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Metal hydrides (MH) are recognized as one of the most suitable material groups for hydrogen storage due to their large hydrogen storage capacity, low operating pressure and high safety. However, their sluggish hydrogen uptake kinetics greatly reduces storage performance. Faster heat removal from the MH storage could play an important role in increasing its hydrogen uptake rate, resulting in improved storage performance. In this regard, this study was aimed at improving the heat transfer characteristics in order to positively influence the hydrogen uptake rate of the MH storage system. The new semi-cylindrical coil was first developed and optimized for hydrogen storage and incorporated as an internal air-as-heat exchanger (HTF). Based on the different pitch sizes, the effect of the new heat exchanger configuration is analyzed and compared with the conventional helical coil geometry. In addition, the operating parameters of the storage of MG and GTP were numerically studied to obtain optimal values. For numerical simulation, ANSYS Fluent 2020 R2 is used. The results of this study show that the performance of an MH storage tank can be significantly improved by using a semi-cylindrical coil heat exchanger (SCHE). Compared to conventional spiral coil heat exchangers, the duration of hydrogen absorption is reduced by 59%. The smallest distance between the SCHE coils resulted in a 61% reduction in absorption time. As regards the operating parameters of MG storage using SHE, all the selected parameters lead to a significant improvement in the hydrogen absorption process, especially the temperature at the inlet to the HTS.
There is a global transition from energy based on fossil fuels to renewable energy. Because many forms of renewable energy provide power in a dynamic manner, energy storage is necessary to balance the load. Hydrogen-based energy storage has attracted a lot of attention for this purpose, especially because hydrogen can be used as a “green” alternative fuel and energy carrier due to its properties and portability. In addition, hydrogen also offers a higher energy content per unit mass compared to fossil fuels2. There are four main types of hydrogen energy storage: compressed gas storage, underground storage, liquid storage, and solid storage. Compressed hydrogen is the main type used in fuel cell vehicles such as buses and forklifts. However, this storage provides a low bulk density of hydrogen (approximately 0.089 kg/m3) and has safety issues associated with high operating pressure3. Based on a conversion process at low ambient temperature and pressure, the liquid storage will store hydrogen in liquid form. However, when liquefied, about 40% of the energy is lost. In addition, this technology is known to be more energy and labor intensive compared to solid state storage technologies4. Solid storage is a viable option for a hydrogen economy, which stores hydrogen by incorporating hydrogen into solid materials through absorption and releasing hydrogen through desorption. Metal hydride (MH), a solid material storage technology, is of recent interest in fuel cell applications due to its high hydrogen capacity, low operating pressure, and low cost compared to liquid storage, and is suitable for stationary and mobile applications6,7 In addition, MH materials also provide safety properties such as efficient storage of large capacity8. However, there is a problem that limits the productivity of the MG: the low thermal conductivity of the MG reactor leads to slow absorption and desorption of hydrogen.
Proper heat transfer during exothermic and endothermic reactions is the key to improving the performance of MH reactors. For the hydrogen loading process, the generated heat must be removed from the reactor in order to control the hydrogen loading flow at the desired rate with maximum storage capacity. Instead, heat is required to increase the rate of hydrogen evolution during discharge. In order to improve the heat and mass transfer performance, many researchers have studied the design and optimization based on multiple factors such as operating parameters, MG structure, and MG11 optimization. MG optimization can be done by adding high thermal conductivity materials such as foam metals to MG layers 12,13. Thus, the effective thermal conductivity can be increased from 0.1 to 2 W/mK10. However, the addition of solid materials significantly reduces the power of the MN reactor. With regard to operating parameters, improvements can be achieved by optimizing the initial operating conditions of the MG layer and coolant (HTF). The structure of the MG can be optimized due to the geometry of the reactor and the design of the heat exchanger. Regarding the configuration of the MH reactor heat exchanger, the methods can be divided into two types. These are internal heat exchangers built into the MO layer and external heat exchangers covering the MO layer such as fins, cooling jackets and water baths. With regard to the external heat exchanger, Kaplan16 analyzed the operation of the MH reactor, using cooling water as a jacket to reduce the temperature inside the reactor. The results were compared with a 22 round fin reactor and another reactor cooled by natural convection. They state that the presence of a cooling jacket significantly reduces the temperature of the MH, thereby increasing the absorption rate. Numerical studies of the water-jacketed MH reactor by Patil and Gopal17 have shown that hydrogen supply pressure and HTF temperature are key parameters influencing the rate of hydrogen uptake and desorption.
