Life-cycle uses of water in U.S. electricity generation
Introduction
Although water is an indispensable resource for economic development, its availability in the United States has not been assessed comprehensively in 25 years [1]. Nevertheless, the current trend indicates that demands on the nation's supplies are growing, while our capacity to store surface-water is becoming more limited, and ground water is being depleted. Predicted drought in some areas might well exacerbate this shortage. Electricity generation via conventional pathways accounts for a major part of water demand. The United States Geological Survey (USGS) estimated that in 2005 thermoelectric power plants withdrew approximately 41% of our freshwater, closely followed by 37% for agricultural irrigation [2]. In an effort to reduce the specter of water shortage in the future, new thermoelectric power plants are instituting water-saving technologies based on recirculating their cooling water or dry cooling. A national level appraisal by the U.S. DOE in 2009 predicts that by 2030 the freshwater water withdrawal for generating electricity could fall 4–23% from the level of 2005 if this trend continues [3].
In contrast, renewable energy sources, such as photovoltaic- and wind-power, use no water during their operation. However, every energy-generation technology does use water sometime throughout their entire life-cycle. For example, during the photovoltaic life-cycle, water is used for cleaning silicon wafers and glass substrates, and preparing chemical solutions. In addition, a significant amount of the electricity used to purify silicon and other semiconductor materials is generated by thermoelectric power plants that rely on a water-cooling system. Conversely, as well as using water during their operation, such plants need water both directly and indirectly during fuel acquisition, plant construction, and disposal stages. In an early study, Gleick reviewed water requirements during the life cycles of electricity-generation technologies, i.e., mining, fuel preparation, and construction, operation, and the maintenance of power plant [4]; this analysis was limited to consumptive water use. Recently, Sovacool and Sovacool evaluated the life-cycle water use of U.S. thermoelectric power plants that encompassed both withdrawal and consumption [5]. Neither study, however, evaluated the parameters of upstream water usage associated with energy and material inputs to the life cycle of electricity-generation technologies.
In this paper, we evaluate the life-cycle water usages of conventional and new electricity-generation technologies, including those in a demonstration stage, i.e., coal gasification with carbon sequestration. We consider both upstream (indirect) and on-site (direct) usages in our assessment to encompass completely the water used in the entire supply chain for electricity generation. Then, we compare the life-cycle water withdrawal factors across electricity-generation options, followed by a discussion on policy implications.
Section snippets
Scope of the analysis
In general, life cycles of thermoelectric power, including the biomass cycle, consist of the acquisition of fuel, its preparation, construction of the plant and equipment, device/product manufacturing, power generation, and fuel disposal stages, as depicted in Fig. 1. Renewable cycles do not entail the front two stages. Fuel disposal particularly is important for the nuclear-fuel cycle, although its environmental impacts are not characterized fully, as disposal has not yet been implemented. In
Thermoelectric fuel cycles
The fuel cycles of conventional thermoelectric power, i.e., coal, nuclear, natural gas, and oil, begin by extracting fuel from the earth and processing them into a form suitable for combustion, so-called beneficiation. Then, during the plant's operation, the fuel is burned to operate the turbine or steam generators. Later parts of the fuel cycles include decommissioning the power plant, and disposing of the spent fuel.
We evaluated water usages during the acquisition and preparation of the fuel.
Water usage factors of power-plant operation
In thermoelectric power plants, water cools and condenses the steam generated by burning fossil- or nuclear-fuels, and replenishes lost steam generated in the boilers; it also is used for cleaning flue gases. These power plants use fossil fuel or biomass, or fission uranium fuel to turn water into high-pressure steam to operate a turbine generator. The steam subsequently is cooled, condensed in a heat exchanger or condenser through which cooling water flows, and returned to a steam generator.
Comparison of life-cycle water factors
Here, we use the data presented in the previous sections to estimate the total life-cycle water factors and compare them across fuel cycles. We consider only water withdrawals for this comparison because there is minimal information on upstream consumptive water usages. For the coal-fuel cycle, the US average water withdrawals are estimated for mining, beneficiation, and plant construction, based on the sources listed in Table 1, along with statistics on the method of coal production, i.e.,
Discussion
Water is used in various stages of the life cycle of electricity-generation technologies, that is, fuel acquisition, fuel treatment, plant construction, operation, decommission, and disposal. Water is also used upstream for energy and material inputs into each stage; although we attempted to quantify the latter, there are few data on upstream water use, and databases often list only water inputs into the process, considering them equal to water withdrawal [52]. A full water accounting,
Conclusion
We reviewed and evaluated the life-cycle water use factors per unit electricity generated across thermoelectric- and renewable-technology options in the United States. These factors can be used for further region-specific analyses as energy technology choice for a region is often constrained by the local availability of natural resources. PV- and wind-technologies in addition to providing clean, abundant energy, they can also prevent a foreseeable water-shortage crisis at local or regional
Acknowledgments
This work was supported by the Solar Technologies Program, US Department of Energy, under Contract DE-AC02-76CH000016 with the US-DOE.
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