The Air Car Rides Again?

The air-powered car: It?s one of those elusive but irresistible ideas. PM highlighted one attempt at a pneumatic car as early as 1945, yet none has managed to come close to the internal combustion engine in terms of range.

That hasn?t stopped modern companies from trying. Indian automaker Tata?s range of Air Cars, powered by 4350 psi of compressed air, were announced in 2007. But the technology, licensed from Luxembourg-based Motor Development International, never quite made it. When the company failed to deliver in 2009, onlookers, including PM, began to cast doubts. "We found that it wouldn?t be able to perform in normal driving conditions," says Andrew Papson, a senior associate at technology consulting firm ICF International. He co-authored the study that debunked the compressed-air car. "You wouldn?t get the range that would make a car feasible."

With so many years of air-car technology being five years away, you can understand the modern skepticism that this technology will ever come to fruition. But here comes a new and interesting idea to the fray.

Soft-spoken Peter Dearman has been quietly toiling away in a garage in the Midlands of Britain, developing his own take on the pneumatic engine. Instead of relying on compressed air?with an energy density that pales in comparison to that of gasoline or lithium-ion batteries?his Dearman Engine uses liquid air, which, according to his Dearman Engine Co., has a density similar to lithium-ion batteries?an important hurdle to overcome if air-powered cars are to travel any significant distance before refueling.

Rather than behaving like a volatile propellant such gasoline or hydrogen, liquid air acts more as a non-chemical battery, functioning as a medium for energy transfer. Industrial gas companies have long produced liquid gases using what?s called the Joule-Thomson effect. Essentially, it works by compressing the air, whisking away any heat produced by compression, and then expanding it back to ambient pressure, further cooling the gas. When cooled to minus 321 F, the air turns into a liquid?effectively storing the energy required to cool it.

While it doesn?t produce the liquid air onboard, Dearman?s engine injects the liquid from a vacuum-sealed tank into a cylinder at ambient temperature; the liquid sublimates, instantly transitioning to a gas and releasing a surge of energy through gas expansion that drives a piston. It?s the elementary mechanics of an engine, minus an ignition system and combustion cycle.

And so far it works, in practice. Dearman retrofitted his engine to a small hatchback and was able to hit speeds of up to 35 miles per hour, he told Sky News, with nothing but cold air coming out of the tailpipe.

According to Mika?l Toussaint, a spokesman for Dearman, the engine can output a maximum of 94 hp. Not too shabby for a car that runs on the very same air we breathe. Engineering firm Ricardo is refining a demonstration version of the Dearman engine for a late 2013 release?though we?ve heard that one before.

Is Dearman?s engine ready for the road? Papson says that the technology makes sense to him, but he?s skeptical of its efficiency. In terms of energy density?a crucial metric for determining a car?s range?liquid air and lithium-ion batteries are not equals, Papson says. He cites one study done in 1988, in which a team of Stanford researchers pinned the energy density of liquid air at 181 watt-hours per liter (w-hr/l). Although that?s much greater than compressed air?s 16.7 w-hr/l, it?s less than a quarter of what modern lithium-ion batteries can do.

Energy-density issues aside, there?s still the issue of efficiency. "The big question about this technology is not the fuel itself, but how it?s expanded and how it goes through the engine," Papson says. "Even if you have a liquid-air tank that has the same energy as automotive batteries, if it?s expanded poorly you could use half of that just in the thermodynamic process."

Indeed, Dearman?s numbers point toward this conclusion. Running at ambient temperature, the specific energy?a measure of how much power is in one kilogram?of the liquid air in Dearman?s engine peaks at 87 w-hr/l; less than half what the Stanford researchers reported possible. Even introducing low-grade waste heat to improve the efficiency of the reaction with a small-diesel-engine hybrid system bumps the energy availability up to only 104 w-hr/l. That?s a drastic improvement over compressed-air cars in terms of energy density. But compared to EVs?which tout roughly 80 percent efficiency?the Dearman Engine just doesn?t make sense in a transportation application, Papson says.

However, just because liquid-air-powered engines might not work in cars doesn?t mean they can?t work elsewhere.

Highview Power Storage, another UK-based company, took the idea of liquid air functioning as a battery, and adapted it to store energy generated by wind turbines and other sources of renewable energy. "Essentially, what we?ve done is married two industries," Highview COO Toby Peters says.

Like Dearman?s engine, Highview?s system uses liquid air as a nonchemical battery, using off-peak generated power to cool air into a liquid and storing energy in the process. When demand rises, the liquid air is boiled off, spinning a turbine and regenerating energy during peak hours. When used in tandem with the waste heat produced from an industrial plant, the system is up to 70 percent efficient, Peters says.

Storing power is not a major issue for thermal power plants that can increase generation to meet high demands. But it?s a huge concern for "wrong time" renewable energy. "The wind is blowing at three o?clock in the morning, but you need the power at seven o?clock in the morning," Peters says. "Electricity is essentially the only commodity that you can?t warehouse."

For a year, Highview?s pilot program has been storing energy generated from a wind farm in the southeast of England and supplying Britain?s National Grid with 2.5 megawatts per hour during high demand, Peters says. Aside from being relatively location-agnostic, Highview?s technology is built from components that already exist?what Peters calls mature components. "Compressors, turbines, heat exchangers?stuff that comes from a GE catalog," he says. The benefits from using existing components are twofold, he says. "First, you don?t need to invest in new manufacturing plants?which is quite important given the state of the economy. Second, you?ve got components that are well-known, robust, and proven in their performance and longevity."

In an industry that?s in dire need of an inexpensive, zero-emissions system for storing "wrong time" energy, Highview?s solution could be more feasible than any automotive applications for liquid air. "The primary restriction on the car is you have limited space to store fuel," Papson says. "In an industrial application or an energy application you have much more freedom to have large fuel tanks?whatever the fuel may be?then you don?t have to worry about these efficiency issues as much."

Source: http://www.popularmechanics.com/cars/news/auto-blog/the-air-car-rides-again-14543822?src=rss

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