An emerging class of heat-storing gels is out to reduce the energy our buildings burn on heating and cooling. Initial tests look promising, so researchers are lining up roofs, walls, windows and even hot-water and AC modules as the next stops.
The materials are called salogels, polymer gels loaded with hydrated salts that carry water trapped inside their crystals. In a recent study, researchers at Texas A&M University review how these gels have evolved and how they might be used in construction.
Heat-storing gels rely on the same principle that lets an ice cube keep a drink cold. As it melts, the ice absorbs thermal energy without getting any warmer. Phase-change materials, which switch between solid and liquid, work the same way. They absorb heat as they melt and release it as they solidify. The family includes waxes such as paraffin as well as hydrated salts, which have water trapped inside their crystals. The researchers favor the salts, which can store more than twice as much heat per volume as paraffin, the wax long used as the benchmark for heat storage. They also resist burning, and they can be reasonably cheap. Calcium chloride hexahydrate, for example, is a leftover from making soda ash (sodium carbonate).
There's a catch, though, and it has held these salts back from construction so far. When they melt, they turn to liquid, leak away and sometimes separate into layers. Engineers have tried to contain them in porous supports or seal them in capsules, but these fixes usually limit the salt to under 70% of the material, and encapsulation complicates manufacturing.
Salogels take a different route, holding the salt in place thanks to adding a little polymer. Versions that use reversible bonds, which can break and re-form, keep 80% to 95% of the pure salt's heat-storage capacity.
"Salogels represent a shift from simply storing heat to engineering multifunctional materials that combine thermal energy storage and conversion," said Kartik Kumar Rajagopalan of the university's Soft Matter Facility, one of the study's authors. "By understanding how polymers interact with salt hydrates at the molecular level, we can begin to design materials with properties tailored for real-world energy applications."
The team suggests two ways to use the gels in buildings. A passive route tucks the material into roofs, walls or windows, where it rides the swing between day and night temperatures. A dynamic route loads it into modules such as air-conditioning units or water heaters.
Testing has stayed small scale but promising so far. In a model house, a sodium acetate trihydrate gel filled three pipes under the floor, and the heat release could be triggered in one, two or all three. In another, a salogel coating on the walls of a 3D-printed house kept the interior notably cooler during the melting phase, and much warmer during crystallization, compared to an identical house without it. In windows, the gel melts by day, turns transparent and cools. At night it crystallizes, gives off heat and turns opaque.
Despite the excitement, authors admit several obstacles remain. Firmer gels store less energy, while the best heat hoarders are softer. The gels also have to release heat on cue, because some salts stay liquid even after the temperature drops low enough to solidify them. The polymer network helps, but how it does so isn't well understood. Humidity protection, large-scale manufacturing and durability over many heating and cooling cycles are still unknowns, as is how much energy a real building would save.
For the next step the authors propose the use of simulation and AI to predict which polymer dissolves in which salt. They also suggest salt blends with adjustable melting points, polymers that store heat themselves, and new fabrication methods such as 3D printing and gel fibers. The goal, they say, is to have a tough, recyclable material that holds up for years inside our buildings.
Source: Texas A&M University
