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Application of metal hydride sheet to thermally driven cooling system
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Application of metal hydride sheet to thermally driven cooling system Naoto Yasuda, Tohru Tsuchiya, Noriyuki Okinaka, Tomohiro Akiyama* Center for Advanced Research of Energy & Materials, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
article info
abstract
Article history:
This paper describes an application of a metal hydride (MH) sheet, which consists of MH
Received 24 January 2013
powder, carbon fiber, and aramid pulp, in a metal hydride heat pump (MHHP) system with
Received in revised form
a TiFe0.9Ni0.1/La0.6Y0.4Ni4.9Al0.1 working pair (MH1/MH2). In the experiments, the effect of
1 April 2013
the use of MH sheet on the system performances was investigated, in which the MH sheets
Accepted 3 April 2013
were used to replace part of the MH powder to improve the heat exchange performance.
Available online 3 May 2013
The sheets and powder were packed alternately into the MH beds in layers with an aspect ratio less than one. The MH sheet significantly accelerated the heat exchange ratio of both
Keywords:
MH packed beds. Using the MH sheet in both reactors, the specific cooling power increased
Metal hydride heat pump
by 1.2 times. The results also indicated that the role of heat exchange in an MH2 reactor as
Thermally driven cooling system
a cooling output side was more important in the enhancement of system performance
Hydrogen storage material
than that in an MH1 reactor as a heat source side. In addition, the proposed MH sheet was
Composite material
effective not only for improving the system performance but also for decreasing the stress on the reactor vessel due to the expansion of MH during the hydrogen absorption/ desorption. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Some metals and alloys can react reversibly with a large amount of hydrogen under certain conditions. They are called metal hydrides (MHs), which have expected to be used not only as hydrogen storage carriers but also as energy conversion materials. The applications of hydrogen storage alloys are more advanced, for example, in Ni-MH battery [1], heat storage [2], purification of hydrogen gas [3], and chemical heat pump. The thermally driven metal hydride heat pump (MHHP) system is a promising candidate to utilize low-grade heat, because it has no mechanical moving parts and uses an environmentally friendly working fluid. Moreover, the system
can be applied in wide operating-temperature ranges by varying the MH composition. Many experimental and theoretical studies have been conducted to evaluate the performance of MHHP [4e8]. In addition, several proposed systems based on MHHP would utilize low-grade heat, such as industrial waste heat, exhaust gas of automobiles, liquefied natural gas cold energy, and solar energy [9e12]. However, the MHHP systems have not achieved practical use because of high initial cost of MHs and insufficient heat and/or mass transfer within the reaction beds. Usually the intrinsic reaction kinetics of MHs is relatively fast. Thus, the reaction rate of hydrogen absorbing/desorbing is limited by heat and/or by mass transfer. Various composite materials have been proposed to enhance the thermal
* Corresponding author. Tel.: þ81 11 706 6842; fax: þ81 11 726 0731. E-mail address: [email protected] (T. Akiyama). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.011
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cooling temperature, K, Tl Wcooling cooling output, W/kg-MH2, reaction heat caused by desorbing hydrogen of DHde MH2, J/mol-H2, total hydrogen flow volume, NL, VH2 half-cycle time, s, t95 amount of MH2 alloy, kg-MH2, mMH2 standard gas volume at 0 C and 1 atm, NL/mol. VSTP
Nomenclature H S P R T Th Tm
enthalpy, kJ/mol, entropy, J/mol K, pressure, MPa, gas constant, temperature, K, heat source temperature, K, heat sink temperature, K,
conductivity of the reaction bed [13e16]. A large amount of stress, on the other hand, is generated on the walls of vessels by the volume expansion of MH during the hydrogen absorption. Several investigations have been conducted on the expansion of MH and its effects on the reaction vessels [17e19]. The packing fraction of MH powder should be controlled to reduce the stress on the walls of vessels. However, the effective thermal conductivity of an MH bed decreased with decreasing the packing fraction. To solve these problems, it is important to improve the heat transfer of the MH bed, to prevent sediment accumulation of finely powdered MH onto the bottom of the reactor, and to control the packing fraction of the MH bed. In our previous study, a sheet-like composite was developed using MH powder, aramid pulp, and carbon fiber. We called it “MH sheet” and conducted an experiment to evaluate its thermal conductivity and cycle characteristics [20]. The purpose of this study is to investigate the effects of the use of MH sheet on the heat exchange performance in each MH bed, on the behavior of hydrogen transfer, and on the cooling output in system. The filling fraction was also evaluated to verify that the parameters could be controlled by applying an MH sheet.
2.
Experimental
2.1.
Metal hydride working pair
Considering the plateau properties and equilibrium pressure, two MHs were selected for the experiments. TiFe0.9Ni0.1 and
La0.6Y0.4Ni4.9Al0.1 were used as a heat source side alloy (MH1) and a cooling output side alloy (MH2), respectively. Both alloys were synthesized by a self-ignition combustion synthesis [21e24]. Fig. 1 shows the pressure-composition-isotherm (PCT) properties of MHs used for an MHHP system. The PCT properties of the products were evaluated after repeating the vacuuming/ pressurizing procedure five times with 4.1 MPa of hydrogen. The alloys demonstrate different equilibrium pressures, which is the driving force to transfer hydrogen between two reactors. Fig. 2 shows van’t Hoff plots of applied MHs. The reaction heat DH and entropy DS were calculated from the following equation. ln P ¼ DH=ðRTÞ DS=R
(1)
Table 1 lists the physical and thermodynamic properties of applied MHs. In this study, a single-stage single-effect system [25] was applied for cold generation. It consists of two reaction beds coupled by a connection pipe. Each reactor bed contains a different hydride, indicated as MH1 and MH2 in Fig. 2. Lowgrade heat, such as solar heat or waste heat, can be used as a driving heat source at Th. A fluid that is close to the environmental temperature is usually used as a heat sink at Tm. When heat source at Th is supplied to the MH1 reactor, MH1 desorbs hydrogen with increasing pressure in the system, and then, MH2 absorbs hydrogen at Tm. This period is called a regeneration process, indicated as (3)e(4) in Fig. 2. As shown in Figs. 1 and 2, the equilibrium pressure of MH2 is lower than that of MH1 at Tm. Therefore, MH1 absorbs hydrogen with decreasing pressure by a change in temperature from Th to Tm.
