Low-temperature Metal Hydrides Research Articles

DESIGN AND PERFORMANCE OF FUNCTION PROVING AUTOMOBILE METAL HYDRIDE REFRIGERATION/AIR CONDITIONING SYSTEMS DRIVEN BY EXHAUST GAS

A function proving automobile metal hydride(MH) refrigeration/air conditioning system is designed. The MH working pair LaNi4.61Mn0.26Al0.13/La0.6Y0.4Ni4.8Mn0.2 with working temperature 150℃/20~50℃/0℃is researched, their Van't-Hoff diagram is drawn and reaction enthalpy, entropy, theoretical cycling coefficient of performance (e0) as well as minimum refrigeration temperature (θm) are deduced. Based on this working pair, reaction beds are designed, a function proving MH intermittent refrigeration water cooling system driven by heat conducting oil is constructed. The regulations of cycling performance andθm are determined by altering the average temperature of high temperature reaction beds. The test results show that the MH working pair has satisfied reaction dynamics and small hysteresis; The low temperature MH reaction enthalpy reaches -27.1 kJ/mol H2. The reaction beds have perfect mass transfer ability, while their heat conduction coefficients are merely 1.5 W/(m·K) which results in long cycle period and low cooling power per MH. Intermittent refrigeration cycle is achieved, 238 W cooling power is obtained at 150℃/30℃/0℃withe0 being 0.26. In the test range, with the growth of average temperature of the high temperature reaction beds, cooling power and systeme0 increase whileθm decreases, which agrees well with MH refrigeration theories.

Recent developments in hydrogen storage applications based on metal hydrides

Metal hydrides have been commercialized for battery applications for more than 8 years. In case of storage applications, metal hydrides were extensively evaluated in combination with combustion engines. The relatively low gravimetric energy density of hydride tanks based on low temperature metal hydrides prevented the commercial use of that technology. Recently, lasting progress in the PEM fuel cell technology offers chances for metal hydride storage systems mainly for low power applications, but also for niche markets. The paper describes promising projects on metal hydride storage technology and gives an outlook about improvements of both the metal hydride alloy performance and the performance of metal hydride storage tanks.

Measurement of the pressure-composition isotherms of high-temperature and low-temperature metal hydrides

Many interesting applications are based on the knowledge of the thermodynamic equilibrium of the metal–metal hydride–hydrogen system. Practical applications for this system include hydrogen storage for stationary and vehicular purposes, hydrogen compression, heat pumps, cooling apparatus as well as heat stores. One of the most advantageous metal hydrides for high-temperature heat stores is nickel-doped magnesium, a material with excellent kinetic properties, high hydrogen concentration, low density and high heat of reaction. A disadvantage of magnesium is the minimum reaction temperature of about 423 K. The objective of this research work is to find low-temperature metal hydrides whose reaction enthalpy ΔrH is able to heat up the magnesium, starting from ambient temperature. In this study the results of an isochoric measurement method for determining pressure–composition isotherms (PCI), reaction enthalpies and reaction entropies are presented on the basis of already investigated as well as hitherto unsurveyed metal hydrides. The measuring method considers the real gas behavior in addition to a test reactor, which employs an improved sealing system. Presented in this study are the results of five metal–metal hydride–hydrogen systems in thermodynamic equilibrium. The measuring method has proved to be reliable because of the test reactor's sealing system.

Feasibility study of a metal hydride hydrogen store for a self-sufficient solar hydrogen energy system

The feasibility of using metal hydride hydrogen storage in a self-sufficient solar hydrogen energy system is studied. Several potential commercial and non-commercial metal hydrides are considered to find a material having a low Δ H value, a low hysteresis effect, gentle P- C- T, plateau slopes and a high hydrogen storage capacity. A 1 N m 3 metal hydride container employing a commercial Hydralloy C15 metal hydride with the proper P- C- T curves is analysed in more detail. As the thermal behaviour of the container is crucial in our application, steady-state and time-dependent thermal properties of the container are measured and the respective models are derived. The metal hydride container is also tested under realistic conditions to get further operational experience on its technical feasibility. Based on this study, low-temperature metal hydrides seem to be technically and economically feasible for small-scale self-sufficient solar hydrogen systems in which high volumetric energy density is needed due to limited space.

The effect of mechanical stress on the absorption of hydrogen by metal hydrides

Measurements by other investigators and the research presented here have shown that the amount of hydrogen absorbed by metal hydrides depends, among other factors, also on the tensile forces within the bed of metal hydrides caused by external mechanical pressure. This dependency of the hydrogen uptake in the metal from the tension could quantitatively be demonstrated by means of equilibrium data derived from isochoric measurements in a specially constructed reactor for a low temperature metal hydride. Furthermore the term isothermal and isobaric isotense was newly introduced to replace the term concentration-pressure isotherm (CPI).

Research on kinetics of hydrogen sorption in low-temperature metal hydrides

The storage of hydrogen gas in metal hydrides depends critically on the sorption kinetics and the sorption isotherm. A special alloy sample (Ti 0.98Zr 0.02V 0.43Fe 0.09Cr 0.05Mn 1.5, in relation to the kinetics similar to LaNi 5) was investigated as a non-pressed bulk on the kinetic and equilibrium behaviour in the range 0–50°C and hydrogen pressures 0–30 bar in an isochoric measuring-equipment. A theoretical model for the description of the measurements is formed, which assumes homogeneity of the alloy sample, ideal behaviour of the hydrogen gas, that no local profiles occur, that the hydrogen sorption current is only determined by the cooling rate of the sample and that the sorption equilibrium settles instantaneously (no set-up for the reaction rate!), thus the anisothermy is taken into account by means of empirical equilibrium function.