Biofuels Can Be Electrifying
by Bill Chameides | May 12th, 2009
posted by Erica Rowell (Editor)
With ethanol looking less and less like a quick fix, are electric vehicles a better way forward? (NREL)
It was a bad week for ethanol. First EPA proposed disqualifying corn ethanol as a renewable fuel (see yesterday’s Grok post). Then, a paper in ScienceExpress questioned the whole concept of powering cars with ethanol.
Fossil Fuels: The Good, the Bad, and the Ugly
Fossil fuels are, in principle, a great source of concentrated energy. Their many carbon atoms are chemically bonded to other carbon atoms or hydrogen atoms. But those carbon atoms are more stable when they are attached to oxygen atoms as carbon dioxide (CO2). When those carbon atoms convert to CO2, they release energy in the form of heat as if emitting a sigh of relief a la a heat burp. That heat has found myriad uses over the last century, including the powering of internal combustion engines which in turn power a car. Because liquid fuels are easily portable, gasoline and diesel have been the fossil fuel choices for cars and trucks.
But there are problems. Most of the gas we use lies beneath foreign soil in countries not always friendly to the United States. And burning gasoline adds enormous amounts of CO2 to the atmosphere, thus contributing to global warming.
Hence the search for an alternative fuel.
Libation for Autos
Material from crops is an obvious replacement candidate. The carbon in plants, like fossil fuels, is chemically bound to other carbon atoms and hydrogen atoms and so possesses chemical energy that can be released when burned. But unlike gasoline, crops are plentiful right here in the U.S.A., so there’s no need to buy the stuff from foreign countries. Moreover, burning crops does not add new CO2 to the atmosphere; it just returns the CO2 that came from the atmosphere as the plant grew via photosynthesis.
But there is a problem. You can’t fill up your gas tank with wood or corn stuffs and zip down the highway. You have to convert them to a usable form of fuel first.
Ethanol, or more specifically bioethanol, is one way to go — it’s a liquid, easily transported fuel that’s been used in motors since the nineteenth century. And well before that, for millennia, humans have converted plant material to alcohol — think grapes for wine. The standard way to turn plants stuffs into fuel (or alcohol) is through fermentation, which converts a plant’s sugars (and starches) into ethanol.
|C6H12O6||→||2 C2H5OH||+||2 CO2||+||Energy|
In the U.S. agricultural world corn is king and so thus far, most of our ethanol is made from the sugars and starches in corn — we call that corn ethanol.
But there are problems with corn ethanol:
- Sugars and starch make up a small percentage of the biomass in corn; so just using the sugars and starches wastes most of the crop’s stored chemical energy.
- Making ethanol from corn is not very efficient because growing corn is very energy intensive.
Fortunately for the ethanol boosters, there is another way to make ethanol — cellulosic ethanol is produced from the plant’s hard, woody, inedible parts, which make up a large percentage of the plant. Because of this, producing cellulosic ethanol is a much more efficient plant-to-fuel process than producing corn ethanol. And because producing corn ethanol is so energy-intensive, it’s estimated that it will increase greenhouse gas emissions, while cellulosic will decrease emissions.
But there are problems even with cellulosic ethanol:
- Ethanol from cellulose is not yet commercially available.
- Ethanol and gasoline don’t mix well (to put it technically, they are not miscible). When the amount of ethanol in gasoline is small, say 10 percent or so, this immiscibility is not a problem, but at larger percentages, it is. What that means is that to use cellulosic ethanol for our cars, we’ll need to develop a completely new fuel distribution and storage system. Think lots of dollars, lots of hassle.
- Producing cellulosic ethanol will require lots of land to put a dent in our dependence on
Gosh, so what’s a car driver (or engineer) to do?
Eureka! There Is an Alternative — and It’s Electric
Liquid fuels are not the only way to power an automobile. Electric engines also work and in fact have an important advantage over internal combustion engines — efficiency. You know how hot the hood of your car gets after a long drive? Remember how noisy you car’s motor can be? The heat and noise represent wasted energy. The quiet cool of electric motors are signs that they are more efficient.
Only about 20 percent of the energy stored in gasoline powers a car and its accessories. By contrast, electric motors convert 75 percent of the chemical energy stored in batteries; thus, they generally run cooler and quieter. (Note: Batteries still have downsides. For example, despite the fact that a smaller fraction of the energy in gasoline is used, the energy density of gas is so much larger than what our current batteries store, that vehicles with internal combustion engines have a longer driving range.)
OK. Great, but what does all that have to do with crops? A lot. Instead of turning plant material into ethanol, take that same plant material and burn it. The heat produced can be used to drive a turbine and generate electricity — electricity that can be used to charge a battery and run an electric car.
But, is it any better than cellulosic ethanol?
J. E. Campbell of the University of California, Merced, and colleagues report “yes” in a recent issue of ScienceExpress. Using a life-cycle model, Campbell’s team estimates that using “bioelectricity” is a much more efficient use of cropland than “bioethanol.” For example, for a mid-sized car running on energy derived from switchgrass, one can drive about 25,000 kilometers per year of city driving and about 30,000 km per year of highway driving for each hectare (about 2.5 acres) of land devoted to growing the switchgrass. Using bioelectricity, the amounts are more than 60,000 km per year per hectare for both city and highway driving.
Bioelectricity also saves a lot more greenhouse gas emissions. For the same switchgrass example, bioethanol offsets about eight tons of CO2 per year per hectare for city and highway driving, while bioelectricity offsets about 27 and 21 tons of CO2 per year per hectare for city and highway driving, respectively.
Hmm. This is depressing. When I think about all the money we’ve spent incentivizing and subsidizing the ethanol industry, and the administration’s plans for future ethanol handouts, I want to cry in my beer. I always knew that alcohol and driving don’t mix.filed under: automobile, carbon dioxide emissions, faculty, fossil fuels, science, transportation
and: biofuels, cars, cellulosic ethanol, corn ethanol, electric cars, gasoline, renewables, research