Numerical modeling and performance comparison of high-temperature metal hydride reactor equipped with bakery system for solar thermal energy storage

https://doi.org/10.1016/j.ijhydene.2020.08.206Get rights and content

Highlights

  • Optimal design strategy of metal hydride thermal storage reactor for solar bakery.

  • Parametric analysis of annular truncated hollow conical fins type reactor.

  • Performance comparison of different innovative reactor designs.

  • Analyses pointed out the optimal design process and the non-homogeneous reaction.

Abstract

The current study provides an optimal design strategy for the metal hydride reactor for the thermal energy storage system for solar bakery applications. The sensitivity analysis of the annular truncated hollow conical fins type reactor was conducted by using COMSOL Multiphysics 5.3a software. The gravimetric exergy output rate and exergy output of different designs were compared. It was found that the optimal geometric parameters and operating conditions for the annular truncated hollow conical fins type reactor are fin angle (θ)=20°, fin radius (Rf) = 0.07 m, number of fins (Nf)= 32 and fin spacing (Sf) = 0.02 m, fin thickness (thf) = 0.008 m, hydrogen supply pressure (Pin, H2) = 5 MPa and inlet heat transfer fluid velocity (VHTF,in) = 20 m/s. Finally, it was concluded that the Deign IV is the most favorable design, which has a uniform reaction, faster reaction time (15,000 s), and higher values of gravimetric exergy output rate and exergy output (1.23 W/kg, 0.028 kW).

Introduction

In recent years, the scarcity and hazardous environmental impacts of the primary energy resources have globally diverted the interest of the scientist community towards the exploitation of the renewable energy resources such as solar, wind, hydel, etc. due to their extraordinary advantages like nonhazardous carbon-emission, environment-friendly and abundant resource. Among all the renewable energy resources for thermal energy generation, concentrated solar thermal technologies have attained a tremendous consideration. Despite the fact, solar energy is the best choice because of the plentiful advantages, there are some challenges like intermittent supply and instability. In order to overcome these difficulties, the energy storage system has been introduced as an effective means to store concentrated solar thermal energy for a solar-based baking system [1,2]. Generally, the thermal energy storage system can be divided into a) sensible heat storage system, (b) latent heat system, and (c) thermochemical solar thermal energy storage system [3]. The sensible heat storage systems have been effectively employed in concentrated solar power (CSP) plants [4,5]. However, the sensible heat storage method is found to be quite low energy storage densities, i.e. 110 kJ kg−1 for Solar Salt and 156 kJ kg−1 Hitec [6]. The utilization of phase change material (PCM) is extensively explored in building energy-saving and CSP [[7], [8], [9]]. Currently, phase change material (PCM) with high latent heat, such as NaNO3 (199 kJ kg−1) and MgCl2 (452 kJ kg−1) is taken under consideration [10]. The limitation for the PCM is its poor thermal conductivities and excessive temperature deterioration [11,12]. Compare with latent heat storage, one most important benefit of utilizing a thermo-chemical storage system is its capability to offer a higher amount of thermal energy storage per kg of storage material: metal hydride alloy materials delivers 15–20 times more energy densities than that of phase change materials (PCMs) [13]. The considered important characteristics of TES systems for selection are given in Table 1.

