Elsevier

Intermetallics

Volume 57, February 2015, Pages 60-67
Intermetallics

Studies on La based intermetallic hydrides to determine their suitability in metal hydride based cooling systems

https://doi.org/10.1016/j.intermet.2014.10.004Get rights and content

Highlights

  • Suitability of La based intermetallics in metal hydride cooling system was studied.

  • Effect of composition change on transferable amount hydrogen was studied.

  • Fine tuning of the La based intermetallics for MHCS was discussed.

Abstract

The La0.9Ce0.1Ni5, La0.8Ce0.2Ni5, LaNi4.7Al0.3 and LaNi4.6Al0.4 intermetallic hydrides were characterised to determine their suitability in metal hydride based cooling system (MHCS). The performance of the MHCS depends on the driving potential, and rate and amount of hydrogen transfer between coupled metal hydride (MH) beds. Therefore, the effect of compositional changes on the variation of these parameters due to the variation in pressure – concentration isotherm (PCI) properties (storage capacity, plateau slope, hysteresis effect, etc.) of metal hydrides employed was studied. Compared to other possible MH pairs, La0.9Ce0.1Ni5 – LaNi4.7Al0.3 hydride pair exhibited high driving potential and transferrable amount of hydrogen, during cooling and regeneration processes, thereby better MHCS performance. In addition, the effect of change in hydrogen concentration (due to hydrogen transfer between coupled MH beds) on reaction enthalpy (ΔH) consequently on MHCS performance was found significant.

Introduction

The unfavourable environmental effects from a conventional refrigeration system due to use of chloro – fluro – carbon (CFC) as a refrigerant have drawn attention to metal hydride based cooling systems (MHCSs), which can operate using low grade energy, such as solar heat, industrial waste heat, geothermal, etc. LaNi5 based metal hydrides are widely used to develop MHCSs due to their excellent hydrogen storage characteristics, thermal stability and reaction kinetics. To generate the desired PCI, thermodynamic and kinetic properties for this application, partial substitutions for either Lanthanum (La) or Nickel (Ni) are employed [1], [2], [3], [4], [5], [6]. Typically, Ni is partially replaced with Al, Cu, Cr, Sn, Fe, Mn, Co, Ga or Zn. Among these elements, Al, Co and Mn are commonly substituted for Ni. Partial substitution with Al improves the cycling performance and significantly reduces the plateau pressure. The decrease in plateau pressure is due to an increase in the unit cell volume [1]. Additionally, the kinetic properties of Al substituted LaNi5 hydrides were studied by several investigators [7], [8], [9] and reported that the reaction rates of La–Ni–Al intermetallics are faster than that of LaNi5 hydride. Partial substitution with Co decreases the reaction rates [10], and partial substitution with Mn reduces the plateau pressure [11] without affecting the material's hydrogen storage capacity. The LaNi4.5T0.5 (T = Fe, Co and Cu) reaction rates were studied by Zarynow et al. [12], they observed the decrease in desorption rates and plateau pressure in the order Ni > Cu > Co > Fe.

Typically, La is partially replaced with other rare earth elements, such as Ce, Pr, Nd, Sm, Gd and Er. Vanmal et al. [2] showed that partial substitution of other rare earth metal(s) in LaNi5 increases the plateau pressure and decreases the hydride stability without affecting its hydrogen storage capacity [13]. For all AB5 type intermetallic hydrides, the Ce to La ratio strongly affects the plateau pressure and hysteresis [14]. High plateau pressure can be generated through partial substitution of La by Ce and Sm; such intermetallics can be used to build metal hydride based hydrogen compressors. Uchida and co-workers [15] studied the hydriding properties of La1−xCexNi5 and found that adding cerium increased hysteresis during hydrogen absorption and desorption, whereas the hydrogen storage capacity was less affected. Recently, it was reported that the hydrogen storage capacity of La1−xCexNi5 hydrides increased for x = 0–0.5; beyond x = 0.5, it led to a decrease in the hydrogen storage capacity. It was also reported that the plateau pressure increases with a decrease in the unit cell volume through Ce substitution [16], [17]. Additionally, studies on hydrogen storage, thermodynamic and kinetics properties of different La–Mg–Ni intermetallics have been done by several researchers [18], [19], [20] and it was reported that La–Mg–Ni based hydrides has low dissociation temperature and are good candidate for reversible hydrogen storage applications.

