Liquid fuel powering internal combustion engines is inherently inefficient. This is because innumerable explosions causing kinetic work to be done also makes piles of heat, and for other reasons. The same amount of energy put into an electric motor and an internal combustion motor produce more usable work for the former than the latter. Also, electric motors can operate at similar efficiencies across a range of speeds, while internal combustion motors require more messing around to change speeds. And then there is torque. Torque is apparently at the center of coolness for many vehicle aficionados. If you can get your hot car or motorcycle to go from zero to fast in a second or two, that is considered cool, even if it has almost no day to day applications. An electric motor has that ability out of the box, an internal combustion motor has to be a super motor to do that well.
Also, liquid fuels spill and smell bad and can explode, and all that. On the other hand, electricity has its limitation too. In the long run, we probably need to change most of our moving things, vehicles, planes, etc. over to mostly electric (with energy recovery from brakes, etc.). But liquid fuel will still be important in certain applications. Mission critical backup generators that you hardly ever need but are life or death are probably best run on liquid fuels stored long term, like at the South Pole research station or in any hospital. We probably will eventually see electric airplanes, but for long time we are probably going to have to put liquid fuel in flying machines. So, in order to not destroy the essential yet merely good enough in pursuit of an unrealistic simplistic perfection of some sort, we need to keep liquid fuels on the table. But, having said that, we need to entirely stop using fossil petroleum based liquid fuels and switch entirely to non-fossil molecules.
One way to do that is to simulate the production of burnable liquids (as nature does) in machines, using non fossil based raw materials. Obviously biodiesel and ethanol are example of this, but these fuel sources have a serious limitation. They take up agricultural resources, and over the next few decades we are likely to hit a ceiling in our agricultural productivity. There are a lot of ways to address that problem, and one of the key ways on the table right now is to not convert much more agricultural land to ethanol or diesel production.
So what about a machine that takes sunlight, CO2 from the atmosphere, and some water and produces a burnable liquid?
The current issue of Science has a writeup on recent research in this area. I’m fairly certain it is not behind a paywall, and can read it yourself: Tailpipe to tank by Robert F. Service.
The writeup talks about multiple alternative research projects that are approaching this problem with various difficulties and various levels of success. This is all very early research but it is all very promising.
The task essentially boils down to running combustion in reverse, injecting energy from the sun or other renewables into chemical bonds. “It’s a very challenging problem, because it’s always an uphill battle,” says John Keith, a chemist at the University of Pittsburgh in Pennsylvania. It’s what plants do, of course, to make the sugars they need to grow. But plants convert only about 1% of the energy that hits them into chemical energy. To power our industrial society, researchers need to do far better. Keith likens the challenge to putting a man on the moon.
The basic method seems to be about the same in all cases. You take a CO2 molecule and convert it to CO by knocking off one Oxygen atom, then combine the CO with H2) to produce “syngas” which can be converted to methanol (a kind of alcohol) which can then be converted into a variety of products. A similar process in widespread use uses fossil methane as a base molecule instead of atmospheric CO2.
A paper about to be published in Advanced Science details a process that uses CO and H2 and photovoltaic generated electricity.
It focuses a broad swath of sunlight onto a semiconductor panel that converts 38% of the incoming energy into electricity at a high voltage. The electricity is shunted to electrodes in two electrochemical cells: one that splits water molecules and another that splits CO2. Meanwhile, much of the remaining energy in the sunlight is captured as heat and used to preheat the two cells to hundreds of degrees, a step that lowers the amount of electricity needed to split water and CO2 molecules by roughly 25%. In the end, Licht says, as much as 50% of the incoming solar energy can be converted into chemical bonds.
This and other methods of making a sun, water, and air based liquid fuel would at least initially be expensive. But who cares? If we convert most of our energy to motion machinery to electric, we won’t need that much, and the remaining uses will be relatively specialized. So what if a hospital has to pay $10.00 a gallon to have a thousand gallons of fuel for use as a backup source of energy to run generators during emergencies? That would be a tiny fraction of the cost of running a hospital. A tiny fraction of a fraction.
And, it need not be super expensive. There is not a rare substance that must be mined from third world war torn client states, or taken away from some other critical use, involved. Go read the original writ-up for a lot more detail on various processes and their potential (and potential costs).
I want to make this point: This is not a way of forestalling climate change by removing CO2 from the air. It would remove CO2, but the amount of CO2 humans have added is huge, and the use of sun/air/water liquid fuels would be small, and their use would return the CO2 to the air. So this is not carbon capture.
Also, this. An industry that produces a synthetic liquid fuel can preferentially use a peak energy. I think we need to explore this idea more. For example, imagine collecting piles of recycled aluminum at a plant that uses great amounts of electricity to melt it down and turn it into ingots for industrial use. The entire plant could be designed to operate on demand and only now and then, when there happens to be piles of extra electricity in a clean-energy rich energy ecosystem, perhaps because it is sunny and windy and other demands happen to be low. The employment structure of the plant would also be designed to do this, drawing on-call workers off of other activities to run the plant. This would essentially amount to carrying out a high energy demand industrial task with free energy. Well, a sun/air/water liquid fuel system could work this way as well. This idea has not gone un-thought:
…Paul Kenis, a solar fuels researcher at the University of Illinois, Urbana-Champaign, argues that the broad penetration of solar and wind power offers hope. Denmark, for example, already produces some 30% of its electricity from wind farms and is on pace to reach 50% by 2020. On a particularly blustery day in July, the nation’s wind turbines generated as much as 140% of the country’s electrical requirements. The excess was sent to its neighbors, Germany, Norway, and Sweden. But the oversupply added to utilities’ fears that in times of peak renewable power production, the value of electricity could fall to zero or even below, as producers would have to pay others to take it so as not to damage their grid.
That’s where solar fuel producers could stand to benefit, Kenis says: By absorbing that power and using it to make fuels and other commodities, they could essentially act as energy banks and perhaps earn some cash as well. For now, Kanan argues, it still makes the most economic sense simply to shunt excess renewable power into the grid, displacing fossil energy. But someday, if renewable power becomes widespread enough and the technology for making renewable fuels improves, we may be able to guzzle gas without guilt, knowing we are just burning sunlight.
Service Robert F. Feature Article. Tailpipe to Tank. 2015. Vol. 349 no. 6253 pp. 1158-1160. DOI: 10.1126/science.349.6253.1158