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Thermodynamics analysis of hydrogen storage based on compressed gaseous hydrogen, liquid hydrogen and cryo-compressed hydrogen
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 4 ( 2 0 1 9 ) 1 6 8 3 3 e1 6 8 4 0
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Thermodynamics analysis of hydrogen storage based on compressed gaseous hydrogen, liquid hydrogen and cryo-compressed hydrogen Zhao Yanxing a, Gong Maoqiong a,b,*, Zhou Yuan a,**, Dong Xueqiang a, Shen Jun a,b a
Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100039, China
article info
abstract
Article history:
Safe, reliable, and economic hydrogen storage is a bottleneck for large-scale hydrogen
Received 12 January 2019
utilization. In this paper, hydrogen storage methods based on the ambient temperature
Received in revised form
compressed gaseous hydrogen (CGH2), liquid hydrogen (LH2) and cryo-compressed
16 April 2019
hydrogen (CcH2) are analyzed. There exists the optimal states, defined by temperature
Accepted 19 April 2019
and pressure, for hydrogen storage in CcH2 method. The ratio of the hydrogen density
Available online 15 May 2019
obtained to the electrical energy consumed exhibits a maximum value at the pressures above 15 MPa. The electrical energy consumed consists of compression and cooling down
Keywords:
processes from 0.1 MPa at 300 K to the optimal states. The recommended parameters for
Hydrogen storage
hydrogen storage are at 35e110 K and 5e70 MPa regardless of ortho-to parahydrogen
Cryo-compressed hydrogen (CcH2)
conversion. The corresponding hydrogen density at the optimal states range from 60.0 to
Liquid hydrogen (LH2)
71.5 kg m3 and the ratio of the hydrogen density obtained to the electrical energy
Ambient temperature compressed
consumed ranges from 1.50 to 2.30 kg m3 kW1. While the ortho-to para-hydrogen con-
gaseous hydrogen (CGH2)
version is considered, the optimal states move to a slightly higher temperatures comparing
Power consumption
to calculations without ortho-to para-hydrogen conversion. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is considered the next generation energy carrier with high calorific value (per kilo) and zero-pollution. Exploitation of hydrogen energy consists of production, storage and utilization. Production and utilization of hydrogen are mature technologies. Large-scale production of hydrogen is currently
achieved thermally, electro- or photolytically [1e3] using variety of raw materials such as coal, natural gas, biomass, boron hydrides, hydrogen sulfide, water, etc. Hydrogen utilization mainly includes hydrogen-based fuel cells and internal combustion engines. The latter is a very mature technology, and its energy utilization efficiency is much higher than that for gasoline engines [4]. Usage of hydrogen-powered fuel cells is currently the best way to utilize hydrogen, and it is believed
* Corresponding author. Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. ** Corresponding author. E-mail addresses: [email protected] (G. Maoqiong), [email protected] (Z. Yuan). https://doi.org/10.1016/j.ijhydene.2019.04.207 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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to be wildly applied in next-generation vehicles. Significant progress in this area have been achieved in Japan [5,6]. Unfortunately, hydrogen is the lightest substance in nature. Its density is only 0.081 kg/m3 at 300 K and 0.1 MPa, which is ten thousand times less than that of water. Moreover the liquefaction of hydrogen with a normal boiling point of ~20 K has a Carnot efficiency of 0.073 and so the liquefaction requires a lot of energy. Thus, highly reliable, economically feasible and safe hydrogen storage technology prevents largescale hydrogen utilization. This problem needs a near-term solution. Various hydrogen storage technologies are at different stages of their development. However, none of commercial products meet all the requirements set by the US Department of Energy (DOE) in 2017 [7]. Five main factors (such as gravimetric and volumetric capacities, operating temperature, cycle life and system fill time) need to be taken into account to develop the most efficient hydrogen storage system [8]. Hydrogen storage materials are required to have at least 5.5 wt % of hydrogen. Currently, the compressed gaseous hydrogen (including ambient temperature compressed gaseous hydrogen, CGH2 and cryo-compressed hydrogen, CcH2), liquid hydrogen (LH2) and material-based hydrogen storage (MH2, such as metal hydrides) are three primary storage methods [9]. The CGH2 and LH2 have been commercialized, while the CcH2 developed by the automobile company BMW [10] is a promising method and the MH2 attracts attention since it operates at low pressure.
Material-based storage In material-based storage (MH2), hydrogen atoms or molecules are tightly bound with other elements either by physisorption and/or chemisorption. Physisorption involves adsorption of hydrogen atoms or molecules on to the surface of, e.g., nanomaterials [11] such as carbon nanotubes and metal organic framework. Less than 1 wt% of hydrogen adsorption capacities were reported for porous materials at ambient conditions [12]. Chemisorption involves absorption of hydrogen molecules and their disintegration into hydrogen atoms followed by their incorporation into the material lattice. Examples of such materials are metal hydrides, liquid organic hydrogen carriers [13,14], etc. When absorbed, hydrogen can potentially be stored at high density and low pressure, which is safer comparing to CGH2 and LH2. However, hydrogen must be released at high temperatures or low external pressures [15]. These processes often have slow reaction kinetics, low reversibility and high dehydrogenation temperatures. Thus, these weaknesses yet remain to be solved [16,17]. In fact, there are still no materials that can fulfil all of the target parameters for the hydrogen storage systems set by the US DOE [18,19].
Liquid hydrogen storage Liquid hydrogen storage (LH2) can achieve higher density than CGH2. Usually, hydrogen is liquefied at ~20 K at atmospheric pressure or higher. Both volumetric and gravimetric capacities
of LH2 are satisfactory. LH2 is an ideal method, however, it consumes a lot of electricity during the liquefaction stage. Theoretically, it requires about 4e10 kWh to produce 1 kg of liquid hydrogen [20]. This accounts for over 30% of the combustion energy stored in hydrogen. This percentage will be even bigger during practical applications. The boil-off characteristic is another unfavorable factor that will further reduce LH2 efficiency [21]. Due to the unavoidable heat influx into the storage vessels, ~2e3% of evaporated hydrogen will be lost per day [19]. Thus, LH2 is more preferred for the high-tech industries, which are concerned more with performance than cost (like aerospace industry).
