Direct Energy Conversion
Gang Chen
Mechanical Engineering Department
Massachusetts Institute of Technology
Office: Room 3-260
Tel: [phone]
Email: [e-mail]
URL: http://web.mit.edu/nanoengineering
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
Direct Thermal-to-Electric
Energy Conversion Technologies
Thermionic Converter
Thermoelectric Converter
Thermophotovoltaic
Converter
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
Thermionic Power Generation
" Electron Distribution is
EXTERNAL LOAD
EXTERNAL LOAD
Ta
Ta f(E) ~ exp(-E/kBT)
Tc
Tc
e
e
" Ec, Ea are working functions
CATHODE
CATHODE
ANODE
ANODE
at cathode and anode
E E
E E
" Only electrons with energy
Ec
Ec
larger than working function
Ea
Ea
or barrier height can flow
from one electrode to another
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
Performance of Thermionic Converters
Hatsopoulos and Kaye, JAP, 1958
USSR TOPAZ
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN 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, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
THERMOPHOTOVOLTAICS
104
Useful
Useless
103
5600 K
102
2800 K
101
1500 K
100
800 K
10-1
02468 10
EG WAVELENGTH (�m)
" Frequency Selective Emitter
" Frequency Selective Filters
" Photon Recycling Structures
" Evanescent Wave Structures
" High Efficiency PV Cells
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
2
Filter
Heat Source
Photovoltaic Cells
EMISSIVE POWER (W/cm
�
m)
Potential Performance
Badalsaro et al., JAP, 89, 3319 (2001)
Experimentally Demonstrated ~ 18%
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN 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, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
Photonic Crystal Selective Emitter
Alternating layers of
tungsten and alumina
Si substrate
A. Narayanaswamy and G. Chen, PRB 70,125101, 2004
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
Near Field Energy Conversion
Wavelength (�m)
9 8.75 8.5 8.25 8
1.2 108
d = 5 nm
d = 1 nm
SiC
8 107
d = 0 nm
Source (BN, SiC) PV material
d = 10 nm
103
4 107
102
Power absorbed
Blackbody
101
0
100
0.14 0.145 0.15 0.155 0.16
Frequency (eV)
10-1
0100 200 300
Vacuum gap (nm)
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
-2
-1
-2
Flux (Wm eV )
Power absorbed (Wcm )
Near-Field Effect on Efficiency
Laroche et al., JAP, 100, 063704 (2006)
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
Radioisotope Powered
Thermoelectric Generators
10 Earth orbit (Transit, Nimbus, LES)
10 Earth orbit (Transit, Nimbus, LES)
Voyager 2
7 planetary (Pioneer, Voyager, Galileo, Ulysses, Cassini) Voyager 2
7 planetary (Pioneer, Voyager, Galileo, Ulysses, Cassini) Voyager 2
(1977)
(1977)
(1977)
6 on lunar surface (Apollo ALESEP)
6 on lunar surface (Apollo ALESEP)
Radioisotope Missions
Radioisotope Missions
Radioisotope Missions
4 on Mars surface (Viking 1& 2)
4 on Mars surface (Viking 1& 2)
3 RHUs on Mars Pathfinder
3 RHUs on Mars Pathfinder
Voyager 1
Voyager 1
Voyager 1
(1977)
(1977)
(1977)
Ulysses
Ulysses
Ulysses
Apollo 11 (1969) Pioneer 11
Apollo 11 (1969) Pioneer 11
Apollo 11 (1969) Pioneer 11
(1990)
(1990)
(1990)
Apollo ALSEP (1969-1972) (1973)
Apollo ALSEP (1969-1972) (1973)
Apollo ALSEP (1969-1972) (1973)
Cassini
Cassini
Cassini
(1997)
(1997)
(1997)
Transit 4 A
Transit 4 A
Transit 4 A
(1961)
(1961)
(1961)
Transit 4 B LES 9
Transit 4 B LES 9
Transit 4 B LES 9
(1961) (1975)
(1961) (1975)
(1961) (1975)
Transit 5BN-1 LES 8
Transit 5BN-1 LES 8
Transit 5BN-1 LES 8
(1963) (1976)
(1963) (1976)
(1963) (1976)
Galileo
Galileo
Galileo
(1989)
(1989)
(1989)
Transit 5BN-2 Transit
Transit 5BN-2 Transit
Transit 5BN-2 Transit
(1961) Triad-01-0X
(1961) Triad-01-0X
(1961) Triad-01-0X
(1972)
(1972)
(1972)
Pioneer 10
Pioneer 10
Pioneer 10
Nimbus 3
Nimbus 3
Nimbus 3
Viking 1 & 2 (1975)
Viking 1 & 2 (1975)
Viking 1 & 2 (1975)
(1972)
(1972)
(1972)
(1969)
