potrero view
Photograph by Gregory R Thomas.

Photograph by Gregory R Thomas.

April 2009

Cable May Shutter Potrero Power Plant

By Greg Thomas

The Trans Bay Cable (TBC) – a 53-mile transmission line that will connect San Francisco to Pittsburgh – is in its 17th month of construction at the Pittsburgh Converter Substation on the northern fringe of the City of Pittsburgh. It’s expected to plug into a substation at the intersection of Illinois and 23rd streets roughly a year from now.  According to San Francisco Public Utility Commission (SFPUC) officials, the cable will provide a reliable way to transmit clean energy to San Francisco.  The cable is “a giant extension cord,” said Karen Kubick, SFPUC manager of infrastructure development.  

In 2004, the California Independent System Operators (Cal-ISO), a quasi-governmental agency that manages the state’s power grid, selected TBC as the preferred approach to meeting the City’s long-term electricity needs.  According to SFPUC staff, the cable will be capable of conveying power for at least the next 40 years.  

Cal-ISO has indicated that completion of the cable would enable it to release the Potrero Power Plant’s Unit Three generator from its contract to continually run. Unit Three is the plant’s largest generator, capable of kicking-out more than 200 megawatts (MW) of electricity.  It’s shutdown “is a big deal,” said Kubick.  Closure of the unit would help the City reach its goal of cutting greenhouse gas emissions by 30 percent by 2020 from 1990 levels.

Recent analyses by Cal-ISO and SFPUC show that with TBC, and meeting stringent reliability criteria, the City faces a supply shortage of well less then 35 MW.  When recent declines in energy demand are considered that gap drops to zero. “As we’ve been saying for the past two years, with Trans Bay the entire Potrero Power Plant can be immedietly closed,” said View publisher and San Francisco Community Power executive director Steven Moss.

TBC will be capable of transmitting 400 MWs of high voltage direct current (DC) into San Francisco’s electrical grid. That’s enough power to supply more than one-third of the City’s power demand during peak hours.  The benefit of a DC line, as opposed to alternating current (AC), which is what powers household wall outlets, is a reduction in transmission line losses. With AC transmission, electricity starts to dissipate after traveling about 25 miles.  “You can put that power into it but it doesn’t [all] come out the other end,” said David Parquet, TBC project manager.  

 “Hopefully we’ll get to the point where San Francisco power is not produced in-City,” said Kubick, alluding to the fact that Cal-ISO has yet to agree to close the Potrero Power Plant’s other units, four, five and six, which currently run on diesel.  The TBC “is part of (that) incremental process,” she said.

Trans Bay Cable LLC, a subsidiary of international investment and advisory firm Babcock and Brown, is undertaking the project.  Parquet’s team broke ground in Pittsburgh in November 2007.  The cable is being manufactured in Italy, and is roughly 70 percent complete, Parquet said.  Upon completion, the finished cable will be shipped to San Francisco, implanted roughly one foot deep into the Bay floor, and hooked-up at each end in Dogpatch and Pittsburgh.  Stations in both locations will convert the current from DC to AC, and vice versa, though Pittsburgh and San Francisco officials indicated that power would only flow into, and not out of, San Francisco. Parquet expects the cable to be operational a year from now.

According to Jaime Michaels of the San Francisco Bay Conservation and Development Commission – the last of several government agencies to approve the project – the cable will have minimal impacts on the bay’s ecology.  “The areas [the cable] is going through are mostly sandy bottom and benthic habitat, which is basically your average critters in the bay,” she said. “They’re important, but it wasn’t [harming] sensitive habitat or a threatened species or anything like that.”

San Francisco currently draws power primarily from the Potrero Power Plant - the dominant feature of which is the soaring brick smoke stack that towers above Dogpatch – and transmission lines running from the Peninsula that originate in the East Bay. TBC will add another supply pathway, creating a more flexible and reliable network of electricity flowing into the City, a Cal-ISO priority.   “In an earthquake-prone region it’s better to not have all your eggs in one basket with all that transmission on one corridor,” said Cal-ISO spokesman Gregg Fishman.  The City’s existing system “is just not enough capacity to keep pace with anticipated growth and demand in San Francisco and to meet some of the community goals, which have been to close the older power plants in San Francisco,” he added.

Fishman claimed that his agency will still look to Unit Three for emergency generation even after TBC is operational.  The unit “is still necessary for local reliability” should an outage in Pittsburgh cut power to the cable, he said, in which case it could be temporarily be brought back online.  However, according to Kubick Unit Three will be shut down when the cable becomes operational as part of Cal-ISO’s action plan for San Francisco – “[it] has been a goal of the City and neighbors for a long time,” she said.  In 2006 construction of the Jefferson-Martin transmission line, which runs up the Peninsula from Redwood City, enabled the closure of the Hunters Point power plant.  Similarly, the TBC should be able to close generation at the Potrero Power Plant – “the biggest nasty power plant in the City,” Kubick said.

B&B conceived of TBC in 2003 as a complement to the Jefferson-Martin transmission line.  Noting that project’s success, B&B began scouting new routes to bring power into the City. They chose Pittsburgh over other Bay Area cites due to the existence of heavy power infrastructure there, and its waterfront location, Parquet said.  However, some environmental groups believe there was another motive.  “They planted it in Pittsburg, and called it a municipal project, in part to escape California Public Utility Commission (CPUC) jurisdiction,” said Moss.  “Avoiding the CPUC saved them time and reduced the risk that the project would be denied.”

B&B Planners crossed paths with SFPUC officials in 2004 – the same time Kubick’s team was researching the development of City-owned peaker plants to meet Cal-ISO’s reliability criteria – and started sketching out project details.  At the time, the peaker plants were considered the preferred alternative to the Potrero Power Plant. But the peaker plant idea, Fishman said, “holds you even [and] doesn’t gain you ground… You’d be trading older, more polluting generation for newer, less polluting generation with the peakers.”

TBC will be capable of conveying power from any generating resource.  However, City officials say the transmission line will tap renewable energy sources at wind farms in Rio Vista.  Perhaps just as likely, though, is for the line to convey fossil fuel power from the cluster of older natural gas-fired generating facilities located in Contra Costa County.

According to Kubick, SFPUC is moving forward on a citywide energy plan that includes installing solar panels on municipal buildings.  Panels capable of powering roughly 1,000 homes have already been installed on Moscone Center.  SFPUC hopes to complete similar smaller solar projects at Davies Symphony Hall. “The City has taken an awful lot of leadership in focusing on green power and trying to reduce any future need on the municipal side for fossil fuel generation power,” she said. “The City is trying to make an example for the people who live here and for the future.”

 

 

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