This seems like the kind of task that might be possible with a solar or renewable fuel.

Given the target speed of 1.5knots, I wonder if a large enough solar installation on a pair of modified tugs could achieve a decent runtime. Or perhaps focus on using the solar to generate hydrogen for a hydrogen burning engine given the availability of water.

I'm hoping for a future version that burns the waste to propel itself, yet while somehow containing (or something-ing) all the foul combustion byproducts.