In the United States we use a lot of energy to move people and cargo from place to place. In 2010, about 28 percent of our total energy consumption was spent on transportation. Of that energy, 93 percent came from burning petroleum products: 61 percent was gasoline, 20 percent diesel, and 12 percent aviation fuel. The remaining seven percent is split about evenly between natural gas and renewables, most of the latter in the form of ethanol.
This means that 97 percent of our transportation energy budget comes from burning fossil fuels. A renewable energy solution to transportation energy use is thus both crucial, if we're to stop adding carbon to the atmosphere, and a long way away.
Much of the problem stems from the way our society is set up. Especially in California, our urban and suburban landscape was built on the assumption that the predominant mode of passenger transportation would be the private auto, with trucks moving most of the cargo. Our dispersed land use patterns make centralized mass transportation much more difficult for passengers or cargo. Unsurprisingly, as the table below shows, highway vehicles consume more than four fifths of the energy used in US transportation.
|Buses (Transit, Intercity and School)||<1%|
|International air carriers||1.4%|
|Domestic air carriers||5.5%|
|Water (Freight and Recreational)||4.7%|
Passenger (Transit, Commuter, and Intercity)
The most straightforward way to convert our transportation to a renewable energy base would be to reorganize our cities and towns so that people lived closer to each other and to services, allowing a shift to walking, bicycling and renewable-powered mass transit for more of our daily transportation needs. Despite significant inertia and even outright opposition, this shift is taking place: even Los Angeles, the very definition of a car-centric city, is rethinking its transportation and settlement patterns.
In the interim, however, our society remains automobile-dependent. Attempts to convert our cars and trucks to renewable power sources come in two basic forms: electric vehicles and biofuels.
Biofuels are combustibles that are made from recently-living things. Their advantage over fossil fuels is that at least in theory, their production and use can be carbon-neutral, putting only as much carbon dioxide into the atmosphere when burned as the fuel's plant feedstock removes from the atmosphere when it is grown.
The most commonly used biofuel in the US is ethanol, a.k.a. ethyl alcohol. Ethanol is mainly used in the US as an additive in gasoline. "Gasohol" is gasoline that has been = diluted slightly with about 10 percent ethanol. E85 is 85 percent ethanol with the remainder of the fule made up of gasoline.
Ethanol contains much less usable energy per liter than does gasoline, and it also readily absorbs water. For both these reasons, fuel as high in ethanol as E85 sometimes performs poorly in standard gasoline engines, especially in cold weather.
Ethanol in the US is largely derived from corn, and has a very low Energy Return on Energy Invested (EROEI). Corn ethanol in the US returns only 130 percent of the energy it takes to grow, process and extract the ethanol. Other crops, such as the sugar cane basasse generally used for ethanol stock in Brazil, can return 800 percent of the energy invested in producing it, and a promising new range of technologies to create ethanol from cellulose may return as much as 3,600 percent of the energy invested.
Ethanol production has been criticized for competing with human hunger in the marketplace for edible crops such as corn and sugarcane, thus raising food prices. Using ethanol as automotive fuel can also increase tailpipe emissions of smog precursors.
Biodiesel is a catch-all phrase for vegetable and animal fats modified for use as fuel in diesel engines. Strictly speaking, biodiesel is oil that has been transesterified; some vegetable diesel fuels are made through simple refining without transesterification, and are thus sometimes called something other than "biodiesel," such as "green" or "renewable diesel." The latter fuel can be used straight in unmodified diesel engines, while biodiesel in the strict sense will degrade natural rubber gaskets and hoses.
Another fuel source sometimes confused with biodiesel is unmodified vegetable oil, which can be burned in diesel engines that have had minor modifications.
Biodiesel in the broader sense was one of the first automobile fuels, and engineer Rudolf Diesel actually designed his first liquid fueled engines to run on modified vegetable oil. Used over the past century in places with petroleum shortages, biodiesel started gaining new popularity in the 1990s in Europe, with California following suit the first years of the 21st Century. Californian enthusiasts often "home-brewed" biodiesel from waste oil gathered from restaurants and simliar small businesses. Global biodiesel production has grown dramatically in the last decade and outstripped the supply of salvageable waste oil in many locations. Especially in Europe, which accounts for 85 percent of biodiesel production, oil crops are increasingly grown specifically for use as biodiesel feedstock. This raises some of the same concerns as the use of food crops as ethanol fuel sources.
Running a vehicle on electric power offers the possibility of using the renewable portion of the electrical supply system as transportation fuel. The main obstacle to an increase in electric vehicle use is that batteries using currently available technology have a much lower energy density than most liquid fuels, restricting the vehicles' potential power or range or both. What's more, it can take significantly longer to "refuel" an electric vehicle than it does its conventionally fueled counterparts. This isn't a dealbreaker for about 90 percent of private vehicle use in the US, as most drivers stay well within the range of existing electric vehicles during the course of their typical day, and could conceivably charge their vehicles while at work or parked at home. Using regular household current, most available models of electric car would require ten hours to charge fully.
That's fine for a commute car, but not so great for picking up and driving to Denver. Dedicated fast charging stations could cut down the time needed to recharge electric car batteries to less than an hour. If enough charging stations are built in a large area, long trips could be managed with about half the time spent driving and half recharging. This would certainly require a social shift in the way we think about road trips. Increases in battery capacity and advances in charging technology could make that ratio a little more palatable to long-distance-driving habituated Americans, though perhaps not by much. If manufacturers standardize to a much greater degree than they currently do, a "battery swap" system could be set up in which drivers would enter a facility, have their depleted battery removed by machine and a fully charged one swapped in in its place.
Of course, the renewability of an electric car network depends primarily on the source of the electricity: if we were to remain with coal-fired power plants generating the bulk of our power, electric cars would actually represent a step in the wrong direction from their gasoline-fired cousins.
Fuel Cell Vehicles
A fuel cell can be thought of as a liquid-fueled battery, which generates electrical power through direct chemical reaction of the fuel without involving combustion. Hydrogen is often mentioned as a fuel cell power source, but organic liquid fuels such as alcohol and other hydrocarbons can also be used. The process oxidizes the fuel, so a fuel cell using a hydrocarbon fuel will emit carbon dioxide, but biofuels with little or no carbon footprint can be used as fuel.
Fuel cells thus offer a possible answer to the low energy density of electrical storage batteries, by taking advantage of liquid fuels' energy density to power an electric car.
Though fuel cell technology has remained out of the limelight since 2000 or so, a number of manufacturers plan commercial fuel cell vehicle releases in the next few years.