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Low-pressure adsorption storage of hydrogen
036(~3199/94 $6.00 + 0.00 Pergamon Press Ltd. © 1993 International Association for Hydrogen Energy.
Int. ,L Hydrogen Energy, Vol. 19, No. 2, pp. 161 164, 1994. Printed in Great Britain.
LOW-PRESSURE A D S O R P T I O N STORAGE OF H Y D R O G E N R. CHAHINEand T. K. BOSE Groupe de Recherche sur les Dirlectriques, Universitb du Qurbec h Trois-Rivirres, Case Postale 500, Trois-Rivirres (Qurbec), Canada G9A 5H7
(Receivedfor publication 16 April 1993) Abstract--The amount of gas that can be stored in an adsorption system is dictated by the nature of the adsorbent material and the operating conditions of the storage system, namely the storage pressure and temperature. This paper describes the impact of these factors on H 2 storage. The discussion is based on measurements of hydrogen adsorption on commercially available and densified adsorbents. The measurements were carried out in the pressure range 1-80 atm and at different storage temperatures using a high-pressure volumetric method. The results show that the adsorption technique could provide a viable method for hydrogen storage.
INTRODUCTION Due to growing concern about the negative impact of fossil fuel production and usage on the environment, sources of clean energy are urgently needed. Of the alternative sources that are being considered, hydrogen has been found to have several advantages. It is renewable, environmentally safe and has the potential for becoming a most versatile energy vector. Nevertheless, the technology for many of its uses, especially as a fuel, has yet to be fully developed. In the search for a new and better means of storing hydrogen, adsorption storage on activated carbons is emerging as a very promising technology [1, 2]. It has the same known advantages of the compression storage technology, but fewer of its drawbacks. The biggest advantage of adsorption over compression resides in its potential to store the same quantity of gas at a much lower pressure. This could result in lower capital and operating costs for compression, and could reduce some of the technical problems associated with high-pressure storage [3] (high pressure tanks, hydrogen embrittlement, etc.). Physical adsorption of a gas by a solid is the condition in which the concentraion of the gas molecules at the gas/ solid interface is greater than the bulk concentration. This enrichment is caused by the Van der Waals interactions between the solid atoms and the gas molecules. The amount of gas usually stored in a high-pressure cylinder can thus be enhanced by placing a high-surface-area material in the cylinder. When the cylinder is filled with compressed gas, a major portion of the gas is adsorbed on the surface of the solid substrate, thereby lowering the storage pressure (see Fig. 1). The operation of an adsorption storage system is basically similar to that of an ordinary compression storage system with one noticeable
exception. Adsorption storage requires thermal management of the exothermic heat of adsorption and the endothermic heat of desorption. For hydrogen, the heat of adsorption is about 4 kJ m o l - ~ at 77K. A D S O R P T I O N STORAGE O F H Y D R O G E N The amount of gas that can be stored in an adsorption system is dictated by the nature of the adsorbent material and the operating conditions of the storage system, namely the storage pressure and temperature. In the following we assess the impact of these factors on H 2 storage. Our assessment is based on measurements of hydrogen adsorption on commercially available and modified adsorbents. The measurements were carried out in the pressure range 1-80 atm and at different storage temperatures using a high-pressure volumetric method.
(.9
Compression Storage
Pressure
Fig. 1. Comparison of adsorption storage and compression storage. 161
162
R. CHAHINE and T. K. BOSE
Adsorbent materials
T=77K
The adsorption properties of a porous solid depend primarily on the size of its pores, which are classified as micropores ( < 2 nm), mesopores (between 2 and 50 nm) and macropores ( > 50 nm). Above the critical point, gas adsorption takes place in the micropores only, and the density of the adsorbed phase is much greater than the density of the unadsorbed gaseous phase (bulk phase) in the macropores or voids. Therefore, adsorption goes up with the amount of micropore surface area available. The nature of the surface area also plays a role in the adsorption process. Certain sites on the surface are more favorable to hydrogen adsorption than others. Two major types of high-surface-area adsorbents are available commercially: zeolites and microporous carbons. Molecular-sieve zeolites are aluminosilicates based on an atomic framework which is arranged to form molecular-size channels. Microporous carbons are obtained by the activation of organic and carbonaceous substrates. Figure 2 shows the excess adsorption isotherms of hydrogen for different zeolites and carbon adsorbents. Excess adsorption, expressed on a per mass basis, signifies the amount of gas present in the pores and on the surface of the adsorbent over and above that corresponding to the bulk phase at the measured temperature and pressure. We can easily see from Fig. 2 that activated carbons are better adsorbents of hydrogen than zeolites. Figure 2 also shows the special performance of AX-21 activated carbon which adsorbs twice as much as regular grade activated carbons. AX-21-type carbons are produced by the reaction of cokes with potassium hydroxides (KOH) and have a cage-like type of porosity in which pore filling occurs by the cooperative effect, thus giving them an unusually high surface area of the order of 3000 m z g ~. The surface area of regular grade microporous carbons lies between 700 and 1800 m 2 g - ~. Our studies [-4, 5] and others [6] in the field of adsorption storage of natural gas for vehicles (NGV) clearly show that if a high surface area is desirable in order to maximize adsorption per unit mass, it is also critical to have an adsorbent with high bulk density in order to minimize the volume of the storage system.
