In recent years, energy conservation and environmental protection have become most important issues for humanity. Phase change materials (PCMs) for thermal energy storage can solve the issues of energy and environment to a certain extent, as PCMs can increase the efficiency and sustainability of energy. PCMs possess large latent heat, and they store and release energy at a constant temperature during the phase change process. Thereby PCMs have gained a wide range of applications in various fields, such as buildings, solar energy systems, power systems and military industry. However, low thermal conductivity of PCMs leads to low heat transfer rate, thus, numerous studies have been carried out to improve thermal conductivity of PCMs. The main purpose of this paper is to review the methods for enhancing thermal conductivity of PCMs, which include adding additives with high thermal conductivity and encapsulating phase change materials. It is found that addition of thermal conductivity enhancement fillers is a more effective method to improve thermal conductivity of PCMs, where carbon-based material additives possess a more promising application prospect. Finally, the applications of PCMs in solar energy system, buildings, cooling system, textiles and heat recovery system are also analyzed.

Review on thermal conductivity enhancement, thermal properties and applications of phase change materials in thermal energy storage

A review on macro-encapsulated phase change material for building envelope applications

Modelling and experimental study of latent heat thermal energy storage with encapsulated PCMs for solar thermal applications

To date, numerous phase change materials (PCM) have been developed for application in latent heat storage systems. There are many issues in the process from the development of PCM to using them in storage systems, which should be resolved. The problem of heat transfer in PCMs during the phase change process is the most important one. Latent heat storage containers usually have simple geometrical forms such as a sphere, cylinder, cylindrical annulus, rectangular enclosure, etc. A large number of papers were published on melting and solidification processes in PCMs. Therefore, there is a pressing necessity for generalizing the art of the state in this field and establish how accumulated knowledge meets practical requirements. The present review considers the current state in investigations of heat transfer in a spherical shell. Heat transfer in PCMs during constrained melting (solid PCM fixed inside the vessel), unconstrained (unfixed) melting and solidification, and phase change in finned shells are analyzed. It is shown that currently, there is no satisfactory description of the constrained melting process. For unconstrained melting and solidification, some correlations are suggested, describing these processes. The applicability range of the proposed correlations, as well as their accuracy were investigated and established. To intensify the process of phase change inside the spherical container, the use of orthogonal fins is appropriate option compared to the employ of circumferential fins.

Melting and solidification of PCMs inside a spherical capsule: A critical review

Low thermal conductivity is the main drawback of phase change materials (PCMs) that is yet to be fully addressed. This paper studies several efficient, cost-effective, and easy-to-use experimental techniques to enhance thermal conductivity of an organic phase change material used for low-temperature thermal energy storage applications. In such applications, the challenges associated with low thermal conductivity of such organic PCMs are even more pronounced. In this investigation, polyethylene glycol (PEG-1000) is used as PCM. To improve the thermal conductivity of the selected PCM, three techniques including addition of carbon powder, and application of aluminum and graphite fins, are utilized. For measurement of thermal conductivity, two experimental methods—including flat and cylindrical configurations—are devised and increments in thermal conductivity are calculated. Melting and solidification processes are analyzed to evaluate melting and solidification zones, and temperature ranges for melting and solidification processes respectively. Furthermore, latent heat of melting is computed under constant values of heat load. Ultimately, specific heat of the PCM in solid state is measured by calorimetry method considering water and methanol as calorimeter fluids. Based on the results, the fin stack can enhance the effective thermal conductivity by more than 40 times with aluminum fins and 33 times with carbon fins. For pure PCM sample, Initiation of melting takes place around 37 °C and continues to above 40 °C depending on input heat load; and solidification temperature range was found to be 33.6–34.9 °C. The investigation will provide a twofold pathway, one to enhance thermal conductivity of PCMs, and secondly ‘relatively easy to set-up’ methods to measure properties of pure and enhanced PCMs.

Wei Li

Papers

Experimental study on melting/solidification and thermal conductivity enhancement of phase change material inside a sphere