"storage in convenient ... form"

If you can't store it, you can't use it, so it doesn't matter how green it is, it won't be used.

The ideal transportable liquid fuel is diesel with gasoline a very close second. Some peculiar and temporary engineering and economic issues make gasoline ideal for passenger cars but in a very long run perspective the whole world is going diesel, its just the future has arrived a little quicker for some parts of the world than others. A common sci fi (and tech) theme.

Originally the diesel engine was developed by the Diesel bros (no kidding) to run on nut oil not crude oil. The Diesel bros had a political axe to grind about farms having self sufficient tractors much like they had self sufficient ox drawn plows. To say things didn't turn out that way would be an understatement, but there's no scientific reason it won't work that way in the future.

The primary problem with nut based biodiesel is you can't scale the production up to reasonable economic levels by shoving entire walnut trees thru a pipeline... but you can pump algae thru a pipeline...

So that's the big pix of why people are trying to turn algae into diesel.

Sure, but people use nonstorable electricity just fine, so I disagree with your first sentence.

Biogas rather than biodiesel might appear as a farm fuel in the future, either from manure or wood gasification. But not while underground gas is still cheap enough to flare.

Ah I think you're correct. I reread your post and I got stuck circular. Yes fixed installations would almost certainly continue not to use liquid fuels just like now. Probably direct solar like you say. And just like now, the transport industry would continue to heavily rely on liq fuels. I'd worry about practical engineering issues with biogas either leaking out (especially if pressurized) or oxygen contamination leaking into the flammability limit range.

If you unleash a chemist and tell him to do anything he wants to make the best energy store, you end up with a liquid hydrocarbon, they really are awesome by both mass and volume energy density and working temp range and corrosion resistance and shelf stability and a bunch of other parameters, even if cost is not an issue. True, weird corner cases like solid rocket boosters do end up with weird fuel components, but they're weird, so that's OK.

Science fiction story just popped into my head:

The story starts with people terraforming a planet and turning it into a giant algae-growing factory, for the purpose of creating oil from it. Whole plots with interesting characters occur. Then, suddenly, towards the end, those people abandon the whole project to go fight a war in their own galaxy. Then, the story ends with the shocking twist ... somehow you find out the abandoned algae factory is Earth. :)

...and then in the sequel they show up millions of years later when the war is over, only to find that sentient creatures have evolved and have almost entirely used up the oil reserves :)

BRILLIANT!

In Larry Niven's "Known Space" stories, one bit of backstory is that billions of years ago the Slaver empire colonized planets with yeast to feed to their livestock -- giant bioengineered slugs. In his "World of Ptavvs" he describes a survivor coming out of stasis on earth, long after the fall of the Slaver empire, very confused about what happened to his planet...

Somebody needs to come up with a better way of converting solar energy into a transportable liquid. I'm guessing this is as good as it gets for now.

Phonons. Phonon pipelines, phonon tanks... Investors please form an orderly line for me to fleece you go millions.

Srsly though. Liquids are extremely good at this kind of density storage trade off, it's the reason petroleum won over electric cars in 1900 ( look it up, electric cars aren't new, we just decided about a. Hundred years ago we liked oil more.)