BMW has played around with recovering heat energy from exhaust gas and turning it into usable energy before.
It involved using exhaust gas to heat up water to produce steam, which passes through an expansion unit that transfers kinetic energy to a crankshaft, which in turn helps power the engine. This was a concept called the BMW TurboSteamer, and was shown back in 2005.
BMW is now harvesting energy from exhaust gas to be turned into electricity instead of kinetic energy, thanks to new technology pioneered by NASA. The materials used by NASA in thermo-electric generation increases electricity generation to a maximum of 1kW, instead of previous limit of 0.2kW.
This electricity can be stored for later use the same way BMW uses electricity regained from its EfficientDynamics brake energy recovery.