A lot of energy in a vehicle is turned into (waste) heat. Heat can be converted into mechanical work and / or electrical power. Fundamental research has been done in the field of thermoelectric power generation, though it still requires much more research to be perfected.
The conversion of heat to mechanical work is already seen in different vehicles, for example using combined cycles with turbo-compound engines. The exhaust gas drives a turbine, which then delivers the power to the crankshaft.
As a result, the engine output power is increased without increasing the fuel consumption. These mechanical concepts are already available and will improve in the future. However, compared to other technologies this is a complex technique which contains a lot of mechanical components.