Researchers at Sandia National Laboratories in Albuquerque and the operators of Arizona Public Service's Palo Verde Nuclear Generating Station near Phoenix believe the comprehensive system-dynamics analysis software that is helping to save millions of gallons of water each day at the 3.3-GW plant could help other thermal power plants—whether gas, coal or nuclear—do the same.
Thermoelectric power generation accounted for more than 40 percent—133 billion gallons per day—of the nation's total water withdrawals in 2015, according to the U.S. Geological Survey. That number, calculated every five years, has been trending down since 2005, but drought and increasing population pressure have pushed up the demand—and cost—of water since that time.
Jeff Brown, senior consulting engineer at Palo Verde, the only nuclear plant in the world that does not sit on an existing body of water, said the treated municipal effluent the plant uses to operate and cool its system used to be a small piece of its operating budget, but its costs for water are now increasing faster than any other element necessary to run the plant.
Before the plant first went on line in 1973, APS worked out a deal to take treated effluent from a consortium of nearby municipalities. For decades, the arrangement provided PVGS operators with inexpensive water and the communities with a revenue source that might otherwise have cost them money to dispose of.
Population in the Phoenix metropolitan area has roughly tripled since Palo Verde opened, and towns in the so-called subregional operating group that supply the plant with wastewater have come to see new value in their treated effluent. They now use it for irrigation and to replenish the aquifer, Brown said.
When APS renegotiated its contract with the communities in 2010, those water costs went up significantly. Now, Brown says, they are increasing a little more than 10 percent per year. Meanwhile, the cost to treat water for use in the plant's reactors, while still higher than the cost of the water itself, is merely keeping pace with inflation.
Aware that Sandia had the capacity to develop dynamic simulation models, Brown approached the lab in 2015.
Bobby Middleton, a nuclear engineer at Sandia, listened to Brown and developed software that combines the physics of the cooling process—such as fluid flow, heat transfer, atmospheric evaporation and water treatment—with the financial impact of different solutions. By tailoring the inputs, Middleton explained in a telephone interview, plant operators can best determine how to create greater efficiency and cost savings across numerous factors. Palo Verde is now using the software to help it save 9 million gallons of water every day, Brown said.
"No one has created a system dynamics analysis that simultaneously considers all these factors before," Middleton said in a release. "It helps us predict the benefits we might see from a particular technology so that we spend time only testing the most promising approaches."
Middleton and Sandia chemist Pat Brady believe their approach, currently in its first phase with the software, could help other U.S. power plants reduce water use in an era of increasing drought and population pressure. They are using software modeling to identify less expensive ways to remove ions at different points in the cooling cycle, for example, and are also examining the feasibility of desalinating discharged cooling water so that it can be reused rather than evaporated from large ponds.
The next phase of their research involves using the software to identify the most promising water-reduction approaches, including alternative water treatment and hybrid or dry-cooling technologies.
Middleton, along with Sandia colleagues Matthew Carlson and Marie Arrieta, in 2017 received a patent for an air-cooler design that uses supercritical carbon dioxide rather than a refrigerant. Supercritical CO2—which under appropriate pressure and temperature conditions above 88 degrees Fahrenheit transforms from liquid to gas without boiling—makes dry cooling possible across a wider temperature range than traditional refrigerants. The supercritical fluid can make dry cooling a reality in environments such as the Arizona desert, where it has traditionally been impractical if not impossible.
The supercritical CO2 contained within the cooling system condenses into liquid, becoming heavier as it cools and lighter as it heats up. It can therefore circulate throughout the cooling system without the use of a pump, creating further efficiency and cost savings, the researchers said. Supercritical CO2 also uses less air to cool water to the same temperature as a traditional dry cooler with a subcritical refrigerant, reducing the need for fans and thereby adding even more efficiency.
Middleton and Brady are now refining the cooling mechanism and design on their way to a working prototype. They hope to install a cost-effective system at Palo Verde no later than 2026 and to see the technology expand to serve other plants and save water across the nation.
"Water-saving technologies for energy production are critical for scientists and engineers to consider today," Brady said.