2018 Volume 59 Issue 5 Pages 855-857
In this study, we developed a thermochemical technique to compress hydrogen gas up to more than 80 MPa. The results showed that Ti1.1CrMn alloys can generate a hydrogen pressure of 82 MPa upon being heated to ∼200°C. In order to evaluate the hydrogen absorption and desorption properties of the Ti1.1CrMn alloy at elevated temperatures, its pressure-composition (PC) isotherms were measured at 100, 140, and 180°C. To examine the durability of the alloy, hydrogen compression cycle tests were performed at pressures ranging from 14 to 80 MPa by heating the alloy from 35 to 200°C. In order to determine the temperature required for achieving the dissociation pressure of 82 MPa, we generated an isobar plot based on the PC isothermal measurements.
The construction cost of hydrogen fueling stations in Japan is extremely high because of the costly design of their components such as compressors and high-pressure storage tanks. Mechanical compression is widely used to store hydrogen at hydrogen stations. However, the hydrogen embrittlement caused by this compression significantly affects the moving parts of the compressor. Strict design regulations make it necessary for the components of hydrogen fueling stations to possess excellent strength. This is the reason for the high cost of hydrogen stations.
Non-mechanical techniques using the hydrogen absorption/desorption reactions of hydrogen storage alloys to thermochemically compress hydrogen gas have been studied since 1970s. The developments of the metal hydride hydrogen compression technologies have been recently reviewed by Lototskyy et al.1) It is known that hydrogen storage alloys can absorb hydrogen gas at ambient temperatures. These hydrogenated alloys release hydrogen gas with a high pressure upon being heated. The generation of high-pressure hydrogen gas using TiFe alloys has been reported.2) The hydrogen desorption pressure depends on the alloy used. Therefore, one can design hydrogen compressors operating over the desired pressure and temperature ranges by selecting appropriate hydrogen storage alloys. Compressors with output pressures as high as 653) and 70 MPa4) have been reported. Hydrogen storage tanks with a hydrogen pressure of 82 MPa are required for differential pressure filling of fuel cell (FC) vehicle tanks. To the best of our knowledge, thermochemical compressors which can discharge a hydrogen pressure of 82 MPa have not been reported previously.
The hydrogen absorption-desorption properties of hydrogen storage alloys at around room temperature (RT) have been investigated in detail.5–7) However, the hydrogen absorption-desorption properties of these alloys at high temperatures have not been studied much. It is believed that at high temperatures, the pressure-composition (PC) isotherms of hydrogen storage alloys show a higher plateau slope and smaller plateau width (i.e. hydrogen capacity) than those observed at RT. Therefore, it is important to investigate the fundamental properties of hydrogen storage alloys at high temperatures in order to realize their practical applications.
In this study, we used a Ti1.1CrMn alloy capable of releasing hydrogen at more than 80 MPa at a relatively low temperature (less than 200°C). Y. Kojima et al.8) have reported that the hydrogen dissociation pressure of Ti1.1CrMn at 23°C is 11 MPa, and the standard enthalpy difference for hydrogenation is −22 kJ/mol H2. According to the van’t Hoff equation, a high dissociation pressure of more than 80 MPa can be achieved by heating the hydrogenated alloy at around 200°C. In this study, a thermochemical hydrogen compressor was designed using Ti1.1CrMn. In order to investigate the hydrogen absorption and desorption properties of the alloy at high temperatures, PC isothermal measurements were carried out over the temperature range of RT–180°C. An isobar plot was generated to determine the temperature required to achieve a dissociation pressure of 82 MPa. Furthermore, a hydrogen compression cycle test was carried out to evaluate the durability of the alloy.
