Cold storage is a strategy for energy storage and utilization that capitalizes on low-demand periods of cost-effective energy to transfer heat into a storage medium. This stored heat can then be released during high-demand periods to address increased energy consumption requirements [1]. This approach is frequently integrated with cooling systems used for various applications such as building air conditioning, urban cooling, data center temperature management, and industrial processes [2]. The objective is to realize energy efficiency by harnessing clean energy sources or utilizing off-peak affordable energy for thermal storage purposes [3]. On the other hand, the increasing disparity between electricity production and consumption on the supply and user sides has led to a growing imbalance in electricity load, resulting in a triple price difference between peak and off-peak electricity prices in China. In developed countries, the peak-to-off-peak electricity price ratio reach 8–10 times. Public buildings over 3000 square meters in Korea must be equipped with energy storage air conditioning systems [4,5]. In order to address this issue and facilitate load shifting for the power grid, thermal energy storage (TES) technology has become crucial in reducing electricity consumption for air conditioning during peak electricity consumption period [6,7]. Currently, the primary methods of TES for air conditioning include water storage [8,9], ice storage [10,11], and eutectic salt storage [12,13]. Water storage involves sensible heat storage but has a low energy storage density (7–11.6 kW·h/m³) and requires a larger area [14,15]. Ice storage, on the other hand, utilizes latent heat storage and boasts a high cold storage density (45–50 kW·h/m³). However, the low temperature at which ice is stored (−7 to −5 °C) necessitates a low evaporating temperature for the refrigeration unit, leading to high energy consumption in the refrigeration system [16,17]. Eutectic salt storage, another form of latent heat storage, exhibits a relatively lower cold storage density (20.8 kW·h/m³) and encounters challenges such as material degradation and corrosion [18,19]. Given the limitations of traditional TES mediums that impede the development of TES technology, there is an urgent need to develop new TES materials with strong energy-saving applicability [20,21]. Novel TES materials are required to overcome these challenges and pave the way for the advancement of TES technology.

Hydrates are crystalline compounds formed when water molecules interact with solute molecules by hydrogen bonding under temperature and pressure [[22], [23], [24]]. These compounds possess a suitable phase change temperature, which aligns well with the cooling temperature range of air conditioning systems [25,26]. Moreover, hydrates exhibit a high latent heat of phase change comparable to that of ice, making them a highly promising medium for cold storage [27,28]. Hydrate-based phase change thermal storage technology offers several advantages. Firstly, the release or absorption of latent heat energy during hydrate phase change surpasses that of traditional cold storage materials, resulting in improved cold storage performance. Secondly, hydrate phase change is a reversible process, enabling the cyclic use of the cold storage system and enhancing its sustainability and economic viability. Furthermore, hydrate-based phase change thermal storage systems have minimal environmental impact as they do not involve the use or release of hazardous substances. This characteristic aligns with the requirements of environmental protection and sustainable development. Despite the significant potential of hydrate-based phase change thermal storage technology in energy conservation and environmental protection, it still faces certain challenges. One of these challenges pertains to the nucleation and stability of hydrates, necessitating the search for or design of suitable hydrate materials that ensure stability and reversibility during the phase change process. Additionally, further research and optimization are required to improve the understanding of heat transfer and control mechanisms in hydrate-based phase change thermal storage systems. This will enhance energy conversion efficiency and system stability. Currently, scientists are studying and applying hydrate-based phase change energy storage systems as ideal solutions for phase change energy storage. Their application in cold storage has the potential to significantly improve the electricity load of the power grid system, playing a crucial role in achieving peak emissions targets. Simultaneously, the incorporation of thermal storage systems enables the exploitation of disparities in electricity rates, leading to operational cost reduction.

The process of hydrate nucleation entails the aggregation of solute molecules in water, leading to the formation of small clusters that subsequently grow and transform into stable crystal structures [29]. However, despite progress in understanding hydrate nucleation, many questions remain unanswered, particularly concerning nucleation behavior at different reaction scales and its correlation with solute properties. Thus, conducting further research on the fundamental principles and mechanisms of hydrate nucleation is crucial for addressing practical issues and challenges that arise in this field.

