杏吧原创

LIVING underwater is about to become a lot easier. The US Navy has developed
a way to clear carbon dioxide from air using the most convenient substance
颈尘补驳颈苍补产濒别鈥攕别补飞补迟别谤.

鈥淩emoving CO₂ is very important,鈥� says Gareth Toft, a research
scientist with the Submarines Atmospheres Group at the British defence lab
Qinetiq鈥攆ormerly the Defence Evaluation and Research Agency鈥攊n
Hampshire. 鈥淲ith levels over 5 per cent you鈥檙e in immediate trouble. At 10 per
cent it鈥檚 fatal.鈥�

But at the moment, the chemicals used to remove exhaled carbon dioxide in
submarines and underwater habitats have to be replaced at frequent intervals.
Lew Nuckols at the US Naval Academy in Annapolis, Maryland, says the idea of
using seawater came to him when he heard about the idea of using the oceans as a
sink for CO₂to curb global warming.

Nuckols has successfully tested the technique, which has been granted a US
patent, in an underwater habitat called Aquarius off Key Largo in Florida. It
works by pumping seawater into the top of a 2-metre-high tower, where it is
atomised into tiny droplets by spray nozzles.

Cabin air is pumped into the bottom of the tower. From there it flows through
the water spray and out through a filter at the top of the tower. The water
absorbs CO₂ from the air, and is then pumped out of a drain at the
bottom of the tower and back into the sea.

The trick is getting the right water and gas flows to optimise how much gas
is absorbed. Seawater absorbs more CO₂ if it is pressurised or cooled,
so by varying the temperature and pressure, the system can be regulated to scrub
the appropriate amount of CO₂ for the number of crew on board.

Ideally you would want to make it work without cooling or pressurising, says
Nuckols, since these processes increase its energy consumption. On the Aquarius,
he has successfully showed that this is possible for small underwater
stations.

The seawater tower technology would allow aquanauts to stay submerged for
much longer than is now possible. At present, Aquarius needs more than 25
kilograms of chemicals a day to scrub CO₂ from the air. This has to be
brought down from the surface and hauled back up again. 鈥淚t鈥檚 a severe
logistical problem,鈥� he says. The oxygen supply would still need to be
replenished, but underwater habitats such as Aquarius can get oxygen by
periodically pumping fresh air down from the surface through a vent.

Nuckols now wants to develop a version to fit into submersibles. Some subs
already have chemical scrubbers that don鈥檛 need to be replenished. One design
uses monoethanolamine (MEA), which has a high affinity for CO₂ when
cooled and releases it again when heated. But these scrubbers consume so much
energy that they can only be used on nuclear submarines. 鈥淲ith our system you
only need a little pump and a circulation fan to make it work,鈥� Nuckols says.