Exploring Tidal Power in the San Francisco Bay

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In a 2006 study, the Electric Power Research Institute (EPRI) identified San Francisco Bay as a potential resource for electricity generation using tidal power technology. Pacific Gas and Electric Company (PG&E), the City and County of San Francisco (CCSF) and the Golden Gate Energy Company signed a statement of collaboration in June 2007 to conduct the most comprehensive study yet undertaken to assess the possibilities for harnessing the tides in San Francisco Bay to create a new source of zero-emissions, renewable electric power.

This multifaceted study will include thorough analyses of:

• the Bay's energy potential
• existing and emerging technologies to capture energy from tidal flows
• possible environmental effects
• economic feasibility and other costs and benefits of tapping this new energy supply

Tidal-power technologies are in their infancy. No commercially viable tidal-power generation operates in the United States currently. But ongoing efforts to increase California's renewable power supplies and address climate change require investigation into untapped, environmentally friendly energy sources, even if the potential benefit is not yet known.

The study draws on the combined resources and expertise of PG&E, CCSF and Golden Gate Energy. PG&E has committed up to $1.5 million and CCSF has committed up to another $346,000 to fund feasibility studies and stakeholder outreach. Golden Gate Energy holds federal regulatory permits necessary to study the San Francisco Bay and supports the study.

Benefits of tidal power are:

• clean
• renewable
• predictable
• reliable
• aesthetically unintrusive (underwater)
• no waste of land

If findings are positive a full-fledged project could be launched. The project would require regulatory approval (including public participation). It is estimated that project operation would be 3 - 5 years out.

Measuring the Bay's Tidal Energy

There are several computer models that are frequently used to estimate the tidal flow at any location in the Bay. Using one of these models, "desktop" studies of the tidal velocity and energy content at a more detailed level than the EPRI study showed the tidal currents contain less extractable energy than expected.

Water moves faster at the surface of the Bay than at greater depth. Most of the fast-moving water is in shipping lanes. There are few areas deep enough for tidal turbines to be installed below the draft of the largest ships (where the tidal flow is strong enough to generate a significant amount of electricity). Although there are some areas outside the shipping lanes with strong tidal currents, the flow at those locations is often too turbulent to be useful.

However, one area has been identified as a possible location for installation of tidal-generation equipment. New data on the speed and direction of the tidal flow in that area will be gathered to confirm the estimated amount of energy available in that area. This data will be used to validate and adjust the computer model. The model will then be run at a higher resolution to identify specific locations where the conditions are most likely to generate electricity. The measurement activities are scheduled for December 2007.

Two types of measurements will be collected:

• Stationary Acoustic Doppler Current Profile (ADCP) instruments will be placed on the seabed for one full tidal cycle (about a month).
• Boat-mounted ADCP instruments will obtain a "snapshot" profile of tidal current velocities in a wider area around the selected site for two days.

The two bottom-mounted ADCPs will be deployed in 60 meters and 40 meters of water. These ADCPs will be mounted inside of a buoy designed to keep the ADCP oriented upwards into the prevailing current. The bouy is streamlined to ensure no pitch and roll affects. These buoys have been used in currents over 5 knots with very high data quality.

The internal ADCP compass determines the absolute current direction during rotation. The bottom of the buoy is attached via a swivel shackle to an acoustic release which is attached to an anchor. The system holds the buoy firmly in place while keeping the ADCP off of the bottom but within ~ 1.5 meters of the bottom. This system will negate the effects of rock outcroppings and large, moving features (e.g. sand waves) on the ADCP operation and retrieval.

What is ADCP?

ADCP instruments are used extensively by research organizations and agencies that ensure navigational safety, and have previously been used in San Francisco Bay. ADCP instruments measure the speed of the water by emitting high-frequency sound waves and analyzing the sound reflected off particles in the water. The sound waves used are so highly pitched that humans and dolphins cannot hear them.

The instruments measure water speed at several different distances at the same time. ADCP range is up to 1000 meters (about 10 football fields), though accuracy decreases with distance. Instruments can be bottom-mounted or boat-mounted. Analysis of data from boat-mounted instruments is complicated because the calculations must account for the movement of the boat.

Further information on ADCP is available from the Woods Hole Oceanographic Institute.