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.