I wonder how viable it is to synthesize Kerosene or some other carbon fuel using renewable energy and some carbon source (stored factory exhaust?). Renewables are intermittent, but if the fuel synthesis process can also be intermittent then isn't that fine? Viola: you got a form of storage. Why does this not seem to be viable despite dropping costs of renewables?
It is possible, the search keywords are "synthetic fuels" or "e-fuels" ("electro-fuels). Currently it's a lot more expensive than fossil fuel, which is why it's mostly a research project and not a large-scale commercial business.
One problem is that while you can get an unlimited amount of hydrogen via electrolysis of water, you have to get the carbon from somewhere to produce hydrocarbons. There is a lot of CO2 in the atmosphere, but it's still a pretty low concentration so it takes a lot of energy to filter it out. And yes, using factory exhaust etc. is an easier option, but if we're gonna solve climate change most CO2 exhaust sources have to be replaced at some point so that might not be a very long term plan.
Energy input is the other. It takes at minimum the combustion energy to reconfigure the molecules but most of the processes happen at high temperature and pressures which takes energy to do.
I understand input is an issue, but why aren't renewables cheap enough to solve the energy input part of the problem? Don't solar panels cost next to nothing nowadays?
How much must renewables decrease in cost in order for such fuels to be economically competitive?
Gasoline is maybe 5 dollars a gallon and contains some 35kwh of energy. When burning gas in an engine we don't get 35kwh [1], but that is what is there.
Lets imagine we are building a gallon of gas with perfect efficiency. 35kwh is already about $3.15-10.50 depending on the market [2]. Lets assume however, we are willing to take the capital risk and invest in our own solar plant which gives us a price of $2.10-2.80. We can take this even further.
Lets say we buy our own panels, and don't even connect it to the grid. These panels are only used for this, and we only make gas during the day. Here we pay $1.2/watt of capacity [0]. Solar panels have a life of about 10 years. 8x265x10 -> 29.2Kwh/dollar if we use the energy from the panels entirely efficiently. This is about $1.2 on the absolute low end. Note that panels require some maintenance, and don't just die after 10 years.
Now that we have the energy, lets get our raw materials. Gasoline has a chemical formula of C8H18, a ratio by weight of 96:18 or 16:3. A Gallon of gas weighs about 6 pounds, meaning about 5 pounds of carbon to 1 pound of hydrogen.
Hydrogen is not cheap [4], Electrolysis costs some 2.4K USD per ton, 1.2 dollars a pound. Electrolysis is more than energy in, hydrogen out. It uses up electrodes for instance. If we are allowed to use fossil fuel derived hydrogen the price comes down to 0.2 dollars a pound [5].
Carbon is $35/ton, but this increases to $75/ton if we want beverage grade [6]. You could argue that we will need to purify the CO2 ourselves either way so we can use the cheaper carbon. Carbon to oxygen by weight is 12 : 30 or 2 : 5. So for 1 pound of carbon we need 3.5 pounds of CO2. This brings our cost to $122.5/ton or about 0.12 cents a pound.
Thus our material cost, including no synthesis is already in the ball park of $2 -> $12.9 per gallon.
We should also consider that gasoline has costs that our hypothetical fuel would also need. Additives make up 30-70 cents per gallon. Distribution another 30-60 cents a gallon. Taxes another 40-60 cents a gallon [7].
Adding these costs, our hypothetical fuel now costs $3 -> $14.8 per gallon. But we still need to actually add the costs for turning the ingredients into fuel. We also don't have any labor costs, or costs of capital.
