A new chemical composite could be used to store heat from the Sun during the day in a thermal battery, and release the heat when needed. A common approach to thermal storage is to use a phase change material (PCM), where input heat melts the material, and its phase change — from solid to liquid — stores energy. When the PCM is cooled back down below its melting point, it turns back into a solid, at which point the stored energy is released as heat. There are many examples of these materials, including waxes or fatty acids used for low-temperature applications, and molten salts used at high temperatures. But all current PCMs require a great deal of insulation, and they pass through that phase change temperature uncontrollably, losing their stored heat relatively rapidly.
Instead, the new system uses molecular switches that change shape in response to light. When integrated into the PCM, the phase-change temperature of the hybrid material can be adjusted with light, allowing the thermal energy of the phase change to be maintained even well below the melting point of the original material.
The composite serves as an add-on for traditional phase change materials — molecules that undergo a structural change when light shines on them. Integrating the molecules with conventional PCM materials to release the stored energy as heat, on demand, was accomplished by combining the fatty acids with an organic compound that responds to a pulse of light. With this arrangement, the light-sensitive component alters the thermal properties of the other component, which stores and releases its energy. The hybrid material melts when heated, and after being exposed to ultraviolet light, it stays melted even when cooled back down. Next, when triggered by another pulse of light, the material resolidifies and gives back the thermal phase-change energy.
The system could make use of any source of heat, not just solar. The availability of waste heat is widespread — from industrial processes, to solar heat, and even the heat coming out of vehicles — and is usually wasted. Harnessing some of that waste could provide a way of recycling that heat for useful applications.
In its chemically stored form, the energy can remain for long periods until the optical trigger is activated. Stored heat can remain stable for at least 10 hours, whereas a device of similar size storing heat directly would dissipate it within a few minutes. In the initial proof-of-concept system, the temperature change or supercooling achieved for the thermal storage material can be up to 10 °C (18 °F), but could go higher. The material can store about 200 joules per gram.