ENERGY NANOTECHNOLOGY --- A Few Examples Gang Chen Nanoengineering Group Rohsenow Heat and Mass Transfer Laboratory Massachusetts Institute of Technology Cambridge, MA 02139 Email: gchen2@mit.edu http://web.mit.edu/nanoengineering
Thermal-Electrical Energy Conversion COEFFICIENT OF PERFORMANCE Temperature (K) 0 50 100 150 200 250 300 10 500 1000 1500 2000 5000 2500 5500 6000 1 8 6 4 2 0 REFRIGERATION Household Refrigerator Thermoelectrics Environmentally Benign Refrigeration POWER GENERATION Thermo- PV THERMODYNAMIC LIMIT Power Plant Auto Energy Efficiency Waste Heat Recovery 0.8 0.6 0.4 0.2 Solar PV 0 Renewable Energy EFFICIENCY Grand Challenges: Efficiency and cost effective mass production
Nano for Energy Increased surface area Interface and size effects Molecules Λ = 1 100 nm λ=1 nm Λ---Mean free path λ---wavelength Photons Λ > 10 nm λ=0.1-10 µm Electrons Λ=10-100 nm λ=10-50 nm Thermodynamics Kinetics Phonons Λ=10-100 nm λ=1 nm
Phonon and Electron Engineering for Thermoelectric Materials
Thermoelectric Devices COLD SIDE COLD SIDE - + HOT SIDE I N P HOT SIDE Nondimensional Figure of Merit Joule Heating Seebeck Coeff. Electron Cooling GPHS Radioisotope Thermoelectric Generator 2 σs T ZT = k Reverse Heat Leakage Through Heat Conduction
State-of-the-Art in Thermoelectrics FIGURE OF MERIT (ZT)max 4 3.5 3 2.5 2 1.5 1 0.5 0 PbSeTe/PbTe Quantum-dot Superlattices (Lincoln Lab) Bi 2 Te 3 /Sb 2 Te 3 Superlattices (RTI) Bi 2 Te 3 alloy PbTe alloy Si 0.8 Ge 0.2 alloy (Michigan State) Skutterudites (Fleurial) Dresselhaus 1940 1960 1980 2000 2020 PbTe/PbSeTe Nano Bulk S 2 σ (µw/cmk 2 ) 32 28 k (W/mK) 0.6 2.5 ZT (T=300K) 1.6 0.3 Harman et al., Science, 2003 Bi 2 Te 3 /Sb 2 Te 3 Nano Bulk S 2 σ (µw/cmk 2 ) 40 50.9 k (W/mK) 0.6 1.45 ZT (T=300K) 2.4 1.0 Venkatasubramanian et al., Nature, 2002. YEAR
Heat Conduction Mechanisms Unit Cell of Superlattice Layer B Layer A Unit Cell of B Layer A New Crystal? Inhomogeneous Multilayers?
Heat Conduction Mechanisms in Superlattices Normalized Thermal Conductivity THERMAL CONDUCTIVITY (W/mK) 10 0 80 60 40 BULK SPECULAR (p=1) p=0.95 Ideal Superlattices In-Plane LD Cross-Plane LD T=300K 20 10-1 AlAs/GaAs DIFFUSE Yao 1987 (p=0) p=0.8 Yu et al. 1995 Capinski et al. 1999 Yao (1987) Capinski et al. 1999 Yu et al. (1995) 0 10 1 10 2 10 3 10 4 10 1 10 2 10 3 LAYER Period THICKNESS Thickness (Å) (Å) In-Plane Cross-Plane Major Conclusions: Ideal superlattices do not cut off all phonons due to pass-bands Individual interface reflection is more effective Diffuse phonon interface scattering is crucial Coherent Structures Are Not Necessary, Nor Optimal!
