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34 MICROPOWER: THE NEXT ELECTRICAL ERA IS SMALLER CLEANER? 35 The rate of global warming in the past century, which scientists agree is at least partially human induced, is pro- jected to triple or quadruple over the next 100 years, causing a range of impacts, including sea level rise, more frequent and intense extreme weather events, flooding coastal low- lands, and shrinking freshwater supplies. Carbon dioxide (CO2) is the most important greenhouse gas, and electricity generation is the largest single source of global carbon emis- sions, accounting for more than one third of the roughly 6 billion tons emitted annually. All fossil fuel combustion emits carbon, though coal releases 29 percent more than oil and 80 percent more than natural gas per unit of energy.73 Power generation also imposes environmental burdens on land, water, and wildlife. Coal mining removes forest cover, contributes to soil erosion, and blocks stream flow, and increasingly it entails mountaintop removal. It also dis- places poor populations, as do large-scale hydropower pro- jects. Uranium mining releases radioactive gas, dust, and seepage from piles of waste rock. Both forms of mining cre- ate acidic mine drainage and discharge substantial amounts of heated water that cause long-term damage to aquatic ecosystems.74 In the case of wildlife impacts, a comparison between large-scale and micropower options is useful. The cooling systems of thermal and nuclear power plants can trap and kill fish. Large hydroelectric dams can directly cause fish fatalities or block migration patterns, leading to substantial population declines: U.S. dams are primarily responsible for a reduction in Pacific Northwest salmon from 16 million to 300,000 wild fish per year. Single events can also have an impact: the Exxon Valdez oil spill killed between 90,000 and 270,000 seabirds. Documented bird deaths related to wind turbines, by contrast, have been confined to less than 200 during the late 1980s, and the problem has since been addressed by careful siting and other practices.75 Solid waste and heavy metals provide additional envi- ronmental criteria for comparing micropower and larger sys- tems. Two PV technologies, cadmium telluride and cooper indium diselenide, use semiconductors that employ heavy metals instead of silicon; but toxic cadmium and selenium releases are small and can be further reduced by improved fabrication, construction, and recycling procedures. While waste from biomass is not toxic, the flue gas and solid waste from coal plants contain high levels of arsenic, cadmium, and other toxic heavy metals.76 Large-scale power generation can pose radiation threats. In addition to the danger of catastrophic accidents, radioactive elements from nuclear fission increase the risk of cancer, damage organs, and affect cell development. Nuclear waste from irradiated fuel rods can also cause cancer or genetic damage and has prompted the creation of costly underground repositories. Nuclear reactors release low levels of radioactivity and lower-level waste that require expensive disposal and storage. Even coal-fired plants release some background radiation.77 Energy use can involve many processes, from mining and transportation to combustion and cleanup, and “life- cycle” assessments that take all these stages into account help gauge a power source’s total environmental impact. Particularly relevant are the life-cycle air and climate impacts from coal, the fuel accounting for a 40 percent share of glob- al electricity—and shares of 73, 75, and 95 percent in India, China, and South Africa, respectively. (See Table 6.) Producing and using limestone to remove sulfur from coal combustion, for example, releases more particulates than federal standards allow for U.S. coal plants. It is also the largest source of coal-related carbon emissions other than combustion, with emissions more than double those for transporting the fuel.78 Life-cycle analysis of micropower systems is also reveal- ing. Solar PV has the highest life-cycle emissions among non-combustion options mostly because of the energy need- ed to make silicon, but they are much lower than those of combustion-based systems. The life-cycle impacts of recipro- cating engines will depend on whether old, diesel-based or modern, natural gas-based systems are used; those from fuelPDF Image | Micropower: The Next Electrical Era
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