Nitrogen cars

 

The Economist/ by N.V.

A COUPLE of dozen electric cars with fuel cells under the bonnet (in place of the more usual flat-pack of batteries beneath the floor) have been zipping around your correspondent’s neighbourhood for the past few years. Most are FCX Clarity models from Honda, all in the same rich crimson colour. A couple of others are silver F-Cell station wagons made by Mercedes-Benz. These experimental vehicles are leased to selected users for trial periods while their manufacturers see how the hydrogen-fuelled cars survive the cut and thrust of Los Angeles' traffic.

So far, most seem to have acquitted themselves rather well. Meanwhile, their drivers can feel rightly smug about the only emission from the exhaust pipes being water vapour. Another plus is that the fuel-cell vehicles are largely free of the “range anxiety” that plagues battery-powered electric cars, such as the Nissan Leaf. Both the Honda and the Mercedes have ranges not that far short of comparable petrol cars—ie, 190 to 240 miles (300 to 380km).

Sooner or later, though, they have to return to one of only five hydrogen-refuelling stations open to the public in the greater Los Angeles area. But once there, their tanks can be refilled in minutes, rather than the hours needed to recharge a battery car.

And there’s the rub. Given further refinement, plus economies of scale, fuel-cell vehicles ought to be an attractive alternative to present-day motoring, if only hydrogen-refuelling facilities were more common. As it is, outlets are fewer and farther between than charging stations for electric vehicles or even pumps for compressed natural gas.

Apart from the usual chicken-and-egg problem, the plant and equipment needed for producing, distributing and storing hydrogen is hugely expensive. Unlike the industrial hydrogen used to make ammonia fertiliser, or for converting heavy oil fractions into petrol, the hydrogen needed for fuel cells must be 99.999% pure. That rules out all the cheaper ways of making it, other than electrolysis of water.

There are problems on the distribution side, too. Because hydrogen has the smallest molecule of all, it leaks through practically everything. In particular, it embrittles steel and causes corrosion, hastening crack propagation in the process. Pipelines and storage tanks have to be specially lined at additional cost.

Unlike fossil fuels such as petrol or diesel, hydrogen is not a source of energy in its own right. It is merely a means for storing electricity generated in a power station and delivering it to the motor driving the wheels of an electric vehicle—in much the same way as a battery works. And as free hydrogen does not occur in useful quantities in nature, it has to be made by using electricity to crack water into its constituent elements.

In California, despite the many solar installations and wind farms, the electricity coming out of the plug is neither green nor clean, being derived predominantly (ie, 62%) from fossil fuel. During cheap-rate periods at night—when electric vehicles tend to be recharged and electrolysis plants are running flat out—most of California’s electricity is imported from coal-fired power stations out of state. Thus, like electric vehicles, hydrogen cars contribute their share of greenhouse gases as well.

Certainly, moving the emissions from the vehicle’s exhaust pipe to the power station makes it easier to control the pollution. So, the 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 -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 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.

If the nitrogen vehicle—with its lower overall cost and rapid refuelling capacity—proves to be the solution to zero-emission motoring that electric vehicles once promised, where does that leave the new generation of hydrogen fuel-cell cars?

According to Honda, a fuel cell driving an electric motor is two to three times more efficient than an internal-combustion engine. Unfortunately, a fuel-cell vehicle is even more expensive to build than a battery car. The third generation of Honda’s 100 kilowatt fuel-cell stack developed for the Clarity is reckoned to have cost around $350,000 a unit. The Mercedes stack probably cost much the same. That is one reason why Honda built only 200 Clarity cars and Mercedes a mere 70 F-Cells.

In recent years, carmakers are reckoned to have more than halved the manufacturing cost of their fuel-cell stacks. Installed, they now probably run to $1,500 a kilowatt of capacity. Nevertheless, that still leaves them an order of magnitude or more pricier than internal-combustion engines. Squeezing yet more cost out of their fuel cell’s proton-exchange-membrane technology will not be easy.

That said, seven new fuel-cell cars were exhibited at the recent Paris Motor Show. Admittedly, most were concept models, but all were fully functional. So the industry seems to be hedging its bets on battery-powered electrics. But is it backing the right technology?

While he is at odds with some of the best brains in the business, your correspondent cannot help but think that—like battery-powered electric vehicles before them—the fuel-cell cars running around his neighbourhood today are destined to occupy at most a 3% niche in the market. And though still a long shot, nitrogen would seem to have a better chance of fulfilling the role hydrogen was once expected to play.