What’s Current? Issue #15 – Pump Storage and the Energy/Water Nexus
Energy is water. A filtration plant producing one million acre feet (MAF) of desalinated water over one year requires 400 megawatts of continuous power. Pumping 1 MAF per year from the pumps immediately south of the Delta through the California Aqueduct all the way into the Los Angeles Basin requires about 300 megawatts of continuous power, at least once you’ve factored back into the equation the hydroelectric power that’s generated when the water tops the Tehachapi summit and goes downhill again. Recycling wastewater, depending on what’s in it, requires around 200 megawatts of continuous power to purify 1 MAF in one year.
And so on. Meanwhile, while energy is water, water is energy. During a very rainy 2019, California’s in-state hydroelectric dams delivered on average 3.8 gigawatts; during a very dry 2015, they still delivered 1.3 gigawatts. With pump storage, the same water can be used over and over. When surplus photovoltaic electricity surges during broad daylight, it powers reversible turbines on the San Luis Dam that lift water from the O’Neill Forebay 200 feet up into the reservoir, and then for several hours during peak demand after sunset, the water is sent back down through these same turbines to generate 450 megawatts.
Here are articles on the nexus between water and energy.
500 Megawatt Pump Storage Proposed Near San Diego
If this project is built, an 8,000 acre foot reservoir will be built up the hill from the already existing 247,000 acre-feet San Vicente Reservoir in the Cuyamaca Mountains east of San Diego. The two reservoirs would be connected by a tunnel and an underground powerhouse containing four reversible pump turbines, sized to generate up to 500 megawatts. If the intended design cycle is to have 4 hours of daily uphill pumping using photovoltaic surplus energy, followed by 4 hours of downhill power generation during daily peak demand, then we may conclude that four acre feet are required to produce one megawatt-hour of electricity. Many variables affect this, of course, the biggest one being the amount of lift.
Seawater Pump Storage Proposed on Coast North of San Francisco
If you’re wondering how the California Energy Commission intends to store 25 gigawatt surges of electricity once their proposed offshore wind projects are completed, here’s a piece of the puzzle. Who needs fresh water when we have an ocean? Based on the design of an experimental 30 megawatt pump storage plant in Okinawa that operated for 16 years and cost around $20 million, developers seek a permit to construct a lake in the coastal hills north of Ft. Ross, with a pipe descending to a powerhouse with reversible pumps that will pump ocean water uphill using surplus electricity, then generate power by releasing the salt water back into the ocean during peak demand. Unsurprisingly, the proposal has already attracted apoplectic opposition. This comment from a representative of the Ocean Foundation says it all: “While generally supportive of renewables, our rural communities are obviously going to need to armor themselves in a regulatory sense against these emerging types of inappropriately-sited resource exploitation schemes that ignore our coastal economies based on visitor-serving businesses, artisanal commercial fisheries, and healthy coastal habitat.” What? You mean we’re not allowed to destroy the environment in order to save it? Expect more cognitive dissonance as California’s policymakers march resolutely towards “net-zero.”
Pumped Storage in California – An Overview of Each Major Plant
Follow this link to an excellent report on California’s existing pump storage plants, and contemplate the engineering vision and decisive policymaking that made these marvels possible. There are seven major pumped storage plants operating in California: Lake Hodges, Castaic Lake, Helms, San Luis, Big Creek, and Oroville. At capacity, they can generate just under 4 gigawatts. That’s a lot, considering California’s in-state power production only averages 21 gigawatts. But California’s average generating capacity will need to rise to at least 80 gigawatts if we actually go through with full electrification of our economy, and according to the current plan, nearly all of that will come from intermittent sources – wind and solar. Unlike batteries which degrade over time, pump storage facilities can operate indefinitely. Retrofitting California’s existing reservoirs to deliver pump storage is a path potentially more sustainable than massive battery farms.
State-of-the-art technology requires around 3.5 megawatt-hours to desalinate one acre foot of seawater. We recently reported on OceanWell, a company testing a design that takes advantage of the pressure differential between shallow and deep water to bring the energy requirement down to just over 2 MWh/AF (ref. WC #8). Using filtration technology, the theoretical minimum is 1.2 MWh/AF. But what if solar energy could be used to distill salt water? That’s what engineers at MIT are developing, in a design that not only saves electricity, but avoids filter clogging. Using MIT’s numbers, a one square mile array could produce around 90,000 acre feet a year of fresh water from seawater. The design also may be able to extract fresh water while releasing only slightly saltier brine, and by avoiding filtration, would avoid any contaminants filters might otherwise introduce.
Once-Through Cooling Policy Amendment Protects Marine Life
To ensure reliable electricity, earlier this summer California officials permitted 3 natural gas powerplants on the Southern California coast to keep running (ref. WC #9). There are new combined cycle technologies that could bring the conversion efficiency of natural gas powerplants to over 80 percent, and emerging synthetic and/or carbon neutral fuel blends that could bring their emissions profile down to almost negligible levels, but for the moment let’s focus on the other concern – water. At peak, California had 19 coastal natural gas powerplants that used 16.7 billion gallons per day of ocean water for cooling. That’s 18.7 million acre feet per year. But the State Water Resources Control Board enacted regulations in 2020 that require plants to either adopt new cooling technology to reduce water consumption by 93 percent, or develop new intake designs that enable a proportionate reduction in entrained marine life. Some plants shut down, and others made these modifications. But one must ask: Why can’t desalination plant proposals incorporate similar design alterations and gain approval? Desalination proponents don’t wish to create 18.7 MAF/year of brine. A half-million acre feet per year of desalinated water (requiring a mere 200 megawatts) would throw the water supply/demand equilibrium into surplus in urban Southern California forever. Why not find the acceptable design and make the investment?