Micro-encapsulated paraffin in phase-change slurries (original) (raw)
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Journal of Thermal Analysis and Calorimetry, 2020
In modern heat transfer systems, thermal storage not only causes the balance between demand and supply, but also improves the heat transfer efficiency in these systems. In the present study, a comprehensive review of the applications of micro-or nano-encapsulated phase change slurries (MPCMs/NPCMs), as well as their effects on thermal storage and heat transfer enhancement, has been conducted. MPCMs/NPCMs have a myriad of applications and various usages such as pipe and channel flows, photovoltaic/thermal, solar heaters, air conditioning systems, storage tanks and heat pipes that have been categorized and studied. It was found that there are many advantageous adding MPCM/NPCM to the base fluid. The most important effect is that the addition of PCMs to the base fluid can intensify the capacity of energy absorption in the base fluid. These materials can absorb a high proportion of received energy by changing their phase and prevent temperature increment of the base fluid. Thereupon, the specific heat of the fluid in the presence of the micro-/nano-capsules increases. Moreover, in most studies reviewed, heat transfer coefficient and Nusselt number increase by the addition of micro-/nano-capsules to the base fluid. Also, the addition of MPCM/NPCM to the base fluid could make this material pumpable, although increment in the concentration of micro-/nano-capsules raises the viscosity of the working fluid and thereupon the pumping power. On the other hand, for a same heat load, the pumping power decreases due to the lower required flow rate in comparison with pure working fluid. The most important factor that must be considered in the application of MPCMs/NPCMs is the complete phase change of the material. Given the favorable thermal and fluid characteristics of MPCMs/NPCMs, the utilization of these materials could be a promising method to transfer heat and store it with high efficiency and low pumping power.
Thermophysical Properties Characterization of Microencapsulated Phase Change Material Slurry
Current chilled water systems require vast amount of water and pumping power to meet increasing cooling demands. Existing cooling and heating distribution systems have an inherent thermal capacity limitation (e.g., specific heat, mass flow rate, delta T), which is often neglected when adding new buildings to military, industrial or commercial facilities, resulting in higher equipment and infrastructure costs. Through the use of an advanced material concept, namely Microencapsulated Phase Change Materials (MPCM), performance enhancement of an improved heat transfer fluid is now being pursued. This paper discusses the status of experimental efforts using a linear alkane phase change material intended for a secondary coolant for space cooling applications. Initial quantitative characterization of MPCM material properties including latent heat of fusion, melting and freezing points, and temperature-and concentration-dependent viscosity data are presented. State-of-the-art equipment was used to characterize the MPCM slurry including the use of a differential scanning calorimeter and a temperature-controlled concentric viscometer. Results indicate that the freezing and melting points of microencapsulated n-Tetradecane differed by 5° C or more when no effective nucleating agent was used. Current efforts have yielded the identification of a very effective nucleating agent, which can suppress supercooling almost entirely. Other experimental results indicate that MPCM slurry viscosity significantly depends not only on volume fraction but also on temperature, which can have an impact on the heat transfer process. MPCM slurry has the potential to become a successful heat transfer fluid, which may result in significant energy and cost savings.
Experimental study on melting/solidification characteristics of a paraffin as PCM
Energy Conversion and Management, 2007
An experimental study is conducted in order to investigate the melting and solidification processes of paraffin as a phase change material (PCM) in a tube in shell heat exchanger system. The motivation of this study is to design and construct a novel storage unit responding to the melting/solidification characteristics of the paraffin. The PCM is stored in the vertical annular space between an inner tube through which the heat transfer fluid (HTF), here water, is flowing and a concentrically placed outer shell. This study focuses on the possibility of the heat transfer enhancement in the heat storage geometry. Enhancement is achieved by tilting the outer surface of the storage container, i.e. the surface of the outer shell with a tilting angle of 5°. The paraffin (P1) is used as the PCM because of its low cost, high energy storage density and large scale availability. At first, the thermophysical properties of the paraffin used are determined through differential scanning calorimeter (DSC) analysis. A series of experiments are conducted to investigate the effect of increasing the inlet temperature and the mass flow rate of the HTF both on the charging and discharging processes (i.e. melting and solidification) of the PCM.
