- Email: [email protected]
Pressure artifacts at isothermal operation of a metal hydride tank
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 ) 7 4 2 2 e7 4 2 7
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Pressure artifacts at isothermal operation of a metal hydride tank D.O. Dunikov*, D.V. Blinov a
Joint Institute for High Temperatures of the Russian Academy of Sciences, Moscow, Russia
article info
abstract
Article history:
The paper presents the results of experimental investigations on hydrogen absorption and
Received 15 November 2018
desorption in the metal hydride reactor RS-1 containing
Received in revised form
Al0.1Fe0.4Co0.2Ni4.3 intermetallic compound at 60 C isothermal conditions. During the
24 January 2019
reactor-scale measurements of pressure-composition isotherms we observe pressure ar-
Accepted 30 January 2019
tifacts; pressures for absorption are higher and for desorption are lower than equilibrium
Available online 21 February 2019
pressures obtained after cooling down and reheating of the reactor. This thermal relaxa-
81 kg of La0.5Nd0.5-
tion procedure removes the pressure artifacts. The observed effect is similar to the large Keywords:
aliquot effect, and mostly affects the desorption isotherm. The highest measured pressure
Metal hydrides
difference is 0.7 bar (11%).
PCT measurement
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Reactor-scale Isothermal operation Pressure artifacts Large aliquot effect
Introduction Metal hydrides (MH) offer several advantages for energy storage, such as high volumetric storage density, increased safety and operation at moderate temperatures and pressures. To be competitive in practical applications metal hydride devices have to be energy efficient. Today, good outcomes are obtained only at laboratory prototype scale levels, and scale-up studies are essential to take metal hydride applications into the commercial market [1]. From a thermodynamic point of view the MH devices are heat machines working in a very narrow temperature window between absorption and desorption temperatures, thus their Carnot
efficiencies are very low. In massive reactors, transient heat transfer processes between two temperature levels require a large amount of heat energy and substantially restrain efficiency of the system, thus being the main source of efficiency losses [2]. However, thermal energy for operation could be obtained from an external reservoir, preferably from the environment or a waste heat flow, decreasing losses of useful energy (exergy). Contemporary commercial alkaline and PEM electrolyzers can deliver hydrogen at pressures exceeding 10 bar [3]. The difference between output pressure of an electrolyzer and input pressure of a fuel cell can be used to eliminate temperature difference between absorption and desorption and perform the isothermal operation. The MH reactor can even be
* Corresponding author. E-mail addresses: [email protected], [email protected] (D.O. Dunikov). https://doi.org/10.1016/j.ijhydene.2019.01.278 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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 ) 7 4 2 2 e7 4 2 7
Nomenclature C N P q V R T DH
hydrogen concentration (% wt.) number of measurements pressure (bar) hydrogen flow rate (st.L/min) hydrogen volume at standard conditions (st.L) gas constant (8.314 J/mole K) temperature ( C) desorption enthalpy (kJ/mole H2)
Subscripts in at the RS-1 reactor inlet RS inside the RS-1 reactor out at the RS-1 reactor outlet hot hot water cold cold water ds dead space abs absorption des absorption Abbreviations JIHT RAS Joint Institute for High Temperatures of the Russian Academy of Sciences RS-1 reactor for hydrogen storage RS-1 (81 kg AB5type alloy) RSP-3 reactor for hydrogen storage and purification 1 (5 kg AB5-type alloy)
charged at temperatures higher than discharge temperature [4]. Such a scheme can be used for utilization of intermittent renewable energy source, for example, the adaptability of a Totalized Hydrogen Energy Utilization System (THEUS) for the intermittent solar energy was successfully demonstrated [5]. A domestic hot water supply system can be the heat reservoir to operate MH devices. Russian building code SNiP 2.04.01e85 requires maintaining temperatures not less than 60 C for open water supply systems, which is enough to operate an MH hydrogen storage system. Predicting the behavior of an MH tank can be a challenging task. Capacity measurements on large samples can produce different results [6] due to the inability to maintain uniformity over the bed during the measurements, especially at reactor scale. Non-uniformity can arise from several causes: Heat effect of reaction and insufficient heat transfer creates temperature gradients, which result in compositional nonuniformity along the bed after absorption or desorption of a portion of hydrogen [7]. Large aliquot effect makes measuring “true” pressure-composition isotherms (PCI) experimentally difficult in isochoric conditions [8], composition gradients are preserved long after both temperature and the reaction have reached a stable equilibrium [9]. Scale effect [10] may also contribute to stresses arising in confined beds. Granular separation and interaction with reactor walls during cycling results in bed nonuniformities [11,12], which change equilibrium pressure. Finally, a large-scale bed may be non-uniform from the start, e.g., composed from several ingots, and small scale
7423
samples taken for PCI measurements may not represent the whole bed. The goal of the present study is to obtain PCI experimentally for absorption and desorption of hydrogen in a largescale metal hydride reactor and confirm the ability to operate in isothermal conditions with the use of output pressure of an electrolyzer as a driving force for hydrogen absorption.
