Potrero Generating Station
Encyclopedia
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
and diesel burning electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
generating station owned by Mirant
Mirant
Mirant Corporation, an Atlanta-based energy company, produces and sells electricity in the United States. The company was spun off from its former parent, Southern Company, on April 2, 2001...
and located on a 23 acres (93,077.8 m²) site in Potrero Point
Potrero Point
Potrero Point San Francisco is the location of the earliest and most important industrial facilities in the Western United States on the eastern extension of San Francisco's Potrero Hill, a natural land mass extending into San Francisco Bay south of Mission Bay. Potrero Point, the point of Potrero...
, San Francisco, California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
. The plant's primary power source is a 206 MW, natural gas burning steam turbine providing baseload power and referred to as "Unit 3". In addition, three 52 MW peaking power diesel generators provide additional power during times of highest consumption. Since the closure of the Hunters Point Generating Station in 2006, Potrero is the only remaining large power plant within the confines of San Francisco and can provide approximately 1/3 of the City's peak electrical power needs.
On Dec. 21, 2010, San Francisco Mayor Gavin Newsom
Gavin Newsom
Gavin Christopher Newsom is an American politician who is the 49th and current Lieutenant Governor of California. Previously, he was the 42nd Mayor of San Francisco, and was elected in 2003 to succeed Willie Brown, becoming San Francisco's youngest mayor in 100 years. Newsom was re-elected in 2007...
announced that the Potrero plant would cease operations by the end of the year. In a press conference announcing the closure, Newsom stated that "this is a monumental step towards cleaner air, environmental justice and our future of renewable energy and healthier communities."
History and capacity
The Potrero power station is located at a site in Potrero PointPotrero Point
Potrero Point San Francisco is the location of the earliest and most important industrial facilities in the Western United States on the eastern extension of San Francisco's Potrero Hill, a natural land mass extending into San Francisco Bay south of Mission Bay. Potrero Point, the point of Potrero...
originally used by San Francisco Gas Light, a provider of gas for cooking and lighting in the late 19th century. Circa 1890 they constructed a small electrical generator at the site, which was the first power plant for the company that would later become Pacific Gas and Electric. This original generator no longer exists though the ruins of buildings associated with it are still on the property. Unit 3 was constructed in 1965, making it one of the oldest power plants still operated in California. The peaking units 4, 5, and 6 were constructed in 1976.
Unit 3, the primary power generator, consists of an eight story natural gas powered boiler that produces superheated high pressure steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...
. San Francisco Bay water is purified and heated to produce high pressure steam. This steam is run through a turbine that subsequently turns a 206 MW generator made by Westinghouse
Westinghouse Electric (1886)
Westinghouse Electric was an American manufacturing company. It was founded in 1886 as Westinghouse Electric Company and later renamed Westinghouse Electric Corporation by George Westinghouse. The company purchased CBS in 1995 and became CBS Corporation in 1997...
. The steam that exits the turbine is then cooled to allow it to condense back into water. The cooling water used in this process operates on a once-through exchange with San Francisco Bay
San Francisco Bay
San Francisco Bay is a shallow, productive estuary through which water draining from approximately forty percent of California, flowing in the Sacramento and San Joaquin rivers from the Sierra Nevada mountains, enters the Pacific Ocean...
. At full power approximately 10 m3/s (226 million gallons per day) of water are pumped from the subsurface of the Bay, passed through screens and filters to remove debris and prevent biological uptake. The cooling water then passes through the condenser to cool steam used to power the steam turbine. The water is returned to the Bay at about 10 C warmer, usually no warmer than 30 C (86 F).
In addition to the main unit, three 52 MW diesel powered peaking generators can be brought online in 4 minutes and to peak load in 10 minutes notice to meet extra electricity demands as specified by the California Independent System Operator. They are generally operated for only a few hours at a time, usually during peak power consumption in the afternoon. In total, they are generally utilized less than 200 hours per year, though they may be online longer if Unit 3 is offline for an extended period (for example due to maintenance). Fuel for these generators is stored on site and is currently delivered by tanker truck, though in the past it was delivered directly by ship. There are three large fuel storage tanks at the Potrero facility, though only one is presently in use storing diesel for the peakers.
Future
For many years an effort has been made by Mirant to expand or upgrade the Potrero Point facility, while many in the local community feel the site should be closed due to health and pollution concerns. In particular, there is concern that the flow of water through the plant is damaging the local environment in the Bay, though Mirant disputes this. On March 2, 2006, the California Public Utilities CommissionCalifornia Public Utilities Commission
The California Public Utilities Commission is a regulatory agency which regulates privately owned public utilities in the state of California, including electric power, telecommunications, natural gas and water companies...
announced its rejection of Mirant's expansion plans. At that time, the plant was tentatively scheduled to be shut down sometime in 2007 so that a more modern replacement of similar size could be built at the site, but subsequently plans were scaled back and now call only for the diesel peaking power generators to be upgraded. This upgrade, if completed, would convert the peaking generators to run on natural gas. This would decrease pollution and remove the need to store large quantities of diesel fuel on site. The upgrade is expected to cost $80–100 million.
Mirant expects that the main Unit 3 will be permanently shut down sometime between 2010 and 2014 once upgrades to the electrical transmission grid going into San Francisco are completed and the city is able to import sufficient power from other parts of the state to meet all of its needs. Meanwhile, the plant continues to generate thermal pollution
Thermal pollution
Thermal pollution is the degradation of water quality by any process that changes ambient water temperature.A common cause of thermal pollution is the use of water as a coolant by power plants and industrial manufacturers...
by the discharge of heated water to San Francisco Bay, and Mirant has given no indication that it will take any steps to further control these discharges.
It is unclear whether the peaking generators will continue to operate beyond that time. Since those generators can be "black start
Black start
A black start is the process of restoring a power station to operation without relying on the external electric power transmission network.Normally, the electric power used within the plant is provided from the station's own generators...
ed", they are capable of powering essential city services even in the event of a total transmission line failure (such as due to an earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
). Given this capacity, the City may consider keeping them operational in case of emergencies.
After the closure of Unit 3, the bulk of the 23 acres (93,077.8 m²) site is likely to be deconstructed, with a plan towards making the area available for other industrial activities.
On December 21, 2010, San Francisco Mayor Gavin Newsom
Gavin Newsom
Gavin Christopher Newsom is an American politician who is the 49th and current Lieutenant Governor of California. Previously, he was the 42nd Mayor of San Francisco, and was elected in 2003 to succeed Willie Brown, becoming San Francisco's youngest mayor in 100 years. Newsom was re-elected in 2007...
announced that the Potrero plant would cease operations by the end of the year.