# Measurements of Operating Parameters of a Metal Hydride Compressor with a Heat Pump
## 1 Introduction  
Typically, hydrogen is not found in nature in an unbound molecular form. It is most frequently produced by partial oxidation of natural gas, steam reforming, or electrolysis of water [1, 2, 3]. Any hydrogen that is not used immediately after it is produced must be stored [4, 5, 6, 7]. However, due to the low density of hydrogen, most storage methods require the use of a compressor to increase the hydrogen pressure to the required level. At high flow rates, the compression of hydrogen may be achieved by using a turbo- or reciprocating compressor. At low flow rates, it is possible to use a pneumatic pressure booster, which is also based on the reciprocating compression [8, 9, 10]. In the cases mentioned above, the energy demand for the compression process is very high [11]. The amount of the energy used in the compression reduces the overall transformation efficiency and thereby increases the cost of the fuel [12]. One of the possible means of hydrogen compression is the use of reversible hydrogen storage in metal alloys, while applying a method based on thermal changes [13]. This technology was also discussed in paper [14], which pointed out a continuous growth of the R&D activities in the development of the metal hydride hydrogen compression technology. A compressor works in an enclosed thermodynamic cycle, in which the hydrogen isstored at a low temperature due to the catalytic effects of metals [15]. This is followed by dissociation of the hydrogen molecules and diffusion of the atoms into an intermetallic structure of the alloy. Once the alloy has been saturated with hydrogen, it is necessary to supply heat from an external source, which increases the pressure as the operating point shifts to a higher isothermal curve. At a higher temperature, it is then possible to desorb hydrogen from the alloy and reduce the storage capacity to the baseline value. Once the temperature has been decreased, the baseline operating point will be restored. The principle of hydrogen compression through the absorption into a metal hydride has been known for more than 40 years. Many scientific teams have been working on developing metal hydride compressors that are suitable for real operations [16; 17]. The importance of this issue is also demonstrated by numerous studies that have been carried out in the field of hydrogen storage and compression globally over the last years. A three-stage metal-hydride-based hydrogen compressor (MHHC) has been designed for the equipment intended for hydrogen production using a low-pressure alkaline electrolyser, in which the compression system performs hydrogen desorption using the heat obtained from an electrolyser. This type of compressor achieved a compression ratio of 2/0.1 MPa while working with three different metal hydrides of the AB5 type at temperatures ranging from 20 to 80 \({}^{\circ}\)C [18]. The American Laboratory of Chemical Sciences and Material Systems carried out the investigation into thermodynamics and kinetics of a prototype Metal Hydride Hydrogen Compressor (MHHC). During their tests, the compressor worked without any problems while compressing 320 g of hydrogen. However, this solution may only become an alternative to mechanical or electrolytic compression if the process is fully automated and the waste heat is utilized [19]. The team led by [PERSON] developed a six-stage metal hydride compressor (MHC) that was able to compress hydrogen while shifting the pressure value from 7 bar to over 220 bar. This MHC exhibited a stable operation at temperatures ranging from 10 to 80 \({}^{\circ}\)C. The specific productivity of the metal hydride compressor achieved as much as 67.2 L\({}_{\mathrm{H2}}\)-kg\({}^{-1}\cdot\)h\({}^{-1}\)[20]. Deployment of a metal hydride compressor in a real operation was analysed in paper [21; 22]. Paper [21] provided an assessment of the technological and energy aspects related to the potential integration of a metal hydride compressor in the existing energy systems.