Experimental technique Metal hydride reactor for hydrogen storage RS-1 We use an RS-1 reactor, designed in JIHT RAS [13]. The reactor consists of 49 cartridges made from two coaxial stainless steel tubes with metal hydride placed in a space between them. Each cartridge is cooled from inside and outside by liquid, and inserted into a common hydrogen collector from one butt and hermetically sealed from another. The reactor is filled with 81 kg of La0.5Nd0.5Al0.1Fe0.4Co0.2Ni4.3 intermetallic compound (IMC), which was provided by Dr. S. Mitrokhin's team from the Lomonosov Moscow State University. By supplier estimations desorption pressures are 1.1 bar at 25 С and 11.6 bar at 80 С, maximum capacity is 1.1%wt., desorption enthalpy is DH ¼ 35.3 kJ/mol H2. The nominal capacity of the reactor is 1% wt. or 9000 st.L of hydrogen, and heat from an exhaust airflow from a commercial PEM FC is sufficient to maintain the steady flow of desorbed hydrogen needed for operation at 1.1 kW(e) power [14]. The reactor is installed in an experimental test bench 12-04 JIHT RAS (Fig. 1). The gas supply is provided from cylinders (hydrogen, nitrogen for purge, helium for dead space measurements), or from a PEM electrolyzer HPAC 10 ITM Power (rated capacity 10 st.L/min at 15 bar) and a metal hydride reactor RSP-3 (maximum capacity 750 st.L H2) [15]. Tap water (from 0.05 to 0.3 kg/s) is used as a heat exchange agent. The test bench has connections to a vacuum system and an automatic control system using LabView software. Inlet gas flows are controlled and measured by Bronkhorst EL-FLOW Select mass flow meter/controllers F-202AC-RAA55-V, outlet gas flows are measured by a Bronkhorst EL-FLOW Select F-112AC flow meter, pressures inside the reactors and the gas supply are measured by Aplisens pressure transmitters model PC28, and water temperatures are measured by thin film platinum sensors Heraeus M422, 1 kU.
Reactor-scale PCI measurements During the experiments, hot water was used to keep the RS-1 reactor at a nearly constant temperature of 60 C. Measurement of each point during the experiments included equilibration of the reactor before and after a portion of hydrogen was added or removed from the RS-1. Thermal equilibration was considered to be reached if the temperature inside the reactor was a stable 60 ± 0.5 C, the temperature difference between the inlet and outlet of the water heat exchanger was smaller than 1 K, and pressure remained constant for at least 1 h. Before absorption measurements, the RS-1 reactor was fully evacuated by a vacuum pump. To increase inlet
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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 ) 7 4 2 2 e7 4 2 7
Heater
Tap water
P RS
T hot
Out
F out
RS-1 FC RS
T out
T cold P RSP T out1
RSP-3
P in
Reducer
HPAC 10
Vacuum system
H2
N2
FC in
He
Fig. 1 e Scheme of the experiments: RS-1 and RSP-3 - metal hydride reactors; HPAC 10 e electrolyzer; P, T e pressure and temperature gauges; FC e flow controller; F- flow meter.
hydrogen flow and avoid fluctuations, the RSP-3 reactor was used as hydrogen buffer between the RS-1 and the electrolyzer. The RSP-3 was fully charged, heated, and a portion of hydrogen ca. 500 st.L was fed into the RS-1, and the reactor was equilibrated without any additional measures. Desorption measurements were started from the last point of the absorption isotherm. Portions of hydrogen (200e500 st.L) were released from the RS-1 into the atmosphere, and the reactor was equilibrated after each point. Pressure artifacts due to sample size effects become obvious from the start of desorption, and an additional equilibration procedure for thermal relaxation was added: the reactor was cooled down by cold water and equilibrated, heated to 60 C and equilibrated again, thus removing the pressure artifacts.
Isothermal operation of the RS-1 The RS-1 reactor was charged in 4 days with pure hydrogen from the PEM electrolyzer HPAC 10. Hydrogen was fed to RS-1 via the pressure reducer at maximum pressure 10 bar. Measured hydrogen flow was qin ¼ 8.8 st.L/min and oscillated between 5.5 and 12.5 st.L/min. On the 4th day, the hydrogen pressure inside the reactor had reached 9.9 MPa and the absorption reaction had gradually stopped. The thermal relaxation procedure was applied after each day. On the 5th day, the reactor was discharged into the atmosphere at flow rate qout ¼ 30 st.L/min via a pressure reducer set to 1.5 bar.
Measurement issues It is impossible to reach true thermodynamic equilibrium and measure “true” isotherms without the compositional homogeneity. For reactor-scale metal hydride beds, it is practically impossible without special efforts, and we measured some apparent PCIs, which may not correspond to real isotherms.
