America is awash in clean-burning natural gas, with around 320 trillion cubic feet of the stuff, and we discovered more of it than we extracted in 2009 and 2010 (the latest years on the Department of Energy site). It produces more energy per carbon molecule than other fuels; it pollutes less than most other hydrocarbons; and it’s easier on engines. Win-win-win! So why is Honda’s Civic CNG the only factory-built natural gas car? Because storing the energy content of a single gallon of gasoline at atmospheric pressure would consume a Civic’s entire cabin and trunk—and stink to high heaven. Therefore, it has to be pressurized. A lot. At 3600 psi, it’s still 3.8 times as bulky as gasoline, and containing it at that pressure is expensive and requires outsized cylinders that don’t conform to underfloor space like cheap blow-molded plastic gasoline tanks do.
Science to the rescue! The DOE’s Advance Research Projects Agency recently doled out $30 million in grants to various organizations to figure out how to make natural gas storage more car-friendly, with a target onboard energy density of 12 MJ/kg at an installed price of $1500. (For reference, a Civic gasoline tank holds about 40 MJ/kg, for less than $100.) Here are three of my favorites:
Gas-tro Intestinal San Francisco’s Otherlab is working on a cylindrical high-pressure tank that loops back on itself like human intestines. Lead engineer Tucker Gilman explains that shrinking the diameter and increasing the length of a pressure vessel increases surface area, reducing stress so wall thickness can shrink and mass doesn’t increase much. Smaller diameter tubes can be bent (respecting a 1:8 ratio of tube radius to bend radius), and by stacking and nesting such bent tubes, the storage tank can better conform to underbody packaging. Tucker claims Otherlab can meet ARPA’s target with high-strength steel.
Metal Air Mattress Pacific Northwest National Laboratory’s approach layers sheets of metal and welds lines in them like an air mattress, then heats the metal and “inflates” it with air in a superplastic forming process. Senior research scientist Kevin Simmons explains that friction-stir welding produces a metal grain structure as formable as the unwelded metal, and that air pressure naturally forces the hot metal into an ideal shape for a pressure vessel, with hemispheres at the ends of each tube. So far, experimentation with a “high-strength lightweight alloy” is yielding 13-15 MJ/kg, with 80 percent better conformability.
Metal Sponge Ford is working on a low-pressure (500-1000 psi) alternative in which a tank is filled with powder or pellets of “metal organic” or “covalent organic” materials. These porous, high-surface-area materials grab natural gas at one temperature and release it at another. The most promising materials operate at near room temperature. The reaction creates heat while refilling, so it must be cooled during the recharge and heated slightly while releasing the gas. But fuel systems technology expert Mike Veenstra says this heating and cooling energy is far less than what would be required to compress gas to 3600 psi for standard tanks. (ARPA grants also were awarded to develop cheaper pumps.)
If one of these ARPA investments hits, T Boone Pickens’ CNG dreams may at last come true.
Illustration: Pep Montserrat
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