## 1 Introduction  
Energy requirements are increasing rapidly due to fast population growth. The burning of fossil fuels with emissions of harmful gases has brought severe damage to that environment and has caused global warming [1]. To solve the accelerating pollution from consumption of traditional fossil fuels, scientists are focusing on improving both the efficiency of utilization and researching renewable resources [2]. Solar, the source of most energy on earth, has almost infinite reserves and can be a promising alternative. However, a large part of solar energy is converted to other forms and wasted during transition. Moreover, solar is not effectively controllable and their exits a mismatch between availability and utilization [3]. Therefore, appropriate thermal energy storage (TES) systems are needed to conserve the waste energy. The use of TES in a solar system can promote overall efficiency, reduce investment, decrease running costs, and have a positive impact on the environment [4]. Excess solar can be stored in the daytime by TES devices, and then used at night. Typically, the forms of TES include: Sensible heat, latent heat, chemical reaction heat storage, and latent heat storage (LHS). The LHS system is considered to be the most promising method with providing huge potential for the storage of solar energy [5]. Compared with sensible heat storage (SHS), the process of LHS saving and releasing thermal energy is based on the heat of solidification and fusion when the medium undergoes phase transformation [6]. The medium is called phase change materials (PCMs) and the heat during phase change is latent heat. The PCMs usually have constant melting and crystallization temperatures and offer large energy storage density within a limited temperature change, which has made PCMs applied in various fields, such as building, refrigeration, textile, automobile, space, and food industry [7; 8; 9; 10; 11; 12; 13; 14]. Paraffin, with the chemical formula of \(\mathrm{C_{n}H_{2n+2}}\), possesses the traits of high latent heat, low supercooling degree, and is non-corrosive. Moreover, the melting point and latent heat increase with the increasing value of carbon atom number, which make paraffin well known as an organic PCM. In most instances, paraffin used for solar TES is required to have a melting point between 50 and 60 \({}^{\circ}\)C [15]. Considering economic benefits and safety, thermal conductivity, density, degradation, volume change during phase transformation, toxicity, and cost are all important performance factors [16]. Microcrystalline wax meets most of the above performance factors, and the volume change that occurs as the working temperature increases can be completely controlled by microencapsulation techniques, which also prevents leakage and enlarges the heat transfer area [17]. [PERSON] et al. used a sol-gel method to synthesize paraffin/SiO\({}_{2}\) microcapsules, which possessed the highest capsulation ratio of 78.1% and a latent heat of 156.0 J/g [18]. [PERSON] et al. prepared paraffin/SiO\({}_{2}\) microcapsules by in-situ polymerization with a melting temperature of 56.5 \({}^{\circ}\)C and enthalpy of 45.5 J/g [19]. [PERSON] et al. investigated novel paraffin/polyurethane microcapsules prepared by interfacial polymerization with isophorone diisocyanate and ethylene diamine and demonstrated that the latent heat could reach 95.5 J/g when the core/shell ratio was 75/25 [20]. Moreover, physical methods, including spray drying and solvent evaporation, also can be used in microencapsulation [21; 22]. The shell is another important component of microcapsules. Urea-formaldehyde stands out from numerous shell materials due to it being a cheap raw material, convenient in regards to preperation preparation, having low toxicity, and good thermal conductivity [23]. [PERSON] et al. summed up the methods of fabricating and designing confined microenvironments and the relevant applications after the first publications of [PERSON] and [PERSON] on polymer vesicles [24]. [PERSON] et al.