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Hydrogen storage
CHAPTER 4
Hydrogen storage Muhammet Kayfecia, Ali Kec¸ ebaşb a
Department of Energy Systems Engineering, Karab€ uk University, Karab€ uk, Turkey Department of Energy Systems Engineering, Mug˘la Sıtkı Koc¸man University, Mug˘la, Turkey
b
Contents 4.1 4.2 4.3 4.4 4.5
Introduction Hydrogen storage methods Pressurized hydrogen storage Liquefied hydrogen storage Metal hydrides 4.5.1 Types of metal hydrides 4.6 Hydrogen storage in nanostructured/porous material 4.6.1 Carbon nanotubes 4.6.2 Zeolites 4.6.3 Metal organic framework 4.6.4 Covalent organic framework 4.7 Glass microspheres 4.8 Boron-based storage 4.9 The storage in underground 4.10 Methanol 4.11 Petrol and other hydrocarbons References Further reading
85 87 88 91 93 97 101 101 102 102 103 104 105 106 107 107 109 110
4.1 Introduction Energy, one of the fundamental elements of economic development, is one of the indispensable requirements of humanity. The energy requirement increasingly raises in parallel with the world population and industrial developments; notwithstanding, the reserves of fossil energy resources rapidly depletes. According to the latest statistical evaluations, crude oil that meets 38.5% of the world’s energy needs has a reserve utilization period of 41 years, whereas natural gas that meets 23.7% has 62 years, and coal that meets 24.7% has 230 years. In 1990, the world had a population of 1.6 billion and primary energy consumption was about 1000 Mtep. In 1997, the population reached 6.5 billion and the primary commercial energy consumption increased to 11,700 Mtep [1]. Thus, the primary energy consumption of the world has increased over eight times within a century.
Solar Hydrogen Production https://doi.org/10.1016/B978-0-12-814853-2.00004-7
© 2019 Elsevier Inc. All rights reserved.
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Using fossil fuels as a main source for meeting world’s energy demand, evaluating of fossil fuels by combustion reaction and occurring of harmful emissions such as carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen oxide (NOx) in this reaction leads to enormous environmental problems. The main reason for global warming, which is the most important environmental problem in the world today, is the strengthening of the greenhouse effect of the atmosphere with increasing CO2 emissions. Because of the limited reserves of primary energy resources, increases in fuel price, the population growth, industrialization, obligation to assess national resources, socioeconomic structuring of the 21st century, the negative effects of existing fuels on the environment (such as greenhouse effect, global warming, climate changes, rainfall abnormalities, and acid rain, health problems); hydrogen energy, which is a portable energy source that has high calorific value, has come forward in the context of new energy technologies. Hydrogen energy is considered one of the most important energy sources in the next century. This energy can be obtained from water and can be transformed into a useful energy with high efficiency and no negative impact on the environment. Hydrogen energy, which will be used to solve the energy problem of the world, will be able to be produced continuously for billions of years. One of the most important advantages of hydrogen gas as an energy is that it does not contain toxic gas and cause corrosion. Therefore, the use of hydrogen energy is safe and very simple when necessary precautions are taken. As a result of burning the hydrogen as nonpolluting fuel, water vapor emerges as waste [2]. In addition to not harming the natural environment, the formation of water vapor as waste can help to eliminate the damages caused by other energy wastes, and it is able to benefited effectively from hydrogen wastes as natural recycling in many areas of technology [3]. As an energy source, the most important feature that hydrogen has is that it can be stored. However, some problems arise during storage due to the fact that it is the lightest gas ever known (the density rate is 0.0838 kg/m3). Today, in order to widespread use of hydrogen that is the energy of the future, the effective and efficient methods for hydrogen storage should be developed. There are many methods used in the hydrogen storage. Hydrogen can be stored in gaseous state by compressing or in liquid state by being cooled to very low temperatures liquefied. However, the investigations have shown that the most effective storage method is the solid storage. Hydrogen can be physically stored in carbon nanotubes and also chemically stored in hydrides as a solid state. Among storage methods, the most effective one is the storage in the form of hydrides because of their high storage capacities, suitable working environments and being safer for working at low pressures [4]. In storage as metal hydride, hydrogen is stored in spaces between metals. Hydrides are compounds which absorb hydrogen by reacting with hydrogen at a certain temperature and also release the hydrogen when they are heated.
Hydrogen storage
Hydrogen reacts with many transition metals and alloys at elevated temperatures in the form of hydrides. Electropositives from these alloys, such as Sc, Yt, Lanthanides, Actinides, and Ti, also the elements of group 5A are very reactive metal hydrides [5].
4.2 Hydrogen storage methods The major obstacle to the widespread use of hydrogen is the storage. Hydrogen in the form of gas takes up 3000 times more space than a gas with the same amount of energy at room temperature and pressure. For this reason, it is necessary to be used of hydrogen in compression, liquefaction or other methods in order to utilize in stationary systems, vehicles, and mobile applications. There are four main techniques used for the storage. These are compressed gas, cryogenic liquid, metal hydride, and carbon adsorption. In the short term, the most feasible ones are the storage in cryogenic liquids and metal hydrides. Despite the fact that the metal hydride method is an advanced method, further research is required for this to be competitive. Carbon adsorption is not yet a mature technique, but it is seen as a feasible method if targets are achieved at the end of research and development studies. The end-use storage techniques of hydrogen are different for each application. The hydrogen quantities that can be stored by some storage techniques, and energy densities are given in Table 4.1 (Fig. 4.1). Table 4.1 Comparison of hydrogen storage methods Method
ρm (wt%)a
ρv (kg/m3)b
T (K)c
P (bar)d
Compressed gas Liquid
13
<40
273
800
Varies
70.8
21.5
1
Physisorption
2
20
77
100
Interstitial metal hydrides Complex hydrides
2
150
273
1
<18
150
>100
1
Chemical hydrides
<40
> 150
273
1
a
Gravimetric storage density. Volumetry storage density. c Operational temperatures for storage method. d Operational pressures for the storage method. b
Description
Compressed hydrogen gas; lightweight, high-pressure cylinder Liquid hydrogen, continuous loss of a few % per day at RT Physical adsorption by porous materials, fully reversible Atomic hydrogen occupies interstitial sites, fully reversible, metals are heavy Complex compounds [BH4]or [AlH]4, desorption at elevated temperature, adsorption at high pressure Thermal decomposition of chemical hydrides, not directly reversible
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Solar hydrogen production
Fig. 4.1 Hydrogen storage methods [6].
Fig. 4.2 Compressed hydrogen gas storage tank [7].
4.3 Pressurized hydrogen storage This storage is performed in a high-pressure resistant tank at room temperature. The compressed gas storage tank is shown in Fig. 4.2. In the compressed gas storage, 1%–7% of hydrogen by weight is stored depending on the weight of the tank and
Hydrogen storage
therefore on the type of it. Tanks that are lighter, more durable and able to store more hydrogen are more expensive. Up to 20% of the energy content of the fuel is spent to be compressed the hydrogen gas at the filling station. Storage in pressurized tanks as compressed gas is a well-known storage method. Hydrogen is compressed into high-pressure tanks. Energy is needed to carry out this process, and this volume filled by the compressed gas is usually quite large. This results with hydrogen having a lower energy density according to the conventional petrol tanks. A hydrogen gas tank will be 3000 times bigger than the petrol tank when it contains the equal amount of energy stored by a petrol tank [8]. Hydrogen is nowadays stored under the pressure of 200–250 bar in 50-L cylindrical tanks. However, the storage pressure as the gas can able to increase up to 600–700 bar. The volumetric energy density of the hydrogen is low due to the nature of being very light even if it is stored in the 50 L tank. On the other hand, storage tanks are heavy due to high pressure [9]. Volumetric density of compressed hydrogen gas a function of gas pressure shown in Fig. 4.3. Hydrogen shows ideal gas properties at very high pressures and low temperatures. Therefore, the number of molecules and mass at a given pressure and temperature can be calculated from the ideal gas equation.
50
Pressure (MPa) 100
150
200
H2liq
0.25
60 0.20 H2 gas
Ideal gas
0.15
40 sv = 460 Mpa (steel)
0.10
Casing Hemisphere
20
0.05
di do dw
0
500
1000 Pressure (bar)
1500
0.00 2000
Fig. 4.3 Volumetric density of compressed hydrogen gas a function of gas pressure.
dw /do
Volumetric H 2 density (kg.m –3)
0
89
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Solar hydrogen production
n¼
PV ! m ¼ nM Ru T
In this equation, n is the number of mole of hydrogen, M for the molecular weight, T the absolute temperature and Ru for the universal gas constant, respectively. The amount of energy to be obtained from hydrogen can be calculated as follows: E ¼m4H In this correlation, ΔH stands for the combustion temperature of the hydrogen gas, which is 140 MJ/kg for hydrogen. The volumetric energy density of the hydrogen can be also calculated from the following equation: Wvolumetric ¼
E 4HP P 4 H ¼ ¼ V nRu T RT
As it is seen in this correlation, the energy density increases with the pressure. However, the increase in pressure is limited by the strength of the storage material. In practice, austenitic steel and aluminum alloys are used as pressurized storage material, but the disadvantage of these storages is to being very heavy. The ratio of the stored hydrogen to the weight of the whole tank is about 2%–3%. These disadvantages of storages can be accomplished by using composite material. These storages contain a thin layer of metal in contact with hydrogen, followed polymer envelope strengthen by a carbon fiber. In such a container, the hydrogen weight ratio increases to 5%. Multilayered polymer is used instead of metal envelope in more advanced storages. The wall thickness of a cylinder capped with two hemispheres is given by the following equation: dw 4P ¼ do 2σ v + 4 P where dw is the cylinder wall thickness, do the outer diameter of the cylinder, ΔP is the over pressure, and σ v is the tensile strength of the cylinder material. Fig. 4.3 shows the volumetric density of compressed hydrogen gas a function of gas pressure. The volumetric density increases with hydrogen pressure and reaches a maximum above 100 MPa, depending on the tensile strength of materials. The tensile strength of materials varies from 50 MPa for aluminum to more than 1100 MPa for steel [10]. One kilogram of hydrogen currently costs $1.20. The same 1 kg of H2 is equivalent to 1 gallon of gasoline. The cost to store and transport hydrogen more than triples its cost. The cost comparison of compressed hydrogen and cryogenic hydrogen is shown in Table 4.2.
