December 21, 2018
Existing water-splitting methods rely on highly purified water, which is a precious resource and costly to produce. To overcome this issue, the Stanford-led team demonstrated a new way of separating hydrogen and oxygen gas from seawater via electricity.
How Hydrogen fuel is better than other fuels?
Scientists from the U.S. Naval Research Laboratory (NRL) have demonstrated significant progress in their novel gas-to-liquid process, which simultaneously recovers carbon dioxide and hydrogen from seawater, and report that it can produce a fuel-like hydrocarbon liquid which may eventually offer a renewable replacement for petroleum based fuel in jet engines.
The ability of Naval Vessels to generate fuel conveys numerous advantages. First off, during conflict this means that the aircraft carrier can maintain a constant supply of fuel without having to spend time away from the mission by returning to land to re-fuel. This time could be significant if the surrounding countries are not friendly forces. Second, fuel supplies are often targeted during conflict, which puts certain countries at an immediate disadvantage if a sufficient amount of fuel cannot be sourced. Generating fuel on-board would immediately remove this risk which has the potential to jeopardize missions.
Seawater is a particularly attractive carbon source for fuel not only because of its obvious abundance, but it contains carbon in the form of CO2 in much higher concentrations than in the air. Scientists at NRL have developed a way to remove CO2 from seawater with a concomitant production of hydrogen (H2), which are the building blocks of hydrocarbons. They achieved this through the use of electrochemical acidification cells.
The production of hydrocarbons, which are compounds solely made up of hydrogen and carbon, from the recovered gases is a two-step process. First, the CO2 and H2 are converted into unsaturated hydrocarbon starter molecules called olefins using an iron-based catalyst. Next, these olefins are converted into a liquid containing larger hydrocarbon molecules with a carbon range suitable for use in jet engines by polymerization. It should be stressed that this is currently a lab-based model system, although the team say that they have made significant advances in this gas-to-liquid system, and proved that the fuel-like liquid generated contained molecules in the required C9-C16 range.
The team claim that the efficiency of this process is far superior to previously developed techniques for CO2 recovery from seawater; this technology removes CO2 at 92% efficiency. Obviously energy will be required as an input to drive the system, and currently this energy is going to come from fossil fuels. It's not a miracle "green" system that can create renewable energy from nothing, so at the moment there is still a reliance on fossil fuels. But it is not all doom and gloom- if this system can be coupled with a renewable energy source which is also built on the aircraft carrier, for example solar cells, or perhaps even more ideally a small nuclear reactor, then the system has the potential to be very sustainable in the long-term.
They predict that this technology could produce jet fuel at around only three to six dollars per gallon, which is impressive. With enough investment, they believe that it could become commercially viable in less than 10 years.
The U.S. Department of Defense (DoD) is the single-largest consumer of fuel in the world and Jet fuel accounts for 71% of the entire military’s petroleum consumption. Therefore, DoD needs to maintain a strong logistical support to provide jet fuel for its equipment across the world.
It is particularly more challenging to maintain a supply of fuel to aircraft carriers as they tread international waters across the globe. So, many aircraft carriers, such as the Nimitz class carriers are powered by nuclear energy. Nuclear powered carriers can run for decades before refueling is required. However, they still need jet fuel supply for the fighter jets on-board the carriers. For example, a Nimitz class carrier can carry about 50 fighter jets and it can store about 3M gallons of jet fuel. This amount of jet fuel can refuel the 50 on-board fighter jets about 20 times each. Therefore, additional ships are required to periodically supply jet fuel to the aircraft carriers, which limits their operational flexibility as they require frequent refueling.
It would be advantageous if the jet fuel could be synthesized on the aircraft carriers themselves. In theatre of war, synthetic fuel production would offer significant logistical and operational advantages by reducing the vulnerabilities resulting from unprotected fuel delivery at sea. A carriers’ ability to produce a significant fraction of the battle group’s fuel for operations would increase the operational flexibility and time on station by reducing the mean time between refueling.