Nanostructuring is very effective in enhancingthe thermal to electrical energy conversion efficiency of mostmaterials, including in particular silicon which presents aperfect trade-off between technical potentialities andenvironmental/political/social issues. Heat sources, thatnowadays are exploited only in a small fraction, could becomekey players in energy production thanks to thermoelectricdevice based on nanostructured silicon.Successful strategies, including on-chip devices for microharvestingand cost-effective large-area vertical nanowireforests for macro-harvesting will be presented. An estimationof the efficiency will be reported, highlighting thecompetitiveness of these devices with respect to conventionaltechniques.