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3.5.2. Reviewofappliedenhancementmethods 3.5.2.1. Fins. Fins are being used in order to increase the heat trans- fer surface area with the storage media [97], therefore enhancing the rate at which thermal energy is distributed within it. They can be axial or radial and are usually attached to tubes [98]. Depend- ing on their shape and their configuration inside the storage tank with the storage media, this mechanism has different results. For example, according to Zhang and Faghri [99], for internally finned tubes, the melting fraction volume can be significantly increased by adding more fins and increasing their thickness and height. Castell et al. [98] experimented adding longitudinal graphite fins to PCMs (sodium acetate trihydrate whose melting point is 58 °C) and tested if these fins could decrease the solidification time of PCMs. They concluded that despite fins altered natural convection within the PCM and they did not enhance the heat transfer coefficient of the material, it was possible to decrease the solidification time of the PCM. Two cases where analysed: PCMs with small fins and PCM with longer fins. Both were compared with a base case module which had no fins. In the first case, a lower temperature difference was necessary to achieve the same heat transfer coefficient as with no fins. Therefore, the needed time to solidify the PCM decreased. In the second case, a lower heat transfer coefficient was obtained in comparison to the PCM with no fins. This was attributed to the width of the fins that should be interfering with natural convection of the PCM. Despite that the heat transfer coefficient decreased, it is im- portant to notice that the solidification time of the PCM was reduced. Lamberg [100] analysed the two phase solidification problem in a low temperature finned PCM storage system (it used aluminium fins and paraffin as PCM) by using an approximate analytical model. It is important to note that the geometry of the storage module had a big influence in the accuracy of the analytical model. Hence, through this analysis, it was concluded that the solidification was dominated by conduction while natural convection existed only during the beginning of the solidification process. In terms of com- parison, the natural convection is negligible compared to conduction. Erek et al. [97] numerically and experimentally investigated latent heat thermal energy storage with phase change around a radially finned tube. The results showed that the stored energy increases with increasing fin radius and decreasing fin space. Velraj et al. [101] investigated the solidification of PCM in a cy- lindrical vertical tube with radial internal fins arrangement. It was concluded that this configuration, which forms a V-shaped enclo- sure for the PCM, gives maximum benefit to the fin arrangement. With an imposed constant heat flux as a boundary condition, Talati et al. [102], and Mostaffa et al. [103] studied solidification of a finned rectangular PCM storage analytically and numerically, with the salt hydrate ClimSel C23 as the PCM and aluminium for the fins. Results showed that when the length of fin to the height of storage ratio is smaller than unity, the PCM solidified more quickly in the storage. Within the framework of the DISTOR project [104], a prototype of high temperature thermal energy storage module with graph- ite fins was tested: to increase the transfer area, expanded graphite was adhered to tubes through which flowed a bi-phase medium. It was concluded that for the same amount of PCM, area and metal material, heat transfer was enhanced by using fins [104]. 3.5.2.2. Fluidized bed. The term “fluidized bed” is unavoidably con- nected to the term “particulate solid material” [105–110]. The geometrical, physical and aerodynamical properties of the par- ticles affect the onset of fluidization, the characteristics, behaviour and the main parameters of fluidized beds. The most important solid properties are particle density, diameter, and porosity. Recently, this technology has been used to store energy through PCM. Izquierdo-Barrientos et al. [111] studied the performance of an air-fluidized bed of micro-encapsulated PCM as a thermal storage system and compared it with a fluidized bed of sand and a fixed bed with sand and PCM. They concluded that PCM is an alterna- tive material that can be used to increase the efficiency of storing thermal energy in the form of latent heat in fluidized bed systems. In fact, under the experimental conditions tested in this work, greater charging efficiencies are observed for the PCM than with just sand in fixed and fluidized beds. Furthermore, when the charging effi- ciency is stabilized, the PCM shows similar results for both beds. The PCM also presents a better recovery efficiency for both beds after the solidification time. Moreover, the influence of the bed height was tested. This study showed that it takes longer to achieve a certain temperature when more mass is used, but higher efficiency values are obtained. The analysis of the influence of the flow rate showed that a higher flow rate allows a faster achievement of the set tem- perature. At the end of the charging process, similar efficiencies are measured at the different studied flow rates. The cycling study re- vealed that the PCM suffers attrition during the fluidization process, although no loss of PCM is observed under the experimental con- ditions tested in this work (75 h of continuous operation with 15 charging/discharging cycles). Pitié et al. [7] introduced PCMs and their potential application in high temperature energy capture and storage, using a circulat- ing fluidized bed (CFB) as transfer/storage mode. Thermal considerations determined the optimum size range for the applied particles (<400 μm) when convection heat transfer dominates. In the design of a CFB heat collector, the heat transfer coefficient between the riser wall and the flowing suspension is an impor- tant design parameter to determine the required heat exchange surface area. Peng et al. [112] predicted the thermal behaviour of the system with a concentric-dispersion model for a packed bed latent heat thermal energy storage using PCM capsules and a molten salt HTF. The radial heat transfer and wall heat losses were considered. The effects of PCM capsule diameter, fluid inlet velocity and storage tank height on the temperature profiles and efficiency of a packed bed were investigated for the charging process. The heat transfer between PCM capsules and molten salt was shown to be significantly influ- enced by the capsule diameter, with finer sizes shortening the effective charging time, thus increases the charging efficiency. The numerical concentric-dispersion model established by the authors enables a fair prediction of the thermal behaviour in a packed bed latent heat thermal energy storage. The fluid inlet velocity has a smaller influence on the charging process, whereas a higher thermal storage tank increases the charging efficiency. 3.5.2.3. Microencapsulation. Microencapsulation is defined as a process in which tiny particles or droplets are surrounded by a coating, or embedded in a homogeneous or heterogeneous matrix, to produce small capsules with many useful properties [113–116]. Unless the matrix encapsulating the PCM has a high thermal con- ductivity, the microencapsulation system suffers from low heat transfer rate. The rigidity of the matrix prevents convection cur- rents and all heat transfer needs to occur by conduction. This can reduce seriously the heat transfer rates, especially in the charging process. The most important parameters in microencapsulation are the thickness of the shell; the geometry of the encapsulation; and the encapsulation size [117]. Microencapsulation of phase change materials refers to a tech- nique in which a large number of small PCM particles are contained within a sealed, continuous matrix which ranges in size from less than 1 mm to more than 300 mm [118]. Nowadays the cost of the microencapsulation is high compared to other storage methods, and it is used only in thermal control applications [116]. Microencap- sulation of PCMs is an effective way of enhancing their thermal conductivity and preventing possible interaction with the surround- ings and leakage during the melting process. This method is classified H. Zhang et al./Progress in Energy and Combustion Science 53 (2016) 1–40 19PDF Image | Thermal energy storage: Recent developments
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