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Transient thermodynamic analysis of a novel integrated ammonia production, storage and hydrogen production system
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 8 2 1 4 e1 8 2 2 4
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Transient thermodynamic analysis of a novel integrated ammonia production, storage and hydrogen production system Maan Al-Zareer*, Ibrahim Dincer, Marc A. Rosen Clean Energy Research Laboratory, Department of Automotive, Mechanical and Manufacturing Engineering, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada
article info
abstract
Article history:
A transient thermodynamic analysis is reported of a novel chemical hydrogen storage
Received 20 October 2018
system using energy and exergy approaches. The hydrogen is stored chemically in
Received in revised form
ammonia using the proposed hydrogen storage system and recovered via the electro-
6 April 2019
chemical decomposition of ammonia through an ammonia electrolyzer. The proposed
Accepted 9 April 2019
hydrogen storage system is based on a novel subzero ammonia production reactor. A single
Available online 8 June 2019
stage refrigeration system maintains the ammonia production reactor at a temperature of 10 C. The energy and exergy efficiencies of the proposed system are 85.6% and 85.3%
Keywords:
respectively. The proposed system consumes 34.0 kJ of work through the process of storing
Ammonia production
1 mol of hydrogen and recovering it using the ammonia electrolyzer. The system is
Energy
simulated for filling 30,000 L of ammonia at a pressure of 5 bar, and the system was able to
Exergy
store 7500 kg of ammonia in a liquid state (1% vapor) in 1500 s. The system consumes
Chemical hydrogen storage
nearly 45.3 GJ of energy to store the 7500 kg of ammonia and to decompose it to reproduce
Efficiency
the stored hydrogen during the discharge phase. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Two of today's significant issues are global warming (climate change) and the air quality of cities, both of which affect our lives and the environment [1]. Hydrogen energy can help mitigate global warming and address air quality problems of cities. Hydrogen is a clean energy carrier that has a large impact if it is produced in sustainable, low-cost processes and in large capacities. Hydrogen and hydrogen-derived fuels can help facilitate the use of renewable energy sources, which can help reduce greenhouse gas emissions [2]. Hydrogen can be produced from various energy sources, including coal [3e5],
nuclear energy [6], even wastes such as waste tires [7], and renewable energy sources [8]. Hydrogen has a wide range of uses such as in petrochemical, agricultural (ammonia fertilizers), manufacturing, food processing, electronics, aerospace, plastics and many other industries. In Alberta, Canada large amounts of hydrogen are used to upgrade the bitumen in oil sands to synthetic crude oil while removing the impurities. A recent commercial example of a use of hydrogen is as a backup power system for communication towers for cell phones, through fuel cells [1]. Also, Hyundai and Audi have been pushing hydrogen fuel cell vehicles, where Hyundai next generation fuel cell SUV is coming soon to vehicle market “NEXO” [9]. Hydrogen is
* Corresponding author. E-mail addresses: [email protected] (M. Al-Zareer), [email protected] (I. Dincer), [email protected] (M.A. Rosen). https://doi.org/10.1016/j.ijhydene.2019.04.085 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature a COP ex ex _ Ex ERef F h h hf ho hamb k LHV _ m m _ N P Po Q_ q Re R Pr s T To Tbs t u
Dimensionless number given in Table 1 Coefficient of performance Specific exergy (kJ/kg) Specific molar exergy (kJ/mol) Exergy rate (kW) Electrical potential of one electrode relative to another electrode (kJ) Faraday's constant (96,485 C/mol) Specific enthalpy (kJ/kg) Specific molar enthalpy (kJ/mol) Specific heat of formation (kJ/kg) Specific enthalpy at the reference environment conditions (kJ/kg) Convection heat transfer coefficient (kW/m2.K) Thermal conductivity (kW/m.K) Lower heating value (kJ/mol) Mass flow rate (kg/s) Mass (kg) Mole flow rate (mol/s) Pressure (kPa) Reference environment pressure (kPa) Heat rate (kW) Specific thermal energy (kJ/kg) Reynolds number Ideal gas constant Prandtl number Specific entropy (kJ/kg K) Temperature (oC) Reference environment temperature (oC) Temperature of boundary surface where heat transfer takes place (oC) Time (s) Specific internal energy (kJ/kg)
expected to become important as an energy carrier and as an energy storage medium. Hydrogen is advantageous as an energy storage medium since it has one of the highest energy densities (on a mass basis) and it can be produced from electrical energy via electrolysis as well as from other energy sources [10]. Also, hydrogen is a unique energy carrier because it can be easily converted to other forms of energy and can be used to produce hydrogen-derived fuels such as ammonia and methanol. Dincer and Rosen [11] reported that hydrogen as an energy currency has the significant market potential, that is hydrogen then can be used for electricity generation via fuel cells, as a chemical fuel for combustion, and as a chemical feedstock for the production of other chemical fuels such as ammonia and methanol. The energy stored in the production of hydrogen (for chemical energy storage) can easily be recovered in various forms, such as thermal energy through combustion or electrical energy using fuel cells. One of the main potential users for hydrogen energy is the transportation sector. One of the fuels that can be produced from hydrogen (ammonia) has been the subject of extensive research [12e14]. Ezzat and Dincer [14] proposed a vehicle
v∞ vi ve _ W w x
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Upstream velocity (m/s) Stoichiometric number for species i Number of electrons Work rate (kW) Specific work (kJ/kg) Mass vapor fraction
Greek letters h Energy efficiency j Exergy efficiency r Density (kg/m3) m Dynamic viscosity (Pa s) Subscripts bs Boundary where heat transfer occurs d Destruction e Electrical is Isentropic in Input (flowing into system boundary) max Maximum net Net out Output (flowing out of system boundary) o Reference environment conditions Q_ Heat flow rate ST Steam turbine tank Ammonia tank W Work _ W Work rate Acronyms API American Petroleum Institute PSAPS Pressurized subzero-cooled ammonia production system RKS Redlich-Kwong-Soave
power system that is hydrogen fueled, and showed that an energy efficiency up to 40% and an exergy efficiency up to 56% can be achieved. But the use of hydrogen in vehicles is not limited to fuel cells; hydrogen can also be used as a combustion fuel for internal combustion engines [15]. The transportation sector is responsible for a significant part of the total greenhouse gas emissions in Canada [16]. Running vehicles on hydrogen produced through sustainable technologies can mitigate greenhouse gas emissions and urban air pollution. Currently available electrical and hybrid vehicles contribute to reducing this problem but, due to the challenges that electrical batteries are facing such as limited storage capacity and reduced performance after a number of charging and discharging cycles, hydrogen is expected to have a significant role as a vehicle fuel in the future. One of the problems slowing the progress of the hydrogen technology is hydrogen storage and transport. Hydrogen storage is a concern since storing hydrogen at ambient pressure and temperature requires 12.18 m3 of volume to store 1 kg of hydrogen. Also, safety is a concern since hydrogen is a combustible gas. One currently available technology for
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storing and transporting hydrogen is by compressing it to high pressures [9,14]. The large energy consumption of the hydrogen compression process and the requirement for high strength storage tanks are some of the disadvantages of hydrogen storage through compression. Boggs and Botte [17] discuss the use of ammonia as a hydrogen storage medium, and the recovery of the hydrogen through electrochemical decomposition. Storing hydrogen in ammonia is referred to as chemical hydrogen storage. One of the advantages of ammonia over hydrogen in terms of storage and transportation is that ammonia has 8.5 times the density of hydrogen at ambient conditions and that ammonia is a liquid at 10 bar and ambient temperature. When ammonia and hydrogen are at a pressure of 10 bar and ambient temperature, the density of ammonia is 744 times that of hydrogen. Another advantage is that significant amounts of hydrogen can be stored in the form of slightly pressurized ammonia. The conversion of various forms and sources of energy to hydrogen (for storing the energy in chemical form) is the subject of extensive research. Although hydrogen production is importance, the process of storing the produced hydrogen is also of high importance. The aim of this paper is to develop and analyze a novel hydrogen storage technology that stores hydrogen in a chemical storage medium (ammonia). Most of research has been carried out on directly using ammonia as a fuel [18,19] since we can use the ammonia directly in solid oxide fuel cells [19,20], and combustion engines [21]. However, in this paper ammonia is used as a storage medium for hydrogen. The developed system is a pressurized subzerocooled ammonia production system. In the system, recovering the stored hydrogen in ammonia is done by electrochemical decomposition of ammonia. The main novelty of this paper is the proposed pressurized ammonia production system for hydrogen storage purposes at an operating temperature of 10 C. The proposed system is analyzed comprehensively through a transient thermodynamic model, using the energy and exergy methods. The developed system is examined for a particular case of filling a tank with a volume of 30,000 L, which is similar to the size of the tanks on gasoline tanker trucks. The system proposed and analyzed in this paper is a unique system as it produces ammonia uniquely at sub-zero temperatures, such as 10 C. This study further aims to validate the proposed system and its conceptual design. In addition, this study involves energy and exergy analyses of the system comprehensively along with its simulation through the engineering process simulation software (i.e., Aspen Plus).
