Alloy selection for dual stage metal-hydride hydrogen compressor: Using a thermodynamic model to identify metal-hydride pairs

Metal-hydrides offer a potentially competitive method for compressing hydrogen, particularly where waste heat is available. Metal hydrides are metal alloys or intermetallic compounds that react reversibly with H2. They readily absorb low-pressure H2 at low temperature, and then release H2 at a higher pressure when the temperature is raised. The high pressure H2 is released at a pressure above that expected from standard press-temperature relations. The absorption and desorption pressures of the hydrides are determined by their thermodynamic properties (enthalpy and entropy). To achieve compression ratios above about 10, more than one stage of compression is typically required. The challenge is to find alloy pairs that can work together effectively to achieve the desired compression from the heating/cooling available. Previously, a thermodynamic model has been proposed for identifying suitable metal hydrides that can be paired together to achieve a desired compression. This paper describes methods used previously in the literature to select alloy pairs, and applies the current method to an example selection of 33 hydrides with potential for hydrogen compression. The example application aims to find pairs that can compress a H2 stream from 10 to 350 bar using a temperature range of 30–150 °C, however the theory could readily be adapted to different compression ratios and temperature ranges. For the specific example evaluated none of the potential pairs were able to meet the compression target, however, modification of the parameters (heating/cooling availability) or alloy properties could resolve this issue.