Sierra Club opposes Tempe power plant plan

The Sierra Club has filed a 57-page critique of a plan to upgrade a natural-gas fired power plant in Tempe with county air-quality regulators.

Arizona Public Service Co. plans to upgrade the Ocotillo Power Plant near Tempe Marketplace by replacing some of the old generators on the site with newer, faster-starting generators.

The Sierra Club said in its comments to the Maricopa County Air Quality Department that APS should consider adding battery storage for electricity, rather than the polluting gas generators.

The environmental group is particularly concerned at the high levels of greenhouse-gas emissions projected from the upgraded plant. Natural-gas plants can emit far less of the pollution associated with climate change than coal-fired power plants, but the APS application indicates that Ocotillo will offer only marginal benefits compared with a coal plant.

"The proposed Ocotillo plant would be allowed to emit greenhouse gases at a rate of 1,690 lb CO2/MWh," Sierra Club said in a press release. "That's right — almost the same as a coal plant."

APS, which has received a Certificate of Environmental Compatibility from the Arizona Corporation Commission for the upgrade in November, says the plant is needed to meet the fluctuating power demand created by relying heavily on solar power in its territory.

Rooftop solar panels and large solar power plants generate enough electricity during midday that APS can turn down other power plants when the sun shines. But as the sun sets, and solar panels stop producing power, the utility must quickly ramp up additional power plants to continue to meet demand. The upgrades at Ocotillo are designed to meet that need.

Sierra Club says in its letter to the Air Quality Department that APS should consider using batteries to capture excess demand at midday and dispatch that power as solar production drops off in the evening.

The state consumer advocate Residential Utility Consumer Office also suggested APS review alternatives such as battery storage when the issue was before the Corporation Commission.

As part of an agreement with RUCO, the utility issued a request for proposals Jan. 30 for power resources to meet peaks in demand and accomodate the variability of renewable power sources such as solar.

But APS still is moving ahead with the plant expansion. APS plans to spend $600 million to $700 million dismantling the two steam generators built in 1960 and replacing them with five shorter generators that use less water and fuel to make electricity. The company hopes to complete the project in 2018.

APS will maintain two smaller "simple-cycle" generators at Ocotillo that were built in the 1970s. Simple-cycle gas plants burn natural gas and force hot air through a turbine to make electricity, somewhat akin to giant jet engines.

The net capacity of the site will increase to 620 megawatts from 330 megawatts.

One megawatt is enough power to supply about 250 homes at once while the plant is running.