is done at relatively low temperatures, for example, around 96% of the ammonia decomposes to hydrogen and nitrogen at a temperature of 300 C (based on ideal conditions of Gibbs free energy minimization approach). The reaction of hydrogen and nitrogen is exothermic, causing the reactor internal temperature to rise and lowering the hydrogen conversion percentage. To show the effect of reactor temperature on the hydrogen conversion percentage a parametric study is carried out. The parametric study results are shown in Fig. 1, where the effect of varying the reactor temperature and the amount of nitrogen injected into the reactor on the conversion percentage of hydrogen is examined. As shown in Fig. 1, as the reactor temperature increases the hydrogen conversion percentage drops dramatically. The effect of the amount of nitrogen is only noticeable at conversion percentages less than 90%, and when more than 10% hydrogen is converted. Fig. 1 also shows that as the temperature of the reactor decreases to below 0 C, a conversion percentage of more than 99% can be achieved, without any sensitivity to the amount of nitrogen entering the reactor. The results in Fig. 1 might seems contradicting with the current well established ammonia production process (Haber Bosch process), however it is not. The ammonia production process at low temperature as shown in Fig. 1 is favored by the Gibbs energy, however the speed of the reaction is slow and that is why the ammonia production plants they are not using it. Instead they increase the temperature of the reactants to a temperature from 400 C to 500 C and high operating pressures, where at these temperatures the molecules will have a very high kinetic energy which translate to more collusion and higher reaction rates. However, at these temperatures the Gibbs energy favor the decomposition of ammonia, and that is why they use a catalyst to insure ammonia production at these high temperatures with a low conversion percentage of the hydrogen and nitrogen. For cases of energy storage
System description The proposed system for hydrogen energy storage is based on chemical hydrogen storage involving charging and discharging phases. During the discharge phase, the stored hydrogen is reproduced by reversing the charging reaction. The chemical hydrogen storage medium ideally has a high lower heating value, and may be used more economically than a system for storing hydrogen directly. The chemical substance to store the hydrogen in the proposed system is ammonia. Ammonia production from hydrogen and nitrogen is an exothermic reaction and ammonia decomposition to hydrogen and nitrogen
Fig. 1 e Effect of varying the ammonia production reactor operating temperature and the amount of nitrogen entering the reactor, on the percentage of the converted hydrogen.
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options a low rate of ammonia production is not a problem especially for long term energy storage option. This paper investigate the possibility of ammonia production at low temperatures and the possible advantages in terms of energy. Based on the parametric study, an ammonia production system is developed which produces ammonia from hydrogen and nitrogen in a subzero reactor. The viability of using a reactor at 10 C is extracted from the result in Fig. 1, where at the temperature the reaction will have over 99% conversion percentage. The subzero reactor is maintained at 10 C by a refrigeration cycle, in which the working fluid is R134a. The concept of producing ammonia from both hydrogen and nitrogen at 10 C differs from the conventional Haber-Bosh process, which uses a catalyst and high operating temperature to achieve a high conversion percentage. In the concept proposed here, based on the Gibbs free energy minimization approach, there is no need for a catalyst to provide the high conversion percentage, since the Gibbs free energy highly favors the forward reaction. The reactor operating conditions are listed in Table 1 and the main operating parameters of the developed pressurized subzero-cooled ammonia production system (PSAPS) are shown in Fig. 2. Fig. 2 presents a schematic diagram of the proposed PSAPS for chemical hydrogen storage. The hydrogen and nitrogen entering the subzero reactor are compressed to 5 bar. The compressed hydrogen and nitrogen are cooled by small ambient temperature coolers, which produce cooled hydrogen and nitrogen at 30 C. The reason behind compressing the hydrogen and nitrogen before entering the reactor is to increase the operating pressure of the subzero reactor, which in turn increases the saturation temperature of the ammonia making liquid ammonia production less energy intensive. The ammonia exiting the reactor enters the vapor-liquid separator. The liquid exiting the vapor-liquid separator is stored in the ammonia tank. The vapor exiting the vapor-liquid separator is recycled back to the subzero reactor. The reason for recirculating the vapor released by the vapor liquid separator, which is rich in ammonia, is that the vapor is not pure
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ammonia. Thus, as the ammonia is liquefied, its saturation temperature is reduced since the ammonia partial pressure decreases. The choice to recirculate the vapor is made because the system was found to consume less energy if the ammonia is recirculated, compared to trying to liquefy all the produced ammonia. This was found to be the case even though when the ammonia is recirculated to the reactor, the conversion percentage of hydrogen is reduced. During the discharge phase, the ammonia is released from the tank to the ammonia electrolyzer, which decomposes the ammonia to hydrogen and nitrogen. The decomposition energy is considered as part of the energy used to store and recover the hydrogen. The proposed PSAPS is modeled and simulated with Aspen Plus and Engineering Equation Solver (EES). The Aspen Plus flowsheet of the modeled PSAPS is shown in Fig. 3. The nitrogen stream A enters the nitrogen compressor (A2), which discharges the nitrogen at a pressure of 5 bar. The subzero ammonia production reactor (A6) is maintained at a constant temperature of 10 C by the refrigeration cycle (A9, A10, A11, A12, A13). The produced liquid ammonia exits the system in stream J exiting the vapor-liquid separator (A8).
Analysis Throughout the modeling, simulation, and analysis of the proposed hydrogen storage system using the PSAPS, the following assumptions are made for analysis and calculations accordingly: The hydrogen storage technology operates at steady-state conditions, since the start up and shut down times for the system are much smaller than its normal operation duration. The filling process of ammonia into the tank is transient in nature. The potential energy variations due to the changes in the elevation are not negligible and thus are taken into consideration.
Fig. 2 e Pressurized subzero-cooled ammonia production system (PSAPS), consisting of two compressors, a temperature controlled reactor working at a subzero temperature, a refrigeration cycle and an ammonia electrolyzer to recover the stored hydrogen.
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Fig. 3 e Aspen Plus flowsheet of the pressurized subzero-cooled ammonia production system (PSAPS), with the subzero ammonia reactor A6 and the refrigeration cycle A9, A10, A11, A12, and A13 with the working fluid R134a.
The subzero reactor loses thermal energy only to the refrigerant in the cooling jacket of the reactor. The fluids in the heat exchangers do not experience any pressure drops.
PSAPS The mass flow rate balance equation for a steady state process can be expressed as follows: X
The selected reference environment model used in the thermodynamic analysis of the proposed system is the natural-environment-subsystem model, having the following characteristics [22]: The reference environment temperature (To) and pressure (Po) are 298.15 K and 101.325 kPa respectively. The reference environment chemical makeup includes atmospheric air consisting of the following constituents (mole fraction): Nitrogen (0.7567), Oxygen (0.2035), Water (0.0303), Argon (0.0091), Carbon dioxide (0.0003) and Hydrogen (0.0001). The reference environment atmospheric air is saturated with water at To and Po. The reference environment water, limestone (CaCO3) and gypsum (CaSO4.2H2O) are condensed phases at To and Po. The selected property model in Aspen Plus to simulate the hydrogen storage technology is Redlich-Kwong-Soave (RKS) cubic equation of state for the thermodynamic properties (except the liquid molar volume, which is calculated based on the API model). The PSAPS part of the energy storage system operates at steady state, and the tank filling process is analyzed using transient thermodynamics. The performance of the proposed hydrogen storage system using PSAPS is assessed with energy and exergy analyses [22].
_ in m
X
_ out ¼ 0 m
(1)
where the mass flow rate entering the control volume of a system equals the mass flow rate exiting, which means that there is no mass accumulation in the control volume. The _ and the subscripts in and out mass flow rate is denoted m refer to the control volume inlet and outlet, respectively. The rate balance equation for energy under steady-state conditions can be expressed as follows: _ in þ Q_ in þ W
X X _ out þ m _ hho þhf ¼ Q_ out þ W _ hho þhf m
(2)
out
in
where the left side represents the energy rate entering the control volume in the form of heat, work and mass, respectively. The right side represents the energy rate leaving the control volume in the form of rate, work and mass, respectively. Also, Q_ is the heat rate transferred into or out of the _ is the work rate into or out of the system, h is system, W the specific enthalpy, and hf is the heat of formation, and the subscript o indicates that a property is at the reference environment conditions. The rate balance equation for exergy under steady-state conditions can be expressed as follows: _ w_ þ _ _ þ Ex Ex Q in in
X X _ _ þ Ex _ w_ þ _ d _ in exin ¼ Ex _ out exout þ Ex m m out Q out in
out
(3) where the left side represents the exergy rate entering the control volume and the right side represents the exergy rate
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leaving the control volume plus the exergy destruction rate. _ d _ _ is work exergy rate and Ex Also, ex is the specific exergy, Ex W is the exergy destruction rate. The exergy rate associated with a heat rate Q_ can be written as _ _ ¼ Ex Q
1
To _ Q Tbs
(4)
where Tbs is the temperature of the boundary surface at which the heat transfer takes place. The performance of the proposed hydrogen chemical storage system using the PSAPS is evaluated through the application of the three balance equations on the system components and the PSAPS as a single unit. This permits energy and exergy efficiencies to be defined. The third performance measure used in this paper is the work expended to store and recover 1 mol of hydrogen. This parameter is important for determining the most efficient and least energy intensive hydrogen storage system. The PSAPS consumes work in the initial compression of the hydrogen and nitrogen entering the system and in maintaining the reactor temperature at 10 C. Since the PSAPS is a hydrogen storage system, the produced and stored ammonia must be decomposed to reproduce the stored hydrogen. The ammonia is decomposed using the ammonia electrolyzer, and the energy it consumes is considered a part of the total energy consumption by the PSAPS. According to Boggs and Botte [17], the ammonia electrolysis consumes 1.55 Wh per H2 g. The energy and exergy efficiencies of the PSAPS are expressed as follows: hPSAPS ¼
_ NH N 3
3 2
_ in; net W _e LHVH2 W _ H LHVH N 2
(5)
2
jPSAPS ¼
_ NH N 3
3 2
_ in; net W _ e exH2 W _ NH exH 2
(6)
2
where the factor 3/2 is used due to the assumption that 100% of the ammonia sent to the ammonia electrolyzer is decom_ NH is the mole flow posed to hydrogen and nitrogen. Also, N 3 rate of the ammonia produced by the PSAPS system (stream J _ e is the electrical power consumed by the in Fig. 3), W _ in; net is made up of three terms: ammonia electrolyzer, and W _ A1 þ W _ A2 þ W _ A10 _ in; net ¼ W W
(7)
Here, A1, A2 and A10 denote the three compressors shown in Fig. 3. The exergy destruction rate through the process of storing and recovering the hydrogen can be expressed by _ in; net þ W _e _ d; PSAPS ¼ W Ex
(8)
Filling the tank with produced liquid ammonia by the PSAPS is a transient (unsteady) process, and the methodology used in analyzing this part of the hydrogen storage process is presented in the next section.
