Heat-storing gels keep rooms steadier, still in small-scale tests

An ice cube in a drink keeps the drink cold: as it melts, it soaks up heat without getting any warmer. Texas A&M University researchers want buildings to do the same, with a family of gels that could be built into roofs, walls and windows. They call them salogels: polymer gels loaded with hydrated salts, meaning salts that carry water trapped inside their crystals.

These materials belong to the family of phase-change materials, which switch between solid and liquid and absorb or release heat in the process. Paraffin, a wax long used as the benchmark for heat storage, is one member. The team favors the salts, which can store more than twice as much heat per volume as paraffin, resist burning and be reasonably cheap. Calcium chloride hexahydrate, for example, is a leftover from making soda ash.

The catch has kept salts out of construction. When they melt, they turn to liquid, leak away and sometimes separate into layers. Engineers have tried to contain them in porous supports or sealed capsules, but those fixes usually limit the salt to under 70% of the material, and capsules complicate manufacturing. Salogels take a different route: a little polymer holds the salt in place. Versions with reversible bonds, which can break and re-form, keep 80% to 95% of the pure salt's heat-storage capacity.

The tests remain small. 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 a 3D-printed house, a salogel coating on the walls kept the interior notably cooler during melting and much warmer during crystallization than an identical house without it. In windows, the gel melts by day and turns transparent, then crystallizes at night, gives off heat and turns opaque.

Firmer gels store less energy, while the best heat stores are softer. Some salts also stay liquid even after the temperature drops low enough to solidify them, so the release has to be reliable on cue, and how the polymer network achieves this is not well understood. Humidity protection, large-scale manufacturing, durability over many heating and cooling cycles, and how much energy a real building would save are all still open.

Owners and builders would gain if these gels hold up, through walls and roofs that smooth out day-night swings, and through air-conditioning units and water heaters where the gel could be loaded as a second route. The sources give no date and no saving figure. The next steps are simulation and AI to predict which polymer dissolves in which salt, salt blends with adjustable melting points, and 3D printing or gel fibers, with the aim of a tough, recyclable material that lasts for years inside buildings.