Phase-change materials

Natural Air Conditioning with latent Heat Storage

form 230
January/February 2010


Birkhäuser (Basel)

We are familiar with them as hand and pocket warmers, namely phase-change materials (PCMs), which are also referred to as latent heat accumulators. They are capable of emitting heat to their environment when changing from a fluid to a solid state.

For some years now, researchers have been working on new applications for PCMs. The phase-change material developed by BASF and the ISE Fraunhofer Institute which goes by the brand name of Micronal® consists of microscopically small plastic balls, at the core of which is a storage medium containing paraffin wax. Rising temperatures melt the wax and the heat is thus absorbed. If the temperature drops, the opposite process takes place: The wax sets and heat is released. Micronal® can be invisibly integrated into building materials such as wall plaster or building boards which can then influence the ambient climate as desired. Energy demand for air conditioners is also clearly reduced so that, on the one hand, the additional outlays for the building material will already have been recouped within five years and, on the other, a greater contribution is made to climate protection. For this reason the team of developers were nominated for “Deutscher Zukunftspreis” this year, a federal award which goes to people who have future-oriented ideas.

Micronal® passed a 16-month endurance test with 24 phase changes per day, implying a minimum life-span of 30 years. There are already multiple Micronal® applications available on the market for the building industry. Lebast®, for instance, is a clay building board equipped with waxes which in spite of its minimal thickness is capable of increasing heat storage capacity many times over, and preventing buildings from overheating in summer. Maxit clima® is a machine-applied plaster coating with a temperature-regulating affect suitable for direct use at the building site and as dry mortar. And in the green porous CelBloc Plus concrete developed by H+H Deutschland GmbH, the PCM prevents heat escaping from the stone and reduces temperature fluctuations on the inner wall surface. Indeed, the heat storage capacity of a 1.5 cm thick gypsum wallboard from Knauf Gips KG that contains PCM is comparable at least to a seven cm thick concrete or 10 cm thick brick wall.

Quite apart from the building sector, PCM is now also being used in many other fields, such as skiwear. Researchers at the Fraunhofer Institute for Bauphysik (IBP) are currently developing a PCM cup: The PCMs are enclosed here in honeycomb-like cavities whose surface is clad in ceramic, metal or plastic. If you pour hot coffee into the cup, it first cools to a suitable temperature for the mouth, and the PCM which is originally fluid, hardens. The material then slowly returns the absorbed energy reserve to the drink and in this way keeps the liquid at drinking temperature. The technique can be used for both cold and hot foods and drinks.

image source: microtek