Ammonia tank The ammonia produced by the PSAPS is in the liquid state and at a pressure higher than the reference environment pressure. The tank properties are presented in Table 1, based on the assumption that there is ammonia in the tank at the initial state. The transient analysis of the tank filling process is an important step in determining the required supply, which is based on the initially set minimum mass of ammonia to be transported in the tank.
Table 1 e Main parameters of the pressurized subzero-cooled ammonia production system (PSAPS). Parameter Ammonia production reactor Ratio of nitrogen mole flow rate to hydrogen mole flow rate entering the system Operating pressure of subzero ammonia production reactor Operating temperature of subzero ammonia production reactor Catalysts Supporting system and components Discharge pressure of the hydrogen compressor Discharge pressure of the nitrogen compressor Operating fluid in the refrigeration cycle Condenser operating pressure of the refrigeration cycle Boiler operating pressure of the refrigeration cycle Isentropic efficiency of the compressors [3] Energy required by the ammonia electrolyzer (validated with [17]) Volume of the tank [25] Length of the tank [25] Diameter of the tank [25] Initial internal pressure of the tank Initial temperature of the tank COP of the cooling unit after the nitrogen compressor (N2C) COP of the cooling unit after the hydrogen compressor (H2C) Operating fluid in the N2C and H2C cycles Condenser operating pressure of the N2C and H2C cycles Boiler operating pressure of the N2C and H2C cycles a
Value
Unit
1/3 5 10 No catalyst is requireda.
mol N2/mol H2 bar o C
5 5 R134a 7 1.9 72 1.55 30,000 10.27 1.82 1 25 40.6 40.6 R134a 7.79 6.73
bar bar
No catalyst is required based on the highly favorable ammonia production reaction as presented in Fig. 1.
bar bar % Wh per H2 g L m m bar o C [dimensionless] [dimensionless] bar bar
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The mass balance equation for the transient process of filling the ammonia tank with the ammonia produced by the PSAPS is expressed as follows: _ out _ in m dmtank =dt ¼ m
(9)
where the left side of the equation presents the rate change of _ in is the the ammonia mass in the tank at the conditions, m _ out is the ammonia mass flow rate entering the tank and m ammonia mass flow rate leaving the tank. The energy balance equation of the process of storing the ammonia produced by the PSAPS is expressed as follows: _ in hin m _ out hout hamb Atank ðT To Þ dðmtank utank Þ=dt ¼ m
(10)
where the left side represents the variation of the ammonia in the tank internal energy, and the right side includes the net energy entering the system through mass minus the thermal energy leaving the tank due to forced conversion. Also, hamb is the external heat transfer convection coefficient and Atank is the surface area of the tank. The shape of the ammonia tank is approximated as a cylinder and hamb is calculated using the cross flow over a cylinder Nusselt number as follows [23]:
Nucyl
" 5=8 #4=5 hamb D 0:62 Re1=2 Pr1=3 Re ¼ 0:3 þ h ¼ i1=4 1 þ k 282; 000 1 þ ð0:4=PrÞ2=3 (11)
Here, the Reynolds number (Re) is calculated by Re ¼
rv∞ D m
(12)
where D is the diameter of the cylinder, the upstream velocity with flow direction perpendicular to the cylindrical tank axis (v∞ ) is considered as high at 12.0 m/s as a worst-case scenario for heat transfer calculations [23], and m and r are the dynamic viscosity and the density, respectively.
Ammonia electrolysis process During the discharge phase, which recovers the hydrogen stored chemically in ammonia, the ammonia is sent to the ammonia electrolyzer. There, the ammonia is decomposed electrochemically into hydrogen and nitrogen, as described below. The aqueous ammonia (NH3(aq)) is fed to the electrolyzer on the anode side of the cell, and in the presence of the electrolyzer electrolyte potassium hydroxide (KOH) the following NH3 oxidation (anode half reaction) takes place: 2NH3 þ 6OH /N2 þ 6H2 O þ 6e
(13)
The cathode side of a KOH and water solution is reduced according to the following reaction: 6H2 O þ 6e /3H2 þ 6OH
(14)
Combining the two half reactions of the electrolysis process, the following reaction results: 2NH3 /N2 þ 3H2
(15)
Nernst's law is used to calculate the potential of an electrode relative to a reference electrode, i.e., to express the
potential difference at the state of equilibrium across an interface, for which both elements of a redox couple are present. The Nernst law used here can be expressed as follows: ERef ¼ EoRef þ
Y v RT ln i ai i ve F
(16)
Here, i refers to the type of species involved in the half redox reaction, but with electrons not considered as one of the species; ERef is the electric potential of one of the electrodes relative to the potential of another electrode; R is the ideal gas constant; T denotes temperature (in Kelvin); F is Faraday's constant (96,485 C/mol) [24]; ve is the number of electrons in the half redox reaction; ai is a dimensionless number, calculated based on Table 1; and vi is the stoichiometric coefficient in the half redox reaction for the species i. Although Equation (16) calculates the potential voltage of one of the electrodes relative to the other electrode in the electrolyzer, in a real electrolyzer other losses should be considered. The effect of doing so is an increase in the overall voltage required to be supplied to the electrolyzer. The losses are adopted from the experimental work of Boggs and Botte [17], as presented in Table 1.
Results and discussion The simulation and analysis results of the proposed chemical hydrogen storage system are presented. Then the system performance is described in terms of energy efficiency, exergy efficiency, exergy destruction rate and the instantaneous total work energy required to store 1 mol of hydrogen. The overall energy and exergy efficiencies of the chemicalbased hydrogen storage system are 85.6% and 85.3% respectively. The PSAPS consumes 34.0 kJ of work per mole of hydrogen stored by the PSAPS and recovered through the use of the ammonia electrolyzer. The exergy destroyed during the process of storing and recovering the hydrogen is equal to the work consumed, since the system input is hydrogen and the final output is hydrogen at same conditions. The PSAPS is distinguished by its novel subzero cooled ammonia production reactor, which is maintained at a subzero temperature of 10 C with the help of a simple refrigeration unit. The refrigeration unit coefficient of performance (COP), based on simulation of a unit with R134a as the working fluid, is 3.58. The COP of the refrigeration unit is less than half of the ideal (Carnot) COP. The flow rates and properties of the streams of the PSAPS for the case of the production of 1 mol of ammonia (for which 3 mol of atomic hydrogen are stored) are presented in Tables 2a and 2b. From the stream properties in Table 2a, the system is seen to achieve a 99.999% conversion of the hydrogen sent to the chemical-based hydrogen storage system [26]. The produced ammonia from the PSAPS as shown in Table 2a (based on temperature and pressure) is in the liquid phase (at 5 bar and 4.22 C). The exergy efficiency, the unit exergy destruction, and the specific thermal energy or work interactions per mole of hydrogen stored and recovered, for each of the components in the PSAPS, are listed in Table 3. The condenser of the refrigeration unit has the highest exergy efficiency in the system, mainly because the exergy lost due to
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Table 2a e Properties of pressurized subzero-cooled ammonia production system (PSAPS) streams, including the mole flow rate of the streams chemical components (locations of listed Aspen Plus streams are shown in Fig. 3) (for the case of 1 mol of ammonia is produced from the system). kJ kg kJ a Stream T (oC) P (bar) Stream components mole flow rate (mol/s) r h ex name mol m3 mol _N _ NH _H N N N 2
A B C D E F G H I J K a
25.0 25.0 265 266 30.0 30.0 4.83 10.0 4.22 4.22 4.22
1.00 1.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00
0.006 0.001 7.03 7.05 0.116 0.15 44.4 47.1 47.1 69.1 46.6
1.13 0.081 3.13 0.224 5.56 0.399 3.75 4.11 3.96 633 3.87
a
0.00 0.00 0.00 0.00 0.00 0.00 81.6 83.6 83.6 1.99 81.6
2
0.00 3.00 0.00 3.00 0.00 3.00 3.49 0.494 0.494 0.00 0.494
0.687 236 6.56 242 4.68 240 333 341 341 343 340
The specific exergy contains both the physical exergy and the chemical exergy [22] of the chemical compounds or elements. The specific exergy is per mole of the flowing stream.
