DARPA has been instrumental in cracking the code for the affordable conversion of plant oils into a drop-in replacement for JP8 jet fuel. Currently the agency is tackling the tougher task of turning cellulosic and algal feedstocks - which would not compete for water or land with food crops - into a viable alternative to petroleum-based JP8.
But all that wasn't DARPA-hard enough, it seems. Now the agency is seeking ideas on how to convert seawater to liquid fuel.
Turning water into thrust? (Photo: US Navy)
According to the solicitation, DARPA is "seeking ideas to utilize the abundance of carbon and hydrogen sources in ocean water to create liquid fuels. The objectives are to: (1) identify a novel catalytic route for converting seawater to liquid fuels or intermediates in a known liquid fuel pathway, and (2) improve conversion efficiency and conversion rate, as well as increase the process scale beyond the bench top. Submissions may focus on harvesting carbon dioxide from the ocean or using the primary form of dissolved carbon dioxide, bicarbonate, directly."
The problem with biofuels is the water required for cultivation and processing of the feedstocks, with fresh water expected to become an increasingly valuable resource in years to come. Take jatropha, one of the most promising feedstocks for second-generation "sustainable" biofuels because it grows on arid land. Figures cited by NASA indicate it takes 20,000 gallons of water to produce one gallon of biodiesel from jatropha. For more information see here.
With some of the other options it would require huge areas of land to meet demand for biofuels. Switchgrass is a cellulosic feedstock that requires only low to medium use of water, but NASA figures show it would take all of the land currently farmed in the US to meet half the nation's fuel needs.
Algae looks so promising because it can produce a lot of oil from a small area, but NASA points out that the two main ways of cultivating algae for fuel will consume lots of water - through evaporation for open ponds or raceways, and for cooling of photobioreactors. It can be waste water, but it's still water.
So using seawater would be great. NASA engineers are working on one idea, to grow algae in plastic bags of sewage towed behind trawlers in the ocean (honest!). Others are working on "saline agriculture" ideas that would produce fuels from halophilic (salt-loving) plants cultivated and harvested conventionally on otherwise unusable land irrigated with seawater.
But the ability to produce liquid fuels directly from seawater would appear to be the Holy Grail of the alternative-fuels movement. Over to you DARPA!