Will Liquid Nitrogen at One Tenth the Cost of Milk, Power the Next Generation of Automobiles?

The air we breathe is composed of almost 80% nitrogen. What if we could find a way to use that nitrogen to power the next generation of automobiles? We are not likely to see "peak nitrogen" for at least a billion or more years.

Air was first liquefied in 1883, using essentially the same process as today—ie, compressing it to 200 atmospheres, cooling it to -190ºC, and then letting it suddenly expand and condense. The process turns 1,000 litres of transparent gas into 1.4 litres of light blue liquid.

As long as its storage container is well insulated, liquid air can be kept at atmospheric pressure for long periods. But on exposure to room temperature, it will instantly boil and revert back to its gaseous state. In the process, it expands 700-fold—providing the wherewithal to operate a piston engine or a turbine.

Liquid nitrogen does an even better job. Being considerably denser than liquid air, it can store more energy per unit volume, allowing cars to travel further on a tankful of the stuff. Weight for weight, liquid nitrogen packs much the same energy as the lithium-ion batteries used in laptops, mobile phones and electric cars. In terms of performance and range, then, a nitrogen vehicle is similar to an electric vehicle rather than a conventional one.

The big difference is that a liquid-nitrogen car is likely to be considerably cheaper to build than an electric vehicle. For one thing, its engine does not have to cope with high temperatures—and could therefore be fabricated out of cheap alloys or even plastics.

For another, because it needs no bulky traction batteries, it would be lighter and cheaper still than an electric vehicle. At present, lithium-ion battery packs for electric vehicles cost between $500 and $600 a kilowatt-hour. The Nissan Leaf has 24 kilowatt-hours of capacity. At around $13,200, the batteries account for more than a third of the car’s $35,200 basic price. A nitrogen car with comparable range and performance could therefore sell for little more than half the price of an electric car.

A third advantage is that liquid nitrogen is a by-product of the industrial process for making liquid oxygen. Because there is four times as much nitrogen as oxygen in air, there is inevitably a glut of the stuff—so much so, liquid nitrogen sells in America for a tenth of the price of milk.

Finally, a breakthrough in engine design has made liquid nitrogen an even more attractive alternative than the lithium-ion batteries used in electric cars. An invention made by an independent British engineer called Peter Dearman dispenses with the costly heat exchanger that is needed to vaporise the liquid nitrogen quickly. Instead, a small amount of water and anti-freeze (eg, methanol) is injected into the cylinder just as the liquid nitrogen is drawn in, causing it to boil and expand rapidly—thereby forcing the piston down inside the the cylinder. “Without that,” says Mr Dearman, “you had to have a multi-stage engine, which is cumbersome, inefficient and expensive.” _Economist

Liquid nitrogen is the byproduct of the production of liquid oxygen from air. The price of liquid nitrogen is relatively inexpensive, and likely to become less costly as production scales up.

There are questions about travel range between refills, and the need for protection around the cryogenic fuel tanks. But liquid nitrogen is not flammable -- unlike gasoline, diesel, or natural gas. And the "explosions" from the sudden exposure of liquid nitrogen with the atmosphere would be easier to contain than explosions of fossil fuels -- and will not start secondary fires.

It is not a great idea, but it seems to be an improvement over today's versions of electric cars -- in terms of economics and technological feasibility.