To pump, or not to pump? It's a question.

By Scott Jespersen MS '20
Hometown: Mountain View, CA
Major: Computational and Mathematical Engineering
Graduate Intern, California Department of Water Resources

Without warning, a sharp honk pierces my ears, echoes decaying for a full thirty seconds off the stark concrete walls of the half-mile-long tunnel. We enter. I'm facing backwards off the rear of a small electric golf cart, the honk a necessary precaution, a signal to anyone emerging from the tunnel in the other direction that we are headed deep underground, into the belly of Oroville Dam, to Hyatt Power Station. I give a brief thanks that wisdom prevailed decades ago, that we no longer ride gas-powered golf carts into such tightly enclosed spaces.

After the minutes-long drive, around a wide leftward bend, we emerge into what looks like a factory, covering several acres. A cast iron shutoff valve, three stories tall, sits with its joints wrapped in plastic film on a set of metal rails, in the only place on the floor that will support its gargantuan weight. The unit's rotor has been removed from the shaft to reveal a circular array of thousands of pounds of red-painted magnets. "Those must be the wicket gates down below!" I yell. "What?" A perfectly efficient generator would make no noise, give off no byproduct. But here, thermodynamics win out. The ambient noise level is just below high school garage band. I yell louder: "I said, those must be the wicket gates down there!" George corrects me. "No, those are still a few floors down!" A cutaway diagram of an electric motor in a science book can be difficult to reconcile with an electric generator of this size.

Each floor is labeled by its height above sea level. At about 220', the ventilation pulls so strongly that it's hard to open the door from the staircase. Wind hurtles past. Water rushes through tubes above, dripping through the tiniest of gaps in the joints. Only a few of Hyatt's six units are running right now. Each unit is capable of putting out upwards of 130 megawatts. The scale is astonishing: 800,000 typical homes powered by this plant, at full throttle. Three of the units, able to pump water back up into Lake Oroville, for use in energy storage.

This summer, energy storage has become an all-consuming obsession for me. The most striking thing about the analysis that I've been engaged in is the sheer scale available at Hyatt, and Thermalito, its little sibling. Consider: Tesla makes headlines around the world for its 100 MW battery installation in the Australian outback. In the month of June, the California ISO grid had a grand total of 156 MW of battery storage attached, smoothing short-term fluctuations in the energy balance. Yet here, at Hyatt-Thermalito, lies a system capable of charging at a rate of 480 MW and discharging at about 930 MW—in other words, a giant battery more than three times larger than all the batteries currently serving the CAISO grid, and one that can hold a charge for a very long time. The case seems clear: Why aren't we utilizing this incredible energy storage resource to the fullest extent?

For one thing, in the case of the State Water Project, water delivery obligations preempt any use of Hyatt-Thermalito for energy storage, severely limiting the potential.

For another: A 400-megawatt Montana pumped hydro project recently in the news would be effectively a closed-loop system. By contrast, Lake Oroville, on the Feather River, has much more serious environmental considerations; salmon and trout spawn and rear just downstream of the dam.

In a paper at the end of my summer internship, I will show that it’s not entirely clear—even from a purely economic perspective, admittedly myopic—that the energy price arbitrage opportunity of using Lake Oroville for pumped storage even makes up for the massive capital investment needed to mitigate the downstream water temperature impacts of such operation. All this speaks to the great challenge of matching supply with demand in California’s electricity grid, one that is seeing proposed solutions from the high-tech but straightforward (lithium-ion batteries, writ large) to the simple and yet outlandish (gravity storage in stacks of concrete blocks). The roles of all these potential solutions, including pumped hydro storage, are still very much in flux, and it has been a great experience to see California’s changing energy landscape up close and personal.

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