Compressed gaseous hydrogen storage Ambient temperature compressed gaseous hydrogen storage (CGH2) is the most mature technology widely adopted in variety of practical application. In 2010, over 80% of the total 215 operating hydrogen refueling stations worldwide adopted CGH2 method [22]. CGH2 is simple, provides fast filling/ releasing rate and has low costs. However, the biggest weakness of CGH2 is its low volumetric density, which makes CGH2 less popular in practice. Current on-board hydrogen storage tanks have operational pressures up to 70 MPa but they provide hydrogen density of only 39.1 kg/m3. Volumetric density does not increase proportionally to the pressure increase, which makes it extremely difficult to increase volumetric density by pressurization only. Additionally, high pressures present a serious safety problem [20]. Sun et al. [23] suggested that 50e55 MPa storage pressures will provide the most optimized trade-off with cost effectiveness. Recently, supercritical cryo-compressed hydrogen storage (CcH2) has been proposed by BMW [10] company and developed by many others [24,25]. Their concept consists of storing hydrogen in a pressure vessel that can operate at cryogenic temperatures (as low as 20 K) and high pressures (e.g. ~35 MPa). CcH2 can store high-density hydrogen, similar to LH2, without evaporative losses in routine use. Since this method involves no liquefaction, power consumption is expected to be significantly reduced. In this paper, the thermodynamics analysis, including the storage density and power consumption, of hydrogen storage methods based on CGH2, LH2, CcLH2 and CcH2 is performed. The optimal parameter for hydrogen storage is expected to be put forward.
Modeling Schematics of the CcH2 system is shown in Fig. 1. Hydrogen will undergo compression, refrigeration and storage processes. Compression process consists of several compressing and cooling units, which produce high-pressure and nearroom temperature hydrogen. Then the hydrogen is cooled in a refrigerator or using, for example, liquid nitrogen, after which it is stored in specially designed containers. The thermodynamic properties of hydrogen are calculated by Refprop 9.1 [26]. It is considered the most accurate computation tool to obtain thermodynamic and transport properties of hydrogen
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Fig. 1 e Schematics of the cryo-compressed hydrogen storage (CcH2).
[27]. Uncertainty associated with density determination is 0.1% at temperatures from the triple point to 250 K and at pressures up to 40 MPa. Between 250 and 450 K and at 300 MPa uncertainty in density determination is 0.04%. Estimated uncertainty for heat capacity values is 1.0%. Estimated uncertainties of vapor pressures and saturated liquid densities calculated using the Maxwell criterion are 0.2% [28].
Results and discussion Analysis of multistage hydrogen compression process Isentropic exponent of hydrogen is 1.41, which will lead to a high discharge temperature even at a moderate pressure ratio of 4e8. Thus, multistage compression is preferred for practical applications. At the same time, the infinite multistage compression approaches isothermal compression, thereby decreases the energy consumption, but increases initial investment and system complexity. Therefore, it is essential to optimize the compression stage. Power is consumed by the compression unit mainly during compressing and cooling processes. In our calculation, adiabatic efficiency of the compressor was set to 70%. The inlet and outlet air temperatures of the air cooler were set to 298 and 310 K, respectively. The temperature of the hydrogen at the outlet of the air cooler was set to 300 K. Fig. 2 shows power consumption of the compressors and the fans during multistage hydrogen compression process. The mass flow rate of hydrogen is set to 5 kg/h and the initial and final pressures are set to 0.1 and 35 MPa, respectively. Two or more additional stages compression processes significantly decrease power consumption. However, power consumption does not change significantly when more stages exist in the process (for example six). The different in power consumption is ~3% when five and six compression stages are implemented. We selected a five stage compression process. Power consumption of such five-stage processes occupies 47% of that of single stage compression. In the calculation, the pressure ratio of each compression stage is set to the same in a certain multistage compression processes.
Analysis of hydrogen cooling Cooling load of hydrogen above the critical temperature is almost linear (see Fig. 3). To reduce energy consumption during cooling process, it is essential to match the cooling temperature and thermal load of hydrogen. Ideally, these two parameters must demonstrate constant temperature difference. Usually, the temperature difference should be neither too big nor too small. At the same time, exergy efficient of the cooling system and pressure loss of the heat transferring media need to be considered as well. However, in practice, it is hard to achieve small temperature difference just using one single refrigerator. Thus, multi-temperature cooling process is generally preferred for this purpose (such as cascade refrigeration cycle used in natural gas liquefiers, which is achieved by multi-stage pure refrigerant cycling [29]). Mixed refrigerant cycle [30], Reverse Brayton cycle [31] and Claude cycle [32] could maintain relatively small heat transfer temperature difference between the cold and warm media. Therefore, it is possible to achieve near constant and small temperature difference between the cooling fluid and the hydrogen. Under
Fig. 2 e Power consumption of the compressors and the fans during multistage hydrogen compression and air cooling processes. X-axis shows the number of stages.
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P¼
1 x
Q Troom 1 Troom 1 dq ¼ lnðKq þ TR Þ q x TR þ Kq K 0
ZQ 0
1 Troom 1 Troom lnðKQ þ TR Þ Q lnðTR Þ ¼ x x K K 1 Troom KQ þ TR Q Troom TR ln ln Q ¼ 1 ¼ x x TR Tobj K TR Tobj Q Troom Troom DT 1 ln ¼ x Troom TH2min TH2min DT
(3)
where DT is the temperature difference between the cooling medium and the hydrogen.
