Experimental and numerical investigating a new configured thermal coupling between metal hydride tank and PEM fuel cell using heat pipes

https://doi.org/10.1016/j.applthermaleng.2020.115490Get rights and content

Highlights

  • In this research an analysis of subsequent effects of an increase in the number of finned heat pipes and wire number (wick layer) inside the heat pipes during thermal coupling is presented.

  • A new novel geometric configuration of heat pipes for the desorption of H2 from a metal hydride storage tank is introduced.

  • A 4 finned heat pipe gives a better desorption performance than 2 heat pipes without fins.

  • At the end of the desorption step maximum H2 flow rate of 0.263slpm was achieved using 4 finned heat pipes while the minimum H2 flow rate of 0.05slpm is related to the 2 heat pipe without fin.

  • The results of RSM analyze show that the best performance of the desorption process at 25 bar achieved using 4 heat pipes covered by 10 fins.

Abstract

This paper presents an experimental investigation of a fuel cell coupled with the metal hydride (MH) tank to study the effects of heat pipe number, wire number and the type of heat pipes with and without fin. The effect of the number of heat pipes and fins, as well as the number of copper wires as a wick layer, has not been investigated in previous similar studies. The computational fluid dynamics (CFD) calculations were carried out using the finite element method (FEM) in COMSOL. Some parameters including condenser length, adiabatic length, evaporator length, heat pipe diameter and heat pipe power were optimized. In the experiments, 30 and 40 strands of copper wire were selected as the wick layer and experiments were repeated for 5 and 10 finned heat pipes. Experimental results also compared with simulation data and it was found that results were consistent with little deviation. By using the relative deviation method, the deviations amounting to 7.45% and 6.76% were obtained in adsorption and desorption step, respectively. Also, sensitivity analysis of the capillary limit and wire diameter of heat pipes were carried out and results illustrated that the reduction of boiling limit has a more significant impact on the heat pipes thermal power than the capillary limit and wire diameter. Response surface methodology was used which showed that the best performance of the desorption process at 25 bar is achieved using 4 heat pipes covered by 10 fins.

Introduction

In recent years, the environmental problems caused by fossil fuels have led researchers to study the possibility of using new technologies for clean fuels. Hydrogen is a suitable alternative to fossil fuels due to its capability of energy carrying [1], [2], [3]. Hydrogen can be used to generate power in a Proton exchange membrane fuel cell (PEMFC) which leads to producing water, heat and electricity [4]. PEMFC is an appropriate option for various static and dynamic applications where a quick set-up is needed. Because it works at relatively low temperatures (60 °C to 80 °C) [5], [6]. Recently, the use of hydrogen as a clean fuel has received much attention for various applications. Corgnale et al. [7] investigated a prototype of hydrogen storage in a fuel cell for vehicles. In their design, the honeycomb finned heat transfer system powered with a resistive heater was used to discharge hydrogen. The results show that the system can achieve extra adsorption capacities of 6.5 wt% at specific conditions. Kwona et al. [8] investigated a high-storage-density hydrogen generator to supply hydrogen for unmanned aerial vehicles. NaBH4 has been used in this study. The results indicate that the gravimetric and volumetric specific energy densities of the hydrogen generator were 739.1 W hr/kg and 272.8 W hr/L, respectively. Lin et al. [9] studied the effects of particle radius and porosity on hydrogen adsorption. They considered the effects of volume expansion on mass, heat transfer and kinetics in the model calculation which solved using finite element method (FEM) in COMSOL. The results demonstrate that hydrogen adsorption can perform better by using larger particle radius and higher porosity.

