I think the primary idea is that eventually we'll still need carbon source inputs for making certain things (fertilizer for one, most types of rocket fuel for another) and so this is an alternative to that. If we're not getting hydrocarbons from fossil fuels, we need to get it from somewhere else for the petrochemical industry.
It's also a nice thing to do when the spot price of energy goes negative or extremely low when there's tons of surplus renewable energy.
I also heard one example assuming a future with nuclear fusion being cheap and you put a nuclear fusion plant at larger airports to produce jet fuel on-site.
Fertilizers don't contain carbon. Plants need nitrogen, potassium and phosphorus, which they can't easily get from their surroundings, so we provide these via fertilizers. They get their own carbon out of the CO2 in the air.
We do need hydrogen to make fertilizers, and currently the cheapest way to make hydrogen is to start with methane, CH4. But this is just a matter of convenience.
You don’t need carbon for fertilizer. Hydrogen is harvested from methane which is added to atmospheric nitrogen to make ammonia, the basis for all nitrogen compounds used for fertilizers. You can change the process slightly to use other sources of hydrogen but they’re considerably more expensive. If you were synthesizing anyway, you would not take a detour to methane.
Biodiesel and ethanol (form corn, or better from sugarcane). Instead of a lot of tall gras, just imagine them as self-assembly solar panels with a mini chemistry factory attached.
I guess you are comparing the efficiency of solar panels (20%) with the efficiency of photosynthesis (1%-2%). The problem is that solar panels have a 20% producing electricity, but using that electricity to make fuel has a very low efficiency.
From the article:
> When air was bubbled through potassium hydroxide dissolved in ethylene glycol and the CO2-loaded solution subsequently hydrogenated in the presence of H2 and a metal catalyst, complete conversion to methanol was observed at 140 °C.
Even with catalyst, organic reactions have an awful efficiency. In many of them the reactives produce not only what you want but also other molecules. In this case it's hard to imagine, because methane is very small and all side products I imagine like COH2 can probably be converted to methane inside a hot recipient full of H2.
Anyway, even if you lose no CO2 as side products, the reaction may lose a lot of energy as heat. Photosynthesis has many intermediate steps that help the conversion to be more efficient (and also because photosynthesis builds a bigger molecule that is harder).
The photosynthesis isn't the end of the biofuel equation. Corn ethanol is energy negative (switchgrass, sugar cane or beets are better but still abysmal).
If you consider a hybrid PV-CSP plant, the H2 step can be done at 20% from air. CO2 capture is under 10%.
So you'd need to show that the ethylene step is under 5% efficient when you also had an available free low quality steam input (or high quality at 50% sunlight-efficiency).
> One hectare of sugar cane yields 4,000 litres of ethanol per year (without any additional energy input, because the bagasse produced exceeds the amount needed to distill the final product). This, however, does not include the energy used in tilling, transportation, and so on. Thus, the solar energy-to-ethanol conversion efficiency is 0.13%.
but I'm still pessimistic about the efficiency of the new method.
The thing that will kill as an energy storage method it is capital and labour expenses more than efficiency. I'd expect something like it to be viable for chemical feedstock once the fossil fuel subsidies start to die out and less investment goes into extracting more and worse sources cheaply though.
It's also a nice thing to do when the spot price of energy goes negative or extremely low when there's tons of surplus renewable energy.
I also heard one example assuming a future with nuclear fusion being cheap and you put a nuclear fusion plant at larger airports to produce jet fuel on-site.