On the other hand, the nucleation and growth of hydrates are influenced by various factors, including undercooling [[30], [31], [32]], promoters [33,34], and additives [[35], [36], [37]]. Based on that numerous researchers have dedicated their efforts to studying hydrate nucleation and growth. Among the hydrates, TBAB hydrate has shown excellent properties and is well-suited for application in cold storage systems at atmospheric pressure. As a result, there has been a proliferation of studies exploring the nucleation and growth properties of hydrates in different TBAB-containing systems. Iwai et al. [38] investigated the crystallization and heat transfer characteristics of TBAB hydrate and discovered that under a reaction volume of 500 ml, TBAB hydrate has been studied and found to form two different forms of hydrate under different conditions and type A TBAB hydrate crystals could only form when the TBAB mass concentration exceeded 22.5 %. When the temperature was significantly lower than the equilibrium temperature of Type B TBAB hydrate, the subcooling led to morphological changes in hydrate formation by affecting the temperature difference driving force. Wang et al. [20] focused on the growth characteristics of 40 % TBAB hydrates and observed that pure TBAB hydrates exhibit a columnar shape, which varies with undercooling. The aforementioned studies primarily concentrated on the nucleation and growth characteristics of hydrates in small-scale spaces. In such confined spaces, the nucleation and growth behaviors of hydrates are influenced by factors like subcooling and gas-liquid interfaces, leading to distinctive changes. The limited space hinders gas-liquid contact and restricts the formation of a gas-liquid interface, resulting in the formation of a hydrate film that impedes further nucleation and growth. However, in pilot-scale spaces, hydrates have a higher probability of nucleation, which accelerates their growth. Consequently, the nucleation, growth, and dissociation characteristics of hydrates are subject to alterations influenced by the scale of the space. At the same time, at the laboratory scale, there is a lack of research reports on the investigation of cycling stability, which is essential for ensuring the reliability, durability, and controllability of hydrates in multiple cycles. Stable cycling properties are crucial for enabling sustainable and efficient applications of hydrates.

Furthermore, low thermal conductivity of hydrated materials is also an important factor to limit hydrate formation, the investigation of heat transfer mechanisms on hydrates is of paramount importance, and researchers have extensively studied the impact of heat transfer characteristics on the intrinsic behavior of hydrates. In laboratory-scale conditions, the growth rate of hydrates be influenced by specific subcooling conditions. External heat transfer is typically employed for cooling hydrates in such settings. Wang et al. [26] conducted a study on TBAB hydrates in a reaction volume of 18 L. They observed that under a subcooling of 7 °C, the induction time for hydrate formation during an 8 h formation process was 100 min at a temperature of 5 °C. These findings highlight the impact of subcooling conditions on the formation and stability of hydrates in laboratory settings. Moreover, Ma et al. [39] conducted a study on the heat transfer characteristics of TBAB hydrate slurry with volume fractions ranging from 0 to 20 % flowing through copper pipes with inner diameters of 6 mm and 14 mm (with a wall thickness of 1 mm). The results demonstrated that the local heat transfer coefficient initially decreases within the slurry region until the volume fraction reaches a certain small value. Afterward, the local heat transfer coefficient increases with increasing Reynolds number. These findings align with the observations made by Kumano et al. [40]. In addition, Ma et al. [41] explored the effect of heat transfer during the formation and decomposition processes of TBAB hydrate by altering the arrangement of the heat exchanger tubes, owing to the diverse heat transfer modes influencing hydrate formation. They observed that the subcooling state of the TBAB water solution had an insignificant impact on the flow and heat transfer characteristics. Additionally, during hydrate formation, the heat transfer coefficient was lower than 800 W/m²·K for Type B hydrate formation and lower than 400 W/m²·K for Type A hydrate formation due to the adherence of hydrate crystals to the heat transfer surface. In conclusion, while researchers have made significant efforts to investigate the influence of heat transfer characteristics on the formation and decomposition processes of TBAB hydrates, these studies have primarily focused on small-scale spaces. Therefore, it is crucial to explore the nucleation and growth patterns of hydrates, non-direct heat transfer, and the impact of heat transfer mechanisms in pilot-scale spaces.

Based on the aforementioned context, this study presents an innovative approach by developing a 4000 L hydrate phase change experimental system. The primary objective is to comprehensively investigate the nucleation, growth characteristics, and accumulation regularity of TBAB hydrates in a pilot-scale space. The research aims to analyze the impact of different degrees of subcooling on TBAB hydrate formation during the formation process while exploring the mechanisms underlying the influence of the cyclic formation process on hydrate growth characteristics. Furthermore, the study delves into the time-varying nature of heat transfer during hydrate formation and decomposition processes, providing insights into the intrinsic heat transfer phenomena during hydrate formation.