Based on the above-mentioned characteristics and the property of high energy storage density as compare to sensible and latent heat, thermochemical thermal storage becomes the more prominent system [15,16]. Compared with metal carbonates and metal hydroxide based thermochemical system, the metal hydride based thermochemical systems is found to be superior depending on the gravimetric energy storage density. E.g. ΔH(CaH2) = 181 kJ/mol, ΔH (CaCO3) = 167 kJ/mol, ΔH (Ca(OH)2) = 112 kJ/mol [17]. The benefits of metal hydrides are high hydrogen capacities, secure management of hydrogen as compared to gaseous storage which requires high pressures of approximately 350–700 bar. Furthermore, hydrogen storage in metal hydrides demands lower energies in contrast to that in the liquid state at cryogenic temperatures (3.23 kWh/kg, T < 150 K). The thermochemical heat storage system mainly consists of 2 processes: hydrogen absorption and hydrogen desorption process. During the absorption process, the metal reacts with hydrogen at a certain temperature and pressure, accompanied by an enormous amount of heat due to the exothermic reaction. Similarly, during the desorption process, heat is imparted to the metal hydride at certain operating conditions, and an endothermic reaction takes place, accordingly, hydrogen gets separated from the metal lattice [18]. Metal hydride based on Mg [[19], [20], [21]], LaNi5 [[22], [23], [24]], and FeTi [25,26], etc. offer sustainable alternatives for hydrogen storage (thermal heat discharging process) and hydrogen delivery (thermal heat storage process) based on the requirement. Mg-based metal hydrides are considered as an auspicious choice for high-temperature TES due to some important properties, such as high hydrogen storage capacity, low price, efficient reversibility, and cyclic stability [3]. Besides, the high hydrogen storage capacity and the ultrafast hydrogenation-dehydrogenation kinetics are necessary for worthwhile commercial applications. Nevertheless, firstly, the sluggish hydrogenation-dehydrogenation kinetics and low thermal conductivity are an issue. In the literature, different approaches like doping of metal hydride with nanomaterials and expanded natural graphite (ENG) and the use of an efficient heat exchanger were adopted to improve the kinetics and the effective thermal conductivity of the bed. Kumar et al. [27] used Nb2O5 as an additive for optimized reaction kinetics of Mg-based metal hydride for thermal energy storage under vacuum at 1.8 MPa and 323–573 °C. Kumar et al. [28] used vanadium doping to improve the reaction kinetics of Mg bed and to significantly reduce the activation energy. It was found that there is a noteworthy rise in temperature (up to 192.5 °C) in just 40 s during the absorption process. Secondly, heat and mass transfer in the metal hydride bed is an essential controlling aspect during the heat charging and discharging process.

