World's highest heat storage density achieved in first ever water-based heat storage material — Developed by Mitsubishi and Science Tokyo

Mitsubishi Electric Corporation, and a research group including Professor Teruaki Hayakawa of the Department of Materials Science and Engineering at the Institute of Science Tokyo, announced that they developed a heat storage material primarily composed of water that exhibits the world's highest heat storage density. Utilizing a thermosensitive polymer gel newly developed, the material can store heat at low temperatures of 30℃-60℃, demonstrating the world's highest heat storage density of 562 kJ/L. The developed heat storage material is effective in recovering and reusing low-temperature waste heat from factories, automobiles, and other sources that has previously been exhausted into atmosphere. The material is expected to contribute to realizing a carbon neutral society through energy conservation and decarbonization. The results were presented at the 33rd Polymer Materials Forum of the Society of Polymer Science, Japan, held at Miyako Messe in Kyoto City on November 14 and 15.

The development and promotion of new energy-saving technologies are currently underway to realize a carbon neutral society that is the government's policy. To effectively recover and reuse waste heat, inexpensive heat storage materials are required that can store low-temperature waste heat, especially below 80℃, at a high density of 333 kJ/L (latent heat of melting water) or higher. In conventional heat storage materials, however, as the heat storage temperature decreases, the heat storage density decreases, making it difficult to enhance the performance of heat storage materials.

The researchers developed a heat storage material that uses a new thermosensitive polymer gel composed primarily of inexpensive water. The material can store heat at high density even at low temperatures by introducing the macromolecular crowding effect that refers to intracellular environments. Mitsubishi Electric employed its molecular simulation technology for molecular structure design of a thermosensitive polymer gel. The Institute of Science Tokyo with its advanced hierarchical polymer synthesis technology, designed chemical reaction in synthesis of a thermosensitive polymer gel. Meanwhile the company synthesized and characterized the polymer gels. The two parties have been conducting joint research since 2016.

It was confirmed that a coupled reaction was performed combining a polymer structural phase transition reaction of changing hydrophilic to hydrophobic states, with a hydrogen-bond rearrangement reaction of water in a thermosensitive polymer gel. They also confirmed that a hydrogen-bond network was disordered, by a macromolecular crowding environment derived from the coupled reaction, increasing enthalpy of the reaction and density of the heat storage. With increasing temperature, the thermosensitive polymer gel is transformed swollen to shrunk structures inducing a macromolecular crowding environment, in which the hydrogen-bond in water is unstably rearranged to increase enthalpy of the rearrangement reaction, leading to high-density heat storage even at low temperatures.

It is thus demonstrated that the world's highest heat-storage density is achieved at low heat storage temperatures of below 60℃, more than twice that of conventional commercial products. The researchers succeeded in large-scale synthesis of the thermosensitive polymer gel for a pre-production that is almost the same quality as a laboratory prototype. The developed thermosensitive polymer gel is composed of safe and inexpensive water (comprising 60 to 90 %) and a nontoxic, nonflammable polymer, containing no substances in conflict with the Law for Promotion of Chemical Management.