Realistic simulation in a single stage hydrogen compressor based on AB2 alloys

https://doi.org/10.1016/j.ijhydene.2016.01.125Get rights and content

Highlights

  • The simulation evidenced the need for realistic behavior of P-c-T curves in a single stage compressor.

  • The effect of some real equation of state on the compressor simulation is addressed.

  • The ideal model presents problems in the P-c-T determination at some extreme situations.

Abstract

A simulation analysis was performed to study the thermodynamic behavior of a realistic single stage hydride compressor, taking into account real H2 gas behavior by implementing several H2 equations of state, as well as realistic features in the hydrogen sorption properties of the hydride, like sloping plateau and hysteresis. The model was validated with experimental data from two different AB2 hydride systems, a low pressure Ti0.9Zr0.1Mn1.34V0.3 and a high pressure Ti0.9Zr0.1Mn1.47Cr0.4V0.2 alloys. The materials were characterized by X-ray powder diffraction (XRPD), Scanning Electron Microscopy (SEM) and Energy Dispersion X-ray spectroscopy (EDX). The numerical study was divided in three main parts: the effect of implementing real/ideal hydrogen gas models, a study of how the equations of state affect the Pressure composition Isotherm (P-c-T) determination and the influence of each model and realistic features of the P-c-T on the compressor simulation. Among all the models tested, Hemmes and Joubert real models give the more confident values for the compression ratio and final moles in the system volume at a final temperature of 353 K and pressures <50 bar.

Introduction

Hydrogen compressors are an excellent approach to integrate the generation and storage with the distribution of hydrogen as an energy carrier for different applications [1], [2], [3]. Specifically, metal hydride hydrogen compression gives great improvements in comparison to other technologies. The main advantages include simplicity in design and operation, absence of moving parts, compactness, safety and reliability, the possibility to consume waste heat instead of electricity, and low energy cost [1], [2].

Several experimental and numerical investigations have been performed to enhance the performance of hydride compressors. Talagañis et al. developed and analyzed several reduced and simplified lumped models to simulate cyclic hydrogen storage and thermal compression processes in metal hydrides and compared its outcome with experimental results [4]. Laurencelle et al. developed a three-stage AB5 metal hydride based hydrogen compressor prototype, which was designed for a hydrogen production facility using a low-pressure alkaline electrolyzer [5]. Wang et al. developed a two-stage 70 MPa hydride compressor using AB2 hydrides by comparing the thermodynamic behavior of different compositions of the AB2 alloys Ti–Cr–Mn and Ti–Zr–Cr–Fe–V, for the first and second stage of the compressor respectively [6]. Lototsky et al. created a prototype MH compressor operating in a temperature range 20–150 °C and providing compression of hydrogen from 10 to 200 bar with an average productivity up to 1 m3/h STP. The compressor has a two stage layout where the first part uses AB5- and the second AB2-type hydride-forming intermetallic compounds [7]. Bhuiya et al. made an experiment-driven design procedure for optimizing the combination of stages of a dual-stage hydrogen compressor with an enhanced compression ratio, where the compressor system was composed by a LaNi5 in first stage and a Ca0.2Mm0.8Ni5 in the second stage, which attainted a 53% higher compression ratio that the one obtained in the first stage [8]. Gkanas et al. optimized the compression ratio by combining of hydride materials AB5 and AB2 for the first and second stage of the compressor, while maximizing the hydrogen rate using numerical simulations to predict the real behavior of the system [9].

All of the investigations and numerical approaches mentioned take into account non-realistic P-c-T features and the ideal gas as the basic approximations to describe the hydrogen behavior at the stages of absorption and desorption of the hydride in a compressor. In order to describe the real behavior of gaseous hydrogen, authors like Joubert evidenced that the non-ideal contribution has to be observed as the difference of the real molar volume and the ideal one, and as a result, they came to a conclusion that this difference is not zero at low pressures due to the non-ideal behavior of this gas, and, as the pressure rises this difference prevails and increases up to a value of 1000 bar [10]. Moreover, Hemmes et al. and Joubert noticed that there is a weak temperature dependence on the non-ideal contribution to the molar volume for temperatures above 300 K [10], [11].

The main objective of the paper is to show the need of realistic simulation of metal hydride compressors. A first aspect of the problem to be taken into account relates to the real hydrogen gas behavior, because the ideal gas model usually employed in this type of simulations leads to inaccurate results under certain circumstances, for example, high pressure operation. In addition, flat and hysteresis free Pressure composition Isotherms (P-c-T) usually assumed in this type of work are far from realistic, because metal hydrides generally present sloping plateaus and absorption/desorption hysteresis. Those effects are included in the present study by using the Fang et al. model [12]. The inclusion of those features in the simulations produce several relevant effects that modify completely the results obtained from the simulation work. Taking into account those considerations and because simulation should be validated by experimental work, we describe in Section Microstructure and composition of the alloys the crystal structure and chemical composition of two metal hydride systems, a low and high equilibrium pressure AB2 hydrides, that could be the materials used in the first and second stages of a metal hydride compressor. Then in Section Real vs. ideal equations of state, we describe several equation of state (EOS) that can be used to replace ideal gas behavior in the simulation and discuss advantages and drawbacks of their practical use. Those EOS are used in Section Influence of the equation of state in P-c-T measurements to show how P-c-T determination differs from one another. Finally in Section Influence of the equation of state in the compressor simulation performance, we include all those effects in the simulation of a single stage metal hydride compressor and it is discussed how the equation of state used and the included realistic features in the P-c-T's are affecting the working point of the compressor.

Section snippets

Experimental

The samples selected for the compressor were synthesized from the elements (with a purity of Ti = 99.99%, Zr = 99.5%, Mn = 99.9%, Cr = 99.2%, V = 99.7%) by an arc melting process under an argon atmosphere of 0.7 bar. A 10% weight excess of Mn was used for the synthesis. The fusion of the materials was performed by turning the obtained ingots several times in order to homogenize their composition. The microstructure and elemental chemical composition were investigated by scanning electron

Microstructure and composition of the alloys

Fig. 1 evidences the Rietveld refinement of the XRPD data of both samples, showing the usual hexagonal C14-type Laves phases (SG P63/mmc; Nº 194) characteristic of TiMn2 based-type alloys [18], [19], [20], [21]. It has to be highlighted that the Ti0.9Zr0.1Mn1.34V0.3 is not a stoichiometric AB2, however it has the same main phase as the Ti0.9Zr0.1Mn1.47Cr0.4V0.2, which is due to the fact that the TiMn2 base-type alloys can vary its B2 (Mn) composition from 1.2 to 2 without changing the main C14

Conclusions

Several aspects of the behavior of the H2 gas should be taken into account in a metal hydride compressor simulation. Large pressures and accumulation of errors can generate great errors in the P-c-T determination if an ideal gas model is implemented. The difference between the real gas models and the ideal one is mainly driven by the real behavior of the H2 gas. The realistic P-c-T features of a material, addressed by the simulation, have a great effect on the final moles desorbed and the

Acknowledgments

The authors thank the Spanish MINECO for financial support under contract (MAT 2011-22780) and Mr. F. Moreno for technical assistance. One of the authors (A.R. Galvis E) thanks the COST action MP1103 for its financial and administrative support and also the Colombian Government and the Administrative Department of Science, Technology and Innovation -Colciencias-, for the financial support under contract 568.

References (28)

Cited by (0)

View full text