Recently, Numerous experimental, design optimization, simulation, and feasibility studies have been presented regarding the different heat exchanger designs and metal hydride tank design for the TES system. In the previous study conducted by our research group, Feng et al. [29] developed a multi-level configuration of the reactor and performed its mathematical modeling by using COMSOL Multiphysics v5.1. A decrease of discharging time from 17,350 s to 14,688 s and an increase of output temperature by ~5 °C was observed. Another research conducted by the same research group, Feng et al. [30] adopted an optimal design methodology for designing a helical heat transfer tube in the metal hydride reactor. It was observed that the GEOR was improved from 198.4 to 306.1 W kg−1. Li et al. [31] performed a comparative study with helical tubes of different crosssection, minor and major diameter passed through the metal hydride bed. Finally, it was concluded that efficient operating condition for the hydrogenation process is Ps > 10 atm, Ts < 288 K. Urbanczyk et al. [32] constructed a tube bundle heat storage tank filled with Mg2FeH6 as high-temperature metal hydride for long and short-term storage applications at temperatures around 500 °C. It was found that 1.6 kWh of heat could be released, and 1.5 kWh of heat could be stored. Melloulia et al. [33] developed a 2D mathematical model in Fortran-90 to analyze heat and mass transfer of metal hydride reactor. It was concluded that this system can recycle 96% of solar energy stored. In another research work, Mellouli et al. [34] described different reactors designs filled with Mg2Ni metal hydride material. In these designs, three different configurations (spherical, hexagonal, and circular cross-sections) for PCM (NaNO3) were proposed. The results illustrate that the circular cross-section design enhanced the heat transfer rate effectively. Another research work was conducted by the same group, Mellolui et al. [35] performed the comparative numerical study of reactor design for cooling purpose. It was found that a single helical cooling pipe takes almost 1500 s to reach the reacted fraction up to 0.9 at the operating condition of Ps = 10 bar and Ts = 297 K. By the addition of copper fins on the helical tube, the hydrogenation time reduces up to 500 s. Inadvertently, a reactor consist of two helical cooling tubes also reaches up to 0.9 of the reacted fraction in 500 s. Though, the insertion of copper fins to the above mentioned two helical tubes decreases the hydrogen absorption time only to 420 s. Consequently, it was suggested that there is a constraint of improvement for any specific model design approach. The study conducted by the same group, Mellouli et al. [36] conducted another study in which they experimentally assessed the hydrogenation performance of a metal hydride based reactor equipped with an internal spiral heat exchanger. After testing, it was observed that the hydrogenation and dehydrogenation time was remarkably decreased. Mathematical modeling has been performed of a multi-tubular heat exchanger equipped with a metal hydride reactor. The research outcomes evidently showed the rate of charging/discharging rise with the intensification of tubular heat exchanges. Askri et al. [37] analyzed the performance of the hydrogenation process of LaNi5 reactor by using different types of cooling systems. It was observed that the average reacted fraction reached up to 0.9 in approximately 2700s without any thermal management. The hydrogenation process was completed in approximately 2050 s by the addition of external fins. The presence of a single central cooling tube, further, decreased the hydrogen absorption time to 980 s. Kaplan [38] conducted research work on a comparison of three metal hydride based reactors designs (unfinned, externally finned, and externally jacketed). It was found that metal hydride reactor design having an external heat transfer fluid jacket enhances the overall heat transfer coefficient up to 133 W/m2. K as compared to externally finned (35 W/m2.K) or unfinned (5.5 W/m2. K) reactor designs. Sekhar BS et al. [39] developed the embedded cooling tubes reactor design to enhance the heat transfer rate of Mg + 30% MmNi4 bed for thermal energy storage. Dong D et al. [40] designed a stainless-steel coil reactor for the thermal energy storage system at 0.2–2.0 MPa and 303–710 °C, which effectively transports supercritical water through MgH2 bed as HTF. Nyamsi et al. [41] derived a semi-analytical expression for a single fin to calculate the optimum dimensions of the finned tube heat exchanger. Then, numerical simulation was performed to further assess the performance of the finned tube heat exchanger. Finally, it was concluded that using a finned tube heat exchanger contributes to the reduction of 42% in the charging time and 13% in the thermal resistance. Wang et al. [42] proposed an innovative radiation mini channel reactor with or without a heat transfer fluid (HTF) jacket. 3D Numerical model was developed and solved by COMSOL for system optimization. The operating conditions such as fluid inlet temperature of 293 K and hydrogen supply pressure of 1 MPa were selected. The optimized dimension was calculated as follows: spread branch number of 3, main tube radius of 2 mm, branch tube radius of 2 mm, axial pitch of 5 mm, the mounting distance of 8.5 mm, and inclination angle of 0°. The results showed that radiation mini channel reactor with jacket (RMCR-J) was verified to offer better performance than radiation mini channel reactor without jacket (RMCR). Raju et al. [43] performed mathematical modeling for optimization of multi-tube heat exchangers equipped with aluminum fins, helical heat transfer pipe, and shell-and-tube type heat exchanger, respectively. Mohan et al. [44] performed the simulation study of LaNi5 based metal hydride reactor for hydrogen storage consist of embedded filters for hydrogen distribution and the cooling tubes The purpose of heat exchanger tubes was to cool and heat the metal hydride bed during the absorption and desorption process respectively. A sensitivity analysis of the geometric and operating parameters has been conducted to analyze their effect on hydrogenation performance. Kumar et al. [45] conducted the activation and experimental testing of a large scale metal hydride based hydrogen storage system for industrial application. The metal hydride reactor is made up of SS316 material having 99 embedded cooling tubes filled with 40 kg of LaNi4.7Al0.3. The experimental test of the system was conducted by varying H2 supply pressure, hydrogenation and dehydrogenation temperatures, and the velocity of heat transfer fluid. It was noticed that the hydrogen supply pressure has a considerable effect on the absorption rate, and the optimal supply pressure was noted in the range of 10–15 bar. Similarly, during the dehydrogenation cycle, the optimal desorption temperature was observed in the range of 80–90 °C. The optimal flow velocity for HTF was noticed in the range of 20–30 lpm. Chung et al. [46,47] conducted experimental and simulation studies to investigate the hydrogenation and dehydrogenation process of the proposed LaNi5 based tank equipped with internal heat pipes. The reduction of hydrogenation time from 2100 to 900 s was observed by using a heat pipe. The results showed that this design enhances hydrogen storage during the hydrogenation and dehydrogenation processes. Krokos et al. [48] conducted a research work that offers a uniquely efficient methodology for the optimal design of a multi-tubular metal hydride based hydrogen storage tank, containing up to nine tubular metal hydride reactors. The reactor was proposed to store a sufficient quantity of hydrogen for a 25 km range, for a fuel cell vehicle. A comprehensive 3D Cartesian, mathematical model was established and validated. The purpose is to obtain the optimum procedure design to boost the overall thermal efficiency and therefore reduce the storage period. Optimization outcomes reveal that approximately 90% upgrade of the storage time can be attained, over the case where the tank is not optimized and for a minimum storage capacity of 99% of the maximum value.