3O AX-21 (0,3 g/c
20
!
60
Fig. 3. Amount of H 2 stored in 11 cylinders loaded with different adsorbents. Unfortunately, the best commercially available highsurface-area adsorbents have, in general, a low bulk density. This is clearly demonstrated in Fig. 3 which shows the amount ofH 2 that can be stored in 11 cylinders loaded with some of the above adsorbents. For example, the AX-21 carbon has a surface area of 3000 m 2 g- ~ and a bulk density of 0.3 gem -3, while Calgon's BPL carbon has a surface area of 1100 m 2 g-~ and a bulk density of 0.47 gem -3. On a mass basis, AX-21 adsorbs 125~ more hydrogen than BPL (at 77K and 35atm), but on a volume basis AX-21 will only store 25~o more hydrogen than BPL under the same conditions.
Operatin9 conditions The development of the adsorption technology for natural gas storage is geared toward its use at around 35 atm and at ambient temperatures. These temperatures may not be suitable for hydrogen storage, as can be seen from Fig. 4, which compares the adsorption of hydrogen and methane (the main constituent of natural gas) as a function of pressure at 298K. Adsorbent storage systems designed to operate at room temperature will be much more efficient in the case of methane. In order to increase the hydrogen storage capacity of the material, one should operate at a much lower temperature. Figure 5 shows the
I • ~21
T=77K
40
Pressure (arm)
T=298K
_.- . . . . . . . . . . . .
~ CNN-201
~--~ 14
o "1-
•
4 3
... _1'~.~-.
"-
2
~
t
Norff
Methane
• Unde 5A • Linde 13X . . . . . . ;---~-
-:
.......... --,.--,_.~ - -
- ~ _
.. Y "
10
g
-~
.~ff.-=-" ~......... y ................................... 7.?. . . . . . . . . . . . "" """ Regular Grade Carbons ................
20 o "O ,<
t
J Hydrogen f
. . . . . . Zeolites 0
20
40
60
= 4O
80
Pressure (atm)
Fig. 2. Excess adsorption isotherms of H z on zeolites and carbon adsorbents.
r 50
6~0
Pressure (atm)
Fig. 4. Excess adsorption isotherms of CH4 and activated carbon.
H 2 on
AX-21
ADSORPTION STORAGE OF HYDROGEN 60
~
be more suitable. This is especially true if the storage system is charged with compressed hydrogen. Figure 7 shows the storage capacity of AX-21 at 175K. At 35 arm the storage density of H2 is around 8 kg m-3 which is equivalent to 100atm of pure compression at room temperature and to about 60 atm at 175K leading to an enhancement factor of about 1.7.
[]
~ 40 ~.
