New tech lets you drink exhaust fumes
Handy in Afghanistan – or in an airship
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.
The other neato thing about this so-called Transport Membrane Condenser tech is that the ceramic micropores also act as a very effective filter, removing all the contaminants found in diesel exhaust and producing nice clean water.
DeBusk seems to hint that the ceramic water-making kit needn't be installed solely at electrical generator plants. She points out that a Humvee-tankful of diesel could also yield a day's supply of water for three soldiers, suggesting that US military vehicles might be able to make drinking water for their crews in future.
It's all an intriguing idea, then, right enough – but what's all this got to do with airships?
Well, as regular dirigible-fancying readers will know, one of the main problems with helium-filled blimps (or rigid ships as in the great days before the Hindenburg disaster) is that as the ship flies along and burns fuel it gets lighter. As the airship becomes more and more buoyant it will not only become more or less impossible to bring it in for a landing – after a while it will rise through its "pressure height", where the lifting gas has expanded to fill the entire envelope. Gas will then be lost through safety valves designed to prevent the ship bursting.
Helium being expensive stuff, it isn't acceptable to vent off gas (or lose it during excursions above pressure height). Thus, helium ships intended to make longhaul flights have generally had to have some means of dealing with increased buoyancy due to burning fuel.
In the 1930s the US Navy's mighty aircraft-carrier dirigibles  and the German Hindenburg and Graf Zeppelin II* were all designed with condenser-type machinery intended to recover water from their engine exhausts, rather in the fashion of the "undeployable" equipment recently rejected by the US military. This kit, which naturally had to dump heat into the air passing by the ships' hulls, was heavy and took up capacity which could otherwise have carried more fuel or payload. It was also troublesome and difficult to maintain in operation.
The modern fashion is for "hybrid" airships which are meant to operate in a heavier-than-air condition, aided by vertical thrust from their engines or lift from their hulls moving through the air. These ships have no need, perhaps, to accumulate water ballast to make up for burned fuel.
At least one modern contender, however – the Bullet 580 ship about to go to work for NASA  – uses "water condensate" methods to control its buoyancy. Other non-hybrid designs such as the Zeppelin NTs – successors to the great zeppelins of old – might find the new capillary-action water recovery tech to be of use, especially if it really can be made small enough to go on Humvees.
So airship fans might also find the new microporous ceramic technology interesting, as well as watchers of cunning military solutions.
Definitely one to keep an eye on. ®
*LZ-129 Hindenburg was designed and intended to be helium filled. In the event the US government, controlling the entire world helium supply, refused to furnish any to the Nazi regime and she was filled with explosive hydrogen.
LZ-130 Graf Zeppelin II which followed the Hindenburg, was also meant to be helium filled: her water-recovery equipment was thought to be the best yet developed at that point. However she too wound up filled with hydrogen. After the Hindenburg disaster, the German authorities refused to allow her to carry paying passengers and she had only a short flying career before being broken up for her aluminium.