Increasing the heat transfer area by adding fins and heat exchangers built into the MH is the key to improving the heat and mass transfer performance and hence the storage performance of the MH18. Several internal heat exchanger configurations (straight tube and spiral coil) have been designed to circulate the coolant in the MH19,20,21,22,23,24,25,26 reactor. Using an internal heat exchanger, the cooling or heating liquid will transfer local heat inside the MH reactor during the hydrogen adsorption process. Raju and Kumar [27] used several straight tubes as heat exchangers to improve the performance of the MG. Their results showed that absorption times were reduced when straight tubes were used as heat exchangers. In addition, the use of straight tubes shortens the hydrogen desorption time28. Higher coolant flow rates increase the rate of hydrogen charging and discharging29. However, increasing the number of cooling tubes has a positive effect on MH performance rather than coolant flow rate30,31. Raju et al.32 used LaMi4.7Al0.3 as an MH material to study the performance of multitube heat exchangers in reactors. They reported that the operating parameters had a significant effect on the absorption process, especially the feed pressure and then the flow rate of the HTF. However, the absorption temperature turned out to be less critical.
The performance of the MH reactor is further improved by the use of a spiral coil heat exchanger due to its improved heat transfer compared to straight tubes. This is because the secondary cycle can better remove heat from the reactor25. In addition, the spiral tubes provide a large surface area for heat transfer from the MH layer to the coolant. When this method is introduced inside the reactor, the distribution of heat exchange tubes is also more uniform33. Wang et al. 34 studied the effect of hydrogen uptake duration by adding a helical coil to an MH reactor. Their results show that as the heat transfer coefficient of the coolant increases, the absorption time decreases. Wu et al. 25 investigated the performance of Mg2Ni based MH reactors and coiled coil heat exchangers. Their numerical studies have shown a reduction in reaction time. The improvement of the heat transfer mechanism in the MN reactor is based on a smaller ratio of screw pitch to screw pitch and a dimensionless screw pitch. An experimental study by Mellouli et al.21 using a coiled coil as an internal heat exchanger showed that HTF start temperature has a significant effect on improving hydrogen uptake and desorption time. Combinations of different internal heat exchangers have been carried out in several studies. Eisapur et al. 35 studied hydrogen storage using a spiral coil heat exchanger with a central return tube to improve the hydrogen absorption process. Their results showed that the spiral tube and the central return tube significantly improve the heat transfer between the coolant and the MG. The smaller pitch and larger diameter of the spiral tube increase the rate of heat and mass transfer. Ardahaie et al. 36 used flat spiral tubes as heat exchangers to improve heat transfer within the reactor. They reported that the absorption duration was reduced by increasing the number of flattened spiral tube planes. Combinations of different internal heat exchangers have been carried out in several studies. Dhau et al. 37 improved the performance of the MH using a coiled coil heat exchanger and fins. Their results show that this method reduces the hydrogen filling time by a factor of 2 compared to the case without fins. The annular fins are combined with cooling tubes and built into the MN reactor. The results of this study show that this combined method provides more uniform heat transfer compared to the MH reactor without fins. However, combining different heat exchangers will negatively affect the weight and volume of the MH reactor. Wu et al.18 compared different heat exchanger configurations. These include straight tubes, fins and spiral coils. The authors report that spiral coils provide the best improvements in heat and mass transfer. In addition, compared with straight tubes, coiled tubes, and straight tubes combined with coiled tubes, double coils have a better effect on improving heat transfer. A study by Sekhar et al. 40 showed that a similar improvement in hydrogen uptake was achieved using a spiral coil as the internal heat exchanger and a finned external cooling jacket.