Table 2b e Properties of the main refrigeration unit of the pressurized subzero-cooled ammonia production system (PSAPS) streams, including the mole flow rate of the streams chemical components (locations of listed Aspen Plus streams are shown in Fig. 3) (for the case of 1 mol of ammonia is produced from the system). _ R134a kg kJ a Stream T (oC) P (bar) h kJ N r ex mol m3 (mole/s) mol name L M N O P Q
3
1.00 0.00 1.00 0.00 1.00 0.00 1.17 0.165 0.165 0.00 0.165
12.7 10.3 45.6 26.3 26.3 12.7
1.80 1.80 7.00 7.00 7.00 1.80
915 899 895 897 915 915
33.7 8.83 30.3 33.2 1200 33.6
8.53 8.53 8.53 8.53 8.53 8.53
3.93 1.58 4.58 4.51 4.43 3.93
The specific exergy contains only the physical exergy for the refrigerant R134a.
the decrease in temperature is relatively small relative to the high exergy content of the fluid due to its high pressure. The subzero cooled reactor's cooling jacket has the lowest exergy efficiency of all of the PSAPS components, primarily because
the rise in coolant temperature due to the addition of thermal energy brings the temperature closer to the reference environment temperature and because the fluid pressure is already close to the reference environment pressure. This means the addition of thermal energy to the fluid decreases its exergy content. The largest contributor to the exergy destruction of the PSAPS (A5, A6, A7, A8) is the subzero cooled reactor, which contributes nearly 40% of the total exergy destruction. The PSAPS system performance and energy consumption are examined for the case of filling the ammonia in a pressurized tank, which has the dimensions of currently available gasoline tanks [25] developed by Vervetank. The properties of the tank are presented in Table 1. When delivering ammonia to the ammonia electrolyzer, the remaining ammonia in the tank is at ambient pressure and temperature (the operating pressure of the ammonia electrolyzer [17]). The tank is filled until the pressure in the tank reaches the supply pressure. The variation of the temperature and the pressure inside the ammonia tank as the ammonia entering the tank from the supply at a constant mass flow rate of 5.0 kg/s is shown in Fig. 4. Note that the variation of the temperature in the tank is
Table 3 e Exergy efficiency, specific exergy destruction, and the thermal energy or work energy (per kg of hydrogen stored and recovered) associated with components in the PMAPS (locations of these Aspen Plus blocks are shown in Fig. 3). kJ kJ kJ a q or w Component j ð%Þ exd mol H2 mol H2 mol H2 A1 A2 A3 A4 A5-A8 (subzero cooled reactor) A9 A10 A11 & A12 (condenser) A13 a
79.3 79.3 67.8 67.8 94.4 27.0 66.0 96.8 88.7
1.54 0.51 0.63 1.89 13.5 12.2 4.40 1.25 1.42
Positive refers to thermal or work energy consumed, and negative refers to thermal or work energy released.
22.3 7.41 6.92 20.7 138 139 38.7 173 0.00
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Fig. 4 e Variation of the internal tank pressure and internal tank temperature throughout the process of storing the PSAPS produced ammonia in the tank.
of a quite importance since the ammonia changes phase throughout the filling process and it will directly affect the energy consumption of the hydrogen storage system. Throughout the storage process it is expected that the storage tank temperature will vary from as low as 40 C to a high temperature of 20 C. The temperature of the ammonia in the tank drops dramatically from 25 C to 35.0 C, because the ammonia entering the tank is flashed from a pressure of 5 bar and a low temperature of 4.13 C, and it is in the liquid state. After the sudden drop in the temperature of the ammonia in the tank, the temperature starts gradually increasing until reaching the final temperature of 4.13 C. The tank internal pressure experiences a small pressure drop from 1 bar to 0.929 bar; the pressure then increases until reaching the supply pressure (5 bar). The vapor fraction (quality) of the ammonia in the tank throughout the filling process of producing ammonia by the PSAPS is shown in Fig. 5. The tank is filled with vapor before the filling process. As the filling process starts, the vapor fraction drops dramatically in the first 5% of the total filling time. After, the rate at which the vapor fraction decreases becomes much lower. The rate keeps decreasing until reaching a value of nearly 0. The vapor fraction at the end of the
Fig. 5 e Quality (vapor fraction) of the ammonia in the tank throughout the process of storing the ammonia in the tank.
Fig. 6 e Cumulative ammonia mass in the tank and the mass flow rate of the ammonia supply coming from the PSAPS throughout the storing process of the ammonia in the tank.
storing process reaches a low value of approximately 1%. Having the vapor fraction reaching a very low value of nearly zero shows the efficiency of the storage system in utilizing almost 99% of the possible liquid storage mass. Fig. 6 shows the cumulative ammonia mass in the tank and the supply mass flow rate throughout the storing process. The system is able to fill the 30,000 L tank with 7521 kg of ammonia at 5 bar and 4.13 C. Fig. 6 shows that the supply mass flow rate is constant throughout the filling process. Finally, the work consumed and the instantaneous heat loss or gain by the tank during the filling process are shown in Fig. 7. There, the total work consumed by the system including the energy required to separate the ammonia into hydrogen and nitrogen is 45.3 GJ. The tank continually gains thermal energy during the filling time. Finally, the performance of the proposed system is compared to that for the hydrogen storage technology manufactured by the company Linde [26]. If the Linde system operates at its maximum mass flow rate, over the same
Fig. 7 e Cumulative consumed work during the filling process, taking into account the energy required to separate the ammonia to reproduce the stored hydrogen, and the instantaneous thermal energy gained or lost during the filling process.
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operation time as for the proposed system, only 14 kg of hydrogen is stored. However, in terms of energy requirements to store the same mass, the Linde system consume 9.96 GJ of electrical energy, which is nearly 22% of the total energy consumed by the proposed system. However, the Linde compression system requires 94,800 s (26.3 h) to compress the same amount of hydrogen the proposed system was able to compress to fill the tanker, which is much larger than the 1500 s the proposed system requires. Fig. 7 shows that the power accumulates on the basis of a linear relation meaning that the power consumption of the operation of the system on a constant mass flow rate supply to the tank is independent of the amount of ammonia mass in the tank. Having this advantage works in favor of the proposed hydrogen storage system, since the slow nature of the proposed reaction can be accommodated by not having to fully fill up the storage tank and changing the supply mass flow rate of ammonia can be another solution. Wang et al. [27] developed a compression system that produces 700 bar hydrogen based on metal hydrides. The system is able to increase the pressure from 40 bar to 700 bar, with an energy efficiency of 12.5%. However, Wang et al. [27] did not report the exergy efficiency of the process, which is expected to have a high value since thermal energy of a low quality (at a temperature of 423 K) is used. Usually researchers argue that the filling time or the storing time of hydrogen, which reflects the mass flow rate of hydrogen, is not a primary concern. That is because, in a future hydrogen economy, large scale hydrogen production and compression systems likely will be able to handle large flow rates and have the ability to adjust these flow rates. The process of producing hydrogen on a large scale will need fast and efficient hydrogen storage systems. Other researchers have examined large scale hydrogen production from nuclear energy [6,28], and have reported high mass flow rates.
Conclusions An efficient hydrogen storage technology is needed for the hydrogen production sector and a hydrogen economy in which hydrogen is the main chemical energy carrier. A promising hydrogen storage option is chemical hydrogen storage, in which the hydrogen is stored in a chemical substance, and later the hydrogen is recovered from the storage medium. This paper proposes and analyzes a novel chemicalbased hydrogen storage technology, where the storage medium is ammonia. The proposed storage system is a pressurized subzero cooled ammonia production system (PSAPS). In the PSAPS, the ammonia is produced in a reactor that is maintained at a subzero temperature (10 C) and a pressure of 5 bar. The produced ammonia is later decomposed, to recover the stored hydrogen, in an ammonia electrolyzer. The main results of the energy and exergy analyses of the chemical hydrogen storage system are as follows: The PSAPS overall energy and exergy efficiencies including the electrolyzer energy are 85.6% and 85.3% respectively. The PSAPS consumes 34.2 kJ of work to store and recover 1 mol of hydrogen.
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The performance of the proposed hydrogen storage system is examined when it is required to fill a 30,000 L tank with pressurized ammonia, demonstrating the following: (a) The PSAPS is able to store 7521 kg of ammonia in the ammonia tank in 1500 s while, in terms of H2, it stored 885 kg of H2. (b) The PSAPS uses 45.3 GJ of work to store the 7521 kg of ammonia and to decompose it to reproduce the stored hydrogen. It is important to note that the ammonia electrolyzer is based on ideal electrochemical water decomposition and operates at a room temperature of 25 C, which results in 1.55 Wh for each gram of hydrogen produced. Careful maintenance of the electrolyzer is likely needed to avoid any degradation of the discharge system efficiency with time due to electrode poisoning. From the initial assessment of the sub-zero ammonia production system it may be considered a promising technology to be used as a hydrogen storage system. Note also that based on the literature and the chemistry of the reaction, the process is expected to have a slow reaction rate for ammonia production and thus experimental investigation of the sub-zero ammonia production system is recommended to further study the performance criteria and the ammonia production rates.
Acknowledgement The authors acknowledge the support provided by the Natural Sciences and Engineering Research Council of Canada.
references
[1] Naterer GF, Suppiah S, Stolberg L, Lewis M, Wang Z, Daggupati V, et al. Canada's program on nuclear hydrogen production and the thermochemical Cu-Cl cycle. Int J Hydrogen Energy 2010;35:10905e26. https://doi.org/10.1016/ j.ijhydene.2010.07.087. [2] Zhao L, Brouwer J, Jahnke F, Lambrech M, Patel P. A novel hybrid reformer-electrolyzer-purifier (REP) for distributed production of low-cost, low greenhouse gas hydrogen. ECS Trans 2016;71:179e92. https://doi.org/10.1149/07101.0179ecst. [3] Giuffrida A, Romano MC, Lozza G. Thermodynamic analysis of air-blown gasification for IGCC applications. Appl Energy 2011;88:3949e58. https://doi.org/10.1016/ j.apenergy.2011.04.009. [4] Ozturk M, Ozek N, Yuksel YE. Gasification of various types of tertiary coals: a sustainability approach. Energy Convers Manag 2012;56:157e65. https://doi.org/10.1016/ j.enconman.2011.11.008. [5] Shen CH, Chen WH, Hsu HW, Sheu JY, Hsieh TH. Cogasification performance of coal and petroleum coke blends in a pilot-scale pressurized entrained-flow gasifier. Int J Energy Res 2012;36:499e508. https://doi.org/10.1002/er.1821. [6] Al-Zareer M, Dincer I, Rosen MA. Development and assessment of a novel integrated nuclear plant for electricity and hydrogen production. Energy Convers Manag 2017;134:221e34. https://doi.org/10.1016/ j.enconman.2016.12.004. [7] Hasan A, Dincer I. Comparative assessment of various gasification fuels with waste tires for hydrogen production. Int J Hydrogen Energy 2019.
