Thermochemical technology shows promising path for heating indoor spaces

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). Working with industry representatives and researchers from Lawrence Berkeley National Laboratory, the scientists determined a realistic configuration for integrating thermochemical materials (TCMs) into a building’s HVAC system. Salt-hydrate TCMs are considered promising candidates for providing load flexibility to a building’s heating system. This flexibility could allow for reduced electrical requirements for the heating system or load shifting to times when electricity is less expensive and/or cleaner. The TCM is discharged and charged through hydration and dehydration reactions, respectively. Hydrating the salt releases heat, which is used to heat the building, and extra heat from the heat pump at other times of day is required to dehydrate, or charge, the TCM. This means the reactor needs to interact with water vapor. This water vapor could come directly from the ambient air, in which case the TCM is an open system. Or the TCM could be in an isolated chamber, evacuated of air, which is known as a closed system. In this case, the water vapor comes from evaporating liquid water from a second chamber. Open systems are simpler but present a challenge during the winter. Water vapor is typically scarce, and using indoor air to drive the hydration reaction can reduce the building’s humidity to an uncomfortable level while the cold outside air contains limited moisture. “The way we integrated the reactor into the building, we’re able to do that without drying out the house,” said Jason Woods, a senior research engineer within NREL’s Advanced Building Equipment Research Group and co-author of the new paper on this topic. “It’s important to think about where the moisture comes from, because performance can be significantly impacted based on how it’s integrated.” The paper, “Open-cycle thermochemical energy storage for building space heating: Practical system configurations and effective energy density,” is published in the December issue of the journal Applied Energy. Woods’ collaborators are Yi Zeng and Adewale Odukomaiya, both of NREL. Other co-authors are from Lawrence Berkeley and NETenergy LLC, a Chicago company. The research, which was financed by the Department of Energy’s Building Technologies Office, arose out of funding priorities established by the office in 2019 regarding thermal energy storage. Buildings require considerable energy to heat and cool, so thermal energy storage offers an opportunity to shift and shape the electrical load. This supports decarbonization by aligning electric heat pump operation with times when low-carbon energy is available. The researchers examined the thermal performance of a TCM reactor powered by strontium chloride, which gives off heat as it reacts with water vapor in the air. They considered a range of climates and building types, examined several configurations, and paid particular attention to the source of water vapor. The research used computer modeling that was then verified by experimental data. The configuration with the best results allowed the TCM reactor to heat the air exiting the building, which is at the same temperature and humidity as the indoor air. Once heated, the air then indirectly heats the incoming ventilation via a heat exchanger. This prevents the reactor from dehumidifying the indoor air and provides a sufficient humidity level. In addition to offsetting the energy required to heat the necessary ventilation air, the air can be heated above the indoor temperature, reducing the energy required by a furnace or heat pump to maintain the indoor temperature. This configuration, however, only works for buildings that have the exhaust air vent located near the incoming ventilation. Woods said the reactor is not intended to replace a heat pump or furnace but to store energy for use later. In modeling the TCM reactor, the researchers assumed the indoor temperature to be 21°C (69.8°F). The relative humidity proved the key factor affecting the reactor performance. They calculated how well the reactor would work in four climates: Atlanta, New York, Minneapolis, and Seattle. Among those cities, the reactor would perform the worst in Minneapolis because of the colder, drier weather in the winter. “There’s little moisture in cold air, so the humidity indoors is lower and it’s more difficult to drive the TCM reaction,” Woods said. With its greater humidity, a TCM reactor in Seattle would have a higher thermal performance, the researchers calculated. In addition to considering a single-family home, the research also examined how well the technology would work in the lobby of a small hotel, a medium-sized office building, and hospital patient rooms. The marginal capital cost for a TCM system decreases as the size of the building increases, with the levelized cost of storage (LCOS) estimated to be lower than 10 cents per kilowatt-hour. Going forward, the researchers will continue to advance this technology. The low LCOS indicates the technology has a feasible path to commercialization, but additional work is needed to quantify the reactor manufacturing, integration, packaging, and installation costs. Making this a cost-effective technology will require addressing each of these costs. The researchers are also exploring other options for integrating TCMs into HVAC systems, including the closed-cycle systems mentioned above. These systems are not constrained by ambient humidity but come with a separate set of challenges they hope to solve with further research.