Hunter's Generator, via Oak Ridge National Laboratory
Theophrastus discovered an electrical effect of tourmaline when heated in 314 BC, it attracted bits of straw and ash. Johann Georg Schmidt rediscovered at the electrical effects of tourmaline in 1707. Sir David Brewster named this effect Pyroelectricity in 1824. More research in this area led to brothers Pierre and Jaques Curie discovering the mechanisms behind Piezoelectricity in the late 1880s.
Recently, Scott Hunter of the Oak Ridge National Laboratory has delved into using some Pyroelectric effects to harvest heat for energy. In the past such devices only produced a 1 – 5 % heat to energy conversion. Hunter developed a piezoelectric generator using modern manufacturing techniques shrinking a tiny pyroelectric cantilever, like in a room thermostate. The output of each Hunter Piezo-generator is in the area of 10 – 30% heat to energy conversion bracket. The cantilever is attached to a piezoelectric substrate. When heated the bi-material effect moves the lever from the hot side to the cold. When cool enough, it flips back, and so on. The device is 1mm square. For reference, 1000 of these devices can be attached to a common PC CPU. Each outputs 1 – 10 milliwatts, depending on the heat source. That CPU could return 10 watts of power in ideal conditions.
There is a type of moving part in this device, so I would say it is quite limited in its rigidity. However, they are easy and cheap to manufacture. In the end, if it doesn't cost more to make than they recover, it is worth it. Until something else better comes along.