[thijson]

Brazil has had a pretty active program of converting cane sugar to ethanol for a while now.

https://en.wikipedia.org/wiki/Ethanol_fuel_in_Brazil

Sugar cane doesn't require replanting every year either, like corn does.

Plants are actually not a good converter of solar energy to chemical energy though. They capture a few percent of it.

Solar cells are able to capture about 10 times that, a much smaller footprint.

[jl6]

Ethanol is quite a useful thing to have though, as a multi-season stable store of energy. We will need to synthesise it (or other synfuels and feedstocks), to fully transition away from fossil sources, and that 10x efficiency factor will be essential, as synthesis is highly energy-lossy.

[busterarm]

> Ethanol is quite a useful thing to have though, as a multi-season stable store of energy.

Am I missing something? Ethanol is hydrophilic and hygroscopic. In concentrations used as a fuel (e.g., E85), it acts like a desiccant and spoils quickly. In a closed system this ends up with phase separation and the freed water causes engine corrosion.

I'm not sure we want people running a still or molecular sieve in their homes to deal with fixing long-term-stored ethanol.

[jandrewrogers]

Ethanol doesn't "spoil". It is a very stable molecule and miscible with water.

The main issue is that it has a strong affinity for water so it needs to be stored in containers that are sealed from the environment. The same issue exists with the ubiquitous ethanol/gasoline blends.

[busterarm]

The cheap thin-walled gas containers you find in the auto parts store or on Amazon sell a heck of a lot better than the good stuff.

This just doesn't meet up with the day to day reality of your average consumer.

Even your gas station underground tanks aren't airtight. The problem is that the air around us has tons of water vapor in it.

[bluGill]

> In concentrations used as a fuel (e.g., E85), it acts like a desiccant and spoils quickly

Citation needed. (hint you won't find one because it isn't true). Be careful here - this myth has been repeated enough that a search will find plenty of claims that don't check out.

High concentration alcohol doesn't spoil. Even lower concentrations don't spoil, but they mix with poor quality gas that does spoil. Well when you get very low it will, but alcohol is poison to living things and so it won't spoil. (I'm not sure how ethanol stands up to UV - but we generally keep it in a tank so that isn't an issue)

Ethanol will absorb water, but it doesn't take it out of the air anymore than anything else.

[red-iron-pine]

nah, it loves to absorb water out of the ambient air.

ethanol that is distilled forms an azeotrope has a hard time getting past 98% on its own. even if you used advanced techniques and additives, it has a strongly hygroscopic nature, meaning it actively attracts and absorbs water vapor directly from the air.

in other words, it will do everything it can to get back to 98%.

to keep ethanol above 98%, you need airtight seals or "molecular sieves" (zeolite beads) inside the tank to constantly "bead up" and trap any incoming water molecules.

[don_esteban]

What is the conversion efficiency for electricity + C02 + H20 -> ethanol/hydrocarbons?

Because that is the overall path (for long-term storable chemical energy, i.e. usable for transport or seasonal energy storage in countries where solar is highly seasonal).

[thijson]

There's an alternative path that removes carbon from the cycle:

electricity + H20 + N2 -> NH3 + O2

Ammonia can be liquified and stored similar to Propane, it does attack copper and brass.

It can be burned in an internal combustion engine, it's about half as energy dense as hydrocarbons though.

There's a danger to humans from it though, it requires sprinkler systems if there is ever a leak.

I think that a large part of the energy budget in a plant is harvesting and concentrating CO2 from the air. N2 is a lot more abundant in the air.

There is work currently on using giant sodium batteries in these large container ships. That might be more cost effective than the above longer term.

[don_esteban]

Yeah, concentrating CO2 from atmospheric concentrations is not easy. The benefit is that it actually removes carbon from the atmosphere. Whether it can ever be done on a large scale is a question, though.

Is there any work on doing that, at a low energy cost? (I mean concentrating CO2, not removing it by weathering rocks?)

Yeah, ships are not really weight constrained, unlike airplanes, really cheap sodium batteries should be feasible.