Hydrogen storage
Table 4.2 The cost comparison of compressed and liquid hydrogen storage methods [11] Raw cost of H2 ($/kg)
Storage method
Compressed hydrogen
1.2
Liquid hydrogen
1.2
Cost to process H2 ($/kg)
0.70 (compression to 45 MPa) 1.11 (cryogenic liquifaction)
Cost to transport H2 ($/kg)
Cost of on-site storage of H2 ($/kg)
0.70 (pressurized tank) 0.21 (Cryogenic tank)
0.45 (high pressure storage tank) 0.24 (cryogenic storage including boil off losses)
Refuelling station cost H2 ($/kg)
Delivered cost of H2 ($/kg)
0.75
3.80
0.66
3.42
4.4 Liquefied hydrogen storage In this technique, the hydrogen is stored at –253°C in atmospheric pressure in fairly wellinsulated tanks. Because the hydrogen is in the liquid form, it contains 3 times more energy than the equivalent weight petrol and 2.7 times more volume is required in case of containing the equivalent energy. The well-known liquefaction process is the precooled Linde cycle flowsheet as shown in Fig. 4.4. Liquefaction is done by cooling a gas to form a liquid. Liquefaction processes use a combination of compressors, heat exchangers, expansion engines, and throttle
LN 2 Makeup gas
GN2
LN 2
GN 2
. Q 2 . m
1
I
3
II
4
III
5
IV
6
9
10 . W
11 V
m mf
g . mf Fig. 4.4 Linde-Hampson liquefaction cycle.
f
Liquid
91
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Solar hydrogen production
valves to achieve the desired cooling [12]. Initially, the gas is compressed at ambient pressure and subsequently cooled to 80 K in a counterflow heat exchanger using liquid nitrogen. Heat exchangers are used to lower the temperature even further, below its inversion temperature by transferring heat from the hydrogen stream to the returning cooled hydrogen. Ultimately, the cooled and compressed gas is forced to pass through a throttle valve or a mechanical expander where it undergoes an isenthalpic expansion to ambient pressure, producing some liquid. The liquid is removed and the cooled gas is returned to the compressor via the heat exchangers. The work needed in this theoretical process is called the ideal work of liquefaction and has been calculated as 11.88 MJ/kg by [13]. Linde liquefaction process temperature entropy diagram is shown in Fig. 4.5. This technique stores 16% of hydrogen by weight, including the tank and insulation. In addition, the liquefaction requires up to 28% of the energy content of the fuel. Another disadvantage is the heat transfer to the tank, despite the insulation. As a result of this heat transfer, hydrogen evaporates. However, this problem can be solved by using a pressurized tank. In this case, this solution also increases the weight and size. Even though the energy used for the hydrogen liquefaction is high, the costs of the liquefaction in spacecraft and rockets are overlooked. If the liquid hydrogen is stored in large tanks, 0.06% of hydrogen per day is lost, in case of storing it in small tanks, 3% of
Fig. 4.5 Precooled liquefaction temperature-entropy diagram.
Hydrogen storage
Table 4.3 Liquid hydrogen physical and chemical properties Chemical formula
H2
Molecular weight Boiling point, 1 atm Freezing point, 1 atm Critical temperature Critical pressure Density, liquid, B.P., 1 atm Density, gas, 20°C, 1 atm Specific gravity, gas (air ¼ 1), 20°C, 1 atm Specific gravity, liquid, B.P. [water ¼ 1, 20°C] Specific volume, 20°C, 1 atm Latent heat of vaporization Flammable limits, 1 atm in air (by volume) Flammable limits, 1 atm in oxygen (by volume) Detonable limits, 1 atm in air (by volume) Detonable limits, 1 atm in oxygen (by volume) Autoignition temperature 1 atm Expansion ratio, liquid to gas, B.P. to 20°C
2.016 252.9°C 259.3°C 240.2°C 12.7 atm 70.8 kg/m3 0.0838 kg/m3 0.0696 0.0710 11.923 m3/kg 389 Btu/lb. mole 4.00%–74.2% 3.90%–95.8% 18.2%–58.9% 15%–90% 571°C 1–845
hydrogen per day evaporates. The reduction of this ratio depends on the insulation. Liquid hydrogen thermophysical properties and chemical properties are shown in Table 4.3.
4.5 Metal hydrides Metal hydrides are made up of metal atoms with lattice imperfections, and hydrogen atoms embedded in the interstices of lattice cavities. The intermediate region can be in the form of a space or a lineal error. In the case of a lineal error, the sequence of the hydrogen atoms performs by collecting throughout errors. Thus, they increase the lattice strain especially when two adjacent atoms recombine in the form of molecular hydrogen. It increases the lattice size during hydrogen adsorption [14, 15]. There are two different ways in hydration, the first of these is the chemical adhesion and the second one is also the electrochemical separation of the water. These reactions are respectively as follows [16]: x M + H2 $ MH x + Q 2 x x x M + H2 O + e $ MH x + OH 2 2 2
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Solar hydrogen production
H2
H2
Charge a-Phase
a-Phase H2
Charge
Discharge
94
H2 H2
Discharge b-Phase
b-Phase
H2
Fig. 4.6 The phase transformation scheme in metal hydrides [17].
In these equations, M and Q stand for the metal, and the heat generated by the exothermic reaction, respectively. In the electrochemical separation, a catalyst such as palladium is needed. The chemical adhesion of the hydrogen is schematically given in Fig. 4.6. As it is shown in the figure, the molecular hydrogen reaches a minimum shallow potential close to the surface, and the atomic hydrogen is in a region that is deeper than the surface but is almost near the surface. In the metal lattice, the hydrogen has a minimum periodic potential within the crack regions in the lattice. While the hydrogen molecules approach to the metal surfaces, they close together due to the effect of the weak van der Waals forces. At the distance of zp, the molecule reaches the point of Ep which is the potential well, and the very large forces approach the surface in the molecular form. Thus, the dissociation energy of the hydrogen molecules is exceeded by the chemisorption energy. The hydrogen molecule is then separated, and the individual hydrogen atoms are attracted to the surface by the effect of the chemisorptive forces and reach the potential of ECH. After this point, the thermal energy of the ambient air is sufficient to increase the vibrational width of hydrogen atoms, thus the hydrogen can reach the metal surfaces [14]. The metal and hydrogen are usually in the form of two different hydrides, as α-phase and β-phase. Only a little hydrogen is absorbed in the α-phase and the hydride is completely formed in the β-phase. Mg2NiH0.3 and Mg2NiH4 can be given as example for the hydrides of the Mg2Ni form. The most general characteristic method of metal hydrides is PCT (pressureconcentration-temperature), the curve in the graph of P-C isotherms. The theoretical
Hydrogen storage
P-C isotherms with α- and β-phases are given in Fig. 4.7. The concentration is usually determined by the amount of hydrogen atoms H/M in the unit metal molecule. The optimal parameter to define metal hydrides is the maximum hydrogen capacity (H/ M)max. The reversible capacity is determined by using the plateau width Δ(H/M) as shown in Fig. 4.8 and utilized to determine the theoretical capacity of metal hydrides. Tc 100
β-Phase
–
80
ΔS R
60
α-Phase
40 10 ΔH R
α + β-Phase 25°C
20
1
E 0(mV)
Peq (bar)
100°C
0 0°C –20 0.0
0.2
0.4 0.6 cH (H/M)
0.8
1.0 2.4
2.8 3.2 3.6 T –1(10–3 K–1)
Fig. 4.7 The theoretical P-C curve of the metal hydride [18].
lnP
Hysteresis=ln
PA PB
Plateau slope = d lnP d(H/M)
Reversible capacity Δ(H/M)
Hydrogen/metal ratio Fig. 4.8 The real P-C isotherm diagram [19].
Capacity (H/M) max
H/M
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Solar hydrogen production
The concentration can be expressed as a percentage mass in total mass, especially in energy intensity comparisons. The real metal hydrides exhibit some hysteresis between adsorption/desorption in the P-C curve. Besides, the plateau has a bit of slope. In the thermodynamic reaction equilibrium equation, the reaction constant K can be expressed as [20] RT ln K ¼ 4H 4S Here, ΔH is the reaction enthalpy and ΔS is the reaction entropy. For the solid-gas reaction, the equilibrium is reduced from the constant to the gas pressure. Thus the van’t Hoff equation is obtained as [20] Peq 4H 4S ln ¼ P0 RT R where R is the gas constant Peq and P0 are the equilibrium pressure and the normal pressure (100 kPa), respectively. The equilibrium pressure is a function of the temperature T and the constants: reaction enthalpy ΔHR and entropy ΔS, being the slope and y-intercept of the plot in Fig. 4.9. ln P versus 1/T, the equilibrium diagram (P, T) values scale are called the van’t Hoff diagram. The reaction enthalpy can be determined from the angular coefficient in the diagram along with the its equation and is called with the P-T harmony of hydride behaviors for practical applications in the diagram. The theoretical van’t Hoff diagram generally describes very well the actual properties of metal hydrides. The representation of van’t Hoff diagram is given in Fig. 4.10 [19]. The reaction enthalpy for the hydride formation is an important quantity. This quantity is usually negative if the reaction is exothermic, and thus the hydride formation releases the energy. Therefore, the energy should be given to the hydrogen with an aim to be released again by the hydride. In most of the applications, the ambient air has been used or at least a temperature between 0°C and 100°C is required. In order to be released the hydrogen by taking the heat from the ambient air of the hydride, the reaction enthalpy should be quite small. ln P
DH a
1/T
Fig. 4.9 The schematic representation van’t Hoff diagram.
Hydrogen storage
Temperature (°C) 1000
600
400 300
200
50
100
25
1000 NiH
Dissociation pressure (atm)
1000 100
VH2
10 1
PdH0.6 ThH2 ZrH2 CaH2
MgH2
0.1 0.01 0.001
Th4H15 UH3
LaH2 YH2
Th4H15
ThH2 0.5
1.0
1.5
2.0
2.5
3.0
3.5
1000/T (K –1) Fig. 4.10 The van’t Hoff diagram for some natural elements [19].
4.5.1 Types of metal hydrides Hydrogen is a highly reactive element. It is known that hydrogen as the hydride is in the form of a solid solution together with thousands of metals and alloys. Many natural elements absorb hydrogen under the favorable conditions. However, as it is shown in Table 4.3, PCT features are not suitable for the most of them to be able to be used in practical applications. In the following graph, the temperature range shown in the box is given in the range between 0°C and 100°C, and the pressure range is given between 100 and 1000 kPa. In order to obtain the desired PCT properties, it is necessary to combine some strong hydride elements (called A) with elements having weaker hydriding properties (called B). In various types of intermetallic compounds, the compounds contain different amounts of elements A and B, the compounds are mostly in the form of solid solutions, and the hydride compounds are in the form of transition metals. The element A is generally rare elements or alkaline earth metals which can constitute stable hydrides. The element B is a transition metal, which can often constitute unstable hydrides. In intermetallic compounds, some defined ratios between B and A are x ¼ 0.5, 1, 2, 5. The hydride families in different types are listed in Table 4.4.
