Prakash M Maiya

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.

Indoor thermal comfort levels in underground metro platforms and trains in Chennai metropolitan city are assessed in the present research study to determine the comfort perception of passengers. The field measurements were conducted in 7 underground metro stations during summer (Mar-Jun 2019) and winter seasons (Dec 2019 - Feb 2020). The thermal comfort of metro passengers is investigated through environmental parameters and a subjective survey. Results indicated that the percentage of unacceptability in metro platforms is in the range of 36.7–98.8%, while inside the metro trains it is less than 12%. This indicates the efficiency of the in-train air-conditioning systems compared to metro platforms. Good agreement is found between the findings of the thermal comfort investigation and the comfort perception of passengers. A linear and very strong positive correlation is found between operative temperature and the percentage of passengers dissatisfied. No relation is found between the metro stations' indoor and outdoor thermal conditions. Thus, it is inferred that internal factors such as the functioning of stations, operation of metro trains, and high density of passengers contribute significantly to indoor heat generation. During both seasons, the thermal conditions in all metro stations surpassed the ASHRAE-55 and ISO7730 guidelines. Stations with low passenger traffic are maintained with high operative temperatures to save energy costs. Based on the prevailing thermal conditions and ASHRAE recommendations of operative temperature, a Predicted Percentage of Dissatisfied (PPD) value of up to 40% could be deemed a thermal comfort metric that can be acceptable in underground metro stations.

Heat management during the absorption/desorption process is a key aspect in improving the performance of large-scale hydrogen storage systems. In this article, the absorption and desorption performance of a multi-tubular hydride reactor is numerically investigated and optimized for 60 kg mass of LaNi5 alloy. The 90% absorption with 7, 14, and 19 tubes is achieved in 985, 404, and 317 s with an overall reactor weight of 78.46, 88, and 88.2 kg, respectively. The 14-tube reactor performance is investigated by introducing the longitudinal fins inside the tubes. The reactor performance is enhanced by allocating fins into different pairs of half and full fins constrained by overall fin volume. A thermal resistance network model is presented to investigate the effect of fin distribution and coolant velocity on equivalent resistance of the metal hydride reactor. Storage performance obtained from numerical model validates the thermal resistance analysis from heat transfer viewpoint. With six full fins, 90% hydrogen absorption is achieved in 76 s. However, tubes with 6, 8, and 12 pairs of half and full fins require 74, 58, and 54 s, respectively. The 14-tube reactor with 8 pairs of half and full fins is used for quantifying the augmentation in the absorption performance in response to operating conditions (supply pressure and heat transfer fluid temperature). A design methodology is outlined for the development of a large-scale multi-tubular hydride reactor based on a heat transfer optimization strategy.

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.

Miniaturization of an alcohol-based absorption refrigerator requires an air-cooled absorber and condenser and the replacement of customary solution pumps by the bubble pump. Evaluation of such a refrigerator requires thermodynamic (specific heat and heat of mixing) and thermophysical (vapour pressure, density, viscosity, surface tension and solubility) properties of refrigerant-absorbent solution. These property correlations for five alcohol-based working combinations, majority of them obtained by curve fitting, have been complied and presented in this paper along with their validity ranges and percentage of error. The working fluids have been analyzed and compared with reference to the solution density governing the hydrostatic height, viscosity and specific heat affecting the heat and mass transfer and solubility to avoid crystallization. Further the variations of performance parameters like cut-off temperature, circulation ratio, coefficient of performance and efficiency ratio of these working fluids with respect to various operating conditions are discussed.

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.

Mischmetal (Mm) nickel (Ni) alloys with different aluminium (Al) contents are tested for hydrogen storage characteristics under dynamic operating conditions. The compositions considered are MmNi5, MmNi4.7Al0.3, MmNi4.5Al0.5, MmNi4.2Al0.8 and MmNi4Al. Pressure concentration isotherms (PCI), plateau slopes and absorption rates are measured. Activation energies are calculated by fitting Arrhenius equation to the absorption data. As expected, the plateau pressure of MmNi5 decreased with increase in Al content. Apparent reaction enthalpy values (as distinct from enthalpy of formation calculated from static measurements), plateau slope, and activation energies increased, and absorption rate decreased with increase in Al content. The study confirms MmNi4.5Al0.5 as a preferred candidate for reversible hydrogen storage at room temperature with nominal supply pressure and fast kinetics. © 2008 Elsevier B.V. All rights reserved.

The effective thermal conductivity (ke) of a MmNi 4.5Al0.5 hydride powder bed was measured using an one-dimensional steady-state axial heat transfer comparative method. The experiments showed that ke lies between 0.1 and 1.2 W/m K in the pressure and the temperature ranges of 0-50 bar and 0-100 °C, respectively. While hydrogen pressure and concentration showed strong influence on k e over the complete range, the temperature effects were minimal. Thermal conductivity of the solid materials, MmNi4.5Al0.5 alloy and its hydrides, was also back calculated from the experimentally measured ke values. © 2011 American Chemical Society.

Hydrogenation of metals is an exothermic and reversible process. Thus, metal hydride reactors/devices become essentially heat-driven. Excellent heat control in the MH reactor is required to develop metal hydride devices such as H2 storage systems successfully. Few attempts at nature-inspired designs have proven to have good heat transfer capabilities. Based on this idea, the present study investigates novel bio-inspired leaf-vein type fins for the metal hydride reactor. Two reactor designs are proposed for heat transfer fluid flow, namely (i) central straight tube and (ii) narrow trapezoidal channels with 10 kg of LaNi5 as a sample alloy. Compared to longitudinal finned single tube reactors (LFSTR), these designs provided better heat transmission and temperature uniformity. For LFSTR, Case-1, and Case-2, 90% storage capacity was reached in 210, 145, and 80 s. Different fin configurations, such as parallel, inclined fins, and fins of different thicknesses, are investigated further in the design with narrow trapezoidal channels. The inclined fin configuration shows better performance, and it is further optimized by varying the inclination angle from 3 to 9° and the fin number from 2 to 4. The optimized design with a 7° inclination angle and four fins required 57 s to attain 90% storage capacity and reduced absorption time by 73% compared to LFSTR. The influence of operating parameters such as hydrogen supply pressure, inlet temperature, and velocity of the heat transfer fluid on the performance is evaluated for the optimized design.

Mischmetal-nickel (Mm-Ni) alloys with single (Al) and multiple (Al, Co, Mn, Fe) substitutions for Ni are studied for their structural, hydrogen storage and thermodynamic properties. The alloys considered are MmNi5, MmNi4.7Al0.3, MmNi4.5Al0.5, MmNi4.2Al0.8 and MmNi4Al for single substitution, and MmNi3.9Co0.8Mn0.2Al0.1, MmNi3.8Co0.7Mn0.3Al0.2, MmNi3.7Co0.7Mn0.3Al0.3, MmNi3.6Co0.6Mn0.3Al0.3Fe0.2 and MmNi3.5Co0.4Mn0.4Al0.4Fe0.3 for multiple substitutions. The XRD patterns of all the alloys show single phase with the reflection peaks related to the CaCu5 hexagonal structure. All the multiple substituted alloys absorb and desorb hydrogen at sub-atmospheric pressures. The equilibrium pressure and hysteresis decrease, while enthalpy of formation (ΔH) and plateau slope increase with increase in unit cell volume, indicating an increase in the stability of the alloys. © 2008.