Which is a better fuel--gasoline or ethanol? Which fuel provides better gas mileage? Produces fewer greenhouse gas emissions? Requires less energy to produce? With the exception of the gas mileage question (ethanol produces two-thirds the energy of gasoline per gallon), none of these questions has an easy answer.
Concerns about climate change, energy independence, and fossil fuel supplies are increasingly in the news. Biofuels such as biodiesel and cellulosic ethanol could help cut U.S. dependence on petroleum-based transportation fuels by up to 30% (U.S. DOE 2006). But with all of the options for alternative fuels out there, how do we determine which is the "best"?
When asked to compare fuel alternatives, many of us think only about what happens in the car itself (e.g., how many miles per gallon it gets and its tailpipe emission ratings). However, a more accurate comparison Would be to examine the processes from "cradle to grave," accounting for all of the steps before fuel is even put into the car--beginning, for example, with petroleum extraction, ocean transport, refinery operation, and the gasoline's transportation to the pump.
A life-cycle assessment (LCA) is a tool used by engineers to make measurements of net energy, greenhouse gas production, water consumption, and other items of concern. The Environmental Protection Agency's National Renewable Fuels Standard sets limits for net greenhouse gas emissions in biofuel manufacturing--compared to gasoline; it also requires biofuel producers to conduct LCAs that evaluate emissions during farming, transportation of raw materials, refining, and so on (EPA 2010).
As with all sustainability issues, getting people to think about the life cycle, or full system, is a challenging yet valuable approach. The first step is to develop a clear picture of all the steps in the system in which materials and energy are used or transformed.
This article describes an activity designed to walk students through the qualitative part of an LCA. It asks them to consider the life-cycle costs of ethanol production, in terms of both energy consumption and carbon dioxide emissions. In the process, they trace matter and energy through a complex fuel-production system.
Once students understand what a qualitative LCA looks like, they are better able to assess the quantitative results and critically consider the modes of production for different fuel types. By the end of the activity, students can suggest efficiency improvements in the biofuel production system and ways to reduce net greenhouse gas emissions. In addition, it often gets them thinking about other products they use and how these choices might affect the environment.
Formative assessment questions.
A. Which is a better fuel--gasoline or ethanol? For each of the
criteria below, place a check mark below which fuel you think is
Energy per gallon/miles per gallon
Energy required to produce a gallon
Carbon dioxide emissions
For the environment
To create ethanol, sugars in plant biomass (i.e., the leaves,
stems, and other plant parts) are harvested and converted into fuel
([C.sub.2][H.sub.5]OH). Based on what you know, check the
statement(s) with which you agree.
-- Carbon dioxide is released when ethanol is produced from plant
-- Creating ethanol from plant biomass is carbon neutral.
-- Creating ethanol from plant biomass contributes to climate
B. Create a drawing or cycle that explains the production of
ethanol from plant biomass. Show the movement of carbon or carbon
dioxide from location to location as best you can.
I designed this lesson, which can be used in an integrated science, biology, or environmental science course, to take two 50-minute class periods. …