Doc and his not-so-good-smelling fuel pellet.
The hydra-waste fuel conversion system was a system devised and built by Emmett Brown and his team at the Institute of Future Technology.
The system was able to convert organic matter such as manure into clear fuel pellets. One pellet was capable of providing heat for an entire season.
Unfortunately, the clear fuel pellets still smelled exactly like what they were before conversion — in this case, manure.