Petroleum fuels have exacted a heavy environmental toll on the planet, and their impact is likely to worsen as “dirtier” energy supplies, such as heavy oil and coal, are tapped. As an alternative, biofuels offer the opportunity to reduce the emissions of both greenhouse gases (GHGs) and urban air pollutants. Their cultivation could cause huge disruptions in land use, but, if managed properly, the cultivation of energy crops could also facilitate the sequestration of carbon in the soil and provide an economic incentive to protect and restore ecosystems previously degraded by human activities.
One of the largest questions raised about biofuels is their net energy balance, particularly the question of whether the bio-based fuels produced contain more useful energy than the (fossil) fuels required to make them. This was a greater concern a decade ago than it is today, since advances in technology have improved production efficiency, giving virtually all current commercial biofuels a positive fossil energy balance. (See Table 4 on the following page.) Plants use photosynthesis to convert solar energy into chemical energy, and as technologies improve and facilities begin to use more biomass energy (e.g. from agricultural residues like sugar cane bagasse and corn stover), the amount of fossil energy used to produce the crops and convert them to biofuels will continue to decline.
There are two primary measures for evaluating the energy performance of biofuel production pathways. These are:
Energy balance—the ratio of energy contained in the final biofuel to the energy used by human efforts to produce it. Typically, only fossil fuel inputs are counted in this equation, while biomass inputs, including the biomass feedstock itself, are not counted. A more accurate term for this concept is fossil energy balance, and it is one measure of a biofuel’s ability to slow the pace of climate change.
Energy efficiency—the ratio of energy in the biofuel to the amount of energy input, counting all fossil and biomass inputs as well as other renewable energy inputs. This ratio adds an indication of how much biomass energy is lost in the process of converting it to a liquid fuel, and helps to measure more- and less-efficient conversions of biomass to biofuel.
Ethanol feedstock such as sugar beets, wheat, and corn have been criticized because their fossil energy balance is close to 1.0, a threshold many consider the line between an energy sink and an energy source. (Diesel and gasoline have fossil energy balances between about 0.8 and 0.9, numbers that may be more relevant for comparison than 1.0.) But this approach fails to account for two important nuances. First, ethanol is a liquid fuel that has qualities that make it useful in the existing transportation infrastructure. Since the natural gas and coal used to produce ethanol do not have this quality, it can be practical to lose energy in the process of converting these fuels into ethanol. Second, even crude petroleum must be refined into usable liquids.