Water and energy are interdependent resources. As the population grows and the climate changes the United States and the world will need to carefully manage these resources in order to meet future demand. Over the last half century it has become clear that one of the greatest challenges to meeting this demand will be the spatial incongruence of water resources and population growth. Unfortunately, climate change will place further water stress on the most arid regions where US population is currently expanding the most rapidly. And, as we know, the threat of severe climate change is influencing our energy supply mix. The term “energy water nexus” recognizes dual realities; that energy generation uses large quantities of water and that water distribution and sanitization uses large quantities of energy. Stated quantitatively we can say that 41% of all US water withdrawals are for thermoelectric power generation and 13% of all US energy use is for water distribution and sanitization.
Thermoelectric power derives electricity from a heat source used to create steam that then turns a turbine. As of 2005, when the most recent United States Geological Survey (USGS) report was released, thermoelectric power represented the greatest share of total US water withdrawals with agriculture being second at 37%. A report produced by the Department of Energy’s National Laboratories notes that coal fired power plants require 25 gallons of water per kilowatt of electricity produced, meaning that for US citizens turning on the lights and powering appliances indirectly uses as much water as taking showers and watering lawns. While it is relevant to note that only about 8% (2 gallons per kilowatt hour out of the total of 25) of the water withdrawal for thermoelectric power generation is considered a consumptive use, much of the water that is returned is thermally polluted. This can negatively impact aquatic life by encouraging algal blooms that reduce oxygen levels through eutrophication, which involves changes in nutrient cycling that can devastate native ecosystems accustomed to colder water. For example, some of the most prized species of freshwater fish can only live in colder waters. As top predators, declines in game fish populations resonate throughout the freshwater ecosystem in a trophic cascade. Beyond catastrophic impacts to native biodiversity the consequences of thermal pollution can include economic losses associated with recreational fishery collapses, increased treatment requirements for drinking water supplies and other externalities.
Energy generation’s consumption of water is not restricted to the generation stage. The extraction of inputs for fossil fuel energy plants also uses substantial amounts of water. As we recently detailed in a separate piece on S&S, the expansion of hydraulic fracturing for shale gas has had limited national impacts on water withdrawals but does represent a significant share of withdrawals in some locales. Hydraulic fracturing, however, does present other threats to water quality beyond withdrawal volumes. These threats include chemical mixing, induced seismicity from injection, contamination of surface water by flowback and produced water, and solid waste pollution all of which S&S will detail in other posts as part of our Matter of Frack series.
The surge in fracking has not been all negative with regard to water use however. One water related benefit of the recent surge has been the increasing use of new natural gas combined-cycle (NGCC) power plants with wet-recirculating or closed-loop cooling systems, which withdraw less than 1% what traditional coal fired plants withdraw using once-through-cooling. Wet-recirculating plants recycle cooling water in towers after each use. This increases evaporative losses but decreases both withdrawals and thermal pollution associated with once-through plants where heated water is more immediately returned to natural bodies. So despite the increased use of water in the extraction phase, if hydraulic fracturing continues to increase the use of NGCC plants, net water use in energy generation may decline. This underlines the complexity of these systems and emphasizes the importance of viewing the water-energy nexus holistically.
No discussion of water use in energy development would be complete without a discussion of hydropower. Though it does not involve the direct removal of water from streams, the dams that create the necessary hydraulic head for power generation are controlled by reservoirs that enhance evaporative water losses in very significant ways. Compared to the 2 gallons of consumptive use associated with thermoelectric power generation, 18 gallons are consumed though evaporation in reservoirs used for hydropower generation in the United States. For the famously large dams used for hydropower and other purposes in arid states such as Arizona, hydropower reservoirs consume over 200 times as much water as thermoelectric power plants, averaging 65 gallons per kilowatt hour of electricity produced. Further, the impoundment of rivers dramatically impacts the natural flow regimes by preventing sediment transport, disconnecting habitat, creating thermal gradients and regulating flow levels in ways that do not align with the locally adapted biota conditioned for seasonal variations. As 30- to 50-year dam contracts with the Federal Energy Regulatory Commission (FERC) expire, the negative impacts of impoundment can be so severe that many communities are choosing to restore their rivers by removing dams and seeking alternative forms of energy generation. While hydropower is less problematic in terms of greenhouse gas emissions, rampant damming still represents one of the more thoughtless impacts the 20th century has bestowed upon US waterways.
