per kilowatt-hour (kWh). Finally, it should be noted that developing complete LCAs of electricity sources is beyond the scope of this panel. There are, however, a wide range of earlier assessments, and these form the basis of this section.

A major complication in comparing LCAs is that there is no set standard for carrying out such analyses. While it is the goal in using LCAs to cover technologies from cradle to grave in a systematic way, there is variability in the assumptions, boundaries, and methodologies used in these assessments. Therefore, caution should be used in comparing LCAs; each is an approximation of a technology’s actual impact. Discussion of the attributes and assumptions used in life-cycle analysis is found in Appendix E.

The renewable energy technologies are wind, solar, geothermal, hydroelectric, tidal, biopower, and storage. Appendix F contains LCA studies for coal, natural gas, and nuclear technologies as a benchmark against which to assess the performance of renewables. LCA information for solar energy is limited to photovoltaic (PV) technologies, and no LCA studies were reviewed for concentrating solar power (CSP) technologies such as solar trough, power tower, or dish–engine technologies. No LCA information is included for enhanced geothermal systems. The life-cycle impacts considered here include net energy usage; atmospheric emissions of greenhouse gases expressed in units of carbon dioxide (CO2) equivalents (CO2e);1 atmospheric emissions of sulfur dioxide (SO2), nitrogen oxides (NOx), and particulate matter;2 land use; and water withdrawals and consumption. To provide a sense of the variability of the LCAs found in the literature, the maximum, minimum, and average energy usage and environmental impact for each technology are shown in figures discussed below in this chapter.

Energy

Energy input and output calculations, the basic building blocks for any life-cycle evaluation of greenhouse gas emissions, can be used to evaluate the energy inten-

1

Equivalent carbon dioxide emissions (CO2e) are the amount of greenhouse gas emissions expressed as carbon dioxide, taking into account the global warming potential of non-carbon dioxide greenhouse gases (e.g., methane and nitrous oxides).

2

All energy technologies are included in the CO2 section even if CO2 emissions were low. Other pollutants with emissions less than 100 mg/kWh are not included in the data and discussion. Studies used to compile CO2 data often make up a different data set from the studies used to compile other emissions. Often LCA studies focus only on CO2. This required building another data set for other emissions.



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