Hydro ยท Analysis
The tide through the Golden Gate, and the turbines that never came
The current under the Golden Gate is one of the most predictable power sources on the West Coast. Here is why the city has studied it for the better part of two decades and still has not dropped a single turbine into it.
I. How a turbine in the Gate would actually work
Picture an underwater wind turbine. A tidal in-stream turbine (a rotor fixed to the seabed, turned by moving water rather than moving air) works on the same idea, except water is roughly 800 times denser than air, so a current that would barely stir a weather vane carries real force. Through the Golden Gate, currents run on the order of 4 to 6 knots on the strongest tides, which is fast for open water. That is the draw: dense, fast, and on a timetable you can read a year ahead.
Where you would put one is the hard part. The channel under the bridge runs more than about 300 feet deep in places, and a turbine has to sit low enough to clear the keels of deep-draft ships yet high enough off the bottom to catch the flow, anchored to a seabed those same currents are forever scouring. Add the need for water that genuinely moves fast, and the usable real estate shrinks to a few patches rather than the whole Gate.
II. The honest math
Saltwater is hard on machinery. Anything bolted to the floor of the Gate faces corrosion, marine growth that fouls the blades, and repairs that mean sending a boat and divers into one of the roughest stretches of water on the coast. None of this is mass produced the way solar panels now are, so every megawatt is close to hand-built. That is the core of why the cost per kilowatt-hour has never penciled out.
San Francisco's own feasibility work, done in the mid-2000s, landed on a modest resource, on the order of tens of megawatts at best, a slice of the city's load rather than the bulk of it. Over the same two decades the price of solar and wind fell sharply, and CleanPowerSF filled much of its supply with those cheaper contracts. At last public report, tidal through the Gate still loses that comparison, and it is the comparison, not the physics, that keeps it unbuilt.
III. Who else is using that water
Only one deep-water door leads into San Francisco Bay, and this is it. Container ships bound for the Port of Oakland, tankers, and cruise vessels all thread the same narrows, guided by the bar pilots who board them offshore. A turbine field cannot sit anywhere that crowds the shipping lane or snags an anchor, and the Coast Guard and the Army Corps of Engineers both have a say over anything placed on the bottom.
Just outside the Gate lies the Greater Farallones National Marine Sanctuary, and the channel itself is a migration route for salmon, a feeding ground for seals and sea lions, and a corridor for gray whales. Spinning blades in that water raise questions that NOAA, the California Coastal Commission, and FERC all weigh before anything gets built. That review is measured in years, and for the Gate it has never finished, because no project ever made the economics worth pushing through it.
IV. Worth watching this year
1. Whether the SFPUC's next long-range power plan for CleanPowerSF and Hetch Hetchy Power so much as mentions tidal, which would be a real signal though the odds are it will not.
2. The FERC marine and hydrokinetic permit list for any fresh preliminary permit naming San Francisco Bay or the Golden Gate, almost always dormant but quick to check.
3. Any U.S. Department of Energy Water Power Technologies Office funding round for tidal hardware, which is national news rather than local but is where cheaper machines would come from first.
4. Updates to the Greater Farallones sanctuary management plan that touch energy development, routine housekeeping most years and worth a glance when it happens.
5. The California ISO interconnection queue for any Bay Area marine generator, which is very likely empty but is the first public place a serious project would appear.
Frequently asked questions
Has anyone ever actually put a turbine in the Golden Gate?
Not a working power project. There were pilot-scale proposals and federal permit filings in the 2000s, alongside the city's feasibility studies, but nothing was built. At last public report the idea remains on paper.
How much power could it realistically make?
On the order of tens of megawatts at best, per the city's feasibility work, enough for a slice of San Francisco's demand rather than the bulk of it. The part of the channel that is fast, deep, and clear of the shipping lane is small.
Why tides and not waves?
Tidal currents here are predictable almost to the minute, and the narrow Gate concentrates them. Wave energy off the coast is more diffuse and has proven even harder to build at a price anyone will pay.
What would stop fish or ships from being harmed?
Those two worries are exactly why nothing has been built. The Gate is the Bay's only deep-water shipping entrance and sits beside a national marine sanctuary, so any project faces long federal and state review before a single blade turns.
References
- San Francisco Public Utilities Commission (SFPUC), The city agency that runs Hetch Hetchy Power and CleanPowerSF, where San Francisco's long-range power planning and its past tidal feasibility work live., www.sfpuc.org.
- Federal Energy Regulatory Commission (FERC), The federal office that issues preliminary permits and licenses for marine and hydrokinetic projects; this is where any filing naming the Bay would show up., www.ferc.gov.
- NOAA Tides and Currents, Official U.S. current predictions; you can pull the Golden Gate current tables a year ahead and read the twice-daily schedule yourself., tidesandcurrents.noaa.gov.
- NOAA Greater Farallones National Marine Sanctuary, The federally protected waters just outside the Gate, and the habitat concerns any project would have to clear., farallones.noaa.gov.
- U.S. Department of Energy, Water Power Technologies Office, Federal research and funding for tidal and wave energy; where cheaper marine turbines would most likely come from first., www.energy.gov/eere/water/water-power-technologies-office.
- California Independent System Operator (CAISO), California's grid operator; its public interconnection queue is the first place a serious new generator would appear., www.caiso.com.