10
(b) MH2
(a) MH1
Pressure [MPa]
1
0.1
0.001
45 ºC 25 ºC 15 ºC 5 ºC
125 ºC 95 ºC 25 ºC 15 ºC
0.01
0
0.3 0.6 0.9 1.2 Stored hydrogen [mass%]
1.5 0
0.3 0.6 0.9 1.2 Stored hydrogen [mass%]
1.5
Fig. 1 e PCT curves of two metal hydrides used for the MHHP system: (a) TiFe0.9Ni0.1 for MH1; (b) La0.6Y0.4Ni4.9Al0.1 for MH2.
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10 Tl= 5 ºC
Tm= 15 ºC
Th=125 ºC
and 50 g of MH2 in order to maximize the amount of transferred hydrogen.
Pressure [MPa]
2.2.
1
(4) H2
(1) H2
0.1
(3)
MH1-Absorption MH1-Desorption MH2-Absorption MH2-Desorption
(2)
0.01 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 -1 1000/T [K ] Fig. 2 e Van’t Hoff plots of applied MHs, which were used to calculate the reaction heat DH and entropy DS. The diagram also indicates the hydrogen flows and recommended operation temperatures for the cooling system; (1)e(2) cooling process, (3)e(4) regeneration process, Th/Tm/Tl [ 125/15/5 C. Then, MH2 desorbs hydrogen, generating a cooling effect due to an endothermic reaction. This is called a cooling process. Experimental conditions were determined according to the PCT properties of applied MHs. Considering that operating pressure is lower than 1.0 MPa, the heat source temperature Th and the initial pressure were determined as 125 C and 1.0 MPa, respectively. The heat sink temperature Tm can range from 5 to 30 C in thermodynamic terms. We confirmed that 15 C was a desirable temperature as Tm, based on the result of a preliminary experiment. Assuming that the system operates in a hydrogen pressure range from 0.1 to 1.0 MPa, the effective hydrogen storage capacity of each MH was almost MH1:MH2 ¼ 1:1.5 in mass ratio. Therefore, we used 75 g of MH1
Table 1 e Physical and thermodynamic properties of applied MHs. Alloy Density [g/cm3] Before activation Bulk density (max) [g/cm3] Before activation (min) After activation (max) After activation (min) Specific heat [J/g K] DH [kJ/mol-H2] Absorption Desorption DS [J/K$mol-H2] Absorption Desorption
MH1 TiFe0.9 MH2 La0.6Y0.4 Ni0.1 Ni4.9Al0.1 6.40 4.04
7.07 3.25
3.25
2.45
3.15
2.96
2.29
2.07
0.396 33.8 37.4 91.3 98.0
0.479 26.3 27.4 86.9 87.8
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Packed-bed reactors of MH
A stainless steel tube of f19.05 200 mm in size was used as a packed-bed reactor as shown in Fig. 3(a). A K-type thermocouple was inserted for measuring the changes in temperature at the center of the reactor. Fig. 4 shows SEM images of the MH sheets, which were composed of MH powder, aramid pulp, and carbon fiber in a molar ratio of 7:1:2. The MH sheets were produced by using a wet paper method, in which agglutinated slurry of raw materials was dispersed onto a stainless steel mesh in water and then the sheet was dehydrated and dried. A detailed explanation of the preparation procedure has already been published in reference [20]. Table 2 lists the packed amounts of MH powder and MH sheet in four runs. The MH1 sheet was used to partially replace the MH powder at Runs 2 and 4. The MH2 sheet was used at Runs 3 and 4. Note that the mass ratio of MH1/MH2 was equal to 1.5, and the mass ratio of CF/Total MH was equal to 0.02. The sheet and powder were packed alternately into the reaction beds to be arranged in layers. The aspect ratios of powder and sheet layers were 0.4e0.6 and 0.3, respectively.
2.3.
Experimental procedure
Before the experiment was started, activation and stabilization treatments of used MHs were carried out. In the activation treatment, the MH1 reactor was first heated up to 300 C and the MH2 reactor was kept at room temperature (around 20 C). Both reactors were evacuated for more than 2 h using a rotary pump and were then charged with 4.0 MPa of pure 7 N hydrogen. 2 h later, the MH1 reactor was naturally cooled to room temperature. Both reactors were maintained at 4.0 MPa for 36 h. After the activation, hydrogen was discharged out of the system. Simultaneously, the MH1 and MH2 reactors were maintained at 125 C and 15 C, respectively, using a thermostatic bath for desorbing hydrogen. Then, both reactors were evacuated for at least 1 h, after which they were charged again with 1.0 MPa of hydrogen at 15 C for absorbing hydrogen. This procedure of desorbing/absorbing was carried out five times. Fig. 3(b) shows a schematic diagram of the experimental apparatus for an MHHP system that mainly consists of two MH reactors connected to each other through a mass flow meter. Before starting the measurement, the MH1 and MH2 reactors were charged with 1.0 MPa of hydrogen and were maintained for 1 h at 125 C and 15 C, respectively. The cooling process was started by moving the MH1 reactor from a thermostatic bath at 125 C to another one at 15 C. During this process, hydrogen flowed from MH2 reactor to MH1 reactor with decreasing pressure in the system. Then, the regeneration process was started by changing the holding temperature of MH1 from 15 C to 125 C, and hydrogen flowed in the inverse direction with increasing pressure in the system. Hydrogen mass flow and pressure variation were measured using a mass flow meter and pressure gauges. Changes in temperature at the center of both reactors were measured using inserted thermocouples. The measurements were
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Fig. 3 e SEM image of a metal hydride sheet, which is composed of (a) metal hydride powder (La0.6Y0.4Ni4.9Al0.1), (b) Aramid pulp, and (c) high thermal conductive carbon fiber in a mass ratio of 7:1:2.
conducted for 1 h in each process. The cycle of cooling and regeneration processes was performed three times for all experimental conditions shown in Table 2. According to a preliminary experiment, the reaction behavior varied only slightly after the third cycle. Thus, the results derived at third cycle were compared in this study.
