# Hydrates for Cold Storage: Formation Characteristics, Stability, and Promoters
## 1 Introduction  
Energy shortage and environmental pollution are some of the most challenging issues worldwide. Cold storage is an important means of saving energy in industry. Cold storage critically impacts macro-control, rational management, utilization efficiency, pollutant emission control, effective load transmission, and effective energy use in industry. The internal mechanism and macro-distribution of working substances used for cold storage were investigated to impart properties that can be exploited to meet energy demands [1]. Depending on the temperature range, cold-storage technology can be utilized in air-conditioning, preservation of food and pharmaceuticals, industrial processes, peak shaving power stations, air separation plants, supercritical air energy storage systems, and low-temperature refrigerators [2]. China has been facing an unbalanced electricity market, with a large peak-valley load gap. The proportion of electricity consumed for air-conditioning exceeds 15%, being the main factor affecting these peak-valley charge differences [3]. Especially in summer, air conditioning electricity consumption accounts for 25-30% of the total electricity demand, where this demand reaches a maximum during the daytime peak, placing severe pressure on the power plants [4]. To effectively minimize the peak-valley power differences in an unbalanced grid supply, methods such as power peak shaving at the supply or user side can be adopted [3, 4, 5]. Air-conditioning and cold-storage technologies are important means of peak shaving on the user side, where the sensible or latent heat of a material is harnessed to store cold energy by using off-peak electricity and the energy is released when needed [1,6]. At present, common cold-storage methods are divided according to the storage media, which include water, ice, eutectic salts, and hydrates [7,8]. Among these, the first three have numerous shortcomings. For example, the cold-storage density of water is low, and large-area equipment is required. In contrast, the cold storage of ice is significantly denser than water, but has a low cooling efficiency and high energy consumption. Eutectic salts cause equipment corrosion and deterioration. In recent years, hydrates have received much attention as a new clean energy source, and hydrate-based technology is considered as a promising approach to solve energy and environmental problems, e.g., through global warming mitigation by capturing and storing carbon dioxide [9,10], and in heavy metal separation for wastewater treatment [11] and desalination [12]. Hydrates are also considered ideal working substances for cold storage due to the latent heat of gas hydrates being equivalent to that of ice, with a high phase-change temperature of 5-12 \({}^{\circ}\)C, high cold-storage density, and small specific volume, thereby overcoming disadvantages such as the low heat storage density, low storage temperature, and susceptibility to aging and failure of water, ice, and eutectic salts, respectively [6, 7, 8, 13,14]. Since the inceptive proposal of gas hydrates as a working substance for cold storage in the early 1980s, scientific and industrial interest worldwide has led to the further development of this concept. The working substance of a cold-storage system directly determines the capacity and efficiency of the system. Therefore, hydrates used as working substances for cold storage should be environmentally friendly, safe, and abundant, and present a high reaction heat, good thermodynamic properties, low subcooling degree, and high growth rate. Freon hydrate has attracted significant attention as a cold-storage working substance because of its properties. The temperature of formation of freon hydrate can be controlled at 5-12 \({}^{\circ}\)C, and the cold-storage density of this hydrate is approximately 302.4-464 kJ/kg [15]. However, many freon substances are restricted by the Kyoto Protocol because of their high global warming potential [16].