Energy consumption in our society is increasing rapidly. A significant fraction of the energy is lost in the form of heat. In this talk we introduce thermoelectric devices that allow direct conversion of heat into electricity. Some new physical concepts and nanostructures make it possible to modify the trade-offs between the bulk electrothermal material properties through the changes in the density of states, scattering rates, and interface effects on the electron and phonon transport. The potential to increase the energy conversion efficiency and bring the cost down to $0.1-0.2/W will be discussed. We also describe how similar principles can be used to make micro refrigerators with cooling power densities exceeding 500 Watts per square centimeter. Hybrid liquid/solid-state cooling will be shown to have the potential to reduce the total cooling power requirement significantly by selective removal of hot spots. Finally, experimental results will be presented for thin film thermal conductivity of nanostructured materials using a femtosecond laser pump-probe technique. We describe how the ballistic and diffusive components of heat transport can be identified. The transition between energy and entropy transport in nanoscale devices will be discussed.< Leer menos