Ti1.1CrMn was purchased from Japan Metals & Chemicals Co. Ltd. The experiments were conducted at Hydrogen Energy Test and Research Center and Fukushima Renewable Energy Institute, National Institute of Advanced Industrial Science and Technology (FREA-AIST). The alloy (80–200 g) was loaded in a vessel and was activated in a 40 MPa hydrogen environment at RT. The PC isothermal curves of the alloy were recorded by using a Sievert’s-type apparatus at a maximum pressure of 100 MPa. The amount of hydrogen desorbed during the PC isothermal measurements at FREA was estimated by using a mass flow controller (MODEL 3660 SERIES, KOJIMA INSTRUMENTS INC.). The details of the apparatus at FREA have been described elsewhere.9)
Figure 1 shows the PC isotherms of Ti1.1CrMn at RT, 100°C, 140°C, and 180°C. The PC isotherm obtained at RT was consistent with that reported previously.8) The absorption/desorption plateau pressures (at midpoint of the plateau) were 31.0/25.7 MPa at 100°C and 56.1/45.3 MPa at 140°C. The width/pressure differences at the beginning and end of the desorption plateaus were roughly estimated to be 0.9 mass%/6.5 MPa at 100°C and 0.6 mass%/19.9 MPa at 140°C. At 180°C, only the desorption PC isotherm was measured because of the pressure limitation of the apparatus. The hydrogen desorption pressure of 82 MPa was achieved at 180°C. The hydrogen content in the alloy was 0.9 mass% at this temperature. The desorption pressure dropped down to 57 MPa towards the end of the desorption plateau (0.5 mass%).
Pressure-composition isotherms of Ti1.1CrMn for hydrogen absorption/desorption at RT, 100°C, 140°C, and 180°C. The isotherms measured at RT after 100 cycles of hydrogen compression tests were also plotted.
The pressure at the end of the absorption plateau (at 1.6 mass%) was around 9.6 MPa at RT, which was assumed to be the inlet pressure of the thermochemical compressor. The pressure at the end of the desorption plateau obtained at elevated temperatures was taken as the outlet pressure of the compressor. The difference in the hydrogen contents at these points corresponded to the effective hydrogen capacity per cycle of the compressor. The hydrogen pressure of a FC forklift tank is 35 MPa, and a hydrogen compressor capable of discharging a hydrogen pressure of 45 MPa is required to fill this tank. The hydrogen pressure of 45 MPa could be achieved at 140°C using the Ti1.1CrMn thermochemical compressor used in this study. The available hydrogen capacity at this temperature was 0.8 mass% per cycle. In order to fill FC vehicle tanks (70 MPa) a hydrogen compressor with a discharge pressure of 82 MPa is required. The Ti1.1CrMn thermochemical compressor investigated in this study could discharge the hydrogen pressure of 82 MPa at 180°C. The available hydrogen capacity at this temperature was 0.7 mass% per cycle. We believe that a further increase in temperature would increase the available hydrogen capacity and hydrogen dissociation pressure of the compressor.
3.2 Cycle testIn some hydrogen storage alloys, absorption-desorption cycles cause disproportionation and/or introduce dislocations. These changes significantly affect the performance of the alloy (for example, decrease in the reversible hydrogen capacity and change in the dissociation pressure). In order to realize the practical applications of thermochemical hydrogen compressors, it is necessary to evaluate the cyclic durability of the alloys which are used for designing these compressors.
The reaction vessel containing the hydrogenated alloy was heated to around 200°C in a closed system to compress hydrogen from 11–14 to 80 MPa. The vessel was then cooled down to 35°C, which resulted in a decrease in the pressure. The corresponding hydrogen desorption/absorption amount was estimated to be 0.6–0.7 mass%. This compression cycle was repeated 100 times. We compared the PC isotherms obtained after the first and 100th cycles to evaluate the cycle durability of Ti1.1CrMn. Figure 1 shows that there was hardly any change in the plateau pressure or the hydrogen content of Ti1.1CrMn even after 100 cycles. This indicates that Ti1.1CrMn has excellent cyclic durability, and there occurred no change (such as disproportionation) affecting the alloy properties.
3.3 Isobar plotAs shown in Fig. 1, the slope of the PC isotherm plateau increased with an increase in temperature, indicating that the desorption pressure of the thermochemical compressor decreased with a decrease in the hydrogen content of the hydrogenated alloy under isothermal conditions. However, a slight increase in temperature resulted in an increase in the desorption pressure. The desorption pressure can vary exponentially with the reciprocal of temperature according to the van’t Hoff equation. Because the hydrogen dissociation pressure of hydrogen compressors at equilibrium depends on the temperature and hydrogen content of their hydrogenated alloys, it is necessary to investigate the temperature at which the desired pressure can be obtained as a function of the hydrogen content of the alloys. Isotherm plots are commonly used to study the relationship between the pressure and composition of metal-hydrogen systems at a constant temperature. Isobar plots10) on the other hand, are used to study the relationship between the temperature and composition of metal-hydrogen systems at a constant pressure.
