# Thermal Energy Storage Performance of Tetrabutylammonium Acrylate Hydrate as Phase Change Materials
## 4 Conclusions  
Kinetic characteristics of thermal energy storage using tetrabutylammonium acrylate (TBAAc) hydrate were practically and experimentally evaluated for practical use as Phase Change Materials (PCMs). During the experiments, the adhesion of hydrate on the surface of the heat exchanger coil was found, which increased the thermal resistance between the heat exchanger and thermal energy storage medium. This was why, as the external forces, we added a mechanical agitation (rotation rate of 100 rpm, 300 rpm, and 600 rpm) and ultrasonic vibration (frequency of 28 kHz and 56 kHz) on each system. It was revealed that the external forces improved the thermal energy storage kinetic characteristics. When the agitation rate was 600 rpm, the system achieved thermal energy storage of 140 MJ/m\({}^{3}\) in 2.9 h. This value is comparable to the ideal performance of ice thermal energy storage when its solid phase fraction is 45%. The energy consumption for agitation was 28 MJ/m\({}^{3}\) to achieve this value. The thermal energy storage of 100 MJ/m\({}^{3}\), which is a TBAAc solid phase fraction of 50%, was achieved in 1.5 h and 5.8 h with mechanical agitation (600 rpm) and ultrasonic vibration (28 kHz), respectively. The energy consumptions to achieve this target value were 15 MJ/m\({}^{3}\) and 5.2 MJ/m\({}^{3}\), respectively. In summary, the agitation increased the rate of heat and mass transfer by creating forced convection, thereby shortening the hydrate formation time and increasing the growth rate of the hydrate. In addition, ultrasonic vibration could keep the hydrate growth time longer by the effect of cavitation.  
In this system, the UA/V (U: thermal transfer coefficient, A: surface area of the heat exchange coil, V: volume of the thermal energy storage medium) was approximately 1.0 \(\times\) 10\({}^{-4}\)-10\({}^{-3}\) W m\({}^{-3}\) K. This UA/V of the heat exchanger was comparable to that of other common heat exchangers. Even if the system would be scaled up and different from this study, the same level of thermal energy storage as this study could be obtained by using a heat exchanger of which UA/V is comparable to this study. As for the ultrasonic vibration system, thermal energy storage density was one-tenth of the agitated system. A device that enables an oscillator to resonate and break the hydrate adhesion is necessary to increase the thermal energy storage density. Then, that system would be most favorable in terms of the storing performance and energy power consumption.  
Conceptualization, H.K., H.T. and R.O.; methodology, H.T., S.I., H.N. and R.O.; formal analysis, H.T., S.I., H.N. and R.O.; investigation, H.K., H.T., S.I., H.N. and R.O.; resources, H.T. and R.O.; data curation, H.K., H.T., S.I., H.N. and R.O.; writing--original draft preparation, H.K. and R.O.; writing--review and editing, H.K. and R.O.; visualization, H.T. and R.O.; supervision, R.O.; project administration, R.O.; funding acquisition, R.O. All authors have read and agreed to the published version of the manuscript.  
This work has been supported by a Keirin-racing-based research promotion fund from the JKA Foundation (Grant Number: 2020M-195) and a part of Low Carbon Technology Research and Development Program for \"Practical Study on Energy Management to Reduce CO\({}_{2}\) emissions from University Campuses\" from the Ministry of the Environment, Japan.  
Not applicable.