Gkanas et al. [49] conducted experimental work on three types of rectangular metal hydride tanks equipped with a plain tube heat exchanger. They performed the numerical simulation to analyze the effects of the reactor dimensions and the arrangement of cooling tubes. Three types of metal hydride bed LaNi5, MmNi4.6Al0.4, and a novel AB2 material were used to fill the reactor tank. Finally, the optimal hydride thickness was found to be 10.39 mm and U = 2000 W/m2 K showed the most excellent outcomes. Muthukumar et al. [50] presented a 2D modeling of a 50 mm diameter reactor having multiple internal cooling tubes. The number of tubes varies from 12 to 20 and hydrogen was supplied through a central pipe. The simulation model was validated by an experimental work [18]. It was observed that by varying the number of tubes 12, 14, 16, 18, and 20, the reacted fraction of metal hydride material (MmNi4.6Al0.4) was reached up to 0.9 in 296, 222, 195, 184, and 155 s respectively. The same research group [51] conducted another experimental work on the meatal hydride reactor filled with 2.75 kg of LmNi4.91Sn0.15. Reduction in hydrogenation reaction time from 480 to 360 s was observed by increasing the number of embedded cooling tubes from 36 to 60 for 1.04 wt% of hydrogen at Ps = 25 bar. Moreover, the same authors performed the 2D and 3D mathematical modeling which demonstrate that the greater number of cooling tubes shows efficient heat transfer at the operating condition of uniform inlet temperature and flow rate of heat transfer fluid [52].

Singh et al. [53] used a sequence of the cooling tubes and porous circular internal fins. It was observed that there is a reduction in heat discharging time of 13% without porous fins. Another research work conducted by the same group [54] designed a system having two U-shaped pipes with porous circular fins. Additionally, they also used copper flakes to enhance the effective thermal conductivity of metal hydride (MH) bed. Their research results [55] suggested that more contact surface area was found between cooling tubes and MH bed if they increase the number of fins (13, 20, 26, 34) and decrease the fin thickness (0.325, 0.25, 0.19 mm). Finally, it was summarized that there is a decrease in the discharging time (610, 495, 467, 415 s) respectively for 1.2 wt% of hydrogen gas at Ps = 1.5 MPa, Ts = 298 K and vs = 1 m s −1. Lewis and Chipper [56] proposed an innovative MH reactor having an embossed plate heat exchanger (EPHX). They also performed the numerical simulation to investigate the effect of flow field layout and coolant flow direction on reactor performance. A comparative study between the embossed plate heat exchanger (EPHX) and helical coil heat exchanger (HCHX) was also conducted. Finally, it was concluded that the EPHX exhibited a marginally lower heat transfer rate, the same hydrogen absorption rate, and extraordinary homogeneity in temperature distribution. Visaria et al. [57,58] presented the optimization study of metal hydride reactor having longitudinal fins attached with the U-shaped pipe to reduce the hydrogenation time by varying the hydrogen supply from 70 to 280 bar and 70–330 bar. It was concluded that 90% of the reacted fraction was achieved in 280 s by varying hydrogen pressure setting with the heat transfer fluid entering the finned U-tube at 273 K and 4.9 g/min. Chibani et al. [59] investigated the hydrogenation process of a large-scale MH reactor having multiple pipes fixed MH bed form by developing the 2D-mathematical model. Chandra et al. [60] conducted research work on numerical simulation of a novel low-temperature metal hydride (LaNi5) reactor having internal conical fins and water-cooling tubes. After the numerical simulation, it was concluded that the reactor with internal conical fins improves the heat transfer rate due to increased surface area. It was observed that the reactor design having 19 fins + 6 tubes design completes the hydrogenation process of 80% and 90% within 290 and 375 s, respectively.

