IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Large batches (800 g) of nanostructured Mg - x wt % MmNi5 (x = 10 - 70) composites were prepared by ball milling elemental Mg with MmNi5 in an attritor for 12 h under hydrogen atmosphere. There was no alloy formation between Mg and MmNi5 within the milling times employed. The grain size of Mg in the composite varied from 28 to 45 nm, increasing with Mg content, whereas that of MmNi5 was constant at 12 nm, irrespective of its concentration in the composite. BET surface area of the milled composites was in the range of 25 to 58 m2 / g, increased with MmNi5 content. The absorption kinetics and capacities of these composites were measured at 100, 200 and 300 {ring operator} C under 30 bar hydrogen pressure on a small sample of about 8 g. The samples absorbed at all the temperatures, the absorption rate increases with increase in MmNi5 content. But the rates decreased marginally with temperature and attained steady state in less than 400 s even at 100 {ring operator} C. However, the hydrogen absorption capacity of the composites followed the Mg content at all the temperatures, with Mg - 10 wt % MmNi5 showing the highest capacity of 5.1 wt%. MmNi5 has a significant effect on the absorption of hydrogen by Mg at temperatures as low as 100 {ring operator} C, even though hydride of MmNi5 was not present in the hydrogenated composite. The nanostructure of Mg together with distribution of MmNi5 on grain surface/grain boundary of Mg appears to have enhanced the absorption, with MmNi5 probably acting as a conduit for hydrogen diffusing into Mg grain. The performance of hydrogen storage device with larger quantities of Mg - MmNi5 composites (350-500 g) was evaluated in the temperature range of 100 - 150 {ring operator} C with supply pressure of 10-30 bar. The absorption rates and quantity of hydrogen absorbed increased with supply pressure and decreased with temperature. The fraction α was more than 70% in compositions with up to 10 - 30 wt % MmNi5, but was much lower at higher MmNi5 content. The absorption behaviour of these composites followed the same trend as in the case of testing on smaller sample size. © 2006 International Association for Hydrogen Energy.

Department of Mechanical Engineering

International Journal of Hydrogen Energy

Application of Mg - x wt % MmNi5 (x = 10 - 70) nanostructured composites in a hydrogen storage device

A CFD analysis of heat and mass transfer in cylindrical metal hydride beds is carried out using the commercial code Fluent 6.2. The effect of bulk diffusion is considered for mass transfer in the solid phase. Temporal and spatial variations of temperature and concentration in hydride bed are plotted. Emphasis is given to monitor the motion of hydrogen within the bed and to the influence of the L / D ratio and porosity. It is observed that a concentration variation in the bed is the driving force for hydrogen flow in hydride beds. The gas movement is observed to be from saturated cooler peripheral region towards the unsaturated hotter core region of the bed. © 2006 International Association for Hydrogen Energy.

GENERAL

Simulation of transient heat and mass transfer during hydrogen sorption in cylindrical metal hydride beds

Pressure-concentration isotherms (PCI) are measured by both static and dynamic methods for mischmetal based alloys, MmNi3.9 Co0.6 Al0.5 and MmNi4 Al. The effects of hydrogen flow rate on plateau pressure, enthalpy of formation and entropy of formation are studied. For MmNi3.9 Co0.6 Al0.5 both shape of the PCI and plateau pressure are dependent on the flow rate. The effect on plateau pressure is not significant in the case of MmNi4 Al. Both materials showed significant variations in enthalpy of formation with flow rate due to variation in reaction rate. The effect of flow rate on desorption isotherm is negligible as the desorption equilibrium pressure is much lower compared to absorption equilibrium pressure due to large hysteresis in these materials. While the basic nature of variation of pressure with concentration is same for both static and dynamic PCIs, the thermodynamic properties estimated based on these data vary significantly. © 2006 International Association for Hydrogen Energy.

Influence of transient operating conditions on pressure-concentration isotherms and storage characteristics of hydriding alloys

A practical metal hydride based hydrogen storage device would consist of many filters to distribute hydrogen gas and heat exchanger tubes to cool or heat the hydride bed depending on whether hydrogen is being absorbed or desorbed. This paper presents the simulation of such a device with LaNi5 as the hydriding alloy. A study of the geometric and operating parameters has been carried out to identify their influence in the hydriding performance of the storage device. © 2007 International Association for Hydrogen Energy.

