Here we examine how the hydriding time of a metal hydride reactor (MHR) varies with the volume fraction, φmf, of a metal foam installed in the reactor. Technically, an experimentally validated mathematical model accounting for the hydrogen absorption kinetics of LaNi5 is used to compute the heat and mass transport in a cylindrical MHR. We then demonstrate that, with a fixed amount of metal hydride powder sealed in the reactor, saving a relatively small fraction (say, 1%) of the MHR internal volume to accommodate a metal foam usually suffices to substantially facilitate heat removal from the reactor, thereby greatly shortening the MHR hydriding time. However, for a metal foam of fixed apparent size, increasing φmf would reduce the metal hydride content, and hence the maximum hydrogen storage capacity, of the MHR. Consequently, if a prescribed amount of hydrogen is to be stored in the MHR, the hydriding time would decrease with increasing φmf at first (due to heat conduction augmentation), reach a minimum at an “optimal” φmf value, and then increase drastically due to metal hydride underpacking.
In the development of hydrogen power systems, fuel (hydrogen) storage and transportation are an important issue. Currently, because of its relatively high hydrogen storage capacity and safety implications, hydrogen storage in metal hydrides appears to be a promising option [1]. In practice, a hydriding intermetallic compound is pulverized into powder form (to increase the reaction surface area per unit apparent volume), and then sealed in a metal hydride reactor (MHR), which usually takes the shape of a cylindrical container [2] and [3]. As hydrogen absorption and desorption of metal hydrides involve heat release and consumption, respectively, thermal management of an MHR strongly affects its hydriding/dehydriding time, and therefore is an important issue in MHR design and optimization [4] and [5]. Meanwhile, the hydriding/dehydriding process in an MHR is further complicated by the gaseous hydrogen flow through the porous metal hydride bed. (Other key issues in the technical design of MHRs are discussed recently by Yang et al. [6]).
Due to the synergetic interactions of the aforementioned factors, the performance of an MHR is highly sensitive to its operating conditions (such as the heat transfer fluid temperature [7], [8] and [9] and reactor inlet/exit pressure [9], [10] and [11]). Also, thermal management of MHRs presents some practical challenges, since metal hydride powders typically have low effective thermal conductivities (on the order of 0.1 W/m-K [12]). Various methods of heat conduction augmentation therefore have been proposed for MHRs. Basically, such methods can be classified into two categories [13], namely using extended surfaces (in the forms of fins, foams, or meshes; see [13], [14] and [15] for example) and binding metal hydrides into a solid matrix formed by a high-conductivity material (such as copper, aluminum, or nickel [14], [16], [17] and [18]). Recently, to further facilitate MHR heat removal, Mellouli et al. [19] inserted a spiral heat exchanger (and a finned spiral heat exchanger instead in a follow-up work [20]) in an MHR, and demonstrated that the hydriding time can be significantly reduced. (Note, however, that when the hydriding and dehydriding processes are mass transfer controlled, instead of heat transfer controlled, the MHR performance may not be improved as significantly by sophisticated thermal management strategies [21]).
Here, using a 1-D theoretical model that accounts for the mass and energy balance and the hydrogen absorption kinetics in a cylindrical MHR, we have examined how the charing time (tch) and hydrogen-to-metal weight percentage (wt.%) of an MHR are affected by the volume fraction (φmf) of a metal foam installed in the MHR to enhance heat conduction. After validating the theoretical model by comparing its numerical results with existing experimental data, two cases were studied numerically. First, in Section 4.2, the amount of metal hydride powder contained in the MHR (m′M per unit reactor height) was specified, and it was found that the metal foam’s presence indeed can substantially reduce the time (tch) required to charge the MHR to 95% of its maximum hydrogen storage capacity, with relatively insignificant reduction in wt.%. It was also shown that the effective thermal resistance of the MHR, Rth, is highly correlated with the hydrogen charging time of the MHR. Moreover, for a given m′M there exists a particular φmf value that would minimize Rth, which is rather close to the value of φmf that minimizes tch. A specific example also was discussed in Section 4.2 to demonstrate how the numerical results of the present model can be used to help identify the design parameters of the MHR and metal foam that would meet the specified hydrogen storage capacity and charging time of the MHR.
Meanwhile, specifying the amount of hydrogen to be stored in an MHR (m′H per unit reactor height) instead, the effects of varying φmf on tch and wt.% were examined in Section 4.3. As the apparent size of the metal foam was fixed in the computations, increasing φmf would reduce the amount of metal hydride sealed in the MHR, thereby reducing the maximum hydrogen storage capacity of the MHR. For a specified m′H, there therefore exists a maximum admissible value of φmf that could meet that specification. Indeed, the numerical results discussed in Section 4.3 indicated that if φmf becomes too close to such a maximum admissible value, the hydrogen charging time tch of the MHR would increase drastically due to metal hydride underpacking. On the other hand, for smaller values of φmf, tch would decrease with increasing φmf because of heat conduction augmentation. Consequently, for a specified m′H there exists an optimal value of φmf that would minimize tch of the MHR. In Section 4.3, the optimal values of φmf were found for various m′H values, and the corresponding MHR thermal resistance and wt.% of the MHR were discussed. Moreover, a specific example was discussed to illustrate that, in practical applications, installing a metal foam of suitable volume fraction in an MHR not only would substantially reduce the hydrogen charging time, but also would slightly increase the hydrogen-to-metal weight percentage of the MHR.