(1969)
(1969)
Mars Pathfinder (1996)
Mars Pathfinder (1996)
Mars Pathfinder (1996)
(RHU s only)
(RHU s only)
(RHU s only)
GPHS Radioisotope
Thermoelectric Generator
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
Thermoelectric Power Generation
HOT SIDE
COLD SIDE
I
I
-
+
NP
I
HOT SIDE
COLD SIDE
Figure of Merit:
Electrical
Seebeck
Conductivity
Coefficient
�S2T
ZT =
k
Thermal Conductivity
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
ZT DILEMMA
INSULATOR
Methods of Reducing k
SEMICONDUCTOR
In Bulk Materials:
SEMIMETAL
METAL
" Alloy, 1950s (Ioffe)
�
S
ZT " Rattlers, 1990 (Slack)
k
�S2T
ZT =
k
square array of Pn
Wanted:
(not to scale)
T vacant
Phonon Glass / Electron Crystal
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
State-of-the-Art in Thermoelectrics
PbSeTe/PbTe
Quantum-dot
AgPbmSbTe2+m
Superlattices
3.0
(Kanatzadis)
(Lincoln Lab)
2.5
Bi2Te3/Se2Te3
2.0
Superlattices
(RTI)
1.5
Skutterudites
Bi2Te3 alloy
1.0
(Fleurial)
PbTe alloy
0.5
Si0.8Ge0.2 alloy
Dresselhaus
0.0
1940 1960 1980 2000 2020
YEAR
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
max
FIGURE OF MERIT (ZT)
Nanocomposites Approach
 Increase interfacial
scattering by mixing
nano-sized particles.
 Enable low-cost, large scale
application.
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
Nanocomposite Synthesis
50 nm
Si
Ge
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
Electron Transport Over Potential Barriers
5 nm
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
Thermal Conductivity of Si0.8Ge0.2
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN 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, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
Comparison of Technologies
0.6
0.6
0.6
0.6
ZTm
ZTm
CARNOT CYCLE
CARNOT CYCLE
10
10
0.5
0.5
0.5
0.5
Power
7
7
Plant
0.4
0.4
0.4
0.4
4 THERMAL
4 THERMAL
THERMAL
POWER
POWER
POWER
PLANT
PLANT
PLANT
Diesel
0.3
0.3
0.3
0.3
2
2
Plant
STIRLING
STIRLING
GENERATOR
GENERATOR IC
1
1
0.2
0.2
0.2
0.2
Engine
AUTOMOTIVE
AUTOMOTIVE
THERMIONIC
TPV
ENGINES
ENGINES
GENERATORS
0.5
0.5
0.1
0.1
0.1
0.1
Thermionic
Converter
Thermoelectric
THERMOELECTRIC
Converter
POWER GENERATORS
01
01
0110
0110
23 5 6 7 8 9 10
23 5 6 7 8 9 10
4
4
TEMPERATURE RATIO (T /T )
TEMPERATURE RATIO (T /T )
hot cold
hot cold
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
POWER GENERATION EFFICIENCY
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, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
ACKNOWLEDGMENTS
" Current Members
" Collaborators (partial list)
H. Asegun (Molecular Dynamics)
M.S. & G. Dresselhaus (MIT, NW&CNT, Theory)
V. Berube (hydrogen storage)
J.-P. Fleurial (JPL, Thermoelectric Devices)
J.W. Gao (nanofluids)
J. Joannopoulos (MIT, Photonic Crystals)
S. Goh (nanowires and polymers)
Z.F. Ren (BC, Thermoelectric Materials, CNT)
T. Harris (Thermoelectrics&Nanomaterials)
X. Zhang (Berkeley, Metamaterials)
Q. Hao (Thermoelectrics)
D. Kramer (Solar thermoelectrics)
" Past Members (Partial List)
H. Lee (Thermoelectric Materials)
Prof. A. Narayanaswamy (Columbia Univ)
H. Lu (TPV and PV)
Dr. Zony Chen (McKinsey)
A. Minnich (thermoelectrics)
Prof. C. Dames (Nanowires, UC Riverside)
A. Muto (nanowires and thermoelectrics)
Prof. D. Borca-Tasciuc (Nanowires, RPI)
A. Schmidt (ps pump-and-probe)
Prof. T. Borca-Tasciuc (Thermoelectrics,RPI)
S. Shen (near field transfer)
Dr. F. Hashemi (Nano-Device Fabrication)
Dr. M. Chieso (nanofluids)
Dr. A. Jacquot (TE Device Fabrication)
Dr. X. Chen (optics, Pump-and-Probe)
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)
Sponsors: DTRA, DOE, NASA,
Prof. R.G. Yang (Nanocomposites, U. Colorado)
NSF, ONR, Ford, Seagate, and
Prof. D.-J. Yao (TE Devices, Tsinghua Univ.)
Prof. T. Zeng (Thermionics, NCSU)
others
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT
 WARREN M. ROHSENOW HEAT AND MASS TRANSFER LABORATORY, MIT