Phase change materials (PCMs) are important for thermal energy storage applications, especially in buildings and in solar thermal systems. The use of PCMs can increase energy efficiency by storage of solar thermal energy, and reducing heating and cooling demands. Suitable phase change materials need to be inexpensive and reliable, with high latent heats and a phase change at a temperature appropriate to the application. Organic phase change materials can satisfy many of these criteria and are suitable for latent-heat thermal energy storage systems operating in the ambient-to-moderate temperature range. Non-paraffin organic PCMs such as esters, fatty acids and fatty alcohols are particularly attractive because these materials are non-toxic and can be extracted from renewable sources, such as from plant and animal fat. They are also abundant and inexpensive, with a commodity cost of around $1/kg for fatty acids, for example. Their economic and embodied energy payback times can be very favorable.
The optimal design of an energy storage system requires accurate data regarding the thermophysical properties of PCMs. Furthermore, for phase change materials to be useful and reliable in their applications, they must be chemically and thermally stable after many melt-freeze cycles, and must be chemically inert with the materials with which they are in contact. For the very promising fatty acid PCMs, there were significant knowledge gaps and even inconsistencies. For example, there are considerable discrepancies between the values reported by different researchers for the latent heat of fusion and the phase change temperature of the same fatty acid PCM. In addition, thermal properties, such as the thermal conductivity and heat capacity, which are especially necessary for numerical studies for optimization, were largely unknown.
Researchers led by Professor Mary Anne White at Dalhousie University in Canada presented a comprehensive analysis of the thermophysical properties, thermal stability and chemical compatibility of six important organic PCMs: dodecanoic acid, decanoic acid, hexadecenoic acid, tetradecanoic acid, 1-octadecanol, and octadecanoic acid. Their research work is published in the journal Solar Energy Materials and Solar Cells. “We focused on fatty acid PCMs with melting temperatures between 30 and 70 °C because these materials span a wide range of thermal energy storage applications, from integration in building materials and residential solar water heating systems to cooling of portable electronic devices. They are cheap and can be sustainably sourced. They are also useful for preparing new PCMs by forming eutectic mixtures with lower melting temperatures”.
The research team implemented consistent procedures and experimental methods for all six phase change materials to provide direct comparisons. They accurately measured the thermal properties of the PCMs in the solid and liquid phases and related physical properties. In addition to thermophysical characterization, the authors determined the thermal stability of phase change materials over thousands of freeze-thaw cycles, as well as their chemical compatibility with 16 different materials. “Chemical compatibility of PCMs with materials is very important information but had been significantly overlooked in this field. For instance, many studies consider the addition of metallic fillers to enhance the thermal conductivity of fatty acid PCMs, but it is not known whether the fatty acids will react with these fillers over time”.
From the long-term cycling results, the authors observed that all the six phase change materials studied were thermally stable over 3000 melt-freeze cycles. In fact, there were no significant changes in the heats of fusion and melting temperatures, even for a low-purity sample of hexadecenoic acid. For this reason, it was determined that these PCMs are thermally reliable for long-term thermal energy storage applications.
The authors also investigated chemical compatibilities of the phase change materials with nine metal alloys and seven plastic materials. “We chose a wide range of materials which are found in most of the thermal energy storage applications that these PCMs were considered for in previous studies, or will likely be used for in the future. For example, copper and aluminum are typical filler materials, whereas the magnesium alloy, Mg AZ91D, and polycarbonate are commonly used in today’s portable electronic devices”. They found that two copper alloys, namely Cu110 and Cu101, and one magnesium alloy, Mg AZ91D, were incompatible with the fatty acids, while the nickel alloy Ni C7521 was compatible only with octadecanoic and hexadecenoic acids. They observed that octadecanol was compatible with all the alloys. Polycarbonate was the only plastic material that did not react significantly with any of the phase change materials investigated.
The in-depth information provided in the study conducted at Dalhousie University regarding the thermophysical properties, thermal stability and chemical compatibility of organic non-paraffin phase change materials provides all the required data to design and optimize thermal energy storage systems in the temperature range 30 to 70 °C, for applications from the built environment to compact electronic devices.
Samer Kahwaji, Michel B. Johnson, Ali C. Kheirabadi, Dominic Groulx, Mary Anne White. Fatty acids and related phase change materials for reliable thermal energy storage at moderate temperatures. Solar Energy Materials and Solar Cells, volume 167 (2017), pages 109–120.
Go To Solar Energy Materials and Solar Cells