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This Arizona utility company wants to build a new lake in the middle of the desert

About an hour east of Phoenix, nestled amid dramatic, rocky cliffs at the edge of Canyon Lake reservoir, sits Mormon Flat Dam, operated by Salt River Project, one of the major utility providers for the Phoenix area. 

When SRP releases water through the dam, a 55-ton turbine inside spins, generating enough power for about 13,000 homes. But this dam is more complicated than a typical hydropower system, where water flows one direction with the current of a river. Since an update in the 1970s, Mormon Flat Dam has operated more like a giant rechargeable battery. 

During times when energy is abundant and cheap to produce, SRP pumps water from Saguaro Lake - one of the reservoirs it manages at a lower elevation - uphill to this reservoir. Then at times when energy production slows, SRP can release that same water back downhill to produce power. Then the process starts over again. 

This method of storing electricity has become especially critical as SRP has invested more in solar power.

“A pumped storage project is really unique,” said SRP principal engineer Eric Hannoush.  “It can take that energy that’s generated during the day when customers don’t necessarily need it, and store it for the nighttime, when solar power falls off and our customers are still using the air-conditioners, especially in the summertime when it’s really hot.”

Now, SRP wants to massively expand its ability to do this. 

The utility recently  announced plans to reach net-zero carbon emissionsby 2050. To stay on track toward that goal, SRP says it will need to triple its renewable power output by 2035, and with that, its energy storage capacity. 

SRP is considering two sites on federally owned land near Apache Lake  to construct a new dam and flood a portion of the desert to create the upper reservoir in what would be Arizona’s largest pumped storage hydropower system. 

Currently, SRP has a combined pumped storage hydropower capacity of 150 megawatts at two pumped storage facilities, Mormon Flat Dam and Horse Mesa Dam. The proposed new pumped storage system would have a 1,000 to 2,000 megawatt capacity. 

It would be a monumental undertaking, but Hannoush said it makes more sense than investing only in utility scale batteries to meet the supplemental nighttime needs of SRP's growing solar power portfolio. 

“The benefit of pumped storage is we expect this to be a centurylong resource,” Hannoush said.  “When you compare battery systems that are continuously being replaced, compared to a resource that’s going to last 100 years, there’s a real economic benefit to building a pumped storage facility.” 

Not only that, but batteries last about four hours per charge. The proposed pumped storage system could generate enough energy to power about 450,000 homes for 10 hours each pump cycle. 

That’s important for SRP, not just for sustainability goals, but for keeping up with  soaring demand.

“The Phoenix metropolitan area and the state of Arizona in general are going through a period of really high growth,” said Amanda Ormond, who runs the Arizona-based energy consulting agency Ormond Group.

All of the state’s utilities are looking for ways to generate more power right now, Ormond said. A decades-long population boom paired with expansion of energy-intensive data centers and other industrial users is driving record power use. And as climate change makes Arizona summers even hotter, the demand for electricity to keep air-conditioners running will increase even more.  SRP projects it will need to produce 40% more power by 2035. 

Ormond isn’t surprised pumped storage hydropower is part of the utility’s growth plan.   

“I think why pumped storage is being looked at now is because the electric system as a whole is moving from generators that turn on and run all the time, like coal and nuclear, to a more flexible system,” Ormond said. “What pumped storage brings is flexibility, because you pump the water up and then you have flexibility to produce power whenever you want.”

More and more utilities, especially in the sunny Southwest, are racing to adopt the technology. Pumped storage hydropower has been used in the U.S. since the 1930s, but in nearly a century since, it has accounted for just a tiny fraction of the country’s energy production. There are currently only about 40 pumped storage systems in use nationwide. But that’s changing. The  U.S. Department of Energy reports more than 90 of these projects are now in development across the country. 

Ormond said SRP’s proposed pumped storage system is ambitious because of its immense size, but she doesn’t think it’s unrealistic. 

“The viability of this project I think is pretty good,” Ormond said. 

Still, it’s likely at least 10 years down the road. SRP expects construction on the project might start in 2027 and the system might come online in 2033. The utility has not announced an estimated price tag for the project, but expects it will be in the billions. And before it can break ground, the utility faces lengthy review processes with federal and state agencies.

The U.S. House of Representatives in November  passed legislation to allow SRP to begin exploring the two sites it is considering for the project. The utility is now waiting on similar approval from the U.S. Senate.  

SRP must also seek support from the public. 

Among those keeping an eye on the project is Don Steuter, conservation chair for the Sierra Club’s Grand Canyon Chapter.

“We have been carefully following it because it is in a rather environmentally sensitive area,” Steuter told KJZZ News. “When it comes to creating an upper lake in the desert, which is basically what we’re talking about here, it does immediately raise all kinds of questions.” 

Steuter said the two sites SRP is considering for the lake, the dam, the transmission lines and other new infrastructure are currently home to saguaro cactuses, desert tortoises and bighorn sheep. As of now, he said, it’s all untouched by Phoenix’s urban expansion. 

“We’re losing open space quickly everywhere, we have been losing open space for a long time,” Steuter said. 

At least six Native American tribes have connections to the areas under consideration. And  surveys last year turned up multiple archaeological findings at both sites – 18 ancient Native American sites at Site 1, which is south of Apache Lake, and seven at Site 2, which is northeast of Apache Lake. 

Steuter said he wants to see thorough environmental and cultural reviews from SRP before anything gets built. 

“We realize that we need energy storage, but the devil is always in the details,” Steuter said. 

SRP says public input is the next step. It plans to host stakeholder meetings and open houses about the project in late April.

Hannoush is optimistic members of the public and regulatory agencies will agree that more pumped storage hydropower would benefit the Phoenix area. 

“We’re in a really unique position to be able to produce a power plant like that, not everybody has the capability,” Hannoush said. 

Peering over the edge of Mormon Flat Dam, Hannoush points out the mountainous desert terrain that flanks the Salt River. The water is there, the gravity is there. These steep canyons, he said, are full of energy potential.  

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Katherine Davis-Young is a senior field correspondent. She has produced work for NPR, New England Public Radio, Southern California Public Radio, PRI's The World, Washington Post, Reuters and more.She has a master’s degree in radio journalism from the USC Annenberg School of Journalism.She lives in central Phoenix with her husband, two daughters, and ill-behaved cat and dog. Her side-passions include photography, crosswords and hot sauce.