 |  Environmental Science: A Global Concern, 7/e William P. Cunningham,
University of Minnesota
Mary Ann Cunningham,
Vassar College
Barbara Woodworth Saigo,
St. Cloud State University
Sustainable Energy
BE ALERT FOR: Net Energy Efficiency
Pay careful attention to the concept of energy conversion efficiencies in the text. To evaluate the effectiveness of any device designed to yield useful energy, you need to consider all the energy consumed in the process as well. For example, assume a farmer can produce 10 units worth of energy (as alcohol) off a cornfield in a year. To assess the relative efficiency of raising corn for energy requires that we take into account the amount of energy used by the farmer to produce the crop. If the fossil fuel used to run the tractor to plow, plant, cultivate, fertilize, and harvest the corn crop and all the other associated energy costs (such as manufacturing the fertilizer and herbicide used to raise the corn) is 9 energy units, what is the net energy gain? One unit of energy, not 10.
BE ALERT FOR: Role of Firewood
Humankind's first energy source was firewood, a form of biomass. Energy from the sun is converted by photosynthesis to chemical energy, which is stored in the molecules that make up wood. This energy gets released as heat during combustion.
Most of us think of wood as an old-fashioned energy source. Surprisingly, it still provides the vast majority of energy used in developing countries. In fact, the removal of trees for fuelwood, with the subsequent erosion of soil, is one of the most important environmental problems of the developing world. Also note that, because of its renewable nature, there is increasing interest in biomass in the developed countries like ours, but with some technological twists:
• production of methane gas from decomposition of such things as manure, garbage, and other organic wastes • creation of fuel alcohols from grains and anaerobic digestion of plant material
The more things change, the more they stay the same.
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2003 McGraw-Hill Higher Education