It must take more energy to break the bonds of water, extract CO2, and recombine into usable fuel than you will ever get out of fuel so produced.  We’ve known for a long time how to split hydrogen from water. But we don’t do that because it takes so much energy to do it that it’s not worth it, which is why 96% of hydrogen comes from natural gas, so this must be a very energy intensive process.

Nor has the Navy hasn’t overturned the laws of physics. Defense One writes that it takes twice as much electricity to convert the water into fuel components as the process yields in terms of power.  The potential energy in the synthetic fuel is much lower than the energy inputs needed to make it.

The EROI of the process is certainly negative – more energy will be used to create the synfuel than what it contains.  Even if the plan is to use nuclear power, then the energy to mine, process, and deliver new nuclear fuel to keep this process going must be subtracted from the overall EROI, not the mention the ship itself, the metal that made the ship and nuclear reactor, and so on.

And is building a bunch of ships with nuclear reactors on board really a good idea? These would be sitting duck floating bombs, tempting terrorist or war targets.

Overly excited non-science writers have made it sound like this will solve Peak oil, but as Mark Draughn at windypundit writes in “Not Quite the End of Big Oil”, that is not the case:

The Navy $3 to $6 per gallon price is the expected price once the process is industrialized. We’re not there yet.

This won’t lead to energy independence for the United States because this is not a new energy source. It’s a process for extracting hydrogen and carbon dioxide from the ocean and “un-burning” them to create a hydrocarbon fuel. However, the principle of conservation of energy tells us that if a fuel produces energy when burned, then the process of creating the fuel must consume energy. Ultimately you can’t get any more energy out of a fuel than you put into creating it, and in practice you’ll get somewhat less, due to inefficiencies in the process.

This will not overthrow big oil because if you have to put energy in to get energy out, then what you’re describing is really an energy storage system, not an energy source. The energy that you put into the storage system still has to come from somewhere else. We could use electrical power to synthesize fuel, but that electrical power still has to be generated, and here in the U.S., over 80% of our energy comes from fossil fuels, and almost half of that is from oil.

Switching our transportation system to use electrical energy would be difficult, because the elements of our transportation system — cars, trucks, trains, planes, ships — all have to carry their energy sources around with them, which means they need an energy source that is portable. More to the point, most modes of transportation require an energy source that is lightweight, which means they must use a storage medium that has a high energy density — that stores a lot of energy per pound of added weight.

Willmott, D. Dec 16, 2014.  Fuel from Seawater? What’s the Catch? Smithsonian.

Scientists at the U.S. Naval Research Laboratory have demonstrated the ability to recover carbon dioxide and hydrogen from seawater and turn it into a liquid hydrocarbon fuel—the kind of stuff that can power a jet engine.

Using a proprietary electrochemical device, researchers were able to pull carbon dioxide from the water, get hydrogen as a byproduct, and then bounce the two gases off each other to manufacture the liquid fuel. The scientists say they can pull about 97 percent of the dissolved carbon dioxide from the water and convert about 60 percent of the extracted gases into hydrocarbons that can be made into fuel

So what’s the catch? Well, there are many.

First, carbon dioxide concentration in seawater is about 100 milligrams per liter. That’s 140 times greater than that of air, but still not very much in real terms. One report calculates that you’d have to process close to nine million cubic meters of water to make 100,000 gallons of fuel, and that’s assuming 100 percent efficiency. Assume far less efficiency, and you have to assume much more water. And the more water you process, the more plankton and other little critters you remove from the food chain—with potentially catastrophic results for marine life.

Secondly, you’d have to pump all that water into the conversion machine using some form of energy, and if the ship uses fuel to make the electricity to do the conversion job, then the whole process would be pointless. So the conversion would need to take place on a nuclear-powered aircraft carrier.

Then, if 60 percent of the gas is converted, what happens to the other 40 percent, including the 25 percent that becomes environmentally unfriendly methane?

And doesn’t flying jets simply put the carbon back into the atmosphere?