What’s better, Biodiesel or Ethanol?

Biodiesel vs. Ethanol - In the world of renewable energy it’s not uncommon for natural allies to turn into unlikely foes.  In order to support a particular point of view, an advocate of one form of renewable energy will often take issue with some other form of renewable energy and adopt the arguments of those who seek to discredit all forms of renewable energy.  For example, I was reading a website that was enumerating the benefits of a vertical axis wind turbine by calling it ‘bird and bat’ safe, thereby implying that horizontal wind turbines are not bird and bat safe.  The bird and bat issue is a specious argument used by people who oppose all forms of wind turbines because of aesthetic reasons, not because of their concern for wildlife.  So adopting and embracing an argument from a group that is likely to oppose vertical axis turbines as well is not a wise decision.  In other words, the enemy of your enemy is not necessarily your friend.

I’ve noticed the same phenomenon with biofuels.  I’ve witnessed several biodiesel proponents compare biodiesel not only to conventional diesel fuel and gasoline, but also take the opportunity to criticize ethanol, a fuel you’d think they’d be somewhat sympathetic toward since biodiesel has a lot in common with ethanol.   Both biodiesel and ethanol use biomass as a feedstock and both are working to gain acceptance.  They are better for the environment than petroleum-based fuels that release carbon to the atmosphere that has been buried for millions of years.   When it comes to biofuels, it’s not wise for advocates of either fuel to circle the wagons and then point their guns inward at each other.  I’m sure biofuel opponents of all stripes must become giddy when they see such self-defeating tactics.

The U.S. uses approximately 140 billion gallons of gasoline per year.  Most of that could be replaced by ethanol with only minor changes to the majority of the U.S. automotive fleet. I’m only considering the demand and not the supply infrastructure.  But since demand drives supply more than the reverse, it’s an important consideration. Cars manufactured in the last 15 years have incorporated fuel injection systems whose air-to-fuel ratio can be adjusted to a much greater degree than their predecessors that used carburetors.  There are kits available to make a non flex-fuel vehicle compatible with E85 ethanol.  The materials used in modern automotive fuel systems are compatible with ethanol because it’s been used as a gasoline additive for a long time.  So, while it may not be as easy as flipping a switch, the existing automotive fleet conversion to ethanol is an economically viable scenario.  As more flex-fuel vehicles begin to comprise the U.S. automotive fleet, it will become even easier to convert to ethanol-based fuel.

Diesel fuel demand is 60 billion gallons per year in the U.S. and is used primarily in the trucking industry.  This means that U.S. gasoline demand is more than twice diesel demand.  However, in Europe the picture is nearly reversed.  They use twice as much diesel as they do gasoline, primarily because a high percentage of cars have diesel engines because automotive emissions regulations are not as strict as they are in the U.S..  Diesel contains about 17% more thermal energy than gasoline per gallon, and can be combusted at higher temperatures, making it possible to convert more of that energy into horsepower, so it’s not unusual to get about 25% better fuel economy out of diesel engine than you can out of a gasoline engine.  Since diesel has historically been priced similarly to gasoline, this advantage hasn’t gone unnoticed in the U.S. and its annual demand growth has been outpacing that of gasoline.

But diesel doesn’t burn as cleanly as gasoline which is a disadvantage.  Ethanol has a lower thermal energy content, about 45% less per gallon than diesel, but it’s also priced lower and burns cleaner than either gasoline or diesel.  In addition, if you could assure its widespread availability, new automotive engines could eventually take advantage of higher compression ratios to help make up for some of what ethanol lacks in thermal energy with increased thermal energy conversion efficiency afforded with a higher compression ratio.

Biodiesel and ethanol have advantages and disadvantages when compared with each other, but there’s good reason to believe that both can and will co-exist in the future.  It will not be an “either or” decision when it comes to biofuels, but rather “how” to take advantage of the strengths of various forms of biofuel.  Biofuels will eventually need to replace fossil fuels.  Fossil fuels are exhaustible and contribute to increased carbon dioxide concentrations in the atmosphere, whereas the biofuels are inexhaustible and, when all things are taken into consideration, they are carbon neutral.  Ethanol and biodiesel can function as a stepping stones for each other, and it’s impossible to predict which may eventually dominate or if they will both find a permanent place as transportation fuels.

I do realize some people feel we need to skip biofuels all together and go right to hydrogen fuel cell and/or battery powered electric vehicles.  I think that electric vehicles will play a part in the future for short distance transportation needs, but there is no current practical path for powering ships, trains, trucks, or aircraft with batteries or hydrogen.  These are the modes of transport that move the majority of goods around the earth.  When it comes to transportation fuels, there are many important considerations such as energy density (both mass and volumetric), expense, safety, and refill/recharge time.  Biofuels still have a 50x mass energy density advantage over any existing battery technology.  They also have a 20x advantage over hydrogen from a volumetric density assuming one uses a 150-bar pressurized container. In case the concept of volumetric density isn’t clear, a 20x disadvantage means that with compressed hydrogen, the vehicle’s fuel tanks may take up most or all of the cargo space to provide equivalent range. Biofuels also have advantages over hydrogen in the areas of infrastructure compatibility, refill/recharge time, and safety.

Since Moore’s law has never extended to improvements in the field of energy, the advantages of hydrocarbon biofuels may exist indefinitely unless something akin to a miraculous discovery occurs.  It’s always nice to take advantage of miraculous discoveries when they do occur, but it makes little sense to plan on them for our future energy needs.