We've all seen articles about massive container ships of the future using kites to supplement their engines, but I suspect a really solarpunk future would look a bit further afield, or perhaps further back in time for their ship designs.
I think in any future with humans and continents, people are going to be trying to cross the ocean. There might be less shipping in a world without our abundance of cheap energy, or more of a focus on reducing consumption and producing necessities locally, but people will still trade goods and travel. So what might the ships look like? Return to tallships? Solar panels and electric motors? I love reexamining traditional technologies to see how they can fit with modern engineering and design principles, safety features, and electronics, but I don't know much about ships, and especially not much about modern sailing.
So what do you think will be bringing back holds full of old world fashions harvested from the Chilaen desert?
Helium is an inert gas. It's one of two gasses that have an average molecular velocity high enough to escape Earth if let into the atmosphere. Any terrestrial helium comes from heavy elements radioactively decaying deep in the earth, and is trapped in the same rock formations that collect petroleum.
Hydrogen is also capable of escape, but reacts with ozone to create water. One of the concerns with a hydrogen economy is that hydrogen production on a global industrial scale will deplete holes into the ozone layer again.
Neon, pure nitrogen gas (air is 78% N), carbon monoxide, ammonia, hydrogen cyanide, acetylene, ethylene, and methane are all lighter than air. Methane, acetylene, and ethylene are extremely flammable. Carbon monoxide, ammonia, and hydrogen cyanide are poisonous and corrosive. Neon exists in the atmosphere in small amounts but is extremely expensive to isolate and vastly inferior to helium for buoyancy. Maybe in a post scarcity society hot-neon balloons would be a thing. Hot-air balloons could also become slightly more effective if the carbon-dioxide and oxygen were filtered from the envelope.
Hear me out: solar-collector heated hot air balloons!
https://en.wikipedia.org/wiki/Solar_balloon
Yes, I know, my kid had a toy like that - but why limit this to toys? You could also use solar heat and support with other heat sources. How about a solar hot air ballon rail, where the rail is a solar collector trough that sends sunlight up to heat the air inside the dirigible?
Oh look the Brazilians at it again