3.
Results and discussion
Fig. 5 shows the changes in the temperature of at the center of MH packed-bed during the cooling and regeneration
(a)
processes. The MH1 reactor was cooled from 125 C to 15 C at the cooling process and was heated up again to 125 C during the regeneration process. The temperature of MH1 packedbed rapidly changed in the beginning of each process. At around 1.5 ks, the heat exchange rate decreased considerably, owing to the reaction heat of MH1 alloy that absorbed or desorbed hydrogen at the cooling/regeneration processes, respectively. It should be noted that the heat exchange rates of Runs 2 and 4 were faster than those of Runs 1 and 3. This result indicates that the MH sheet in MH1 reactor accelerated the heat exchange rate. In contrast, the MH2 reactor remained at 15 C in both processes. Thus, the temperature changes
Hydrogen cylinder
(b)
Helium cylinder
M.F.
P
P
Reservoir tank
MH1 reactor T.C.
MH2 R.P.
reactor T.C.
Fig. 4 e (a) Packed-bed reactor used in the experiment and (b) Schematic diagram of the experimental apparatus for a metal hydride heat pump (MHHP) system: M.F., mass flow meter; P, pressure gauge; T.C., thermocouple; R.P., rotary pump.
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Table 2 e Packed amounts of MH powder and MH sheet in four runs. Run
1 2 3 4
MH1 reactor [g]
MH2 reactor [g] a
b
Powder
Sheet
Total MH
CF
Powder
Sheet
Total MHa
CFb
75.0 69.8 75.0 69.8
0.0 7.5 0.0 7.5
75.0 75.0 75.0 75.0
0.0 1.5 0.0 1.5
50.0 50.0 46.5 46.5
0.0 0.0 5.0 5.0
50.0 50.0 50.0 50.0
0.0 0.0 1.0 1.0
Note that the mass ratio of MH1/MH2 ¼ 1.5, and mass ratio of CF/Total MH ¼ 0.02. a (Total amount of metal hydride) ¼ powder þ 0.70 sheet. b (Amount of carbon fiber) ¼ 0.20 sheet.
were attributed to the reaction heat of the MH2 alloy. The endothermic hydrogen desorption at MH2 in the cooling process is utilized as a cooling output. The reaction heat due to the hydrogen desorption/absorption was quickly removed at Runs 3 and 4 in contrast to Runs 1 and 2, indicating that the heat exchange performance was improved by using the MH sheet. As mentioned above, the effective thermal conductance of the MH bed limits the reaction kinetics in the MHHP system. Therefore, the improved heat exchange rate contributed to the enhancement of the system output. Fig. 6 shows the changes in the hydrogen pressure during (a) cooling and (b) regeneration processes. In the cooling process, a rapid pressure decrease due to the absorbing hydrogen of MH1 was observed soon after the start of the measurement. After showing a minimum at around 0.3 ks, the hydrogen pressure gradually increased and reached an equilibrium value. The pressure increase means that the reaction rate of MH2 became higher than that of MH1. The pressure changes at all runs showed the same tendency. In the regeneration process, a rapid pressure increase due to the desorbing hydrogen of MH1 was observed, having a maximum at around 0.4e0.6 ks except for Run 3. Then, the hydrogen pressure gradually decreased owing to the absorbing hydrogen of MH2 and reached an equilibrium value. The fact that Run 3 had no local maximum values was explained by the heat exchange performance of each reactor. The MH2 sheet was partially used as a reaction bed in the MH2 reactor at Run 3. In this case,
the MH2 reactor had higher heat exchange performance than the MH1 reactor. Therefore, the hydrogen released from MH1 was immediately absorbed by MH2 close to the equilibrium pressure. Thus, the hydrogen pressure did not increase beyond the equilibrium value. Fig. 7 shows the changes in the hydrogen flow rate during (a) cooling and (b) regeneration processes. In the cooling process, the hydrogen flow rate increased rapidly and showed a maximum value immediately after the start of the measurement. The maximum values of Runs 1e4 were 1.27, 1.54, 1.34, and 1.54 NL/min, respectively. This result suggests that the peak intensity depended on the heat exchange rate of the MH1 reactor; the observed peak was mainly attributed to the hydrogen absorption in MH1. After the maximum, the flow rate decreased abruptly and had an inflection point at around 0.08e0.15 ks, above which it decreased gradually. The inflection point indicated that MH2 started to absorb hydrogen. In the regeneration process, all runs showed first (dashed arrow) and second (solid arrow) peaks, which were mainly attributed to desorbing hydrogen in MH1 and absorbing hydrogen in MH2, respectively. The values of the first peaks of Runs 1e4 were 0.55, 0.64, 0.56, and 0.57 NL/min, and the value of second peaks of that were 0.74, 0.78. 0.77, and 0.86 NL/min, respectively. Thus, the intensity of the first peak depended on the heat exchange rate of the MH1 reactor in the regeneration process. In contrast, the second peak was related to the heat exchange performance of both reactors. The hydrogen flow
Fig. 5 e Changes in the temperature at the center of MH packed-bed during cooling and regeneration processes. Note that the MH sheet accelerated the heat exchange rate in each reactor.
