Motivation. Energy and Nanotechnology. Gang Chen Rohsenow Heat and Mass Transfer Laboratory Mechanical Engineering Department Massachusetts Institute of Technology Cambridge, MA 02139. Sources. http://www.sc.doe.gov. Molecules L = 1-100 nm l =1 nm. Photons L > 10 nm l =0.1-10 m m.
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Energy and Nanotechnology Gang Chen Rohsenow Heat and Mass Transfer Laboratory Mechanical Engineering Department Massachusetts Institute of Technology Cambridge, MA 02139
Molecules L = 1-100 nm l=1 nm Photons L > 10 nm l=0.1-10 mm L---Mean free path l---wavelength Electrons L=10-100 nm l=10-50 nm Phonons L=10-100 nm l=1 nm Nano for Energy • Increased surface area • Interface and size effects
Nanoscience Research for Energy Needs • Catalysis by nanoscale materials • Using interfaces to manipulate energy carriers • Linking structures and function at the nanoscale • Assembly and architecture of nanoscale structures • Theory, modeling, and simulation for energy nanosciences • Scalable synthesis methods National Nanotechnology Initiative Grand Challenge Workshop, March, 2004
Examples Grätzel cell for photovoltaic generation and water splitting Catalytic nanostructured hydrogen storage materials • Mass transport • Heat transfer (intake and release) • Small scale thermodynamics • Two phase flow • Multiscale and multiphysics • Radiation transport to maximize absorption • Two phase flow • Electrochemical transport • Multiscale, multiphysics transport
Hot Side I I Diffusion COLD SIDE N P I Cold Side HOT SIDE Power Generation Thermoelectrics Devices • Refrigeration • Power Generation: • T(hot)=500 C, T (cold)=50 C • ZT=1, Efficiency = 8 % • ZT=3, Efficiency =17 % • ZT=5, Efficiency =22 % Figure of Merit: Electrical Conductivity Seebeck Coefficient • Critical Challenges: Electron Phonon Reduce phonon heat conduction while maintaining or enhancing electron transport Thermal Conductivity
Phonons L=10-100 nm l=1 nm Electrons L=10-100 nm l=10-50 nm Nanoscale Effects for Thermoelectrics Interfaces that Scatter Phonons but not Electrons Electron Phonon Molecular Dynamics (Freund)
PbSeTe/PbTe Quantum-dot Superlattices (Lincoln Lab) PbTe/PbSeTe Nano Bulk AgPbmSbTe2+m (Kanatzadis) S2s (mW/cmK2) 32 28 k (W/mK) 0.6 2.5 ZT (T=300K) 1.6 0.3 Harman et al., Science (2003) Bi2Te3/Sb2Te3 Superlattices (RTI) Bi2Te3/Sb2Te3 Nano Bulk S2s (mW/cmK2) 40 50.9 k (W/mK) 0.6 1.45 ZT (T=300K) 2.4 1.0 Skutterudites (Fleurial) Bi2Te3 alloy Venkatasubramanian et al., Nature, 2002. PbTe alloy Si0.8Ge0.2 alloy Dresselhaus State-of-the-Art in Thermoelectrics
10% energy conversion efficiency = 26% increase in useful energy Mechanical losses Mechanical losses Exhaust 9kJ 9kJ Entropy Losses Driving Entropy 10kJ Driving 10kJ Gasoline 100kJ Oil or Nat’l Gas Oil or Nat’l Gas Gasoline 100 kJ Residential Thermal Power Heating Thermal Power Heating 6kJ 6kJ Auxiliary Auxiliary In US, residential and commercial buildings consume ~35% energy supply 30kJ 35kJ TPV & TE Recovery 30kJ 35kJ 10kJ 10kJ Refrigeration & Appliances Refrigeration & Appliances Electrical Power Electrical Power Parasitic heat losses Parasitic heat losses Exhaust Coolant Electricity Coolant Exhaust Electricity PV Potential Applications Transportation In US, transportation uses ~26% of total energy.
Nanomaterials are trans-boundary • Basic energy research leads to breakthroughs • Transports (molecular, continuum) are crucial • Inter-departmental collaborations Challenges and Opportunities • Mass production of nanomaterials • Energy systems: high heat flux