MIT scientists are developing a new chemical composite material that can absorb heat from the sun or other sources, store it, and release it again in a controlled fashion when exposed to light. The Phase Change Material (PCM) is a mixture of fatty acids and organic compounds and could one day provide, among other things, a solar-powered cooking stove for the developing world that could absorb heat from the sun during the day and release it on command at night.

PCMs interest scientists and engineers for their wide range of potential applications as they can absorb or release large amounts of heat when changing from a solid to liquid state, or vice versa. They have already been explored as a way of regulating heat in buildings, creating "heat batteries," and even keeping coffee hot on the drive to work.

The principle behind using PCMs as a heat source is simple. When, for example, you melt ordinary ice, it takes significant heat to change it into liquid water. This is one reason why snow doesn't suddenly all melt at once on an unseasonably warm day and why ice cubes cool drinks better than dropping in stainless steel cubes kept in the freezer.

The difficulty lies in getting a PCM, like waxes, fatty acids, or molten salts, to change its phase at a reasonable temperature, remain in its liquid state for a long time, and to make it release the heat on command. For many substances, the second requires heavy insulation, while the third depends on creating a PCM that releases its energy when slightly heated or exposed to a catalyst. Unfortunately, once the average PCM starts to release its heat, it does so all at one go as the PCM shifts from liquid to solid.

Led by postdocs Grace Han and Huashan Li and Professor Jeffrey Grossman, the MIT approach is to essentially introduce little molecules that act as phototriggers into conventional PCMs. In this case, it's the fatty acid tridecanoic acid mixed with an organic compound of an azobenzene dopant and a tridecanoic ester group.

This hybrid material liquefies when heated, but when it's exposed to ultraviolet light, it remains liquid even when it cools down. This means that most of the heat is locked up in the compound at 200 joules per gram, which is good for an organic PCM. Another pulse of light triggers the fatty acid and the PCM solidifies, releasing the heat.

What this means is that the material can be used as a chemical heat "battery" that allows heat to be released on demand and in the quantity desired. In addition, the PCM can store the heat for over 10 hours and the researchers say that this time could be improved upon.

At the moment, the MIT PCM is only in the proof of concept stage, though it can already handle temperature changes of 10° C (18° F). The hope is that it can not only find applications in solar-powered cookers, but also as a means of drying grain and to recover waste heat from factories and vehicles for later use.

The research was published in Nature Communications.

Source: MIT

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