Reports on the science behind our energy concerns have become a regular feature on NPR's "Science Friday" program, and downloading its podcasts is now an established part of my weekly routine. Its first segment last Friday focused on the pros and cons of ethanol and biodiesel and their various production techniques. These issues are important aspects of the larger questions concerning the fuel(s) of the future, which might derive from chemistry, synthetic biology, or electricity. Long experience and numerous applications bias us toward liquid fuels, and the current emphasis on biofuels reflects that, though this is not a foregone conclusion, particularly for supporters of plug-in hybrids and electric cars.
The "Science Friday" segment drew on recent articles in Wired and National Geographic, and included Professor Dan Kammen, the Director of U.C. Berkeley's Renewable and Appropriate Energy Laboratory, whom I've mentioned previously. The discussion provided many interesting perspectives on biofuels, including why it is so difficult to break down the cellulose and lignin in plant material, and how current approaches to cellulosic ethanol differ from those begun in the 1960s and '70s, and are thus more promising. They also considered the utility of ethanol as a fuel, at least during a transition to a more optimized energy carrier for transportation. Yields, energy density, and compatibility with infrastructure and end-use devices are all important considerations, along with whether government or the market will make the ultimate choice.
After listening to the program, I became curious about the relative comparison between transportation energy and food agriculture. Multiplying a daily diet of 2500 Calories by 300 million of us equates to 1 quadrillion BTUs per year. That works out to the energy contained in 500,000 barrels per day of oil. Americans stand at the apex of a vast agricultural pyramid, and its net result supplies our bodies with only about 1/20th of the fuel that our cars use. Now, there are all sorts of inefficiencies on both sides of that comparison, such as those relating to our consumption of animal protein and the use of inefficient engines and other devices. But it still says something about the relative upper boundary of biofuels' potential, compared to the higher energy-density sources on which we currently rely.
And since liquid fuels for transportation still retain a big edge over gases and electricity, we can't ignore the current incumbent. Could the fuel of the future actually be gasoline? Take a look at last week's Economist, before you write me off as a shill for the oil industry. The article covers a variety of biofuel alternatives, starting with ethanol and working its way up the molecular chains, ultimately suggesting that something very similar to gasoline could be produced by advanced biological synthesis, and that this would provide significant advantages over current biofuels, without many of the drawbacks of petroleum products or liquids from coal. Based on my own analysis of ethanol's limitations, I agree, if the synthesis could be done economically--the $64,000 question for any of these alternatives.
Nor should we write off hydrogen entirely, as many energy pundits have. While the cost and infrastructure obstacles it faces are truly Herculean, in the long run all of those could be overcome, if the incentive were large enough. Hydrogen's fate, then, depends on an end-use application that is so superior to the internal combustion engine in performance and cost that it justifies all that extra expense and effort, including throwing away large quantities of primary energy from natural gas, nuclear power or renewables, to produce it. Today's fuel cells don't meet those criteria, but tomorrow's just might. The potential is certainly there. If it can be unlocked, then hydrogen's role as a common-denominator energy carrier made from a wide variety of primary sources--including biological synthesis--could become an advantage, rather than a drawback.
When it comes to predicting the fuel of the future, to replace petroleum and reduce our global carbon footprint, we still lack good answers. The best we can do today is to keep trying out alternatives, along the lines of the "clinical trials" suggested by Dr. Craig Ventor in the Economist article, progressively refining our questions along the way. In the meantime, improving our energy efficiency would greatly reduce the height of the mountain any new fuel must climb.
On November 1, 2007 I'll be participating in a webcast discussion on "Fuels for Now and the Future" with Scott Sklar of The Stella Group, Ltd., hosted by Cleantech Collective. For more information and to register for the webcast, please follow this link.