Of the examples mentioned above, the use of spiral coils as internal heat exchangers provides better heat and mass transfer improvements than other heat exchangers, especially straight tubes and fins. Therefore, the aim of this study was to further develop the spiral coil to improve the heat transfer performance. For the first time, a new semi-cylindrical coil has been developed based on the conventional MH storage helical coil. This study is expected to improve hydrogen storage performance by considering a new heat exchanger design with a better heat transfer zone layout provided by a constant volume of MH bed and HTF tubes. The storage performance of this new heat exchanger was then compared to conventional spiral coil heat exchangers based on different coil pitches. According to existing literature, operating conditions and spacing of coils are the main factors affecting the performance of MH reactors. To optimize the design of this new heat exchanger, the effect of coil spacing on hydrogen uptake time and MH volume was investigated. In addition, in order to understand the relationship between the new hemi-cylindrical coils and operating conditions, a secondary goal of this study was to study the characteristics of the reactor according to different operating parameter ranges and determine the appropriate values for each operating mode. parameter.
The performance of the hydrogen energy storage device in this study is investigated based on two heat exchanger configurations (including spiral tubes in cases 1 to 3 and semi-cylindrical tubes in cases 4 to 6) and a sensitivity analysis of operating parameters. The operability of the MH reactor was tested for the first time using a spiral tube as a heat exchanger. Both the coolant oil pipe and the MH reactor vessel are made of stainless steel. It should be noted that the dimensions of the MG reactor and the diameter of the GTF pipes were constant in all cases, while the step sizes of the GTF varied. This section analyzes the effect of the pitch size of HTF coils. The height and outer diameter of the reactor were 110 mm and 156 mm, respectively. The diameter of the heat-conducting oil pipe is set at 6mm. See Supplementary Section for details on the MH reactor circuit diagram with spiral tubes and two semi-cylindrical tubes.
On fig. 1a shows the MH spiral tube reactor and its dimensions. All geometric parameters are given in table. 1. The total volume of the helix and the volume of the ZG are approximately 100 cm3 and 2000 cm3, respectively. From this MH reactor, air in the form of HTF was fed into the porous MH reactor from below through a spiral tube, and hydrogen was introduced from the top surface of the reactor.
Characterization of selected geometries for metal hydride reactors. a) with a spiral-tubular heat exchanger, b) with a semi-cylindrical tubular heat exchanger.
The second part examines the operation of the MH reactor based on a semi-cylindrical tube as a heat exchanger. On fig. 1b shows the MN reactor with two semi-cylindrical tubes and their dimensions. Table 1 lists all the geometric parameters of semi-cylindrical pipes, which remain constant, with the exception of the distance between them. It should be noted that the semi-cylindrical tube in Case 4 was designed with a constant volume of HTF tube and MH alloy in the coiled tube (option 3). As for fig. 1b, air was also introduced from the bottom of the two semi-cylindrical HTF tubes, and hydrogen was introduced from the opposite direction of the MH reactor.
Due to the new design of the heat exchanger, the purpose of this section is to determine the appropriate initial values for the operating parameters of the MH reactor in combination with SCHE. In all cases, air was used as a coolant to remove heat from the reactor. Among the heat transfer oils, air and water are commonly chosen as heat transfer oils for MH reactors due to their low cost and low environmental impact. Due to the high operating temperature range of magnesium-based alloys, air was chosen as the coolant in this study. In addition, it also has better flow characteristics than other liquid metals and molten salts41. Table 2 lists the properties of air at 573 K. For the sensitivity analysis in this section, only the best configurations of the MH-SCHE performance options (in cases 4 through 6) are applied. The estimates in this section are based on various operating parameters, including the initial temperature of the MH reactor, the hydrogen loading pressure, the HTF inlet temperature, and the Reynolds number calculated by changing the HTF rate. Table 3 contains all operating parameters used for sensitivity analysis.
This section describes all the necessary control equations for the process of hydrogen absorption, turbulence and heat transfer of coolants.
To simplify the solution of the hydrogen uptake reaction, the following assumptions are made and provided;
During absorption, the thermophysical properties of hydrogen and metal hydrides are constant.
Hydrogen is considered an ideal gas, so local thermal equilibrium conditions43,44 are taken into account.
where \({L}_{gas}\) is the radius of the tank, and \({L}_{heat}\) is the axial height of the tank. When N is less than 0.0146, the hydrogen flow in the tank can be ignored in the simulation without significant error. According to current research, N is much lower than 0.1. Therefore, the pressure gradient effect can be neglected.
