Nano for Energy Increased surface area Interface and size effects Photons L > 10 nm l=0.1-10 mm Phonons L=10-100 nm l=1 nm Electrons l=10-50 nm L---Mean free path l---wavelength Molecules L = 1-100 nm

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 Radiation transport to maximize absorption Two phase flow Electrochemical transport Multiscale, multiphysics transport Mass transport Heat transfer (intake and release) Small scale thermodynamics Two phase flow Multiscale and multiphysics

Thermoelectrics Devices Power Generation I N P Cold Side Hot Side Diffusion COLD SIDE HOT SIDE 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

Nanoscale Effects for Thermoelectrics Interfaces that Scatter Phonons but not Electrons Phonons L=10-100 nm l=1 nm Electrons l=10-50 nm Electron Phonon Molecular Dynamics (Freund)

State-of-the-Art in Thermoelectrics Bi2Te3 alloy PbTe alloy Si0.8Ge0.2 alloy Skutterudites (Fleurial) PbSeTe/PbTe Quantum-dot Superlattices (Lincoln Lab) Bi2Te3/Sb2Te3 (RTI) AgPbmSbTe2+m (Kanatzadis) Dresselhaus PbTe/PbSeTe S2s (mW/cmK2) 32 28 k (W/mK) 0.6 2.5 ZT (T=300K) 1.6 0.3 Bulk Nano Harman et al., Science (2003) Bi2Te3/Sb2Te3 S2s (mW/cmK2) 40 50.9 k (W/mK) 0.6 1.45 ZT (T=300K) 2.4 1.0 Bulk Nano Venkatasubramanian et al., Nature, 2002.

Potential Applications Transportation Gasoline 100 kJ 10kJ 30kJ 35kJ Parasitic heat losses Coolant Exhaust 9kJ 6kJ Auxiliary Driving Mechanical losses In US, transportation uses ~26% of total energy. Coolant Exhaust Gasoline 100kJ 10kJ 30kJ 35kJ 9kJ 6kJ Auxiliary Driving Mechanical losses Parasitic heat losses 10% energy conversion efficiency = 26% increase in useful energy Heating Refrigeration & Appliances TPV & TE Recovery PV Electricity Oil or Nat’l Gas Entropy Thermal Power Electrical Power In US, residential and commercial buildings consume ~35% energy supply Residential Heating Refrigeration & Appliances Losses Electricity Oil or Nat’l Gas Entropy Thermal Power Electrical Power

Challenges and Opportunities Mass production of nanomaterials Energy systems: high heat flux Nanomaterials are trans-boundary Basic energy research leads to breakthroughs Transports (molecular, continuum) are crucial Inter-departmental collaborations