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Table 4.4 The prototype and structure of some important intermetallic compounds [5] Intermetallic compounds
Prototype
Hydride
Structure
AB5 AB2 AB3 A2B7 A6B23 AB A2B
LaNi5 ZrV2, ZrMn2, TiMn2 CeNi3, YFe3 Y2Ni7, Th2Fe7 Y6Fe23 TiFe, ZrNi Mg2Ni, Ti2Ni
LaNiH6 ZrV2H5.5 CeNi3H4 Y2Ni7H3 Ho6Fe23H12 TiFeH2 Mg2NiH4
Hook phase, hexagon Lave phase, hexagon or cubic Hexagon, PuNi3-type Hexagon, Ce2Ni7-type Cubic, Th6Mn23-type Cubic, CsCl- or CrB-type Cubic, MoSi2- or Ti2Ni-type
4.5.1.1 AB5 intermetallic compounds The compounds AB5 are generally in the form of the hydride at the temperature of 100°C and a few atmospheric equilibrium pressures. Other important features are the low hysteresis, tolerances in gas pollution and the ease of activation in the initial cycle. Thus, the most of these alloys contain Ni as the base metal. AB5 family has hexagonal structure with CaCu5-type lattice. One of the most important examples of AB5 class alloys is the LaNi5 alloy. These class alloys have extraordinary versatility because many different element types can take the place of A and B lattice gaps. The typical examples for metal A can be given as Al, Mn, Si, Zn, Cr, Fe, Cu, and Co for Mm, Ca, Y, Zr, and B. Compounds substituting for A and B cause significant changes in the macrostructure of the alloy. For instance, if Ni is replaced with Co and Fe in the LaNi5 alloy, the volume expansion occurs on the hydriding, the corrosion velocity decreases, and it provides to improve the life cycle of the hydride [13]. The PCT properties of AB5 alloys are given in Fig. 4.11. The hysteresis is quite low in AB5 alloys except for MmNi5 [19]. The most important advantage of AB5 alloys is that they are not in the form of a protective oxide and are resistant against to the small O2 and H2O pollution in H2. The hydrogen capacities of AB5 compounds are low. The maximum capacity with LaCo5 and Ca0.7Mm0.3Ni5 with nominal quantity of nearly 1% has performed as 1.90% by weight. The reversible capacity has been determined as 1.28% by weight with LaNi5, and the nominal quantity has also been about 0.7%–1% by weight [19]. On the other hand, the cost of AB5 alloys is also quite high. The costs of the alloy are in the range of 8%–11 €/kg considering only raw material in a case where the reactor is not included. The cost become about 0.7–2.0 €/gH in a case when this cost is translated to the cost of the hydrogen storage by being added the reversible capacity of this hydrogen storage [19]. 4.5.1.2 AB2 intermetallic compounds AB2 alloys, such as AB5 alloys, also have wide and versatile positive PCT properties at ambient temperatures. The element A is generally Ti, Zr, Hf, TH, or a lanthanide
Hydrogen storage
Temperature (°C) 200 150
100
50
0
25
–25
–50
100 MmNi4.15Fe0.85
LaNi5 CaNi5
MmNi5
Pd (atm)
10
LaNi4.25Al0.75
1
MmNi4.5Al0.5 MmNi3.5Co0.7Al0.8
LaNi4.8Sn0.2
0.1 2
2.5
3
3.5
4
4.5
1000/T (K–1
(
Fig. 4.11 The van’t Hoff diagram for various AB5 alloys [19].
(atomic numbers 57–71). The element B may be one of transition such as V, Cr, Mn, and Fe or nontransition metals. The PCT properties of some AB2 alloys are given in Fig. 4.12. Hydrogen capacities of AB2 alloys are comparable to AB5 on a reversible basis, but generally have high capacity. In these alloys, especially large pressures and temperatures are possible. The maximum capacities of AB2 alloys are usually between 1.5% and 2% by weight and 3.4% by weight with TiCr1.2V0.8 pairs. The reversible capacities are also between 0.9% and1.3% by weight [19]. The costs of raw materials are between 3 and 12 €/kg. The hydrogen-based costs of AB2 alloys are about 0.5–1.3 €/gH. Thus, the costs of AB2 alloys are somewhat lower according to the costs of AB5 alloys. For instance, the maximum and reversible capacity for Ti0.92Zr0.02V0.43Fe0.09Cr0.05Mn1.5 is 1.9% and 1.3% by weight, respectively and the cost is 0.41 €/gH [13]. 4.5.1.3 AB intermetallic compounds The AB alloys are generally TiFe based. These tend to have two plateaus, the upper one is not very stable, and the hysteresis is quite wide. The PCT properties can be partially corrected using Mn or Ni instead of Fe, as shown in Fig. 4.13. In this figure, U shows the top plateau and L shows the bottom plateau.
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Solar hydrogen production
Temperature (°C) 100
200 150
50
0
25
–25
–50
100 TiMn1.5
TiCr1.8
Ti0.98Zr0.02V0.43 Fe0.09Cr0.05Mn1.5 ZrFe1.5Cr0.5
Pd (atm)
10
1 ZrMn2 TiMn1.4V0.62 0.1 2
3
2.5
3.5
4
4.5
–25
–50
1000/T (K–1
(
Fig. 4.12 The van’t Hoff diagram for various AB2 alloys [19].
Temperature (°C) 200 150
100
50
25
0
100
TiFe (L) TiFe (U)
Pd (atm)
10
1
TiFe0.8Ni0.2 TiFe0.85Mn0.2(L) 0.1 2
2.5
3
3.5
1000/T (K–1
(
100
Fig. 4.13 The van’t Hoff diagram for various AB alloys [19].
4
4.5
Hydrogen storage
Oxide films are usually in the form of TiFe-based alloys. This reduces the susceptibility to the pollution in hydrogen. Oxidation, on the other hand, does not create a tendency for combustion behaviors. The maximum hydrogen capacity for AB alloys varies by less than 1%–2% by weight. The interesting two alloys here, TiFe and TiFe0.85Mn0.15, have a maximum capacity of about 1.9% and a reversible capacity of 1.5%. The costs of these alloys are very low as being 0.34 and 0.35 €/gH, respectively. The main reason for not being able to use of AB alloys as commercial is that the top plateau, and the susceptibility to pollution is unstable. In some studies, it has been aimed to overcome these problems using so-called binary hydrides such as (ZrCo)1 x(TiNi)x; however, very clear efficiency increases have not been achieved. For instance, H/M ¼ 1.6 in the alloy of (ZrCo)0.7(TiNi)0.3 and the hydrogen capacity is 1.16% by weight [13, 18]. 4.5.1.4 Other intermetallic compounds In addition to the alloys shown above, many intermetallic families herein have the hydrogen absorption capacity, but these are not of commercial interest. These include A2B, AB3, A2B7, and A3B alloys. Some of these alloys have good hydrogen capacities, but the PCT properties are not suitable. Besides the PCT properties, it has poor hydrogen capacity in practical application areas or has a very narrow plateau tendency.
4.6 Hydrogen storage in nanostructured/porous material 4.6.1 Carbon nanotubes Carbon is one of the most suitable substances for the gas storage, because it can be especially made into very small particles with high porosity, and due to the attractive force between the carbon atoms and the gas molecules. Nanotubes have many special superior features. For instance, the modulus of elasticity is five times higher than the steel. Besides, depending on the structure of the tube, some of them behave as a semiconductor and some as a conductor. Because of these features, it is possible to reduce electronic devices to micro and nanodimensions using nanotubes. Hydrogen is stored in nanotubes in chemical or physical ways. Carbon nanotubes were discovered in 1991 by Lijima. Carbon nanotubes are briefly in the form of graphite plates converted into the form of tube. Diameters are in the range of a few nanometers or 10–20 nm, and the lengths are also in the micron level. Nanotubes can be produced in multiwalled tubes like it can be produced in single walled. The single and multiwalled nanotubes and the yarn structure consisting of nanotubes is given in Fig. 4.14. There are also nanotubes made up of various additives such as alkali-added (Li-K) [22]. Hydrogen absorption in carbon nanotubes occurs on highly porous super active graphite surfaces. Absorption process occurs with Van Der Waal’s force, which carbon atoms apply to hydrogen molecules. For this reason, the absorption process is not a chemical, but a physical event. Hydrogen absorbed at a given temperature is only a function of
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Solar hydrogen production
SWCNT
SWCNT (yarns)
MWCNT
Fig. 4.14 The single and multiwalled nanotubes [21].
the pressure. The desired amount of hydrogen is released when the pressure is reduced. The nano structure is used in pressurized tanks, and therefore the absorbed hydrogen contributes to hydrogen stored as gas form. These systems store approximately 4% hydrogen by weight.
4.6.2 Zeolites Zeolites are a group of crystalline microporous aluminosilicate compounds of both natural and synthetic origin that are defined as a gap and channel type pore structures in molecular dimensions. Zeolites were first described by the Swedish mineralogist Baron Axel Cronstedt in 1756. Their names are derived from the word of Greek “zeo” and the word of “lithos” meaning to boil and the rock. The general formula of zeolite compounds can be given as h i Mx=n ðAlO2 Þx ðSiO2 Þy mH2 O Here, the negatively charged structures of the alkaline or alkaline earth metals are generally balanced with n-valence M cations, but they can be able to change. Zeolites can store considerable amounts of hydrogen, thanks to their high surface area, except that they are widely used in the petrochemical shredding and water softening as ion exchangers. It is possible to produce zeolites constituting from more than 150 different types of tetrahedral Si(Al)O4 with different components.
4.6.3 Metal organic framework Metal organic frames (MOFs) are compounds containing of metal ions or the bundles of organic binders. MOFs have made great progress in recent years due to their potential applications such as the gas storage with physisorption, separation, catalysis, and medication.
Hydrogen storage
MOFs can be constructed as a building block with many different organic binders and metal bundles or metal ions (Fig. 4.15). Historical developments of MOFs are shown in Fig. 4.16.
4.6.4 Covalent organic framework Covalent organic frames are highly porous crystalline materials and have been formed by building blocks connected with very strong covalent bond together (Fig. 4.17). As being different from MOF, COFs do not contain metal ions or bundles, and similarly can store very high amounts of hydrogen at low temperatures.
Fig. 4.15 Crystal structures of few typical MOF materials.
Fig. 4.16 The development process of MOFs as hydrogen storage material
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Solar hydrogen production
COF growth
NH2-f-SiO2
Core etching
NH2-f-SiO2 COF
COF hollow sphere
Fig. 4.17 The covalent organic framework structure.
Fig. 4.18 The hydrogen storage in micro glass spheres [23].
4.7 Glass microspheres In this method, small, hollow glass spheres with diameters ranging from 25 to 500 μm, and the wall thicknesses of 1 μm are used in the hydrogen storage. The glass microspheres shown in Fig. 4.18 are filled with the hydrogen gas at high pressure and high temperatures such as 200°C–400°C. At high temperature, the glass walls become permeable and the gas fills into the microspheres. The hydrogen is trapped inside the spheres
Hydrogen storage
when the glass is cooled to the room temperature. It is provided to release the hydrogen gas by reheating the spheres when the hydrogen is to be used. The storage capacity of glass spheres is about 5%–6% at 20–49 MPa pressure [24].
4.8 Boron-based storage Sodium boron hydride-based storage is based on the use in the liquid form, unlike alanates and solid-state storage methods. Sodium borohydride in the form of the solution gives hydrogen according to the following reaction and turn into sodium metaborate. This transformation is shown in Fig. 4.19. NaBH4 ðsÞ + 2H2 O ! 4H2 + NaBO2 ðcatalyzerÞ As it can be seen in the equation, the amount of hydrogen occurred as result of the reaction is twice that of bonded hydrogen in the form of hydride. The equal amounts of hydrogen in the presence of NaBH4 are come up with water splitting. The reaction given in the equation is exothermic. One consequence of this is that the hydrogen obtained from the system is humid. The most important advantage of the hydrogen storage in sodium borohydride is that the stored hydrogen can be recovered at the room temperature and the recovery can be
Fuel pump
Hydrogen on demand catalyst chamber
NaBH4 fuel tank
NaBO2 return tank
Gas/liquid separator
H2
Borate
Hydrogen gas + steam
Heat exchanger coolant loop
Pure, humidified, hydrogen gas
Fig. 4.19 The sodium borohydride-based hydrogen storage [25].
Fuel cell or hydrogen engine
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Solar hydrogen production
easily controlled with the aid of a catalyst. As a matter of fact, the liquid solution is safe even in contact with flame, but hydrogen release is provided in case of contacting with the solution of catalyst. The biggest disadvantage of the system is the necessity of being recycled of occurring NaBO2 to NaBH4 in another environment.
