# Measurements of Operating Parameters of a Metal Hydride Compressor with a Heat Pump
## 5 Analysis of the Results and Discussion  
During the measurements, the average hydrogen flow rate at the compressor inlet was 5.96 L-min\({}^{-1}\) and the average value of the output flow rate was 6.73 L-min\({}^{-1}\) (Figure 6). The hydrogen flow rate was regulated in order to achieve a balanced ratio of the absorption/desorption temperatures in MH1 and MH2 tanks. The heating power supplied from the heat pump condenser was 20.7% higher than the cooling power of the evaporator, and this difference was also manifested by an increase in the total volume of hydrogen at the outlet from the MH compressor. With the aim to ensure the continuous operation, it is necessary to achieve identical volumes of the absorbed and the desorbed hydrogen in the tandem pair of MH tanks; this may only be achieved by the optimal regulation, which will result in identical hydrogen flow rates at the condenser inlet and outlet. During the measurements, the average temperature of the MH1 tank, in which hydrogen desorption took place, amounted to 54.9 \({}^{\circ}\)C (Figure 8). The average temperature of the MH2 tank, in which the hydrogen was absorbed, was 12.3 \({}^{\circ}\)C. During the experiment, the absolute temperature shifted to the levels above the designed value; however, the difference between the temperatures in the tanks was 42.6 \({}^{\circ}\)C, whereas the designed value was 40 \({}^{\circ}\)C. Such a shift in the operating temperatures resulted in a shift in the operating pressures to the respective isothermal curves of the selected alloy. The average operating pressure in the MH1 tank was 1.08 MPa, while in the MH2 tank it was 0.56 MPa. As the hydrogen was gradually absorbed in the MH2 tank, the hydrogen concentration in the alloy was rising; consequently, the hydrogen pressure increased from the baseline value of 0.27 MPa to the final value of 0.58 MPa despite a minor change in the operating temperature. Similarly, the pressure in the MH2 tank decreased from the baseline value of 1.48 MPa to 0.81 MPa. If it is necessary to maintain a constant pressure at the outlet during the compression, the temperature in the tank must be gradually elevated. The average difference between the temperatures of the heat-transfer fluid at the MH1 inlet and outlet was 5.6 \({}^{\circ}\)C at a flow rate of 1.55 L-min\({}^{-1}\). As for the MH2 tank, the temperature difference was 6.6 \({}^{\circ}\)C at a coolant flow rate of 0.66 L-min\({}^{-1}\). The increased flow rate of the heat-transfer fluid resulted in a smaller difference in the coolant temperatures, which led to more even thermal fields of the MH tanks and hence also a higher compression ratio of the compressed hydrogen; however, the consequences of this process also included an increase in the consumption of the electric energy by the pump. In order to create an  
Figure 12: Cooling and heating powers supplied to the tanks. optimal design of a MH compressor, it is necessary to find a compromise between the individual operating parameters with the aim of maximising the compressor's performance. The average achieved cooling and heating powers on the side of the condenser that heated the MH1 tank was 362 W, while the cooling power on the evaporator side was 300 W (Figure 12). The final average COP of the heat pump was 2.92; at such a COP, the consumption of the primary source of the electric energy becomes significantly lower. The measured values of the pressure in MH1 and MH2 tanks were used to calculate the average value of the compression ratio, which amounted to 2.5. The \(\varepsilon\) curve may be seen in Figure 13. The compression ratio was affected by a number of parameters, such as the hydrogen flow rates at the inlet and outlet, heat pump power, current amounts of hydrogen in the tanks, as well as the hydrogen pressure. Therefore, the value of the compression ratio was not constant during the measurements. The pressure at the outlet gradually decreased due to the regulation, which was aimed at achieving a constant temperature.