Thermal and pressure equilibrium for the RS-1 reactor is hard to reach as well, and it took about 1e2 h to obtain constant pressure for every point. We have managed to reach DT ¼ 1 K temperature uncertainty of our experiments with RS-1, which corresponds to the following measurement error in equilibrium pressure of the 60 C isotherm: DP DH DT 35:3 kJ=mol 1K ¼ ¼ z4% P RT T 8:314 J=mol K ð273:15 þ 60Þ2 K2
(1)
In sorption measurements, the determination of the dead space usually accounts for a large element of uncertainty [16]. Dead space measurements with helium were made for fully outgassed and the half-charged states to estimate the dependence of the dead space volume on the hydrogen concentration. Before each measurement, the reactor was fully discharged and outgassed under vacuum conditions. Helium properties were calculated using REFPROP [17]. At first, the empty reactor was heated to 29.9 C, helium was charged from a gas cylinder, and the reactor was kept under constant temperature for 1 h reaching the final pressure 5.15 bar. Total volume of the dead space including pores, reactor parts and tubes after the flow controller FCRS is 16.0 ± 0.1 L. Next, the empty reactor was filled with 4500 st.L of hydrogen from a gas cylinder and equilibrated at 30.5 C. The half-charged reactor was filled with helium and kept under constant temperature for 2 h. Measured partial pressure of He was 5.74 bar after 2 h of equilibration at a constant temperature. Dead space for the partially charged reactor is 14.7 ± 0.1 L. The density of a non-hydrogenated LaNi5-type alloy is ca. 8300 kg/m3 [12], thus the volume of the metal particles is 9.76 L and increases by 1.3 L for C ¼ 0.5 %wt. The estimated density of the hydride phase is 6100 kg/m3 for H/M ¼ 1, which is lower than values reported in literature, e.g., 6590 kg/m3 for LaNi5H6
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 ) 7 4 2 2 e7 4 2 7
[18], but this difference is a second-order correction in our measurements and can be neglected. Thus the correction for the dead space of the RS-1 reactor is: Vds ½st:L ¼ ð16 2:6C ½%wt:Þ
273:15 PRS ½bar TRS ½ C þ 273:15
(2)
For long run experiments, a precision of gas mass flow measurement starts to play an important role. Each flow controller and flow meter produces some noise during the measurement even if valves are closed, and there is no gas flow. For example, during the 4-day charging of the RS-1 reactor from the HPAC-10 electrolyzer, the measured value of hydrogen volume charged into RS-1 was 8930 st.L, while the systematic error was 220 st.L (2.5%), and corrected value is 8710 st.L. To eliminate the systematic error in flow measurement, at each point we measured the flow controller/meter noise with closed valves and excluded its value from experimental results. In addition, we used higher hydrogen flow rates to minimize charging times. Measurement error for the volume of a portion of hydrogen is 0.5%, the error accumulates during the consecutive N measurements at absorption by the factor N1/2, and thus the final error for the absorption curve is about 2% (18 portions) or ±90 st.L for the full reactor. The uncertainty of temperature at isothermal conditions is 1 K, and the measurement error for the equilibrium pressure is 4%.
Experimental results Results for the absorption experiments are presented in Fig. 2. During 4 days the reactor has successfully consumed 8500 st.L of hydrogen at the constant flow rate 8.8 st.L/min from the electrolyzer at 60 C and 10 bar. When the pressure inside the reactor has become close to the pressure in the supply line, the hydrogen flow has dropped down, and the reactor has been slowly charged at almost constant pressure up to 8700 st.L. It took about a month to measure an apparent absorption isotherm, which appeared to be smooth and at first glance was not affected by pressure artifacts. Nevertheless, the apparent isotherm has higher pressure than equilibrium points obtained after the thermal relaxation of the RS-1 during the
Fig. 2 e Experimental results for absorption.
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experiments with the electrolyzer. Indeed, considering the measurement uncertainty, several points definitely differ, but the systematic behavior is obvious. Experimental results for desorption are presented in Fig. 3. From the reactor 7960 st.L of H2 has been discharged at constant flow rate 30 st.L/min at 60 C and outlet pressure 1.5 bar, which is enough to operate a 2 kW PEM fuel cell for more than 4 h. When the pressure inside the reactor has reached 1.5 bar, and residual hydrogen volume was 740 st.L, a crisis has occurred, and flow rate dropped down, and discharge has been stopped. Thus, the ability of the RS-1 reactor to operate at isothermal conditions using pressure difference between the electrolyzer and the fuel cell is confirmed. The apparent desorption isotherm (Fig. 3) differs significantly from the absorption isotherm. Equilibrium pressures at isothermal PCI measurements have not formed a smooth curve. Pressure has been decreasing much faster than expected, and measured points tend to the dynamic discharge curve, and equilibrium pressures depend on a history of discharge. Thus, the pressure artifacts at desorption are evident. To obtain the desorption isotherm the reactor was cooled down and reheated after the release of several portions of hydrogen. An example for a point at C ¼ 0.71 wt% is presented in Fig. 4. At the start, the reactor is heated by hot water Thot ¼ 60 C. During the equilibration, temperature difference with outlet water Tout is within 1 K, and pressure inside the reactor is at the constant value PRS ¼ 6.41 bar. Then the outlet valve opens and releases 203 st.L of hydrogen, the reactor is equilibrated again, and pressure reaches 5.95 bar. After the thermal relaxation (the reactor was cooled by cold water Tcold ¼ 10 C, reheated and equilibrated) pressure rises to 6.33 bar, thus the pressure artifact is DPRS ¼ 0.38 bar. The equilibrium points after the thermal relaxation form a smooth curve. We applied a Lengmuir-like fit [19] to apparent absorption isotherm and equilibrium points: C ¼ a1
a2 P1=a3 1 þ a2 P1=a3
(3)
and obtained a good correspondence with the results for C > 0.2%wt. The fit parameters are presented in Table 1. Absorption Pabs and desorption Pdes curves for the equilibrium
Fig. 3 e Experimental results for desorption.
