# Measurements of Operating Parameters of a Metal Hydride Compressor with a Heat Pump
## 2 Description of the Designed MH Compressor Mechanism
The compressor contained a touch screen for the control and visualization of the measured data. The compressor prototype was designed with independent modules in order to facilitate future applications within the research into the optimization of MH compressors, where component replacements and design optimization might be required. The control PC was connected to 36 temperature sensors BD18B20 with accuracy of \(\pm\)0.5 \({}^{\circ}\)C, which facilitated the monitoring of thermal fields of the individual components. The heat pump with a water circuit contained 14 temperatures sensors (t1 through t14), which facilitated analysing the condition and transient performance of the compressor during operation. Figure 2: Hydrogen circuit diagram. The measurement system also included 7 pressure sensors (5 sensors for H\({}_{2}\) and 2 sensors for a coolant in the heat pump) with accuracy of 0.4% and 5 flow meters with accuracy of 1% (2 hydrogen mass flow rate meters and 3 heat-transfer fluid flow meters). At the outlets from I/O ports, there were 27 relays that facilitated controlling all the compressor devices (electronic valves, devices, coolers, etc.). Figure 4 shows the final arrangement of the components and Table 1 contains a description of the basic parameters of the MH compressor. Figure 4: MH compressor with a heat pump. Figure 3: Diagram of a heat pump with a water circuit. Each of the used tandem-arranged MH tanks contained 3.125 kg of the alloy and an internal tubular heat exchanger, through which the heat-transfer fluid flew from the condenser or the evaporator of the heat pump. Description of the Compressor's Thermal Cycle  
Figure 1 shows the basic thermal cycle of the compressor that used the La\({}_{0.85}\)Ce\({}_{0.15}\)Ni\({}_{5}\) alloy, which is capable of forming metal hydrides. The cycle was plotted using the measured PCIs (Pressure-Concentration Isotherms) at the temperatures ranging from 10 to 50 \({}^{\circ}\)C. The baseline point A represents the baseline alloy status, in which the concentration of the absorbed hydrogen was 0.16% at a pressure of 0.12 MPa. In the zone between points A and B, hydrogen was absorbed into the alloy while the heat was released. The amount of the released heat of reaction, specific for the La\({}_{0.85}\)Ce\({}_{0.15}\)Ni\({}_{5}\) alloy, was identified by an experimental measurement and amounted to 1 MJ\(\cdot\)m\({}^{-3}\) of H\({}_{2}\). After the alloy was saturated to a concentration of 0.66%, the pressure increased to 0.2 MPa. Due to an increasing pressure gradient, a further increase in the concentration would be unreasonable as it would lead to a reduced compression ratio and an unnecessary increase in the operating pressure of the compressor (point C). The heat necessary for maintaining the isothermal conditions is defined as follows:  
A \(\Rightarrow\) B  
\[Q_{\rm AB}=-q_{\rm abs~{}H_{2}}\cdot\Delta V_{\rm absH_{2}}+k\cdot A\cdot(t-t _{\rm a})\cdot\Delta\tau\ (\rm J) \tag{1}\]  
After the required concentration was achieved, it was necessary to increase the alloy temperature by adding the heat. It may be divided into the calorimetric heat, the heat necessary to cover the hydrogen desorption, and the effects of the heat loss:  
B \(\Rightarrow\) C  
\[Q_{\rm BC}=\sum_{i}\left(m_{i}\cdot c_{i}\right)\cdot\Delta T+q_{\rm des~{}H_{2}} \cdot\Delta V_{\rm desH_{2}}+\int k\cdot A\cdot(t-t_{\rm a})\cdot\rm d\tau\ (\rm J) \tag{2}\]  
where in \(\Sigma(m_{i}\cdot c_{i})\) is the overall heat capacity of the MH tank, including the integrated heat exchanger and the absorbed hydrogen (J\(\cdot\)K\({}^{-1}\)); \(\Delta T\) is the temperature difference between the working isotherms (K); \(q_{\rm des~{}H2}\) is the heat of reaction in hydrogen desorption (J\(\cdot\)m\({}^{-3}\)); and \(\Delta V_{\rm des~{}H2}\) is the amount of the hydrogen released due to a temperature increase by \(\Delta T\) (m\({}^{3}\)). After the heat was supplied (in the zone between points B and C), the temperature increased by 40 \({}^{\circ}\)C and the pressure increased to a value of ca 1.22 MPa.