Friday, July 19, 2013
In recent years, global issues such as drought and the scarcity of natural resources have put a spotlight on two critical resources that make most of life’s daily activities possible: energy and water.
Two researchers from The University of Texas at Austin’s Cockrell School of Engineering believe that increasing awareness about water and energy interdependencies — an area referred to as the energy-water nexus — could help regions and communities manage the resources they have more efficiently, as well as help shape water and energy conservation policy. But, until recently, no one understood exactly what the energy cost was for water that’s delivered to homes, businesses and industry.Associate professor Michael Webber in the Department of Mechanical Engineering and Kelly T. Sanders, a civil engineering doctoral student, calculated the total energy consumed for water use on a national scale in their 2012 peer-reviewed article “Evaluating the Energy Consumed for Water in the United States.”
The report’s main conclusion is that roughly 13 percent of energy used in the U.S. is consumed in the treatment, distribution, reclamation or usage of water. The report is the first to estimate the energy needs of the nation’s water system starting with where it originates and following it through its delivery to four end-use sectors: residential, commercial, industrial and power.
This story first appeared on the Cockrell School of Engineering website. It was written by Sandra Zaragoza.“It turns out that we use more energy for water than most people would anticipate,” Sanders said.
The researchers also found that there is wasted energy in heating water. This is attributed to the process of transferring thermal energy into the water we use for bathing and cooking, as well as inefficiencies in plumbing networks.
They concluded that one of the largest uses of energy for water stems from residential and commercial water heating, which contradicts a popular belief that water treatment is the most energy-intensive water-related activity. Water heating represents 30 percent of the 13 percent of energy consumed for water-related uses in the U.S., while treating water for distribution in the public water supply represents 4 percent of the 13 percent.
Calculating the Energy Cost of WaterAs part of the report’s methodology, Webber and Sanders grouped water-related usage into two main categories: direct water usage and direct steam usage.
For direct water usage, the researchers accounted for the energy used to pump, heat, chill, treat and pressurize water directly across all sectors, including residential. In total, direct water usage accounted for approximately 8 percent of U.S. annual primary energy consumption.
Similarly, chemical manufacturers, oil refineries and other industries use energy to generate steam, which is injected in intermediary manufacturing stages to produce the desired end product, such as oil or gasoline. Making steam for direct use accounted for approximately 4 percent of national energy consumption.
“A major takeaway from the report is the role of water conservation in reducing energy consumption,” Sanders said. “It wasn’t always intuitive where conservation policies could make a difference. When it comes to energy conservation, incentivizing more efficient water-heating technologies might be the low-hanging fruit. There’s a lot of room for improvement in energy and water management at the home.”
Webber and Sanders also delved into some of the regional differences in energy consumption for water influenced by a variety of factors, including source water quality, proximity to a water-treatment facility and desired sanitation level.
In the United States, our “national water-related energy use is expected to increase as water-stressed states such as Texas, Florida, Arizona and California shift toward more energy-intensive technologies,” according to the report.
States like Texas, Florida and California have built desalination facilities that, on average, require about 10 times more energy per unit of water treated than standard surface-water treatment operations.
And in California, a lack of proximity to surface water makes the energy cost of water that much higher. In some cases, water is extracted from Northern California then transported more than 400 miles and over two mountain ranges to reach end consumers in San Diego. To compare, tap water in Massachusetts has an energy intensity that is only 14 percent as energy intensive as tap water in Southern California.
While the report provides important benchmarks, Sanders and Webber continue to build on their work by looking more closely at these regional differences.
“We want to see what water-related energy use looks like in water-scarce states such as Texas versus water-rich states in the Northeast,” Sanders said. “The Northeast uses more energy for water heating than in the Southwest, where temperatures are warm. However, water scarcity has prompted several Southwestern states to turn to energy-intensive desalination processes and pumping projects to deliver adequate volumes of water to the people who need it. There are interesting tradeoffs.”
Mom Was Right
On a basic level, Sanders said the report serves to instill the lessons mothers teach their children about conservation and costs.
“Our mothers urged us to take shorter showers to save water,” Sanders said. “The report shows us that shorter showers are also important in saving energy.”
Danny Reible, professor in the Department of Civil, Architectural and Environmental Engineering, adds that the Webber-Sanders report helps set the framework for discussing the various energy costs associated with water.
“We often think of the implication of water scarcity on our ability to generate power,” Reible said. “This report makes it clear that the reverse, ‘energy for water,’ is equally important. And the availability and economic cost of water is largely driven by energy costs.”