Performance simulation of metal hydride hydrogen storage device with embedded filters and heat exchanger tubes

Nanocrystalline Mg-x wt.% Cr2O3 (x = 5-20) composites were prepared by ball milling Mg and nanocrystalline Cr2O3 under toluene. The Cr2O3 used (particle/crystallite size of 80 nm) was prepared by combustion synthesis of chromium nitrate and sucrose. Grain sizes of Mg and Cr2O3 in the composites, based on XRD peaks, were in the range of 32-37 and 76-90 nm, respectively. Absorption/desorption kinetics of these composites at 100, 200 and 300 °C were measured. In all cases absorption rates increased with Cr2O3 content at each temperature studied and absorption capacity decreased with decrease in temperature. Maximum hydrogen absorption at 300 and 200 °C was in Mg-5 wt.% Cr2O3 composite whereas at 100 °C it was in Mg-15 wt.% Cr2O3 composite. The higher absorption capacity of Mg-15 wt.% composite at 100 °C was probably due to smaller interparticle distance of Cr2O3 on Mg surface/grain boundary. All the composites exhibited a large absorption in the initial few seconds at all the temperatures. This has been attributed to adsorption aided by the catalyst, and subsequent surface and grain boundary diffusion. Desorption was incomplete in all the composites at 300 °C and only marginal at 200 and 100 °C. Desorption rates at 300 °C increased with increase in Cr2O3 content in all the compositions, while the desorption capacity increased with increase in Cr2O3 content till 15 wt.% and decreased with further increase in Cr2O3 content. The catalytic effect of Cr2O3 was only in absorption process and does not seem to have as large an influence on desorption process. © 2006 Elsevier B.V. All rights reserved.

Journal of Alloys and Compounds

Hydrogen storage properties of Mg-Cr2O3 nanocomposites: The role of catalyst distribution and grain size

Metal hydride-based hydrogen storage devices are tested using AB 5 alloys, namely MmNi4.6Fe0.4 and MmNi 4.6Al0.4. Performance studies are carried out by varying the supply pressure, absorption temperature and overall heat transfer coefficient. At any given absorption temperature, hydrogen absorption rate and storage capacity are found to increase with supply pressure for both the alloys. At a supply pressure of 35 bar and a cold fluid temperature of 15°C, MmNi4.6Fe0.4 alloy stored about 1.6 wt%, while MmNi 4.6Al0.4 stored 1.3 wt%. Cold fluid temperature is found to have a significant effect on hydrogen storage capacity at lower supply pressures. The overall heat transfer coefficient has a negligible influence on the hydrogen storage capacities of both the alloys. However, higher values of overall heat transfer coefficients yield better rates of absorption and desorption. © 2005 International Association for Hydrogen Energy. © 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Experiments on a metal hydride-based hydrogen storage device

A metal hydride based hydrogen compressor is studied experimentally by varying the operating parameters such as supply pressure and heat source temperature using MmNi4.6Al0.4 alloy. The compressor performance is found to improve at higher heat source temperatures. For a given hot fluid temperature, the influence of supply pressure on system performance is prominent up to about 10 bar. Maximum efficiency of 7.3% is obtained at a pressure ratio of 8.8 at 95°C heat source temperature. Results obtained by a simulation compared well with the experimental data. © 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Experiments on a metal hydride based hydrogen compressor

Intermetallic Mg2Ni-based alloys of different compositions, namely stoichiometric (Mg2Ni), superstoichiometric (60 wt%Mg) and substoichiometric (40 wt% Mg), were synthesised by mechanical alloying in a high-energy planetary ball mill. While the latter two were wet milled, the former was prepared by both wet and dry milling. The alloyed powders were characterised for particle size by laser diffraction and phase content by XRD. Thermal annealing preceded by 12 h of dry milling was found to accomplish the complete formation of Mg2Ni phase. Dry milling is found to be better in terms of milling time and purity of the alloy formed. The lower the Mg content, the faster is the formation of Mg2Ni phase. Pressure composition isotherms revealed the presence of hysterisis. The plateau pressure increased with increase in Ni content at 250°C and 300°C. The maximum absorption of hydrogen was close to the theoretical limit at all temperatures in stoichiometric and superstoichiometric compositions (∼94% and ∼90%, respectively). However, the substoichiometric composition absorbed considerably less than its theoretical limit (∼84%). The absorbed and desorbed quantities were similar in the given timeframe at the temperatures 330°C and 300°C, with the superstoichiometric composition showing the highest value. The absorbed quantity and absorption rate at 250°C were comparable for all the compositions to the values at the higher temperatures, but the desorption capacity was poorer, with Mg2Ni showing the highest value. © 2004 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Comparative evaluation of Mg-Ni hydrogen absorbing materials prepared by mechanical alloying

