There are many different approaches to transfer thermal energy to electricity, and the technologies to do so have existed for several decades. 
An established approach is by using a thermoelectric device, where a change in temperature across a semiconductor material creates a voltage through a phenomenon known as the Seebeck effect.
A related approach is the use of thermogalvanic cells, where a temperature difference gives rise to an electric current in an electrochemical cell.
The organic Rankine cycle, offered by companies such as Ormat, is a very known approach, whereby an organic substance is used as working medium instead of water.  The benefit is that this process can reject heat at lower temperatures for the production of electricity than the regular water steam cycle.  An example of use of the steam Rankine cycle is the Cyclone Waste Heat Engine.