The US navy has been investigating the production of fuel from seawater using electricity from ship’s nuclear power systems for a number of years. This process would allow aircraft carrier task forces to stay at sea longer without depending on vulnerable fuel tankers to keep the planes flying. The navy has now announced that they have successfully used the fuel from their pilot plant to fly a plane with an internal combustion engine. (Well, OK a model mustang.)
The process used involves electrolysis of sea water to produce CO2 and hydrogen followed by a catalytic reaction to produce hydrocarbons. There is nothing radically new here. Hydrogen has been produced commercially using electrolysis for a long time. There are also well established commercially available processes for converting mixtures of hydrogen and nitrogen or hydrogen and CO2 into a range of useful chemicals and fuels. My guess is that most of the effort taken by the US navy has been focused on developing a process that could fit into a small part of an aircraft carrier.
The potential of these types of development go well beyond the needs of the US navy. Think about it: Unless there is an amazing breakthrough, renewable power plus batteries are not going to be able to deliver 100% renewable transport. Renewable power + batteries is not going be suitable for long distant flights, travel in the Australian outback or long distance sea travel. There is a need for energy intensive transportable fuels to cover these needs.
Bio-fuels are not the answer. Diversion of land to the production of bio-fuels is already causing starvation of people in some countries as well as damage to the environment. (Think jungle clearing for palm oil production.) In addition, the production of bio-fuels is vulnerable to climate change and pests as well as posing potential problems if the organisms used escape into the wild. What is needed are low impact renewable fuels produced by inorganic processes such as the US Navy process mentioned above.
The easiest low impact renewable fuel to produce is liquid ammonia. It is easy to produce because all it needs is water, nitrogen from the air. It may be practical to locate both power source and fuel production close to where it is needed thus doing away with the need to transport fossil fuel based fuels all the way from well to consumer. Ammonia can be used as a transport fuel in internal combustion engines and gas turbines with only minor engine modifications. (It needs about 5% biodiesel in the mix for diesel engines.) Ammonia used as a fuel can be stored and handled in a similar way to LPG. However, ammonia has lower energy densities compared with conventional fossil fuels. (43% by weight compared with Jet A fuel.) This is a serious disadvantage if we are talking about air transport.
It is worth noting that most of the ammonia produced today is made from dirty hydrogen produced from natural gas. The major use of ammonia is nitrogenous fertilizers. Cleaning up the production of nitrogenous fertilizer and other nitrogenous compounds is a bonus that comes with replacing dirty hydrogen with renewable hydrogen.
In the short term cleaner hydrocarbons could be produced using waste CO2 from sources such as steel and cement production. In the longer term the main source of CO2 would have to be the ocean. Keep in mind that renewable hydrogen + CO2 can be used to clean up the petrochemical industry as well as transport fuel.