Analysis of density and power consumption of CcH2 Fig. 3 e Cool down of hydrogen (a) 0.4 MPa and (b) at 70 MPa.
this assumption, the smallest power consumption is calculated by the following equation (using data from Fig. 4). ZQ P¼ 0
dq ¼ hq
ZQ 0
1 x
Tevap Troom Tevap
dq ¼
ZQ 1 Troom 1 dq x Tevap
(1)
0
where hq is the refrigeration efficiency, dq is the thermal load under constant temperature, Q is the total thermal load, Tevap is the cooling temperature, Troom is the room temperature, and x is the relative Carnot efficiency. Considering typical practical refrigeration efficiency, x is set to 0.3 for the whole temperature zone. Since the cooling load of hydrogen is almost linear as function of the increasing temperature, it can be represented by: Tevap ¼ TR þ
Tobj TR q ¼ TR þ Kq Q
(2)
where Tobj and TR represent the maximum and minimum cooling temperature, respectively. Combining Eqs. (1) and (2), power consumption of hydrogen cooling is as follows:
Fig. 4 e Cool down of hydrogen: sensible heat þ latent heat.
In this work we analyzed densities of gaseous hydrogen at 5e70 MPa and at 35e300 K and of liquid hydrogen at 0.1e20 MPa and 20e26 K (see Fig. 5). Even at 70 MPa and 300 K, the density of hydrogen is only 39.1 kg/m3. Apparently only by pressurization, it is very hard to achieve high density of hydrogen. Liquid hydrogen has a relatively high density: 71.0 kg/m3 at 20 K and 0.4 MPa. However, gaseous hydrogen above 15 MPa can have higher density than liquid hydrogen. However, simply pursuing high density can be uneconomic, especially for large-scale or long-term storage situations. Power consumption to achieve high density of hydrogen must be considered. The ratio of the hydrogen density obtained to the electrical energy consumed consisting of compression and cooling down processes from 0.1 MPa at 300 K to the optimal states is defined, that is j ¼ hydrogen density/power consumption, as shown in Fig. 6. In this section, the ortho-to para-hydrogen enthalpy of conversion is ignored but it will be calculated in the next section. At room temperature, j increases as hydrogen pressure increases at 5e70 MPa (see Fig. 6). Thus, it is a good choice to increase the pressure for storage of hydrogen at room temperature from both density and power consumption points. Since the coefficient of performance (COP) of the refrigerator declines sharply at very low temperatures, it is ineffective to liquefy hydrogen at any pressure. Index j of liquid hydrogen is noticeable lower than that of most of CcH2 because of the
Fig. 5 e Isobaric densities of hydrogen at various temperatures.
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Fig. 6 e Density of hydrogen per total power consumption at different temperatures and pressures.
latent heat. Although the latent heat accounts for about 10% of the total cooling heat, the isothermal condensation process makes this part of refrigeration power noticeable (see Fig. 3). The latent heat consumes 41% of the total cooling power. That is also why CcH2 has much more efficient electricity consumption than LH2. A maximum value of j exists at each pressure except for 5 and 10 MPa. Thus, an optimal temperature for hydrogen storage is above 15 MPa, and the bigger the pressure, the bigger the optimal storage temperature is. CGH2 at 70 MPa has very efficient power consumption, which also helps to achieve higher j value comparing to LH2. CcH2 is even more efficient at 20 MPa, and has higher j value than CGH2 at 70 MPa. The total power consumption per total combustion energy stored in the hydrogen at different temperatures and pressures is also interesting index (see Fig. 7). Index u represents ratio between total power consumption and heat of combustion of hydrogen. Power consumed during hydrogen storage during LH2 process corresponds to ~35% of the energy stored in this hydrogen. CcH2 method shows good performance as it consumes only 25% of the total energy, stored in the hydrogen. CcH2 at 80e160 K need to be analyzed in detail. This temperature zone is wildly used in Liquefied Natural Gas and Air
Fig. 7 e Total power consumption per heat of combustion of hydrogen at different temperatures and pressures.
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Separation industries. Thus, mature, reliable and highefficient refrigeration technologies are already available, such as mixed refrigerant cycle, cascade refrigeration cycle, and nitrogen Reverse Brayton cycle, etc. Additionally, liquid nitrogen cooling is also an efficient way, because of the small temperature difference between nitrogen and hydrogen. In the future, large-scale refrigeration based on pulse tube refrigerator will be also available. Therefore, it is important to optimize CcH2 at this temperature zone to obtain high hydrogen density as it can be then easily transferred to these mature existing technologies. Recommended CcH2 parameters for this temperature range together with those for CGH2 and LH2 are shown in Table 1. At 70 MPa, CGH2 has the best u and j values because power consumption is the lowest. However, intensity is also low. Densities of the recommended parameters in cases 1e14 vary from 60.0 to 71.5 kg/m3; j values vary from 1.50 to 2.30. Cases 4 to 14 achieve better densities and j values than CGH2 at 70 MPa. Although densities of hydrogen gas of these cases are slightly lower than those for LH2, their j values are much better than LH2. In the case of fuel cell applications, hydrogen storage in cars and in hydrogen refilling stations need to be seriously considered as well. The biggest challenge in these applications may be the heat leakage problem. Heat leakage problem has a more serious effect on CcH2 than on CGH2 and LH2. For example, if the temperature is 80 K (such as in case 7), the pressure will rise from 35 up to 42 MPa. Fortunately, in these applications, the hydrogen will be consumed quickly. Consequently, the temperature increase due to unexpected heat leakage should be easily offset. Additionally, thermal storage devices could be added to CcH2 to decrease the heat leakage. In some cases (such as cases 8e14), CcH2 can be put into a liquid nitrogen Dewar. The heat leakage will have no effect on the hydrogen because the heat from the hydrogen will be transferred to the liquid nitrogen.