A metal hydride water pumping system for low head and high discharge applications is analyzed. A gear system converts the large desorption to absorption pressure differential into large volume displacement. A parametric study is carried out for LaNi5 alloy for various pump heads, absorption temperatures and gear ratios. It is observed that higher the desorption temperature, the higher is the optimum desorption pressure maximizing the specific water discharge. © 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.

Metal hydride water pumping system for low head-high discharge applications

The desiccant assisted air-conditioners yield lower humidities in the conditioned space compared to the conventional systems. In this paper, the effects of design variables of the air-conditioner, namely supply airflow rate, compressor pumping capacity (compressor speed) and desiccant wheel speed on its performance are studied. It is found that an optimum wheel speed of about 17.5 rph exists at which both moisture removal capacity and COP are maximum. © 2004 Elsevier Ltd. All rights reserved.

Applied Thermal Engineering

Parametric studies on a desiccant assisted air-conditioner

Composites based on Mg with intermetallic FeTi (up to 30 wt.%) and FeTiMn (40 wt.%) were prepared by mechanical alloying for evaluation of their hydrogen storage characteristics. In the first case FeTi was prepared by mechanical alloying and subsequently milled with Mg under argon in different proportions. Powders milled under argon had to be activated by remilling under hydrogen. In the second case FeTiMn prepared by melting was milled together with MgH 2 under hydrogen. Hydrogen storage characteristics, pressure-composition isotherms and kinetics were investigated under different parameters. Among the FeTi composites, Mg-5 wt.% FeTi had the maximum absorption capacity of 5.80 wt.% at 300°C and 5.12 wt.% at 200°C. At 300°C the initial desorption rate was the highest for Mg-10 wt.% FeTi, but its final desorption capacity was less than that of Mg-5 wt.% FeTi. None of the compositions showed any desorption at 200°C. In the case of composites with FeTiMn, absorption and desorption temperatures decreased by ball milling and higher storage capacities were achieved. In Mg-40wt.% FeTiMn, the lowest absorption and desorption temperatures were 80 and 240°C, respectively, with the material absorbing 4 wt.% hydrogen even at 80°C. The kinetics improved with increasing milling time. © 2004 Elsevier B.V. All rights reserved.

Characterisation of Mg-x wt.% FeTi (x = 5-30) and Mg-40 wt.% FeTiMn hydrogen absorbing materials prepared by mechanical alloying

Control of relative humidity is an essential aspect of maintaining indoor air quality in an air-conditioned space. In this paper, a desiccant wheel integrated air-conditioner is studied for low humidity air-conditioning. The desiccant wheel dehumidifies and heats the supply air. The regeneration of desiccant is accomplished by the return air, which gets cooled and humidified. The study shows that the proposed system can deliver supply air at much lower dew point temperature compared to the conventional system with a marginal penalty on COP. Its performance is better than the typical reheat system to provide the same low humidity levels. © 2004 Published by Elsevier Ltd.

Application of desiccant wheel to control humidity in air-conditioning systems

The concept of using metal hydrides in cold storage modules is discussed. Refrigeration can be stored in such modules at ambient temperature for subsequent cooling applications. Analysis of a system based on ZrMnFe/LaNi5 is done for various operating conditions and its performance characteristics are presented. Optimum heat source temperatures to yield maximum COP are computed. In view of the high cost of hydrogen valves, cold storage module without hydrogen valve is also comparatively studied. © 2002 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy.

