Pumped Storage at Apache Lake

Salt River Project (SRP) of Arizona plans to build a “pumped storage” facility at a site on Apache Lake, downstream from Roosevelt Dam and upstream from Horse Mesa Dam. Apache Lake is the first of a series of lakes along the Salt River, all of which provide hydroelectricity for the Phoenix metro area.

The graphic shows how pumped storage works. It actually functions more like a “battery.” When renewable energy isn’t needed to meet grid demand, it can be used to pump water uphill into a reservoir. This “stored hydro” can then be used during periods of high demand by allowing water to flow back downhill and spin a turbine to generate electricity. Conventional electrochemical batteries could be used, but pumped storage requires less maintenance and up-front cost.

SRP estimates that, with the size of the planned reservoir it could power 250,000 homes for 10 hours. The overall process is 80% efficient, returning 0.8 kWh of electricity for every 1.0 kWh pumped in. By comparison, the best lithium ion batteries return up to 0.96 kWh, cost about the same, but don’t last nearly as long as a pumped storage facility. Still, SRP plans to add some large chemical batteries for storage and evaluation.

To make pumped storage a viable option you need topography near a lake that allows for a reservoir to be constructed nearby, and at a significantly higher elevation. SRP is considering two possible sites where natural basins would allow creation of a reservoir with no construction of dams or levees. Both are at elevations around 1000 feet higher than Apache Lake (click to enlarge):

We should distinguish pumped storage from existing “pumpback” facilities on other dams in the SRP system. Pumpback does the same thing, but returns water to the lake just upstream of the dam when excess power is available from the grid, so there is no additional reservoir to construct. But where the water and renewable energy are available, and the topography is favorable, pumped storage makes sense.

Water recreation and sports enthusiasts would notice the water level rising and falling over a pumping cycle, but not enough to affect activities. The pumped storage reservoir “only” takes 6.52 billion gallons for a full fill, and Apache Lake holds some 80 billion gallons. The intake would be 30 feet below the surface of Apache Lake, but the water level doesn’t vary much compared to other lakes on the system.

There are some downsides to the project. By increasing the surface area of the Salt River system, the reservoir would increase water loss from evaporation. But that can be mitigated by floating a layer of white reflective spheres — or a floating solar farm. Obviously, the reservoir would not be stocked for fishing, and recreational use would be prohibited. The article I read was not clear on this, but I suspect it will not be fenced to allow free access for wildlife.

The project is at least 10 years from operation, but SRP is already well into the engineering and design work. Arizona congressional representatives have introduced legislation to place the project under control of the US Bureau of Reclamation. That would make additional federal funding available.

With all the news about exotic battery technologies, it’s good to be reminded that Nature’s “gravity battery” is still a viable mechanism for energy storage: weight × height = energy, and water weighs a hefty 8.3 lb/gal. Sure, it’s not as heavy as rock, but it’s a lot easier on the turbines.

Next Week in Sky Lights ⇒ Density Altitude Revisited

Density Altitude Revisited