Since many military aircraft are going to need chemical fuels indefinitely, it makes sense that the Air Force is interested in electricity-to-fuels. There was an analogous surge in researching coal-to-liquids in the 1970s after the oil price shocks. Liquid fuels are going to have at least military demand for a very long time, and planners are interested in mitigating supply threats before they become supply emergencies.
Flexibility and utility are gained, in the form of a high-enegy-density (by both volume and mass) energy store with excellent storage capabilities, a widely-developed extant handling and utilisation infrastructure and knowledge, and, considering the capabilities, extremely positive safety and interactions properties: hydrocarbon fuels are non-explosive, non-toxic with incidental exposure, and do not erode metallic or polymer components of fuel and power systems.
Net energy loss is at least 40% of input (hydrolysis to produce hydrogen), as well as the energy cost of carbon capture (this varies by methods). For seawater-based carbon sourcing, net synthesis costs of about 50% are what I've seen (see my earlier long comment on sources and references).
For seawater extraction: "CO2 extraction from seawater using bipolar membrane electrodialysis", Matthew D. Eisaman et al is a 2011 paper discussing CO2 extraction via a "BPMED" (bipolar membrane electrodialysis) process. It delivers CO2 at 1.52 kWh/kg, vs. a value of 0.54 kWh/kg cited by Terry, though it's not clear on first reading if Terry performed actual experiments or used theoretical values.
http://talknicer.com/co2extraction.pdf
My recollection is that atmospheric extraction costs are 1.5--3x higher, though I'm uncertain of that precise figure.
Recovered energy through Carnot and Rankin-cycle engines is on the order of 20--40%. The round-trip energy recovery from input electricity to delivered motive power is about 10--20%. Those are inherent to the processes and cannot be improved on.
Note that electric generation itself converts only about 30--40% of input thermal energy (from a nuclear reactor in the US Navy's scenario). Again, this is an unavoidable energy loss given the physics of thermal-to-mechanical energy conversion
Military aircraft and marine vessels can make at best limited use of direct electrical power given limitations of batteries, extension cords, and transmission lines, particularly with regard to energy and power demands. Any fuel, even
For a large established base there might be some benefit. But there's also the last-mile problem.
Aircraft are effectively addressing the last-mile fuel delivery problem directly (by carrying it with them). They tend not to loiter long over the engagement area.
Long-mission drones might be an exception to this. If sufficiently lightweighted and dedicated largely to surveillance, these could benefit by solar + battery electric power. Personnel risks would be low, and aircraft flight dynamics improve as scale is decreased (square-cube law of lift (square) vs. mass (cube) relation. This is the inverse of aerostats, which are more efficient at providing lift with size.
Solar and wind have the issue of land usage, in that the large land requirements of renewables mean more land to secure. That and military bases do not have the luxury of being conveniently sited next to the best places for such things.
It would hardly be the first environmental disaster happening on a military base. Even as recently as Iraq and Afghanistan: https://www.military.com/benefits/veteran-benefits/what-burn...
Even in non-combat environments (McMurdo Station, Project Iceworm), proved nuclear power solutions (adaptations of naval reactors used successfully in submarines) have proved nonviable.
The risks of a reactor in a live-fire zone are ... considerable.
Land-based actions have a last-mile problem in that to put and establish boots on the ground, they need to traverse that mile directly, and extend supply lines to that last mile.
Tactical aircraft have an effective combat range measured in hundreds or thousands of miles. Mechanised infantry in miles or tens of miles. Foot soldiers in yards to miles.
A Naval task force's operation is at sea, outside the effective range of virtually any of the opponents the US has faced in combat since WWII.[1] Aircraft and crew depart a carrier or other base, conduct a mission, and return to base, outside the area of engagment. The logistics chain occurs through what has been for nearly three quarters of a century non-hostile territory.
An FOB is right in the stinkin' middle of the mess. It's within the area of engagement, supply lines move through hostile territory, are subject to ambush attacks, both by live opponents and remotely-activated or passively-triggered IEDs and mines. Total supply requirements are too great for aerial supply alone.
That's the last-mile problem.
At the same time, FOBs and other installations are subject to enemy attack, and large-scale renewables deployments and synfuel equipment would be attractive and viable targets for relatively simple attacks (mortars, drones, missles), which could easily degrade, disable, or entirely destroy such equipment.
I'm unsure of what a major Army or Marine unit's fuel requirements are, but assuming a 40% conversion efficiency from sunlight and 8 hours at 200W per m^2 of PV array, creating 1 barrel of oil per day (42 gallons) would require on the order of 2,700 m^2 of PV array, or a square roughly 50 m on a side. A 100m square might provide 4 barrels/day.
There might be some land-based operations which could support this, but I strongly suspect many could not.
Actual solar performance would also likely be far lower, likely yielding only 25--50% of the output I'm listing here (spacing factor, overcast, and other standard reductions on nameplate capacity), even before accounting to combat-based degradation.
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Notes:
1. Five ships were lost to mines during the Korean war, the USS Liberty was scrapped after it was attacked by Israeli forces in 1967. The USS Cole was damaged, but not lost, in a suicide-bombing attack in 2000. Numerous other vessels have been lost largely through accidents and occasional sabotage (all by US nationals or service members). The USS Pueblo was captured intact in 1967. https://news.usni.org/2012/08/28/notable-us-navy-ships-lost-...
That said:
- The pilot projects are essentially small-scale deployments. The principles are being developed at what's effectively "personal scale", plus or minus.
- I expect that there would probably be economies of scale in several regards, including in operations, maintenance, and training.
- At small scale, if you want fuel, growing it may be the more robust and reliable option. This gets back to the fundamental problem that fuels require considerable energy inputs. For an off-grid site, a firewood-fueled stove, possibly driving a generator through some mechanism (turbine, steam engine) or a biomass-based oil (olive, corn, canola, algae, ...) or alcohol (via fermentation) production serving needs for which liquid fuels were absolutely essential might be more reliable.
Keep in mind that a little over a century ago, the US was effectively biofuel powered, with a principle prime mover in the form of the horse. Horsefeed consumed roughly 20% of total US agricultural output at the time. When I first started looking into alternatives to a fossil-fuel based world, my assumption was that biomass would be a readily-viable substitute. In reality, there simply is not enough net primary production (plant growth) to reasonbly support present levels of human energy consumption, let alone projected growth through economic advancement and population increase through the 21st century. If humans are to live on biomass (as we once did), either standards of living, or population, or both, will have to decrease quite considerably. (This give rise to some of the more modest suggestions of a maximum viable long-term human population, ranging from about 500 million to 2 billion, suggested by some experts on the topic.)
One of the prime positie attributes of hydrocarbon fuels, especially the slightly heavier grades (kerosene, diesel, fuel oil, etc.) is that they travel and store exceedingly well. So long as 1) a synfuel capability exists and 2) your off-grid site has some commerce with the outside world, trading for fuel would likely be your best bet.
Tell me why this is a bad idea?
Of course, where possible the solution is to just use batteries, but try telling frequent fliers that they'll need to take a train in the future. We're going to need some synthetic fuels where energy density is paramount.
You would have to spend trillions of dollars though.