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| Fig. 1: Simplified schematic of the water use cycle. |
Americans use over 400 billion gallons of water, or 1,300 gallons per person, each day. Nearly half (49%) of this water use is dedicated to cooling systems in thermoelectric power plants that produce electricity. [1] Other important energy-related water uses include surface mining (quarrying) and hydraulic fracturing (fracking). In many ways, the energy sector is vitally dependent on readily available water. Similarly, modern water infrastructure depends on multiple energy-intensive processes to deliver water from source to tap and to eventually return non-contaminated water to a source after use. Energy, typically in the form of electricity, is required to supply, treat, distribute, and recycle water, as illustrated by the water use cycle in Fig. 1. In California, nearly 20% of the state's electricity consumption goes toward water-related uses. [2] This report focuses on the latter part of this energy-water interdependence, examining the energy costs of water infrastructure, particularly in California.
Each energy intensive step in the water use cycle can be effectively classified into one of two fundamental categories (color-coded in Fig. 1): water transport (light blue) or water purification (light red). End-uses of water (green) vary considerably from toilet flushing to watering crops, and are not considered part of the basic water infrastructure assessed here.
Water transport processes include all parts of the water infrastructure that move water from one place to another. Pumping power (W=VdP) is required both to overcome gravity for elevation change and to overcome friction in water piping. These energy costs depend heavily on the distance and topography between the source and end user, which varies significantly between regions. A stark example of such variability is the comparison between Northern and Southern California water supplies. In Northern California, urban water supplies are primarily sourced locally from shallow wells or conveyed by gravity from mountain reservoirs, yielding a near-zero energy cost for transport to local distribution stations. By contrast, Southern California imports some 50% of its water from the Colorado River and Northern California, requiring hundreds of miles of transport and more than 50 times the energy (~9,000 kWhr/million gallons), on average, than Northern California water supplies. [2] The California State Water Project (SWP) transports surface water from the Sacramento-San Joaquin Delta to 20 million residents in Southern California, nearly 400 miles to the south, lifting water more than 2,000 ft. over the Tehachapi Mountains along the way. This water transport makes California SWP the single largest energy consumer in the state, accounting for approximately 3% of total electricity consumption. [3] Energy costs from local water distribution and other transport mechanisms (discharge, collection, recycle) do not vary much (~1,000 kWhr/million gallon) region to region, and can be more (Northern California) or much less (Southern California) than the supply and conveyance steps. [2] Looking forward, population growth in the United States will increase demand on limited fresh water sources, requiring that water be pumped from greater distances and deeper wells, increasing the energy-intensity (due to transport) per gallon of water used. [4]
Water purification processes include treatment prior to use and wastewater treatment after use. The energy requirement to treat water depends both on the initial quality of the source and the required purity post-processing. Fundamentally, energy (via a separation process) is needed to overcome the chemical potential difference between pure and diluted (or dirty) water. Separation or purification processes can be mechanical (filtration), chemical (chlorination), or thermal (distillation), and are often used together in staged treatment facilities. Historically, fresh water sources have required relatively modest energy input to be made potable (< 500 kWhr/million gallons), but increasingly stringent water quality standards and less pristine sources over time have led to more significant energy requirements (~1,400 kWhr/million gallons) for potable treatment. [2,4] Wastewater treatment is similar to source water treatment with the typical addition of a biological (decomposition) process for treating solid waste. Also, wastewater filtration processes are typically more energy intensive due to the higher pumping pressures required to handle solids in the flow stream. Wastewater treatment energy costs are therefore slightly higher (~2,000 kWhr/million gallons) than initial source water treatment costs. [5] A particularly energy-intensive water purification process is desalination (~15,000 kWhr/million gallons), separating salts from seawater via distillation or osmotic (membrane) processes. [6] Though desalination has high energy costs, it can be competitive in certain seaside municipalities in Southern California that rely on expensive water imports.
These aggregate (transport plus purification) energy costs represent approximately 8% of total state energy consumption, equating to about 20 gigawatt hours per year, not counting end uses. [2] Comparably, California yields around 25-30 gigawatt hours of in-state hydroelectric energy production each year. [7] So while the state's water resources produce a significant amount of renewable hydroelectric power, most of this energy is actually nullified by the energy requirements to manage California's water infrastructure.
© Mitchell Spearrin. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] J. F. Kenny et al., "Estimated Use of Water in the United States in 2005," U.S. Geological Survey, Circular 1344, September 2009.
[2] "California's Water-Energy Relationship," California Energy Commission, CEC-700-2005-011-SF, November 2005.
[3] R. Cohen, B. Nelson and G. Wolff, "Energy Down the Drain: The Hidden Costs of California's Water Supply," Natural Resources Defense Council, August 2004.
[4] R. Goldstein and W. Smith, "Water and Sustainability (Volume 4): U.S. Electricity Consumption for Water Supply and Treatment - The Next half Century," Electric Power Research Institute, Technical Report No. 1006787, March 2002.
[5] R. Wilkinson, "Methodology for Analysis of the Energy Intensity of California's Water Systems, Lawrence Berkeley Laboratory, Agreement No. 4910110, January 2000.
[6] "Water Desalination: Findings and Recommendations," California Department of Water Resources, October 2003.
[7] "Energy Aware Planning Guide," Section II, California Energy Commission, CEC-600-2009-013, February 2011.