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Fig. 6 e Changes in the hydrogen pressure during (a) cooling and (b) regeneration processes, showing rapid changes due to absorbing/desorbing hydrogen at MH1 soon after the start of the measurement.
rate of Run 4 showed the highest maximum value among all runs. According to the pressure changes and hydrogen flow rates during the experiments, the reaction rate of each MH can be described as follows: Shortly after the start of measurement, MH1 reacted rapidly in both processes. Next, the reaction rate of MH2 gradually increased and became higher than that of MH1 at 0.3e0.6 ks. Finally, absorbing/desorbing hydrogen in each reactor was finished and the hydrogen pressure reached an equilibrium state. The difference in the reaction rates was attributed to the driving forces of each MH: MH1 utilized large temperature changes as a driving force for the reaction, whereas MH2 required a pressure change caused by the reaction of MH1. Fig. 8 shows the integrated hydrogen flow volumes during (a) cooling and (b) regeneration processes. The total flow volume in each run is almost the same, because it is independent of the heat exchange performance. In order to evaluate the cooling output at all experimental conditions, a
half-cycle time was defined as the time required for achieving 95% of the total flow volume at 3.6 ks. According to a preliminary experiment, the standard error of halfcycle time was estimated at less than 2% after third cycle. Fig. 9 shows the half-cycle time and cooling power output under the experimental conditions. The cooling output, Wcooling, was calculated using the following equation. Wcooling ¼
DHde VH2 t95 mMH2 VSTP
(2)
where DHde, VH2 , t95, mMH2, and VSTP denote the reaction heat caused by desorption of hydrogen of MH2, total hydrogen flow volume, half-cycle time, the amount of MH2 alloy, and the standard gas volume at 0 C and 1 atm, respectively. The MHHP system is not able to supply a continuous cooling effect. As the two reaction beds work either in a cooling or regeneration process, two identical reaction bed couples working in the opposite directions are necessary to realize a
Fig. 7 e Changes in the hydrogen flow rate during (a) cooling and (b) regeneration processes. In the regeneration process, all runs show first (dashed arrow) and second (solid arrow) peaks, which are mainly attributed to the desorbing hydrogen at MH1 and absorbing hydrogen at MH2, respectively.
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Fig. 8 e Integrated hydrogen flow volume during (a) cooling and (b) regeneration processes. Half-cycle time was defined as the time required to achieve 95% of total flow volume.
1.5
Half cycle time [ks]
Cooling power 195
200
182 162
170
0.9
150
0.6
100
0.3
50
0
Run 1
Run 2
Run 3
Run 4
Cooling power [W/kg-MH2]
1.2
250 Half-cycle time of Cooling Regeneration
0
Fig. 9 e Half-cycle time and cooling power output under the experimental conditions. The applied MH sheets improved the cooling power output by accelerating the heat exchange rate.
quasi-continuous cold output. Thus, the cooling output is limited by longer half-cycle time. The half-cycle time in the regeneration process was used to estimate the cooling output under the experimental conditions. Upon applying
the MH sheet to both reactors (Run 4), the specific cooling power was as high as 195 W/kg-MH2, which was 1.2 times higher than when using only MH powder (Run 1). The result clearly shows that applying MH sheets improved the cooling power output, owing to the accelerated heat exchange. When Runs 2 and 3 were compared, the cooling output of Run 3 was considerably higher than that of Run 2. The heat exchange in the MH2 reactor had more important role in the enhancement of system performance than that in the MH1 reactor did. Table 3 lists the filling fractions and total volumes of each packed-bed reactor used in the experiments. The filling fraction of MH is decreased and porosity is increased by an MH sheet. This parameter can be easily controlled by changing the amount of MH sheet. The volume expansion ratio of MH is higher than 20% after absorbing hydrogen. The stresses induced by the deformation lead to a pulverization of MH, and thus, to self-compaction and concentration of stresses. Qin et al. have reported the pulverization and the expansion characteristics of La0.6Y0.4Ni4.8Mn0.2, and their influences in horizontal and vertical thin-wall reactors [19]. According to their paper, the packing fraction should not exceed 35 vol% when the hydrogen contents is around 1.0 mol-H/mol-alloy. The stress on the reaction vessel, caused by the expansion of MH during the hydrogen absorption/desorption, could be reduced since the packing fraction was easily controlled by using an MH sheet.
Table 3 e Filling fraction and the total volume of packed-bed reactors. MH1
Filling fraction [Vol.%]
Volume [cm3]
Metal hydride Aramid Carbon fiber Porosity
MH2
Only powder
Powder þ sheet
Only powder
Powder þ sheet
49.2 0.0 0.0 50.8 23.8
33.6 1.5 1.9 62.9 34.9
41.9 0.0 0.0 58.1 16.9
30.3 1.5 1.9 66.3 23.4
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Conclusions
An experimental study was conducted to evaluate the effectiveness of a metal hydride (MH) sheet applied to a metal hydride heat pump (MHHP) system. The applied MH sheet significantly increased the heat exchange ratio in both reactors. The heat exchange ratio at temperature between 15 C and 125 C was accelerated in the MH1 reactor as a heat source side. Meanwhile, the heat of reaction was effectively removed in the MH2 reactor as cooling output side. Upon applying the MH sheet to both reactors (Run 4), the specific cooling power was as high as 195 W/kg-MH2, which was 1.2 times higher than in the case of using only MH powder (Run 1). According to the comparison between Runs 2 and 3, the role of heat exchange for the enhancement of system performances in the MH2 reactor was more important than that in the MH1 reactor. Moreover, the stress on the reaction vessel, caused by the expansion of MH during hydrogen absorption/desorption, could be reduced since the packing fraction was easily controlled by using MH sheet. These results also showed that the application of MH sheet for packed-bed reactor offers many benefits for improving heat exchange performances and reducing the stress on reaction vessel, and that an MH sheet can be used with existing heat transfer methods.