7426
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 ) 7 4 2 2 e7 4 2 7
Fig. 4 e Inlet Thot and outlet Tout water temperatures, pressure inside the reactor PRS and hydrogen flow q for the desorption isotherm at C ¼ 0.71 wt%, the pressure artifact is DPRS ¼ 0.38 bar.
Table 1 e Parameters for the Lengmuir-like fit (3) for the PCIs at 60 C. Experiment Apparent absorption isotherm Absorption with thermal relaxation Desorption with thermal relaxation
a1
a2
a3
1.04 0.994 1.01
2.64e-4 4.42e-5 4.16e-5
0.222 0.177 0.170
points after thermal relaxation conform to each other with almost constant hysteresis ln (Pabs/Pdes) ~0.06. Unfortunately, it is impossible to open the RS-1 reactor and obtain a sample for PCT measurement in a Sieverts apparatus without exposing the activated IMC to the air and compare our results with “true” isotherms. Nevertheless, we can conclude that the thermal relaxation removes pressure artifacts. During reheating, the MH bed releases some hydrogen, which redistributes more uniformly, thus decreasing concentration gradients over the bed.
During the reactor-scale measurements of the pressurecomposition isotherms the pressure artifacts are observed, pressures for absorption are higher and for desorption are lower than equilibrium pressures obtained after the thermal relaxation of the MH bed. The effect is similar to the large aliquot effect, and mostly affects the desorption isotherm, the highest measured pressure difference is 0.7 bar (11%); The pressure artifacts are removed by the thermal relaxation procedure. During the thermal relaxation, which includes cooling down and reheating of the reactor, some hydrogen is released, and hydrogen concentration redistributes more uniformly over the bed; Without thermal relaxation, the desorption isotherm tends to the dynamic desorption curve.
Acknowledgements Conclusions We have performed experimental investigations on hydrogen absorption and desorption in the metal hydride reactor RS-1 containing 81 kg of La0.5Nd0.5Al0.1Fe0.4Co0.2Ni4.3 intermetallic compound at 60 C isothermal conditions. From the experimental results we conclude: It is possible to operate the MH reactor in isothermal conditions using the electrolyzer output pressure as the driving force for hydrogen absorption and hot water as the heat reservoir, thus avoiding transient heat transfer processes, which are the main source of efficiency losses;
The work has been funded by the Russian Science Foundation (project No. 17-19-01738).
references
[1] Muthukumar P, Kumar A, Raju NN, Malleswararao K, Rahman MM. A critical review on design aspects and developmental status of metal hydride based thermal machines. Int J Hydrog Energy 2018;43(37):17753e79. https:// doi.org/10.1016/j.ijhydene.2018.07.157. [2] Lototskyy MV, Yartys VA, Pollet BG, Bowman Jr RC. Metal hydride hydrogen compressors: a review. Int J Hydrog Energy
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 ) 7 4 2 2 e7 4 2 7
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2014;39(11):5818e51. https://doi.org/10.1016/j.ijhydene.2014. 01.158. Felgenhauer M, Hamacher T. State-of-the-art of commercial electrolyzers and on-site hydrogen generation for logistic vehicles in South Carolina. Int J Hydrog Energy 2015;40(5):2084e90. https://doi.org/10.1016/j.ijhydene.2014. 12.043. Tange M, Maeda T, Nakano A, Ito H, Kawakami Y, Masuda M, Takahashi T. Experimental study of hydrogen storage with reaction heat recovery using metal hydride in a totalized hydrogen energy utilization system. Int J Hydrog Energy 2011;36(18):11767e76. https://doi.org/10.1016/j.ijhydene.2011. 06.023. Bhogilla SS, Ito H, Kato A, Nakano A. Experimental study on a laboratory scale Totalized Hydrogen Energy Utilization System for solar photovoltaic application. Appl Energy 2016;177:309e22. https://doi.org/10.1016/j.apenergy.2016.05. 145. Gross KJ, Carrington KR, Barcelo S, Karkamkar A, Purewal J, Ma S, Zhou H-C, Dantzer P, Ott K, Burrell T, Semeslberger T, Pivak Y, Dam B, Chandra D. Recommended best practices for the characterization of storage properties of hydrogen storage materials: V3.34 Feb 21. 2012. Available: http:// energy.gov/sites/prod/files/2014/03/f12/best_practices_ hydrogen_storage.pdf. Pons M, Dantzer P. Heat transfer in hydride packed beds. III. Significant interactions between the temperature and composition gradients*. Z Phys Chem 1994;225. https://doi. org/10.1524/zpch.1994.183.Part_1_2.225. Gray EMA, Buckley CE, Kisi EH. Stability of the hydrogen absorption and desorption plateaux in LaNi5H Part 2: effects of absorbing and desorbing large aliquots of hydrogen. J Alloy Comp 1994;215(1):201e11. https://doi.org/10.1016/09258388(94)90841-9. Mohammadshahi SS, Webb TA, Gray EM, Webb CJ. Experimental and theoretical study of compositional inhomogeneities in LaNi5Dx owing to temperature gradients and pressure hysteresis, investigated using spatially resolved in-situ neutron diffraction. Int J Hydrog Energy 2017;42(10):6793e800. https://doi.org/10.1016/j.ijhydene.2016. 12.061. Malyshenko SP, Mitrokhin SV, Romanov IA. Effects of scaling in metal hydride materials for hydrogen storage and compression. J Alloy Comp 2015;645(Supplement 1):S84e8. https://doi.org/10.1016/j.jallcom.2014.12.273.