163
175 K .~ . . . . . .~--10
298 K 20
30
40
50
Pressure (atm)
Fig. 5. Adsorption isotherms of H 2 as AX-21 at different temperatures. variation of H 2 adsorption with temperature. Lowering the storage temperature to 175K will increase the adsorption by a factor of 5, and by a factor of 15 if the temperature is lowered further to 77K. The direct added cost of this option to the storage system will be in the use of the cryogenic reservoirs. The other added cost comes from cooling of the filling gas if it is compressed H 2. However, this cost will not be considered if the filling gas is vaporized liquid hydrogen. The total H 2 storage density of AX-21 as a function of the pressure at 77K is shown in Fig. 6. The total capacity is the sum of the excess adsorption, which is mainly confined to micropores, and the bulk phase, which is found in macropores and voids between the carbon particles. At 35 atm, the hydrogen storage density of AX21 is 25 g I- 1 which is about 30~ of the liquid hydrogen density and equivalent to 300atm of compression at room temperature and to about 78 arm of compression at 77K, thus giving an enhancement factor of about 2.2. One should also notice that the relative efficiency of the adsorption storage is greater at lower pressures. From Fig. 6 we can see that halving the storage pressure to 17.5 atm will only reduce the storage capacity by 20)~ and will further simplify the design of the cryogenic storage vessel. It could be argued that a storage temperature of 77K is too low, and temperatures in the range 150-175K would
T=77K
Modified adsorbents Short of developing specific adsorbents for hydrogen storage, the performance of commercially available adsorbents could be substantially improved by the chemical modification of their surface properties and by increasing their bulk density. In the case of hydrogen, it was shown El] that the metal assisted modification of the surface of activated carbon could lead to a 20~o enhancement in H 2 uptake. In order to improve the packing of low-density adsorbents, we developed a solidification method I-7, 8-1 that squeezes the voids out of the carbon particles without considerably reducing the specific adsorption. With this method we were able to increase the bulk density of the AX-21 carbon by a factor of more than 2 and to improve the volumetric storage capacity of methane by more than 50~. The capacity improvement in the case of hydrogen storage at 77K is shown in Fig. 8. The volumetric storage capacity increase is 54~o at around 17 atm, and 34~o at around 35 atm. Also shown in Fig. 8 is the amount of H 2 that can be stored at 77K by pure compression. The storage capacity of the solidified adsorbent at 17 atm is 5.2 times greater than that obtainable by compression storage alone under the same conditions. It is around 40~o of the LH 2 density and it is equivalent to the density of compressed H 2 at 90 atm and 77K. At 35 atm, the adsorption storage capacity is about 3 times greater, and is equivalent to 110 atm of pure compression. ECONOMICS OF HYDROGEN STORAGE ON ACTIVATED CARBON The economics of H 2 adsorption storage has been studied recently by Amankwah et al. [9], for a storage
12
30
T = 175 K
1 Total
'
.~:,........ ' 8 Total
20
~ , ~'
Adsorbed
. . . . . . . . .
.~
_~,~. . . .
~.~.___..~-----.... ' - A d s o r b e d
o
,
~
10
10
20
30
40
50
Pressure (arm)
Fig. 6.
H2
storage density of AX-21 activated carbon at 77K and as a function of the pressure.
to
2o
3o
2o
Pressure (atm)
Fig. 7. H 2 storage density of AX-21 activated carbon at 175K and as a function of the pressure.
164
R. CHAHINE and T. K. BOSE designed for hydrogen storage. The availability of such an adsorbent will greatly increase the competitivity of this storage option.
T= 77 K --~- 40
Solidified AX-21 . . . . . . . . . . . .
30~
...................
CONCLUSIONS
~' 2o
N ~
10
........ "°°'i °° "" i .....
oo
Pure Compression
20
~o
6'o
~o
Pressure (atm)
Fig. 8. Comparison of the total H 2 storage density at 77K and as a function of the pressure.
system operating at filling conditions of 150K and 54 atm and using AX-31M activated carbon which is similar to the one used in our experiments. The study also compared the costs of adsorption storage with those of the more traditional alternatives: compressed gas storage at 200 atm (GH2), liquefaction (LH2) and metal hydride (MH2) systems. The storage costs were divided into three major factors: the filling utility cost, the storage system cost, and the energy consumption cost. An overview of these costs is reproduced in Table 1, where we can see that the adsorption storage option (referred to as SA-M) compares very favorably with the other options. The results shown here are for the as-received carbon. If we consider the 4 0 ~ improvement in the storage density that could be obtained by solidifying the adsorbent and the additional 20~o increase that could be obtained by surface modification as mentioned earlier, we can greatly enhance the economic outlook of adsorption storage, which is already positive. The other point that should be made here is that none of the activated carbons available commercially have been optimized for hydrogen adsorption, and the best activated carbons available commercially are K O H based and they are expensive. Therefore, there is a need to develop a lowcost/high-capacity solid adsorbent which is specifically
We have shown that hydrogen adsorption on activated carbon holds much promise. The adsorption technique could provide a viable method for hydrogen storage. Low-pressure adsorption storage is not only economical, but also has a psychological advantage over high-pressure compression storage as far as consumers are concerned. Finally, hydrogen storage on activated carbon is rather new and much research needs to be done for further improvement of this technology.