Photon Engineering: Thermophovoltaics
Thermophotovoltaics Heat Source Filter Photovoltaic Cells EMISSIVE POWER (W/cm 2 µm) 10 4 10 3 10 2 10 1 10 0 Useful Useless 5600 K 2800 K 1500 K 800 K 10-1 0 2 4 6 8 10 E WAVELENGTH (µm) G Frequency Selective Emitter Frequency Selective Filters Photon Recycling Structures Evanescent Wave Structures High Efficiency PV Cells
Surface Waves and Near Surface Energy Density ω Free Space Surface Modes k x Energy Density (Jm -3 ev -1 ) 10 5 10 3 10 1 10-1 10-3 10-5 Wavelength (µm) 5 10 15 20 25 d = 10 nm d = 100 nm d = 1 µm d = 10 mm 0.1 0.2 0.3 0.4 0.5 0.6 Energy (ev) blackbody Energy density in the vicinity of a half-plane of BN. High Energy Density, Monochromatic EM Fields Exists Near Surfaces When ε is Equal but of Opposite Signs. But They Are Non-Emitting! Surface Plasmons and Surface Phonon Polaritons
Near Field Energy Conversion 1.2 10 8 9 Wavelength (µm) 8.75 8.5 8.25 d = 5 nm 8 Power absorbed (Wcm -2 ) SiC Source (BN, SiC) PV material 10 3 10 2 Power absorbed 10 1 Blackbody 10 0 10-1 0 100 200 300 Vacuum gap (nm) Flux (Wm -2 ev -1 ) 8 10 7 4 10 7 d = 1 nm d = 0 nm d = 10 nm 0 0.14 0.145 0.15 0.155 0.16 Frequency (ev)
Coupled Conduction and Radiation Nonequilibrium Thermoelectric Devices
Nonequilibrium Transport Conventional TE Cooler Conventional Micro TE Cooler T 2 Electron Temperature T 1 Cooling Target Thermoelectric Element T 1 Cooling Target Phonon Temperature Thermoelectric Element T 2 Battery Battery Proposed Nonequilibrium Thermoelectric Devices Explore nonequilibrium between electrons and phonons couple the cooling target with thermoelectric element without direct lattice contact ZT 2 σs T = k e + Xk p T 1 Cooling Target Vacuum Gap Electron Temperature, T e Phonon Temperature, T p Battery T 2
Surface Plasmon Coupling of Electrons d Model Based on Fluctuation-Dissipation Theorem d=10 nm Macroscale gap T 1 Far-Field Nanoscale gap T 1 Surface Waves Three orders of magnitude increase in energy transfer flux due to surface plasmon resonance
Surface-Plasmon Enabled Nonequilibrium Thermoelectric Refrigerators COLD END TEMPERATURE (K) 260 240 220 200 180 160 140 G InSb k e /k=0.1 Z=0.002K -1 T H =300K G=10 8 W/(m 3 K) G=10 10 W/(m 3 K) G=10 12 W/(m 3 K) Conventional 120 10 0 10 1 10 2 10 3 10 4 THERMOELECTRIC ELEMENT LENGTH (µm) COP 1 0.8 0.6 0.4 0.2 Cooling Load q = 50 W/cm 2 G = 10 8 W/(m 3 K) G = 10 10 W/(m 3 K) G = 10 12 W/(m 3 K) Conventional k e /k = 0.1 Z = 0.002K -1 T H = 300K 0 1 10 100 THERMOELECTRIC ELEMENT LENGTH (µm) Performance is determined by the doping concentration and operation temperature. Principle works for both refrigerators and power generators.
Key Points Nanoscale effects are enabling breakthroughs in energy technologies. Need cost-effective and mass producible nanotechnology for energy applications. Fundamental understanding leads to new manufacturing paradigms. Fundamental research problems exist in both individual nanostructures and mesoscopic nanostructures. Multidisciplinary research and interdisciplinary researchers are needed.
ACKNOWLEDGMENTS Current Members H. Asegun (Molecular Dynamics) V. Berube (hydrogen storage) Z. Chen (Metamaterials, TPV) S. Goh (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) S. Nakamura (nanowires and thermoelectrics) A. Narayanaswamy (Metamaterials, TPV) G. Radtke (hydrogen storage) A. Schmidt (ps pump-and-probe) E. Skow (polymers) S. Shen (lubrication, rarefied gas dynamics) Dr. M. Chieso (nanofluids) Dr. X. Chen (thermoelectrics, Pump-and-Probe) Dr. D. Vashee (thermoelectrics) Prof. Y.T. Kang (nanofluids) Collaborators 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. 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: ARO, DOE, NASA, NSF, ONR, Industries