Transonic Combustion's fuel injection system. Credit: Technology Review.

A conference put on by the new Advanced Research Projects Agency for Energy (ARPA-E) this week was packed with companies exhibiting intriguing approaches to clean energy. I'll be looking into some of them in more detail in upcoming stories, but here's a few that caught my eye.

Transonic Combustion, based in Camarillo, CA, has developed a gasoline fuel injection system that can improve the efficiency of gasoline engines by 50 to 75 percent, beating the fuel economy of hybrid vehicles. A test vehicle the size and weight of a Toyota Prius (but without hybrid propulsion) showed 64 miles per gallon for highway driving. The company says the system can work with existing engines, and costs about as much as existing high-end fuel injection.

American Superconductor, of Devens, MA, is developing massive 10 megawatt wind turbines that are only possible with the use of extremely lightweight superconducting generators. (Today's turbines typically generate around 2 to 3 megawatts.)

A group out of Michigan State University is developing a natural gas electricity generator for use in hybrid vehicles. The goal: give natural gas cars the same driving range as conventional gasoline cars, making way for their wide adoption.

Oscilla Power, based in Salt Lake City, UT, plans to start testing a novel wave power generator. Wave power is notoriously difficult to harness because of the damage waves can cause to mechanical systems. Oscilla has found a way to use an inexpensive iron-aluminum alloy to generate electrical current, without the need for any moving parts.

Palo Alto Research Center (PARC), spun out of Xerox PARC, is developing a new form of refrigeration that could be three times as efficient as existing forms. It's based on thermoacoustics, a technology that works for cooling at extremely low temperatures (such as for liquefying gases), but hasn't been used for cooling at room temperature (what you need for household refrigeration). The company thinks it's found a way around previous limits to the technology.