Consideration of ortho-to parahydrogen conversion Parahydrogen (or orthohydrogen) content in gaseous hydrogen is a function of an equilibrium temperature. Parahydrogen concentration at equilibrium, xpH2, as function of temperature, T (K), as well as normal-hydrogen to equilibrium-hydrogen enthalpy of conversion in the hypothetical state of ideal gas, DHN-E, are shown in Fig. 8 xpH2 values at 300 and 20 K are ~25% and 99.8%, respectively, which implies noticeable enthalpy of conversion especially at very low temperatures. According to Valenti [33], enthalpy of conversion in the hypothetical state of ideal gas can be well approximated as conversion in the real state at any pressure. DHN-E and the enthalpy of vaporization of parahydrogen at 20 K is ~532 and 447 kJ/kg, respectively. For a long-term storage, the process from normal-hydrogen to equilibriumhydrogen should be completely finished, otherwise the temperature will increase. Enthalpy of conversion will significantly increases cooling efforts required for efficient hydrogen storage. Catalytic conversion of orthohydrogen to parahydrogen is needed for a quick conversion and can be executed in a batch mode or in a continuous mode. Continuous mode, in which
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Table 1 e Parameters proposed for storage of hydrogen (case 1e14) without consideration of ortho-to parahydrogen conversion.
T (K) p (MPa) r (kg$m3) j (kg$m3$kW1) u (%)
T (K) p (MPa) r (kg$m3) j (kg$m3$kW1) u (%)
case1
case2
case3
case4
case5
case6
case7
case8
35 5 60.0 1.50 20.3
35 10 68.2 1.60 21.6
40 15 69.3 1.68 20.9
50 20 66.8 1.75 19.3
60 25 65.0 1.83 18.1
60 30 68.9 1.90 18.4
70 35 67.3 1.96 17.4
80 40 66.1 2.02 16.6
case9
case10
case11
case12
case13
case14
case15
case16
80 45 69.0 2.08 16.8
90 50 67.9 2.13 16.2
90 55 70.4 2.18 16.4
100 60 69.3 2.22 15.8
100 65 71.5 2.26 16.0
110 70 70.5 2.30 15.5
300 70 39.3 1.71 11.7
20 0.4 71.7 1.01 36.0
Fig. 8 e Parahydrogen concentration at equilibrium, xpH2, as a function of temperature, T (K), and normal-hydrogen to equilibrium-hydrogen enthalpy of conversion in the hypothetical ideal gas state, DHN-E, as a function of temperature, T (K), defined with respect to the universal gas constant R [33].
the catalyst is placed within the heat exchangers, is more efficient because the energy released by the conversion itself is extracted at the highest possible temperature of the liquefaction. In this study, the liquefaction work caused by
Fig. 9 e Density of hydrogen per total power consumption at different temperatures and pressures.
catalytic conversion is calculated through a continuous conversion according to the following equation: PðTa KÞ ¼
DHTa DHTaþDT þ PðTa þ DT KÞ; PðTa ¼ 250 KÞ ¼ 0 Ta x Troom Ta
(4)
where DHTa is the DHN-E at equilibrium temperature Ta and x is the relative Carnot efficiency equal to 0.3. Consideration of ortho-to parahydrogen conversion with Eq. (4), Fig. 6 is translated to Fig. 9. Since the power consumption increases at low temperature results in j decrease,
Table 2 e Proposed parameters for storage of hydrogen (cases 1e14) considering ortho-to para-hydrogen conversion. Case 15 and case 16 represent CGH2 and LH2, respectively.
T (K) p (MPa) r (kg$m3) j (kg$m3$kW1) u (%)
T (K) p (MPa) r (kg$m3) j (kg$m3$kW1) u (%)
case1
case2
case3
case4
case5
case6
case7
case8
35 5 60.0 1.22 25.0
40 10 63.4 1.32 24.3
50 15 61.3 1.44 21.6
60 20 60.1 1.56 19.5
70 25 59.3 1.68 17.9
80 30 58.8 1.78 16.8
90 35 58.5 1.87 15.9
100 40 58.4 1.95 15.2
case9
case10
case11
case12
case13
case14
case15
case16
100 45 61.2 2.02 15.5
110 50 61.2 2.08 15.0
110 55 63.8 2.14 15.2
110 60 66.3 2.19 15.4
120 65 65.7 2.23 14.9
120 70 67.8 2.28 15.1
300 70 39.3 1.71 11.7
20 0.4 71.7 1.01 36.0
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the maximum point of j curve move to a slightly higher temperature, in comparison without considering of ortho-to parahydrogen conversion. The recommended CcH2 (as well CGH2 and LH2) parameters by consideration of ortho-to parahydrogen are presented in Table 2.
Conclusions Hydrogen storage methods based on the ambient temperature compressed gaseous hydrogen (CGH2), liquid hydrogen (LH2) and cryo-compressed hydrogen (CcH2) are analyzed. CcH2 method can achieve high storage density and low power consumption. Latent heat consumes ~41% of the total power, though it only accounts 10% of the total thermal load. This is the most important reason why CcH2 is superior to LH2. Based on the thermodynamic calculations, it can be concluded that: (1) Maximum of j (the ratio of the hydrogen density obtained to the electrical energy consumed) exists at certain pressures and temperatures. When the ortho-to para-hydrogen conversion is considered, the maximum of j moves to a slightly higher temperatures comparing to calculations without ortho-to para-hydrogen conversion consideration. (2) Recommended temperature and pressure parameters for hydrogen storage regardless of ortho-to parahydrogen conversion are 35e110 K and 5e70 MPa. Corresponding hydrogen densities are 60.0e71.5 kg m3, and j ranges from 1.50 to 2.30 kg m3 kW1.
[5] [6]
[7]
[8]
[9] [10]
[11]
[12]
[13]
[14]
[15]
Acknowledgements This work is financially supported by the National Key R&D Program of China (No. 2018YFB0904400), the National Natural Sciences Foundation of China (No. 51876215), and the Key Research Program of Frontier Sciences, CAS (No. QYZDY-SSWJSC028).