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[11] Kazakov AN, Romanov IA, Kuleshov VN, Dunikov DO. Experimental investigations of adsorption characteristics and porosity of activated metal hydride powders. J Phys Conf 2017;891(1), 012115. https://doi.org/10.1088/1742-6596/891/1/ 012115. [12] Blinov DV, Dunikov DO, Kazakov AN, Romanov IA. Influence of geometrical non-uniformities of LaNi5 metal hydride bed on its structure and heat and mass transfer at hydrogen absorption. J Phys Conf 2017;891(1):012119. https://doi.org/10. 1088/1742-6596/891/1/012119. [13] Malyshenko SP, Borzenko VI, Dunikov DO, Nazarova OV. Metal hydride technologies of hydrogen energy storage for independent power supply systems constructed on the basis of renewable sources of energy. Therm Eng 2012;59(6):468e78. https://doi.org/0.1134/ S0040601512060055. [14] Borzenko V, Eronin A. The use of air as heating agent in hydrogen metal hydride storage coupled with PEM fuel cell. Int J Hydrog Energy 2016;41(48):23120e4. https://doi.org/10. 1016/j.ijhydene.2016.10.067. [15] Blinov DV, Borzenko VI, Dunikov DO, Romanov IA. Experimental investigations and a simple balance model of a metal hydride reactor. Int J Hydrog Energy 2014;39(33):19361e8. https://doi.org/10.1016/j.ijhydene.2014. 07.048. [16] Thommes M, Kaneko K, Neimark Alexander V, Olivier James P, Rodriguez-Reinoso F, Rouquerol J, Sing Kenneth SW. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl Chem 2015;1051. https://doi. org/10.1515/pac-2014-1117. [17] Lemmon EW, Huber ML, McLinden MO. NIST standard reference database 23: reference fluid thermodynamic and transport properties-REFPROP, version 9.1. National Institute of Standards and Technology; 2013. https://doi.org/10.18434/ T4JS3C. [18] Matsushita M, Monde M, Mitsutake Y. Experimental formula for estimating porosity in a metal hydride packed bed. Int J Hydrog Energy 2013;38(17):7056e64. https://doi.org/10.1016/j. ijhydene.2013.04.005. € m H, Suda S, Lewis D. A numerical expression for [19] Bjurstro the P-C-T properties of metal hydrides. J Less Common Met 1987;130:365e70. https://doi.org/10.1016/0022-5088(87) 90130-5.
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Short Communication
Pressure artifacts at isothermal operation of a metal hydride tank D.O. Dunikov*, D.V. Blinov a
Joint Institute for High Temperatures of the Russian Academy of Sciences, Moscow, Russia
article info
abstract
Article history:
The paper presents the results of experimental investigations on hydrogen absorption and
Received 15 November 2018
desorption in the metal hydride reactor RS-1 containing
Received in revised form
Al0.1Fe0.4Co0.2Ni4.3 intermetallic compound at 60 C isothermal conditions. During the
24 January 2019
reactor-scale measurements of pressure-composition isotherms we observe pressure ar-
Accepted 30 January 2019
tifacts; pressures for absorption are higher and for desorption are lower than equilibrium
Available online 21 February 2019
pressures obtained after cooling down and reheating of the reactor. This thermal relaxa-
81 kg of La0.5Nd0.5-
tion procedure removes the pressure artifacts. The observed effect is similar to the large Keywords:
aliquot effect, and mostly affects the desorption isotherm. The highest measured pressure
Metal hydrides
difference is 0.7 bar (11%).