If generated heat removes effectively from the fuel cell, it can be used in various heating applications (such as hot water supply, heating of different spaces, and heating of various reactants in cold air conditions) [10], [11]. In the process of generating heat by a fuel cell, hydrogen storage is one of the most essential steps that should be done effectively. Using a metal hydride tank (MH) is one of the most convenient methods to overcome hydrogen storage difficulties [5], [12]. A PEMFC can be coupled to an MH tank to control the operating temperature above the ambient temperature which leads to a remarkable increase in the rate of hydrogen release from the MH storage system. For this purpose, it is possible to apply heat pipes in order to transfer effectively generated heat in the fuel cell to the MH tank [13], [14]. Heat pipes due to their high conductivity and strength can carry most of the heat generated in the fuel cell into an MH without the need for pressure equipment such as pumps [15]. The structure of heat pipes is simple due to the lack of a moving component in their construction which has better performance for temperature control mechanism than other passive cooling methods [16]. The heat transfer operation using heat pipes is based on the heat pipe specified equilibrium pressure and liquid boiling point. It is worth to note that it is impractical to employ the full hydrogen storage capacity of the MH tank [17]. In addition, the capability of stack cooling is reduced by increasing the effective length and reducing the diameter of the heat pipes [5]. Hydrogen can be adsorbed on a metal hydride bed in an MH tank and the heat transfer is improved by that physical mixing [18]. Also the mass and heat transfer of the reactor is enhanced using a spiral coil heat exchanger in the desorption step [19]. The porosity of the metal bed influenced the heat transfer. The porosity is decreased during hydrogen injection and is increased when hydrogen is removed [20]. A comparison between three serpentines, maze and parallel configurations of flow plates of a proton exchange membrane fuel cell was presented by Carton et al. [21]. The results showed that the serpentine flow plate design is more effective than other configurations. So it can be concluded that the efficiency of fuel cells is increased using the effective thermal coupling of fuel cells to a hydrogen storage tank by employing heat pipes. W. Li et al. [22] developed a hybrid PCM using foam/nano-PCMS composite to provide high heat transfer efficiency. In contrast to previous similar articles, they also considered the effect of natural convection on the heated-surface temperature for both nano-PCMS and foam/nano-PCMS modules. They concluded that the heat transfer efficiency in nano-PCMS was dramatically related to the relative angle between the thermal management module and the heat source. Finally they proved that the feasibility of foam/nano-PCMS composite was enhancing heat transfer efficiency and decreasing reliance on the convection coefficient of nano-PCMS [22].

A small number of researches have been carried out in the area of the thermal coupling of PEM fuel cell to the metal hydride storage tank using heat pipes [13], [23]. Some of them investigated the effect of heat pipes in the adsorption step without coupling fuel cell with MH tank but they did not examine the impact of the number of fins [15], [23]. The result of voltage and current density is also investigated by researchers [24], [25]. Few studies have been carried out on heat pipe technology and system design and their combination with experimental studies lead to great results. In these articles [26], [27], the main objectives are to get transient operation of high temperature solid oxide cell (SOC) systems and to develop a new thermal control approach for solid oxide cell (SOC) stacks and systems, respectively. Diling et al. [26] considered updated design concepts, size reduced fabrication, and filling procedure after. The experimental part leads to great knowledge about the heat spreading capabilities and power limitations of planar heat spreaders in which high heat transfer rates have been shown by planar sodium heat pipe prototypes over 700 W for axial cross sections down to 4 × 120 mm2 under isothermal operation and it was also true for 2D heat spreading conditions. Another study [27] investigates the thermal effects of integrated heat pipes in solid oxide cell stacks by using a 3-D CFD-modelling that resulted in a great understanding of temperature gradient reduction, heat removal, and a good knowledge of the system.

Marius Dillig et al. [28] have gone through different details including thermal management, start-up and shut-down processes and thermal contact resistances. Precise experimental and theoretical investigation is required for the mentioned subjects, that relatively depends on temperature, geometry and material properties of the contact. They have investigated some information on contact resistance and certain results have been achieved on numerical status that gives useful information about Ni-mesh, Ni-mesh and anode, cathode and interconnector so does between interconnector and sealing. Significant results have been achieved include 800 °C resistance between interconnector and Ni-mesh 4.2 (±0.8) 10−4 (m2 K W−1), between Ni-mesh and NiO-GDC anode electrode 8.2 (±2.8) 10−4 (m2K W−1), and between LSCF cathode and interconnector using contact paste 3.3 (±0.5) 10−4 (m2K W−1). Geometry has been improved by choosing to a heat transfer coefficient of 383 (W m−2 K−1) instead of approximate 1685 (W m−2 K−1), in which thermal contact resistance is negligible. On the other hand, many studies have focused on the utilization of heat exchanger for coupling PEMFC with metal tank [29], [30]. S. Mohammadshahi et al. [29] have developed a 3D mathematical model for intermetallic metal hydride tank . COMSOL multiphysics software was used to solve this model. A heat exchanger with controlled circulating bath and varied cooling system fluid temperature including two concentric tubes equipped with eleven fins was determined as thermal coupling system. At last, the equilibrium pressure and thermal conducing equation were modified. B. Delhomme et al. [30] studied a solid oxide fuel cell (SOFC) which is based on combined heat and power (CHP) system. In their work a particular design of a CHP system based on 1 KW SOFC stack integrated with a MgH2 tank was tested in experimental set- up. During this test, a finned heat exchanger was used to provide heat to the MH tank. These experiments confronted with difficulties that lead to the system failure and resulted in a better system design.

