# Measurements of Operating Parameters of a Metal Hydride Compressor with a Heat Pump
## 2 Description of the Designed MH Compressor Mechanism  
The underlying principle of hydrogen compression is its absorption into the La\({}_{0.85}\)Ce\({}_{0.15}\)Ni\({}_{5}\) metal alloy; Figure 1 shows two of the alloy's PCI characteristics at various temperatures and a plotted thermal cycle. The absorption process is characterized between points A and B. The absorption ran at a temperature of 10 \({}^{\circ}\)C and the heat was removed from the tank using a heat pump evaporator. After the absorption was completed, the alloy was heated to a temperature of min. 50 \({}^{\circ}\)C, which significantly increased the pressure. Subsequently, the hydrogen was desorbed at a temperature of 50 \({}^{\circ}\)C, but at a significantly higher pressure (between points C and D). After the hydrogen desorption at higher pressures, the alloy was refrigerated back to the baseline temperature of 10 \({}^{\circ}\)C. In order to maintain the continuous operation of the compressor, two MH tanks were arranged in series and the heat was transported between them using a heat pump. The hydrogen absorption in the MH1 tank and the hydrogen desorption from the MH2 tank ran simultaneously (Figure 2). However, during the intervals when the temperature was changing, between points BC and DA, the hydrogen absorption did not take place in any of the tanks; that is why an additional tank was used (MH3) together with a compressor to absorb hydrogen while the operating temperatures of MH1 and MH2 tanks were changing. This facilitated a continuous hydrogen flow at the inlet into the compressor. This additional tank had a separated thermal management, which was based on a pair of (PT) TEC1-12708 Peltier modules with a cooling power of 63 W at a zero-temperature  
Figure 1: PCI characteristics of the La\({}_{0.85}\)Ce\({}_{0.15}\)Ni\({}_{5}\) alloy with a plotted operating cycle of the compressor. difference between its hot and cold sides. Peltier modules facilitate switching between the heating and cooling modes by simply reversing polarity of the power supply. They may be used to regulate the temperature within the range from 0 to 70 \({}^{\circ}\)C. The purpose of such a design, which was used at the operating temperatures of 10-50 \({}^{\circ}\)C, was to approximate, as much as possible, the ideal compression ratio \(\varepsilon\) = 4. The expected mean COP of the heat pump was 3; this means that 1/3 of the electric energy was expected to be used for the compression compared to the thermal energy produced in the heat pump condenser. This significantly reduced the consumption of electric energy during hydrogen compression. In one cycle, the compressor used 0.166 m\({}^{3}\) of hydrogen in one MH tank. Figure 2 shows the basic diagram of the hydrogen circuit of the metal hydride compressor. It contained the electrically controlled JAKSA Magnetni Ventili hydrogen valves (0-150 bar; 290,100 TMEx35/D), Parker HPRVSM6A-V-K2-435 safety valves, and pressure sensors with analogue output signal, BD SENSORS 110-4002-E-1-100-N40-1-111 type. A maximum heating power of the heat pump was 1.2 kW and the maximum cooling power was 1 kW. Electronically controlled 3-way zone valves were used to change the direction of the heat-transfer fluid stream from the condenser to the MH1 or MH2 tanks (Figure 3). This facilitated a cyclical change in the temperatures of both tanks, as was required by the designed control algorithm. In order to ensure the removal of excess heat, produced as a result of a higher heating power on the condenser side than the cooling power on the evaporator side, this circuit was equipped with an auxiliary cooler \"C\". This prototype MH compressor with the heat pump was designed with an autonomous control system. For this purpose, a Raspberry Pi control minicomputer with additional cards (1-wire; A/D converter-16 bit; I/O ports--32\(\times\)) was integrated with the Linux distribution Raspbian operating system. The control software was written in C++ language in Qt Creator environment.