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• The hysteresis degree of DSP is decreased after encapsulation. • The reason for the hysteresis is the difference in the molecular structure during the heating and cooling process. • The system energy of encapsulated DSP is mainly decided by "randomly distributed" molecules. • Molecular dynamics simulation of pure DSP and encapsulated DSP. There is a phenomenon of inconsistent melting and solidification behavior of PCM, which is called phase change hysteresis, generally expressed by the difference between melting temperature (T m) and solidification temperature (T s). The hysteresis degree (T m -T s) is proportional to the energy loss of the system, if the temperature is beyond the operating temperature range of the system, the latent heat of the PCM cannot be exploited. The phase change hysteresis and supercooling were regarded as the same concept in previous studies no matter simulations or experimental research, which will lead to large errors. It is believed that there is a large hysteresis degree of inorganic PCM besides higher supercooling degree, poor chemical stability, and serious phase separation phenomenon, like Na 2 HPO 4 ·12H 2 O(DSP), which is prone to dehydrate from Na 2 HPO 4 ·12H 2 O to Na 2 HPO 4 ·7H 2 O during the heating and cooling process.DSP is adsorbed by SiO 2 via hydrogen bond, van der Waals force, and capillary effect, which can solve the above shortcomings. There are certain limitations in studying the hysteresis properties of PCM via experimental methods. As so far, the hysteresis properties of inorganic hydrated salts are rarely discussed in related papers, the reason for the hysteresis phenomenon is not explained, and the nucleation mechanism is not clear. Therefore, a composite phase change material (CPCM) model with DSP as the core and mesoporous silica MCM-41 as the shell was established in this work. The phase change hysteresis (PCH) characteristics are studied by analyzing the radial distribution function (RDF), self-diffusion coefficient(D), and density. The molecular motion of the interface layer was studied by the molecular dynamics(MD) method, and the arrangement of microscopic particles before and after adsorption was compared. The results showed that the hysteresis degree of DSP decreases to 1.5 °C after encapsulation by SiO 2 , which is because the effect of "Near-range ordered" molecules tend to disappear. After experimental verification, there is a 20% error between the simulation and experimental results. In addition, the reason for the hysteresis is the difference in the molecular structure that determines the energy of the system during the heating and cooling process. The purpose of this paper is to explain the reasons for the hysteresis from the view of microscopic and to provide a basis for the design of LHTES systems. [ABSTRACT FROM AUTHOR]

[Display omitted] • The hysteresis is due to the difference of primary nucleation mechanism during heating and cooling. • The PCH is related to the properties of the material itself and the experimental measurement conditions. • The hysteresis temperature is positively correlated with the heating/cooling rate. • The hysteresis effect is more obvious in an incomplete phase transition. • Numerical models of the hysteresis effect of complete and incomplete phase transition are summarized. As an efficient energy storage material, phase change material can be combined with lightweight buildings to reduce the energy consumption of the building envelopes effectively, while it is often accompanied by phase change hysteresis(PCH) and supercoiling (SC) phenomena during the phase transition. SC means that the nucleation does not solidify at the theoretical solidification temperature and needs to be nucleated at a lower temperature. PCH indicates the difference between the melting temperature and the solidification temperature of the phase change materials (PCMs). So far, a lot of research on SC has been done and there is still a lack of research on PCH. On the one hand, the PCH is proportional to the energy loss of the system, the latent heat of PCMs cannot be exploited if PCH is beyond the operating temperature range of the system. On the other hand, when PCMs are combined with building envelopes, the greater the hysteresis temperature, the better the thermal performance of the wall. It's vital to use the hysteresis temperature to improve the performance of the system. Especially, PCMs are equipped in building envelopes, the hysteresis phenomenon of incomplete phase transition will be more obvious because of large temperature fluctuation outside. The hysteresis effect of PCMs in building envelopes is discussed in this paper, the numerical simulation methods and experimental research of PCH are reviewed and analyzed from two aspects of complete and incomplete phase transition. It showed that the PCH should be considered when simulating the thermal behavior of PCMs in building envelopes. The results would be more accurate if two enthalpy-temperature curves (H(T) curve) are used to represent the melting and solidification process, respectively. In addition, the nucleation mechanism of PCH has not been discussed in previous articles and there is no explanation about whether the hysteresis phenomenon is caused by the properties of the material itself or is affected by experimental measurement conditions. Therefore, the related issues are discussed in this paper, it is considered that the PCH is not only related to the properties of the materials but also affected by the experimental measurement conditions. [ABSTRACT FROM AUTHOR]

Adsorption desalination and cooling is a promising technology that has recently been investigated. In this study, a silica gel powder composite (SGPC) with nano-copper (NC) and polyvinyl pyrrolidone (PVP) was synthesized by mechanical grinding and melt blending for the first time. It can improve the thermal conductivity of silica gel powder (SGP) and the operation efficiency of the adsorption desalination and cooling system (ADCS). By measuring the curve of temperature variation to compare the four thermal conductivity enhancers (TCE), graphite powder (GP), carbon nanotubes (CNT), nano-alumina (NA) and NC, it was found that NC had the best thermal conductivity enhancement. The microstructure, adsorption and thermal properties of the SGPC samples were systematically characterized by scanning electron microscopy (SEM), thermogravimetric analyzer (TGA) and thermal constant analyzer. The results showed that the adsorption capacity of SGPC with NC content of 0–1.6 wt% was in the range of 0.29–0.33 g −1. The thermal conductivity of SGPC with NC content of 0.8wt%, 1.2 wt%, and 1.6 wt% was found to be improved by 114%, 156% and 185% compared to the parent SGP, and the thermal conductivity was effectively enhanced. Thus, the developed adsorbent has eximious water adsorbability and thermal conductivity and has good adsorption/desorption cycle stability, which is attractive for ADCS. [ABSTRACT FROM AUTHOR]