REFERENCES 1. J. S. Noh et al., Int. J. Hydrogen Energy 12, 693 (1987). 2. D. L. Block, S. Dutta and A. T-Raissi, Hydrooenfor Power Applications--Task 2. Solar Energy Research Institute, Golden, CO (1988). 3. R. J. Remick et aL, Advanced onboard storage concepts for NG fueled automotive vehicles. Report No. DOE/NASA/ 0327-1 (1984). 4. T. K. Bose et al., Rapport sur la compr6hension du ph6nom6ne d'adsorption: application au gaz naturel comprim& SOQUIP, Quebec, Canada (1984). 5. T. K. Bose, R. Chahine, L. Marchildon and J. M. St-Arnaud, D6veloppement de la technique de l'adsorption en vue de reduire la pression dans un cylindre de gaz naturel comprim& Final Report Ministry of Science and Technology of Qu6bec/ SOQUIP (1987). 6. S. S. Barton et al. The projection and evaluation of composite carbons for the adsorption of methane. Report No. AF-84-04 Ontario Ministry of Energy, Toronto, Ontario (1984). 7. T.K. Bose, R. Chahine, J. M. St-Arnaud, SOQUIP and GMI, High-density adsorbent and method of producing same. U.S. Patent No. 4,999,330 (1991). 8. R. Chahine and T. K. Bose, Proc. 20th Biennial Conf. on Carbon, Santa Barbara, CA, pp. 638-639 (1991). 9. K. A. G. Amankwah, J. S. Noh and J. A. Schwarz, Int. J. Hydrogen Energy 14, 437 (1989).
Table 1. Cost of H 2 storage 1-9]
System GH 2 (200 atm) LH 2 MH 2 (FeTi) SA-M (150K, 54 atm)
Utility unit
Storage unit
Energy consumption
Total cost ($ MB.T.U.- 1)
0.77 1.21 0.57
7.66 2.23 7.40 2.46
2.50 14.50 5.10 2.52
10.93 17.94
1.98
13.07
6.96
Int. ,L Hydrogen Energy, Vol. 19, No. 2, pp. 161 164, 1994. Printed in Great Britain.
LOW-PRESSURE A D S O R P T I O N STORAGE OF H Y D R O G E N R. CHAHINEand T. K. BOSE Groupe de Recherche sur les Dirlectriques, Universitb du Qurbec h Trois-Rivirres, Case Postale 500, Trois-Rivirres (Qurbec), Canada G9A 5H7
(Receivedfor publication 16 April 1993) Abstract--The amount of gas that can be stored in an adsorption system is dictated by the nature of the adsorbent material and the operating conditions of the storage system, namely the storage pressure and temperature. This paper describes the impact of these factors on H 2 storage. The discussion is based on measurements of hydrogen adsorption on commercially available and densified adsorbents. The measurements were carried out in the pressure range 1-80 atm and at different storage temperatures using a high-pressure volumetric method. The results show that the adsorption technique could provide a viable method for hydrogen storage.
INTRODUCTION Due to growing concern about the negative impact of fossil fuel production and usage on the environment, sources of clean energy are urgently needed. Of the alternative sources that are being considered, hydrogen has been found to have several advantages. It is renewable, environmentally safe and has the potential for becoming a most versatile energy vector. Nevertheless, the technology for many of its uses, especially as a fuel, has yet to be fully developed. In the search for a new and better means of storing hydrogen, adsorption storage on activated carbons is emerging as a very promising technology [1, 2]. It has the same known advantages of the compression storage technology, but fewer of its drawbacks. The biggest advantage of adsorption over compression resides in its potential to store the same quantity of gas at a much lower pressure. This could result in lower capital and operating costs for compression, and could reduce some of the technical problems associated with high-pressure storage [3] (high pressure tanks, hydrogen embrittlement, etc.). Physical adsorption of a gas by a solid is the condition in which the concentraion of the gas molecules at the gas/ solid interface is greater than the bulk concentration. This enrichment is caused by the Van der Waals interactions between the solid atoms and the gas molecules. The amount of gas usually stored in a high-pressure cylinder can thus be enhanced by placing a high-surface-area material in the cylinder. When the cylinder is filled with compressed gas, a major portion of the gas is adsorbed on the surface of the solid substrate, thereby lowering the storage pressure (see Fig. 1). The operation of an adsorption storage system is basically similar to that of an ordinary compression storage system with one noticeable
exception. Adsorption storage requires thermal management of the exothermic heat of adsorption and the endothermic heat of desorption. For hydrogen, the heat of adsorption is about 4 kJ m o l - ~ at 77K. A D S O R P T I O N STORAGE O F H Y D R O G E N The amount of gas that can be stored in an adsorption system is dictated by the nature of the adsorbent material and the operating conditions of the storage system, namely the storage pressure and temperature. In the following we assess the impact of these factors on H 2 storage. Our assessment is based on measurements of hydrogen adsorption on commercially available and modified adsorbents. The measurements were carried out in the pressure range 1-80 atm and at different storage temperatures using a high-pressure volumetric method.