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Available online at www.sciencedirect.com
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Transient thermodynamic analysis of a novel integrated ammonia production, storage and hydrogen production system Maan Al-Zareer*, Ibrahim Dincer, Marc A. Rosen Clean Energy Research Laboratory, Department of Automotive, Mechanical and Manufacturing Engineering, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada
article info
abstract
Article history:
A transient thermodynamic analysis is reported of a novel chemical hydrogen storage
Received 20 October 2018
system using energy and exergy approaches. The hydrogen is stored chemically in
Received in revised form
ammonia using the proposed hydrogen storage system and recovered via the electro-
6 April 2019
chemical decomposition of ammonia through an ammonia electrolyzer. The proposed
Accepted 9 April 2019
hydrogen storage system is based on a novel subzero ammonia production reactor. A single
Available online 8 June 2019
stage refrigeration system maintains the ammonia production reactor at a temperature of 10 C. The energy and exergy efficiencies of the proposed system are 85.6% and 85.3%
Keywords:
respectively. The proposed system consumes 34.0 kJ of work through the process of storing
Ammonia production
1 mol of hydrogen and recovering it using the ammonia electrolyzer. The system is
Energy
simulated for filling 30,000 L of ammonia at a pressure of 5 bar, and the system was able to
Exergy
store 7500 kg of ammonia in a liquid state (1% vapor) in 1500 s. The system consumes
Chemical hydrogen storage
nearly 45.3 GJ of energy to store the 7500 kg of ammonia and to decompose it to reproduce
Efficiency
the stored hydrogen during the discharge phase. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Two of today's significant issues are global warming (climate change) and the air quality of cities, both of which affect our lives and the environment [1]. Hydrogen energy can help mitigate global warming and address air quality problems of cities. Hydrogen is a clean energy carrier that has a large impact if it is produced in sustainable, low-cost processes and in large capacities. Hydrogen and hydrogen-derived fuels can help facilitate the use of renewable energy sources, which can help reduce greenhouse gas emissions [2]. Hydrogen can be produced from various energy sources, including coal [3e5],
nuclear energy [6], even wastes such as waste tires [7], and renewable energy sources [8]. Hydrogen has a wide range of uses such as in petrochemical, agricultural (ammonia fertilizers), manufacturing, food processing, electronics, aerospace, plastics and many other industries. In Alberta, Canada large amounts of hydrogen are used to upgrade the bitumen in oil sands to synthetic crude oil while removing the impurities. A recent commercial example of a use of hydrogen is as a backup power system for communication towers for cell phones, through fuel cells [1]. Also, Hyundai and Audi have been pushing hydrogen fuel cell vehicles, where Hyundai next generation fuel cell SUV is coming soon to vehicle market “NEXO” [9]. Hydrogen is
* Corresponding author. E-mail addresses: [email protected] (M. Al-Zareer), [email protected] (I. Dincer), [email protected] (M.A. Rosen). https://doi.org/10.1016/j.ijhydene.2019.04.085 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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Nomenclature a COP ex ex _ Ex ERef F h h hf ho hamb k LHV _ m m _ N P Po Q_ q Re R Pr s T To Tbs t u
Dimensionless number given in Table 1 Coefficient of performance Specific exergy (kJ/kg) Specific molar exergy (kJ/mol) Exergy rate (kW) Electrical potential of one electrode relative to another electrode (kJ) Faraday's constant (96,485 C/mol) Specific enthalpy (kJ/kg) Specific molar enthalpy (kJ/mol) Specific heat of formation (kJ/kg) Specific enthalpy at the reference environment conditions (kJ/kg) Convection heat transfer coefficient (kW/m2.K) Thermal conductivity (kW/m.K) Lower heating value (kJ/mol) Mass flow rate (kg/s) Mass (kg) Mole flow rate (mol/s) Pressure (kPa) Reference environment pressure (kPa) Heat rate (kW) Specific thermal energy (kJ/kg) Reynolds number Ideal gas constant Prandtl number Specific entropy (kJ/kg K) Temperature (oC) Reference environment temperature (oC) Temperature of boundary surface where heat transfer takes place (oC) Time (s) Specific internal energy (kJ/kg)
expected to become important as an energy carrier and as an energy storage medium. Hydrogen is advantageous as an energy storage medium since it has one of the highest energy densities (on a mass basis) and it can be produced from electrical energy via electrolysis as well as from other energy sources [10]. Also, hydrogen is a unique energy carrier because it can be easily converted to other forms of energy and can be used to produce hydrogen-derived fuels such as ammonia and methanol. Dincer and Rosen [11] reported that hydrogen as an energy currency has the significant market potential, that is hydrogen then can be used for electricity generation via fuel cells, as a chemical fuel for combustion, and as a chemical feedstock for the production of other chemical fuels such as ammonia and methanol. The energy stored in the production of hydrogen (for chemical energy storage) can easily be recovered in various forms, such as thermal energy through combustion or electrical energy using fuel cells. One of the main potential users for hydrogen energy is the transportation sector. One of the fuels that can be produced from hydrogen (ammonia) has been the subject of extensive research [12e14]. Ezzat and Dincer [14] proposed a vehicle
v∞ vi ve _ W w x
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Upstream velocity (m/s) Stoichiometric number for species i Number of electrons Work rate (kW) Specific work (kJ/kg) Mass vapor fraction
Greek letters h Energy efficiency j Exergy efficiency r Density (kg/m3) m Dynamic viscosity (Pa s) Subscripts bs Boundary where heat transfer occurs d Destruction e Electrical is Isentropic in Input (flowing into system boundary) max Maximum net Net out Output (flowing out of system boundary) o Reference environment conditions Q_ Heat flow rate ST Steam turbine tank Ammonia tank W Work _ W Work rate Acronyms API American Petroleum Institute PSAPS Pressurized subzero-cooled ammonia production system RKS Redlich-Kwong-Soave
power system that is hydrogen fueled, and showed that an energy efficiency up to 40% and an exergy efficiency up to 56% can be achieved. But the use of hydrogen in vehicles is not limited to fuel cells; hydrogen can also be used as a combustion fuel for internal combustion engines [15]. The transportation sector is responsible for a significant part of the total greenhouse gas emissions in Canada [16]. Running vehicles on hydrogen produced through sustainable technologies can mitigate greenhouse gas emissions and urban air pollution. Currently available electrical and hybrid vehicles contribute to reducing this problem but, due to the challenges that electrical batteries are facing such as limited storage capacity and reduced performance after a number of charging and discharging cycles, hydrogen is expected to have a significant role as a vehicle fuel in the future. One of the problems slowing the progress of the hydrogen technology is hydrogen storage and transport. Hydrogen storage is a concern since storing hydrogen at ambient pressure and temperature requires 12.18 m3 of volume to store 1 kg of hydrogen. Also, safety is a concern since hydrogen is a combustible gas. One currently available technology for
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storing and transporting hydrogen is by compressing it to high pressures [9,14]. The large energy consumption of the hydrogen compression process and the requirement for high strength storage tanks are some of the disadvantages of hydrogen storage through compression. Boggs and Botte [17] discuss the use of ammonia as a hydrogen storage medium, and the recovery of the hydrogen through electrochemical decomposition. Storing hydrogen in ammonia is referred to as chemical hydrogen storage. One of the advantages of ammonia over hydrogen in terms of storage and transportation is that ammonia has 8.5 times the density of hydrogen at ambient conditions and that ammonia is a liquid at 10 bar and ambient temperature. When ammonia and hydrogen are at a pressure of 10 bar and ambient temperature, the density of ammonia is 744 times that of hydrogen. Another advantage is that significant amounts of hydrogen can be stored in the form of slightly pressurized ammonia. The conversion of various forms and sources of energy to hydrogen (for storing the energy in chemical form) is the subject of extensive research. Although hydrogen production is importance, the process of storing the produced hydrogen is also of high importance. The aim of this paper is to develop and analyze a novel hydrogen storage technology that stores hydrogen in a chemical storage medium (ammonia). Most of research has been carried out on directly using ammonia as a fuel [18,19] since we can use the ammonia directly in solid oxide fuel cells [19,20], and combustion engines [21]. However, in this paper ammonia is used as a storage medium for hydrogen. The developed system is a pressurized subzerocooled ammonia production system. In the system, recovering the stored hydrogen in ammonia is done by electrochemical decomposition of ammonia. The main novelty of this paper is the proposed pressurized ammonia production system for hydrogen storage purposes at an operating temperature of 10 C. The proposed system is analyzed comprehensively through a transient thermodynamic model, using the energy and exergy methods. The developed system is examined for a particular case of filling a tank with a volume of 30,000 L, which is similar to the size of the tanks on gasoline tanker trucks. The system proposed and analyzed in this paper is a unique system as it produces ammonia uniquely at sub-zero temperatures, such as 10 C. This study further aims to validate the proposed system and its conceptual design. In addition, this study involves energy and exergy analyses of the system comprehensively along with its simulation through the engineering process simulation software (i.e., Aspen Plus).
