The energy-water nexus: Managing water in an energy-constrained world
Kathleen Cantner, AGI
Water can be tricky. With too little, crops die, industries move away, power plants fail, ecosystems suffer and people go thirsty. With too much, floods ruin infrastructure, destroy crops, spread waterborne diseases, and disrupt flows of clean water, wastewater, power and transportation. We want water at the right time and in the right place because moving and storing water require effort. We also want it at the right quality and the right temperature. Saline water, brackish water and polluted water are abundant but costly to treat. Water that’s too warm won’t cool power plants effectively and can damage ecosystems, while water that’s too cold can burst pipes and damage infrastructure.
Water’s capriciousness, however, can be tempered — with energy. If we had unlimited and perfectly clean energy (to mitigate environmental impacts) at our disposal, we could desalinate the ocean, providing enough potable water for everyone, everywhere. We could build pipelines to move water from where it is abundant to where it is scarce. We could build adequate sanitation infrastructure in communities that lack it so that raw sewage does not flow into freshwater ecosystems and cause sickness and eutrophication. We could build more storage reservoirs so that water could be collected in times of excess to be used in times of shortage.
Of course, we do not have unlimited energy and what we do have isn’t perfectly clean, and consequently, energy has become a constraining factor on our management of water issues.
The corollary is also true: Just as energy constraints become water constraints, in many regions, water has become a constraining factor on the energy supply. Power plant operators sometimes don’t have access to enough water to build new power generation facilities using conventional designs. And they face environmental constraints on the temperature of cooling water that is discharged into local streams based on limits established to protect fish and ecosystems. Water can also be a constraining factor in extracting energy sources such as in oil and gas production, which can use tremendous amounts of water.
This interdependence between water and energy is called the energy-water nexus. And while the relationship can be mutually constraining, it also presents an opportunity to address both energy and water issues together, because conserving one leads to conservation of the other. Consequently, the way we manage the delicate relationship between the two will have major implications on the future of our energy and water crises.
Before the advent of centralized water and mechanical pumping systems, people typically drew water from their local stream, river or well, even in urban areas. The only energy involved was that expended to draw and carry the water. In many rural or poor parts of the world, this is still the case. Breaking from that tradition, the U.S. began developing its centralized drinking water treatment and distribution systems in the early 1900s and now enjoys one of the safest and most abundant public water supplies in the world.
Due to increasing demand, current trends in the United States’ water sector suggest that we are moving toward the use of more energy-intensive water: for example, water that has been desalinated in places like Southern California and Florida; or that has been extracted from deep aquifers as in the Colorado Rockies and Great Plains; or that has been transferred between basins, which is done in the Desert Southwest and California.
These trends raise questions about how much energy we use to treat, move and prepare water for end use today, and what lessons learned from the United States’ water system can be applied to other regions in the world that are still developing large centralized water systems and wastewater infrastructure.
Our recent research has examined these questions. To calculate how much energy we use for water on a national scale, we first tallied the energy used in the U.S. to pump water to treatment facilities and treat it. Then we added up water-related home energy use for purposes such as heating water for bathing and cooking. Finally, we included nonhousehold energy used for water in businesses, public facilities, industrial facilities and power plants. We found that annually the U.S. spends 12.3 quadrillion BTU (12.6 percent of our total annual energy consumption) in one way or another directly on water — much more than we had anticipated.
Energy Used for Water in the U.S.
In the U.S., that 12.3 quadrillion BTU includes uses for everything from extraction through treatment, end uses and disposal. Approximately 30 percent of that energy is consumed exclusively for residential and commercial water heating. Another 30 percent is used to heat and pressurize water for steam-injection in industrial processes such as oil refining and chemical manufacturing. Appliances such as dishwashers, washing machines and dryers, which heat water to clean and heat air to dry, represent about 8 percent of water-related energy use.
Although water treatment is envisioned as the poster child of energy spent on water, water treatment uses a relatively small amount of energy compared to that consumed at the point of use. Energy consumed for water treatment by public water utilities represents only 4 percent of the 12.3 quadrillion BTU (equating to 0.5 percent of total 2010 U.S. energy consumption). The remaining 28 percent is spread across multiple uses.
The energy required for water treatment is dictated by the water quality of the source and specific end use. Groundwater typically requires more energy for pumping than surface water, and those energy requirements increase with well depth. But surface water, which is sometimes degraded from runoff or industrial discharge, often requires more treatment than large groundwater sources, which tend to be cleaner. Once treated, the water is pumped from the treatment facility through the water distribution system to its final end user, where it might be heated, pressurized, pumped or cooled.
