Numerical investigation of hybrid heat transfer enhancement techniques for a shell and tube latent heat storage unit

Abstract

The depletion of the earth’s environment due to the continuous emission of pollution and the increase in energy demand daily due to the increase in the population and lifestyle are important issues that need serious consideration for a better society. To overcome these issues, sustainable and cleaner methods for energy generation are necessary. Solar energy is a potential source of energy for a sustainable society. But solar energy is irregular in nature. Thermal energy storage (TES) systems play a crucial role in solving the demand and supply mismatch. Latent heat storage systems (LHSS) based on phase change materials (PCMs) are prominent techniques for storing thermal energy. These systems have higher energy storage density with less fluctuation in the temperature. Among various heat exchanger configurations, shell and tube type LHSS is chosen due to its minimal heat loss. The thermal performance of the LHSS significantly depends upon the heat transfer enhancement techniques. Usage of extended surfaces (fins), nanoparticles, metal foam, cascading, encapsulation, etc., are a few heat transfer enhancement techniques used in the literature. In the present work performance of the LHSS is analyzed with hybrid heat transfer enhancement techniques. The present work is aimed to analyze the performance of the LHSS due to the usage of (a) Fins (radial, spiral and longitudinal) + GNP (graphene Nano platelets) nanoparticles, (b) metal foam (0.97, 0.95 and 0.93 porosity) + GNP nanoparticles and (c) cascaded metal foam (linearly and radially). The effect of the orientation is also considered in the present research work. To analyze the performance of the LHSS; melting time, solidification time, energy storage and release ratios and exergy efficiency during melting and solidification are considered. The performance of the LHSS is compared with pure PCM shell and tube heat exchanger. The influence of geometric parameters and HTF (heat transfer fluid) conditions on exergy efficiency is also analyzed. Numerical analysis is carried out for the melting and solidification process using ANSYS FLUENT. Initially, the performance of the pure PCM shell and tube LHSS is carried out. The pure PCM shell and tube LHSS results are compared with the hybrid enhanced PCM shell and tube LHSS. The shell and tube LHSS dimensions are considered based on the optimized i geometries specified in the literature. Throughout the analysis of the hybrid enhanced LHSS, the dimensions of the shell and tube LHSS and inlet conditions of the HTF are the same. The radius, pitch, and thickness of the radial fins are considered based on the optimal dimensions obtained from the literature. The same pitch is considered for the spiral finned heat exchanger, and ten fins are considered for the longitudinal finned heat exchanger. The thickness of spiral and longitudinal fins is selected such that PCM in all the finned LHSS is the same. GNP (graphene Nano platelets) nanoparticles are selected as they are compatible with PCM. The usage of fins + GNP nanoparticles resulted in a reduction of melting and solidification time. Maximum reduction of melting time and solidification time by 73.71% and 82.23% are noted in radial finned 1% volume GNP nanoparticle LHSS oriented vertically. Exergy efficiency during solidification has also improved on the usage of fins. The maximum exergy efficiency of 4.55% is noted in radial finned 1% volume GNP nanoparticle LHSS oriented at 45̊. A minimum exergy efficiency of 0.715% is noted in pure PCM LHSS oriented horizontally during solidification. But the usage of fins + GNP nanoparticles reduced the energy storage ratio and exergy efficiency during melting. The energy storage ratio is reduced to 0.89 in 1% volume GNP nanoparticle radial fin LHSS inclined vertically compared to 0.992 in pure PCM LHSS inclined at 45̊. Exergy efficiency during melting is reduced to 44.4% in 1% volume GNP nanoparticle radial fin LHSS inclined vertically compared to 76.16% in pure PCM LHSS oriented at 45̊. Variation in energy release ratio is negligible using fins + GNP nanoparticles compared with pure PCM LHSS. The metal foam + GNP nanoparticles