Compressing Air

PG&E plans to use the $25 million grant for a project to pump compressed air into an underground reservoir then release it to generate electricity during periods of peak demand. The CAES would be designed to utilize wind power in off-peak hours and have an output capacity of 300 MW for 10 hours.

Jonathan Marshall, chief, external communications at PG&E, announced the project on the utility’s blog. I asked him for a schematic or architectural drawing of the project, but he said PG&E doesn’t have one yet.

“It hasn’t been designed yet, that’s what the DOE funding would help out with,” said Marshall. “There will be two years for that phase and construction will take about five years.”

CAES is a hybrid storage/power production system, according to the National Energy Technology Laboratory (NETL). Off-peak electricity powers a motor/generator that drives compressors to force air into an underground storage reservoir. When electric power is needed the process is reversed: compressed air is returned to the surface, heated by natural gas in combustors and run through high-pressure and low-pressure expanders to power the motor/generator to produce electricity.

PG&E’s proposed CAES project doesn’t store the compressed air in a reservoir or cavern but in porous rock. Some fossil fuel is needed to heat the compressed air but it amounts to about a third of what a natural gas-fired plant would use.

“There may be some future generation technologies that we would be interested in testing that don’t use any fossil fuels at all, but we’re not quite there yet,” said Marshall.

Wind would be a major resource for PG&E’s CAES project because it peaks in the evening and morning hours. But the electrons won’t always come from wind power; energy storage looks attractive for any kind of intermittent renewable energy.

“Because solar tends to peak when our natural system peak demand is seen, there is not a whole lot of reason to store it, whereas wind peaks when our demand is low, so storage is a very natural option,” said Marshall. “The primary purpose of this is to store energy that you are buying anyway but don’t need for immediate demand.”

CAES interested PG&E because it can store a lot of energy. “CAES has orders of magnitude more capacity than typical utility batteries and appears to be the most cost-effective form of storage, according to technical experts at the Electric Power Research Institute,” wrote Marshall in his blog.

PG&E isn’t the first utility to be interested in CAES. Plans for large CAES projects have been floating around the industry for years, but a sample of proposed CAES projects reveals many good ideas and little action.

The Norton Energy Storage (NES) facility in Norton, Ohio, was going to be a 2,700 MW CAES facility using underutilized baseload power plants to compress the air. Start-up was anticipated for May 2003. The CAES Development Co. (CDC) obtained all permits necessary to develop the project, which was to be constructed in as many as nine modules of 300 MW each. Due to dislocations in the U.S. energy markets caused by the 2000 California energy crisis and the 2001 Enron bankruptcy, a plan to sell the facility to a utility fell through and CDC delayed the project. The trail is now cold and I can’t find any evidence of further development.

In June 2002, El Paso Corp. subsidiary EP Energy Finance and Ridge Energy Storage and Grid Services announced an agreement for $15 million of capital to develop compressed air energy storage projects. That seems to be the end of the story.

In January 2007, the DOE announced that it was collaborating with a group of municipal utilities in the Midwest to integrate a 75 MW to 150 MW wind farm with compressed air energy storage. The Iowa Stored Energy Park (ISEP) was to be the first plant in the world to use energy from a wind farm plus supplemental off-peak electricity to produce compressed air for storage in an aquifer. According to the ISEP website, the project won’t move into the design phase until testing and analysis of air storage at potential locations is complete “with procurement and construction to follow.” Electricity could be available to utilities in 2011.

Today, only two CAES plants exist, the same two CAES plants cited in most articles written about a planned CAES project: a 290 MW facility in Germany that went online in 1978 and the 110 MW McIntosh Project in Alabama, which began service in 1991. Both use caverns created by salt deposits and neither uses wind power.

Regardless of past experience, CAES could be worth a shot. The economics add up and the technology is proven. Compressed air requires 1/25^th the storage volume of pumped hydro storage to produce the same megawatts and time duration, wrote Steve Blankinship in a June 2008 Power Engineering magazine article, and about 75 percent of the U.S. is geologically suited for air storage caverns, including salt domes, depleted gas fields, depleted mines and deep saline aquifers.

A decision on PG&E’s smart grid funding grant could be made by the end of the year. “This would be our first for this technology, it would be one of the most advanced designs of its kind in the world and the first to use this porous rock formation,” said Marshall.

If PG&E is determined enough, the utility could be first company in almost 20 years to turn “will build” into “under construction,” a positive turn of events for the utility industry and renewables.