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The Environmental Imperative for Renewable Energy: An Update Lifecycle Analysis |
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An accurate comparison of the land, water, air, and climactic
impacts of various electricity generation options requires "life
cycle" analyses, which examine the effects of producing and transporting
fuel, building and subsequently decommissioning facilities,
generating power, and treating and disposing of waste. For
ease of comparison, some studies translate these diverse impacts
into dollars, in keeping with past regulatory practices of attempting
to identify the leastcost resource strategy. 143 Such comparisons
are controversial and, to some readers, unsatisfying, since
many human health and environmental effects have no clear
dollar cost.
Several studies tackle a more modest task - although still a dauntingly complex one - by comparing the air or climate impacts of energy choices on a full life-cycle basis. 144 (See Table 7.) Such a study might include the following major categories:
It is also possible to include the energy required to decommission power plants or recycle equipment. The type of energy used to accomplish these tasks will affect the level of emissions, as will the absolute quantity of steel, cement, aluminum, etc. Note that cement production itself emits CO2 , and that aluminum production releases carbon tetrafluoride, a potent greenhouse gas. 145 Life-cycle analysis reveals interesting issues. For example, a report by the National Renewable Energy Laboratory on a hypothetical combined-cycle biomass gasification plant reckons that growing and transporting biomass, and building biopower facilities, requires about 5% of the energy produced by the cycle. 146 (The report also notes that soils vary in their capacity to accumulate carbon. Certain high-capacity soils may turn biopower from a modest source of CO2 into a carbon-absorbing "sink.") The report calculates lifecycle emissions for this plant at 46 grams of CO2 equivalent per kWh, plus the other pollutants described in Table 7. 147 A separate NREL report analyzes the life-cycle impacts of hypothetical coal-burning facilities. As expected, combustion represents the largest source of CO2 . Perhaps unexpectedly, the majority of noncombustion CO2 in current plants results from producing, transporting, and using limestone to absorb conventional pollutants from flue gas. Most of the SO2 and NOX comes from the power plant, while mining operations release most of the methane. For current coal systems, most of the particulate pollution comes from the production of limestone - ironically, particulate pollution from these operations exceeds the federal air standards set for coal plants. The report figures life-cycle greenhouse emissions of 1,114 g/ CO2 equivalent per kWh for current coal systems, and 1,050 g/ CO2 equivalent for systems capable of meeting the EPA's New Source Performance Standards for conventional pollutants. 148 The plants also produce the pollutants described in Table 7. Life-cycle emissions from noncombustion generation options, such as nuclear, wind, and geothermal power, are generally far lower than combustion options, as described in Table 7, although greenhouse emissions from biopower are also quite low. For example, a 1993 study for the Swiss Department of Energy posits a life-cycle greenhouse impact for nuclear power of 39.1 grams of CO2 equivalent per kilowatt-hour. 149 Other studies of nuclear power range between 8 and 54 g/ kWh, 150 although it is not always obvious what each study considers. (The Swiss study excludes plant decommissioning and radioactive waste storage.) PV systems have much lower life-cycle emissions than combustion options, but are the highest of the noncombustion options, particularly those PV systems including a battery. Fuel cells illustrate the importance of life-cycle analysis. This technology combines hydrogen and airborne oxygen in a chemical reaction yielding water, heat and electricity. Fuel cells entail no combustion; the cells themselves release no conventional pollutants, and few or no greenhouse gases. However, the systems' life-cycle impact depends on the source of the hydrogen and the efficiency of the cell. In the short term, most stationary fuel cells are expected to incorporate a fuel processor able to extract hydrogen from natural gas. In such a system, the processor would emit moderate levels of CO2 , as well as low levels of NOX and volatile organic compounds. For example, one study of phosphoric acid fuel cells fed by natural gas estimates efficiencies of 36%, CO2 emissions at 1000 lb/ MWh, and NOX emissions at 0.02-0.03 lb/ MWh. Exploitation of these cells' waste heat for productive purposes can raise overall efficiency to about 60%, lowering CO2 emissions to about 660 lb/ MWh. 151 In the future, fuel cells may run on hydrogen derived from sustainable biomass or biofuels, or from water split into hydrogen and oxygen by renewablygenerated power. The life-cycle greenhouse profile of such systems would include primarily the energy necessary to manufacture the components.
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The Environmental Imperative for |
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