4.9 The storage in underground The low cost and large-scale storage method of hydrogen gas is to store it in the underground. Two technologies are currently in use for storage of large amounts of gases. The exhausted oil or natural gas reservoirs is similar to the natural gas. In this storage type with a slightly higher cost is also to store the hydrogen in underground in the salt domes, mine quarries, that is, for instance, in the city of Kiel, Germany, the hydrogen has been storing in a cavern under 1330 m since 1971. However, 1%–3% of hydrogen stored in the caverns is lost per year due to the leak [26]. Over the last decades, there have been several examples of underground storage of pure hydrogen or syngas [27]: • England, Teesside, Yorkshire: The British company ICI has stored 1 million Nm3 of nearly pure hydrogen in three salt caverns at a depth of about 400 m. The caverns have operated successfully for many years, and they are now operated by SABIC. • France, Beynes, Ile de France: The gas company Gaz de France has stored a gas with 50%–60% hydrogen in an aquifer of 330 million Nm3 capacity for nearly 20 years. No gas losses or safety problems have been recorded. • Russia: Pure hydrogen was stored underground at 9 MPa for the needs of the aerospace industry. • Germany: 62% of H2 gas was stored in a salt cavern of 32,000 m3 at 8–10 MPa. • Czechoslovakia: 50% of H2 syngas was stored in an aquifer. The other method is storage in aquifers, that is, water carrying layers capped with impermeable layers above and preferably also below the layer into which hydrogen may be pumped so as to replace water. Typically, there would be clay layers with a sand layer in between. The geometry of the sand layer and its contained water would have the form of a waving structure, with some bends that curve upward. These are the locations where a gas such as hydrogen may be pumped into the geological structure, thereby displacing water but still be confined due to the pressure of the water below, when the curvature is such that the gas cannot “run” away to the sides. Decisive parameters are the permeability of the water-carrying layer and of the enclosing clay layers, where the latter determines the leakage rate of the gas. Further, the integrity of the structure, for example, in terms of forming an unbroken upward bend, would determine the rate of losses to the sides. Finally, adsorption or other penetration of the gas, here hydrogen, into the water will have to be considered generally, in order to determine the maximum length of time, that the gas can be expected to remain in the store [28].
Hydrogen storage
4.10 Methanol Methanol is obtained from hydrogen and CO, is liquid under normal conditions, and has high hydrogen content; because of these properties, methanol can be considered as a suitable vehicle fuel. The principle of use states that hydrogen occurs by being broken down methanol and the resulting hydrogen is consumed as fuel. The energy loss in the fragmentation process is quite high, therefore the efficiency of the system is low. In order to overcome this negation, high-efficiency fuel cells that operate directly with methanol have been developed. According to studies conducted, a methanol-reformer fuel-driven vehicle emits 40%–70% more CO2 comparing to a similar petrol vehicle. Besides, there are also CO and hydrocarbon emissions. Methanol is toxic, water miscible, and very corrosive substance. Because of these features, more specific safety precautions are needed when compared to the petrol during the transportation and use. Its negative properties and high emissions prevent methanol from being a highly preferred fuel.
4.11 Petrol and other hydrocarbons These products can be considered as hydrogen storage systems since hydrogen can be produced by being passed of petroleum and other hydrocarbons through a reformer. However, an approximate 30 min is required to be produced hydrogen from petrol which means that it must be heated for 30 min before the vehicle is used, therefore the battery of the vehicle must be at least 30 min durability before moving. Due to the complex, difficult and costly nature of the reformer and battery connection, the obligation to remove hydrocarbons and CO which are formed beside hydrogen production, the possibility of CO to destroy reformer membranes, and the NOx compounds forming due to the necessity of operating at high temperatures, and therefore emissions problems, make the petrol and hydrocarbons move far away from to be ideal hydrogen carrier. Fig. 4.20 shows CO2 emissions measured at different speeds when compressed hydrogen, 300
Gasoline (from petrolium)
250 Methanol (from natural gas) CO2 (g/kg)
200 150 100 50
H2 (electrolyser)
0
Fig. 4.20 CO2 emissions in case of using different fuels as hydrogen storage in a petrol engine vehicle.
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methanol, and petrol are used as the hydrogen storage in a petrol engine vehicle. As it is seen, the lowest CO2 emission is obtained in case of using hydrogen produced via electrolysis. Current approaches for onboard hydrogen storage systems include pressurized hydrogen gas, liquid hydrogen, adsorbents (carbon nanotubes, MOF, COF materials, etc.), metal hydrides, and chemical hydrides. The advantages and disadvantages for each hydrogen storage methods are summarized in Table 4.5. • Storage of hydrogen as compressed gas at high pressures (up to 70 MPa) can be used to store small quantities of gas. Table 4.5 Advantages and disadvantages of different hydrogen storage approaches [29] H2 storage systems
Advantages
Disadvantages
Ongoing efforts
Compressed H2
Commercially available
Develop and design costeffective vessel/container
Liquid H2
Commercially available
Cryocompressed Metal hydride
High volumetric capacity Reversible on-board
Sorbent and carbon-based materials
Reversible on-board
Low volumetric capacity High compression energy Heat management during charging required H2 loss Safety issue High liquefaction energy Heat management to reduce boil-off High compression/ liquefaction energy Low gravimetric/volumetric capacity Heat management during charging required High operating temperature for H2 release Low volumetric density
Loss of useable H2
Optimize material’s properties (pore size, pore volume, surface area, among others)
Chemical hydride
Good volumetric capacity Proper operating temperatures
Low operating temperature for H2 uptake Thermal management required Off-board regeneration
Improve kinetics of hydrogen adsorption/ desorption along with heat management
Increase dihydrogen binding energies.
Develop cost effective and energy efficient Regeneration methods for the spent material
Hydrogen storage
• Storage of hydrogen in liquid form can be used for storing large quantities of gas for longer times, however at high-energy penalties for liquefaction. • Metal hydrides may offer an advantage for storing small quantities of gas in the medium to long term.
References [1] International Energy Agency, World Energy Outlook, www.iea.org, 2015. (Accessed 16 February 2015). [2] J. Pottier, C. Bailleux, A gas of the past, present and future ‘A Drama In 3 Acts’, in: T.N. Veziroglu (Ed.), Hydrogen Energy Progress VI: Proceedings of the 6th World Hydrogen Energy Conference, Vienna, Austria, 1986. € [3] M. Altan, E. Y€ or€ ukog˘ulları, Hidrojen Zeolit Sisteminin Enerji Teknolojisindeki Onemi, in: D€ unya Enerji Konseyi T€ urk Milli Komitesi, T€ urkiye 7. Enerji Kongresi, 1997. [4] G. Barkhordarian, T. Klassen, R. Bormann, Catalytic mechanism of transition-metal compounds on Mg hydrogen sorption reaction, J. Phys. Chem. B 110 (22) (2006) 11020–11024. [5] A. Z€ uttel, Materials for hydrogen storage, Mater. Today 6 (9) (2003) 24–33. [6] Office of Energy Efficiency and Renewable Energy, Hydrogen Storage, https://energy.gov/eere/ fuelcells/hydrogen-storage, 2018. (Accessed 15 May 2018). [7] N. Sirosh, Hydrogen composite tank program, Rev. Lit. Arts Am. 2000 (2002) 1–7. [8] Online Source. Available: www.fuelcellstore.com. [9] The International Coalition of Hydrogen Associations, The History of Hydrogen, http://www.hpath. org/resources/Factsheet-History.pdf, 2016. (Accessed 15 May 2016). [10] Y. Sun, L. Wang, W.A. Amer, et al., Hydrogen storage in metal-organic frameworks, J. Inorg. Organomet. Polym. 23 (2013) 270. [11] Sigma-Aldrich, https://www.sigmaaldrich.com/technical-documents/articles/material-matters/ introducing-hydrnol.html, 2015. (Accessed 10 January 2015). [12] T.M. Flynn, A Liquification of Gases, McGraw-Hill Encyclopedia of Science & Technology, in: seventh ed., vol. 10, McGraw-Hill, New York, 1992, pp. 106–109. [13] T. Hottinen, Technical Review and Economic Aspects of Hydrogen Storage Technologies, Helsinki University of Technology, Helsinki, 2001. [14] T.J. Carter, L.A. Cornish, Hydrogen in Metals, Eng. Fail. Anal. 8 (2) (2001) 113–121. [15] P. Fischer, K. Yvon, Crystal and magnetic structures of ternary metal hydrides: a comprehensive review, in: Hydrogen in Intermetallic Compounds, 1988, pp. 87–138. [16] G. Sandrock, A panoramic overview of hydrogen storage alloys from a gas reaction point of view, J. Alloy. Comp. 293–295 (1999) 877–888. [17] N. Cuı, J.L. Luo, K.T. Chuang, Study of hydrogen diffusion in α- and β-phase hydrides of Mg2Ni alloy by microelectrode technique, J. Electroanal. Chem. 503 (2001) 92–98. [18] L. Schlapbach, Hydrogen in Intermetallic Compounds I, Springer, Germany, 1988. [19] G. Sandrock, S. Suda, L. Schlapbach, Hydrogen in Intermetallic Compounds II, Springer-Verlag, Berlin, Germany, 1992. [20] M. Hangstr€ om, Improved Metal Hydrides for Energy Applications Based on Gas-Solid Reactions, Helsinki University of Technology, Finland, 1999. [21] S. Lee, Y. Lee, Hydrogen storage in single-walled carbon nanotubes, Appl. Phys. Lett. 20 (2002) 2877–2899. [22] M. Hirscher, et al., Hydrogen storage in carbon nanostructures, J. Alloys Compd. 330–332 (2002) 654–658. [23] R. Teitel, Hydrogen Storage in Glass Microspheres, Brookhaven National Laboratories, Brookhaven, 1981. € [24] S. Kaya, Magnezyum Bor Hidr€ ur Sentezi ve Hidrojen C ¸ eviriminde Kullanımı, Gazi Universitesi Fen Bilimleri Enstit€ us€ u Y€ uksek Lisans Tezi, Ankara, 2005 (in Turkish).
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€ urk, Hidrojen depolama amacıyla magnezyum tozlarının € [25] M. G€ uvendiren, H.E. Unalan, T. Ozt€ o €g˘u €t€ ulmesinde katkı maddelerinin etkisi, in: Toz Metal€ urjisi Konferansı, 2002 (in Turkish). [26] Underground Gas Storage, https://hub.globalccsinstitute.com/publications/operating-flexibilitypower-plants-ccs/2-underground-hydrogen-storage, 2016. (Accessed 10 May 2016). [27] D.L. Katz, M.R. Tek, Overview of underground storage of natural gas, J. Pet. Geol. 33 (1981) 943–951. [28] B. Sorensen, Underground Hydrogen Storage in Geological Formations, and Comparison with Other Storage Solutions, in: Hydrogen Power Theoretical and Engineering International Symposium, Merida Technical University, 2007. [29] H.T. Hwang, A. Varma, Hydrogen storage for fuel cell vehicles, Curr. Opin. Chem. Eng. 5 (2014) 42–48.
Further reading [30] P. Selvam, B. Viswanathan, C.S. Swamy, V. Srinivasan, Magnesium and magnesium alloy hydrides, Int. J. Hydrogen Energy 11 (3) (1986) 169–192. [31] Linde Group, Liquid Hydrogen Storage, www.linde-gas.com, 2010. (Accessed 16 May 2010). [32] E. Shimizu, K. Aoki, T. Masumoto, Hydrogen absorption properties of amorphous and crystalline alloys in the pseudobinary ZrCo-TiNi system, J. Alloys Compd. 293 (1999) 526–530. [33] Online Source: https://www.sigmaaldrich.com/technical-documents/articles/material-matters/ introducing-hydrnol.html. [34] M.T. Syed, et al., An economic analysis of three hydrogen liquefaction systems, Int. J. Hydrogen Energy 23 (7) (1998) 565–576.
