Effective handling of reusable waste heat is one of the important fundamental technologies for reducing energy consumption as well as promoting energy conservation. Among possible ways for handling waste heat, thermal energy storage is the key technology in the thermal energy management. Basically, there are two major classes of materials for achieving thermal energy storage, which are known as phase change materials (PCMs) and chemical heat storage materials. Although some of them are familiar to our daily life and are widely used in various social systems, detailed molecular mechanism of thermal energy storage remains elusive at present. In the TherMAT project, we are now investigating the basic mechanism of thermal energy storage for both material systems based on the predictive computer simulations. As for PCM, sugar alcohols are known as one of the promising candidate for achieving large amount of thermal energy storage. Based on the molecular and crystal structures of known sugar alcohols, we have computationally designed and predicted a new organic molecular material which can achieve larger amount of thermal energy storage than ever known. At first step, we carefully analyzed molecular properties of known C4, C5 and C6 sugar alcohols based on the classical MD simulations. Then we have clarified the molecular factors that control physical properties, such as melting point and latent heat. On the basis of these detailed analyses, we proposed molecular design guidelines to achieve effective thermal energy storage; 1) linear elongation of carbon backbone, 2) separated distribution of OH groups, and 3) even numbers of carbon atoms inside the carbon skeleton. Our computational results clearly demonstrated that if we carefully designed molecular structures, non-natural sugar alcohols have potential ability to achieve thermal storage density up to 450-500 kJ/kg, which is larger than the best value of the present known organic PCMs (~350 kJ/kg).
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