Acknowledgments We thank Mr. Shiraishi and Mr. Yamakado of Azumi Filter Paper Co., Ltd. for preparing the MH sheet, and acknowledge Dr. Bae of Waseda University for measuring physical and thermal properties of alloys. This research was supported by a Grant-in-Aid for JSPS Fellows (23 3412).
references
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Application of metal hydride sheet to thermally driven cooling system Naoto Yasuda, Tohru Tsuchiya, Noriyuki Okinaka, Tomohiro Akiyama* Center for Advanced Research of Energy & Materials, Faculty of Engineering, Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628, Japan
article info
abstract
Article history:
This paper describes an application of a metal hydride (MH) sheet, which consists of MH
Received 24 January 2013
powder, carbon fiber, and aramid pulp, in a metal hydride heat pump (MHHP) system with
Received in revised form
a TiFe0.9Ni0.1/La0.6Y0.4Ni4.9Al0.1 working pair (MH1/MH2). In the experiments, the effect of
1 April 2013
the use of MH sheet on the system performances was investigated, in which the MH sheets
Accepted 3 April 2013
were used to replace part of the MH powder to improve the heat exchange performance.
Available online 3 May 2013
The sheets and powder were packed alternately into the MH beds in layers with an aspect ratio less than one. The MH sheet significantly accelerated the heat exchange ratio of both
Keywords:
MH packed beds. Using the MH sheet in both reactors, the specific cooling power increased
Metal hydride heat pump
by 1.2 times. The results also indicated that the role of heat exchange in an MH2 reactor as
Thermally driven cooling system
a cooling output side was more important in the enhancement of system performance
Hydrogen storage material
than that in an MH1 reactor as a heat source side. In addition, the proposed MH sheet was
Composite material
effective not only for improving the system performance but also for decreasing the stress on the reactor vessel due to the expansion of MH during the hydrogen absorption/ desorption. Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Some metals and alloys can react reversibly with a large amount of hydrogen under certain conditions. They are called metal hydrides (MHs), which have expected to be used not only as hydrogen storage carriers but also as energy conversion materials. The applications of hydrogen storage alloys are more advanced, for example, in Ni-MH battery [1], heat storage [2], purification of hydrogen gas [3], and chemical heat pump. The thermally driven metal hydride heat pump (MHHP) system is a promising candidate to utilize low-grade heat, because it has no mechanical moving parts and uses an environmentally friendly working fluid. Moreover, the system
can be applied in wide operating-temperature ranges by varying the MH composition. Many experimental and theoretical studies have been conducted to evaluate the performance of MHHP [4e8]. In addition, several proposed systems based on MHHP would utilize low-grade heat, such as industrial waste heat, exhaust gas of automobiles, liquefied natural gas cold energy, and solar energy [9e12]. However, the MHHP systems have not achieved practical use because of high initial cost of MHs and insufficient heat and/or mass transfer within the reaction beds. Usually the intrinsic reaction kinetics of MHs is relatively fast. Thus, the reaction rate of hydrogen absorbing/desorbing is limited by heat and/or by mass transfer. Various composite materials have been proposed to enhance the thermal
* Corresponding author. Tel.: þ81 11 706 6842; fax: þ81 11 726 0731. E-mail address: [email protected] (T. Akiyama). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.04.011
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cooling temperature, K, Tl Wcooling cooling output, W/kg-MH2, reaction heat caused by desorbing hydrogen of DHde MH2, J/mol-H2, total hydrogen flow volume, NL, VH2 half-cycle time, s, t95 amount of MH2 alloy, kg-MH2, mMH2 standard gas volume at 0 C and 1 atm, NL/mol. VSTP
Nomenclature H S P R T Th Tm
enthalpy, kJ/mol, entropy, J/mol K, pressure, MPa, gas constant, temperature, K, heat source temperature, K, heat sink temperature, K,
conductivity of the reaction bed [13e16]. A large amount of stress, on the other hand, is generated on the walls of vessels by the volume expansion of MH during the hydrogen absorption. Several investigations have been conducted on the expansion of MH and its effects on the reaction vessels [17e19]. The packing fraction of MH powder should be controlled to reduce the stress on the walls of vessels. However, the effective thermal conductivity of an MH bed decreased with decreasing the packing fraction. To solve these problems, it is important to improve the heat transfer of the MH bed, to prevent sediment accumulation of finely powdered MH onto the bottom of the reactor, and to control the packing fraction of the MH bed. In our previous study, a sheet-like composite was developed using MH powder, aramid pulp, and carbon fiber. We called it “MH sheet” and conducted an experiment to evaluate its thermal conductivity and cycle characteristics [20]. The purpose of this study is to investigate the effects of the use of MH sheet on the heat exchange performance in each MH bed, on the behavior of hydrogen transfer, and on the cooling output in system. The filling fraction was also evaluated to verify that the parameters could be controlled by applying an MH sheet.
2.
Experimental
2.1.
Metal hydride working pair
Considering the plateau properties and equilibrium pressure, two MHs were selected for the experiments. TiFe0.9Ni0.1 and
La0.6Y0.4Ni4.9Al0.1 were used as a heat source side alloy (MH1) and a cooling output side alloy (MH2), respectively. Both alloys were synthesized by a self-ignition combustion synthesis [21e24]. Fig. 1 shows the pressure-composition-isotherm (PCT) properties of MHs used for an MHHP system. The PCT properties of the products were evaluated after repeating the vacuuming/ pressurizing procedure five times with 4.1 MPa of hydrogen. The alloys demonstrate different equilibrium pressures, which is the driving force to transfer hydrogen between two reactors. Fig. 2 shows van’t Hoff plots of applied MHs. The reaction heat DH and entropy DS were calculated from the following equation. ln P ¼ DH=ðRTÞ DS=R
(1)
Table 1 lists the physical and thermodynamic properties of applied MHs. In this study, a single-stage single-effect system [25] was applied for cold generation. It consists of two reaction beds coupled by a connection pipe. Each reactor bed contains a different hydride, indicated as MH1 and MH2 in Fig. 2. Lowgrade heat, such as solar heat or waste heat, can be used as a driving heat source at Th. A fluid that is close to the environmental temperature is usually used as a heat sink at Tm. When heat source at Th is supplied to the MH1 reactor, MH1 desorbs hydrogen with increasing pressure in the system, and then, MH2 absorbs hydrogen at Tm. This period is called a regeneration process, indicated as (3)e(4) in Fig. 2. As shown in Figs. 1 and 2, the equilibrium pressure of MH2 is lower than that of MH1 at Tm. Therefore, MH1 absorbs hydrogen with decreasing pressure by a change in temperature from Th to Tm.
