Solar collectors and wind generators hold so much promise for clean energy, but they have a major flaw – they produce no power when the sun doesn't shine or the wind doesn't blow.

"If all we had to do was to generate power when the sun is shining, we would actually be in good shape right now," said Ben Sternberg, a researcher in the University of Arizona's Compressed Air Energy Storage, or CAES, program. "The crucial issue now is finding economical ways to store energy for large-scale use, either home-by-home over the entire country, or utility scale."

Batteries have traditionally been used to store energy, but they're expensive, have a limited number of charge-discharge cycles, and pose resource and disposal problems.

The CAES group is developing cost-competitive energy-storage systems based on compressing air and storing it in man-made containers or below ground in natural reservoirs.

When solar panels shut down and wind generators stop spinning, the compressed air is heated slightly and released to drive turbines that generate electricity. The compressed air also can be released directly to drive mechanical systems without being converted to electricity.

Although CAES researchers are putting a high-tech spin on compressed air storage and its modern materials, sophisticated remote sensing gear, and computer analysis, it's a simple, well-tested and mature technology. Urban systems were built in European cities as early as 1870, and by the 1890s were storing and delivering power to factories and homes.

The UA's CAES research team is working on three projects that range from systems that might power a single air conditioner or refrigerator to building-wide systems, as well as massive storage sites that could store utility-scale energy.

Small-scale storage system

In this system, a low-speed motor uses some or all of the power from a solar panel or wind generator to pump air into a tank similar to those used for propane or oxygen. The energy is later used to power an appliance, such as a refrigerator. Several of these units could be linked together to power a home.

"We hope to develop a single-appliance system that could be built for less than $1,000," said Dominique Villela, a doctoral student in materials science and engineering. "We've had visitors from Alaska, whose villages depend on energy generated from propane. This is very expensive. Systems like ours could save them a lot of money by using solar or wind power for refrigerators or lights, for instance."

These systems could also save lives and taxpayer dollars in combat zones by producing energy on site. A recent segment on NPR's Science Friday program featured military efforts to conserve energy and switch to renewable energy fuels. Program guests noted that a gallon of gas that cost $2.35 in the U.S. could cost between $200 and $400 by the time it reaches outposts in Afghanistan. They also noted that more than 1,000 Americans have been killed moving fuel since that war began.

Villela said the group's research is now focusing on reliability issues, scaling the system to provide more energy storage, and adapting the system to less expensive materials and components.

Building-size systems

UA civil engineers are designing hollow structural members that could be used to store compressed air in load-bearing components, such as foundation piles or the frames of buildings and houses.

"The key to our system is that the loads on structural components coming from compressed air are small compared to building loads, such as the weight of the building and wind loads," said George Frantziskonis, a professor of civil engineering and engineering mechanics. "This makes CAES storage in buildings economically and aesthetically feasible."

The larger the building, the more economical the CAES system and the greater the energy cost savings both in the short and long term, Frantziskonis said.

Underground storage reservoirs

Researchers in the UA's Laboratory for Advanced Subsurface Imaging, or LASI, are developing high-resolution underground imaging systems that can be used to find salt deposits, porous rocks and other natural underground storage reservoirs. These sites could be used to hold large amounts of compressed air to drive utility-scale turbines.

While salt deposits have traditionally been associated with CAES technology, "you don't need a large cavern," said Sternberg, a professor of mining and geological engineering and director of the LASI program. "Rocks that have lots of pores also can provide energy storage. A third option is alluvium in basins, such as those found throughout the Southwest."

All of these possibilities require mapping the Earth's subsurface in high resolution with ground-penetrating electromagnetic waves. "That's where our work comes in because accurate imaging is needed to determine if there are discontinuities in these underground storage areas that will allow too much air to escape," he said.

Sternberg said porosity within the Earth, either from caverns or lots of interconnected pore space, has tremendous potential for low-cost storage that would make renewables cost competitive with fossil fuels.

Recent breakthroughs in the LASI program could help drive exploration and development of these resources. "We're getting data that's an order of magnitude more sensitive than conventional measurements," Sternberg said. "It's a combination of a new approach to collecting data, a new type of antenna array and a very different way of analyzing the data."

Sternberg is anxious to rapidly expand this technology to utility-size exploration. "Right now, so much of our energy is coming from volatile areas of the world, and we've got to overcome that," he said. "Energy security is our biggest risk. That's why this is so pressing. We cannot afford to drag this out and sit on the new developments in energy independence that are being created here in the LASI and CAES programs, as well as in other programs at universities across the country."

Science Foundation of Arizona and the Arizona Research Institute for Solar Energy, or AzRISE, are funding UA's CAES projects. The small-scale project also is being funded by the U.S. Department of Energy and has received help from several local businesses.