I recently discovered that there are theorized devices, called phonovoltaic cells, that also convert heat energy to electric power [1, 2].
In a PHOTOvoltaic cell, incoming photons give electrons the energy that eventually turns into a current. Like the photon, a phonon is also a packet of energy, but in the form of a moving vibration inside a solid. In a PHONOvoltaic cell a phonon gives its energy to electrons to create a current.
Unfortunately, we have been unable to find materials with the right thermodynamic properties to create these devices. But there is progress on showing that graphene could be it.
I guess that a phonovoltaic cell, like a photovoltaic cell, would have efficiencies much higher than TECs (5-8%), and so would make a lot more economic sense.