LHSS analysis is performed considering 0.97, 0.95 and 0.93 porosity copper metal foams in combination with pure PCM, 0.5% and 1% vol fraction GNP nanoparticles. Dimensions of the shell and tube heat exchanger are kept the same as pure PCM shell and tube LHSS. The usage of metal foam + GNP nanoparticles improved melting time, solidification time, and exergy efficiency during solidification. Maximum reduction of melting and solidification time by 78.32 % and 91.75% are noted in 0.93 porosity metal foam 1% volume GNP nanoparticles LHSS oriented vertically. The maximum exergy efficiency of 10.5% is noted in 0.93 porosity metal foam 1% volume GNP nanoparticle LHSS oriented at 45̊ during solidification. As observed in fins + GNP nanoparticles, using metal foam + GNP nanoparticles also resulted in the reduction of energy storage ratio and exergy ii efficiency during melting. The energy storage ratio is reduced to 0.88 using 0.93 porosity metal foam LHSS enhanced with 1% volume GNP nanoparticles oriented vertically. Whereas 0.992 is observed in pure PCM LHSS inclined at 45̊. Exergy efficiency during melting is reduced to 46.28% on the usage of 0.93 porosity metal foam LHSS enhanced with 1% volume GNP nanoparticles oriented vertically compared to 76.16% in pure PCM LHSS oriented at 45̊. Variation in energy release ratio is negligible due to the usage of metal foam + GNP nanoparticles compared with pure PCM LHSS. Thermal performance analysis of the cascaded metal foam LHSS is carried out considering 0.93, 0.95 and 0.97 porosity copper metal foams. Metal foams are cascaded in both radial and linear manner. Cascaded metal foams improved melting/solidification rates and exergy efficiency during solidification. Maximum reduction of melting and solidification time by 76.17 % and 91.75% are noted in radial cascaded 0.93-0.95-0.97 LHSS. The maximum exergy efficiency of 9.22% is observed in radial cascaded 0.93-0.95-0.97 LHSS inclines at 45̊ compared to 0.715% in pure PCM LHSS oriented horizontally during solidification. Also, cascaded metal foams resulted in a reduction of energy storage ratio and exergy efficiency during melting. The energy storage ratio is reduced to 0.86 in linear cascaded 0.97-0.93-0.95 porosity metal foams LHSS oriented vertically. Exergy efficiency during melting is reduced to 45.4% on the usage of 0.97-0.93-0.95 porosity metal foam LHSS oriented vertically compared with 76.16% in pure PCM LHSS oriented at 45̊. The usage of hybrid techniques enabled an improvement in melting/solidification time and exergy efficiency during solidification. Although the energy storage ratio decreased, this decrease in energy storage is not due to improper usage of latent heat of the PCM but ineffective use of the PCM sensible heat. So the reduction in the energy storage ratio has little effect on the performance of LHSS. But the decline in exergy efficiency during melting is an important performance factor. Although improvement in exergy efficiency during solidification is observed, further improvement is necessary as maximum exergy efficiency is only 10.5%. This is a serious concern that limits the usage of LHSS. Using metal foam+ GNP nanoparticles has shown better thermal performance than the other two hybrid techniques. So machine learning model is developed to predict the transient variation of melt fraction in metal foam + GNP nanoparticles enhanced shell and tube LHSS. iii Further study is carried out to analyze the effect on the phase change time and exergy efficiency during melting and solidification by varying porosity of metal foam, the volume fraction of GNP nanoparticles, length of the heat exchanger, the inlet temperature of HTF, length to diameter ratio (l/d) of shell and Reynolds number of HTF. l/d ratio of LHSS is varied such that amount of PCM in LHSS is the same as that in pure PCM LHSS. A significant improvement of 23.32% in exergy efficiency during solidification is obtained. It is observed that the l/d ratio of LHSS and the porosity of metal foam significantly affect the melting and solidification rates. Exergy efficiency during melting depends considerably on the HTF inlet temperature. Whereas exergy efficiency during solidification is noted to be significantly dependent on the Reynolds number of the HTF.