by Denis Dilba April 2017
Whenever the future of mobility is under discussion, it quickly involves the “usual suspects”: electric vehicles – be it automobiles, bicycles, motorcycles or trucks. Even ships and aircraft should be operated fully electrically in the future if at all possible. In a few years – it seems – there’ll hardly be any type of vehicle that won’t have a drive battery in one form or another on board. After all, we have to do something against the emissions from internal combustion engines that are harmful to the climate and to our health. Plus, we’ll hardly have any other options sooner or later because petroleum resources will soon be depleted. This means that the days of liquid fuels like gasoline or diesel are counted, doesn’t it? That’s not completely wrong but not completely right either, says Wolfgang Warnecke, Chief Scientist Mobility of the Durch-British oil corporation Shell: “I think that the future viability of liquid fuels in particular is underrated.” This, he feels, is due to the fact that implications and correlations are often heavily simplified in the discussion of the future of mobility. “A closer look, though, shows that they’re a lot more complex – and then the answer is no longer as simple as originally thought,” says Warnecke, who has a PhD in engineering.
Generally, an in-depth investigation is necessary to determine what combination from the wide variety of the powertrain concepts and energy carriers available today is best suited for what type of mobility. “A single solution optimally suited for all forms of mobility will, unfortunately, not exist in the foreseeable future,” says Warnecke, because everything has its advantages and disadvantages, depending on the application: IC engines, fuel cells, batteries, liquid and gaseous fuels. Batteries, for instance, have the major advantage of not causing harmful emissions locally. But Warnecke adds: “The greater the distance you want to drive with them the larger, heavier and, above all, more expensive the batteries become. And when they’re empty it takes at least 20 minutes to ‘fill up’ on electricity even when using quick-charging systems.” Added to this is the fact that the production of the electricity that is used to charge the batteries causes carbon dioxide emissions as well, depending on the available electric power mix. Of course developers around the world are working on improving all aspects of battery cell technology but Warnecke doesn’t expect any huge leaps in the coming years.
Points-win for the fuel cell
That’s why at the moment fully electric powertrains only make real sense on small city cars where they help improve the frequently poor quality of air today while offering customers an acceptable compromise between range before reaching the next charging station, available space, weight and price, the Shell expert explains. But even for slightly longer distances, there are indications that fuel cell powertrains may have the edge over batteries. On one tank of hydrogen, fuel cell vehicles can cover a longer distance than battery-electric vehicles on their battery charge. Not to be underestimated either is the time advantage when refueling the vehicle, says Warnecke: “Hydrogen is put in a tank just as quickly as gasoline or diesel.” Although hydrogen filling stations are still lacking, this disadvantage applies to quick-charging stations for batteries as well. By contrast, the infrastructure for liquid fuels already exists. But that’s not the only reason why today there’s hardly an alternative to them whenever it comes to even longer distances, higher payload and tight schedules.
» Liquid fuels are underrated
Shell Chief Scientist
One of the main reasons – and also the one why the position of liquid fuels has been uncontested for more than 100 years – is their high energy density compared with all other types of fuel. “When I compare one kilogram (2.2 lb) of gasoline, diesel or kerosene with one kilogram of battery, the liquid fuels contain up to 40 times more energy than even future electricity storage systems will,” says Warnecke. In all fairness, though, he adds that the efficiency of IC engines is clearly below that of electric motors. In other words, clearly less of the energy contained in fuels reaches the driven wheel than in a powertrain combining a battery and an electric motor. But even when factoring this in, about 15 times more energy can be extracted from the same volume of liquid fuel than from a battery. And that, the scientist explains, has crucial advantages: “The volume per energy unit is smaller, so less space for storage is needed, which also means less onboard weight, which additionally lowers overall consumption once more.” Plus, he adds, that more energy would get into the vehicle per unit of time, so refueling is faster.
That’s also why in the foreseeable future there’ll be no serious alternatives to kerosene in aviation. “Batteries and fuel cells as principal propulsion systems are currently out of the question for passenger planes simply due to the required sizes,” says the Shell expert. All the space would be taken up by the propulsion and energy storage systems and the passengers would have to stay on the ground.
Where to put carbon dioxide?
“However, if we have to or want to continue using liquid fuels, we’ll have to think about solutions for dealing with CO2 and pollutant emissions,” says Warnecke, because at least CO2 is produced in any combustion process. Currently being investigated and discussed are so-called carbon capture and storage (CCS) technologies of storing the greenhouse gas in underground rock layers or salt cavities. However, they’d have to ensure safe storage of CO2 for centuries. There are doubts about this really being possible. Therefore, research done by Shell scientists includes a method of converting CO2 into climate-neutral rock, among other things. The most elegant solution, though, would be to just use CO2 again as a raw material for the production of liquid fuels. Indirectly, for instance, this is possible using biofuels from plants. They absorb CO2 from the atmosphere, are harvested and subsequently processed into fuels.
This method, though, is controversial as well, because producing fuel instead of food from sugar beets, corn or rapeseed is questionable. And even growing plants that are not suitable for food production at least requires a lot of space. On the other hand, the utilization of plant residues wouldn’t be harmful but so far scientists haven’t been able to achieve any real breakthrough here. Stefan Jennewein, a biochemist at the Fraunhofer-Institute for Molecular Biology and Applied Ecology (IME) in Aachen, Germany, has therefore abandoned biofuels and developed an alternative, using genetically modified bacteria to produce short-chain alcohols and acetone from CO2. From these substances, kerosene or marine diesel oil can be produced in a comparatively simple process. The potential, the scientist says, is huge. With the CO2 emissions of just one major steel mill an airline providing international service could be supplied with kerosene, according to his estimates. At the moment, he’s working on a pilot plant to convince investors of this method. Investors are absolutely necessary to market the technology because, says Jennewein, we’re talking about millions of tons of fuel per year.
By means of such methods the carbon dioxide emitted by automobiles, aircraft, ships and factories could be captured and reconverted into fuel in the future as well. “Ideally, such processes would consume exactly as much CO2 as we emit in the combustion process of such synthetic liquid fuels,” says Shell scientist Warnecke. “This could then be called CO2-neutral.” However, not all the problems would be solved this way. Local emissions of pollutants such as nitrogen oxides or sulfur oxides are also produced in the combustion process of synthetic liquid fuels.
While research is dedicated to this aspect as well, “new liquid fuels don’t give us a license to simply continue to use them for every form of mobility,” says Warnecke. “Wherever we’re able to use sensible alternatives we need to do so.”
Even if Shell’s chief mobility scientist Wolfgang Warnecke and technology journalist Denis Dilba (39) had spent several hours more on this discussion, the results concerning the future of mobility would just have been snapshots. One thing is clear, though: liquid fuels will be staying with us in this challenging context longer than we may have thought.
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“Neither in the passenger car nor in the commercial vehicle sector will we be able to get around using the internal combustion engine. That’s also why we welcome the development that increasingly puts the focus on synfuels. When looking at the energy chain of synthetic hydrocarbon compounds – produced from solar power for instance – CO2 may become a raw material. Although this debate already took place years ago, it never really progressed. But now it’s being resumed. These new fuels will then require efficient combustion and the diesel engine with its efficiency may be an example in this context. However, changed overall conditions as well as the development of costs will cause the share of diesel engines to decrease.”