Hydrostor, a Canadian company, says it has developed a new compressed air energy storage system that is half the cost of grid-scale batteries and equal to the cost of a new natural gas powered generating plant. Finding ways to store electrical power is critical to making renewable energy sources commonplace. Grid storage soaks up excess power from solar panels or wind turbines so it can be used later in a process known as time shifting. That saves utility companies money because they don’t need to fire up a so-called peaker plant to meet the demand for extra power at certain times of the day.
Called Hydrostor Terra, the system uses excess electricity to compress air and store it underground in specially constructed tanks. Later, when extra electricity is needed, some of that pressure is released and used to turn turbo-expander turbines. What sets the Hydrostor process apart from other compressed air storage systems is this: normally, the air released from storage is heated using natural gas to make the turbo expander turbines more efficient. Naturally, burning a fossil fuel detracts somewhat from the zero emissions premise of using renewable energy in the first place.
But compressing a gas creates heat and the Hydrostor system captures that heat and stores it in a thermal management system. When the pressurized air is released, that harvested heat is used to increase the temperature of the air instead of using BTUs from burning natural gas. Hydrostor says its system is 70% efficient. Traditional systems that use natural gas heating are 42 to 54 percent efficient. (Battery storage systems can be up to 95% efficient.)
AECOM, the world’s largest contracting company, is working with Hydrostor to build special underground tanks for the compressed air and pipe in water from an external source for the compressed air to displace. This helps maintain a constant pressure in the tank, which reduces risk of damage as the system is charged and discharged. Earlier systems generally tried to make use of existing geologic caverns underground or had to construct artificial caverns in salt deposits. According to a Hydrostor press release, “The Terra system, by contrast, can be deployed at any site within proximity to a body of water, including inner city and urban areas.”
Although exact pricing details depend on the project, UtilityDive noted in January that Hydrostor’s systems cost between $1,000 and $2,000 per kilowatt to build. A company spokesperson confirms that figure and adds that the price reflects a “fully installed” system including a warranty. “Many other technologies, including batteries, typically aren’t quoted on a fully installed basis,” the spokesperson said. According to the US Energy Information Administration, new natural gas plants cost about $1,488 per kilowatt to build as of 2014.
The President and CEO of Hydrostor, Curtis VanWalleghem, noted in the company’s press release that it is “engaged with several utilities around the world to deploy systems rated at hundreds of megawatts, delivering gigawatt-hours of storage at durations ranging from four hours up to multiple days.” The company is “targeting dozens of mostly coal-powered facilities of at least 100 megawatt capacity across the US that either shut down in 2016 or will shut this year,” according to the Globe and Mail.
Energy storage is not new. Before the advent of grid scale batteries, which are a fairly recent development, pumped hydro storage systems became common in many parts of the world. A large facility in Wales has been in operation since 1982. A 400 MW pumped hydro system is under construction in Montana. There is even a proposal to drag a train filled with concrete blocks up a mountain in Nevada using excess electricity and regenerate electrical power as the train trundles back downhill after the sun goes down. The Hydrostor system adds one more option for energy storage to the mix.