“Water Battery” Being Considered on Mokelumne River

Pumped storage technology project would connect reservoirs, could raise efficiency of California’s renewable energy sources

Earlier this year, GreenGenStorage received a renewal of their licensing period to submit Pre-Application Documents (PAD) to the Federal Energy Regulatory Commission (FERC) for their proposed Mokelumne Pumped Storage Water Battery Project. The developer hopes to submit their PAD by the end of the year for the project, which would utilize excess solar and wind energy to power their pumped storage technology, thereby generating electricity for the grid at peak energy-usage hours. 

“During the midday peak of solar energy generation, we would pump water uphill to an upper reservoir,” Nicholas Sher, manager at GreenGenStorage said. “Then in the evening where energy usage is consumed the most, we would release that water downstream and it would drive a turbine, thus generating energy. Then by soaking up wind power in the evenings for release in the morning peak hours, we’re essentially just time-shifting renewable energy.”

By using renewable energy to transfer water back and forth between two reservoirs, the project would be a non-consumptive means of using water to generate power. In the case of the Mokelumne Pumped Storage Water Battery Project, the Salt Springs Reservoir has been identified as the lower reservoir, as it already has a small hydro-facility on it that generates about 44 megawatts of energy.

Two options are currently being explored for the upper reservoir that Salt Springs would be connected to via penstock pipes: the Lower Bear or Upper Bear Reservoir.

“Our project, if approved, would generate between 250 megawatts to 800 megawatts and would require an upgrade of that existing 115 kV transmission line to a 230 kV line,” Sher said. “The permit that we have is a preliminary hydro permit that allows GreenGen to analyze developing the site, but it doesn’t give us any ability nor authority to do so yet. We’re still working on our PAD, which sets forth the current existing environment and we will at some point decide on which of the upper reservoirs to use.”

Where is the proposed Mokelumne Pumped Storage Water Battery Project? Use the interactive map below to explore the proposed project area.



There are several factors to consider as it relates to identifying the Lower Bear or Upper Bear for the project’s upper reservoir. 

“The Lower Bear Reservoir has a lot of activity with boat docks and recreation, whereas the Upper Bear is much more pristine and doesn’t have as much human activity. While the Lower Bear is a larger reservoir, the Upper Bear is a smaller reservoir. Fluctuation levels as you move the water back and forth would not be too much on the Lower Bear Reservoir, but that’s still being analyzed,” Sher said. “Yet if we were to use the Upper Bear, the fluctuation levels would be greater because it’s a steeper canyon.”

While a decision has not yet been made on which upper reservoir to use, GreenGen has been in talks with the Calaveras band of Mi-Wuk Indians, the tribe located closest to the proposed project site. 

“These are their traditional lands, so we’re working with them on finding out where cultural or sacred areas are so that we don’t impact or mitigate impacts to those sites,” Sher said. “We’re also working with them on protecting local species such as bears and deer.”

By incorporating tribal and community feedback into their project design, GreenGen hopes to mitigate or address whatever environmental impacts might result from the project. One particular concern that’s been brought forth from The Foothill Conservancy relates to the rise of water temperature in the reservoirs.  

“The Foothills’ biggest concern has been water temperature, and it’s a very valid concern,” Sher said. “We will be moving water back and forth so there may be a mixing of warm water with cold water, thereby warming up the water in the reservoirs and making it worse for downstream species. We have some novel ideas on how to cool the water that will leave the system better off than it currently is, but that’s a part of our analysis and work that’s yet to be done.”

Lower Bear Reservoir – photo: GreenGenStorage

One novel approach GreenGen might consider implementing would be floating solar on the reservoirs. Not only would floating solar provide shading to reduce water temperatures, but it would also provide an option of hanging chillers to use that solar power to further cool the water. 

“Floating solar does come with aesthetic impacts and reduced usage of those reservoirs that need to be looked at, so we’re not coming in blindly saying, ‘This is a solution without impact,’” Sher said. “But the benefit of floating solar would be to basically electrify the boating on the Lower Bear or potentially create a micro grid for that community as well. Another idea we’re looking at is in penstock design and asking, ‘Is there a way to design the tunnel and piping so that it actually cools the water?’ So there’s an opportunity for us to create a larger cold water pool that can then be used to help ameliorate temperature issues downstream.”

Another concern exists in that water batteries such as this pumped storage project would consume more energy that it produces, thereby resulting in a net loss of electricity. But according to Sher, this is inevitable because any technology that produces more energy than it consumes would likely violate the laws of thermodynamics.

“No energy storage solution produces more than or equal to as much energy as it consumes,” Sher explained. “Whether this is lithium batteries or whatever technology you have, we consume more than we produce. But we can use energy storage systems to soak up excess renewable power, energy that would otherwise go to waste and then releasing it when it’s needed during peak hours. So it’s time-shifting, but I’m unaware of any energy solution or storage solution that would be net positive.”

Salt Springs Reservoir – photo: GreenGenStorage

Concerns aside, Sher states that California or the United States as a whole will not be able to reach its climate goals without pumped storage or long duration storage projects such as this one. 

“We’re trying to electrify all our buildings and transportation as decarbonisation is being pushed across the country. With that increasing load, you’re going to need energy storage because pandemic aside, we don’t consume our energy at midday when solar is really producing at its max,” Sher explained. “So whether it’s batteries, pumped storage or whatever the technology is, we need something to soak up that excess power to then give it to us when we use it.”

From his experience at the California Public Utilities Commission, Sher notes renewable power is often being exported out of California because the need for that energy doesn’t always coincide with when that energy is being harnessed.  

Consequently, the state of California curtailed 460,000 megawatt hours of renewable energy in 2018 because there were no storage options available. That level of energy capacity would have been enough to power consumption for an estimated 80,000 homes. 

Then considering California experienced rolling blackouts last year for the first time in twenty years, long duration storage can alleviate or remove that risk. 

“We all want and need renewable energy, but the impacts from those facilities have been sometimes devastating to the local environment,” Sher said. “We’re going to need every arrow in our quiver to meet our [climate] goals so with that in mind, we saw the opportunity here to soak up excess renewable power and mitigate those impacts while using that energy in a way that is not wasteful.”

As climate change drives water levels at several reservoirs across California to record lows, the window of opportunity to implement projects like this one is critical. However, the reservoirs in consideration by GreenGen may still be in a unique position to be appropriately suited for a pumped storage option like this. 

“These reservoirs are at between 4000-6000 feet elevation and are directly where the snow melt takes place,” Sher said. “So the context is dissimilar from other reservoirs that are at very low points. But if there’s not enough water in those reservoirs that would not allow this project to run, it has an economic impact but worse than that, the impact downstream for both species and humans is going to be devastating.”

Top photo caption and credit: The upper bear dam and reservoir – photo: GreenGenStorage

Scott King writes about science, technology, and the environment for the Ally. Support his work.


This site uses cookies to provide you with a great user experience. By continuing to use this website, you consent to the use of cookies in accordance with our privacy policy.

Scroll to Top