Enhancement of PCM solidification using inorganic nanoparticles and an external magnetic field with application in energy storage systems
Introduction
Heat storage provides new solution for waste heat recovery. Researchers continuously have endeavored to ameliorate the efficacy of LHTESS via thermal management of PCM (Dadollahi and Mehrpooya, 2017; De Falco et al., 2017; Ibrahim et al., 2017; Nižetić et al., 2018; Gao et al., 2017; Sheikholeslami, 2018a, Sheikholeslami, 2018b; Yuan et al., 2017).
Nanotechnology has penetrated to all branches of science and technology including energy. Nowadays nanoparticles are used in various energy systems and technologies (Gómez-Villarejo et al., 2017; Pavlovic et al., 2018; Bellos and Tzivanidis, 2018; Mahian et al., 2019a; Mahian et al., 2019b), combustion engines(Ali et al., 2018; Soni and Gupta, 2017), fuel cells (Stoševski et al., 2016), sensor technologies (Abegaz et al., 2017) and so on. LHTESS are another type of energy systems in which nanotechnology application is developing. In these systems, by adding nanoparticles to PCM its ability for heat conduction enhances (Liu et al., 2016), and, hence, the solidification and melting process accelerates. Here, a brief report on the benefits of nanoparticles to improve the performance of PCM and LHTES units has been presented.
In 2007, Khodadadi and Hosseinizadeh (2007) for the first time assessed the solidification process of CuH2O nanofluid and reported that in the presence of nanoparticles the performance of PCM could be enhanced. Fan and Khodadadi (2012) demonstrated both experimentally and theoretically the freezing process of the mixture of cyclohexane and copper oxide nanoparticles. They concluded a linear decrement in solidification time with . For , the freezing time was decreased by about 8%. Hasadi and Khoadadi(El Hasadi and Khodadadi, 2013) evaluated the impact of nanoparticles size (2 and 5 nm) on the solidification time in a cavity where copper/water was selected as the NEPCM. They concluded that to shorten the solidification time, the size of nanoparticles should be bigger.
Sheikholeslami and Ghasemi (2018) simulated a nanofluid-based LHTES in the presence of thermal radiation. They demonstrated that higher values of radiation parameters lead to faster charging process. Mahdi and Nsofor (2016) modelled the discharging process in a triplex-tube storage system where alumina nanoparticles were added to the PCM. They concluded that using nanoparticles at a high concentration (i.e. 8%) can shorten the solidification time by 20%. Later, Mahdi and Nsofor (Jasim M. Mahdi and Nsofor, 2017a) reported the roles of using porous metal foam and nanoparticles on the solidification time in the triplex-tube LHTES system. They found that adding nanoparticles with a concentration of 8% and porous media with a porosity of 95% can decrease the solidification time up to 96.5%. Their results indicate that adding nanoparticles to a porous system is not much effective so that at a porosity of 95%, with the rise of concentration of nanoparticle from 0 to 8% the solidification time reduces just by about 1%. However, adding nanoparticles to a non-porous LHTES system can reduce the solidification time by 20%. In another work, Mahdi and Nsofor (2018) concluded that employing fins for performance enhancement (solidification rate) of the LHTES system is more helpful than the simultaneous use of fins and nanoparticles. Mahdi and Nsofor(Jasim M Mahdi and Nsofor, 2017; Jasim M. Mahdi and Nsofor, 2017c) also investigated the impact of nanoparticles, fins and porous foam on charging rate of pure PCM. Hossain et al. (2015) reported NEPCM melting in a porous cavity. They found adding nanoparticles to PCM leads to energy saving during the melting process.
Tasnim et al. (2015) concluded that adding nanoparticles to a porous cavity filled with PCM delays the melting process. Kohyani et al. (2017) demonstrated the discharging process of copper/cyclohexane as the NEPCM in a porous cavity under an external magnetic field. They found that adding nanoparticles is not effective in the discharging process when a porous media with high thermal conductivity fills the cavity. They also concluded that applying the magnetic field can reduce the melting time. The readers may refer to other related references such as (Bazri et al., 2018; Elbahjaoui and El Qarnia, 2017; Sharma et al., 2017; Sheikholeslami, 2018a, Sheikholeslami, 2018b; Wang et al., 2016) which are not discussed here to save the space.
Conduction is effective mode in solidification. Only in the beginning of solidification, free convection exists and as time progress the impact of free convection becomes negligible in comparison to conduction. Adding magnetic field can enhance the conduction mechanism. So, adding magnetic field can accelerate solidification process. Furthermore, adding nanoparticles into host PCM causes thermal conductivity to increase and in this way, the rate of discharging enhances with rise of nanoparticles concentration. A literature review reveals that there is no article on the influence of Lorentz forces on the solidification of inorganic nanoparticles enhanced PCM inside a porous annulus. In this research, for the first time, the influences of using NEPCM (copper/water) on solidification rate in a porous annulus under an external magnetic field have been studied. The FEM is utilized to model this transient problem. The role of using nanoparticles, Hartmann number and Rayleigh number on solidification process has been investigated.
Section snippets
Problem definition and governing formulas
A diagram of the present problem is depicted in Fig. 1(a). An external magnetic force is employed to speed up discharging process. The geometry is a porous annulus filled with NEPCM i.e. mixture of water and CuO nanoparticles with properties given in Table 1.
Discharging of NEPCM through the porous cavity can be described by the below formulas:
To describe
Solution procedure and validation
Galerkin FEM with adaptive mesh has been employed to solve current process. Implicit approach used for transient terms. Newton-Raphson technique has been utilized in last step. Fig. 2 demonstrates the variations of mesh with time.
For verifying the FEM code, outputs were compared with results of Ismail et al. (2001). In that article, experimental and numerical procedures have been employed to investigate finned tubes thermal storage enclosure. They utilized control volume method to solve the
Results and discussion
In this research, improvement of discharging through a porous thermal storage system is investigated. Dispersing nanoparticles into the pure PCM and applying magnetic field are two methods which are used to accelerate the solidification rate. The impacts of Rayleigh number, nanofluid concentration, and magnetic forces on the rate of solidification have been examined.
Fig. 4, Fig. 5 demonstrate the roles of adding CuO nanoparticles with a volume fraction of 4% in water on the solidification
Conclusion
In this research, for the first time, NEPCM and Lorentz force are employed to augment the efficiency of solidification. Influences of Rayleigh and Hartmann numbers and concentration of nanoparticles on discharging process are demonstrated. The results indicated that employing magnetic field can promote the PCM solidification. Moreover, the required time for freezing the PCM increases with raising the Rayleigh number. With augmenting the Hartmann number from 0 to 10, the solidification time was
Acknowledgment
Authors acknowledge the funding support of Babol Noshirvani University of Technology through Grant program No. BNUT/390051/98.
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