The sun and wind are great sources of clean and renewable energy, but the sun doesn't always shine and the wind doesn't always blow.
A new local research project aims to address such intermittency with energy-storage technologies that can help keep grid power stable and capture electricity for later use.
Tucson-based photovoltaics maker Solon Corp. is teaming up with Tucson Electric Power Co. and the University of Arizona's Arizona Research Institute for Solar Energy (AzRISE) to build an energy storage research site at the UA Science and Technology Park.
The Energy Storage Management Research and Testing site will be hooked up to a 1.6-megawatt solar plant, recently built by Solon for owner TEP at the UA Tech Park's Solar Zone, near South Kolb Road and Interstate 10.
The first phase of the project begins in August, when Solon and AzRISE plan to install a compressed-air energy storage (CAES) system designed and constructed by UA faculty and students.
The second phase will add a lithium-ion battery storage system this fall, followed by additional technologies in spring 2012.
The idea is to find the best storage methods to help utilities deal with voltage variations and other issues created as more large utility-scale solar farms are hooked up to the grid, said Bill Richardson, Solon director of research and development.
Storage systems also allow utilities to tap solar energy at night or during cloudy days.
But adding storage equipment to a solar-energy system adds costs that must be recouped through increased generation. And though some energy-storage technologies have proved cost-effective, there's no one-size-fits-all solution, Richardson said.
For example, battery storage can provide instant power for voltage or frequency support, while compressed-air storage is better suited to providing longer periods of backup power.
"With every (utility) customer, you have different needs and different technologies, and each one will pencil out differently," he said.
The storage research site will help sort out those issues, starting with the UA's compressed-air system.
Two utility-scale compressed-air plants - a 321-megawatt plant built in Germany in 1978 and a 110-MW plant built in Alabama in 1991 - prove compressed air can be cost-effective.
But the AzRISE researchers are looking to improve their efficiency, AzRISE Director Joseph Simmons said.
Because air becomes super-cold as it is expanded, it can quickly freeze up equipment at the point of expansion, Simmons said.
To avoid a freeze-up, CAES systems including the Alabama plant burn natural gas to heat the expanding air, which adds costs.
"The basic concept is a proven technology, but the problem is, they don't have very efficient air expanders," Simmons said.
The UA project's aim is to significantly reduce the need for natural gas by storing the heat generated as air is compressed and using it during the expansion phase.
The research is funded by Science Foundation Arizona, the U.S. Department of Energy and TEP. The UA team is led by Simmons and assistant professor Krishna Muralidharan, with graduate students Dominique Villela and Vijay Nathan. Several undergrads also are involved.
The team is working with an off-the-shelf industrial compressor and a new, high-efficiency rotary engine system made by W2 Energy, based in Ontario, Canada. Such air engines work by using compressed air to turn a driveshaft connected to a generator.
The UA team also is building its own piston air engine - with parts from an old Volkswagen engine. Undergrad Matthew Jordan is leading that effort.
The researchers are designing a separate system to store the waste heat from the compressor for later use in heating the expanding air, using a metal drum filled with thermal oil and rocks, Simmons said.
"Our preliminary calculations show this will easily do a 50 percent reduction in fuel (usage), and we could reach a 75 percent reduction," he said.
The air-storage system initially will be a 500-gallon metal tank, but the system can be scaled up by using additional tanks or scrap such as surplus pipeline, Simmons said.
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