is done at relatively low temperatures, for example, around 96% of the ammonia decomposes to hydrogen and nitrogen at a temperature of 300 C (based on ideal conditions of Gibbs free energy minimization approach). The reaction of hydrogen and nitrogen is exothermic, causing the reactor internal temperature to rise and lowering the hydrogen conversion percentage. To show the effect of reactor temperature on the hydrogen conversion percentage a parametric study is carried out. The parametric study results are shown in Fig. 1, where the effect of varying the reactor temperature and the amount of nitrogen injected into the reactor on the conversion percentage of hydrogen is examined. As shown in Fig. 1, as the reactor temperature increases the hydrogen conversion percentage drops dramatically. The effect of the amount of nitrogen is only noticeable at conversion percentages less than 90%, and when more than 10% hydrogen is converted. Fig. 1 also shows that as the temperature of the reactor decreases to below 0 C, a conversion percentage of more than 99% can be achieved, without any sensitivity to the amount of nitrogen entering the reactor. The results in Fig. 1 might seems contradicting with the current well established ammonia production process (Haber Bosch process), however it is not. The ammonia production process at low temperature as shown in Fig. 1 is favored by the Gibbs energy, however the speed of the reaction is slow and that is why the ammonia production plants they are not using it. Instead they increase the temperature of the reactants to a temperature from 400 C to 500 C and high operating pressures, where at these temperatures the molecules will have a very high kinetic energy which translate to more collusion and higher reaction rates. However, at these temperatures the Gibbs energy favor the decomposition of ammonia, and that is why they use a catalyst to insure ammonia production at these high temperatures with a low conversion percentage of the hydrogen and nitrogen. For cases of energy storage
System description The proposed system for hydrogen energy storage is based on chemical hydrogen storage involving charging and discharging phases. During the discharge phase, the stored hydrogen is reproduced by reversing the charging reaction. The chemical hydrogen storage medium ideally has a high lower heating value, and may be used more economically than a system for storing hydrogen directly. The chemical substance to store the hydrogen in the proposed system is ammonia. Ammonia production from hydrogen and nitrogen is an exothermic reaction and ammonia decomposition to hydrogen and nitrogen
Fig. 1 e Effect of varying the ammonia production reactor operating temperature and the amount of nitrogen entering the reactor, on the percentage of the converted hydrogen.
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options a low rate of ammonia production is not a problem especially for long term energy storage option. This paper investigate the possibility of ammonia production at low temperatures and the possible advantages in terms of energy. Based on the parametric study, an ammonia production system is developed which produces ammonia from hydrogen and nitrogen in a subzero reactor. The viability of using a reactor at 10 C is extracted from the result in Fig. 1, where at the temperature the reaction will have over 99% conversion percentage. The subzero reactor is maintained at 10 C by a refrigeration cycle, in which the working fluid is R134a. The concept of producing ammonia from both hydrogen and nitrogen at 10 C differs from the conventional Haber-Bosh process, which uses a catalyst and high operating temperature to achieve a high conversion percentage. In the concept proposed here, based on the Gibbs free energy minimization approach, there is no need for a catalyst to provide the high conversion percentage, since the Gibbs free energy highly favors the forward reaction. The reactor operating conditions are listed in Table 1 and the main operating parameters of the developed pressurized subzero-cooled ammonia production system (PSAPS) are shown in Fig. 2. Fig. 2 presents a schematic diagram of the proposed PSAPS for chemical hydrogen storage. The hydrogen and nitrogen entering the subzero reactor are compressed to 5 bar. The compressed hydrogen and nitrogen are cooled by small ambient temperature coolers, which produce cooled hydrogen and nitrogen at 30 C. The reason behind compressing the hydrogen and nitrogen before entering the reactor is to increase the operating pressure of the subzero reactor, which in turn increases the saturation temperature of the ammonia making liquid ammonia production less energy intensive. The ammonia exiting the reactor enters the vapor-liquid separator. The liquid exiting the vapor-liquid separator is stored in the ammonia tank. The vapor exiting the vapor-liquid separator is recycled back to the subzero reactor. The reason for recirculating the vapor released by the vapor liquid separator, which is rich in ammonia, is that the vapor is not pure
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ammonia. Thus, as the ammonia is liquefied, its saturation temperature is reduced since the ammonia partial pressure decreases. The choice to recirculate the vapor is made because the system was found to consume less energy if the ammonia is recirculated, compared to trying to liquefy all the produced ammonia. This was found to be the case even though when the ammonia is recirculated to the reactor, the conversion percentage of hydrogen is reduced. During the discharge phase, the ammonia is released from the tank to the ammonia electrolyzer, which decomposes the ammonia to hydrogen and nitrogen. The decomposition energy is considered as part of the energy used to store and recover the hydrogen. The proposed PSAPS is modeled and simulated with Aspen Plus and Engineering Equation Solver (EES). The Aspen Plus flowsheet of the modeled PSAPS is shown in Fig. 3. The nitrogen stream A enters the nitrogen compressor (A2), which discharges the nitrogen at a pressure of 5 bar. The subzero ammonia production reactor (A6) is maintained at a constant temperature of 10 C by the refrigeration cycle (A9, A10, A11, A12, A13). The produced liquid ammonia exits the system in stream J exiting the vapor-liquid separator (A8).
Analysis Throughout the modeling, simulation, and analysis of the proposed hydrogen storage system using the PSAPS, the following assumptions are made for analysis and calculations accordingly: The hydrogen storage technology operates at steady-state conditions, since the start up and shut down times for the system are much smaller than its normal operation duration. The filling process of ammonia into the tank is transient in nature. The potential energy variations due to the changes in the elevation are not negligible and thus are taken into consideration.
Fig. 2 e Pressurized subzero-cooled ammonia production system (PSAPS), consisting of two compressors, a temperature controlled reactor working at a subzero temperature, a refrigeration cycle and an ammonia electrolyzer to recover the stored hydrogen.
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Fig. 3 e Aspen Plus flowsheet of the pressurized subzero-cooled ammonia production system (PSAPS), with the subzero ammonia reactor A6 and the refrigeration cycle A9, A10, A11, A12, and A13 with the working fluid R134a.
The subzero reactor loses thermal energy only to the refrigerant in the cooling jacket of the reactor. The fluids in the heat exchangers do not experience any pressure drops.
PSAPS The mass flow rate balance equation for a steady state process can be expressed as follows: X
The selected reference environment model used in the thermodynamic analysis of the proposed system is the natural-environment-subsystem model, having the following characteristics [22]: The reference environment temperature (To) and pressure (Po) are 298.15 K and 101.325 kPa respectively. The reference environment chemical makeup includes atmospheric air consisting of the following constituents (mole fraction): Nitrogen (0.7567), Oxygen (0.2035), Water (0.0303), Argon (0.0091), Carbon dioxide (0.0003) and Hydrogen (0.0001). The reference environment atmospheric air is saturated with water at To and Po. The reference environment water, limestone (CaCO3) and gypsum (CaSO4.2H2O) are condensed phases at To and Po. The selected property model in Aspen Plus to simulate the hydrogen storage technology is Redlich-Kwong-Soave (RKS) cubic equation of state for the thermodynamic properties (except the liquid molar volume, which is calculated based on the API model). The PSAPS part of the energy storage system operates at steady state, and the tank filling process is analyzed using transient thermodynamics. The performance of the proposed hydrogen storage system using PSAPS is assessed with energy and exergy analyses [22].
_ in m
X
_ out ¼ 0 m
(1)
where the mass flow rate entering the control volume of a system equals the mass flow rate exiting, which means that there is no mass accumulation in the control volume. The _ and the subscripts in and out mass flow rate is denoted m refer to the control volume inlet and outlet, respectively. The rate balance equation for energy under steady-state conditions can be expressed as follows: _ in þ Q_ in þ W
X X _ out þ m _ hho þhf ¼ Q_ out þ W _ hho þhf m
(2)
out
in
where the left side represents the energy rate entering the control volume in the form of heat, work and mass, respectively. The right side represents the energy rate leaving the control volume in the form of rate, work and mass, respectively. Also, Q_ is the heat rate transferred into or out of the _ is the work rate into or out of the system, h is system, W the specific enthalpy, and hf is the heat of formation, and the subscript o indicates that a property is at the reference environment conditions. The rate balance equation for exergy under steady-state conditions can be expressed as follows: _ w_ þ _ _ þ Ex Ex Q in in
X X _ _ þ Ex _ w_ þ _ d _ in exin ¼ Ex _ out exout þ Ex m m out Q out in
out
(3) where the left side represents the exergy rate entering the control volume and the right side represents the exergy rate
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leaving the control volume plus the exergy destruction rate. _ d _ _ is work exergy rate and Ex Also, ex is the specific exergy, Ex W is the exergy destruction rate. The exergy rate associated with a heat rate Q_ can be written as _ _ ¼ Ex Q
1
To _ Q Tbs
(4)
where Tbs is the temperature of the boundary surface at which the heat transfer takes place. The performance of the proposed hydrogen chemical storage system using the PSAPS is evaluated through the application of the three balance equations on the system components and the PSAPS as a single unit. This permits energy and exergy efficiencies to be defined. The third performance measure used in this paper is the work expended to store and recover 1 mol of hydrogen. This parameter is important for determining the most efficient and least energy intensive hydrogen storage system. The PSAPS consumes work in the initial compression of the hydrogen and nitrogen entering the system and in maintaining the reactor temperature at 10 C. Since the PSAPS is a hydrogen storage system, the produced and stored ammonia must be decomposed to reproduce the stored hydrogen. The ammonia is decomposed using the ammonia electrolyzer, and the energy it consumes is considered a part of the total energy consumption by the PSAPS. According to Boggs and Botte [17], the ammonia electrolysis consumes 1.55 Wh per H2 g. The energy and exergy efficiencies of the PSAPS are expressed as follows: hPSAPS ¼
_ NH N 3
3 2
_ in; net W _e LHVH2 W _ H LHVH N 2
(5)
2
jPSAPS ¼
_ NH N 3
3 2
_ in; net W _ e exH2 W _ NH exH 2
(6)
2
where the factor 3/2 is used due to the assumption that 100% of the ammonia sent to the ammonia electrolyzer is decom_ NH is the mole flow posed to hydrogen and nitrogen. Also, N 3 rate of the ammonia produced by the PSAPS system (stream J _ e is the electrical power consumed by the in Fig. 3), W _ in; net is made up of three terms: ammonia electrolyzer, and W _ A1 þ W _ A2 þ W _ A10 _ in; net ¼ W W
(7)
Here, A1, A2 and A10 denote the three compressors shown in Fig. 3. The exergy destruction rate through the process of storing and recovering the hydrogen can be expressed by _ in; net þ W _e _ d; PSAPS ¼ W Ex
(8)
Filling the tank with produced liquid ammonia by the PSAPS is a transient (unsteady) process, and the methodology used in analyzing this part of the hydrogen storage process is presented in the next section.
