Meet EATR (Energetically Autonomous Tactical Robot):
EATR is not a dainty diner:
The system obtains its energy by foraging – engaging in biologically-inspired, organism-like, energy-harvesting behavior which is the equivalent of eating. It can find, ingest, and extract energy from biomass in the environment (and other organically-based energy sources), as well as use conventional and alternative fuels (such as gasoline, heavy fuel, kerosene, diesel, propane, coal, cooking oil, and solar) when suitable.
The belly of the beast-to-be will be Cyclone Power's external combustion engine. The company claims its engine can run on dainties from chicken fat to orange peelings to swamp gas to waste heat.
The first order was for an engine utilizing waste heat. None of its engines have yet been delivered. In the nature of things none may ever be delivered.
Just a caution. DARPA has delivered on such innovations as the internet but has also buried a few things that didn't work out such as - intelligent robots.
In the case of EATR, DARPA's current heart throb will need a brain to distinguish rocks, concrete, soil and other materials from food. Brains are not easy to come by as the GOP is finding out.
Those who think we can only have electricity as fossil fuels are abandoned when the sun is shining or the wind blows may need a brain transplant as well. EATR might help show the way.