US government boffins have come up with a cunning plan to use diesel fuel for two purposes – both conventionally to generate power, then afterwards as drinking water. The technology to be used will also be of interest to airship enthusiasts, as it could be used in one of the major problems facing helium-filled dirigibles.
The current problem facing scientists at the US Oak Ridge National Laboratory is that of logistics for forward operating bases in Afghanistan. Such bases tend to get their power from diesel generators, requiring many truckloads of fuel each month to be hauled in through hostile badlands full of bombs and ambushes. More trucks must haul water; each soldier needs gallons of water each day to survive and fight under a crippling burden of gear in brutal temperatures.
But it's a little-known fact that hydrocarbon fuels make water as they burn. A typical equation (assuming completely efficient combustion) sees dodecane plus oxygen from the air turn into carbon dioxide and water thus:
C12H26 + 25O2 → 12CO2 + 13H2O
Roughly speaking, "one gallon of diesel should produce one gallon of water", ORNL boffin Melanie DeBusk tells MSNBC. This could mean something of a logistical miracle: each tanker of diesel could effectively deliver a tankerful of water as well for free.
The problem is that the water comes out of the diesel engine as hot steam in the exhaust. Steam can, of course, be condensed back into water: but conventional means of doing this – which effectively involve cooling the exhaust down using refrigeration – require a lot of machinery. They also rob power from the engine, and require somewhere handy to dump the heat extracted from the exhaust. Normally this heat dump is a large amount of water, which in this application is – by definition – not going to be around. Dumping heat to the ground or the surrounding air is more problematic still.
Condenser-style water recovery was the initial route tried at ORNL, but the solution was deemed "un-deployable" by the military: it would have been so troublesome to put in place that it would actually involve less effort and danger to simply truck water in. In this respect it seems that today's military logistics may be somewhat less advanced than they used to be: nowadays water is drawn from boreholes and trucked across the desert, but as long ago as 1867 British armies in dry country were using condenser equipment to make water from the air.
The ORNL boffins didn't give up, though. They're now trying a fiendish new idea called "capillary condensation". In this, the exhaust is run through special pipes made of microporous ceramic. The tiny pores in the pipes naturally suck water molecules from the inside to the outside – no cooling, no energy required. This kit has been investigated recently by industry for the purpose of recovering water – and waste heat – from boiler smokestacks. According to the US Department of Energy (four-page PDF/510 KB) in some applications up to 90 per cent of the water in exhaust gases could be recovered.