Advanced Materials − The Basis of Innovative Buildings

New Phase Change Materials Based on Sugar Alcohols for Latent Heat Storage

© Fraunhofer ISE
D-mannitol in crucible after 20 cycles with oxygen contact. Right: D-mannitol in crucible after 20 cycles without oxygen contact.

The objective of the project was the development of new phase change materials (PCM) based on sugar alcohols for thermal energy storage. The main application of the latent heat storage units in the project are long-term heat storage units for buildings. For example, excess heat of a solar thermal system in the summer can be stored for the winter. PCMs store heat in the solid-liquid phase change. Compared to other PCMs, sugar alcohols have a high melting enthalpy. The melting temperature is usually between 100 and 250 °C. To maintain the advantage of the high melting enthalpy and to reduce the melting temperature for the building, the partners developed sugar alcohol mixtures. A focus of the project was the characterization of pure sugar alcohols and the developed mixtures.

The requirements on the storage units and thus, the boundary conditions for material development were defined at Fraunhofer ISE in a simulation of a solar thermal system for building heating and domestic hot water supply with latent heat storage. An important property of PCMs is the thermal stability while passing through the melting and crystallization cycles. For this reason, we have investigated the thermal stability of selected sugar alcohols and their mixtures, and developed degradation determination methods. In the first step, the material was cycled for a short period, where the materials was heated 20 times above its melting point and cooled down again. The melting enthalpy before and after cycling was used as stability measure. If short-term cycling was stable, long-term cycling was performed. No long-term stability could be ensured for sugar alcohol D mannitol. Erythritol and xylitol were maintained constant at temperatures above the melting temperature over a longer period. The melting enthalpy was determined before and after the treatment. They showed stable behavior under the given test conditions. If degradation occurred, it could be demonstrated using further methods that different factors, such as the ambient atmosphere, influence the degree of degradation. Based on the melting temperature of pure sugar alcohol between 100 and 250 °C, an application in process heat systems is also considered for increasing the energy efficiency.