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Composites as Phase Change Materials

Phase Change Materials (PCMs) have been developed for over a decade for use in buildings, electronics and now cars.  PCMs are materials that melt and solidify at certain temperatures with the capability to store or release large amounts of energy during this phase change. Note that solid-gas and liquid-gas PCMs have also been investigated. PCMs may be organic or inorganic compounds, but typically include a conductive filler, which is where graphite comes in. Other PCMs use carbon nanotubes (CNTs).

omposites are being developed as Phase Change Materials (PCMs) using graphite, graphene and multi-walled carbon nanotubes (MWCNTs).
SOURCE: (left) and International Journal of Heat and Mass Transfer, Volume 120, May 2018, Pages 33-41 at (right).

Fraunhofer LBF researchers have experimented with different carrier materials and fillers such as boron nitride and graphite. They ended up with a substrate made of polyethylene, a polymer already commonly used in automotive interiors, and graphite as the filler. The graphite reportedly gives the composite a very good thermal conductivity and rapid heat dissipation as well as excellent energy storage.

The working principles for the graphite/PE composite PCM were completed within the "Smart Cover Panel" par of OPTEMUS. To demonstrate the new material’s benefits, researchers incorporated it into the dashboard of the Fiat 500e prototype and cycled it through heating and cooling phases. Cooling was provided by Peltier elements, which are electrothermal transducers, cooled, in turn, by external fans. Energy for the Peltier elements and EV radiator comes from an additional 12-volt battery powered by photovoltaic cells.

Graphite/polypropylene composite PCM installed in dashboard of Fiat 500e prototype vehicle.
SOURCE: Fraunhofer LBF

LBF researchers used the energy released during the phase transition between heating and cooling to switch on the Peltier elements only for a certain period of time. Thus, the fans do not constantly run at maximum power. The cooling system is coupled to the temperature in the material by appropriate regulation. Fraunhofer believes this will extend the life expectancy of the electronic components. The Smart Cover Panel is actually controlled by the sunlight itself, which means that cooling only starts in hot summer days, while the warmth remains in the dashboard during winter.

Fraunhofer LBF’s new composite PCM was experimentally compared to polypropylene-talcum compound used in conventional dashboards. After 1000 seconds of exposure to 1200 watts/m² of radiant power, the temperature difference on the surface of the specimens was roughly 41°C, which corresponds to a 46% reduction in temperature.