PCT measurement
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Reactor-scale Isothermal operation Pressure artifacts Large aliquot effect
Introduction Metal hydrides (MH) offer several advantages for energy storage, such as high volumetric storage density, increased safety and operation at moderate temperatures and pressures. To be competitive in practical applications metal hydride devices have to be energy efficient. Today, good outcomes are obtained only at laboratory prototype scale levels, and scale-up studies are essential to take metal hydride applications into the commercial market [1]. From a thermodynamic point of view the MH devices are heat machines working in a very narrow temperature window between absorption and desorption temperatures, thus their Carnot
efficiencies are very low. In massive reactors, transient heat transfer processes between two temperature levels require a large amount of heat energy and substantially restrain efficiency of the system, thus being the main source of efficiency losses [2]. However, thermal energy for operation could be obtained from an external reservoir, preferably from the environment or a waste heat flow, decreasing losses of useful energy (exergy). Contemporary commercial alkaline and PEM electrolyzers can deliver hydrogen at pressures exceeding 10 bar [3]. The difference between output pressure of an electrolyzer and input pressure of a fuel cell can be used to eliminate temperature difference between absorption and desorption and perform the isothermal operation. The MH reactor can even be
* Corresponding author. E-mail addresses: [email protected], [email protected] (D.O. Dunikov). https://doi.org/10.1016/j.ijhydene.2019.01.278 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
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 ) 7 4 2 2 e7 4 2 7
Nomenclature C N P q V R T DH
hydrogen concentration (% wt.) number of measurements pressure (bar) hydrogen flow rate (st.L/min) hydrogen volume at standard conditions (st.L) gas constant (8.314 J/mole K) temperature ( C) desorption enthalpy (kJ/mole H2)
Subscripts in at the RS-1 reactor inlet RS inside the RS-1 reactor out at the RS-1 reactor outlet hot hot water cold cold water ds dead space abs absorption des absorption Abbreviations JIHT RAS Joint Institute for High Temperatures of the Russian Academy of Sciences RS-1 reactor for hydrogen storage RS-1 (81 kg AB5type alloy) RSP-3 reactor for hydrogen storage and purification 1 (5 kg AB5-type alloy)
charged at temperatures higher than discharge temperature [4]. Such a scheme can be used for utilization of intermittent renewable energy source, for example, the adaptability of a Totalized Hydrogen Energy Utilization System (THEUS) for the intermittent solar energy was successfully demonstrated [5]. A domestic hot water supply system can be the heat reservoir to operate MH devices. Russian building code SNiP 2.04.01e85 requires maintaining temperatures not less than 60 C for open water supply systems, which is enough to operate an MH hydrogen storage system. Predicting the behavior of an MH tank can be a challenging task. Capacity measurements on large samples can produce different results [6] due to the inability to maintain uniformity over the bed during the measurements, especially at reactor scale. Non-uniformity can arise from several causes: Heat effect of reaction and insufficient heat transfer creates temperature gradients, which result in compositional nonuniformity along the bed after absorption or desorption of a portion of hydrogen [7]. Large aliquot effect makes measuring “true” pressure-composition isotherms (PCI) experimentally difficult in isochoric conditions [8], composition gradients are preserved long after both temperature and the reaction have reached a stable equilibrium [9]. Scale effect [10] may also contribute to stresses arising in confined beds. Granular separation and interaction with reactor walls during cycling results in bed nonuniformities [11,12], which change equilibrium pressure. Finally, a large-scale bed may be non-uniform from the start, e.g., composed from several ingots, and small scale
7423
samples taken for PCI measurements may not represent the whole bed. The goal of the present study is to obtain PCI experimentally for absorption and desorption of hydrogen in a largescale metal hydride reactor and confirm the ability to operate in isothermal conditions with the use of output pressure of an electrolyzer as a driving force for hydrogen absorption.
Experimental technique Metal hydride reactor for hydrogen storage RS-1 We use an RS-1 reactor, designed in JIHT RAS [13]. The reactor consists of 49 cartridges made from two coaxial stainless steel tubes with metal hydride placed in a space between them. Each cartridge is cooled from inside and outside by liquid, and inserted into a common hydrogen collector from one butt and hermetically sealed from another. The reactor is filled with 81 kg of La0.5Nd0.5Al0.1Fe0.4Co0.2Ni4.3 intermetallic compound (IMC), which was provided by Dr. S. Mitrokhin's team from the Lomonosov Moscow State University. By supplier estimations desorption pressures are 1.1 bar at 25 С and 11.6 bar at 80 С, maximum capacity is 1.1%wt., desorption enthalpy is DH ¼ 35.3 kJ/mol H2. The nominal capacity of the reactor is 1% wt. or 9000 st.L of hydrogen, and heat from an exhaust airflow from a commercial PEM FC is sufficient to maintain the steady flow of desorbed hydrogen needed for operation at 1.1 kW(e) power [14]. The reactor is installed in an experimental test bench 12-04 JIHT RAS (Fig. 1). The gas supply is provided from cylinders (hydrogen, nitrogen for purge, helium for dead space measurements), or from a PEM electrolyzer HPAC 10 ITM Power (rated capacity 10 st.L/min at 15 bar) and a metal hydride reactor RSP-3 (maximum capacity 750 st.L H2) [15]. Tap water (from 0.05 to 0.3 kg/s) is used as a heat exchange agent. The test bench has connections to a vacuum system and an automatic control system using LabView software. Inlet gas flows are controlled and measured by Bronkhorst EL-FLOW Select mass flow meter/controllers F-202AC-RAA55-V, outlet gas flows are measured by a Bronkhorst EL-FLOW Select F-112AC flow meter, pressures inside the reactors and the gas supply are measured by Aplisens pressure transmitters model PC28, and water temperatures are measured by thin film platinum sensors Heraeus M422, 1 kU.
