Doesn't matter. TEGs operate at very low efficiency in either application. There are slightly better devices than these Peltier Effect (electric to thermal) devices that use the Seabeck Effect (thermal to electric), but they are only a few percent better at efficiency. Neither device is being used to harvest waste energy because they are just not efficient enough to be practical. If they were, don't you think they would already be widely used? These devices have been around a long, long time and yet they are niche application use only because they are just not good at doing things at scale. Even solar photovoltaic cells (photoelectric effect, light to electricity) are just barely efficient enough to work at scale for their cost, size/weight and lifetime. You're still better off using chemical batteries to power things because they have much greater efficiency than all of these technologies. Physics doesn't give you anything for free and we have to accept that fact.