The reactor walls were well insulated in all cases. Therefore, there is no heat exchange 47 between the reactor and the environment.
It is well known that Mg-based alloys have good hydrogenation characteristics and high hydrogen storage capacity up to 7.6 wt%8. In terms of solid state hydrogen storage applications, these alloys are also known as lightweight materials. In addition, they have excellent heat resistance and good processability8. Among several Mg-based alloys, Mg2Ni-based MgNi alloy is one of the most suitable options for MH storage due to its hydrogen storage capacity of up to 6 wt%. Mg2Ni alloys also provide faster adsorption and desorption kinetics compared to MgH48 alloy. Therefore, Mg2Ni was chosen as the metal hydride material in this study.
The energy equation is expressed as 25 based on the heat balance between hydrogen and Mg2Ni hydride:
X is the amount of hydrogen absorbed on the metal surface, the unit is \(weight\%\), calculated from the kinetic equation \(\frac{dX}{dt}\) during absorption as follows49:
where \({C}_{a}\) is the reaction rate and \({E}_{a}\) is the activation energy. \({P}_{a,eq}\) is the equilibrium pressure inside the metal hydride reactor during the absorption process, given by the van’t Hoff equation as follows25:
Where \({P}_{ref}\) is the reference pressure of 0.1 MPa. \(\Delta H\) and \(\Delta S\) are the enthalpy and entropy of the reaction, respectively. Properties of alloys Mg2Ni and hydrogen are presented in table. 4. The named list can be found in the supplementary section.
The fluid flow is considered turbulent because its velocity and Reynolds number (Re) are 78.75 ms-1 and 14000, respectively. In this study, an achievable k-ε turbulence model was chosen. It is noted that this method provides higher accuracy compared to other k-ε methods, and also requires less computation time than RNG k-ε50,51 methods. See the Supplementary Section for details on the basic equations for heat transfer fluids.
Initially, the temperature regime in the MN reactor was uniform, and the average hydrogen concentration was 0.043. It is assumed that the outer boundary of the MH reactor is well insulated. Magnesium-based alloys typically require high reaction operating temperatures to store and release hydrogen in the reactor. The Mg2Ni alloy requires a temperature range of 523–603 K for maximum absorption and a temperature range of 573–603 K for complete desorption52. However, experimental studies by Muthukumar et al.53 showed that the maximum storage capacity of Mg2Ni for hydrogen storage can be achieved at an operating temperature of 573 K, which corresponds to its theoretical capacity. Therefore, the temperature of 573 K was chosen as the initial temperature of the MN reactor in this study.
Create different grid sizes for validation and reliable results. On fig. 2 shows the average temperature at selected locations in the hydrogen absorption process from four different elements. It is worth noting that only one case of each configuration is selected to test for grid independence due to similar geometry. The same meshing method is applied in other cases. Therefore, choose option 1 for the spiral pipe and option 4 for the semi-cylindrical pipe. On fig. 2a, b shows the average temperature in the reactor for options 1 and 4, respectively. The three selected locations represent bed temperature contours at the top, middle, and bottom of the reactor. Based on the temperature contours at the selected locations, the average temperature becomes stable and shows little change in element numbers 428,891 and 430,599 for cases 1 and 4, respectively. Therefore, these grid sizes were chosen for further computational calculations. Detailed information on the average bed temperature for the hydrogen absorption process for various cell sizes and successively refined meshes for both cases is given in the supplementary section.
Average bed temperature at selected points in the hydrogen absorption process in a metal hydride reactor with different grid numbers. (a) Average temperature at selected locations for case 1 and (b) Average temperature at selected locations for case 4.
The Mg-based metal hydride reactor in this study was tested based on the experimental results of Muthukumar et al.53. In their study, they used a Mg2Ni alloy to store hydrogen in stainless steel tubes. Copper fins are used to improve heat transfer inside the reactor. On fig. 3a shows a comparison of the average temperature of the absorption process bed between the experimental study and this study. The operating conditions chosen for this experiment are: MG initial temperature 573 K and inlet pressure 2 MPa. From fig. 3a it can be clearly shown that this experimental result is in good agreement with the present one with respect to the average layer temperature.
