Every night some 74,000 gallons (280,000 liters) of water are frozen at Norton Audubon Hospital in Louisville, Kentucky. The hospital used to get all of its air conditioning from a conventional system found in most U.S. buildings, but now 27 tanks of ice sustain a network of cold-water pipes keeping operating rooms at safe temperatures and patients comfortable.

This type of thermal energy storage, also known as ice batteries, is being added to buildings in the U.S. for its ability to provide cool air without releasing planet-warming emissions. These systems cut electricity usage and lower the strain on the grid. With rising temperatures and the growing demand for electricity in the U.S., ice thermal energy storage offers a sustainable option for cooling buildings.

Trane Technologies, a company that makes heating and cooling equipment, says it has seen a growing demand for this technology over the past few years. Its ice batteries are mostly used in schools as well as commercial and government buildings. Nostromo Energy, another company that makes ice batteries, is pursuing customers among energy-intensive data centers that have high cooling demands. Smaller systems are also made for homes by companies such as Ice Energy.

Ice thermal energy storage technology varies between manufacturers, but each follows a similar concept: At night when electricity from the grid is at its cheapest, water is frozen into ice that thaws the next day to cool the building. The ice chills water that is circulating through pipes in the building, absorbing heat from the rooms and creating a cooling effect. Air cooled by the system is then pushed through vents.

Norton Audubon Hospital uses a Trane ice battery system. Trane said its ice batteries are often used alongside traditional air conditioning, and the ice-based cooling is used to lower energy costs during peak demand times. The traditional AC components are typically left in place or downsized when ice batteries are added.

The stored ice doesn't require energy to thaw, which reduces the strain on the grid and minimizes the building's electricity usage, ultimately lowering monthly bills.

"Storing energy for further uses is where we're going with the future of the grid," said Ted Tiffany, senior technical lead at the Building Decarbonization Coalition. He said access to air conditioning is a major public health need that is being exacerbated by a warming planet, and ice batteries are a sustainable way to address health risks associated with extreme heat.

Energy costs at Norton Audubon were $278,000 lower for the first year the ice battery system was in operation after it was installed 2018. The hospital estimates that the system and other energy-saving measures have saved it nearly $4 million since 2016.

"The technology has been awesome for us," said Anthony Mathis, a Norton Healthcare executive who oversees sustainability. He said he receives inquiries from other building operators about the technology and thinks more facilities will adopt it as awareness grows.

Experts on sustainable energy say ice thermal energy storage is among the options commercial buildings can use to reduce electricity demand or store excess energy. Some commercial buildings use lithium batteries, which can store excess solar or wind energy that are available intermittently.

Dustin Mulvaney, environmental studies professor at San Jose State University, said ice batteries are a sensible option for health care settings and senior homes because lithium batteries can pose a fire risk.

Manufacturers also see opportunities in data centers, which are increasing in number to support artificial intelligence and have high energy and cooling needs. A December report from the Department of Energy found that data centers consumed more than 4% of the electricity in the country in 2023 and that number could grow up to 12% by 2028.

"Data centers are very energy-hungry and about 30% to 40% of their energy use is for cooling ... that's where a solution like ours could really help," said Yoram Ashery, CEO of Nostromo Energy.

Nostromo Energy said it is discussing deals to install its systems with several large data center operators, but declined to provide further details. Its ice battery technology is also used at the Beverly Hills Hotel in California.

California is currently the biggest market for this technology because the grid there uses a lot of solar power during the day but switches to polluting energy sources such as natural gas after the sun sets. Ice batteries can be used for air conditioning in the late afternoon and evening instead of drawing from the grid.

"A lot of utilities are really interested in this type of load-shifting technology," said Joe Raasch, chief operating officer at Ice Energy, another ice thermal energy storage company. He said summer is typically the most expensive time for utilities to operate because of the air conditioning demand.

"It's really great technology that the grid really needs because so much of the future electric load is driven by cooling," said Raasch.

___

The Associated Press' climate and environmental coverage receives financial support from multiple private foundations. AP is solely responsible for all content. Find AP's standards for working with philanthropies, a list of supporters and funded coverage areas at AP.org.