Batteries that “drink” seawater could power long-range underwater vehicles | Hacker News Reader

Batteries that “drink” seawater could power long-range underwater vehicles

news.mit.edu

79 pointsyurisagalov18 comments

My chemistry's weak, but isn't this basically a way to extract the energy put in when aluminum is refined? A quick internet search suggests that's 75 KWh/kilo. Lithium Ion batteries are apparently 0.2 KWh/kilo, so even assuming a lot of loss in refining and then "burning" the aluminum, it seems plausible.

Well, it's absolutely the case that seawater isn't fueling it.

Edit: Removed incorrect speculation. Sorry, HN.

It turns out that they actually are using the water - in the same way that an IC engine would breathe air.

If fuel cells aren't batteries, I wonder if this is still one?

Based on the article's description, the water is a part of the main chemical reaction. It sounds like the reaction (essentially) H20 + Al -> H2 + Al(OH3), with an unspecified alloy of alluminum.

Basically yeah. It's a primary cell using seawater as one electrode.

Instead of protecting the lithium battery from the environment, the aluminium battery embraces it. At first glance this sounds brilliant.

Aluminium batteries are interesting and may be promising. According to Wikipedia, in 2002 Yang and Knickle concluded:

The Al/air battery system can generate enough energy and power for driving ranges and acceleration similar to gasoline powered cars...the cost of aluminium as an anode can be as low as US$ 1.1/kg as long as the reaction product is recycled. The total fuel efficiency during the cycle process in Al/air electric vehicles (EVs) can be 15% (present stage) or 20% (projected), comparable to that of internal combustion engine vehicles (ICEs) (13%). The design battery energy density is 1300 Wh/kg (present) or 2000 Wh/kg (projected). The cost of battery system chosen to evaluate is US$ 30/kW (present) or US$ 29/kW (projected). Al/air EVs life-cycle analysis was conducted and compared to lead/acid and nickel metal hydride (NiMH) EVs. Only the Al/air EVs can be projected to have a travel range comparable to ICEs. From this analysis, Al/air EVs are the most promising candidates compared to ICEs in terms of travel range, purchase price, fuel cost, and life-cycle cost.

Interestingly, Al batteries are missing from the MIT battery primer [0].

[0] http://web.mit.edu/2.009/www/resources/mediaAndArticles/batt...

I think there's a pretty big difference between an aluminum air battery and this.

They will need a way to cope with: sand grains, lime, phytoplancton, gelatinous zooplancton, marine snow, sharks atracted by electric fields... will need a microporous filter and a way to force the saltwater into the device.

The problem with this concept is that this batteries could fail suddenly. Will the oceanographers want to use it and take the risk? Even a small cube full of Salinity and temperature sensors etc, can be valued in several millions.

A way to solve it could be to design a saltwater circuit totally closed and autonomous. Like a gas deposit. Could even act as a shield for the machine. A hole on this deposit and water entering on it? no harm done. Could be even an automatic activation method for several types of rescue systems (Water entering in the machine/ship after a crash, seawater-batteries activating automatically)

I recently saw a presentation by some college students that investigated the feasibility of running ocean gliders on nuclear power. The proposal was to use a nice sized chunk of strontium 90 for a radioisotopic thermal generator. This would provide 45 watts electrical power for about 10 years.

The interesting part was not the technical feasibility, but how they were going to make a case to the NRC to make such a use of radioisotopes legal. Apparently there is a precedent for this, some scientific ice monitoring equipment has used strontium 90 RTGs.

Perhaps this is something Liquid Robotics should (or already are) looking into. That's the company James Gosling works for and they're doing fascinating stuff with ocean robots.

https://www.liquid-robotics.com/

I don't see any performance metrics.

My thoughts exactly. Tough to make any sort of evaluation of this technology when the article doesn't provide any performance data or discuss the tradeoffs of this solution (first thing that comes to mind is how heavy are these batteries?)

I really don't like that the hydrogen made in process is considered waste and is escaping. We sure have a lot of water on planet but if the technology will become mainstream and we will just throw the hydrogen away it will become a huge problem.

It seems like we're a very long way away from it being a practical problem. And if it ever becomes an issue it doesn't sound like it will be very hard to overcome. After all burning this hydrogen would release even more energy and some water, although it would require a source of oxygen.

If anything I'd be more worried about the large scale effects of massive amounts of aluminum hydroxide being released in the environment.

At least aluminum hydroxide is solid, so it's easy to collect. Hydrogen is gas though.

Hydrogen in low concentrations isn't dangerous and it's gonna be slowly bonded into water again whenever there is a flame or lightning strike.

Sunlight is also important for these reactions. There is quite a bit of ozone which combines with hydrogen to become oxygen + water.

Orders of magnitude are important. There are 326,000,000 cubic miles of water in the oceans. 1 cubic mile = 5280 * 5280 * 5280 * 7.48 gallons/cubic foot * 8.35 pounds per gallon. Aka ~ 3 * 10^ 21 pounds and 21 is one of those exponents that are hard to wrap your head around. By comparison worldwide aluminum production is only 1.1 * 10 ^ 7 pounds. Further, only the fraction of hydrogen that reaches space is lost to space making this a non issue.

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