>>13335769Not him but why are you here? You clearly need to study moar!
To answer your question in a simple way, there is "energy" locked into every bit of matter, extracting it into a viable fuel source is very difficult at this point due to our technological limitations so it takes more energy to free the potential than we get in returns from doing so.
If you mean "utilize heat and unspent fuel as a secondary source" then look up catalytic converters in automobiles and thermoelectric generators (TEG).
