Instead of a battery, the new concept is a kind of fuel cell — which is similar to a battery but can be quickly refueled rather than recharged. In this case, the fuel is liquid sodium metal, an inexpensive and widely available commodity. The other side of the cell is just ordinary air, which serves as a source of oxygen atoms. In between, a layer of solid ceramic material serves as the electrolyte, allowing sodium ions to pass freely through, and a porous air-facing electrode helps the sodium to chemically react with oxygen and produce electricity.
In a series of experiments with a prototype device, the researchers demonstrated that this cell could carry more than three times as much energy per unit of weight as the lithium-ion batteries used in virtually all electric vehicles today.
A great deal of research has gone into developing lithium-air or sodium-air batteries over the last three decades, but it has been hard to make them fully rechargeable. “People have been aware of the energy density you could get with metal-air batteries for a very long time, and it’s been hugely attractive, but it’s just never been realized in practice,” Chiang says.
By using the same basic electrochemical concept, only making it a fuel cell instead of a battery, the researchers were able to get the advantages of the high energy density in a practical form. Unlike a battery, whose materials are assembled once and sealed in a container, with a fuel cell the energy-carrying materials go in and out.
The researchers envision that to use this system in an aircraft, fuel packs containing stacks of cells, like racks of food trays in a cafeteria, would be inserted into the fuel cells; the sodium metal inside these packs gets chemically transformed as it provides the power. A stream of its chemical byproduct is given off, and in the case of aircraft this would be emitted out the back, not unlike the exhaust from a jet engine.
But there’s a very big difference: There would be no carbon dioxide emissions. Instead the emissions, consisting of sodium oxide, would actually soak up carbon dioxide from the atmosphere. This compound would quickly combine with moisture in the air to make sodium hydroxide — a material commonly used as a drain cleaner — which readily combines with carbon dioxide to form a solid material, sodium carbonate, which in turn forms sodium bicarbonate, otherwise known as baking soda.
“There’s this natural cascade of reactions that happens when you start with sodium metal,” Chiang says. “It’s all spontaneous. We don’t have to do anything to make it happen, we just have to fly the airplane.”
As an added benefit, if the final product, the sodium bicarbonate, ends up in the ocean, it could help to de-acidify the water, countering another of the damaging effects of greenhouse gases.
Initially, the plan is to produce a brick-sized fuel cell that can deliver about 1,000 watt-hours of energy, enough to power a large drone, in order to prove the concept in a practical form that could be used for agriculture, for example. The team hopes to have such a demonstration ready within the next year
Is it really cheaper and more practical to produce sodium vs hydrogen?
The typical issue with fuel cells is not energy density, it is the fact that you need to waste a lot of energy to regenerate and transport the fuel.
For example, if you take a classic hydrogen option, you can either get it from natural gas (which is not sustainable/eco-friendly) or from water (which is fully sustainable as you get a closed cycle, but comes with additional energy losses on electrolysis, transportation and usage).
Similarly, here with sodium you either have to produce it over and over from salt, or you’ll have to regenerate soda, with the first option being wasteful and the second too energy-demanding and complicated.
So, overall, you’ll need to spend much more energy (= both recurring and upfront costs) compared to running battery-powered transportation if you want to make it a close cycle similar to batteries.
I’ve never understood that thinking. Yes, it takes energy to produce fuel. So what? We started with a form of energy that couldn’t be stored and transported, and converted it to a form that could be. That’s the entire point.
That’s not actually true.
A 777 can carry up to 320,000 pounds of fuel, which gives it a 9000 mile range. It will land about 300,000 pounds lighter than it took off.
Build an electric version of the 777. Put enough batteries on board to make a 9000 mile flight, and it will weigh the same amount on landing as it did on takeoff. It carries the whole load for the whole flight.
Put that original 777 on the 2600 mile flight from LA to New York, and it doesn’t need a full fuel load. You can drop 200,000 pounds of fuel, and add 200,000 pounds of payload.
The e777 will still have the same weight of batteries needed for that 9000 mile flight.
Swap out the batteries with fuel cells, and you can take on an optimal, sub-maximal fuel load for your shorter flights, radically improving total efficiency over batteries.
The point is, the efficiency of the entire process is much smaller compared to battery. Some estimates say that between electrolysis, transportation and fuel cell conversion it’s almost twice as bad in terms of energy efficiency, so you ultimately need double the energy for the same thing.
Sure, the math on planes is somewhat different as you need to account for battery weight. But really, it might still be more efficient to cram those batteries in. And as we know, it is still too bad to be usable.
“The math is somewhat different” does not give adequate consideration to the importance.
That 777 I mentioned? The fuel weight on a maximum range flight is more than twice its remaining payload capacity. Fuel weight is the primary consideration you need to be looking at. The efficiency gains from charging batteries (relative to electrically-produced fuel) cannot justify the losses from their constant weight.
Only twice? Then its not even a contest. I was assuming fuel production was 1/10th as efficient conversion as battery charging.
Alright, that’s fair on your part. Still, thus needs to be taken into account, as the real competition is not with the battery planes (we know they suck), but with combustion jets.