Model verification. (a) Code verification of the Mg2Ni metal hydride reactor by comparing the current study with the experimental work of Muthukumar et al.52, and (b) verification of the spiral tube turbulent flow model by comparing the current study with that of Kumar et al. Research.54.
To test the turbulence model, the results of this study were compared with the experimental results of Kumar et al.54 to confirm the correctness of the chosen turbulence model. Kumar et al.54 studied turbulent flow in a tube-in-pipe spiral heat exchanger. Water is used as hot and cold fluid injected from opposite sides. The hot and cold liquid temperatures are 323 K and 300 K, respectively. Reynolds numbers range from 3100 to 5700 for hot liquids and from 21,000 to 35,000 for cold liquids. Dean numbers are 550-1000 for hot liquids and 3600-6000 for cold liquids. The diameters of the inner pipe (for hot liquid) and the outer pipe (for cold liquid) are 0.0254 m and 0.0508 m, respectively. The diameter and pitch of the helical coil are 0.762 m and 0.100 m, respectively. On fig. 3b shows a comparison of experimental and current results for various pairs of Nusselt and Dean numbers for the coolant in the inner tube. Three different turbulence models were implemented and compared with experimental results. As shown in fig. 3b, the results of the achievable k-ε turbulence model are in good agreement with the experimental data. Therefore, this model was chosen in this study.
Numerical simulations in this study were performed using ANSYS Fluent 2020 R2. Write a User-Defined Function (UDF) and use it as the input term of the energy equation to calculate the kinetics of the absorption process. The PRESTO55 circuit and the PISO56 method are used for pressure-velocity communication and pressure correction. Select a Greene-Gauss cell base for the variable gradient. The momentum and energy equations are solved by the second-order upwind method. As regards the under-relaxation coefficients, the pressure, velocity, and energy components are set to 0.5, 0.7, and 0.7, respectively. The standard wall functions are applied to the HTF in the turbulence model.
This section presents the results of numerical simulations of improved internal heat transfer of an MH reactor using a coiled coil heat exchanger (HCHE) and a helical coil heat exchanger (SCHE) during hydrogen absorption. The effect of HTF pitch on the temperature of the reactor bed and the duration of absorption was analyzed. The main operating parameters of the absorption process are studied and presented in the sensitivity analysis section.
To investigate the effect of coil spacing on heat transfer in an MH reactor, three heat exchanger configurations with different pitches were investigated. The three different pitches of 15mm, 12.86mm and 10mm are designated body 1, body 2 and body 3 respectively. It should be noted that the pipe diameter was fixed at 6 mm at an initial temperature of 573 K and a loading pressure of 1.8 MPa in all cases. On fig. 4 shows the average bed temperature and hydrogen concentration in the MH layer during the hydrogen absorption process in cases 1 to 3. Typically, the reaction between the metal hydride and hydrogen is exothermic to the absorption process. Therefore, the temperature of the bed rises rapidly due to the initial moment when hydrogen is first introduced into the reactor. The bed temperature increases until it reaches a maximum value and then gradually decreases as heat is carried away by the coolant, which has a lower temperature and acts as a coolant. As shown in fig. 4a, due to the previous explanation, the temperature of the layer increases rapidly and decreases continuously. The hydrogen concentration for the absorption process is usually based on the bed temperature of the MH reactor. When the average layer temperature drops to a certain temperature, the metal surface absorbs hydrogen. This is due to the acceleration of the processes of physisorption, chemisorption, diffusion of hydrogen and the formation of its hydrides in the reactor. From fig. 4b it can be seen that the rate of hydrogen absorption in case 3 is lower than in other cases due to the smaller step value of the coil heat exchanger. This results in a longer overall pipe length and a larger heat transfer area for HTF pipes. With an average hydrogen concentration of 90%, the absorption time for Case 1 is 46,276 seconds. Compared to the duration of absorption in case 1, the duration of absorption in cases 2 and 3 was reduced by 724 s and 1263 s, respectively. The supplementary section presents temperature and hydrogen concentration contours for selected locations in the HCHE-MH layer.
Influence of distance between coils on average layer temperature and hydrogen concentration. (a) Average bed temperature for helical coils, (b) hydrogen concentration for helical coils, (c) average bed temperature for hemi-cylindrical coils, and (d) hydrogen concentration for hemi-cylindrical coils.