Hydrogen storage Muhammet Kayfecia, Ali Kec¸ ebaşb a
Department of Energy Systems Engineering, Karab€ uk University, Karab€ uk, Turkey Department of Energy Systems Engineering, Mug˘la Sıtkı Koc¸man University, Mug˘la, Turkey
b
Contents 4.1 4.2 4.3 4.4 4.5
Introduction Hydrogen storage methods Pressurized hydrogen storage Liquefied hydrogen storage Metal hydrides 4.5.1 Types of metal hydrides 4.6 Hydrogen storage in nanostructured/porous material 4.6.1 Carbon nanotubes 4.6.2 Zeolites 4.6.3 Metal organic framework 4.6.4 Covalent organic framework 4.7 Glass microspheres 4.8 Boron-based storage 4.9 The storage in underground 4.10 Methanol 4.11 Petrol and other hydrocarbons References Further reading
85 87 88 91 93 97 101 101 102 102 103 104 105 106 107 107 109 110
4.1 Introduction Energy, one of the fundamental elements of economic development, is one of the indispensable requirements of humanity. The energy requirement increasingly raises in parallel with the world population and industrial developments; notwithstanding, the reserves of fossil energy resources rapidly depletes. According to the latest statistical evaluations, crude oil that meets 38.5% of the world’s energy needs has a reserve utilization period of 41 years, whereas natural gas that meets 23.7% has 62 years, and coal that meets 24.7% has 230 years. In 1990, the world had a population of 1.6 billion and primary energy consumption was about 1000 Mtep. In 1997, the population reached 6.5 billion and the primary commercial energy consumption increased to 11,700 Mtep [1]. Thus, the primary energy consumption of the world has increased over eight times within a century.
Solar Hydrogen Production https://doi.org/10.1016/B978-0-12-814853-2.00004-7
© 2019 Elsevier Inc. All rights reserved.
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Using fossil fuels as a main source for meeting world’s energy demand, evaluating of fossil fuels by combustion reaction and occurring of harmful emissions such as carbon dioxide (CO2), carbon monoxide (CO), sulfur dioxide (SO2), and nitrogen oxide (NOx) in this reaction leads to enormous environmental problems. The main reason for global warming, which is the most important environmental problem in the world today, is the strengthening of the greenhouse effect of the atmosphere with increasing CO2 emissions. Because of the limited reserves of primary energy resources, increases in fuel price, the population growth, industrialization, obligation to assess national resources, socioeconomic structuring of the 21st century, the negative effects of existing fuels on the environment (such as greenhouse effect, global warming, climate changes, rainfall abnormalities, and acid rain, health problems); hydrogen energy, which is a portable energy source that has high calorific value, has come forward in the context of new energy technologies. Hydrogen energy is considered one of the most important energy sources in the next century. This energy can be obtained from water and can be transformed into a useful energy with high efficiency and no negative impact on the environment. Hydrogen energy, which will be used to solve the energy problem of the world, will be able to be produced continuously for billions of years. One of the most important advantages of hydrogen gas as an energy is that it does not contain toxic gas and cause corrosion. Therefore, the use of hydrogen energy is safe and very simple when necessary precautions are taken. As a result of burning the hydrogen as nonpolluting fuel, water vapor emerges as waste [2]. In addition to not harming the natural environment, the formation of water vapor as waste can help to eliminate the damages caused by other energy wastes, and it is able to benefited effectively from hydrogen wastes as natural recycling in many areas of technology [3]. As an energy source, the most important feature that hydrogen has is that it can be stored. However, some problems arise during storage due to the fact that it is the lightest gas ever known (the density rate is 0.0838 kg/m3). Today, in order to widespread use of hydrogen that is the energy of the future, the effective and efficient methods for hydrogen storage should be developed. There are many methods used in the hydrogen storage. Hydrogen can be stored in gaseous state by compressing or in liquid state by being cooled to very low temperatures liquefied. However, the investigations have shown that the most effective storage method is the solid storage. Hydrogen can be physically stored in carbon nanotubes and also chemically stored in hydrides as a solid state. Among storage methods, the most effective one is the storage in the form of hydrides because of their high storage capacities, suitable working environments and being safer for working at low pressures [4]. In storage as metal hydride, hydrogen is stored in spaces between metals. Hydrides are compounds which absorb hydrogen by reacting with hydrogen at a certain temperature and also release the hydrogen when they are heated.
Hydrogen storage
Hydrogen reacts with many transition metals and alloys at elevated temperatures in the form of hydrides. Electropositives from these alloys, such as Sc, Yt, Lanthanides, Actinides, and Ti, also the elements of group 5A are very reactive metal hydrides [5].
4.2 Hydrogen storage methods The major obstacle to the widespread use of hydrogen is the storage. Hydrogen in the form of gas takes up 3000 times more space than a gas with the same amount of energy at room temperature and pressure. For this reason, it is necessary to be used of hydrogen in compression, liquefaction or other methods in order to utilize in stationary systems, vehicles, and mobile applications. There are four main techniques used for the storage. These are compressed gas, cryogenic liquid, metal hydride, and carbon adsorption. In the short term, the most feasible ones are the storage in cryogenic liquids and metal hydrides. Despite the fact that the metal hydride method is an advanced method, further research is required for this to be competitive. Carbon adsorption is not yet a mature technique, but it is seen as a feasible method if targets are achieved at the end of research and development studies. The end-use storage techniques of hydrogen are different for each application. The hydrogen quantities that can be stored by some storage techniques, and energy densities are given in Table 4.1 (Fig. 4.1). Table 4.1 Comparison of hydrogen storage methods Method
ρm (wt%)a
ρv (kg/m3)b
T (K)c
P (bar)d
Compressed gas Liquid
13
<40
273
800
Varies
70.8
21.5
1
Physisorption
2
20
77
100
Interstitial metal hydrides Complex hydrides
2
150
273
1
<18
150
>100
1
Chemical hydrides
<40
> 150
273
1
a
Gravimetric storage density. Volumetry storage density. c Operational temperatures for storage method. d Operational pressures for the storage method. b
Description
Compressed hydrogen gas; lightweight, high-pressure cylinder Liquid hydrogen, continuous loss of a few % per day at RT Physical adsorption by porous materials, fully reversible Atomic hydrogen occupies interstitial sites, fully reversible, metals are heavy Complex compounds [BH4]or [AlH]4, desorption at elevated temperature, adsorption at high pressure Thermal decomposition of chemical hydrides, not directly reversible
87
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Solar hydrogen production
Fig. 4.1 Hydrogen storage methods [6].
Fig. 4.2 Compressed hydrogen gas storage tank [7].
4.3 Pressurized hydrogen storage This storage is performed in a high-pressure resistant tank at room temperature. The compressed gas storage tank is shown in Fig. 4.2. In the compressed gas storage, 1%–7% of hydrogen by weight is stored depending on the weight of the tank and
Hydrogen storage
therefore on the type of it. Tanks that are lighter, more durable and able to store more hydrogen are more expensive. Up to 20% of the energy content of the fuel is spent to be compressed the hydrogen gas at the filling station. Storage in pressurized tanks as compressed gas is a well-known storage method. Hydrogen is compressed into high-pressure tanks. Energy is needed to carry out this process, and this volume filled by the compressed gas is usually quite large. This results with hydrogen having a lower energy density according to the conventional petrol tanks. A hydrogen gas tank will be 3000 times bigger than the petrol tank when it contains the equal amount of energy stored by a petrol tank [8]. Hydrogen is nowadays stored under the pressure of 200–250 bar in 50-L cylindrical tanks. However, the storage pressure as the gas can able to increase up to 600–700 bar. The volumetric energy density of the hydrogen is low due to the nature of being very light even if it is stored in the 50 L tank. On the other hand, storage tanks are heavy due to high pressure [9]. Volumetric density of compressed hydrogen gas a function of gas pressure shown in Fig. 4.3. Hydrogen shows ideal gas properties at very high pressures and low temperatures. Therefore, the number of molecules and mass at a given pressure and temperature can be calculated from the ideal gas equation.
50
Pressure (MPa) 100
150
200
H2liq
0.25
60 0.20 H2 gas
Ideal gas
0.15
40 sv = 460 Mpa (steel)
0.10
Casing Hemisphere
20
0.05
di do dw
0
500
1000 Pressure (bar)
1500
0.00 2000
Fig. 4.3 Volumetric density of compressed hydrogen gas a function of gas pressure.
dw /do
Volumetric H 2 density (kg.m –3)
0
89
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Solar hydrogen production
n¼
PV ! m ¼ nM Ru T
In this equation, n is the number of mole of hydrogen, M for the molecular weight, T the absolute temperature and Ru for the universal gas constant, respectively. The amount of energy to be obtained from hydrogen can be calculated as follows: E ¼m4H In this correlation, ΔH stands for the combustion temperature of the hydrogen gas, which is 140 MJ/kg for hydrogen. The volumetric energy density of the hydrogen can be also calculated from the following equation: Wvolumetric ¼
E 4HP P 4 H ¼ ¼ V nRu T RT
As it is seen in this correlation, the energy density increases with the pressure. However, the increase in pressure is limited by the strength of the storage material. In practice, austenitic steel and aluminum alloys are used as pressurized storage material, but the disadvantage of these storages is to being very heavy. The ratio of the stored hydrogen to the weight of the whole tank is about 2%–3%. These disadvantages of storages can be accomplished by using composite material. These storages contain a thin layer of metal in contact with hydrogen, followed polymer envelope strengthen by a carbon fiber. In such a container, the hydrogen weight ratio increases to 5%. Multilayered polymer is used instead of metal envelope in more advanced storages. The wall thickness of a cylinder capped with two hemispheres is given by the following equation: dw 4P ¼ do 2σ v + 4 P where dw is the cylinder wall thickness, do the outer diameter of the cylinder, ΔP is the over pressure, and σ v is the tensile strength of the cylinder material. Fig. 4.3 shows the volumetric density of compressed hydrogen gas a function of gas pressure. The volumetric density increases with hydrogen pressure and reaches a maximum above 100 MPa, depending on the tensile strength of materials. The tensile strength of materials varies from 50 MPa for aluminum to more than 1100 MPa for steel [10]. One kilogram of hydrogen currently costs $1.20. The same 1 kg of H2 is equivalent to 1 gallon of gasoline. The cost to store and transport hydrogen more than triples its cost. The cost comparison of compressed hydrogen and cryogenic hydrogen is shown in Table 4.2.