The hydrogen desorption isobar of Ti1.1CrMn at 82 MPa (Fig. 2) was generated from its isotherms at elevated temperatures. Activated Ti1.1CrMn was heated (in a 80 MPa hydrogen environment) from RT to the temperature at which a pressure of 87 MPa was achieved. The hydrogen gas in the vessel was then released to 82 MPa at the same temperature. This corresponds to the measurement of a part of the isotherm at a constant temperature. The heating of the vessel and isotherm measurements were continued until a pressure of 85–88 MPa was achieved. Each data point of the isobar plot was determined from the hydrogen content at 82 MPa in the partial isotherm of the corresponding temperature. In order to compare these data with the isothermal properties of the alloy, the data points at 140 and 180°C for 82 MPa were also plotted on this isobar profile (shown by the solid triangles). Both the measurements were found to be consistent with each other. Moreover, the temperature values at 0.3 and 0.4 mass% were estimated from the temperature dependence of the low-hydrogen region (α-phase region) of the isotherms measured at 100–200°C according to the van’t Hoff equation.
The relationship between the temperature and Ti1.1CrMn composition (isobar plot) required to achieve a hydrogen desorption pressure of 82 MPa. The dashed line is guide to the eyes. Open circles (○), closed squares (■), and closed triangles (▲) are the values obtained by experiments, estimated from calculations, and interpolated from the PC isotherms at 140 and 180°C (for 82 MPa, Fig. 1), respectively.
From the isobar plots of hydrogen storage alloys, one can determine the temperature required to achieve a discharge pressure of 82 MPa depending on the hydrogen content of the alloy. For example, in this study, the hydrogen desorption pressure of 82 MPa could be achieved at temperatures below 180°C when the hydrogen content in the alloy was 0.9 mass% (lower limit of the available hydrogen content). At hydrogen contents greater than 0.9 mass%, an increase in temperature was required to maintain the hydrogen pressure higher than 82 MPa. Therefore, the amount of available hydrogen was 0.7 mass% at 180°C and 1.1 mass% at 220°C by heating of a hydrogenated Ti1.1CrMn (1.6 mass% of hydrogen content at RT). Utilizing the hydrogen from the low-hydrogen content regions of PC isotherms is less efficient for reducing the operation cost of thermochemical hydrogen compressors. This is because the required temperature increases drastically in this case. An efficient approach to reduce the operation cost of these compressors is to utilize waste heat to increase temperature. The isobar results obtained in this study provide guidelines (on the temperature of the waste heat available from the surrounding environment and the required compression ability) to optimally design thermochemical hydrogen compressors.
To realize practical on-board hydrogen storage applications, efforts have been made to improve the hydrogen storage capacity per mass and the properties of M-H systems at near-ambient temperatures. However, for thermochemical compressor applications, the properties of M-H systems at high pressures and temperatures become more important. In this study, hydrogen compression of up to 82 MPa was achieved by heating a hydrogenated Ti1.1CrMn alloy up to ∼200°C. The hydrogen compression test for 100 cycles revealed that Ti1.1CrMn exhibits high cyclic durability. In order to determine the temperature required for achieving a hydrogen desorption pressure of 82 MPa, the isobar properties of the alloy were investigated. It was found that the temperature and hydrogen content required for achieving the desorption pressure of 82 MPa were strongly related to each other. In the future, the discharge pressure of hydrogen stations is expected to increase to 87.5 MPa. The dissociation pressure of 87.5 MPa can be achieved by heating hydrides at around 200°C.
Authors are thankful to Prof. Y. Kojima for his useful discussions and suggestions. This work was partially supported by Hyogo Center of Excellence Program Promotion Project and New Energy and Industrial Technology Development Organization (NEDO), New Energy Venture Business Technology Innovation Program (Fuel cell Storage cell), “Development of energy-saving hydrogen compressor by renewable energy”.