Firstly, all the aforementioned research studies were focused on the modeling of small dimensions design for concentrated solar power plants, there is a lack of any numerical study for design optimization and the non-homogeneous reaction of large-scale system design for solar-based cooking devices. Secondly, Nevertheless, the heat transfer performance and overall discharging efficiency of MH reactors were still inadequate to meet the definite conditions. The knowledge differences in the current study on the optimum arrangement and shape of the fins, heat transfer pipe, and reactor with heat transfer fluid and metal hydride jacket are in both applications and fundamentals stage. The design should emphasize engineering attributes such as shape, size, and arrangement of the fins, arrangement of heat transfer fluid, and metal hydride jacket meanwhile being cost-effective. Furthermore, the design should approach the minimum viable opposition for hydrogen flow inside a metal hydride bed to speed up the hydrogenation process. Presently, this facet is not yet dealt with in review literature extensively. So, the main objective of the study is to introduce an innovative optimized ATHCF heat exchanger design to get the desired targets of less hydrogenation time and a high heat transfer rate.

This article focused on the design optimization and parametric study of nano-engineered composite (MgH2+V2O5) based annular truncated hollow conical fins type HTMH reactor. Five HTMH reactor designs are proposed: a) central heat transfer pipe having annular truncated hollow conical fins; b) central heat transfer pipe having annular truncated hollow conical fins + outer jacket; c) central heat transfer pipe having annular truncated hollow conical fins + outer jacket with conical fins; d) central heat transfer pipe + multiple jackets for MH bed and heat transfer fluid; e) central MH bed + multiple jackets for heat transfer fluid and MH bed. In the first phase of research work, the sensitivity analysis of an annular truncated hollow conical fins type reactor was done based on structural parameters (fin angle, fin thickness, fin spacing, and the number of fins) and operating conditions (hydrogen supply pressure, inlet heat transfer fluid velocity). In the second phase, the gravimetric exergy output rate (GEOR) was compared for the different designs to achieve the desired targets.

Section snippets

The optimal design strategy for MH-TES system for solar bakery

The current research offers an optimal design strategy for the MH-TES system as discussed below:

  • I.

    The optimal design principle is to maintain the homogeneous reaction of the nano-engineered composite (MgH2+V2O5) powder bed with no dead reaction zone.

  • II.

    Innovative design process: Firstly, numeric modeling and parametric study of the ATHCF type reactor was conducted to minimize the dead reaction zone.

  • III.

    Finally, the improvement of the bed structure was done for the enhancement of the heat transfer rate

Grid independency test

Three different types (normal, fine, and finer) of meshes with different grid sizes have been created by using the meshing tool in COMSOL MULTYPHYSICS 5.3a, and the relative tolerance was set as 10−5. The grid independence test was conducted by considering the average reacted fraction (X) and average bed temperature (Tbed) as a performance assessment index. The domain and boundary elements in three different cases were found to be; Case 1: normal (9079, 1789), Case 2: fine (19,914, 2571) Case

Conclusions

This study proposed an optimal design strategy for the metal hydride reactor for a thermal energy storage system for solar bakery applications to enhance the heat transfer and thus to reduce the discharging time, which combines optimal design principles and innovative design process through the comparison of different designs with the same heat exchanger volume. The foremost conclusions are as follows:

  • 1)

    The sensitivity analysis of an annular truncated hollow conical fins type reactor was

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work is financially supported by the National Natural Science Foundation of China (Nos. 51876150 and 21736008). The authors would also like to acknowledge the support from the Department of Energy Systems Engineering, University of Agriculture, Faisalabad-Pakistan.

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