The substitutions offer a wide variety of PCI and thermodynamic properties. Reliable data on the hydrogen storage and thermodynamic properties of these intermetallics are essential for thermodynamic analyses and metal hydride based engineering device design. Many experimental and theoretical studies on performance evaluation of metal hydride based cooling systems working at different operating temperatures are available in the literature [21], [22], [23], [24], [25], [26], [27], [28]. The thermodynamic cycle analysis and heat and mass transfer aspects during hydrogen absorption and desorption were studied. Dantzer et al. [25], [26], [27] have experimentally and theoretically studied the effect of transferrable amount of hydrogen and pressure difference on the performance of cooling system. It was reported that the system performance vary with hydrogen transfer between the beds. Also, the van't Hoff plot constructed at mid plateau pressure not valid for estimation of thermodynamic properties under hydrogen transfer process. Additionally, a broad review on metal hydride based heating and cooling systems [28] and metal hydride based hydrogen compressors [29] were published. They have discussed different single stage and multi stage thermal systems and also reported the procedure for selection of alloys pairs as well as the possible ways for improving system performance. Recently, Vinod and Anil [1] have studied the effect of different measurement parameters on PCI and thermodynamic properties of Al substituted LaNi5 based metal hydrides. It was reported that for the development of any metal hydride based thermodynamic devices, the PCI and thermodynamic properties of metal hydrides employed should be measured at appropriate operating conditions like charging/discharging pressure difference, temperature range, etc.

Study of literature revealed that the important challenge in the development of MHCS is the selection of metal hydride pair, and evaluation of its suitability. The suitability and potential of MH pair for the development of MHCS depends on the achievable driving potential, and rate and amount of hydrogen transfer during cooling and regeneration processes. Therefore, in the present study, we tried to fine tune some La based (La0.9Ce0.1Ni5, La0.8Ce0.2Ni5, LaNi4.7Al0.3 and LaNi4.6Al0.4) intermetallic hydrides to identify the MH pair which gives the best MHCS performance. It brings a systematic study on the effects of MH bed composition and hydrogen concentration on the parameters like reaction enthalpy, driving potential, and rate and amount of hydrogen transfer between metal hydride beds, which influence the cooling system performance. The proposed MH pairs are La0.9Ce0.1Ni5 – LaNi4.7Al0.3, La0.9Ce0.1Ni5 – LaNi4.6Al0.4, La0.8Ce0.2Ni5 – LaNi4.7Al0.3 and La0.8Ce0.2Ni5 – LaNi4.6Al0.4.

Section snippets

Experimental measurements

A Sievert type experimental construction with the operating temperature range between 0 and 240 °C and 100 bar maximum pressure was used for the PCI measurements. A fully sealed reactor composed of an SS – 316 tube with a 12 mm inner diameter and 2 mm wall thickness was used. We ensured that the metal hydride sample was maintained at a constant temperature during the PCI measurements. The amount of hydrogen absorbed and desorbed was calculated based on the mass balance of hydrogen. The

Results and discussion

To achieve best MHCS performance, proper selection of MH pair is important. The MH pairs are selected based on the PCI, thermodynamic and kinetic properties. The following sections present the characterisation of metal hydrides to estimate their suitability for MHCS.

Conclusions

The principle of use of a pair of the metal hydrides for cooling cycle, and its thermodynamic and heat and mass transfer analysis are well known from the literature. The present study enlightens on selection of the best MH pair for the development of MHCS. The studies on La based intermetallic hydride pairs for MHCS development led to the following conclusions:

  • For better MHCS performance (COP, SCP, CR and ηE), the MH pair should possess high hydrogen storage capacity, flat plateau, low

Acknowledgements

The authors would like to thank Dr. V. S. Tiwari (SOG) and Dr. Gurvinderjit Singh (SOF), RRCAT, Indore for providing the XRD characterisation facility for the alloys used.

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