# Thermal Energy Storage Performance of Tetrabutylammonium Acrylate Hydrate as Phase Change Materials
## 1 Introduction  
The use of sustainable, renewable energy sources is becoming increasingly important as global attention on environmental issues increases. However, the amount of electricity generated by renewable energy sources such as wind and solar power fluctuates greatly depending on the natural environment. In recent years, energy consumption has been increasing. It is essential to have a technology to fill the gap between the amount of electricity generated and the demand. These technologies would contribute to the load leveling [1]. In particular, the recent development in information and communication technology (ICT) (e.g., Internet of Things, 5G communications, cloud computing, big data) has produced a great demand for data centers (DCs). In 2020, the rapid spread of remote working due to the pandemic also boosted demand for DCs. The industrial use of these technologies is also driving this trend [2, 3]. [PERSON] et al. [4] estimated that the electricity use in DCs accounted for 1% of worldwide electricity use in 2020. This trend will make DCs even more energy-intensive and will increase the cost of cooling the heat generated and the amount of electricity used [5]. Considering the generalization of ICT and the widespread use of the fifth-generation mobile communication system, the amount of communication will further increase worldwide. The importance of data centers will increase to support the exchange of huge amounts of data. A suitable cooling system will be able to keep the appropriate temperature. In addition, such systems are significant to realize the zero-downtime of a DC operation with low cost. DC cooling systems are classified into two types. One is an air-based cooling system [6; 7]. The other one is a liquid cooling system [8]. As for air-based system, the heat emitted from the computer is removed by air flow. The main drawback of this system is a low thermal conductivity, less than 0.03 W m\({}^{-1}\) K\({}^{-1}\)[9]. On the other hand, in a liquid cooling system, the heat is removed by the thermal conduction and convection into the liquid, mostly water. A liquid cooling system is inclined to be bigger and heavier because that system utilizes only sensible heat, which has low energy density. Thermal energy storage technology is used to store the heat energy with the heat capacity of substance. When the demand of the electricity is low, the surplus electricity is stored as cold energy. Then, the stored energy is changed to electricity when the demand increases, for example, daytime peak in power usage [10; 11]. Phase Change Materials (PCMs) can be a competitive system for thermal energy storage because they can also utilize latent heat, which has a higher energy density [12; 13; 14; 15; 16]. Organic compounds and water are representative examples of PCMs for DC cooling. As for organic PCMs, they often have problems in terms of safety. For example, paraffin is known as an organic PCM, but it is not safe because of its flammability [17]. Water is not suitable because its phase change point is outside the range of operation temperature in DCs, 288 K to 305 K [18]. Utilizing clathrate hydrate for a thermal energy storage medium has been proposed as a better idea than water or an ice-based medium [19; 20; 21; 22; 23]. Clathrate hydrates, ice-like compounds, are formed when some gases have contact with water or ice under high pressure and/or low temperature [24]. The gases encapsulated in clathrate hydrates are called guest compounds. Clathrate hydrates have enormous potential for industrial usage thanks to their thermodynamic properties [25; 26; 27; 28; 19; 22]. Considering the application for thermal energy storage medium, equilibrium temperature should be suitable for the DC operation temperature. Favorable equilibrium temperature can be obtained appropriately by selecting the guest compounds [29; 30; 31]. Until now, only tetrabutylammonium bromide (TBAB) hydrate has been commercially applied as a hydrate PCM for air conditioning.