Hydrogen storage
Table 4.2 The cost comparison of compressed and liquid hydrogen storage methods [11] Raw cost of H2 ($/kg)
Storage method
Compressed hydrogen
1.2
Liquid hydrogen
1.2
Cost to process H2 ($/kg)
0.70 (compression to 45 MPa) 1.11 (cryogenic liquifaction)
Cost to transport H2 ($/kg)
Cost of on-site storage of H2 ($/kg)
0.70 (pressurized tank) 0.21 (Cryogenic tank)
0.45 (high pressure storage tank) 0.24 (cryogenic storage including boil off losses)
Refuelling station cost H2 ($/kg)
Delivered cost of H2 ($/kg)
0.75
3.80
0.66
3.42
4.4 Liquefied hydrogen storage In this technique, the hydrogen is stored at –253°C in atmospheric pressure in fairly wellinsulated tanks. Because the hydrogen is in the liquid form, it contains 3 times more energy than the equivalent weight petrol and 2.7 times more volume is required in case of containing the equivalent energy. The well-known liquefaction process is the precooled Linde cycle flowsheet as shown in Fig. 4.4. Liquefaction is done by cooling a gas to form a liquid. Liquefaction processes use a combination of compressors, heat exchangers, expansion engines, and throttle
LN 2 Makeup gas
GN2
LN 2
GN 2
. Q 2 . m
1
I
3
II
4
III
5
IV
6
9
10 . W
11 V
m mf
g . mf Fig. 4.4 Linde-Hampson liquefaction cycle.
f
Liquid
91
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Solar hydrogen production
valves to achieve the desired cooling [12]. Initially, the gas is compressed at ambient pressure and subsequently cooled to 80 K in a counterflow heat exchanger using liquid nitrogen. Heat exchangers are used to lower the temperature even further, below its inversion temperature by transferring heat from the hydrogen stream to the returning cooled hydrogen. Ultimately, the cooled and compressed gas is forced to pass through a throttle valve or a mechanical expander where it undergoes an isenthalpic expansion to ambient pressure, producing some liquid. The liquid is removed and the cooled gas is returned to the compressor via the heat exchangers. The work needed in this theoretical process is called the ideal work of liquefaction and has been calculated as 11.88 MJ/kg by [13]. Linde liquefaction process temperature entropy diagram is shown in Fig. 4.5. This technique stores 16% of hydrogen by weight, including the tank and insulation. In addition, the liquefaction requires up to 28% of the energy content of the fuel. Another disadvantage is the heat transfer to the tank, despite the insulation. As a result of this heat transfer, hydrogen evaporates. However, this problem can be solved by using a pressurized tank. In this case, this solution also increases the weight and size. Even though the energy used for the hydrogen liquefaction is high, the costs of the liquefaction in spacecraft and rockets are overlooked. If the liquid hydrogen is stored in large tanks, 0.06% of hydrogen per day is lost, in case of storing it in small tanks, 3% of
Fig. 4.5 Precooled liquefaction temperature-entropy diagram.
Hydrogen storage
Table 4.3 Liquid hydrogen physical and chemical properties Chemical formula
H2
Molecular weight Boiling point, 1 atm Freezing point, 1 atm Critical temperature Critical pressure Density, liquid, B.P., 1 atm Density, gas, 20°C, 1 atm Specific gravity, gas (air ¼ 1), 20°C, 1 atm Specific gravity, liquid, B.P. [water ¼ 1, 20°C] Specific volume, 20°C, 1 atm Latent heat of vaporization Flammable limits, 1 atm in air (by volume) Flammable limits, 1 atm in oxygen (by volume) Detonable limits, 1 atm in air (by volume) Detonable limits, 1 atm in oxygen (by volume) Autoignition temperature 1 atm Expansion ratio, liquid to gas, B.P. to 20°C
2.016 252.9°C 259.3°C 240.2°C 12.7 atm 70.8 kg/m3 0.0838 kg/m3 0.0696 0.0710 11.923 m3/kg 389 Btu/lb. mole 4.00%–74.2% 3.90%–95.8% 18.2%–58.9% 15%–90% 571°C 1–845
hydrogen per day evaporates. The reduction of this ratio depends on the insulation. Liquid hydrogen thermophysical properties and chemical properties are shown in Table 4.3.
4.5 Metal hydrides Metal hydrides are made up of metal atoms with lattice imperfections, and hydrogen atoms embedded in the interstices of lattice cavities. The intermediate region can be in the form of a space or a lineal error. In the case of a lineal error, the sequence of the hydrogen atoms performs by collecting throughout errors. Thus, they increase the lattice strain especially when two adjacent atoms recombine in the form of molecular hydrogen. It increases the lattice size during hydrogen adsorption [14, 15]. There are two different ways in hydration, the first of these is the chemical adhesion and the second one is also the electrochemical separation of the water. These reactions are respectively as follows [16]: x M + H2 $ MH x + Q 2 x x x M + H2 O + e $ MH x + OH 2 2 2
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Solar hydrogen production
H2
H2
Charge a-Phase
a-Phase H2
Charge
Discharge
94
H2 H2
Discharge b-Phase
b-Phase
H2
Fig. 4.6 The phase transformation scheme in metal hydrides [17].
In these equations, M and Q stand for the metal, and the heat generated by the exothermic reaction, respectively. In the electrochemical separation, a catalyst such as palladium is needed. The chemical adhesion of the hydrogen is schematically given in Fig. 4.6. As it is shown in the figure, the molecular hydrogen reaches a minimum shallow potential close to the surface, and the atomic hydrogen is in a region that is deeper than the surface but is almost near the surface. In the metal lattice, the hydrogen has a minimum periodic potential within the crack regions in the lattice. While the hydrogen molecules approach to the metal surfaces, they close together due to the effect of the weak van der Waals forces. At the distance of zp, the molecule reaches the point of Ep which is the potential well, and the very large forces approach the surface in the molecular form. Thus, the dissociation energy of the hydrogen molecules is exceeded by the chemisorption energy. The hydrogen molecule is then separated, and the individual hydrogen atoms are attracted to the surface by the effect of the chemisorptive forces and reach the potential of ECH. After this point, the thermal energy of the ambient air is sufficient to increase the vibrational width of hydrogen atoms, thus the hydrogen can reach the metal surfaces [14]. The metal and hydrogen are usually in the form of two different hydrides, as α-phase and β-phase. Only a little hydrogen is absorbed in the α-phase and the hydride is completely formed in the β-phase. Mg2NiH0.3 and Mg2NiH4 can be given as example for the hydrides of the Mg2Ni form. The most general characteristic method of metal hydrides is PCT (pressureconcentration-temperature), the curve in the graph of P-C isotherms. The theoretical
Hydrogen storage
P-C isotherms with α- and β-phases are given in Fig. 4.7. The concentration is usually determined by the amount of hydrogen atoms H/M in the unit metal molecule. The optimal parameter to define metal hydrides is the maximum hydrogen capacity (H/ M)max. The reversible capacity is determined by using the plateau width Δ(H/M) as shown in Fig. 4.8 and utilized to determine the theoretical capacity of metal hydrides. Tc 100
β-Phase
–
80
ΔS R
60
α-Phase
40 10 ΔH R
α + β-Phase 25°C
20
1
E 0(mV)
Peq (bar)
100°C
0 0°C –20 0.0
0.2
0.4 0.6 cH (H/M)
0.8
1.0 2.4
2.8 3.2 3.6 T –1(10–3 K–1)
Fig. 4.7 The theoretical P-C curve of the metal hydride [18].
lnP
Hysteresis=ln
PA PB
Plateau slope = d lnP d(H/M)
Reversible capacity Δ(H/M)
Hydrogen/metal ratio Fig. 4.8 The real P-C isotherm diagram [19].
Capacity (H/M) max
H/M
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Solar hydrogen production
The concentration can be expressed as a percentage mass in total mass, especially in energy intensity comparisons. The real metal hydrides exhibit some hysteresis between adsorption/desorption in the P-C curve. Besides, the plateau has a bit of slope. In the thermodynamic reaction equilibrium equation, the reaction constant K can be expressed as [20] RT ln K ¼ 4H 4S Here, ΔH is the reaction enthalpy and ΔS is the reaction entropy. For the solid-gas reaction, the equilibrium is reduced from the constant to the gas pressure. Thus the van’t Hoff equation is obtained as [20] Peq 4H 4S ln ¼ P0 RT R where R is the gas constant Peq and P0 are the equilibrium pressure and the normal pressure (100 kPa), respectively. The equilibrium pressure is a function of the temperature T and the constants: reaction enthalpy ΔHR and entropy ΔS, being the slope and y-intercept of the plot in Fig. 4.9. ln P versus 1/T, the equilibrium diagram (P, T) values scale are called the van’t Hoff diagram. The reaction enthalpy can be determined from the angular coefficient in the diagram along with the its equation and is called with the P-T harmony of hydride behaviors for practical applications in the diagram. The theoretical van’t Hoff diagram generally describes very well the actual properties of metal hydrides. The representation of van’t Hoff diagram is given in Fig. 4.10 [19]. The reaction enthalpy for the hydride formation is an important quantity. This quantity is usually negative if the reaction is exothermic, and thus the hydride formation releases the energy. Therefore, the energy should be given to the hydrogen with an aim to be released again by the hydride. In most of the applications, the ambient air has been used or at least a temperature between 0°C and 100°C is required. In order to be released the hydrogen by taking the heat from the ambient air of the hydride, the reaction enthalpy should be quite small. ln P
DH a
1/T
Fig. 4.9 The schematic representation van’t Hoff diagram.
Hydrogen storage
Temperature (°C) 1000
600
400 300
200
50
100
25
1000 NiH
Dissociation pressure (atm)
1000 100
VH2
10 1
PdH0.6 ThH2 ZrH2 CaH2
MgH2
0.1 0.01 0.001
Th4H15 UH3
LaH2 YH2
Th4H15
ThH2 0.5
1.0
1.5
2.0
2.5
3.0
3.5
1000/T (K –1) Fig. 4.10 The van’t Hoff diagram for some natural elements [19].
4.5.1 Types of metal hydrides Hydrogen is a highly reactive element. It is known that hydrogen as the hydride is in the form of a solid solution together with thousands of metals and alloys. Many natural elements absorb hydrogen under the favorable conditions. However, as it is shown in Table 4.3, PCT features are not suitable for the most of them to be able to be used in practical applications. In the following graph, the temperature range shown in the box is given in the range between 0°C and 100°C, and the pressure range is given between 100 and 1000 kPa. In order to obtain the desired PCT properties, it is necessary to combine some strong hydride elements (called A) with elements having weaker hydriding properties (called B). In various types of intermetallic compounds, the compounds contain different amounts of elements A and B, the compounds are mostly in the form of solid solutions, and the hydride compounds are in the form of transition metals. The element A is generally rare elements or alkaline earth metals which can constitute stable hydrides. The element B is a transition metal, which can often constitute unstable hydrides. In intermetallic compounds, some defined ratios between B and A are x ¼ 0.5, 1, 2, 5. The hydride families in different types are listed in Table 4.4.