To improve the heat transfer characteristics of the MG reactor, two HFCs were designed for a constant volume of the MG (2000 cm3) and a spiral heat exchanger (100 cm3) of Option 3. This section also considers the effect of the distance between the coils of 15 mm for case 4, 12.86 mm for case 5 and 10 mm for case 6. In fig. 4c,d show the average bed temperature and concentration of the hydrogen absorption process at an initial temperature of 573 K and a loading pressure of 1.8 MPa. According to the average layer temperature in Fig. 4c, the smaller distance between the coils in case 6 reduces the temperature significantly compared to the other two cases. For case 6, a lower bed temperature results in a higher hydrogen concentration (see Fig. 4d). The hydrogen uptake time for Variant 4 is 19542 s, which is more than 2 times lower than for Variants 1-3 using HCH. In addition, compared to case 4, the absorption time was also reduced by 378 s and 1515 s in cases 5 and 6 with lower distances. The supplementary section presents temperature and hydrogen concentration contours for selected locations in the SCHE-MH layer.
To study the performance of two heat exchanger configurations, this section plots and presents temperature curves at three selected locations. The MH reactor with HCHE from case 3 was chosen for comparison with the MH reactor containing SCHE in case 4 because it has a constant MH volume and pipe volume. The operating conditions for this comparison were an initial temperature of 573 K and a loading pressure of 1.8 MPa. On fig. 5a and 5b show all three selected positions of the temperature profiles in cases 3 and 4, respectively. On fig. 5c shows the temperature profile and layer concentration after 20,000 s of hydrogen uptake. According to line 1 in Fig. 5c, the temperature around the TTF from options 3 and 4 decreases due to the convective heat transfer of the coolant. This results in a higher concentration of hydrogen around this area. However, the use of two SCHEs results in a higher layer concentration. Faster kinetic responses were found around the HTF region in case 4. In addition, a maximum concentration of 100% was also found in this region. From line 2 located in the middle of the reactor, the temperature of case 4 is significantly lower than the temperature of case 3 in all places except the center of the reactor. This results in the maximum hydrogen concentration for case 4 except for the region near the center of the reactor away from the HTF. However, the concentration of case 3 did not change much. A large difference in the temperature and concentration of the layer was observed in line 3 near the entrance to the GTS. The temperature of the layer in case 4 decreased significantly, resulting in the highest hydrogen concentration in this region, while the concentration line in case 3 was still fluctuating. This is due to the acceleration of SCHE heat transfer. Details and discussion of the comparison of the average temperature of the MH layer and HTF pipe between case 3 and case 4 are provided in the supplementary section.
Temperature profile and bed concentration at selected locations in the metal hydride reactor. (a) Selected locations for case 3, (b) Selected locations for case 4, and (c) Temperature profile and layer concentration at selected locations after 20,000 s for the hydrogen uptake process in cases 3 and 4.
On fig. Figure 6 shows a comparison of the average bed temperature (see Fig. 6a) and hydrogen concentration (see Fig. 6b) for the absorption of HCH and SHE. It can be seen from this figure that the temperature of the MG layer decreases significantly due to an increase in the heat exchange area. Removing more heat from the reactor results in a higher hydrogen uptake rate. Although the two heat exchanger configurations have the same volumes compared to using HCHE as Option 3, SCHE’s hydrogen uptake time based on Option 4 was significantly reduced by 59%. For a more detailed analysis, the hydrogen concentrations for the two heat exchanger configurations are shown as isolines in Figure 7. This figure shows that in both cases, hydrogen begins to be absorbed from below around the HTF inlet. Higher concentrations were found in the HTF region, while lower concentrations were observed in the center of the MH reactor due to its distance from the heat exchanger. After 10,000 s, the hydrogen concentration in case 4 is significantly higher than in case 3. After 20,000 seconds, the average hydrogen concentration in the reactor has risen to 90% in case 4 compared to 50% hydrogen in case 3. This may be due to the higher effective cooling capacity of combining two SCHEs, resulting in a lower temperature inside the MH layer. Consequently, a more equilibrium pressure falls inside the MG layer, which leads to a more rapid absorption of hydrogen.
Case 3 and Case 4 Comparison of average bed temperature and hydrogen concentration between two heat exchanger configurations.
