Audi has been working on a synthetic diesel fuel and is currently producing test samples, with plans for industrial production. This is potentially a useful technology (depending on the details) but, as is almost always the case, is widely misreported.

For example, Gizmag states: “Audi just created diesel fuel from air and water.” Farther down in the article they do mention that you also need another critical ingredient – energy.

Engadget reports: “The automaker recently produced its first batch of “e-diesel,” a synthetic diesel based solely on carbon dioxide and water — readily available chemicals that are far nicer than sulfur and other typical diesel elements.” They never make mention that the process requires energy.

I don’t think this is a nitpick, because already the Audi story has been mentioned to me by someone who did not understand, until I pointed it out to them, that processes such as this are not a source of energy or fuel, they are simply an energy storage medium. Saying that fuel is made from “carbon dioxide and water,” while not wrong, is incomplete and fosters a fundamental misunderstanding of what is going on.

The same reporting occurred last year when the US Navy announced their plans to make fuel from seawater. Science.dodlive reported, “Energy Independence: Creating Fuel from Seawater.” That was one step more misleading by invoking the notion of energy independence.

The same confusion remains common around hydrogen fuel. Since we have little free hydrogen on earth, hydrogen is not a fuel source, just an energy storage medium. If someone is advertising an engine that can run on water you can be almost guaranteed that what they are doing is electrolysing water into hydrogen and oxygen, then burning that hydrogen back with oxygen. Of course, the laws of thermodynamics ensure that it costs more energy to electrolyse the water than you get back when you burn the hydrogen with oxygen.

The analogy that seems to get people to understand what is going on most easily is that of a battery. No one claims that batteries will make us energy independent because we can run cars on batteries. They more intuitively understand that you need an energy source to charge the batteries. The same is true of synthetic fuels, including hydrogen.

Synthetic fuels are still useful, however. In the case of the Navy, they were not looking for an energy source, but a way to use the power generated by the nuclear reactors on aircraft carriers to make fuel that can then be dispensed to smaller support craft that don’t have their own nuclear power plants.

The big advantage to synthetic fuels, including Audi’s new e-diesel, is that they are carbon neutral in themselves. In the case of the Audi process, they start by using an energy source to electrolyse water into hydrogen and oxygen. They then combine the hydrogen with carbon dioxide taken from the air, creating what they call blue crude. This undergoes further processing into e-diesel. Right now their pilot plant can make about 1,000 liters per month, which is nothing. They will need to ramp up to industrial scale production, something which can never be taken for granted.

The e-fuel, like bio-fuels, is carbon neutral because it takes carbon out of the air and then puts it back when burned. Fossil fuels are a problem because they take carbon sequestered under ground and add it to the carbon cycle, increasing the amount of carbon in the atmosphere.

There are always two major factors that need to be considered, however. The first is – what is going to be your energy source? Stories, like all the stories about the Audi fuel that bother to even mention the need for an energy source, always say that renewable energy sources will be used. That is not something that can blithely be taken for granted, however. Having a massive renewable energy infrastructure is a bigger deal than the ability to use that infrastructure to make synthetic fuel. We could also use it to recharge batteries, or run our homes.

I would like to see hard numbers on how much energy production would be necessary to displace fossil fuels in cars. I suspect it will be challenging. In fact we may need nuclear power plants to produce enough energy to replace a significant amount of fossil fuel with synthetic fuel.

Making synthetic fuels does have an advantage, however. They can be a useful energy storage medium because the renewable energy does not have to be distributed, it can be used where it is made, assuming the fuel plant is located where the renewable energy is made. Also, the energy does not have to be on demand. The plant can simply run when the sun shines or the wind blows.

Also, synthetic fuels, if they are compatible with existing cars, can be used without needing to add any major new infrastructure. Simply convert pumps at gas stations to synthetic fuels, and fill up your tank. They also avoid the problem with batteries of recharge time, or adding a new infrastructure to swap out or recharge batteries.

The second major factor to consider is the efficiency of the entire process, from plant to car. If the process is inefficient then it may use too much energy to realistically scale up. This is the ultimate limiting factor with biofuels – getting the process efficient enough. One way to measure overall efficiency is cost. How much will a gallon or liter of e-diesel cost?

I suspect this is mostly why, even though we have been hearing about synthetic fuels for decades, we aren’t running our cars on synthetic fuels today. How are we going to power the whole process, and can we produce it efficiently at industrial scales so that it will be cost effective?

Because of anthropogenic global warming, however, the question can be reframed as – how much are we willing to spend for a carbon-neutral synthetic fuel to avoid pouring more carbon into the atmosphere? What may happen is that governments will subsidize the production of synthetic fuels to bring their cost down to a level that is competitive with fossil fuels. Eventually, through rising fossil fuel costs or improvement in efficiency, synthetic fuel costs may become competitive on their own.

None of the reporting on Audi’s new e-diesel that I have seen mentions any of these issues or gives us any numbers on cost, efficiency, or needed energy infrastructure. Those are the details, however, that will ultimately determine the success or failure of this endeavor.