flow battery with water soluble, non metal reactants

The interesting question is then, what plant produces the relevant quinone in high yields, and where does it grow?

Synthesising a quinone from CO2 without the help of a plant might just be a little too expensive to be economically attractive. I didn't sit through a ll those lectures on "natural products" in organic chemistry for nothing - it might have been fifty years ago, but the message stuck.

"Total synthesis" was a way of proving a structure, not an economically fea sible route to mass production.

If the reactants are preserved through a charge/discharge cycle then a moderately high manufacturing cost may not be that bad.

Those glossy adverts aren't designed to illuminate your soul, however -- they're designed to attract investment and alumni dollars. Most of them tout technologies that are just around the corner, and probably will be for the rest of your life.

I did not see anywhere in the research paper a value for the possible energy density. But I suspect it might be pretty low. How many litres of chemicals to store the several Gj you might need for a big plant, capable of running for a few days when the sun doesnt shine ?

Existing tech using molten salts works, but the sizes get enormous for more than a few hours storage.

That's what you've got a grid for - the sun is usually shining somewhere du ring the day.

Energy density is unlikely to be impressive. Quinones aren't usually wildly water-soluble, and they tend to be big molecules and don't store energy in particularly high energy bonds.

Burning hydrocarbons to water and CO2 does release a lot of energy per gram of hydrocarbon - nobody counts the mass of the oxygen that comes free from the atmosphere.

The nice thing about an enormous tank of molten salt is that it has a high mass to surface area ratio, so having a thermal time constant of week or so doesn't require you to spend a lot on insulation. You are storing heat, r ather than electric power, but at least you don't have to convert it into e lectric power before you can store it.