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
They’re comparing it to lithium batteries for power density, but ignoring that the sodium metal in this case is a consumable, unlike batteries.
They say it’s 1200 Wh / kg of sodium, however gasoline is a whole 3800 Wh / kg, and somehow I think the carbon dioxide is less harmful than the same amount of sodium hydroxide. Not to mention how much more complicated storing liquid sodium would be since it reacts with air.
So that might not be a big deal. Most of the energy in gasoline is released as heat and not useable.
Not sure it’s enough.
Airplane engines are about 35% efficient. Maybe you can push it upwards 50% with state of the art designs.
Fuel cells hits about 60-70%, state of the art can maybe hit 85% (and the electric engines can be efficient enough to be part of the error margin in this equation). Best case you’re halving wasted energy. That means you need AT LEAST half the energy density, or else you’re carrying more fuel mass for the same flight. Might be tolerable if it is at least cheaper, but you’re also adding stress and wear as you do.
That is about the H2 energy release from sodium reacting with water (perhaps just humidity in air).
H2 has 33000wh/kg, and a fuel cell gets double the energy of a combustion engine/turbine, and so if you were starting with sodium, might as well pour water on it on the ground, and fill the plane up with automatic high pressure H2.
There are no emissions other than water vapour from the sodium process because the reaction leaves solid/liquid byproducts (that are too valuable to discard) other than H2.
From what the article says, this fuel cell produces sodium oxide by reacting sodium with oxygen. There’s no hydrogen gas being produced in the fuel cell.
The emissions are sodium hydroxide, or sodium carbonate after it reacts with carbon in the air.
(Also now I’m not sure where I got 1200Wh/kg from. The article says both 1000 and 1500 Wh/kg)
Article says whole system is 1000wh/kg (including support machinery). There are 5 (including intermediate step) reactions of air and sodium. I’d guess they are using 100% humidity air. H2 is part of the reaction with humidity, and is a much more rapid and “exothermic” reaction than transformation to SaO.