(.9
Compression Storage
Pressure
Fig. 1. Comparison of adsorption storage and compression storage. 161
162
R. CHAHINE and T. K. BOSE
Adsorbent materials
T=77K
The adsorption properties of a porous solid depend primarily on the size of its pores, which are classified as micropores ( < 2 nm), mesopores (between 2 and 50 nm) and macropores ( > 50 nm). Above the critical point, gas adsorption takes place in the micropores only, and the density of the adsorbed phase is much greater than the density of the unadsorbed gaseous phase (bulk phase) in the macropores or voids. Therefore, adsorption goes up with the amount of micropore surface area available. The nature of the surface area also plays a role in the adsorption process. Certain sites on the surface are more favorable to hydrogen adsorption than others. Two major types of high-surface-area adsorbents are available commercially: zeolites and microporous carbons. Molecular-sieve zeolites are aluminosilicates based on an atomic framework which is arranged to form molecular-size channels. Microporous carbons are obtained by the activation of organic and carbonaceous substrates. Figure 2 shows the excess adsorption isotherms of hydrogen for different zeolites and carbon adsorbents. Excess adsorption, expressed on a per mass basis, signifies the amount of gas present in the pores and on the surface of the adsorbent over and above that corresponding to the bulk phase at the measured temperature and pressure. We can easily see from Fig. 2 that activated carbons are better adsorbents of hydrogen than zeolites. Figure 2 also shows the special performance of AX-21 activated carbon which adsorbs twice as much as regular grade activated carbons. AX-21-type carbons are produced by the reaction of cokes with potassium hydroxides (KOH) and have a cage-like type of porosity in which pore filling occurs by the cooperative effect, thus giving them an unusually high surface area of the order of 3000 m z g ~. The surface area of regular grade microporous carbons lies between 700 and 1800 m 2 g - ~. Our studies [-4, 5] and others [6] in the field of adsorption storage of natural gas for vehicles (NGV) clearly show that if a high surface area is desirable in order to maximize adsorption per unit mass, it is also critical to have an adsorbent with high bulk density in order to minimize the volume of the storage system.
3O AX-21 (0,3 g/c
20
!
60
Fig. 3. Amount of H 2 stored in 11 cylinders loaded with different adsorbents. Unfortunately, the best commercially available highsurface-area adsorbents have, in general, a low bulk density. This is clearly demonstrated in Fig. 3 which shows the amount ofH 2 that can be stored in 11 cylinders loaded with some of the above adsorbents. For example, the AX-21 carbon has a surface area of 3000 m 2 g- ~ and a bulk density of 0.3 gem -3, while Calgon's BPL carbon has a surface area of 1100 m 2 g-~ and a bulk density of 0.47 gem -3. On a mass basis, AX-21 adsorbs 125~ more hydrogen than BPL (at 77K and 35atm), but on a volume basis AX-21 will only store 25~o more hydrogen than BPL under the same conditions.
Operatin9 conditions The development of the adsorption technology for natural gas storage is geared toward its use at around 35 atm and at ambient temperatures. These temperatures may not be suitable for hydrogen storage, as can be seen from Fig. 4, which compares the adsorption of hydrogen and methane (the main constituent of natural gas) as a function of pressure at 298K. Adsorbent storage systems designed to operate at room temperature will be much more efficient in the case of methane. In order to increase the hydrogen storage capacity of the material, one should operate at a much lower temperature. Figure 5 shows the
I • ~21
T=77K
40
Pressure (arm)
T=298K
_.- . . . . . . . . . . . .
