Test Bubbles

Nebraska Public Power District is going underground. Three thousand feet below ground, to be precise. They're looking closely at some empty salt domes in Deuel County, Nebraska. NPPD is the state's largest electric utility and they plan to spend more than $8 million conducting engineering and geomorphic tests for a Compressed Air Energy Storage project.

The salt domes in this particular Dakota Sandstone formation were mined in the 1950s and 1960's. The same formation was used as a natural gas storage area in the late '90s which means the subsurface 'breathes'. You can get compressed gases in and extract them back out of the formation. On the 'exhale' NPPD would use the compressed air to help power a turbine and generate electricity.

The latest $8 million project offers a series of engineering tests designed to quantify the 'lung capacity' equivalent from the rock formation. Beginning in 2014, NPPD plans to inject 3 billion cubic feet of air into the underground geological formation over a six-month period to develop "test bubbles" and determine whether the formation will hold air stored at 830 to 1,000 pounds per square inch over a long period of time.

The basic idea of CAES involves using power when it's plentiful (and cheap) to drive an air compressor, store air in a geologic formation and draw it back out when prices are high, heat it up, and then supply the compressed air to a modified gas turbine.

NPPD won't make a decision about building or not building a CAES power plant on the site until mid-2015. Construction would start the following year. If approved the turbine would be operational, or on-line, in 2019.