10
(b) MH2
(a) MH1
Pressure [MPa]
1
0.1
0.001
45 ºC 25 ºC 15 ºC 5 ºC
125 ºC 95 ºC 25 ºC 15 ºC
0.01
0
0.3 0.6 0.9 1.2 Stored hydrogen [mass%]
1.5 0
0.3 0.6 0.9 1.2 Stored hydrogen [mass%]
1.5
Fig. 1 e PCT curves of two metal hydrides used for the MHHP system: (a) TiFe0.9Ni0.1 for MH1; (b) La0.6Y0.4Ni4.9Al0.1 for MH2.
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10 Tl= 5 ºC
Tm= 15 ºC
Th=125 ºC
and 50 g of MH2 in order to maximize the amount of transferred hydrogen.
Pressure [MPa]
2.2.
1
(4) H2
(1) H2
0.1
(3)
MH1-Absorption MH1-Desorption MH2-Absorption MH2-Desorption
(2)
0.01 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 -1 1000/T [K ] Fig. 2 e Van’t Hoff plots of applied MHs, which were used to calculate the reaction heat DH and entropy DS. The diagram also indicates the hydrogen flows and recommended operation temperatures for the cooling system; (1)e(2) cooling process, (3)e(4) regeneration process, Th/Tm/Tl [ 125/15/5 C. Then, MH2 desorbs hydrogen, generating a cooling effect due to an endothermic reaction. This is called a cooling process. Experimental conditions were determined according to the PCT properties of applied MHs. Considering that operating pressure is lower than 1.0 MPa, the heat source temperature Th and the initial pressure were determined as 125 C and 1.0 MPa, respectively. The heat sink temperature Tm can range from 5 to 30 C in thermodynamic terms. We confirmed that 15 C was a desirable temperature as Tm, based on the result of a preliminary experiment. Assuming that the system operates in a hydrogen pressure range from 0.1 to 1.0 MPa, the effective hydrogen storage capacity of each MH was almost MH1:MH2 ¼ 1:1.5 in mass ratio. Therefore, we used 75 g of MH1
Table 1 e Physical and thermodynamic properties of applied MHs. Alloy Density [g/cm3] Before activation Bulk density (max) [g/cm3] Before activation (min) After activation (max) After activation (min) Specific heat [J/g K] DH [kJ/mol-H2] Absorption Desorption DS [J/K$mol-H2] Absorption Desorption
MH1 TiFe0.9 MH2 La0.6Y0.4 Ni0.1 Ni4.9Al0.1 6.40 4.04
7.07 3.25
3.25
2.45
3.15
2.96
2.29
2.07
0.396 33.8 37.4 91.3 98.0
0.479 26.3 27.4 86.9 87.8
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Packed-bed reactors of MH
A stainless steel tube of f19.05 200 mm in size was used as a packed-bed reactor as shown in Fig. 3(a). A K-type thermocouple was inserted for measuring the changes in temperature at the center of the reactor. Fig. 4 shows SEM images of the MH sheets, which were composed of MH powder, aramid pulp, and carbon fiber in a molar ratio of 7:1:2. The MH sheets were produced by using a wet paper method, in which agglutinated slurry of raw materials was dispersed onto a stainless steel mesh in water and then the sheet was dehydrated and dried. A detailed explanation of the preparation procedure has already been published in reference [20]. Table 2 lists the packed amounts of MH powder and MH sheet in four runs. The MH1 sheet was used to partially replace the MH powder at Runs 2 and 4. The MH2 sheet was used at Runs 3 and 4. Note that the mass ratio of MH1/MH2 was equal to 1.5, and the mass ratio of CF/Total MH was equal to 0.02. The sheet and powder were packed alternately into the reaction beds to be arranged in layers. The aspect ratios of powder and sheet layers were 0.4e0.6 and 0.3, respectively.
2.3.
Experimental procedure
Before the experiment was started, activation and stabilization treatments of used MHs were carried out. In the activation treatment, the MH1 reactor was first heated up to 300 C and the MH2 reactor was kept at room temperature (around 20 C). Both reactors were evacuated for more than 2 h using a rotary pump and were then charged with 4.0 MPa of pure 7 N hydrogen. 2 h later, the MH1 reactor was naturally cooled to room temperature. Both reactors were maintained at 4.0 MPa for 36 h. After the activation, hydrogen was discharged out of the system. Simultaneously, the MH1 and MH2 reactors were maintained at 125 C and 15 C, respectively, using a thermostatic bath for desorbing hydrogen. Then, both reactors were evacuated for at least 1 h, after which they were charged again with 1.0 MPa of hydrogen at 15 C for absorbing hydrogen. This procedure of desorbing/absorbing was carried out five times. Fig. 3(b) shows a schematic diagram of the experimental apparatus for an MHHP system that mainly consists of two MH reactors connected to each other through a mass flow meter. Before starting the measurement, the MH1 and MH2 reactors were charged with 1.0 MPa of hydrogen and were maintained for 1 h at 125 C and 15 C, respectively. The cooling process was started by moving the MH1 reactor from a thermostatic bath at 125 C to another one at 15 C. During this process, hydrogen flowed from MH2 reactor to MH1 reactor with decreasing pressure in the system. Then, the regeneration process was started by changing the holding temperature of MH1 from 15 C to 125 C, and hydrogen flowed in the inverse direction with increasing pressure in the system. Hydrogen mass flow and pressure variation were measured using a mass flow meter and pressure gauges. Changes in temperature at the center of both reactors were measured using inserted thermocouples. The measurements were
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Fig. 3 e SEM image of a metal hydride sheet, which is composed of (a) metal hydride powder (La0.6Y0.4Ni4.9Al0.1), (b) Aramid pulp, and (c) high thermal conductive carbon fiber in a mass ratio of 7:1:2.
conducted for 1 h in each process. The cycle of cooling and regeneration processes was performed three times for all experimental conditions shown in Table 2. According to a preliminary experiment, the reaction behavior varied only slightly after the third cycle. Thus, the results derived at third cycle were compared in this study.
