[a] In recent years, interest in the use of phase-change materials (PCMs) has gained momentum for thermal storage applications in the fields of energy conservation and renewable energy.[1] These PCMs typically have one or more phase transitions with a high enthalpy change at the transition temperature. Solid-liquid phase change materials have proven to be versatile and economically attractive for a number of energy storage applications.[2] Recently, it has been reported that blends of paraffins and fatty acids can be used as latent heat storage components in form-stable phase-change materials. [3] These materials offer a low volume change at the phase transition in contrast to traditional solid-liquid phase-change materials. However, the success of a compound in a PCM application relies on a set of desirable thermophysical properties that includes a high heat of fusion (DH f ), a phase transition temperature suited to the application, high thermal conductivity, no tendency to phase separate, good phase-change kinetics (little supercooling, sufficient crystallization rate), chemical stability, low toxicity, and low flammability. [1a, 4] PCMs can be classified as organic, inorganic, or eutectic materials. In the literature there have been a number of systematic studies of the properties of potential inorganic and organic PCMs. [1,5,6] For example, the DH f of organic paraffins has been shown to increase with carbon chain length, [4] varying from the C 14 paraffin, which melts at 5.5 8C with DH f = 228 J g
À1, to the C 34 paraffin, which melts at 75.9 8C with DH f = 269 J g À1 . These enthalpy changes are substantial and useful for a number of low-temperature applications (for example, domestic solar heating), but the low melting point of the materials limits higher-temperature concentrated solarthermal applications. The heats of fusion of various alcohols, fatty acids, and esters, which offer a range of higher melting points, have also been studied. [3,6] The sugar alcohols possess high DH f values (> 200 J g À1 ) with melting transitions in the 100-200 8C range, but they often supercool, which limits their application.[6] Also, organic PCMs are generally volatile and flammable and can cause considerable safety concerns in large-scale energy-storage applications.[7] Their thermal conductivities are also generally low. Inorganic phase-change materials such as salt hydrates [1a, 4] also possess high heats of fusion, in the range of 80-250 J g À1 . For example, KF·4 H 2 O has DH f = 231 J g À1 at the melting temperature of 20 8C, which makes it suitable for thermal energy storage in human comfort applications.[2] However, these hydrate PCMs often suffer from supercooling and phase separation, which can affect their thermal-storage capacity. Additionally, inorganic PCMs are in many cases corrosive. There have also been reports of organic and inorganic eutectic materials as potential energy-storage materials; for instance a mixture of trimethylolethane (38.5 wt %), water (31.5 wt %), and urea (30 wt %) has a DH ...