For the next few decades at least, liquid hydrocarbons–gasoline, diesel, and jet fuel–will continue to be the mainstays of transportation. They’re cheap; refueling is fast; and their energy density, crucial to long-distance travel, is hard to beat.
“Advanced technology is going to happen slowly,” says Daniel Sperling, the director of the Institute for Transportation Studies at the University of California at Davis and a member of the California Air Resources Board. “The focus needs to be on making conventional technology more efficient.”
It should be possible to reduce the fuel consumption of a midsize sedan by up to 60 percent without sacrificing size or performance, using mostly existing technology. Lightweight materials will help. Advanced turbocharging and fuel-injection technology will extract more power from smaller engines that lose less energy to friction (see “Research to Watch”). Similarly, making airplanes lighter and their engines more efficient could cut their fuel consumption 30 to 50 percent by 2020.
Biofuels should help curb petroleum consumption, although the contribution they make will depend on many factors, including the price of oil and the development of new technologies. The International Energy Agency has estimated that by 2050, ethanol and biodiesel could meet 13 percent of global demand for transport fuel. The U.S. Energy Information Administration estimates that biofuel consumption in the United States will increase from 7.7 billion gallons per year in 2007 to 35 billion gallons by 2030 while consumption of gasoline, diesel, and jet fuel combined holds steady at about 220 billion gallons per year.
At first, most biofuels will be ethanol made from corn or sugarcane. The amount of ethanol that can be produced from these sources, particularly corn, is constrained by the need for farmland. What’s more, the greenhouse-gas reductions achieved are minimal, because producing corn ethanol takes a lot of fossil fuel. But cellulosic sources of ethanol, such as switchgrass and wood, can be grown on marginal lands, greatly increasing potential fuel production. And the process of making ethanol from these materials consumes less fossil fuel. Corn ethanol contains roughly 1.3 to 1.7 times the energy of the fossil fuels used to make it; for cellulosic ethanol, it’s about 4.4 to 6.1 times as much. By 2030, a significant portion of biofuels will be synthesized from biomass using biological and thermochemical techniques to create gasoline and diesel fuels. Such biofuels could even eclipse cellulosic ethanol.
It will take decades before anything other than liquid fuels powers a significant portion of the nearly one billion cars on the road. Still, cars will rely more and more on electricity. Hybrids, which accounted for only about 2 percent of U.S. sales of light-duty vehicles in 2007, could account for 40 percent by 2030. Then there are plug-in hybrids, which are just starting to be sold and which could account for 2 percent of sales by 2030. Unlike conventional hybrids, which derive all their power from gasoline-powered internal-combustion engines, plug-in hybrids have batteries that can be charged from the electrical grid, ideally using nighttime excess generating capacity. They can go the distance of an average commute on this energy alone, using an electric motor; an onboard gasoline engine kicks in for longer trips. Since some of the energy for propelling the car comes from power plants, overall greenhouse-gas emissions depend on the fuel those power plants use. Assuming typical driving patterns, a plug-in hybrid with a 20-mile electric range will generate about 325 grams of carbon dioxide emissions per mile if the electricity comes from a coal-powered plant (a conventional vehicle emits about 450 grams per mile). If the electricity comes from wind power, the hybrid will generate 150 grams per mile.
The high cost of batteries will initially slow the adoption of hybrids and all-electric vehicles (see “Scaling Up Is Hard to Do”). Lithium-ion batteries that can provide a 40-mile range currently cost more than $16,000, according to an estimate by Carnegie Mellon University. But technological improvements and mass production could reduce this price by 75 percent or more. Meanwhile, researchers are exploring different chemistries, such as lithium-air batteries. These technologies could store 10 times as much energy as conventional lithium-ion batteries, extending range and potentially lowering costs.