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Abstract
Metal hydrides have been investigated for use in environmentally friendly heat pumps. These systems operate using reversible adsorption and desorption of hydrogen from metallic compounds and can be incorporated in a cycle having a work input (compressor) or thermal energy input (generator). Some challenges faced by compressor driven metal hydride heat pumps are poor heat transfer in the metal hydride beds and high compressor discharge temperatures. To overcome these challenges, this article investigates the use of a metal-hydride slurry in conjunction with various isothermal compression techniques. Liquid-flooded, electrochemical, and liquid piston compressors were modeled and integrated into a system model in order to assess their impact on the performance of the slurry-based metal hydride heat pump system.
Nomenclature
| A | = | area [cm2] |
| ASR | = | area specific resistance [ Ω-cm2 ] |
| c | = | specific heat capacity [kJ-kg-1-K-1] |
| cv | = | constant volume specific heat capacity [kJ-kg-1-K-1] |
| COP | = | coefficient of performance |
| ϵ | = | solution heat exchanger effectiveness |
| F | = | Faraday Constant [Col-mol-1] |
| ΔHf | = | hydride Enthalpy of Formation [kJ-mol-1] |
| h | = | specific enthalpy [kJ-kg-1] |
| ht | = | heat transfer coefficient at time t [kW-(m2)-1-K-1] |
| i | = | current [A] |
| id | = | current density [A-cm-2] |
| = | mass flow rate [kg-s-1] | |
| Min | = | function to calculate the minimum value of a set of expressions |
| P | = | pressure [kPa] |
| ρ | = | density [kg-m-3] |
| = | heat transfer rate [kW] | |
| R | = | gas constant [J-kg-1-K-1] |
| T | = | temperature [K] |
| v | = | specific volume [m3-kg-1] |
| η | = | efficiency |
| V | = | volume [m3] |
| VEC | = | voltage [V] |
| VO | = | reversible voltage [V] |
| ω | = | frequency [Hz] |
| w | = | specific work [kJ- kg-1] |
| = | power [kW] | |
| WO | = | reversible power [W] |
| = | liquid piston velocity [m-s-1] | |
| xo | = | maximum hydrogen adsorption |
| x | = | fraction of maximum hydrogen adsorption |
Subscripts
| ads | = | adsorption |
| c | = | cooling |
| comp | = | compressor |
| des | = | desorption |
| EC | = | electrochemical |
| fric | = | friction |
| H2 | = | hydrogen |
| hyd | = | hydride |
| isen | = | isentropic |
| liq | = | liquid |
| s | = | surface temperature |
| solnHX | = | solution heat exchanger |
| s,s | = | slurry strong |
| s,w | = | slurry weak |