Stored energy is the ability to take generated electricity and store in a form where it can be used later, on-demand, as needed. This is especially important for renewable energy. Wind and solar plants need to be able to store power for use when the wind has stopped blowing or when the sun isn't out. The variability of renewable energy production is one of its biggest challenges now. The ability to store that power is essential for renewable energy to become a reality, with grid-scale storage everywhere.

The Arizona Research Institute for Solar Energy (AzRISE), part of the University of Arizona (UA), is conducting important research into utilizing compressed air energy storage (CAES) as a storage mechanism. Excess energy is used to compress air into chambers, then it is heated slightly and released when needed to power turbines. There are currently two utility-scale CAES plants operational, one in Germany, the other in Alabama. This is a proven technology, but the plants are inefficient. They use natural gas to heat up the compressed air. This heating is needed because air becomes super-cold when it expands and if it stays cold, can damage machinery. But using natural gas can be costly.

AzRISE is opening a new facility in Tucson soon to further test its innovative idea of capturing the heat generated when the air is compressed. The excess heat will be stored in a mixture of oil and rocks. AzRISE Director Joseph Simmons says this will reduce fuel costs by at least 50% and possibly as much as 75%. If this works at utility-scale, it will be a major breakthrough, sharply cutting the cost of CAES. The more we can store power generated from renewable energy, the more efficient and stable the grid will be. All the design and work will be done by UA students and staff working in partnership with private business.

Energy can also be stored by using pumped hydro, batteries, fuel cells, supercapacitors, and thermal energy. Each has advantages and disadvantages. AzRISE is exploring all of them and emphasizes that there is no one solution for all. Each site, whether it is a house, office, factory, or power plant will need whatever stored energy works best for them. Fuel cells and CAES can hold their power for months, while supercapacitors for no more than ten minutes. Thus, short-term storage is best for power plants where grid balancing is needed but is obviously not so good for homes, where much longer storage is important. Thus, CAES could be ideal for homes with solar power. The initial test systems from AzRISE are small enough to be used by a house or small building. They can of course be much larger.

UA is also researching ways to decrease the cost and raise the efficiency of photovoltaics and batteries, how to integrate all of this into a smart grid with smart metering, and much more. Arizona has plentiful sun. A smart grid using renewable solar energy with multiple ways of storing energy could provide the state with clean, renewable energy for decades to come.