[16]
[17]
[18]
[19]
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.04.207.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Thermodynamics analysis of hydrogen storage based on compressed gaseous hydrogen, liquid hydrogen and cryo-compressed hydrogen Zhao Yanxing a, Gong Maoqiong a,b,*, Zhou Yuan a,**, Dong Xueqiang a, Shen Jun a,b a
Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China b University of Chinese Academy of Sciences, Beijing 100039, China
article info
abstract
Article history:
Safe, reliable, and economic hydrogen storage is a bottleneck for large-scale hydrogen
Received 12 January 2019
utilization. In this paper, hydrogen storage methods based on the ambient temperature
Received in revised form
compressed gaseous hydrogen (CGH2), liquid hydrogen (LH2) and cryo-compressed
16 April 2019
hydrogen (CcH2) are analyzed. There exists the optimal states, defined by temperature
Accepted 19 April 2019
and pressure, for hydrogen storage in CcH2 method. The ratio of the hydrogen density
Available online 15 May 2019
obtained to the electrical energy consumed exhibits a maximum value at the pressures above 15 MPa. The electrical energy consumed consists of compression and cooling down
Keywords:
processes from 0.1 MPa at 300 K to the optimal states. The recommended parameters for
Hydrogen storage
hydrogen storage are at 35e110 K and 5e70 MPa regardless of ortho-to parahydrogen
Cryo-compressed hydrogen (CcH2)
conversion. The corresponding hydrogen density at the optimal states range from 60.0 to
Liquid hydrogen (LH2)
71.5 kg m3 and the ratio of the hydrogen density obtained to the electrical energy
Ambient temperature compressed
consumed ranges from 1.50 to 2.30 kg m3 kW1. While the ortho-to para-hydrogen con-
gaseous hydrogen (CGH2)
version is considered, the optimal states move to a slightly higher temperatures comparing
Power consumption
to calculations without ortho-to para-hydrogen conversion. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen is considered the next generation energy carrier with high calorific value (per kilo) and zero-pollution. Exploitation of hydrogen energy consists of production, storage and utilization. Production and utilization of hydrogen are mature technologies. Large-scale production of hydrogen is currently
achieved thermally, electro- or photolytically [1e3] using variety of raw materials such as coal, natural gas, biomass, boron hydrides, hydrogen sulfide, water, etc. Hydrogen utilization mainly includes hydrogen-based fuel cells and internal combustion engines. The latter is a very mature technology, and its energy utilization efficiency is much higher than that for gasoline engines [4]. Usage of hydrogen-powered fuel cells is currently the best way to utilize hydrogen, and it is believed
* Corresponding author. Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China. ** Corresponding author. E-mail addresses: [email protected] (G. Maoqiong), [email protected] (Z. Yuan). https://doi.org/10.1016/j.ijhydene.2019.04.207 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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to be wildly applied in next-generation vehicles. Significant progress in this area have been achieved in Japan [5,6]. Unfortunately, hydrogen is the lightest substance in nature. Its density is only 0.081 kg/m3 at 300 K and 0.1 MPa, which is ten thousand times less than that of water. Moreover the liquefaction of hydrogen with a normal boiling point of ~20 K has a Carnot efficiency of 0.073 and so the liquefaction requires a lot of energy. Thus, highly reliable, economically feasible and safe hydrogen storage technology prevents largescale hydrogen utilization. This problem needs a near-term solution. Various hydrogen storage technologies are at different stages of their development. However, none of commercial products meet all the requirements set by the US Department of Energy (DOE) in 2017 [7]. Five main factors (such as gravimetric and volumetric capacities, operating temperature, cycle life and system fill time) need to be taken into account to develop the most efficient hydrogen storage system [8]. Hydrogen storage materials are required to have at least 5.5 wt % of hydrogen. Currently, the compressed gaseous hydrogen (including ambient temperature compressed gaseous hydrogen, CGH2 and cryo-compressed hydrogen, CcH2), liquid hydrogen (LH2) and material-based hydrogen storage (MH2, such as metal hydrides) are three primary storage methods [9]. The CGH2 and LH2 have been commercialized, while the CcH2 developed by the automobile company BMW [10] is a promising method and the MH2 attracts attention since it operates at low pressure.
Material-based storage In material-based storage (MH2), hydrogen atoms or molecules are tightly bound with other elements either by physisorption and/or chemisorption. Physisorption involves adsorption of hydrogen atoms or molecules on to the surface of, e.g., nanomaterials [11] such as carbon nanotubes and metal organic framework. Less than 1 wt% of hydrogen adsorption capacities were reported for porous materials at ambient conditions [12]. Chemisorption involves absorption of hydrogen molecules and their disintegration into hydrogen atoms followed by their incorporation into the material lattice. Examples of such materials are metal hydrides, liquid organic hydrogen carriers [13,14], etc. When absorbed, hydrogen can potentially be stored at high density and low pressure, which is safer comparing to CGH2 and LH2. However, hydrogen must be released at high temperatures or low external pressures [15]. These processes often have slow reaction kinetics, low reversibility and high dehydrogenation temperatures. Thus, these weaknesses yet remain to be solved [16,17]. In fact, there are still no materials that can fulfil all of the target parameters for the hydrogen storage systems set by the US DOE [18,19].
Liquid hydrogen storage Liquid hydrogen storage (LH2) can achieve higher density than CGH2. Usually, hydrogen is liquefied at ~20 K at atmospheric pressure or higher. Both volumetric and gravimetric capacities
of LH2 are satisfactory. LH2 is an ideal method, however, it consumes a lot of electricity during the liquefaction stage. Theoretically, it requires about 4e10 kWh to produce 1 kg of liquid hydrogen [20]. This accounts for over 30% of the combustion energy stored in hydrogen. This percentage will be even bigger during practical applications. The boil-off characteristic is another unfavorable factor that will further reduce LH2 efficiency [21]. Due to the unavoidable heat influx into the storage vessels, ~2e3% of evaporated hydrogen will be lost per day [19]. Thus, LH2 is more preferred for the high-tech industries, which are concerned more with performance than cost (like aerospace industry).
