Solar power and wind energy are popular in the green movement, but when there's no wind or sun, there's no power.
So University of Arizona researchers are trying to find ways to store those energy sources and regenerate them through something that's always available: air.
The Compressed Air Energy Storage program is working to cheaply store compressed air for energy in man-made structures or in natural underground reservoirs. The stored air, created by compressors powered by another energy source, would power turbines when released and provide energy to homes and buildings when other sources of energy, such as solar power, aren't available.
The technology is a better choice than, for example, batteries, said Ben Sternberg, professor of mining and geological engineering and director of UA's Laboratory for Advanced Subsurface Imaging, part of the program.
"Batteries aren't very economical," he said. "The initial expense is large for batteries as energy-storage means, but worse than that, they have a limited number of charge/discharge cycles."
Batteries also pose disposal problems, said Krishna Muralidharan, an assistant professor in the department of material science and engineering, because of their potentially hazardous nature.
Muralidharan, also part of the project, said its goal is to use air as an economical, safe and long-lasting way to provide energy.
It may complement technologies such as solar power, too. One possibility is to have solar power provide daytime energy as well as power the compressors that pump air into the reservoir, Sternberg said.
The group is working on systems for home use, large underground reservoirs and storage worked into commercial structures. Reservoirs likely would be used on a scale large enough to be utilized by a utility such as Tucson Electric Power Co. instead of individual storage units.
A utility could store air in a reservoir during the day, when sunshine is abundant. The air could be released to drive turbines that generate electricity to provide nighttime power, Sternberg said.
Muralidharan said the group also is trying to take things a step further by making compressed air the direct power source for targeted smaller-scale appliances, such as air conditioners and refrigerators.
Currently, a photovoltaic panel turns sunshine into electricity that runs the compressor, storing air to run turbines that generate electricity.
The group wants to eliminate the last part to make appliances run on air, he said. Much engineering has to happen before this is available, he said, but a basic prototype should be available by 2012.
How much energy the technology can produce is still being researched, Sternberg said: A back-of-the-envelope calculation shows that it takes two swimming pools' worth of compressed air to produce a few megawatt-hours of energy, Muralidharan said, which is a significant fraction of the annual energy consumption of an average household in Tucson.
And researchers are still only getting a 50-percent return on the energy it takes to compress and extract the air, Muralidharan said.
Storing large volumes of air also could be a problem, he said, but Sternberg said there are potentially enough reservoirs to serve the nation's storage needs.
Another concern is finding suitable geology for that storage -- a key being to find homogenous formations, Sternberg said.
The group is developing a new subsurface imaging technology called the Differential Target Antenna Coupling method to map structures with the ability to investigate deeper and with much greater resolution. The technology has many potential uses, including natural resource exploration and water resource mapping, Sternberg said.
For those who think using this energy is lofty, Sternberg said there are places where it's already in use, with demonstration sites in Alabama and Germany.
Sites suitable for compressed air include salt deposits, porous rocks and other natural reservoirs, Sternberg said. One site could potentially be under the Tucson Mountains.
"On the surface you see volcanic rocks," he said. "Below the surface we're finding conducting rocks. We believe it's likely that we have water saturated sedimentary rocks beneath the mountains. That's a potential for, first of all, water resources, and another possible place to store compressed air."
For commercial structures, UA civil engineers are designing storage features in the frames or foundations of buildings. The project is run by Joseph Simmons, director of the Arizona Research Institute for Solar Energy and head of the UA department of materials science and engineering.
The project is funded by the Science Foundation Arizona, the Arizona Research Institute for Solar Energy, the U.S. Department of Energy and several local businesses.
Victoria Blute is a NASA Space Grant intern. E-mail her at firstname.lastname@example.org