[Thermal storage]
Design of Thermal storage material from the aspect of nucleation and their thermophysical properties

RESEARCH OVERVIEW

• In order to design materials that can efficiently store thermal energy, the Thermophysical Properties Measurements of Non-Equilibrium Molten Alloys for Design of Thermal Storage Material (ELF-Thermal Storage) investigation studies the heat energy storage and discharge processes, which are controlled by the mass transport properties and nucleation rates
• Using the Electrostatic Levitation Furnace (ELF), the solidification rate and thermophysical properties can accurately be measured according to the degree of supercooling, temperature, and sample composition under a gas atmosphere that prevents sample evaporation.
• Through this research, by clarifying the physical properties and nucleation rate in liquid alloys, there is the possibility to design materials for the practical use of the phase-separated liquid alloy as a thermal storage material.

DESCRIPTION

Using a differential thermal analyzer and simulation of the material design, the determination of the operating temperature has been performed on materials that can store thermal energy. In order to simulate the performance and appropriate design of these materials, knowledge of the nucleation rate and the thermophysical properties in the liquid state, according to the exact degree of supercooling, is indispensable. However, in ground experiments, heterogeneous nucleation on the liquid-container interface is promoted.

Additionally, the microstructure of the phase-separated alloy is made unstable by convection. The Research team has measured the thermophysical properties of molten alloys on the ground, but it is difficult to measure the viscosity. Moreover, it is considered that the nucleation from the molten state is promoted by the convection existing in the melt, and there are difficulties in obtaining the measurement of the nucleation rate and the viscosity measurement of the molten state.

In order to overcome these difficulties: 1) the actual homogeneous nucleation rate must be determined in an environment where convection is suppressed, 2) the melting that governs mass transfer phenomena such as density, viscosity, and surface tension, are required to be measured without contacting materials under microgravity, which can be achieved in space. Even when using levitation devices, the nucleation rate varies depending on the electromagnetic levitation device (EML) on the ground and the EML under microgravity. This inconsistency causes the nucleation to be promoted by the flow in the melt. Therefore, it is expected that an accurate nucleation rate as a function of supercooling can be obtained by conducting an investigation in the space environment, and using an electrostatic levitation device with a small flow in the melt.

Viscosity has an Arrhenius-type dependence on temperature, and its value changes exponentially with changes in temperature. In order to accurately evaluate the thermophysical properties of molten alloys, it is essential to evaluate the temperature dependence accurately over a wide temperature range. However, ground tests other than the floating method cannot be performed at high temperatures because the sample reacts with the container. Additionally, at temperatures below the melting point, solidification is promoted by the formation of heterogeneous nuclei, so the viscosity cannot be evaluated correctly.

On Earth, the viscosity is measured by the electrostatic levitation method as a non-contact method, but high vacuum conditions are necessary to measure the viscosity to prevent discharge. In a high vacuum condition, accurate viscosity measurements for liquid alloys are hindered because of the compositional change due to evaporation during the measurement. Therefore, it is necessary to make measurements in space where the sample can be levitated in a small electric field under a gaseous atmosphere. The heterogeneous nucleation is supported by the convection in the melt, so the measurement of the nucleation rate (which can suppress the convection) is important for modeling the crystal growth. This nucleation rate measurement can be accurately obtained in space experiments from the relationship between the cooling curve and the degree of supercooling, at which nucleation occurs.

The development of thermal storage materials that can store large amounts of energy at low cost is an important issue for achieving sustainable development goals. The primary research goal involves how much heat energy can be stored, and how to improve the heat exchange efficiency of porous materials. However, if a phase-separated alloy that separates into the low-temperature and high-temperature phase melting is applicable, it is expected that thermal energy can be efficiently stored by utilizing the heat of fusion of the low-temperature phase, while maintaining the structure by high-temperature phase.

In order to utilize the phase separate alloy for the thermal storage material of uniformly dispersed materials, it is essential to measure the physical properties of these materials and simulate the solidification. The Thermophysical Properties Measurements of Non-Equilibrium Molten Alloys for Design of Thermal Storage Material (ELF-Thermal Storage) investigation clarifies the physical properties and nucleation rate in the molten state of the material. Results from this investigation may make it possible to design materials for the practical use of the phase-separated molten alloy as a thermal energy storage material. Not only can this technology for producing energy storage materials in space help achieve Sustainable Development Goals on Earth, but it can also prove vital for the development of novel energy storage materials on future space stations.

SPACE APPLICATIONS

Development of materials that can store large amounts of thermal energy at low cost is important for future space exploration, enabling energy storage on space stations or bases on the Moon or Mars.

EARTH APPLICATIONS

Materials that can store thermal energy could enable the use of unstable sources of renewable energy and unused energy discharged as heat in factories on Earth.

OPERATIONAL REQUIREMENTS AND PROTOCOLS

The Electrostatic Levitation Furnace (ELF) instrument is assembled and installed in the Multi-Purpose Small Payload Rack (MSPR/MSPR2) in Kibo. After setup, the investigation is operated from ground control as required by investigators at Space Station Integration and Promotion Center (SSIPC), Tsukuba Space Center. The investigation procedure is as follows:

• A crew member prepares the investigation by inserting the Sample Holder into the Sample Cartridge, and inserting the Sample Cartridge into the ELF chamber. The ELF is then activated and configured for operation.
• To begin investigation operations, the sample is released into Experiment Volume by the Sample Release Rod. The Sample is charged, position controlled, heated, and melted using electrodes and Power Lasers. During operations, the sample is measured through sensors and cameras.
• At the completion of investigation operations, recorded video, pictures, and data are downlinked to Earth.
• The investigation is closed out by the deactivation of ELF.

KOBATAKE Hidekazu [Doshisha University]