Comparison of the hydrogen concentration after 500, 2000, 5000, 10000 and 20000 s after the start of the hydrogen absorption process in case 3 and case 4.
Table 5 summarizes the duration of hydrogen uptake for all cases. In addition, the table also shows the time of absorption of hydrogen, expressed as a percentage. This percentage is calculated based on the absorption time of Case 1. From this table, the absorption time of the MH reactor using HCHE is about 45,000 to 46,000 s, and the absorption time including SCHE is about 18,000 to 19,000 s. Compared to Case 1, the absorption time in Case 2 and Case 3 was reduced by only 1.6% and 2.7%, respectively. When using SCHE instead of HCHE, absorption time was significantly reduced from case 4 to case 6, from 58% to 61%. It is clear that the addition of SCHE to the MH reactor greatly improves the hydrogen absorption process and the performance of the MH reactor. Although the installation of a heat exchanger inside the MH reactor reduces the storage capacity, this technology provides a significant improvement in heat transfer compared to other technologies. Also, decreasing the pitch value will increase the volume of the SCHE, resulting in a decrease in the volume of the MH. In case 6 with the highest SCHE volume, the MH volumetric capacity was only reduced by 5% compared to case 1 with the lowest HCHE volume. In addition, during absorption, case 6 showed faster and better performance with a 61% reduction in absorption time. Therefore case 6 was chosen for further investigation in the sensitivity analysis. It should be noted that the long hydrogen uptake time is associated with a storage tank containing an MH volume of about 2000 cm3.
The operating parameters during the reaction are important factors that positively or negatively affect the performance of the MH reactor under real conditions. This study considers a sensitivity analysis to determine the appropriate initial operating parameters for an MH reactor in combination with SCHE, and this section investigates the four main operating parameters based on the optimal reactor configuration in case 6. The results for all operating conditions are shown in Fig. 8.
Graph of hydrogen concentration under various operating conditions when using a heat exchanger with a semi-cylindrical coil. (a) loading pressure, (b) initial bed temperature, (c) coolant Reynolds number, and (d) coolant inlet temperature.
Based on a constant initial temperature of 573 K and a coolant flow rate with a Reynolds number of 14,000, four different loading pressures were selected: 1.2 MPa, 1.8 MPa, 2.4 MPa, and 3.0 MPa. On fig. 8a shows the effect of loading pressure and SCHE on hydrogen concentration over time. The absorption time decreases with increasing loading pressure. Using an applied hydrogen pressure of 1.2 MPa is the worst case for the hydrogen absorption process, and the absorption duration exceeds 26,000 s to achieve 90% hydrogen absorption. However, the higher loading pressure resulted in a 32-42% decrease in absorption time from 1.8 to 3.0 MPa. This is due to the higher initial pressure of hydrogen, which results in a larger difference between the equilibrium pressure and the applied pressure. Therefore, this creates a large driving force for the hydrogen uptake kinetics. At the initial moment, hydrogen gas is rapidly absorbed due to the large difference between the equilibrium pressure and the applied pressure57. At a loading pressure of 3.0 MPa, 18% hydrogen rapidly accumulated during the first 10 seconds. Hydrogen was stored in 90% of the reactors at the final stage for 15460 s. However, at a loading pressure of 1.2 to 1.8 MPa, the absorption time was significantly reduced by 32%. Other higher pressures had less of an effect on improving absorption times. Therefore, it is recommended that the loading pressure of the MH-SCHE reactor be 1.8 MPa. The supplementary section shows the hydrogen concentration contours for various loading pressures at 15500 s.