97
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Solar hydrogen production
Table 4.4 The prototype and structure of some important intermetallic compounds [5] Intermetallic compounds
Prototype
Hydride
Structure
AB5 AB2 AB3 A2B7 A6B23 AB A2B
LaNi5 ZrV2, ZrMn2, TiMn2 CeNi3, YFe3 Y2Ni7, Th2Fe7 Y6Fe23 TiFe, ZrNi Mg2Ni, Ti2Ni
LaNiH6 ZrV2H5.5 CeNi3H4 Y2Ni7H3 Ho6Fe23H12 TiFeH2 Mg2NiH4
Hook phase, hexagon Lave phase, hexagon or cubic Hexagon, PuNi3-type Hexagon, Ce2Ni7-type Cubic, Th6Mn23-type Cubic, CsCl- or CrB-type Cubic, MoSi2- or Ti2Ni-type
4.5.1.1 AB5 intermetallic compounds The compounds AB5 are generally in the form of the hydride at the temperature of 100°C and a few atmospheric equilibrium pressures. Other important features are the low hysteresis, tolerances in gas pollution and the ease of activation in the initial cycle. Thus, the most of these alloys contain Ni as the base metal. AB5 family has hexagonal structure with CaCu5-type lattice. One of the most important examples of AB5 class alloys is the LaNi5 alloy. These class alloys have extraordinary versatility because many different element types can take the place of A and B lattice gaps. The typical examples for metal A can be given as Al, Mn, Si, Zn, Cr, Fe, Cu, and Co for Mm, Ca, Y, Zr, and B. Compounds substituting for A and B cause significant changes in the macrostructure of the alloy. For instance, if Ni is replaced with Co and Fe in the LaNi5 alloy, the volume expansion occurs on the hydriding, the corrosion velocity decreases, and it provides to improve the life cycle of the hydride [13]. The PCT properties of AB5 alloys are given in Fig. 4.11. The hysteresis is quite low in AB5 alloys except for MmNi5 [19]. The most important advantage of AB5 alloys is that they are not in the form of a protective oxide and are resistant against to the small O2 and H2O pollution in H2. The hydrogen capacities of AB5 compounds are low. The maximum capacity with LaCo5 and Ca0.7Mm0.3Ni5 with nominal quantity of nearly 1% has performed as 1.90% by weight. The reversible capacity has been determined as 1.28% by weight with LaNi5, and the nominal quantity has also been about 0.7%–1% by weight [19]. On the other hand, the cost of AB5 alloys is also quite high. The costs of the alloy are in the range of 8%–11 €/kg considering only raw material in a case where the reactor is not included. The cost become about 0.7–2.0 €/gH in a case when this cost is translated to the cost of the hydrogen storage by being added the reversible capacity of this hydrogen storage [19]. 4.5.1.2 AB2 intermetallic compounds AB2 alloys, such as AB5 alloys, also have wide and versatile positive PCT properties at ambient temperatures. The element A is generally Ti, Zr, Hf, TH, or a lanthanide
Hydrogen storage
Temperature (°C) 200 150
100
50
0
25
–25
–50
100 MmNi4.15Fe0.85
LaNi5 CaNi5
MmNi5
Pd (atm)
10
LaNi4.25Al0.75
1
MmNi4.5Al0.5 MmNi3.5Co0.7Al0.8
LaNi4.8Sn0.2
0.1 2
2.5
3
3.5
4
4.5
1000/T (K–1
(
Fig. 4.11 The van’t Hoff diagram for various AB5 alloys [19].
(atomic numbers 57–71). The element B may be one of transition such as V, Cr, Mn, and Fe or nontransition metals. The PCT properties of some AB2 alloys are given in Fig. 4.12. Hydrogen capacities of AB2 alloys are comparable to AB5 on a reversible basis, but generally have high capacity. In these alloys, especially large pressures and temperatures are possible. The maximum capacities of AB2 alloys are usually between 1.5% and 2% by weight and 3.4% by weight with TiCr1.2V0.8 pairs. The reversible capacities are also between 0.9% and1.3% by weight [19]. The costs of raw materials are between 3 and 12 €/kg. The hydrogen-based costs of AB2 alloys are about 0.5–1.3 €/gH. Thus, the costs of AB2 alloys are somewhat lower according to the costs of AB5 alloys. For instance, the maximum and reversible capacity for Ti0.92Zr0.02V0.43Fe0.09Cr0.05Mn1.5 is 1.9% and 1.3% by weight, respectively and the cost is 0.41 €/gH [13]. 4.5.1.3 AB intermetallic compounds The AB alloys are generally TiFe based. These tend to have two plateaus, the upper one is not very stable, and the hysteresis is quite wide. The PCT properties can be partially corrected using Mn or Ni instead of Fe, as shown in Fig. 4.13. In this figure, U shows the top plateau and L shows the bottom plateau.
99
Solar hydrogen production
Temperature (°C) 100
200 150
50
0
25
–25
–50
100 TiMn1.5
TiCr1.8
Ti0.98Zr0.02V0.43 Fe0.09Cr0.05Mn1.5 ZrFe1.5Cr0.5
Pd (atm)
10
1 ZrMn2 TiMn1.4V0.62 0.1 2
3
2.5
3.5
4
4.5
–25
–50
1000/T (K–1
(
Fig. 4.12 The van’t Hoff diagram for various AB2 alloys [19].
Temperature (°C) 200 150
100
50
25
0
100
TiFe (L) TiFe (U)
Pd (atm)
10
1
TiFe0.8Ni0.2 TiFe0.85Mn0.2(L) 0.1 2
2.5
3
3.5
1000/T (K–1
(
100
Fig. 4.13 The van’t Hoff diagram for various AB alloys [19].
4
4.5
Hydrogen storage
Oxide films are usually in the form of TiFe-based alloys. This reduces the susceptibility to the pollution in hydrogen. Oxidation, on the other hand, does not create a tendency for combustion behaviors. The maximum hydrogen capacity for AB alloys varies by less than 1%–2% by weight. The interesting two alloys here, TiFe and TiFe0.85Mn0.15, have a maximum capacity of about 1.9% and a reversible capacity of 1.5%. The costs of these alloys are very low as being 0.34 and 0.35 €/gH, respectively. The main reason for not being able to use of AB alloys as commercial is that the top plateau, and the susceptibility to pollution is unstable. In some studies, it has been aimed to overcome these problems using so-called binary hydrides such as (ZrCo)1 x(TiNi)x; however, very clear efficiency increases have not been achieved. For instance, H/M ¼ 1.6 in the alloy of (ZrCo)0.7(TiNi)0.3 and the hydrogen capacity is 1.16% by weight [13, 18]. 4.5.1.4 Other intermetallic compounds In addition to the alloys shown above, many intermetallic families herein have the hydrogen absorption capacity, but these are not of commercial interest. These include A2B, AB3, A2B7, and A3B alloys. Some of these alloys have good hydrogen capacities, but the PCT properties are not suitable. Besides the PCT properties, it has poor hydrogen capacity in practical application areas or has a very narrow plateau tendency.
4.6 Hydrogen storage in nanostructured/porous material 4.6.1 Carbon nanotubes Carbon is one of the most suitable substances for the gas storage, because it can be especially made into very small particles with high porosity, and due to the attractive force between the carbon atoms and the gas molecules. Nanotubes have many special superior features. For instance, the modulus of elasticity is five times higher than the steel. Besides, depending on the structure of the tube, some of them behave as a semiconductor and some as a conductor. Because of these features, it is possible to reduce electronic devices to micro and nanodimensions using nanotubes. Hydrogen is stored in nanotubes in chemical or physical ways. Carbon nanotubes were discovered in 1991 by Lijima. Carbon nanotubes are briefly in the form of graphite plates converted into the form of tube. Diameters are in the range of a few nanometers or 10–20 nm, and the lengths are also in the micron level. Nanotubes can be produced in multiwalled tubes like it can be produced in single walled. The single and multiwalled nanotubes and the yarn structure consisting of nanotubes is given in Fig. 4.14. There are also nanotubes made up of various additives such as alkali-added (Li-K) [22]. Hydrogen absorption in carbon nanotubes occurs on highly porous super active graphite surfaces. Absorption process occurs with Van Der Waal’s force, which carbon atoms apply to hydrogen molecules. For this reason, the absorption process is not a chemical, but a physical event. Hydrogen absorbed at a given temperature is only a function of
101
102
Solar hydrogen production
SWCNT
SWCNT (yarns)
MWCNT
Fig. 4.14 The single and multiwalled nanotubes [21].
the pressure. The desired amount of hydrogen is released when the pressure is reduced. The nano structure is used in pressurized tanks, and therefore the absorbed hydrogen contributes to hydrogen stored as gas form. These systems store approximately 4% hydrogen by weight.
4.6.2 Zeolites Zeolites are a group of crystalline microporous aluminosilicate compounds of both natural and synthetic origin that are defined as a gap and channel type pore structures in molecular dimensions. Zeolites were first described by the Swedish mineralogist Baron Axel Cronstedt in 1756. Their names are derived from the word of Greek “zeo” and the word of “lithos” meaning to boil and the rock. The general formula of zeolite compounds can be given as h i Mx=n ðAlO2 Þx ðSiO2 Þy mH2 O Here, the negatively charged structures of the alkaline or alkaline earth metals are generally balanced with n-valence M cations, but they can be able to change. Zeolites can store considerable amounts of hydrogen, thanks to their high surface area, except that they are widely used in the petrochemical shredding and water softening as ion exchangers. It is possible to produce zeolites constituting from more than 150 different types of tetrahedral Si(Al)O4 with different components.
4.6.3 Metal organic framework Metal organic frames (MOFs) are compounds containing of metal ions or the bundles of organic binders. MOFs have made great progress in recent years due to their potential applications such as the gas storage with physisorption, separation, catalysis, and medication.
Hydrogen storage
MOFs can be constructed as a building block with many different organic binders and metal bundles or metal ions (Fig. 4.15). Historical developments of MOFs are shown in Fig. 4.16.
4.6.4 Covalent organic framework Covalent organic frames are highly porous crystalline materials and have been formed by building blocks connected with very strong covalent bond together (Fig. 4.17). As being different from MOF, COFs do not contain metal ions or bundles, and similarly can store very high amounts of hydrogen at low temperatures.
Fig. 4.15 Crystal structures of few typical MOF materials.
Fig. 4.16 The development process of MOFs as hydrogen storage material
103
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Solar hydrogen production
COF growth
NH2-f-SiO2
Core etching
NH2-f-SiO2 COF
COF hollow sphere
Fig. 4.17 The covalent organic framework structure.
Fig. 4.18 The hydrogen storage in micro glass spheres [23].
4.7 Glass microspheres In this method, small, hollow glass spheres with diameters ranging from 25 to 500 μm, and the wall thicknesses of 1 μm are used in the hydrogen storage. The glass microspheres shown in Fig. 4.18 are filled with the hydrogen gas at high pressure and high temperatures such as 200°C–400°C. At high temperature, the glass walls become permeable and the gas fills into the microspheres. The hydrogen is trapped inside the spheres
Hydrogen storage
when the glass is cooled to the room temperature. It is provided to release the hydrogen gas by reheating the spheres when the hydrogen is to be used. The storage capacity of glass spheres is about 5%–6% at 20–49 MPa pressure [24].
4.8 Boron-based storage Sodium boron hydride-based storage is based on the use in the liquid form, unlike alanates and solid-state storage methods. Sodium borohydride in the form of the solution gives hydrogen according to the following reaction and turn into sodium metaborate. This transformation is shown in Fig. 4.19. NaBH4 ðsÞ + 2H2 O ! 4H2 + NaBO2 ðcatalyzerÞ As it can be seen in the equation, the amount of hydrogen occurred as result of the reaction is twice that of bonded hydrogen in the form of hydride. The equal amounts of hydrogen in the presence of NaBH4 are come up with water splitting. The reaction given in the equation is exothermic. One consequence of this is that the hydrogen obtained from the system is humid. The most important advantage of the hydrogen storage in sodium borohydride is that the stored hydrogen can be recovered at the room temperature and the recovery can be
Fuel pump
Hydrogen on demand catalyst chamber
NaBH4 fuel tank
NaBO2 return tank
Gas/liquid separator
H2
Borate
Hydrogen gas + steam
Heat exchanger coolant loop
Pure, humidified, hydrogen gas
Fig. 4.19 The sodium borohydride-based hydrogen storage [25].
Fuel cell or hydrogen engine
105
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Solar hydrogen production
easily controlled with the aid of a catalyst. As a matter of fact, the liquid solution is safe even in contact with flame, but hydrogen release is provided in case of contacting with the solution of catalyst. The biggest disadvantage of the system is the necessity of being recycled of occurring NaBO2 to NaBH4 in another environment.
