> So you're ignoring the stored energy vs. energy storage distinction. Yes, yes, oil and coal are storing energy from the sun from a billion years ago.

While that's an astute observation, it's not my point.

It's also somewhat imprecise: most petrochemical deposits were made ~650 - 65 million years ago. But that's just a nit.

I'm not concerned with where and when existing hydrocarbons were deposited. Merely their very high energy density and (for now) extremely high prevalence. High enough that it's more feasible to operate cars by combusting petroleum than it is to run them on batteries with 10 year lifetimes and 93% recoverability in recycling, simply because there isn't enough accessible lithium on the planet to create enough batteries to replace the cars already on the road.

Even if you had a cost-effective energy source, lithium-ion batteries aren't going to give us enough storage capacity. Due to a shortage of storage medium, not energy.

Which means something else has to give: we don't all own private automobiles (leasing or hiring autonomous vehicles when needed might make this possible). Or we use other storage media: lead-acid, flywheels, compressed air. Or, as you suggest (and I suspect), synthetically generated hydrocarbons, either liquid or gas.

The known efficiencies of this last process are low: at best, maybe 10% of incident sunlight, more likely somewhere between 0.1% to 1%. Which is actually far better than the net efficiency of the production of existing fossil hydrocarbons, it's just that they've accumulated over hundreds of millions of years. As the paper below illustrates, in one year (1997), human fossil fuel consumption amounted to some 422 times the plant matter fixed via photosynthesis (net primary productivity or NPP). Fossil fuel consumption from 1751 - 1998 corresponds to over 13,300 years of NPP. Which is actually somewhat less than I'd have thought.

Net photosynthetic efficiency is around 2.4%. Conversion of biomass to hydrocarbons is the great unknown. Transportation amounts for roughly 1/4 of global energy use. I'll assume that large measures of this might be substituted by other than liquid fuels (EVs, human power, electrified passenger and freight rail, substituting high-speed rail for air), but a significant portion of long-haul land, water, and virtually all air traffic will likely require a highly dense chemical fuel. That will mean one or more of: synthetic (carbon-neutral) hydrocarbons, carbon offsets while continuing to use fossil fuels (while they last), and/or a hell of a lot less transportation. Or create a runaway greenhouse situation.

Frankly none of the options is great.

Replacing all present fossil fuel use with existing biomass would require 22% of the present total primary productivity of the planet, a 50% increase of the amount of NPP humans already consume. And that's if we're counting on using biomass directly, not relying on conversion to other forms (e.g.: ethanol), which drastically reduces net photosynthetic conversion efficiency. Last time I checked, we don't typically run cars on wood or straw (though ships once ran predominantly on solid coal).

http://globalecology.stanford.edu/DGE/Dukes/Dukes_ClimChange...