Compressed gaseous hydrogen storage Ambient temperature compressed gaseous hydrogen storage (CGH2) is the most mature technology widely adopted in variety of practical application. In 2010, over 80% of the total 215 operating hydrogen refueling stations worldwide adopted CGH2 method [22]. CGH2 is simple, provides fast filling/ releasing rate and has low costs. However, the biggest weakness of CGH2 is its low volumetric density, which makes CGH2 less popular in practice. Current on-board hydrogen storage tanks have operational pressures up to 70 MPa but they provide hydrogen density of only 39.1 kg/m3. Volumetric density does not increase proportionally to the pressure increase, which makes it extremely difficult to increase volumetric density by pressurization only. Additionally, high pressures present a serious safety problem [20]. Sun et al. [23] suggested that 50e55 MPa storage pressures will provide the most optimized trade-off with cost effectiveness. Recently, supercritical cryo-compressed hydrogen storage (CcH2) has been proposed by BMW [10] company and developed by many others [24,25]. Their concept consists of storing hydrogen in a pressure vessel that can operate at cryogenic temperatures (as low as 20 K) and high pressures (e.g. ~35 MPa). CcH2 can store high-density hydrogen, similar to LH2, without evaporative losses in routine use. Since this method involves no liquefaction, power consumption is expected to be significantly reduced. In this paper, the thermodynamics analysis, including the storage density and power consumption, of hydrogen storage methods based on CGH2, LH2, CcLH2 and CcH2 is performed. The optimal parameter for hydrogen storage is expected to be put forward.
Modeling Schematics of the CcH2 system is shown in Fig. 1. Hydrogen will undergo compression, refrigeration and storage processes. Compression process consists of several compressing and cooling units, which produce high-pressure and nearroom temperature hydrogen. Then the hydrogen is cooled in a refrigerator or using, for example, liquid nitrogen, after which it is stored in specially designed containers. The thermodynamic properties of hydrogen are calculated by Refprop 9.1 [26]. It is considered the most accurate computation tool to obtain thermodynamic and transport properties of hydrogen
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Fig. 1 e Schematics of the cryo-compressed hydrogen storage (CcH2).
[27]. Uncertainty associated with density determination is 0.1% at temperatures from the triple point to 250 K and at pressures up to 40 MPa. Between 250 and 450 K and at 300 MPa uncertainty in density determination is 0.04%. Estimated uncertainty for heat capacity values is 1.0%. Estimated uncertainties of vapor pressures and saturated liquid densities calculated using the Maxwell criterion are 0.2% [28].
Results and discussion Analysis of multistage hydrogen compression process Isentropic exponent of hydrogen is 1.41, which will lead to a high discharge temperature even at a moderate pressure ratio of 4e8. Thus, multistage compression is preferred for practical applications. At the same time, the infinite multistage compression approaches isothermal compression, thereby decreases the energy consumption, but increases initial investment and system complexity. Therefore, it is essential to optimize the compression stage. Power is consumed by the compression unit mainly during compressing and cooling processes. In our calculation, adiabatic efficiency of the compressor was set to 70%. The inlet and outlet air temperatures of the air cooler were set to 298 and 310 K, respectively. The temperature of the hydrogen at the outlet of the air cooler was set to 300 K. Fig. 2 shows power consumption of the compressors and the fans during multistage hydrogen compression process. The mass flow rate of hydrogen is set to 5 kg/h and the initial and final pressures are set to 0.1 and 35 MPa, respectively. Two or more additional stages compression processes significantly decrease power consumption. However, power consumption does not change significantly when more stages exist in the process (for example six). The different in power consumption is ~3% when five and six compression stages are implemented. We selected a five stage compression process. Power consumption of such five-stage processes occupies 47% of that of single stage compression. In the calculation, the pressure ratio of each compression stage is set to the same in a certain multistage compression processes.
Analysis of hydrogen cooling Cooling load of hydrogen above the critical temperature is almost linear (see Fig. 3). To reduce energy consumption during cooling process, it is essential to match the cooling temperature and thermal load of hydrogen. Ideally, these two parameters must demonstrate constant temperature difference. Usually, the temperature difference should be neither too big nor too small. At the same time, exergy efficient of the cooling system and pressure loss of the heat transferring media need to be considered as well. However, in practice, it is hard to achieve small temperature difference just using one single refrigerator. Thus, multi-temperature cooling process is generally preferred for this purpose (such as cascade refrigeration cycle used in natural gas liquefiers, which is achieved by multi-stage pure refrigerant cycling [29]). Mixed refrigerant cycle [30], Reverse Brayton cycle [31] and Claude cycle [32] could maintain relatively small heat transfer temperature difference between the cold and warm media. Therefore, it is possible to achieve near constant and small temperature difference between the cooling fluid and the hydrogen. Under
Fig. 2 e Power consumption of the compressors and the fans during multistage hydrogen compression and air cooling processes. X-axis shows the number of stages.
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P¼
1 x
Q Troom 1 Troom 1 dq ¼ lnðKq þ TR Þ q x TR þ Kq K 0
ZQ 0
1 Troom 1 Troom lnðKQ þ TR Þ Q lnðTR Þ ¼ x x K K 1 Troom KQ þ TR Q Troom TR ln ln Q ¼ 1 ¼ x x TR Tobj K TR Tobj Q Troom Troom DT 1 ln ¼ x Troom TH2min TH2min DT
(3)
where DT is the temperature difference between the cooling medium and the hydrogen.
