Every hot object emits a large amount of thermal energy in the form of black-body radiation: the sun, car exhausts, radiator and electronic chips are common examples. However, the energy emitted by low-grade sources at low temperature (200-400 °C) is difficult to convert in useable electricity, and most of it is often wasted. This talk addresses the possibility to harness this energy in the THz and mid-infrared region of the electromagnetic spectrum, using rectennas. A rectenna consists of an integrated broadband antenna to collect radiation, combined with a fast rectifier to convert it to dc power. The rectifiers have to operate at challenging high frequencies (up to 10-30 THz) at room temperature, and recent experimental results based on self-switching nanodiodes will be presented.