Six electricity production systems are evaluated using energy and energy (Environmental Accounting. Emergy and Environmental Decision Making. New York:Wiley, 1996. 370pp.) accounting techniques, in order to rank their relative thermodynamic and environmental efficiencies. The output/input energy ratio as well as the emergy-based emergy yield ratio (EYR) and environmental loading ratio (ELR) have been jointly used to explore and compare system performances. Generation of CO2 has also been accounted for in order to compare renewable and nonrenewable energy sources. The production systems include both plants using nonrenewable energy sources (natural gas, oil, and coal thermal plants) and the so-called renewable energy sources (geothermal, hydroelectric, and wind plants). A method for evaluating the environmental contribution to electric production is shown to provide important information that can be used to support the environmentally sound public policy. Renewable power plants were characterized by high energy return on investment, while fossil fueled plants exhibited average energy efficiency in the 25-36% range. EYR varied from a high of 7.6/1 for hydroelectric generation to about 4.2/1 for the oil thermal plant. The renewable energy plants required the highest environmental inputs per unit of output while fossil fuel plants required relatively small environmental inputs for cooling and to support fuel combustion. Environmental loading was highest with thermal plants. Using an emergy index of sustainability, it is quantitatively shown how renewable energy source plants like wind, hydroelectric, and geothermal had higher sustainability compared to thermal plants. (C) 2002 Elsevier Science Ltd. All rights reserved.