Analysis of density and power consumption of CcH2 Fig. 3 e Cool down of hydrogen (a) 0.4 MPa and (b) at 70 MPa.
this assumption, the smallest power consumption is calculated by the following equation (using data from Fig. 4). ZQ P¼ 0
dq ¼ hq
ZQ 0
1 x
Tevap Troom Tevap
dq ¼
ZQ 1 Troom 1 dq x Tevap
(1)
0
where hq is the refrigeration efficiency, dq is the thermal load under constant temperature, Q is the total thermal load, Tevap is the cooling temperature, Troom is the room temperature, and x is the relative Carnot efficiency. Considering typical practical refrigeration efficiency, x is set to 0.3 for the whole temperature zone. Since the cooling load of hydrogen is almost linear as function of the increasing temperature, it can be represented by: Tevap ¼ TR þ
Tobj TR q ¼ TR þ Kq Q
(2)
where Tobj and TR represent the maximum and minimum cooling temperature, respectively. Combining Eqs. (1) and (2), power consumption of hydrogen cooling is as follows:
Fig. 4 e Cool down of hydrogen: sensible heat þ latent heat.
In this work we analyzed densities of gaseous hydrogen at 5e70 MPa and at 35e300 K and of liquid hydrogen at 0.1e20 MPa and 20e26 K (see Fig. 5). Even at 70 MPa and 300 K, the density of hydrogen is only 39.1 kg/m3. Apparently only by pressurization, it is very hard to achieve high density of hydrogen. Liquid hydrogen has a relatively high density: 71.0 kg/m3 at 20 K and 0.4 MPa. However, gaseous hydrogen above 15 MPa can have higher density than liquid hydrogen. However, simply pursuing high density can be uneconomic, especially for large-scale or long-term storage situations. Power consumption to achieve high density of hydrogen must be considered. The ratio of the hydrogen density obtained to the electrical energy consumed consisting of compression and cooling down processes from 0.1 MPa at 300 K to the optimal states is defined, that is j ¼ hydrogen density/power consumption, as shown in Fig. 6. In this section, the ortho-to para-hydrogen enthalpy of conversion is ignored but it will be calculated in the next section. At room temperature, j increases as hydrogen pressure increases at 5e70 MPa (see Fig. 6). Thus, it is a good choice to increase the pressure for storage of hydrogen at room temperature from both density and power consumption points. Since the coefficient of performance (COP) of the refrigerator declines sharply at very low temperatures, it is ineffective to liquefy hydrogen at any pressure. Index j of liquid hydrogen is noticeable lower than that of most of CcH2 because of the
Fig. 5 e Isobaric densities of hydrogen at various temperatures.
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Fig. 6 e Density of hydrogen per total power consumption at different temperatures and pressures.
latent heat. Although the latent heat accounts for about 10% of the total cooling heat, the isothermal condensation process makes this part of refrigeration power noticeable (see Fig. 3). The latent heat consumes 41% of the total cooling power. That is also why CcH2 has much more efficient electricity consumption than LH2. A maximum value of j exists at each pressure except for 5 and 10 MPa. Thus, an optimal temperature for hydrogen storage is above 15 MPa, and the bigger the pressure, the bigger the optimal storage temperature is. CGH2 at 70 MPa has very efficient power consumption, which also helps to achieve higher j value comparing to LH2. CcH2 is even more efficient at 20 MPa, and has higher j value than CGH2 at 70 MPa. The total power consumption per total combustion energy stored in the hydrogen at different temperatures and pressures is also interesting index (see Fig. 7). Index u represents ratio between total power consumption and heat of combustion of hydrogen. Power consumed during hydrogen storage during LH2 process corresponds to ~35% of the energy stored in this hydrogen. CcH2 method shows good performance as it consumes only 25% of the total energy, stored in the hydrogen. CcH2 at 80e160 K need to be analyzed in detail. This temperature zone is wildly used in Liquefied Natural Gas and Air
Fig. 7 e Total power consumption per heat of combustion of hydrogen at different temperatures and pressures.
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Separation industries. Thus, mature, reliable and highefficient refrigeration technologies are already available, such as mixed refrigerant cycle, cascade refrigeration cycle, and nitrogen Reverse Brayton cycle, etc. Additionally, liquid nitrogen cooling is also an efficient way, because of the small temperature difference between nitrogen and hydrogen. In the future, large-scale refrigeration based on pulse tube refrigerator will be also available. Therefore, it is important to optimize CcH2 at this temperature zone to obtain high hydrogen density as it can be then easily transferred to these mature existing technologies. Recommended CcH2 parameters for this temperature range together with those for CGH2 and LH2 are shown in Table 1. At 70 MPa, CGH2 has the best u and j values because power consumption is the lowest. However, intensity is also low. Densities of the recommended parameters in cases 1e14 vary from 60.0 to 71.5 kg/m3; j values vary from 1.50 to 2.30. Cases 4 to 14 achieve better densities and j values than CGH2 at 70 MPa. Although densities of hydrogen gas of these cases are slightly lower than those for LH2, their j values are much better than LH2. In the case of fuel cell applications, hydrogen storage in cars and in hydrogen refilling stations need to be seriously considered as well. The biggest challenge in these applications may be the heat leakage problem. Heat leakage problem has a more serious effect on CcH2 than on CGH2 and LH2. For example, if the temperature is 80 K (such as in case 7), the pressure will rise from 35 up to 42 MPa. Fortunately, in these applications, the hydrogen will be consumed quickly. Consequently, the temperature increase due to unexpected heat leakage should be easily offset. Additionally, thermal storage devices could be added to CcH2 to decrease the heat leakage. In some cases (such as cases 8e14), CcH2 can be put into a liquid nitrogen Dewar. The heat leakage will have no effect on the hydrogen because the heat from the hydrogen will be transferred to the liquid nitrogen.