According to the literature mentioned above, the application of the heat pipes in coupling has been recently a novel research and their usage in hydrogen fuel cells and solid oxide fuel cells has been discussed recently in few papers. One of the problems that is mentioned in previous research is the difficulty of stack design and inserting heat pipe to fuel cell [31] The effect of the number of heat pipes and fins, as well as the number of copper wires as a wick layer, has not been investigated in previous similar studies. Above all and in spite of the whole efforts they haven’t evaluate some information that some of them can be found in this study as it follows.

In this research, the thermal coupling of PEMFC with an MH hydrogen storage tank using heat pipes was studied experimentally and by means of mathematical modelling and numerical simulation. In experimental set-up, fuel cell coupled to a MH tank and hydrogen stored in metal hydride bed (LaNi5) was supplied to fuel cell at desorption step. Computational fluid dynamics (CFD) was used to analyze and solve the model equations. This study addresses the H2 adsorption and desorption in detail. Also, an analysis of subsequent effects of an increase in the number of finned heat pipes and wire number (wick layer) inside the heat pipes during thermal coupling is presented. In addition, the present work introduces a new geometric configuration for thermal assistance of PEM fuel cell to H2 desorption from a metal hydride storage tank using heat pipes, in which the condenser part is placed inside the MH tank which is a novel design compared to other studies [23], [32], [33]. Furthermore, hydrogen release and consumption are manipulated to investigate the dependence of the generated heat in the fuel cell and the heat required in the MH tank. Response surface methodology (RSM) is one of the most helpful tools for investigating and optimization in many engineering applications especially for Multi-objective optimization [34]. RSM is used to evaluate the response variable based on the input parameters of the process.

Section snippets

Mathematical modelling

A mathematical model is a numerical explanation of a system to formulate problems and recognize underlying assumptions. In mathematical modelling, it is important to consider some assumptions to clarify the basic framework of the model and a better understanding of how the system operates as well as simplifying the model. In this study the main considered assumptions are:

• A three-dimensional unsteady-state model was applied for the entire system.

• Steam is produced in PEMFC at the operating

Experimental Section

Fig. 1 shows the schematics of the metal hydride tank arrangement used in this work. As Fig. 1 shows, the system includes a storage tank, heat pipes, cathode, gas diffusion layer (GDL), electrodes, membrane, anode and cooling water tube. In the design of this system, heat pipes, cooling water tube and the storage tank are made of copper. Also, the metal powder in the storage tank was considered to be LaNi5 according to several scientific sources [19], [20]. Also, Fig. 1c shows that heat pipes

Results and discussion

In this research, the CFD calculation of the Navier Stokes equation, mass and energy conservation equations were carried out using the finite element method (FEM) in COMSOL Multiphysics® Modeling Software 5.3a. Multi-frontal massively parallel sparse direct (MUMPS) algorithm was used in COMSOL. This is a nonlinear equation solving method and a routine form of Gaussian elimination methods in which the set of nonlinear equations are first transformed into system of linear equations and then

Conclusions

An experimental investigation of a proton-exchange membrane fuel cells coupled with an MH tank is presented. In contrast to similar studies that have been carried out before, the novelty of this study is that, the effect of the number of heat pipes and fins, as well as the number of copper wires as a wick layer, have been investigated. First, the validity of this geometry was examined by simulation the model using COMSOL Multiphysics® Modeling Software 5.3a. At the optimization stage of heat

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.

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