Technology

We already do this at every steam based power plant in the world (basically everything but hydro, PV, and wind) and it's done much more efficiently. Doing this stuff with tiny gadgets on micro generators like ICE vehicles is a pretty inefficient implementation, especially as it adds weight to already heavy vehicles, decreasing efficiency and safety, and increasing tire and brake wear. The only place I can imagine this being useful is very heavy vehicles that for some reason still have to be using diesel like long haul trucks/busses, diesel freight trains etc and the like. And EVEN then you're looking at major issues with economy. If you increase the weight of a truck by 2 percent to give it a 2% increase in fuel efficiency, you are hurting not helping. 2% comes off of your GCVWR margins and suddenly you need 51 trucks instead of 50 trucks to transport a given load, not only increasing your fuel use by 2% but also increasing vehicle maintenance and tire and brake pollution by 2%

Edit: I'm not saying relatively miniaturized energy recovery systems don't have a future, but I'm dubious it's in transport or handheld devices. At least for the foreseeable future. Infrastructure scale however has always been a major application for energy recovery development, stirling engine, steam turrbine and TE development keeps getting further pushed to eek efficiency out of power stations, power plants, substations, emergency generators, maybe even HVAC systems and other building scale applications.