# Measurements of Operating Parameters of a Metal Hydride Compressor with a Heat Pump
## 4 Measurements of the Operating Parameters of the MH Compressor with a Heat Pump  
Prior to the measurements, the system was vacuumed with the aim of removing potential undesired gases. This was followed by setting the baseline volumes of hydrogen in individual MH tanks. The MH1 tank was set to the operating point C, as shown in Figure 1, with a baseline hydrogen volume of 280 litres. The MH2 tank was activated in the operating point A, with a baseline hydrogen volume of 80 litres. The MH1 tank was then heated to a temperature of 51 \({}^{\circ}\)C to induce the hydrogen desorption at a higher pressure, and the temperature of the MH2 tank was reduced to 10 \({}^{\circ}\)C to induce a subsequent hydrogen absorption. By heating the MH1 tank, the operating pressure was stabilized  
Figure 5: The MH tank and the thermal balance diagram. at a value of 1.46 MPa. By cooling the MH2 tank, the pressure was stabilized at a value of 0.3 MPa. After opening the V2 valve, the hydrogen was supplied to the MH2 tank for absorption. Simultaneously, the V3 valve was opened to facilitate hydrogen desorption from the MH1 tank. Figure 6 shows the hydrogen flow rates at the compressor inlet and outlet over the period in which the measurements were carried out. During the measurement, which lasted 1700 s, 487 data were recorded from each sensor installed in the MH compressor; the measured data were therefore plotted as smoothed curves. The flow rates were regulated manually using thin dosing valves; that is why the curves exhibit significant degree of dispersion. By integrating the flow rates over time, the total volumes of the hydrogen that flew through the compressor inlet and outlet were identified (Figure 7). Over the entire measurement period, 166 litres of hydrogen were supplied into the MH1 tank, while 188 litres were removed from the MH2 tank. This disproportion was caused by the fact that the MH1 tank was heated more intensively, using the heat pump condenser. Identical volumes may be achieved by transferring a larger amount of the heat to an external cooler. The tank temperatures recorded during the measurements are plotted in Figure 8. The temperature of the MH2 tank could not be maintained at a value of 10 \({}^{\circ}\)C during the hydrogen absorption due to a low intensity of the heat removal at higher hydrogen flow rates. The heating power supplied to the MH1 tank was more intensive and facilitated achieving a temperature higher than 50 \({}^{\circ}\)C; this contributed to achieving higher compression ratios. In order to avoid extremely high temperatures, the external cooler was activated in short intervals corresponding to the local peaks on the curve with the aim of reducing the temperature of the MH1 tank in short time intervals. The heat-transfer fluid flew from the condenser through the external cooler with a maximum cooling power of 1 kW at the flow rate of 2.5 L-min\({}^{-1}\), while the temperatures were measured at the cooler inlet and outlet. The obtained values facilitated calculating the cooler's power using the calorimetric equation. The average temperature of the surrounding air during the measurement was 22.3 \({}^{\circ}\)C. The short intervals when the cooler was in operation are indicated by five vertical dotted lines crossing the curve. Figure 6: Curves of the hydrogen flow rates at the compressor inlet and outlet. Cooling the heat circuit of the heat pump condenser by the auxiliary cooler significantly contributed to maintaining the temperature of the MH2 tank. The reason is that decreasing the temperature in the condenser resulted in a slightly delayed decrease in the temperature of the evaporator that was thermally interconnected with the MH1 tank. Figure 9 shows the curve of hydrogen pressures in the tanks during the measurements. Figure 8: Curves of the temperatures in the tanks during compression. Figure 7: Total volumes of the hydrogen supplied to/removed from the compressor during measurements.