Analysis of a metal hydride cold storage module

The concept of mass recovery in metal hydride systems is studied with a single stage multi-bed cooling system as example. Mass recovery results in variation of bed temperatures due to removal or addition of heat of desorption or absorption respectively. Coefficient of performance and cold output increase while required heat input decreases for the mass recovery cycle. Thus mass recovery between hydride reactors is found to improve system performance compared to that of a basic system. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

International Journal of Thermal Sciences

Performance analysis of a single stage four bed metal hydride cooling system, part A: Influence of mass recovery

Experimental analysis of a H2O-LiBr vapour absorption cooler based on bubble pump technique and hydrostatic principle are presented. Heat input to the bubble pump and boiler, pump lift/driving head and weak solution concentration are varied to analyze the system performance. The cooler is tested at different load conditions, i.e., at no load, pull down load and steady load conditions. Results indicate that coefficient of performance of about 0.50 could be attained for the boiler temperature of 85 °C and condenser and absorber temperatures in excess of 40 °C. © 2003 Elsevier Ltd. All rights reserved.

Experimental analysis of a bubble pump operated H2O-LiBr vapour absorption cooler

The effects of combined heat and mass recovery on the performance of a single stage multi-bed metal hydride cooling system is studied. A combined recovery cycle which effectively utilizes the advantages of both heat recovery and mass recovery is described. Reduction in sensible heating and cooling requirements along with increased hydrogen desorption results in improvement of coefficient of performance. Operating temperatures exert significant influence on the performance of combined recovery cycle. It is found that combined recovery is more effective at lower heat source temperatures and also at higher intermediate temperatures. © 2003 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Performance analysis of a single stage four bed metal hydride cooling system, part C: Influence of combined heat and mass recovery

Influence of heat recovery on the performance of a single stage multi-bed metal hydride cooling system is studied with Zr 0.9 Ti 0.1 CrFe and Zr 0.7 Ti 0.3 CrFe as high and low temperature alloys respectively. The heat recovery cycle effectively utilizes the sensible heat contained in the beds at the end of desorption and resorption processes thereby reducing the heat input and increasing the refrigerating effect. Compared to the basic cycle, the coefficient of performance of heat recovery cycle improves by about 15% to over 75% depending on the operating temperatures. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.

Performance analysis of a single stage four bed metal hydride cooling system, part B: Influence of heat recovery

The principle of metal hydride based thermal sorption compressor is adapted for water pumping application. Desorption of hydrogen at high pressure and its absorption at low pressure by a metal hydride are used to expand and contract a bellow which pumps an equivalent amount of water. A part of the pumped water is used to cool the bed to facilitate the absorption process, simultaneously yielding warm water. A parametric study is carried out for mischmetal alloy (MmNi4Al) at various pump heads, warm water temperatures, dead volumes of bellow and heat loss through bellow. It is observed that hydride based water pumping is suitable for 'low discharge and high head' conditions, particularly due to its capability to heat a portion of pumped water. © 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.

Parametric studies on a heat operated metal hydride based water pumping system

Influence of operating parameters such as heat source and sink temperatures, operating pressures, pressure ratios, cycle time, and bed parameters such as overall heat transfer coefficient, bed thickness and thermal conductivity, on the performance of a single stage hydrogen compressor is presented. Coupled heat and mass transfer and reaction kinetics are considered for the analysis. An AB2-type alloy, Ti0.98Zr0.02V0.43Fe0.09Cr0.05Mn1.5 is chosen as an example. At a given pressure ratio, the hydrogen throughput increases with hot fluid temperature and decreases with increase in cold fluid temperature. Optimum values of thermal conductivity and overall heat transfer coefficient exist for any given hot and cold fluid temperatures. Among the variables studied, heat transfer fluid temperature, bed thickness and supply pressure are found to exert significant influence on the compressor performance. © 2002 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy.

Parametric studies on a metal hydride based single stage hydrogen compressor

The 1201-type nominal (Hg0.9 Re0.1)Sr2CuO4+δ samples, synthesized at ∼ 920°C in partial vacuum, show, from temperature dependent dc-magnetization, Tconset ∼ 54 K. The CuK-edge features obtained from X-ray absorption measurements on the sample show predominantly Cu2+ state and a weak but distinct Cu1+ signature due to superconducting 1201 phase (after taking due account of impurity Sr2CuO3, and SrCuO2 phases). The presence of Cu1+ signature in SC samples is consistent with our earlier observations of Cu1+ in Ba-free superconducting (Hg,Cr)-1201 and (Hg,Mo)-1201 materials.