Reactor-scale PCI measurements During the experiments, hot water was used to keep the RS-1 reactor at a nearly constant temperature of 60 C. Measurement of each point during the experiments included equilibration of the reactor before and after a portion of hydrogen was added or removed from the RS-1. Thermal equilibration was considered to be reached if the temperature inside the reactor was a stable 60 ± 0.5 C, the temperature difference between the inlet and outlet of the water heat exchanger was smaller than 1 K, and pressure remained constant for at least 1 h. Before absorption measurements, the RS-1 reactor was fully evacuated by a vacuum pump. To increase inlet
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Heater
Tap water
P RS
T hot
Out
F out
RS-1 FC RS
T out
T cold P RSP T out1
RSP-3
P in
Reducer
HPAC 10
Vacuum system
H2
N2
FC in
He
Fig. 1 e Scheme of the experiments: RS-1 and RSP-3 - metal hydride reactors; HPAC 10 e electrolyzer; P, T e pressure and temperature gauges; FC e flow controller; F- flow meter.
hydrogen flow and avoid fluctuations, the RSP-3 reactor was used as hydrogen buffer between the RS-1 and the electrolyzer. The RSP-3 was fully charged, heated, and a portion of hydrogen ca. 500 st.L was fed into the RS-1, and the reactor was equilibrated without any additional measures. Desorption measurements were started from the last point of the absorption isotherm. Portions of hydrogen (200e500 st.L) were released from the RS-1 into the atmosphere, and the reactor was equilibrated after each point. Pressure artifacts due to sample size effects become obvious from the start of desorption, and an additional equilibration procedure for thermal relaxation was added: the reactor was cooled down by cold water and equilibrated, heated to 60 C and equilibrated again, thus removing the pressure artifacts.
Isothermal operation of the RS-1 The RS-1 reactor was charged in 4 days with pure hydrogen from the PEM electrolyzer HPAC 10. Hydrogen was fed to RS-1 via the pressure reducer at maximum pressure 10 bar. Measured hydrogen flow was qin ¼ 8.8 st.L/min and oscillated between 5.5 and 12.5 st.L/min. On the 4th day, the hydrogen pressure inside the reactor had reached 9.9 MPa and the absorption reaction had gradually stopped. The thermal relaxation procedure was applied after each day. On the 5th day, the reactor was discharged into the atmosphere at flow rate qout ¼ 30 st.L/min via a pressure reducer set to 1.5 bar.
Measurement issues It is impossible to reach true thermodynamic equilibrium and measure “true” isotherms without the compositional homogeneity. For reactor-scale metal hydride beds, it is practically impossible without special efforts, and we measured some apparent PCIs, which may not correspond to real isotherms.
Thermal and pressure equilibrium for the RS-1 reactor is hard to reach as well, and it took about 1e2 h to obtain constant pressure for every point. We have managed to reach DT ¼ 1 K temperature uncertainty of our experiments with RS-1, which corresponds to the following measurement error in equilibrium pressure of the 60 C isotherm: DP DH DT 35:3 kJ=mol 1K ¼ ¼ z4% P RT T 8:314 J=mol K ð273:15 þ 60Þ2 K2
(1)
In sorption measurements, the determination of the dead space usually accounts for a large element of uncertainty [16]. Dead space measurements with helium were made for fully outgassed and the half-charged states to estimate the dependence of the dead space volume on the hydrogen concentration. Before each measurement, the reactor was fully discharged and outgassed under vacuum conditions. Helium properties were calculated using REFPROP [17]. At first, the empty reactor was heated to 29.9 C, helium was charged from a gas cylinder, and the reactor was kept under constant temperature for 1 h reaching the final pressure 5.15 bar. Total volume of the dead space including pores, reactor parts and tubes after the flow controller FCRS is 16.0 ± 0.1 L. Next, the empty reactor was filled with 4500 st.L of hydrogen from a gas cylinder and equilibrated at 30.5 C. The half-charged reactor was filled with helium and kept under constant temperature for 2 h. Measured partial pressure of He was 5.74 bar after 2 h of equilibration at a constant temperature. Dead space for the partially charged reactor is 14.7 ± 0.1 L. The density of a non-hydrogenated LaNi5-type alloy is ca. 8300 kg/m3 [12], thus the volume of the metal particles is 9.76 L and increases by 1.3 L for C ¼ 0.5 %wt. The estimated density of the hydride phase is 6100 kg/m3 for H/M ¼ 1, which is lower than values reported in literature, e.g., 6590 kg/m3 for LaNi5H6
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[18], but this difference is a second-order correction in our measurements and can be neglected. Thus the correction for the dead space of the RS-1 reactor is: Vds ½st:L ¼ ð16 2:6C ½%wt:Þ
273:15 PRS ½bar TRS ½ C þ 273:15
(2)
For long run experiments, a precision of gas mass flow measurement starts to play an important role. Each flow controller and flow meter produces some noise during the measurement even if valves are closed, and there is no gas flow. For example, during the 4-day charging of the RS-1 reactor from the HPAC-10 electrolyzer, the measured value of hydrogen volume charged into RS-1 was 8930 st.L, while the systematic error was 220 st.L (2.5%), and corrected value is 8710 st.L. To eliminate the systematic error in flow measurement, at each point we measured the flow controller/meter noise with closed valves and excluded its value from experimental results. In addition, we used higher hydrogen flow rates to minimize charging times. Measurement error for the volume of a portion of hydrogen is 0.5%, the error accumulates during the consecutive N measurements at absorption by the factor N1/2, and thus the final error for the absorption curve is about 2% (18 portions) or ±90 st.L for the full reactor. The uncertainty of temperature at isothermal conditions is 1 K, and the measurement error for the equilibrium pressure is 4%.