3.
Results and discussion
Fig. 5 shows the changes in the temperature of at the center of MH packed-bed during the cooling and regeneration
(a)
processes. The MH1 reactor was cooled from 125 C to 15 C at the cooling process and was heated up again to 125 C during the regeneration process. The temperature of MH1 packedbed rapidly changed in the beginning of each process. At around 1.5 ks, the heat exchange rate decreased considerably, owing to the reaction heat of MH1 alloy that absorbed or desorbed hydrogen at the cooling/regeneration processes, respectively. It should be noted that the heat exchange rates of Runs 2 and 4 were faster than those of Runs 1 and 3. This result indicates that the MH sheet in MH1 reactor accelerated the heat exchange rate. In contrast, the MH2 reactor remained at 15 C in both processes. Thus, the temperature changes
Hydrogen cylinder
(b)
Helium cylinder
M.F.
P
P
Reservoir tank
MH1 reactor T.C.
MH2 R.P.
reactor T.C.
Fig. 4 e (a) Packed-bed reactor used in the experiment and (b) Schematic diagram of the experimental apparatus for a metal hydride heat pump (MHHP) system: M.F., mass flow meter; P, pressure gauge; T.C., thermocouple; R.P., rotary pump.
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Table 2 e Packed amounts of MH powder and MH sheet in four runs. Run
1 2 3 4
MH1 reactor [g]
MH2 reactor [g] a
b
Powder
Sheet
Total MH
CF
Powder
Sheet
Total MHa
CFb
75.0 69.8 75.0 69.8
0.0 7.5 0.0 7.5
75.0 75.0 75.0 75.0
0.0 1.5 0.0 1.5
50.0 50.0 46.5 46.5
0.0 0.0 5.0 5.0
50.0 50.0 50.0 50.0
0.0 0.0 1.0 1.0
Note that the mass ratio of MH1/MH2 ¼ 1.5, and mass ratio of CF/Total MH ¼ 0.02. a (Total amount of metal hydride) ¼ powder þ 0.70 sheet. b (Amount of carbon fiber) ¼ 0.20 sheet.
were attributed to the reaction heat of the MH2 alloy. The endothermic hydrogen desorption at MH2 in the cooling process is utilized as a cooling output. The reaction heat due to the hydrogen desorption/absorption was quickly removed at Runs 3 and 4 in contrast to Runs 1 and 2, indicating that the heat exchange performance was improved by using the MH sheet. As mentioned above, the effective thermal conductance of the MH bed limits the reaction kinetics in the MHHP system. Therefore, the improved heat exchange rate contributed to the enhancement of the system output. Fig. 6 shows the changes in the hydrogen pressure during (a) cooling and (b) regeneration processes. In the cooling process, a rapid pressure decrease due to the absorbing hydrogen of MH1 was observed soon after the start of the measurement. After showing a minimum at around 0.3 ks, the hydrogen pressure gradually increased and reached an equilibrium value. The pressure increase means that the reaction rate of MH2 became higher than that of MH1. The pressure changes at all runs showed the same tendency. In the regeneration process, a rapid pressure increase due to the desorbing hydrogen of MH1 was observed, having a maximum at around 0.4e0.6 ks except for Run 3. Then, the hydrogen pressure gradually decreased owing to the absorbing hydrogen of MH2 and reached an equilibrium value. The fact that Run 3 had no local maximum values was explained by the heat exchange performance of each reactor. The MH2 sheet was partially used as a reaction bed in the MH2 reactor at Run 3. In this case,
the MH2 reactor had higher heat exchange performance than the MH1 reactor. Therefore, the hydrogen released from MH1 was immediately absorbed by MH2 close to the equilibrium pressure. Thus, the hydrogen pressure did not increase beyond the equilibrium value. Fig. 7 shows the changes in the hydrogen flow rate during (a) cooling and (b) regeneration processes. In the cooling process, the hydrogen flow rate increased rapidly and showed a maximum value immediately after the start of the measurement. The maximum values of Runs 1e4 were 1.27, 1.54, 1.34, and 1.54 NL/min, respectively. This result suggests that the peak intensity depended on the heat exchange rate of the MH1 reactor; the observed peak was mainly attributed to the hydrogen absorption in MH1. After the maximum, the flow rate decreased abruptly and had an inflection point at around 0.08e0.15 ks, above which it decreased gradually. The inflection point indicated that MH2 started to absorb hydrogen. In the regeneration process, all runs showed first (dashed arrow) and second (solid arrow) peaks, which were mainly attributed to desorbing hydrogen in MH1 and absorbing hydrogen in MH2, respectively. The values of the first peaks of Runs 1e4 were 0.55, 0.64, 0.56, and 0.57 NL/min, and the value of second peaks of that were 0.74, 0.78. 0.77, and 0.86 NL/min, respectively. Thus, the intensity of the first peak depended on the heat exchange rate of the MH1 reactor in the regeneration process. In contrast, the second peak was related to the heat exchange performance of both reactors. The hydrogen flow
Fig. 5 e Changes in the temperature at the center of MH packed-bed during cooling and regeneration processes. Note that the MH sheet accelerated the heat exchange rate in each reactor.
