q. Hao (Thermoelectrics)D. Kramer (Solar thermoelectrics)H. Lee (Thermoelectric Materials)H. Lu (TPV and PV)A. Minnich (thermoelectrics)A. Muto (nanowires and thermoelectrics)A. Schmidt (ps pump-and-probe)S. Shen (near field transfer)Dr. M. Chieso (nanofluids)Dr. X. Chen (optics, Pump-and-Probe)

• Collaborators (partial list)M.S. & G. Dresselhaus (MIT, NW&CNT, Theory)J.-P. Fleurial (JPL, Thermoelectric Devices) J. Joannopoulos (MIT, Photonic Crystals)Z.F. Ren (BC, Thermoelectric Materials, CNT)X. Zhang (Berkeley, Metamaterials)

• Past Members (Partial List)Prof. A. Narayanaswamy (Columbia Univ)Dr. Zony Chen (McKinsey)Prof. C. Dames (Nanowires, UC Riverside)Prof. D. Borca-Tasciuc (Nanowires, RPI)Prof. T. Borca-Tasciuc (Thermoelectrics,RPI)Dr. F. Hashemi (Nano-Device Fabrication)Dr. A. Jacquot (TE Device Fabrication)Dr. M.S. Jeng (Nanocomposites, ITRI)Dr. R. Kumar (Thermoelectric Device Modeling)Dr. W.L. Liu (superlattice)Dr. D. Song (TE and Monte Carlo, Intel)Dr. S.G. Volz (MD, Ecole Centrale de Paris)Prof. B. Yang (TE and Phonons, U. Maryland)Prof. R.G. Yang (Nanocomposites, U. Colorado)Prof. D.-J. Yao (TE Devices, Tsinghua Univ.)Prof. T. Zeng (Thermionics, NCSU)

Sponsors: DTRA, DOE, NASA, NSF, ONR, Ford, Seagate, and others