Throughout the United States, more than 100 million buildings tap into electrical energy to keep heating, ventilation, air conditioning and refrigeration units functioning. HVAC systems cause most of the peak load demand on the electric grid; one way to alleviate the grid burden is to develop new storage options for heating and cooling.
As a partner in the Department of Energy's Stor4Build Consortium, Oak Ridge National Laboratory is co-leading research with the National Renewable Energy Laboratory, Lawrence Berkeley National Laboratory, Pacific Northwest National Laboratory and the American Council for an Energy-Efficient Economy to develop thermal energy storage as a complement to electrical battery storage.
ORNL Director Stephen Streiffer welcomed fellow collaborators and industry stakeholders to the two-day Stor4Build workshop focused on paths forward for the development, demonstration and deployment of novel building equipment technologies and materials that utilize thermal energy.
"ORNL exists to solve big problems that address national needs such as developing solutions that improve the electric grid," Streiffer said. "The Stor4Build consortium will play a critical role in shaping the future of the nation's energy systems."
Sven Mumme, Stor4Build co-director and the DOE technology manager for opaque envelope and thermal energy storage R&D, said thermal energy storage has many benefits. "For example, thermal energy can improve heat pump performance and facilitate their market adoption, and with the utilization of low-embodied-carbon materials, a building's carbon impacts can be reduced too," Mumme said.
On-demand energy
Thermal energy storage, or TES, functions like a battery, keeping energy stored in a material as a source of heat or cold that can be reserved for later use in buildings. Researchers are optimizing the performance of phase-change materials such as wax and salt hydrates that can store and release energy when changed from a solid to a liquid or a liquid to a solid.
Since TES can store or release heat based on the immediate need, it can be coupled with a building's HVAC system to reduce the demand for power from the grid at peak times, such as the late afternoon in the summer or early morning in the winter, while maintaining the room temperature at the thermostat's setting. TES also allows heat pumps to function more effectively in extreme weather.
Finding ways to cost-effectively integrate TES into HVAC systems and within a building's envelope, or walls and roof, is the main goal of Stor4Build. To reduce costs and enable wider adoption, new approaches to TES are needed, said ORNL's Kyle Gluesenkamp, co-director of Stor4Build and a distinguished R&D scientist.
"Today's TES products are mostly used for cooling of large commercial spaces," he said. "If we want to see widespread adoption of this technology, so that anyone can benefit, no matter where you live, we have to reduce the costs, develop better performing storage materials, and design components and modular packaging that can be seamlessly integrated into all kinds of HVAC systems, heat pumps and building envelopes."
Expanding heat pump adoption
Heat pumps provide an energy-efficient alternative to traditional furnaces and air-conditioners. The heat pump transfers thermal energy using a refrigeration cycle to cool and warm spaces.
Through Stor4Build, Gluesenkamp has led a research team to design, fabricate and evaluate a novel heat pump prototype that can shift electricity demand with TES. By modifying a heat pump using only commercially available components, the team was able to reduce costs. Through funding provided by DOE's Building Technologies Office, Gluesenkamp utilized the resources of the Building Technologies Research and Integration Center at ORNL to develop the prototype.
"If deployed at scale, our TES-ready heat pump promises to reduce the need for electrical infrastructure upgrades and shift peak demand more cost effectively than electric batteries alone," Gluesenkamp said. "We built a prototype that shows a heat pump with TES can reduce electric demand for heating at peak times of the grid by as much as three times compared to today's commercially available heat pumps."
The accomplishment is significant, he noted, because it provides proof-of-concept to inform utilities and industry as to what is achievable with heat pumps and TES. In addition to the prototype, Gluesenkamp's team also demonstrated a new defrost method utilizing TES that avoids a common complaint with heat pumps. Heat pumps are prone to blow cold air periodically during periods of defrosting; however, defrosting is necessary to prevent outdoor coils from becoming blocked by frost.
"We reduced the duration and improved occupant comfort by defrosting with TES," he said. "Defrosting can be completed in three minutes versus the standard five to 10 minutes. This equates to no cold-blow-effect indoors, higher heat pump efficiency and lower peak electric demand too."
Ultimately, reducing the blowing of cold air will provide greater comfort for occupants and reduces stress on the grid because minimal electricity is used - all positive attributes that could further enable the mass deployment of heat pumps.
Transforming the market
ORNL's heat pump research is just one of many paths Stor4Build will leverage over the next few years to advance potential TES solutions from early- to medium-stage development to market adoption. By 2028, the program intends to have conducted field evaluations of novel packaged or integrated TES solutions to validate the benefits of the new technologies when installed in homes. In addition, the program plans to target a community-scale demonstration that can show aggregated peak demand reduction of 50-100 kilowatts in 20 to 50 homes while reducing the power consumption for thermal loads of individual homes by at least 50% for a duration of four to six hours.
Mumme, who shared anticipated accomplishments for 2025 with Stor4Build workshop attendees at ORNL, noted that the national laboratories will execute projects in five areas: TES-ready heat pumps for residential buildings; TES-ready equipment for commercial buildings; active TES in building envelopes that improve resilience, reduce peak demand and save energy; TES sizing, benefits and decision open-source tools; and standard testing protocols and procedures for TES products.
"Stor4Build unites stakeholders, including the HVAC industry and utility providers, and our academic and research communities to share knowledge and collaborate on shared goals to accelerate scaled adoption of TES technologies in buildings," Mumme said.
"It will take crosscutting teams to develop equitable solutions that ensure the benefits of TES are clear for everyone," he added.
In addition to Mumme and Gluesenkamp, Stor4Build co-directors include Sumanjeet Kaur, materials scientist and thermal energy group leader at Lawrence Berkeley National Laboratory; and Tim LaClair, distinguished research scientist in building thermal energy sciences at the National Renewable Energy Laboratory.
UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science .