~ CNN-201
~--~ 14
o "1-
•
4 3
... _1'~.~-.
"-
2
~
t
Norff
Methane
• Unde 5A • Linde 13X . . . . . . ;---~-
-:
.......... --,.--,_.~ - -
- ~ _
.. Y "
10
g
-~
.~ff.-=-" ~......... y ................................... 7.?. . . . . . . . . . . . "" """ Regular Grade Carbons ................
20 o "O ,<
t
J Hydrogen f
. . . . . . Zeolites 0
20
40
60
= 4O
80
Pressure (atm)
Fig. 2. Excess adsorption isotherms of H z on zeolites and carbon adsorbents.
r 50
6~0
Pressure (atm)
Fig. 4. Excess adsorption isotherms of CH4 and activated carbon.
H 2 on
AX-21
ADSORPTION STORAGE OF HYDROGEN 60
~
be more suitable. This is especially true if the storage system is charged with compressed hydrogen. Figure 7 shows the storage capacity of AX-21 at 175K. At 35 arm the storage density of H2 is around 8 kg m-3 which is equivalent to 100atm of pure compression at room temperature and to about 60 atm at 175K leading to an enhancement factor of about 1.7.
[]
~ 40 ~.
163
175 K .~ . . . . . .~--10
298 K 20
30
40
50
Pressure (atm)
Fig. 5. Adsorption isotherms of H 2 as AX-21 at different temperatures. variation of H 2 adsorption with temperature. Lowering the storage temperature to 175K will increase the adsorption by a factor of 5, and by a factor of 15 if the temperature is lowered further to 77K. The direct added cost of this option to the storage system will be in the use of the cryogenic reservoirs. The other added cost comes from cooling of the filling gas if it is compressed H 2. However, this cost will not be considered if the filling gas is vaporized liquid hydrogen. The total H 2 storage density of AX-21 as a function of the pressure at 77K is shown in Fig. 6. The total capacity is the sum of the excess adsorption, which is mainly confined to micropores, and the bulk phase, which is found in macropores and voids between the carbon particles. At 35 atm, the hydrogen storage density of AX21 is 25 g I- 1 which is about 30~ of the liquid hydrogen density and equivalent to 300atm of compression at room temperature and to about 78 arm of compression at 77K, thus giving an enhancement factor of about 2.2. One should also notice that the relative efficiency of the adsorption storage is greater at lower pressures. From Fig. 6 we can see that halving the storage pressure to 17.5 atm will only reduce the storage capacity by 20)~ and will further simplify the design of the cryogenic storage vessel. It could be argued that a storage temperature of 77K is too low, and temperatures in the range 150-175K would
T=77K
Modified adsorbents Short of developing specific adsorbents for hydrogen storage, the performance of commercially available adsorbents could be substantially improved by the chemical modification of their surface properties and by increasing their bulk density. In the case of hydrogen, it was shown El] that the metal assisted modification of the surface of activated carbon could lead to a 20~o enhancement in H 2 uptake. In order to improve the packing of low-density adsorbents, we developed a solidification method I-7, 8-1 that squeezes the voids out of the carbon particles without considerably reducing the specific adsorption. With this method we were able to increase the bulk density of the AX-21 carbon by a factor of more than 2 and to improve the volumetric storage capacity of methane by more than 50~. The capacity improvement in the case of hydrogen storage at 77K is shown in Fig. 8. The volumetric storage capacity increase is 54~o at around 17 atm, and 34~o at around 35 atm. Also shown in Fig. 8 is the amount of H 2 that can be stored at 77K by pure compression. The storage capacity of the solidified adsorbent at 17 atm is 5.2 times greater than that obtainable by compression storage alone under the same conditions. It is around 40~o of the LH 2 density and it is equivalent to the density of compressed H 2 at 90 atm and 77K. At 35 atm, the adsorption storage capacity is about 3 times greater, and is equivalent to 110 atm of pure compression. ECONOMICS OF HYDROGEN STORAGE ON ACTIVATED CARBON The economics of H 2 adsorption storage has been studied recently by Amankwah et al. [9], for a storage
12
30
T = 175 K
1 Total
'
.~:,........ ' 8 Total
20
~ , ~'
Adsorbed
. . . . . . . . .
