Stored heat from the sun can provide energy on demand

ENGINEERS at MIT, US, have developed a chemical composite that can store heat from the sun during the day and release it again on demand when it is needed.

Materials science and engineering professor Jeffrey Grossman and his colleagues believe that a ‘thermal battery’ based on the composite could be particularly useful in the developing world. Here, there is ample heat from the sun during the day, but cooking takes place later in the day, using fuel such as wood or dung. The thermal battery could be used for purposes like cooking, heating or drying crops instead, saving time and effort.

The composite developed by the researchers is made of tridecanoic acid, a so-called phase change material, which melts in the heat, storing energy. When it cools back to below its melting point, the energy is released as heat. Most phase change materials require a lot of insulation, and the stored energy is lost very quickly. However, the MIT team overcame this problem by incorporating a light-responsive azobenzene dopant. The azobenzene changes shape in response to light. When it is incorporated into tridecanoic acid, the structure changes and the phase change temperature of the hybrid material can then be changed by exposing it to light. The hybrid material melts when heated, and when exposed to UV light, remains liquid. Another pulse of UV light causes the material to resolidify, giving out the stored energy. As a result, the thermal energy release can be easily controlled on demand.

The composite can retain the stored heat at least ten hours, and the researchers say it could be tuned to last longer than this.

At present the material is simply a proof of concept, as the temperature change achieved is only 10^oC, but Grossman says that the team believe there is “a lot of potential for using light-activated materials to hijack the thermal storage properties of phase change materials.” The material can store around 200 J/g of energy, which researcher Grace Han says is “quite significant”. The researchers now hope to increase the temperature change possible.

One day, thermal batteries made from phase change materials could even be used to harness and recycle other waste heat.

“The availability of waste heat is widespread, from industrial processes, to solar heat, and even the heat coming out of vehicles, and it's usually just wasted,” said Han.

Nature Communications doi.org/cgcj