Experimental results Results for the absorption experiments are presented in Fig. 2. During 4 days the reactor has successfully consumed 8500 st.L of hydrogen at the constant flow rate 8.8 st.L/min from the electrolyzer at 60 C and 10 bar. When the pressure inside the reactor has become close to the pressure in the supply line, the hydrogen flow has dropped down, and the reactor has been slowly charged at almost constant pressure up to 8700 st.L. It took about a month to measure an apparent absorption isotherm, which appeared to be smooth and at first glance was not affected by pressure artifacts. Nevertheless, the apparent isotherm has higher pressure than equilibrium points obtained after the thermal relaxation of the RS-1 during the
Fig. 2 e Experimental results for absorption.
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experiments with the electrolyzer. Indeed, considering the measurement uncertainty, several points definitely differ, but the systematic behavior is obvious. Experimental results for desorption are presented in Fig. 3. From the reactor 7960 st.L of H2 has been discharged at constant flow rate 30 st.L/min at 60 C and outlet pressure 1.5 bar, which is enough to operate a 2 kW PEM fuel cell for more than 4 h. When the pressure inside the reactor has reached 1.5 bar, and residual hydrogen volume was 740 st.L, a crisis has occurred, and flow rate dropped down, and discharge has been stopped. Thus, the ability of the RS-1 reactor to operate at isothermal conditions using pressure difference between the electrolyzer and the fuel cell is confirmed. The apparent desorption isotherm (Fig. 3) differs significantly from the absorption isotherm. Equilibrium pressures at isothermal PCI measurements have not formed a smooth curve. Pressure has been decreasing much faster than expected, and measured points tend to the dynamic discharge curve, and equilibrium pressures depend on a history of discharge. Thus, the pressure artifacts at desorption are evident. To obtain the desorption isotherm the reactor was cooled down and reheated after the release of several portions of hydrogen. An example for a point at C ¼ 0.71 wt% is presented in Fig. 4. At the start, the reactor is heated by hot water Thot ¼ 60 C. During the equilibration, temperature difference with outlet water Tout is within 1 K, and pressure inside the reactor is at the constant value PRS ¼ 6.41 bar. Then the outlet valve opens and releases 203 st.L of hydrogen, the reactor is equilibrated again, and pressure reaches 5.95 bar. After the thermal relaxation (the reactor was cooled by cold water Tcold ¼ 10 C, reheated and equilibrated) pressure rises to 6.33 bar, thus the pressure artifact is DPRS ¼ 0.38 bar. The equilibrium points after the thermal relaxation form a smooth curve. We applied a Lengmuir-like fit [19] to apparent absorption isotherm and equilibrium points: C ¼ a1
a2 P1=a3 1 þ a2 P1=a3
(3)
and obtained a good correspondence with the results for C > 0.2%wt. The fit parameters are presented in Table 1. Absorption Pabs and desorption Pdes curves for the equilibrium
Fig. 3 e Experimental results for desorption.
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Fig. 4 e Inlet Thot and outlet Tout water temperatures, pressure inside the reactor PRS and hydrogen flow q for the desorption isotherm at C ¼ 0.71 wt%, the pressure artifact is DPRS ¼ 0.38 bar.
Table 1 e Parameters for the Lengmuir-like fit (3) for the PCIs at 60 C. Experiment Apparent absorption isotherm Absorption with thermal relaxation Desorption with thermal relaxation
a1
a2
a3
1.04 0.994 1.01
2.64e-4 4.42e-5 4.16e-5
0.222 0.177 0.170
points after thermal relaxation conform to each other with almost constant hysteresis ln (Pabs/Pdes) ~0.06. Unfortunately, it is impossible to open the RS-1 reactor and obtain a sample for PCT measurement in a Sieverts apparatus without exposing the activated IMC to the air and compare our results with “true” isotherms. Nevertheless, we can conclude that the thermal relaxation removes pressure artifacts. During reheating, the MH bed releases some hydrogen, which redistributes more uniformly, thus decreasing concentration gradients over the bed.
During the reactor-scale measurements of the pressurecomposition isotherms the pressure artifacts are observed, pressures for absorption are higher and for desorption are lower than equilibrium pressures obtained after the thermal relaxation of the MH bed. The effect is similar to the large aliquot effect, and mostly affects the desorption isotherm, the highest measured pressure difference is 0.7 bar (11%); The pressure artifacts are removed by the thermal relaxation procedure. During the thermal relaxation, which includes cooling down and reheating of the reactor, some hydrogen is released, and hydrogen concentration redistributes more uniformly over the bed; Without thermal relaxation, the desorption isotherm tends to the dynamic desorption curve.
Acknowledgements Conclusions We have performed experimental investigations on hydrogen absorption and desorption in the metal hydride reactor RS-1 containing 81 kg of La0.5Nd0.5Al0.1Fe0.4Co0.2Ni4.3 intermetallic compound at 60 C isothermal conditions. From the experimental results we conclude: It is possible to operate the MH reactor in isothermal conditions using the electrolyzer output pressure as the driving force for hydrogen absorption and hot water as the heat reservoir, thus avoiding transient heat transfer processes, which are the main source of efficiency losses;
The work has been funded by the Russian Science Foundation (project No. 17-19-01738).
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