.~
_~,~. . . .
~.~.___..~-----.... ' - A d s o r b e d
o
,
~
10
10
20
30
40
50
Pressure (arm)
Fig. 6.
H2
storage density of AX-21 activated carbon at 77K and as a function of the pressure.
to
2o
3o
2o
Pressure (atm)
Fig. 7. H 2 storage density of AX-21 activated carbon at 175K and as a function of the pressure.
164
R. CHAHINE and T. K. BOSE designed for hydrogen storage. The availability of such an adsorbent will greatly increase the competitivity of this storage option.
T= 77 K --~- 40
Solidified AX-21 . . . . . . . . . . . .
30~
...................
CONCLUSIONS
~' 2o
N ~
10
........ "°°'i °° "" i .....
oo
Pure Compression
20
~o
6'o
~o
Pressure (atm)
Fig. 8. Comparison of the total H 2 storage density at 77K and as a function of the pressure.
system operating at filling conditions of 150K and 54 atm and using AX-31M activated carbon which is similar to the one used in our experiments. The study also compared the costs of adsorption storage with those of the more traditional alternatives: compressed gas storage at 200 atm (GH2), liquefaction (LH2) and metal hydride (MH2) systems. The storage costs were divided into three major factors: the filling utility cost, the storage system cost, and the energy consumption cost. An overview of these costs is reproduced in Table 1, where we can see that the adsorption storage option (referred to as SA-M) compares very favorably with the other options. The results shown here are for the as-received carbon. If we consider the 4 0 ~ improvement in the storage density that could be obtained by solidifying the adsorbent and the additional 20~o increase that could be obtained by surface modification as mentioned earlier, we can greatly enhance the economic outlook of adsorption storage, which is already positive. The other point that should be made here is that none of the activated carbons available commercially have been optimized for hydrogen adsorption, and the best activated carbons available commercially are K O H based and they are expensive. Therefore, there is a need to develop a lowcost/high-capacity solid adsorbent which is specifically
We have shown that hydrogen adsorption on activated carbon holds much promise. The adsorption technique could provide a viable method for hydrogen storage. Low-pressure adsorption storage is not only economical, but also has a psychological advantage over high-pressure compression storage as far as consumers are concerned. Finally, hydrogen storage on activated carbon is rather new and much research needs to be done for further improvement of this technology.
REFERENCES 1. J. S. Noh et al., Int. J. Hydrogen Energy 12, 693 (1987). 2. D. L. Block, S. Dutta and A. T-Raissi, Hydrooenfor Power Applications--Task 2. Solar Energy Research Institute, Golden, CO (1988). 3. R. J. Remick et aL, Advanced onboard storage concepts for NG fueled automotive vehicles. Report No. DOE/NASA/ 0327-1 (1984). 4. T. K. Bose et al., Rapport sur la compr6hension du ph6nom6ne d'adsorption: application au gaz naturel comprim& SOQUIP, Quebec, Canada (1984). 5. T. K. Bose, R. Chahine, L. Marchildon and J. M. St-Arnaud, D6veloppement de la technique de l'adsorption en vue de reduire la pression dans un cylindre de gaz naturel comprim& Final Report Ministry of Science and Technology of Qu6bec/ SOQUIP (1987). 6. S. S. Barton et al. The projection and evaluation of composite carbons for the adsorption of methane. Report No. AF-84-04 Ontario Ministry of Energy, Toronto, Ontario (1984). 7. T.K. Bose, R. Chahine, J. M. St-Arnaud, SOQUIP and GMI, High-density adsorbent and method of producing same. U.S. Patent No. 4,999,330 (1991). 8. R. Chahine and T. K. Bose, Proc. 20th Biennial Conf. on Carbon, Santa Barbara, CA, pp. 638-639 (1991). 9. K. A. G. Amankwah, J. S. Noh and J. A. Schwarz, Int. J. Hydrogen Energy 14, 437 (1989).
Table 1. Cost of H 2 storage 1-9]
System GH 2 (200 atm) LH 2 MH 2 (FeTi) SA-M (150K, 54 atm)
Utility unit
Storage unit
Energy consumption
Total cost ($ MB.T.U.- 1)
0.77 1.21 0.57
7.66 2.23 7.40 2.46
2.50 14.50 5.10 2.52
10.93 17.94
1.98
13.07
6.96