Ammonia tank The ammonia produced by the PSAPS is in the liquid state and at a pressure higher than the reference environment pressure. The tank properties are presented in Table 1, based on the assumption that there is ammonia in the tank at the initial state. The transient analysis of the tank filling process is an important step in determining the required supply, which is based on the initially set minimum mass of ammonia to be transported in the tank.
Table 1 e Main parameters of the pressurized subzero-cooled ammonia production system (PSAPS). Parameter Ammonia production reactor Ratio of nitrogen mole flow rate to hydrogen mole flow rate entering the system Operating pressure of subzero ammonia production reactor Operating temperature of subzero ammonia production reactor Catalysts Supporting system and components Discharge pressure of the hydrogen compressor Discharge pressure of the nitrogen compressor Operating fluid in the refrigeration cycle Condenser operating pressure of the refrigeration cycle Boiler operating pressure of the refrigeration cycle Isentropic efficiency of the compressors [3] Energy required by the ammonia electrolyzer (validated with [17]) Volume of the tank [25] Length of the tank [25] Diameter of the tank [25] Initial internal pressure of the tank Initial temperature of the tank COP of the cooling unit after the nitrogen compressor (N2C) COP of the cooling unit after the hydrogen compressor (H2C) Operating fluid in the N2C and H2C cycles Condenser operating pressure of the N2C and H2C cycles Boiler operating pressure of the N2C and H2C cycles a
Value
Unit
1/3 5 10 No catalyst is requireda.
mol N2/mol H2 bar o C
5 5 R134a 7 1.9 72 1.55 30,000 10.27 1.82 1 25 40.6 40.6 R134a 7.79 6.73
bar bar
No catalyst is required based on the highly favorable ammonia production reaction as presented in Fig. 1.
bar bar % Wh per H2 g L m m bar o C [dimensionless] [dimensionless] bar bar
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The mass balance equation for the transient process of filling the ammonia tank with the ammonia produced by the PSAPS is expressed as follows: _ out _ in m dmtank =dt ¼ m
(9)
where the left side of the equation presents the rate change of _ in is the the ammonia mass in the tank at the conditions, m _ out is the ammonia mass flow rate entering the tank and m ammonia mass flow rate leaving the tank. The energy balance equation of the process of storing the ammonia produced by the PSAPS is expressed as follows: _ in hin m _ out hout hamb Atank ðT To Þ dðmtank utank Þ=dt ¼ m
(10)
where the left side represents the variation of the ammonia in the tank internal energy, and the right side includes the net energy entering the system through mass minus the thermal energy leaving the tank due to forced conversion. Also, hamb is the external heat transfer convection coefficient and Atank is the surface area of the tank. The shape of the ammonia tank is approximated as a cylinder and hamb is calculated using the cross flow over a cylinder Nusselt number as follows [23]:
Nucyl
" 5=8 #4=5 hamb D 0:62 Re1=2 Pr1=3 Re ¼ 0:3 þ h ¼ i1=4 1 þ k 282; 000 1 þ ð0:4=PrÞ2=3 (11)
Here, the Reynolds number (Re) is calculated by Re ¼
rv∞ D m
(12)
where D is the diameter of the cylinder, the upstream velocity with flow direction perpendicular to the cylindrical tank axis (v∞ ) is considered as high at 12.0 m/s as a worst-case scenario for heat transfer calculations [23], and m and r are the dynamic viscosity and the density, respectively.
Ammonia electrolysis process During the discharge phase, which recovers the hydrogen stored chemically in ammonia, the ammonia is sent to the ammonia electrolyzer. There, the ammonia is decomposed electrochemically into hydrogen and nitrogen, as described below. The aqueous ammonia (NH3(aq)) is fed to the electrolyzer on the anode side of the cell, and in the presence of the electrolyzer electrolyte potassium hydroxide (KOH) the following NH3 oxidation (anode half reaction) takes place: 2NH3 þ 6OH /N2 þ 6H2 O þ 6e
(13)
The cathode side of a KOH and water solution is reduced according to the following reaction: 6H2 O þ 6e /3H2 þ 6OH
(14)
Combining the two half reactions of the electrolysis process, the following reaction results: 2NH3 /N2 þ 3H2
(15)
Nernst's law is used to calculate the potential of an electrode relative to a reference electrode, i.e., to express the
potential difference at the state of equilibrium across an interface, for which both elements of a redox couple are present. The Nernst law used here can be expressed as follows: ERef ¼ EoRef þ
Y v RT ln i ai i ve F
(16)
Here, i refers to the type of species involved in the half redox reaction, but with electrons not considered as one of the species; ERef is the electric potential of one of the electrodes relative to the potential of another electrode; R is the ideal gas constant; T denotes temperature (in Kelvin); F is Faraday's constant (96,485 C/mol) [24]; ve is the number of electrons in the half redox reaction; ai is a dimensionless number, calculated based on Table 1; and vi is the stoichiometric coefficient in the half redox reaction for the species i. Although Equation (16) calculates the potential voltage of one of the electrodes relative to the other electrode in the electrolyzer, in a real electrolyzer other losses should be considered. The effect of doing so is an increase in the overall voltage required to be supplied to the electrolyzer. The losses are adopted from the experimental work of Boggs and Botte [17], as presented in Table 1.
Results and discussion The simulation and analysis results of the proposed chemical hydrogen storage system are presented. Then the system performance is described in terms of energy efficiency, exergy efficiency, exergy destruction rate and the instantaneous total work energy required to store 1 mol of hydrogen. The overall energy and exergy efficiencies of the chemicalbased hydrogen storage system are 85.6% and 85.3% respectively. The PSAPS consumes 34.0 kJ of work per mole of hydrogen stored by the PSAPS and recovered through the use of the ammonia electrolyzer. The exergy destroyed during the process of storing and recovering the hydrogen is equal to the work consumed, since the system input is hydrogen and the final output is hydrogen at same conditions. The PSAPS is distinguished by its novel subzero cooled ammonia production reactor, which is maintained at a subzero temperature of 10 C with the help of a simple refrigeration unit. The refrigeration unit coefficient of performance (COP), based on simulation of a unit with R134a as the working fluid, is 3.58. The COP of the refrigeration unit is less than half of the ideal (Carnot) COP. The flow rates and properties of the streams of the PSAPS for the case of the production of 1 mol of ammonia (for which 3 mol of atomic hydrogen are stored) are presented in Tables 2a and 2b. From the stream properties in Table 2a, the system is seen to achieve a 99.999% conversion of the hydrogen sent to the chemical-based hydrogen storage system [26]. The produced ammonia from the PSAPS as shown in Table 2a (based on temperature and pressure) is in the liquid phase (at 5 bar and 4.22 C). The exergy efficiency, the unit exergy destruction, and the specific thermal energy or work interactions per mole of hydrogen stored and recovered, for each of the components in the PSAPS, are listed in Table 3. The condenser of the refrigeration unit has the highest exergy efficiency in the system, mainly because the exergy lost due to
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Table 2a e Properties of pressurized subzero-cooled ammonia production system (PSAPS) streams, including the mole flow rate of the streams chemical components (locations of listed Aspen Plus streams are shown in Fig. 3) (for the case of 1 mol of ammonia is produced from the system). kJ kg kJ a Stream T (oC) P (bar) Stream components mole flow rate (mol/s) r h ex name mol m3 mol _N _ NH _H N N N 2
A B C D E F G H I J K a
25.0 25.0 265 266 30.0 30.0 4.83 10.0 4.22 4.22 4.22
1.00 1.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.00
0.006 0.001 7.03 7.05 0.116 0.15 44.4 47.1 47.1 69.1 46.6
1.13 0.081 3.13 0.224 5.56 0.399 3.75 4.11 3.96 633 3.87
a
0.00 0.00 0.00 0.00 0.00 0.00 81.6 83.6 83.6 1.99 81.6
2
0.00 3.00 0.00 3.00 0.00 3.00 3.49 0.494 0.494 0.00 0.494
0.687 236 6.56 242 4.68 240 333 341 341 343 340
The specific exergy contains both the physical exergy and the chemical exergy [22] of the chemical compounds or elements. The specific exergy is per mole of the flowing stream.
