# Measurements of Operating Parameters of a Metal Hydride Compressor with a Heat Pump
## 6 Conclusions  
The publications that discuss the hydrogen compression using MH compressors primarily deal with higher ranges of the operating temperatures of the tanks. The investigating teams all share the same goal--to find a cheap source of heat and cold. The investigation into the concept of the MH compressor with the heat pump discussed in this paper was centred at the performance of the autonomous compressor that produced the range of operating temperatures by the thermal process that takes place inside the heat pump. The range of the operating temperatures of the tandem-arranged tanks was 12.3-54.9 \({}^{\circ}\)C, while the pressure was elevated from the average value of 0.56 to the value of 1.08 MPa. [PERSON] et al. [20] achieved the compressor's specific productivity of 67.2 L\({}_{\text{H2}}\)-kg\({}^{-1}\cdot\)h\({}^{-1}\). In the experiment with the MH compressor and the heat pump, the achieved productivity, expressed as the compressor's volumetric efficiency relative to the amount of the used alloy and the compression time, was 56.24 L\({}_{\text{H2}}\cdot\)kg\({}^{-1}\cdot\)h\({}^{-1}\). In this regard, it should be noted that in this experiment with the heat pump, the difference in the operating temperatures was lower, 42.6 \({}^{\circ}\)C, i.e., 39% less than that applied in paper [20]. Nevertheless, the productivity achieved in this experiment was only 16% lower. Metal hydride compressors have the potential to become economically competitive alternatives to standard volume and centrifugal compressors, with a higher level of safety and a lower consumption of the input energy. The newly designed metal hydride compressor with a heat pump demonstrates the possibility of using suitable metal alloys to increase the hydrogen pressure through the use of thermal energy. The purpose of using the heat pump with the tandem-arranged MH tanks was to reduce the consumption of input electric  
Figure 14: A comparison of the measured and calculated tank temperatures. energy while generating the required amount of thermal energy and refrigeration. The measurement data was used to validate the model describing the thermal balance over time, which must be known in order to design correctly the individual components of the MH compressor. On the other hand, the obtained values of the operating parameters helped to identify the key issues hindering the deployment of these types of compressors in real operations. In particular, the low thermal conductivity of the powder metal hydride leads to significant temperature differences in the alloy volume, and such differences result in the impaired heat removal by the internal heat exchanger. At the same time, it is necessary to maintain sufficient flow rates of the heat-transfer fluid in the heat exchanger in order for the overall heat transfer coefficient to reach the desired level. The achieved average value of the compression ratio, i.e., 2.0, indicates that the compressor can increase the hydrogen pressure: in ideal conditions, this value may be increased to 4 at temperatures ranging from 10 to 50 \({}^{\circ}\)C. The compression ratio may be further increased by replacing the heat pump that uses the R134a coolant with a heat pump that uses CO\({}_{2}\) as the coolant. This may facilitate increasing the typical operating temperature range by more than 30 \({}^{\circ}\)C, while achieving higher values of the COP. The prospect for the future is to replace the MH tanks with the tanks containing more efficient internal heat exchangers, which will contribute to increasing the compression ratio. Additionally, in order to achieve higher operating pressures at the outlet from the MH compressor, the plan is to add a second stage with an interconnection between its heat circuit and the heat pump using CO\({}_{2}\) as the working fluid; this will facilitate increasing the range of operating temperatures while maintaining a high COP. Conceptualisation, T.B. ; Formal analysis, M.L., N.J.; Visualization, T.B., M.L. ; Validation, T.B., N.J., M.L.