Approximately 40 percent of the water that leaves a water treatment plant is returned to the environment through outdoor irrigation or leaks (although this proportion varies a great deal by location and season). Water that is used inside and flushed down the drain (or the toilet) is delivered to a wastewater treatment facility and reconditioned to a cleanliness level that is appropriate for release into the environment. A small fraction of the reconditioned water might be “reclaimed” for nonpotable purposes such as irrigating golf courses or for power plant cooling.
Thermoelectric power producers, agricultural users and many industrial facilities typically extract their own water, as opposed to receiving it from the public water supply. For some activities, raw water quality is sufficient and does not require additional treatment. For other applications, onsite water treatment might be required. Contaminated wastewater from self-supplied water users is still required to be treated to a standard consistent with the EPA’s Clean Water Act before being discharged to the environment.
Regional Challenges, Regional Solutions
Despite these broad characterizations and averages regarding national water-related energy use, the United States is a difficult place to generalize. Disparate climates with varying amounts of precipitation and susceptibility to drought affect the availability of surface water and groundwater, which affects pumping depths and distances. Homeowners in Southern California, for example, are likely to receive water that has been pumped hundreds of kilometers, through two mountain ranges, from the San Joaquin Delta in Northern California. Before the water even reaches its intended customers, it has an energy intensity of about 11 kilowatt-hours (kWh) per 1,000 gallons (though some of that is recovered with in-line turbines as the water flows within pipes back down from the mountain passes). By contrast, customers in Massachusetts, where precipitation and water reservoirs are ample, receive water that has an intensity of about 1.5 kWh per 1,000 gallons, a mere 14 percent of their California counterparts. (However, even in Southern California, end-use activities such as water heating, cooling, pumping and pressurization at the point-of-use still represent nearly 60 percent of the total energy embedded in water over its entire lifecycle.)
The energy consumed by public water and wastewater utilities may currently represent a small slice of water-related energy, but many public water systems around the country are shifting toward more energy-intensive water sources that will likely increase their overall energy use in the future. States like California, Florida and Texas have built desalination facilities that, on average, require about 10 times more energy per unit of water treated than standard surface water treatment operations. Plans for long-distance pipelines to bring water to the drought-stricken West are under way, and although they might temporarily placate water shortages, they have high energetic and financial costs.
In the meantime, droughts in the West, Great Plains and Midwest have increased pressure on over-pumped aquifers, causing water tables to fall and forcing users to draw up from deeper water levels.
Historically, cities and communities grew around available water sources, with the assumption that those sources would last indefinitely. But with new demands on water resources, this planning assumption is challenged. As the water moves to new locations, one question that looms is whether we will move the people to be close to the water or move the water to be close to the people.
There are some bright spots in the United States’ water picture. Reclaimed wastewater is being used for purposes such as landscape and golf course irrigation, power plant and industrial cooling, and toilet flushing. It is also being used to replenish aquifers: In Orange County, Calif., falling aquifers have become increasingly susceptible to saltwater intrusion. In response, scientists and engineers designed a system to reinject treated wastewater into the depleted aquifers to create a barrier to keep saltwater from contaminating valuable freshwater resources.
Water conservation has also led to important reductions in demand. Water-efficient cooling technologies for power plants reduce their water demand. Many industries have reduced the water-intensity of their supply chains to decrease vulnerability to water shortages. More efficient irrigation systems, low-flow appliances, and xeriscaping — landscaping with native plants that do not require irrigation — also offer water savings for municipalities. Significant savings in irrigation remain for the agricultural sector.
Follow the Leader?
Like the U.S., other countries are moving toward more energy-intensive water via large infrastructure projects intended to deliver clean water from water-rich regions to water-scarce regions. In China, the South-North Water Transfer Project, slated for completion in 2050, includes plans for three water pipelines of 500, 1,200 and 1,300 kilometers in length, respectively. Egypt, India, Libya and South Africa, as well as other developing and developed countries, are planning or constructing very large water-supply projects. Similarly, countries such as India, Kuwait, Saudi Arabia, Singapore and the United Arab Emirates have already committed to large desalination facilities to increase potable water supplies. But these projects are very costly and markedly increase the energy consumed to bring water to people around the world.
There is a delicate balance between energy and water resources. Large water infrastructure projects bring water to people who might not otherwise have it, but they also stress the energy infrastructure and impact efforts to move toward alternative sources. However, conservation, reclaimed water projects, and desalination powered with renewable energy could achieve energy, water and climate objectives, simultaneously. For example, Saudi Arabia is building the first commercial-scale solar-powered seawater reverse osmosis desalination plant in the world. Once construction is completed later this year, the plant is anticipated to meet the daily water needs of 100,000 people. In the Desert Southwest of the U.S., something similar could be done to turn the vast brackish water resources into freshwater using renewable wind or solar resources.
How the U.S. and other water-constrained countries manage water during the transition from nonrenewable to sustainable sources of energy poses one of the biggest ongoing challenges of the 21st century.