City departments collectively failed to reduce greenhouse gases by their 2012 target, yet the shortcoming had nothing to do with electricity usage.

The San Francisco Public Utilities Commission is able to boast that it delivers zero-emissions electricity to government properties.

That electricity comes from the hydropower production at the Hetch Hetchy reservoir in Yosemite Valley. Since it depends on rainfall, there are times when there isn't enough output and the SFPUC must fill in the gaps. In the past, that was done with pollution-producing sources.

However, with investment over the years in clean-energy infrastructure such as the 2010 Sunset Reservoir solar array or the 2004 solar installation at the Moscone Center, the SFPUC can provide no-emissions electricity.

Since 2004, the agency has completed 15 solar projects that produce 7.5 megawatts of total output, said spokesman Charles Sheehan. Additionally, the SFPUC produces biogas from its wastewater treatment facilities. Should these sources not meet demand, clean energy is purchased from other sources.

While the agency was heading toward cleaner-energy usage, in 2011 a state law required any gaps in hydroelectricity to be filled by state-certified renewable energy.

In 2005, the Hetch Hetchy system was emitting 76.28 pounds of carbon dioxide per megawatt hour produced. Comparatively, PG&E's emissions factor in 2005 was 489 pounds. And while in 2012 electricity usage by city departments accounted for no emissions, in 2010 it was 7.8 percent of total emissions.

The Hetch Hetchy system provides 130 megawatts of electricity for The City's municipal buildings, including San Francisco International Airport and Port of San Francisco property. Sheehan said the emissions-free hydropower has cut out 175,000 metric tons of carbon dioxide each year.

The City has a multiyear plan to reduce greenhouse gas emissions. Last year, departments achieved a 12.2 percent reduction, short of their 20 percent goal, from the 2005 emissions baseline of 213,808 metric tons per year.

Not all environmentalists celebrate the emissions-free hydropower.

Eric Brooks, a Green Party member, said the Hetch Hetchy Reservoir could serve as an important resource to help launch the proposed city-run CleanPowerSF electricity program. But Brooks said it's not the answer toward a real green-energy future.

Brooks is among those who support draining the Hetch Hetchy Reservoir.

“The best climate crisis solution is to quickly build true clean-energy solutions like solar, wind and efficiency to replace both fossil fuel and large hydroelectricity, and then decommission as many dams as possible worldwide to restore forest watersheds to their primary role of storing carbon,” Brooks said.

San Francisco voters defeated a November measure in support of draining the reservoir, which also provides The City with drinking water.

Bay Area Newsclean EnergyGovernment & PoliticsHetch Hetchy hydroelectric powerhydroelectricPolitics

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