The choice of an appropriate initial temperature of the MH reactor is one of the main factors affecting the hydrogen adsorption process, as it affects the driving force of the hydride formation reaction. To study the effect of SCHE on the initial temperature of the MH reactor, four different temperatures were chosen at a constant loading pressure of 1.8 MPa and a Reynolds number of 14,000 HTF. On fig. Figure 8b shows a comparison of various starting temperatures, including 473K, 523K, 573K, and 623K. In fact, when the temperature is higher than 230°C or 503K58, the Mg2Ni alloy has effective characteristics for the hydrogen absorption process. However, at the initial moment of hydrogen injection, the temperature rises rapidly. Consequently, the temperature of the MG layer will exceed 523 K. Therefore, the formation of hydrides is facilitated due to the increased absorption rate53. From fig. It can be seen from Fig. 8b that hydrogen is absorbed faster as the initial temperature of the MB layer decreases. Lower equilibrium pressures occur when the initial temperature is lower. The greater the pressure difference between the equilibrium pressure and the applied pressure, the faster the process of hydrogen absorption. At an initial temperature of 473 K, hydrogen is rapidly absorbed up to 27% during the first 18 seconds. In addition, the absorption time was also reduced from 11% to 24% at a lower initial temperature compared to the initial temperature of 623 K. The absorption time at the lowest initial temperature of 473 K is 15247 s, which is similar to the best case loading pressure, however, the decrease in initial temperature reactor temperature leads to a decrease in hydrogen storage capacity. The initial temperature of the MN reactor must be at least 503 K53. In addition, at an initial temperature of 573 K53, a maximum hydrogen storage capacity of 3.6 wt% can be achieved. In terms of hydrogen storage capacity and absorption duration, temperatures between 523 and 573 K shorten the time by only 6%. Therefore, a temperature of 573 K is proposed as the initial temperature of the MH-SCHE reactor. However, the effect of the initial temperature on the absorption process was less significant compared to the loading pressure. The supplementary section shows the contours of the hydrogen concentration for various initial temperatures at 15500 s.
The flow rate is one of the main parameters of hydrogenation and dehydrogenation because it can affect turbulence and heat removal or input during hydrogenation and dehydrogenation59. High flow rates will create turbulent phases and result in faster fluid flow through the HTF tubing. This reaction will result in faster heat transfer. Different entry velocities for HTF are calculated based on Reynolds numbers of 10,000, 14,000, 18,000, and 22,000. The initial temperature of the MG layer was fixed at 573 K and the loading pressure at 1.8 MPa. The results in fig. 8c demonstrate that using a higher Reynolds number in combination with SCHE results in a higher uptake rate. As the Reynolds number increases from 10,000 to 22,000, the absorption time decreases by about 28-50%. The absorption time at a Reynolds number of 22,000 is 12,505 seconds, which is less than at various initial loading temperatures and pressures. Hydrogen concentration contours for various Reynolds numbers for GTP at 12500 s are presented in the supplementary section.
The effect of SCHE on the initial temperature of the HTF is analyzed and shown in Fig. 8d. At an initial MG temperature of 573 K and a hydrogen loading pressure of 1.8 MPa, four initial temperatures were chosen for this analysis: 373 K, 473 K, 523 K, and 573 K. 8d shows that a decrease in the temperature of the coolant at the inlet leads to a reduction in the absorption time. Compared to the base case with an inlet temperature of 573 K, the absorption time was reduced by approximately 20%, 44% and 56% for inlet temperatures of 523 K, 473 K and 373 K, respectively. At 6917 s, the initial temperature of the GTF is 373 K, the hydrogen concentration in the reactor is 90%. This can be explained by enhanced convective heat transfer between the MG layer and the HCS. Lower HTF temperatures will increase heat dissipation and result in increased hydrogen uptake. Among all operating parameters, improving the performance of the MH-SCHE reactor by increasing the HTF inlet temperature was the most suitable method, since the end time of the absorption process was less than 7000 s, while the shortest absorption time of other methods was more than 10000 s. Hydrogen concentration contours are presented for various initial temperatures of GTP for 7000 s.
This study presents for the first time a new semi-cylindrical coil heat exchanger integrated into a metal hydride storage unit. The ability of the proposed system to absorb hydrogen was investigated with various configurations of the heat exchanger. The influence of the operating parameters on the heat exchange between the metal hydride layer and the coolant was investigated in order to find the optimal conditions for storing metal hydrides using a new heat exchanger. The main findings of this study are summarized as follows:
With a semi-cylindrical coil heat exchanger, the heat transfer performance is improved because it has a more uniform heat distribution in the magnesium layer reactor, resulting in a better hydrogen absorption rate. Provided that the volume of the heat exchange tube and metal hydride remains unchanged, the absorption reaction time is significantly reduced by 59% compared to a conventional coiled coil heat exchanger.
Post time: Jan-15-2023