Energy contained in thermochemical materials can serve as an effective heating solution for indoor environments, especially in areas with high humidity, according to experts from the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL).
Energy stored in thermochemical materials can effectively heat indoor spaces, particularly in humid regions, according to researchers with the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL).
In collaboration with industry partners and researchers from Lawrence Berkeley National Laboratory, the team identified a feasible way to incorporate thermochemical materials (TCMs) into the heating, ventilation, and air conditioning (HVAC) systems of buildings. Salt-hydrate TCMs are particularly promising because they can enhance flexibility in a building’s heating system, potentially leading to lower energy consumption and enabling the shifting of energy use to periods when electricity is cheaper or cleaner.
TCMs function through hydration and dehydration processes. When the salt absorbs water, it releases heat, which warms the building, whereas additional heat is required at other times to dehydrate the TCM and recharge it. This process necessitates interaction with water vapor, which can be sourced directly from the surrounding air in an open system, or from evaporating water in a sealed, closed system.
While open systems are more straightforward to implement, they face challenges in winter when water vapor is less abundant. Drawing moisture from the indoor air can lead to discomfort due to reduced humidity, while the cold outdoor air is often too dry.
“By designing the reactor’s integration into the building system, we can avoid dehumidifying the indoor environment,” stated Jason Woods, a senior research engineer at NREL and a coauthor of the study. “It’s crucial to consider the moisture source, as this greatly influences overall performance.”
The findings, published in the December issue of Applied Energy, detail “Open-cycle thermochemical energy storage for building space heating: Practical system configurations and effective energy density.” Woods collaborated with Yi Zeng and Adewale Odukomaiya from NREL, along with colleagues from Lawrence Berkeley National Laboratory and NETenergy LLC based in Chicago.
This research, supported by the Department of Energy’s Building Technologies Office, stemmed from prioritization set forth in 2019 focused on thermal energy storage solutions. Buildings consume significant energy for heating and cooling, thus thermal energy storage offers a viable method to manage electrical load and support decarbonization by synchronizing electric heat pump usage with periods of low-carbon energy production.
The research team analyzed a TCM reactor that utilizes strontium chloride, which emits heat during its reaction with water vapor. They evaluated numerous climates and building types, exploring different setups while emphasizing the importance of the water vapor source. Their modeling work was validated through experimental results.
One of the most effective configurations allowed the TCM reactor to heat the outgoing air from the building to match the temperature and humidity of the indoor air. This heated air would then warm the incoming ventilation air via a heat exchanger, preventing the reactor from lowering indoor humidity levels and maintaining comfort. Not only would this setup supplement the energy needed for heating incoming air, but it could also elevate air temperature beyond the indoor temperature, thereby lessening reliance on traditional heating systems.
This strategy is effective only if the exhaust vents are located near the incoming ventilation points. Woods clarified that the reactor is intended to function alongside existing heat pumps and furnaces to store energy for later use.
In their simulations, the researchers set the indoor temperature at 21 degrees Celsius (69.8 degrees Fahrenheit). They identified relative humidity as the crucial variable influencing reactor performance, analyzing how it would operate in four different cities: Atlanta, New York, Minneapolis, and Seattle. Results showed it would be least effective in Minneapolis due to the cold, dry winter conditions.
“When the air is cold, it holds little moisture, causing indoor humidity to drop and making it challenging to engage the TCM reaction,” Woods noted.
Conversely, in Seattle’s more humid environment, the reactor was predicted to perform better thermally.
Beyond single-family homes, the study also assessed the effectiveness of the technology in small hotel lobbies, medium-sized office buildings, and hospital patient rooms. The initial investment for a TCM system is lower for larger buildings, with the levelized cost of storage (LCOS) projected to be under 10 cents per kilowatt-hour.
Looking ahead, researchers intend to advance this technology further. The favorable LCOS suggests a viable route to commercialization, but more work is needed to evaluate production, integration, packaging, and installation costs. Addressing these expenses will be critical for validating the cost-effectiveness of the technology. They are also investigating other methods to incorporate TCMs into HVAC systems, including the previously mentioned closed-cycle systems, which are free from external humidity limitations but pose different challenges that require further research.
