There are many reasons for the renewed interest in CAES, according to Joe Spease, CEO of WindSoHy – for instance, recent research advances that have shown pressurized air can be stored in depleted gas wells. But more recently, utilities and developers have taken a renewed interest in the technology for a completely different reason: the ability to store large amounts of renewable energy for long periods of time. In 2001, a major study of human activity patterns found that people in the US spend roughly 90 percent of their time indoors. Webster, whose company Magnum controls the salt dome adjacent to the Intermountain project under a long-term lease, is eyeing multiple potential customers – including the Intermountain Power Agency and most of the major western utilities – about the possibility of deploying a CAES facility in Utah. The technology is employed in a facility in Greater Manchester, England, and the company that owns it, Highview Power, is in the process of developing a similar project in northern Vermont. At its most basic level, CAES technology involves taking air, compressing it during times of surplus power, and storing it in pressurized form either underground or in above-ground containers – although large-scale facilities are predominantly underground because of cost effectiveness, according to Bailie.

The main benefits of the technology are the attractive costs, flexibility and high energy density, which means it requires a small footprint to put in a large number of MWh, Salvatore Minopoli, vice president of Highview Power, said at the Energy Storage Association’s (ESA) annual conference and expo in August, adding that the optimum scale for a project is anything over 25 MW, and anything over four hours. The project was the first of its kind in the U.S., and had a 26-hour duration. Existing plants tend to be designed in the 24 hour-plus duration range, Bailie said, and have a wide operating range. Another benefit with CAES is that its cost levels out as you scale up, since expanding a CAES plant requires simply making a bigger cavern, said Bailie. During the winter months you can often step out into the snow and look across the serene Lake Wakatipu to the impressive Southern Alps beyond. Another commonality between CAES and liquid air storage technologies is they are both trying to carve out a niche that distinguishes them from lithium-ion batteries.

In fact, LADWP is interested in liquid air storage solutions, said Ashkan Nassiri, the utility’s manager of strategic initiatives. The salt dome is several miles across and about a mile thick, making it the perfect bulk energy storage medium, according to Webster. The remnants of the aircraft fell into the sea approximately 190 kilometres (120 miles) off the coast of Ireland, killing all 329 people on board, including 268 Canadian citizens, 27 British citizens, and 22 Indian citizens. The aircraft must not leave the maintenance site until it is completed. With liquid air, the biggest challenge is getting a good understanding of the operating costs for a plant, in terms of preventative maintenance and potential overhauls, Tolliver said. There is quite a bit of commonality between the technologies, said Todd Tolliver, senior manager of storage technologies at ICF, in terms of using equipment to literally squish air into a smaller space, or liquid form, and storing it until needed. Rather than using a rotating compressor, like a turbojet engine does, the forward velocity and aerodynamics compress the air into the engine.

Like Play-Doh compound, this dough is a mix of starch, water, salt and a lubricant, and it uses heat to help the starch gelatinize. The utility has its eye on two types of compressed air technologies – one that uses a small amount of natural gas along with compressing the air into a large underground cavern like a salt dome, which could be a good fit for the Intermountain project, and another that doesn’t use any fuel but tends to be a little less efficient, which LADWP is evaluating for other parts of its system. Department of Energy, as well as another $25 million from California energy agencies, demonstrated that using an abandoned natural gas reservoir to store high-pressure compressed air is technically feasible, according to the report. Advances in hydrogen technology could also boost CAES since it burns at a far higher temperature than natural gas. For hundreds of years, the only thing we had to empirically measure air temperature reading was a traditional thermometer, often filled with dangerous mercury. And while there are hundreds of salt domes throughout the U.S., their dome is somewhat unusual, both because of its size and because it’s adjacent to pre-existing infrastructure that can be repurposed, Webster said.