Sunday, March 25, 2012

Aluminum air batteries for extended trips in electric cars

I like the idea of owning an electric car, except that we frequently make trips to see family members 100 and 350 miles away.  So I've been scratching my head about ways to let an electric car do that.  (The obvious approach is to buy a Chevy Volt or a plug-in Prius).

Today I read up on aluminum-air batteries, which have some neat properties.  They have very simple chemistry from very plentiful sources: aluminum, water and carbon.  They let the aluminum oxidize using oxygen from the atmosphere, and as the aluminum anode dissolves, you get aluminum hydroxide, which can be reprocessed into pure aluminum.

The other great thing about them is their theoretical energy density of 8 kwh / kg, far higher than we see with lithium-ion batteries.

The downside is that they're single-use -- no recharging.  But for a road trip, that may not be so bad -- instead of gassing up, you trade in the depleted battery modules for fresh ones and head back on your way.  I'm imagining a standardized battery module rack in the trunk that you only fill with modules when you're planning a long trip.

The economics kinda sorta work.  This paper claims they can get about 1.3 kwh / kg of the 8kw theoretical maximum, and that they could recycle the batteries for about $1.10 / kg.  So you'd basically be paying $1 / kwh.  The Nissan Leaf takes about 34 kwh / 100 miles, so your road trip miles would cost about $0.34 / mile.  That's 2x or 3x the price of gas for a traditional car on the highway, but you're only paying it on long trips, and it saves you from having to install a gas motor + generator in your all-electric car.

The other interesting possibility is using solar thermal plants to reprocess the spent aluminum hydroxide.  It melts at 572F, which is a downright easy temperature for a field of mirrors to produce.  The great thing about that process is that the solar thermal plant doesn't have to actually generate any electricity directly -- it's just generating heat to strip off the oxygen atoms, which turns into electricity later when it's in your car.  So the plant is much simpler than a solar thermal electric plant would be.

Here's a paper that proposed that very arrangement back in 2010.

In my studies, it was sad to see a lot of the startups from the 1990's and 2000's working on aluminum-air batteries closed down.  I only found one or two companies selling metal-air batteries at all, and those are zinc-air batteries, which have a lower theoretical energy density limit.

So maybe it's time to start looking into this technology again, especially now that electric cars are hitting the market.


Damien Miller said...

So Al batteries have good energy density by mass, but what about by volume? Aluminium isn't exactly the densest material in elemental form...

Anonymous said...

Aluminum Air Batteries have an energy density that is around the energy density of gasoline. That means that they take up around the same space as a gasoline tank. That's the up side.

The down side is that they are a use once device. The recycling is not quite as easy as was indicated. The residue is Al(OH)3. at room temperature, It is a sticky get. There are several steps in recycling it.

The anode is just activated charcoal, while the cathode is basically aluminum foil. There a salt water bath for the electrolyte. There needs to be some barrier between the anode and the cathode. Something like porous Teflon is often used, according to Wikipedia.

The batteries, like most batteries, have a limited shelf life, due to internal leakage. Where they are used commercially, the electrolyte is drained when the battery is not in use.

In all, it makes for a somewhat complicated system. That is why it hasn't caught on. Building the entire recyclable ecosystem is cost prohibitive for an introductory use case.

It would take millions of electric cars to make it usable, and even then, a great deal of additional electric generation would be needed.

Standardized battery sizes would also be required. It is doubtful if the manufacturers would all agree or take the chance on the enormous capital outlays for a change to such a system.