Alternative energy sources, such as solar and wind power, seem to be losing momentum as the shale gas boom continues, yet these non-fossil fuel sources consume comparatively little water. That is, the only water consumed by solar and wind power is the relatively negligible amount of water involved in the manufacturing of the infrastructure itself. While the economic impracticality of these alternatives is evident at the moment, several studies have shown that they do theoretically offer all of the power we need if the political will to aggressively pursue these options materializes.
You may have noticed that at this point we have only described one half of the energy water nexus — how water is used to generate energy. It is important to remember, however, that the relationship between water and energy goes the other way as well. Pumping water — to supply arid regions, for irrigation and to reach treatment facilities — requires energy. As noted above, 13% of all the energy we use in the U.S. is devoted to moving water. The Central Arizona Project (CAP), constructed in part to supply Greater Phoenix with water and prevent unsustainable aquifer withdrawals, moves water 335 miles from the Colorado River through south-central Arizona. Because the water is moved up over 3,000 vertical feet the project included the construction of a coal-fired power plant to generate the 536 MW of power needed annually. An energy total that represents 4% of all the power generated in Arizona on an annual basis.
As Phoenix, one of the fastest growing urban area in the U.S., continues to expand, the water requirements of the city — and the amount of energy needed to satisfy these requirements — will only continue to grow. Unfortunately, Phoenix is not unique. Five of the ten fastest growing urban areas in the U.S., and five of the ten fastest growing global cities, are in areas the WWF defines as facing physical or economic water scarcity. The interrelated nature of water and energy means that not only will these cities face increased energy demands due to population growth, that demand for energy will be magnified by the need to supply a growing population with energy intensive water. Further, energy intensive water is expensive water. If global population growth continues to be concentrated in areas of water stress, providing access to clean water at affordable prices will be even more challenging.
The direct connection between water supply and energy use also highlights an important, perhaps overlooked, implication of water supply projects: climate change. While it is difficult to link any single weather event to climate change, it is clear that climate change increases water stress in some regions of the U.S. Recognizing that water supply projects require large energy supplies, it should be clear that any attempt to solve the water shortages caused by climate change could further exacerbate future climate change if they are powered by fossil fuels — like the CAP. Climate change thus provides another connection between water and energy. Not only is the supply of water and energy interdependent, but some energy sources can contribute to reductions in long-term water supplies.
While, impressively, overall water withdrawals in the U.S. has stabilized since the 1980’s despite population growth this stabilization has not come without costs. The aforementioned impacts of impounding rivers for hydropower generation apply equally to impoundments in reservoirs used for water supply. Further, beyond impoundment land use manipulation and associated water quality stresses continue to expand. So even though water withdrawals have not increased in recent years US waterways have continued to degrade. Stated simply, aquatic ecosystems in America are in a pathetic state that will require energy for treatment as conditions worsen, as climate change reduces supplies and as population and landscape development continue to expand.
Water and energy are more interrelated than almost any other two resources. It is nearly impossible to supply a large population with one if you do not have the other. This presents a unique management challenge that is compounded by the additional feedback loop between energy generation and long-term water supplies that climate change has created. That population growth stubbornly insists upon occurring in water stressed regions further exacerbates the issue. All of these things taken together suggest that finding creative, and long-term, solutions to the water-energy problem may be the most important challenge facing resource managers in the coming decades.
Water is essential to life and, increasingly, so is energy. Let’s hope that the connections between the two ultimately prove to be a source of creative solutions instead of insurmountable challenges.
Image Credit: By Marianne Gagnon, via Wikimedia Commons