Hydrides hysteresis

There are a number of engineering properties relating to practical applications of hydrides hysteresis, reversible capacity, decrepitation, activation, reaction kinetics, impurity tolerances, chemical stability, heat transfer, safety, cost and... 

Hysteresis, the pressure difference between absorption and desorption, should be small for most practical applications of hydrides. Hysteresis varies markedly from alloy to alloy in a manner not understood by researchers in the field. However, an empirical body of knowledge now exists to show a number of systems where hysteresis can be made quite small. The nickel-aluminum-mischmetal system (e.g., Figure 2) is a good example of one with small hysteresis. 

The plateau region on the PCX cnrves in Fig. 2.43a is very flat, showing only a minimal slope (Sect. 1.4.1). However, each absorption-desorption pair of the PCX curves clearly exhibits a pressure hysteresis. This means that the pressure needed for absorption (hydride formation), is always greater than that of hydride decomposition, p. The cause of pressure hysteresis in metal hydride systems is not fnlly nnderstood. A number of models that attempted to explain this phenomenon... 

S. Qian, D.O. Northwood, Elastic and plastic accommodation effects on hysteresis during hydride formation and decomposition, Int. J. Hyd. Ener. 15 (1990) 649-654. 

The thermodynamics of Reaction 5.43 are comparable to those of other metallic, ionic and covalent hydrides. NaAlH4 exhibits typical low hysteresis and a two-plateau absorption and desorption isotherm. 

There are two basic cycles for heat pump operation conventional (15) and temperature upgrading (3, J/[). It is convenient to visualize the operation of these cycles by following the changes in pressure and temperature of each alloy on a Van t Hoff plot. For ease of narration, these curves are idealized. Many engineering properties of metal hydrides must be considered in a detailed explanation (e.g., hysteresis, plateau slope, cyclic stability, etc.). The two cycles are shown in Figure 17. 

Absorption experiments for Nd2Co7 and Dy2Co7 show that the quantity AP/P is small (<0.20) for the a — / transition in each system. F is the desorption plateau pressure (in atm), and AP is the difference between the absorption and desorption plateau pressures. AP/P at different temperatures for the same hydride essentially was constant. For the 0 y transition, surprisingly, no hysteresis was observed. The hysteretic behavior of the RC03 hydrides differed somewhat from the behavior of the R2C07 hydrides in that AP/P was consistently larger for the 0 —1 y transition than for the a0 transition. For Ce2Co7, AP/P... 

Figure 15-13. Hysteresis during formation and decomposition of hydrides activity aH vs. mole fraction. VH (schematic).

It is necessary tell the difference between the idealized cycle of operation HHP (Camo cycle) and a real cycle. The Camo cycle has maximal COP a for the chosen range of temperatures (7h -Tm -7j). The real cycle is connected to cyclic of HHP operation and always COPleai— COPu, never amount to it. For approach a Camo cycle it is necessary to reduce a hysteresis of absorption[desorption pressure and slope of curves P-C-T for chosen hydrides. If in a range of working loop variables some pairs hydrides get best of them will be pair having the greatest COPKd. ... 

In periodically working cyclic installations it is important to use hydride systems with the maximal contents of active hydrogen. Questions of revealing of new hydride materials are extremely important from the point of view of a ratio of the prices and efficiency. Concentration of active hydrogen depends on hysteresis Fh=Pd/Pa, a plateau slope =ln(Pci/Pcz) (Cj-C2). The hydrides most suitable to... 

HHP under other approximately identical conditions are what have a small plateau slope and a hysteresis of pressure at absorption and desorption. This position is the basic from a point of view of increase in amount of the active hydrogen circulating in HHP, and improvements of HHP efficiency. For example, the amount of hydrogen in inverse proportion also can decrease three times at changeFh with 0.9 up to 0.6. An ideal case is hydride which does not have hysteresis of pressure and plateau slope. 

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