Maybe if we have to dome our cities and thus have no wind to disrupt the drone.
I wonder if it would be cheaper to use a small electric self-driving car like a mobile version of the amazon locker?
That's essentially a blimp. Due to square/cube scaling, blimps have less lift and more drag relative to their size at small scales than large ones. The extra power needed to overcome the drag would by far negate the power savings from reduced lifting requirements.
As a benchmark: the DJI Phantom weighs roughly 1kg, and its battery can store about 80kJ of electrical energy. I found a NASA study [1] which estimated that the energy released when a li-ion battery combusts is roughly 2x the usable energy capacity. So let's say that a battery fire would release 160kJ of energy.
In contrast, a balloon capable of lifting a 1kg drone would need about 830 liters of hydrogen, with a mass of 75 grams. That amount of hydrogen, if burned in room-temperature air, would release about 9MJ -- roughly 50x as much energy as a battery failure.
(Of course, most of the hydrogen would burn itself up quickly and harmlessly, assuming the balloon isn't flying near any flammable objects.)
I'm not so sure that kind of energy release would be entirely harmless.
You're also not going to get anywhere near the theoretical maximum detonation velocity if it ignites because of a leak.
My point is comparing hydrogen to TNT is very misleading.
But (and I have no reason to think that this is true) if it turned out to be commercially viable, I'm sure that perception could be changed.
Also, as far as I know, the cheapest/lightest material to make the balloon out of would be mylar, which is conductive and not something you want around powerlines.
And I'm discarding the pump to put it back, assuming we're just going to vent to the atmosphere when we're done. Things get worse if you want to pump it back in.
Are you proposing that it could somehow hook itself up to a helium or hydrogen source, while hovering?