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Fig. 6 e Changes in the hydrogen pressure during (a) cooling and (b) regeneration processes, showing rapid changes due to absorbing/desorbing hydrogen at MH1 soon after the start of the measurement.
rate of Run 4 showed the highest maximum value among all runs. According to the pressure changes and hydrogen flow rates during the experiments, the reaction rate of each MH can be described as follows: Shortly after the start of measurement, MH1 reacted rapidly in both processes. Next, the reaction rate of MH2 gradually increased and became higher than that of MH1 at 0.3e0.6 ks. Finally, absorbing/desorbing hydrogen in each reactor was finished and the hydrogen pressure reached an equilibrium state. The difference in the reaction rates was attributed to the driving forces of each MH: MH1 utilized large temperature changes as a driving force for the reaction, whereas MH2 required a pressure change caused by the reaction of MH1. Fig. 8 shows the integrated hydrogen flow volumes during (a) cooling and (b) regeneration processes. The total flow volume in each run is almost the same, because it is independent of the heat exchange performance. In order to evaluate the cooling output at all experimental conditions, a
half-cycle time was defined as the time required for achieving 95% of the total flow volume at 3.6 ks. According to a preliminary experiment, the standard error of halfcycle time was estimated at less than 2% after third cycle. Fig. 9 shows the half-cycle time and cooling power output under the experimental conditions. The cooling output, Wcooling, was calculated using the following equation. Wcooling ¼
DHde VH2 t95 mMH2 VSTP
(2)
where DHde, VH2 , t95, mMH2, and VSTP denote the reaction heat caused by desorption of hydrogen of MH2, total hydrogen flow volume, half-cycle time, the amount of MH2 alloy, and the standard gas volume at 0 C and 1 atm, respectively. The MHHP system is not able to supply a continuous cooling effect. As the two reaction beds work either in a cooling or regeneration process, two identical reaction bed couples working in the opposite directions are necessary to realize a
Fig. 7 e Changes in the hydrogen flow rate during (a) cooling and (b) regeneration processes. In the regeneration process, all runs show first (dashed arrow) and second (solid arrow) peaks, which are mainly attributed to the desorbing hydrogen at MH1 and absorbing hydrogen at MH2, respectively.
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Fig. 8 e Integrated hydrogen flow volume during (a) cooling and (b) regeneration processes. Half-cycle time was defined as the time required to achieve 95% of total flow volume.
1.5
Half cycle time [ks]
Cooling power 195
200
182 162
170
0.9
150
0.6
100
0.3
50
0
Run 1
Run 2
Run 3
Run 4
Cooling power [W/kg-MH2]
1.2
250 Half-cycle time of Cooling Regeneration
0
Fig. 9 e Half-cycle time and cooling power output under the experimental conditions. The applied MH sheets improved the cooling power output by accelerating the heat exchange rate.
quasi-continuous cold output. Thus, the cooling output is limited by longer half-cycle time. The half-cycle time in the regeneration process was used to estimate the cooling output under the experimental conditions. Upon applying
the MH sheet to both reactors (Run 4), the specific cooling power was as high as 195 W/kg-MH2, which was 1.2 times higher than when using only MH powder (Run 1). The result clearly shows that applying MH sheets improved the cooling power output, owing to the accelerated heat exchange. When Runs 2 and 3 were compared, the cooling output of Run 3 was considerably higher than that of Run 2. The heat exchange in the MH2 reactor had more important role in the enhancement of system performance than that in the MH1 reactor did. Table 3 lists the filling fractions and total volumes of each packed-bed reactor used in the experiments. The filling fraction of MH is decreased and porosity is increased by an MH sheet. This parameter can be easily controlled by changing the amount of MH sheet. The volume expansion ratio of MH is higher than 20% after absorbing hydrogen. The stresses induced by the deformation lead to a pulverization of MH, and thus, to self-compaction and concentration of stresses. Qin et al. have reported the pulverization and the expansion characteristics of La0.6Y0.4Ni4.8Mn0.2, and their influences in horizontal and vertical thin-wall reactors [19]. According to their paper, the packing fraction should not exceed 35 vol% when the hydrogen contents is around 1.0 mol-H/mol-alloy. The stress on the reaction vessel, caused by the expansion of MH during the hydrogen absorption/desorption, could be reduced since the packing fraction was easily controlled by using an MH sheet.
Table 3 e Filling fraction and the total volume of packed-bed reactors. MH1
Filling fraction [Vol.%]
Volume [cm3]
Metal hydride Aramid Carbon fiber Porosity
MH2
Only powder
Powder þ sheet
Only powder
Powder þ sheet
49.2 0.0 0.0 50.8 23.8
33.6 1.5 1.9 62.9 34.9
41.9 0.0 0.0 58.1 16.9
30.3 1.5 1.9 66.3 23.4
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Conclusions
An experimental study was conducted to evaluate the effectiveness of a metal hydride (MH) sheet applied to a metal hydride heat pump (MHHP) system. The applied MH sheet significantly increased the heat exchange ratio in both reactors. The heat exchange ratio at temperature between 15 C and 125 C was accelerated in the MH1 reactor as a heat source side. Meanwhile, the heat of reaction was effectively removed in the MH2 reactor as cooling output side. Upon applying the MH sheet to both reactors (Run 4), the specific cooling power was as high as 195 W/kg-MH2, which was 1.2 times higher than in the case of using only MH powder (Run 1). According to the comparison between Runs 2 and 3, the role of heat exchange for the enhancement of system performances in the MH2 reactor was more important than that in the MH1 reactor. Moreover, the stress on the reaction vessel, caused by the expansion of MH during hydrogen absorption/desorption, could be reduced since the packing fraction was easily controlled by using MH sheet. These results also showed that the application of MH sheet for packed-bed reactor offers many benefits for improving heat exchange performances and reducing the stress on reaction vessel, and that an MH sheet can be used with existing heat transfer methods.
Acknowledgments We thank Mr. Shiraishi and Mr. Yamakado of Azumi Filter Paper Co., Ltd. for preparing the MH sheet, and acknowledge Dr. Bae of Waseda University for measuring physical and thermal properties of alloys. This research was supported by a Grant-in-Aid for JSPS Fellows (23 3412).
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