October 21, 2012


Difference Engine: End of the electric car? (The Economist, Oct 15th 2012)

[T]he question becomes whether there is a more efficient way of packaging electricity for use in vehicles, other than charging batteries or making hydrogen by electrolysis of water?

A growing body of opinion seems to think liquid air is the answer (or, more specifically, the nitrogen component that makes up 78% of air). It is not exactly a new idea. Air was first liquefied in 1883, using essentially the same process as today--ie, compressing it to 200 atmospheres, cooling it to -190ÂșC, 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."

The Institution of Mechanical Engineers in London, the leading standards-setting and registration body for the profession, was so impressed with the Dearman Engine Company's developments that it has now established a working group comprising engineers, academics, government officials and industry leaders, to explore ways of exploiting liquid-nitrogen technology.

Posted by at October 21, 2012 7:54 AM

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