Table 2b e Properties of the main refrigeration unit of the pressurized subzero-cooled ammonia production system (PSAPS) streams, including the mole flow rate of the streams chemical components (locations of listed Aspen Plus streams are shown in Fig. 3) (for the case of 1 mol of ammonia is produced from the system). _ R134a kg kJ a Stream T (oC) P (bar) h kJ N r ex mol m3 (mole/s) mol name L M N O P Q
3
1.00 0.00 1.00 0.00 1.00 0.00 1.17 0.165 0.165 0.00 0.165
12.7 10.3 45.6 26.3 26.3 12.7
1.80 1.80 7.00 7.00 7.00 1.80
915 899 895 897 915 915
33.7 8.83 30.3 33.2 1200 33.6
8.53 8.53 8.53 8.53 8.53 8.53
3.93 1.58 4.58 4.51 4.43 3.93
The specific exergy contains only the physical exergy for the refrigerant R134a.
the decrease in temperature is relatively small relative to the high exergy content of the fluid due to its high pressure. The subzero cooled reactor's cooling jacket has the lowest exergy efficiency of all of the PSAPS components, primarily because
the rise in coolant temperature due to the addition of thermal energy brings the temperature closer to the reference environment temperature and because the fluid pressure is already close to the reference environment pressure. This means the addition of thermal energy to the fluid decreases its exergy content. The largest contributor to the exergy destruction of the PSAPS (A5, A6, A7, A8) is the subzero cooled reactor, which contributes nearly 40% of the total exergy destruction. The PSAPS system performance and energy consumption are examined for the case of filling the ammonia in a pressurized tank, which has the dimensions of currently available gasoline tanks [25] developed by Vervetank. The properties of the tank are presented in Table 1. When delivering ammonia to the ammonia electrolyzer, the remaining ammonia in the tank is at ambient pressure and temperature (the operating pressure of the ammonia electrolyzer [17]). The tank is filled until the pressure in the tank reaches the supply pressure. The variation of the temperature and the pressure inside the ammonia tank as the ammonia entering the tank from the supply at a constant mass flow rate of 5.0 kg/s is shown in Fig. 4. Note that the variation of the temperature in the tank is
Table 3 e Exergy efficiency, specific exergy destruction, and the thermal energy or work energy (per kg of hydrogen stored and recovered) associated with components in the PMAPS (locations of these Aspen Plus blocks are shown in Fig. 3). kJ kJ kJ a q or w Component j ð%Þ exd mol H2 mol H2 mol H2 A1 A2 A3 A4 A5-A8 (subzero cooled reactor) A9 A10 A11 & A12 (condenser) A13 a
79.3 79.3 67.8 67.8 94.4 27.0 66.0 96.8 88.7
1.54 0.51 0.63 1.89 13.5 12.2 4.40 1.25 1.42
Positive refers to thermal or work energy consumed, and negative refers to thermal or work energy released.
22.3 7.41 6.92 20.7 138 139 38.7 173 0.00
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Fig. 4 e Variation of the internal tank pressure and internal tank temperature throughout the process of storing the PSAPS produced ammonia in the tank.
of a quite importance since the ammonia changes phase throughout the filling process and it will directly affect the energy consumption of the hydrogen storage system. Throughout the storage process it is expected that the storage tank temperature will vary from as low as 40 C to a high temperature of 20 C. The temperature of the ammonia in the tank drops dramatically from 25 C to 35.0 C, because the ammonia entering the tank is flashed from a pressure of 5 bar and a low temperature of 4.13 C, and it is in the liquid state. After the sudden drop in the temperature of the ammonia in the tank, the temperature starts gradually increasing until reaching the final temperature of 4.13 C. The tank internal pressure experiences a small pressure drop from 1 bar to 0.929 bar; the pressure then increases until reaching the supply pressure (5 bar). The vapor fraction (quality) of the ammonia in the tank throughout the filling process of producing ammonia by the PSAPS is shown in Fig. 5. The tank is filled with vapor before the filling process. As the filling process starts, the vapor fraction drops dramatically in the first 5% of the total filling time. After, the rate at which the vapor fraction decreases becomes much lower. The rate keeps decreasing until reaching a value of nearly 0. The vapor fraction at the end of the
Fig. 5 e Quality (vapor fraction) of the ammonia in the tank throughout the process of storing the ammonia in the tank.
Fig. 6 e Cumulative ammonia mass in the tank and the mass flow rate of the ammonia supply coming from the PSAPS throughout the storing process of the ammonia in the tank.
storing process reaches a low value of approximately 1%. Having the vapor fraction reaching a very low value of nearly zero shows the efficiency of the storage system in utilizing almost 99% of the possible liquid storage mass. Fig. 6 shows the cumulative ammonia mass in the tank and the supply mass flow rate throughout the storing process. The system is able to fill the 30,000 L tank with 7521 kg of ammonia at 5 bar and 4.13 C. Fig. 6 shows that the supply mass flow rate is constant throughout the filling process. Finally, the work consumed and the instantaneous heat loss or gain by the tank during the filling process are shown in Fig. 7. There, the total work consumed by the system including the energy required to separate the ammonia into hydrogen and nitrogen is 45.3 GJ. The tank continually gains thermal energy during the filling time. Finally, the performance of the proposed system is compared to that for the hydrogen storage technology manufactured by the company Linde [26]. If the Linde system operates at its maximum mass flow rate, over the same
Fig. 7 e Cumulative consumed work during the filling process, taking into account the energy required to separate the ammonia to reproduce the stored hydrogen, and the instantaneous thermal energy gained or lost during the filling process.
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 8 2 1 4 e1 8 2 2 4
operation time as for the proposed system, only 14 kg of hydrogen is stored. However, in terms of energy requirements to store the same mass, the Linde system consume 9.96 GJ of electrical energy, which is nearly 22% of the total energy consumed by the proposed system. However, the Linde compression system requires 94,800 s (26.3 h) to compress the same amount of hydrogen the proposed system was able to compress to fill the tanker, which is much larger than the 1500 s the proposed system requires. Fig. 7 shows that the power accumulates on the basis of a linear relation meaning that the power consumption of the operation of the system on a constant mass flow rate supply to the tank is independent of the amount of ammonia mass in the tank. Having this advantage works in favor of the proposed hydrogen storage system, since the slow nature of the proposed reaction can be accommodated by not having to fully fill up the storage tank and changing the supply mass flow rate of ammonia can be another solution. Wang et al. [27] developed a compression system that produces 700 bar hydrogen based on metal hydrides. The system is able to increase the pressure from 40 bar to 700 bar, with an energy efficiency of 12.5%. However, Wang et al. [27] did not report the exergy efficiency of the process, which is expected to have a high value since thermal energy of a low quality (at a temperature of 423 K) is used. Usually researchers argue that the filling time or the storing time of hydrogen, which reflects the mass flow rate of hydrogen, is not a primary concern. That is because, in a future hydrogen economy, large scale hydrogen production and compression systems likely will be able to handle large flow rates and have the ability to adjust these flow rates. The process of producing hydrogen on a large scale will need fast and efficient hydrogen storage systems. Other researchers have examined large scale hydrogen production from nuclear energy [6,28], and have reported high mass flow rates.
Conclusions An efficient hydrogen storage technology is needed for the hydrogen production sector and a hydrogen economy in which hydrogen is the main chemical energy carrier. A promising hydrogen storage option is chemical hydrogen storage, in which the hydrogen is stored in a chemical substance, and later the hydrogen is recovered from the storage medium. This paper proposes and analyzes a novel chemicalbased hydrogen storage technology, where the storage medium is ammonia. The proposed storage system is a pressurized subzero cooled ammonia production system (PSAPS). In the PSAPS, the ammonia is produced in a reactor that is maintained at a subzero temperature (10 C) and a pressure of 5 bar. The produced ammonia is later decomposed, to recover the stored hydrogen, in an ammonia electrolyzer. The main results of the energy and exergy analyses of the chemical hydrogen storage system are as follows: The PSAPS overall energy and exergy efficiencies including the electrolyzer energy are 85.6% and 85.3% respectively. The PSAPS consumes 34.2 kJ of work to store and recover 1 mol of hydrogen.
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The performance of the proposed hydrogen storage system is examined when it is required to fill a 30,000 L tank with pressurized ammonia, demonstrating the following: (a) The PSAPS is able to store 7521 kg of ammonia in the ammonia tank in 1500 s while, in terms of H2, it stored 885 kg of H2. (b) The PSAPS uses 45.3 GJ of work to store the 7521 kg of ammonia and to decompose it to reproduce the stored hydrogen. It is important to note that the ammonia electrolyzer is based on ideal electrochemical water decomposition and operates at a room temperature of 25 C, which results in 1.55 Wh for each gram of hydrogen produced. Careful maintenance of the electrolyzer is likely needed to avoid any degradation of the discharge system efficiency with time due to electrode poisoning. From the initial assessment of the sub-zero ammonia production system it may be considered a promising technology to be used as a hydrogen storage system. Note also that based on the literature and the chemistry of the reaction, the process is expected to have a slow reaction rate for ammonia production and thus experimental investigation of the sub-zero ammonia production system is recommended to further study the performance criteria and the ammonia production rates.
Acknowledgement The authors acknowledge the support provided by the Natural Sciences and Engineering Research Council of Canada.
references
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