Consideration of ortho-to parahydrogen conversion Parahydrogen (or orthohydrogen) content in gaseous hydrogen is a function of an equilibrium temperature. Parahydrogen concentration at equilibrium, xpH2, as function of temperature, T (K), as well as normal-hydrogen to equilibrium-hydrogen enthalpy of conversion in the hypothetical state of ideal gas, DHN-E, are shown in Fig. 8 xpH2 values at 300 and 20 K are ~25% and 99.8%, respectively, which implies noticeable enthalpy of conversion especially at very low temperatures. According to Valenti [33], enthalpy of conversion in the hypothetical state of ideal gas can be well approximated as conversion in the real state at any pressure. DHN-E and the enthalpy of vaporization of parahydrogen at 20 K is ~532 and 447 kJ/kg, respectively. For a long-term storage, the process from normal-hydrogen to equilibriumhydrogen should be completely finished, otherwise the temperature will increase. Enthalpy of conversion will significantly increases cooling efforts required for efficient hydrogen storage. Catalytic conversion of orthohydrogen to parahydrogen is needed for a quick conversion and can be executed in a batch mode or in a continuous mode. Continuous mode, in which
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Table 1 e Parameters proposed for storage of hydrogen (case 1e14) without consideration of ortho-to parahydrogen conversion.
T (K) p (MPa) r (kg$m3) j (kg$m3$kW1) u (%)
T (K) p (MPa) r (kg$m3) j (kg$m3$kW1) u (%)
case1
case2
case3
case4
case5
case6
case7
case8
35 5 60.0 1.50 20.3
35 10 68.2 1.60 21.6
40 15 69.3 1.68 20.9
50 20 66.8 1.75 19.3
60 25 65.0 1.83 18.1
60 30 68.9 1.90 18.4
70 35 67.3 1.96 17.4
80 40 66.1 2.02 16.6
case9
case10
case11
case12
case13
case14
case15
case16
80 45 69.0 2.08 16.8
90 50 67.9 2.13 16.2
90 55 70.4 2.18 16.4
100 60 69.3 2.22 15.8
100 65 71.5 2.26 16.0
110 70 70.5 2.30 15.5
300 70 39.3 1.71 11.7
20 0.4 71.7 1.01 36.0
Fig. 8 e Parahydrogen concentration at equilibrium, xpH2, as a function of temperature, T (K), and normal-hydrogen to equilibrium-hydrogen enthalpy of conversion in the hypothetical ideal gas state, DHN-E, as a function of temperature, T (K), defined with respect to the universal gas constant R [33].
the catalyst is placed within the heat exchangers, is more efficient because the energy released by the conversion itself is extracted at the highest possible temperature of the liquefaction. In this study, the liquefaction work caused by
Fig. 9 e Density of hydrogen per total power consumption at different temperatures and pressures.
catalytic conversion is calculated through a continuous conversion according to the following equation: PðTa KÞ ¼
DHTa DHTaþDT þ PðTa þ DT KÞ; PðTa ¼ 250 KÞ ¼ 0 Ta x Troom Ta
(4)
where DHTa is the DHN-E at equilibrium temperature Ta and x is the relative Carnot efficiency equal to 0.3. Consideration of ortho-to parahydrogen conversion with Eq. (4), Fig. 6 is translated to Fig. 9. Since the power consumption increases at low temperature results in j decrease,
Table 2 e Proposed parameters for storage of hydrogen (cases 1e14) considering ortho-to para-hydrogen conversion. Case 15 and case 16 represent CGH2 and LH2, respectively.
T (K) p (MPa) r (kg$m3) j (kg$m3$kW1) u (%)
T (K) p (MPa) r (kg$m3) j (kg$m3$kW1) u (%)
case1
case2
case3
case4
case5
case6
case7
case8
35 5 60.0 1.22 25.0
40 10 63.4 1.32 24.3
50 15 61.3 1.44 21.6
60 20 60.1 1.56 19.5
70 25 59.3 1.68 17.9
80 30 58.8 1.78 16.8
90 35 58.5 1.87 15.9
100 40 58.4 1.95 15.2
case9
case10
case11
case12
case13
case14
case15
case16
100 45 61.2 2.02 15.5
110 50 61.2 2.08 15.0
110 55 63.8 2.14 15.2
110 60 66.3 2.19 15.4
120 65 65.7 2.23 14.9
120 70 67.8 2.28 15.1
300 70 39.3 1.71 11.7
20 0.4 71.7 1.01 36.0
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the maximum point of j curve move to a slightly higher temperature, in comparison without considering of ortho-to parahydrogen conversion. The recommended CcH2 (as well CGH2 and LH2) parameters by consideration of ortho-to parahydrogen are presented in Table 2.
Conclusions Hydrogen storage methods based on the ambient temperature compressed gaseous hydrogen (CGH2), liquid hydrogen (LH2) and cryo-compressed hydrogen (CcH2) are analyzed. CcH2 method can achieve high storage density and low power consumption. Latent heat consumes ~41% of the total power, though it only accounts 10% of the total thermal load. This is the most important reason why CcH2 is superior to LH2. Based on the thermodynamic calculations, it can be concluded that: (1) Maximum of j (the ratio of the hydrogen density obtained to the electrical energy consumed) exists at certain pressures and temperatures. When the ortho-to para-hydrogen conversion is considered, the maximum of j moves to a slightly higher temperatures comparing to calculations without ortho-to para-hydrogen conversion consideration. (2) Recommended temperature and pressure parameters for hydrogen storage regardless of ortho-to parahydrogen conversion are 35e110 K and 5e70 MPa. Corresponding hydrogen densities are 60.0e71.5 kg m3, and j ranges from 1.50 to 2.30 kg m3 kW1.
[5] [6]
[7]
[8]
[9] [10]
[11]
[12]
[13]
[14]
[15]
Acknowledgements This work is financially supported by the National Key R&D Program of China (No. 2018YFB0904400), the National Natural Sciences Foundation of China (No. 51876215), and the Key Research Program of Frontier Sciences, CAS (No. QYZDY-SSWJSC028).
[16]
[17]
[18]
[19]
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.04.207.
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