4.9 The storage in underground The low cost and large-scale storage method of hydrogen gas is to store it in the underground. Two technologies are currently in use for storage of large amounts of gases. The exhausted oil or natural gas reservoirs is similar to the natural gas. In this storage type with a slightly higher cost is also to store the hydrogen in underground in the salt domes, mine quarries, that is, for instance, in the city of Kiel, Germany, the hydrogen has been storing in a cavern under 1330 m since 1971. However, 1%–3% of hydrogen stored in the caverns is lost per year due to the leak [26]. Over the last decades, there have been several examples of underground storage of pure hydrogen or syngas [27]: • England, Teesside, Yorkshire: The British company ICI has stored 1 million Nm3 of nearly pure hydrogen in three salt caverns at a depth of about 400 m. The caverns have operated successfully for many years, and they are now operated by SABIC. • France, Beynes, Ile de France: The gas company Gaz de France has stored a gas with 50%–60% hydrogen in an aquifer of 330 million Nm3 capacity for nearly 20 years. No gas losses or safety problems have been recorded. • Russia: Pure hydrogen was stored underground at 9 MPa for the needs of the aerospace industry. • Germany: 62% of H2 gas was stored in a salt cavern of 32,000 m3 at 8–10 MPa. • Czechoslovakia: 50% of H2 syngas was stored in an aquifer. The other method is storage in aquifers, that is, water carrying layers capped with impermeable layers above and preferably also below the layer into which hydrogen may be pumped so as to replace water. Typically, there would be clay layers with a sand layer in between. The geometry of the sand layer and its contained water would have the form of a waving structure, with some bends that curve upward. These are the locations where a gas such as hydrogen may be pumped into the geological structure, thereby displacing water but still be confined due to the pressure of the water below, when the curvature is such that the gas cannot “run” away to the sides. Decisive parameters are the permeability of the water-carrying layer and of the enclosing clay layers, where the latter determines the leakage rate of the gas. Further, the integrity of the structure, for example, in terms of forming an unbroken upward bend, would determine the rate of losses to the sides. Finally, adsorption or other penetration of the gas, here hydrogen, into the water will have to be considered generally, in order to determine the maximum length of time, that the gas can be expected to remain in the store [28].
Hydrogen storage
4.10 Methanol Methanol is obtained from hydrogen and CO, is liquid under normal conditions, and has high hydrogen content; because of these properties, methanol can be considered as a suitable vehicle fuel. The principle of use states that hydrogen occurs by being broken down methanol and the resulting hydrogen is consumed as fuel. The energy loss in the fragmentation process is quite high, therefore the efficiency of the system is low. In order to overcome this negation, high-efficiency fuel cells that operate directly with methanol have been developed. According to studies conducted, a methanol-reformer fuel-driven vehicle emits 40%–70% more CO2 comparing to a similar petrol vehicle. Besides, there are also CO and hydrocarbon emissions. Methanol is toxic, water miscible, and very corrosive substance. Because of these features, more specific safety precautions are needed when compared to the petrol during the transportation and use. Its negative properties and high emissions prevent methanol from being a highly preferred fuel.
4.11 Petrol and other hydrocarbons These products can be considered as hydrogen storage systems since hydrogen can be produced by being passed of petroleum and other hydrocarbons through a reformer. However, an approximate 30 min is required to be produced hydrogen from petrol which means that it must be heated for 30 min before the vehicle is used, therefore the battery of the vehicle must be at least 30 min durability before moving. Due to the complex, difficult and costly nature of the reformer and battery connection, the obligation to remove hydrocarbons and CO which are formed beside hydrogen production, the possibility of CO to destroy reformer membranes, and the NOx compounds forming due to the necessity of operating at high temperatures, and therefore emissions problems, make the petrol and hydrocarbons move far away from to be ideal hydrogen carrier. Fig. 4.20 shows CO2 emissions measured at different speeds when compressed hydrogen, 300
Gasoline (from petrolium)
250 Methanol (from natural gas) CO2 (g/kg)
200 150 100 50
H2 (electrolyser)
0
Fig. 4.20 CO2 emissions in case of using different fuels as hydrogen storage in a petrol engine vehicle.
107
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Solar hydrogen production
methanol, and petrol are used as the hydrogen storage in a petrol engine vehicle. As it is seen, the lowest CO2 emission is obtained in case of using hydrogen produced via electrolysis. Current approaches for onboard hydrogen storage systems include pressurized hydrogen gas, liquid hydrogen, adsorbents (carbon nanotubes, MOF, COF materials, etc.), metal hydrides, and chemical hydrides. The advantages and disadvantages for each hydrogen storage methods are summarized in Table 4.5. • Storage of hydrogen as compressed gas at high pressures (up to 70 MPa) can be used to store small quantities of gas. Table 4.5 Advantages and disadvantages of different hydrogen storage approaches [29] H2 storage systems
Advantages
Disadvantages
Ongoing efforts
Compressed H2
Commercially available
Develop and design costeffective vessel/container
Liquid H2
Commercially available
Cryocompressed Metal hydride
High volumetric capacity Reversible on-board
Sorbent and carbon-based materials
Reversible on-board
Low volumetric capacity High compression energy Heat management during charging required H2 loss Safety issue High liquefaction energy Heat management to reduce boil-off High compression/ liquefaction energy Low gravimetric/volumetric capacity Heat management during charging required High operating temperature for H2 release Low volumetric density
Loss of useable H2
Optimize material’s properties (pore size, pore volume, surface area, among others)
Chemical hydride
Good volumetric capacity Proper operating temperatures
Low operating temperature for H2 uptake Thermal management required Off-board regeneration
Improve kinetics of hydrogen adsorption/ desorption along with heat management
Increase dihydrogen binding energies.
Develop cost effective and energy efficient Regeneration methods for the spent material
Hydrogen storage
• Storage of hydrogen in liquid form can be used for storing large quantities of gas for longer times, however at high-energy penalties for liquefaction. • Metal hydrides may offer an advantage for storing small quantities of gas in the medium to long term.
References [1] International Energy Agency, World Energy Outlook, www.iea.org, 2015. (Accessed 16 February 2015). [2] J. Pottier, C. Bailleux, A gas of the past, present and future ‘A Drama In 3 Acts’, in: T.N. Veziroglu (Ed.), Hydrogen Energy Progress VI: Proceedings of the 6th World Hydrogen Energy Conference, Vienna, Austria, 1986. € [3] M. Altan, E. Y€ or€ ukog˘ulları, Hidrojen Zeolit Sisteminin Enerji Teknolojisindeki Onemi, in: D€ unya Enerji Konseyi T€ urk Milli Komitesi, T€ urkiye 7. Enerji Kongresi, 1997. [4] G. Barkhordarian, T. Klassen, R. Bormann, Catalytic mechanism of transition-metal compounds on Mg hydrogen sorption reaction, J. Phys. Chem. B 110 (22) (2006) 11020–11024. [5] A. Z€ uttel, Materials for hydrogen storage, Mater. Today 6 (9) (2003) 24–33. [6] Office of Energy Efficiency and Renewable Energy, Hydrogen Storage, https://energy.gov/eere/ fuelcells/hydrogen-storage, 2018. (Accessed 15 May 2018). [7] N. Sirosh, Hydrogen composite tank program, Rev. Lit. Arts Am. 2000 (2002) 1–7. [8] Online Source. Available: www.fuelcellstore.com. [9] The International Coalition of Hydrogen Associations, The History of Hydrogen, http://www.hpath. org/resources/Factsheet-History.pdf, 2016. (Accessed 15 May 2016). [10] Y. Sun, L. Wang, W.A. Amer, et al., Hydrogen storage in metal-organic frameworks, J. Inorg. Organomet. Polym. 23 (2013) 270. [11] Sigma-Aldrich, https://www.sigmaaldrich.com/technical-documents/articles/material-matters/ introducing-hydrnol.html, 2015. (Accessed 10 January 2015). [12] T.M. Flynn, A Liquification of Gases, McGraw-Hill Encyclopedia of Science & Technology, in: seventh ed., vol. 10, McGraw-Hill, New York, 1992, pp. 106–109. [13] T. Hottinen, Technical Review and Economic Aspects of Hydrogen Storage Technologies, Helsinki University of Technology, Helsinki, 2001. [14] T.J. Carter, L.A. Cornish, Hydrogen in Metals, Eng. Fail. Anal. 8 (2) (2001) 113–121. [15] P. Fischer, K. Yvon, Crystal and magnetic structures of ternary metal hydrides: a comprehensive review, in: Hydrogen in Intermetallic Compounds, 1988, pp. 87–138. [16] G. Sandrock, A panoramic overview of hydrogen storage alloys from a gas reaction point of view, J. Alloy. Comp. 293–295 (1999) 877–888. [17] N. Cuı, J.L. Luo, K.T. Chuang, Study of hydrogen diffusion in α- and β-phase hydrides of Mg2Ni alloy by microelectrode technique, J. Electroanal. Chem. 503 (2001) 92–98. [18] L. Schlapbach, Hydrogen in Intermetallic Compounds I, Springer, Germany, 1988. [19] G. Sandrock, S. Suda, L. Schlapbach, Hydrogen in Intermetallic Compounds II, Springer-Verlag, Berlin, Germany, 1992. [20] M. Hangstr€ om, Improved Metal Hydrides for Energy Applications Based on Gas-Solid Reactions, Helsinki University of Technology, Finland, 1999. [21] S. Lee, Y. Lee, Hydrogen storage in single-walled carbon nanotubes, Appl. Phys. Lett. 20 (2002) 2877–2899. [22] M. Hirscher, et al., Hydrogen storage in carbon nanostructures, J. Alloys Compd. 330–332 (2002) 654–658. [23] R. Teitel, Hydrogen Storage in Glass Microspheres, Brookhaven National Laboratories, Brookhaven, 1981. € [24] S. Kaya, Magnezyum Bor Hidr€ ur Sentezi ve Hidrojen C ¸ eviriminde Kullanımı, Gazi Universitesi Fen Bilimleri Enstit€ us€ u Y€ uksek Lisans Tezi, Ankara, 2005 (in Turkish).
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Solar hydrogen production
€ urk, Hidrojen depolama amacıyla magnezyum tozlarının € [25] M. G€ uvendiren, H.E. Unalan, T. Ozt€ o €g˘u €t€ ulmesinde katkı maddelerinin etkisi, in: Toz Metal€ urjisi Konferansı, 2002 (in Turkish). [26] Underground Gas Storage, https://hub.globalccsinstitute.com/publications/operating-flexibilitypower-plants-ccs/2-underground-hydrogen-storage, 2016. (Accessed 10 May 2016). [27] D.L. Katz, M.R. Tek, Overview of underground storage of natural gas, J. Pet. Geol. 33 (1981) 943–951. [28] B. Sorensen, Underground Hydrogen Storage in Geological Formations, and Comparison with Other Storage Solutions, in: Hydrogen Power Theoretical and Engineering International Symposium, Merida Technical University, 2007. [29] H.T. Hwang, A. Varma, Hydrogen storage for fuel cell vehicles, Curr. Opin. Chem. Eng. 5 (2014) 42–48.
Further reading [30] P. Selvam, B. Viswanathan, C.S. Swamy, V. Srinivasan, Magnesium and magnesium alloy hydrides, Int. J. Hydrogen Energy 11 (3) (1986) 169–192. [31] Linde Group, Liquid Hydrogen Storage, www.linde-gas.com, 2010. (Accessed 16 May 2010). [32] E. Shimizu, K. Aoki, T. Masumoto, Hydrogen absorption properties of amorphous and crystalline alloys in the pseudobinary ZrCo-TiNi system, J. Alloys Compd. 293 (1999) 526–530. [33] Online Source: https://www.sigmaaldrich.com/technical-documents/articles/material-matters/ introducing-hydrnol.html. [34] M.T. Syed, et al., An economic analysis of three hydrogen liquefaction systems, Int. J. Hydrogen Energy 23 (7) (1998) 565–576.