A review on effect of phase change material encapsulation on the thermal performance of a system
Renewable and Sustainable Energy …, 2012
This paper presents a detailed review of effect of phase change material (PCM) encapsulation on the performance of a thermal energy storage system (TESS). The key encapsulation parameters, namely, encapsulation size, shell thickness, shell material and encapsulation geometry have been investigated thoroughly. It was observed that the core-to-coating ratio plays an important role in deciding the thermal and structural stability of the encapsulated PCM. An increased core-to-coating ratio results in a weak encapsulation, whereas, the amount of PCM and hence the heat storage capacity decreases with a decreased core-to-coating ratio. Thermal conductivity of shell material found to have a significant influence on the heat exchange between the PCM and heat transfer fluid (HTF). This paper also reviews the solidification and melting characteristics of the PCM and the effect of various encapsulation parameters on the phase change behavior. It was observed that a higher thermal conductivity of shell material, a lower shell size and high temperature of HTF results in rapid melting of the encapsulated PCM. Conduction and natural convection found to be dominant during solidification and melt processes, respectively. A significant enhancement in heat transfer was observed with microencapsulated phase change slurry (MPCS) due to direct surface contact between the encapsulated PCM and the HTF. It was reported that the pressure drop and viscosity increases substantially with increase in volumetric concentration of the microcapsules.
ANALYSIS OF PCM SLURRIES AND PCM EMULSIONS AS HEAT TRANSFER FLUIDS
Microencapsulated Phase Change Materials suspensions and PCM emulsions have been developed during the last 10 years to be utilized as heat transfer fluids. The prime interest in these fluids is the rise of the heat capacity associated to the phase change, with a consequent pumping power reduction as well as a higher heat transfer versus pressure drop ratio. However, several authors present in their experimental researches heat transfer coefficients that are lower than the heat transfer coefficient for water. This heat transfer coefficient is influenced by several factors, and mainly, by the fluid turbulence level and the PCM concentration. Due to such controversial information, an experimental study is reasonable. The objective of the present study is to make an exhaustive analysis of the recent bibliography to enable a decision on how to approach future researches in a strategic way.
Experimental study of the thermal characteristics of phase change slurries for active cooling
Applied Energy, 2012
Phase change materials (PCMs) are increasingly being used for thermal energy storage in buildings and industry to produce energy savings and reduce carbon dioxide emissions. PCM slurries are also being investigated for active thermal energy storage or as alternatives to conventional single phase fluids because they are pumpable and have advanced heat transport performance with phase change. The present study investigates several types of phase change materials for the preparation of PCM slurries which have potential for cooling applications. The thermophysical properties of paraffin in water emulsions, such as latent heat of fusion, melting and freezing temperature ranges, viscosity and the effect of surfactants, have been tested using appropriate experimental techniques. It has been identified that the use of small quantities of higher melting temperature paraffin and surfactants in the emulsion can reduce the effect of supercooling and increase the useful heat of fusion. However there are negative impacts on viscosity which should be considered in heat transport applications.
A microencapsulated phase change material (MPCM) slurry has been formulated as a high heat capacity heat transfer fluid (HTF) for turbulent flow conditions. The MPCM slurry consists of du-rable 5-μm microcapsules. The phase change material inside the microcapsules consists of methyl stearate. Heat transfer and fluid flow properties of the aqueous MPCM slurry in turbulent flow (3000 < Re < 6000) were determined experimentally. Heat transfer data were obtained under con-stant heat flux conditions using a closed heat transfer loop. The experimental system was used to determine effective heat transfer coefficient and pressure drop with and without phase change oc-curring inside the microcapsules. Dynamic viscosity data suggest that at relatively low mass con-centrations the slurry behaves as a Newtonian fluid. The pressure drop data under turbulent flow conditions were also measured at MPCM concentrations of 7 and 11% by weight. It was found that whenever the MPCM experienced a ph...