A 19th-century idea might lead to cleaner cars, larger-scale renewable energy.

Source: www.technologyreview.com

>”Highview Power’s process is 50 to 60 percent efficient—the liquid air can yield just over half as much electricity as it takes to make it. Batteries, by contrast, can be more than 90 percent efficient. But the new process can make up for its inefficiency by using waste heat from other processes (see “Audi to Make Fuel Using Solar Power”). Highview has demonstrated that low-temperature waste heat from power plants or even data centers can be used to help warm up the liquefied air. The system can also last for decades, while batteries typically need to be replaced every few years. This longevity could help reduce overall costs.

Several companies are developing ways to improve the efficiency of compressing air, which could also make the liquefaction process more efficient (see “LightSail Energy Snags $37M in Funding” and “Compressed-Air System Could Aid Wind Power”). Liquefied air is about four times more energy-dense than compressed air, and storing it at a large scale takes up less space.

Liquid air might also prove useful in cars and trucks. An inventor named Peter Dearman has made a compact system that, instead of relying on large heat exchangers, uses antifreeze injected into an engine’s combustion chamber to recycle heat that would otherwise be wasted. He built a ramshackle prototype and showed that it could power a car. Ricardo is working on a version that could eventually be commercialized.

Liquid air stores energy at about the density of nickel–metal hydride batteries and some lithium-ion batteries, the kind used in hybrid and electric cars now. But it has a key advantage—it can be poured into a fuel tank far faster than a battery can be recharged, says Andrew Atkins, a senior technologist at Ricardo. The engine would run on liquid nitrogen—basically liquid air with the oxygen removed—and would emit only nitrogen. The carbon emissions associated with the engine would depend on the power source used to liquefy the nitrogen.”<