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(1)Thermal analysis and Neutron imaging studies of the Metal Hydride Storage tank. May 18, 2011.

(2) Faculty of Physics and Applied Computer Science. Doctoral thesis. Nivas Babu Selvara j. Thermal analysis and neutron imaging studies of the metal hydride storage tank. Supervisor: prof. dr. hab. Henryk Figiel. Cracow, April 2011.

(3) Declaration of the author of this dissertation: Aware of legal responsibility for making untrue statements I hereby declare that I have writtenthis dissertation myself and all the contents of the dissertation have been obtained by legal means.. Date, signature of author. Declaration of the thesis Supervisor: This dissertation is ready to be reviewed.. Date, signature of supervisor.

(4) Contents 1 Introduction. 11. 1.1. Need for hydrogen driven applications. . . . . . . . . . . . . . . . . .. 11. 1.2. Challenges of hydrogen-based energy systems . . . . . . . . . . . . . .. 13. 1.3. Hydrogen basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 14. 1.4. Topic of the thesis. 18. . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 2 Hydrogen Storage Methods 2.1. 2.2. 2.3. 20. Hydrogen in compressed gaseous form . . . . . . . . . . . . . . . . . .. 20. 2.1.1. Recent developments . . . . . . . . . . . . . . . . . . . . . . .. 22. Liquid Hydrogen Storage . . . . . . . . . . . . . . . . . . . . . . . . .. 23. 2.2.1. Challenges in liquid hydrogen storage . . . . . . . . . . . . . .. 24. 2.2.2. Recent developments . . . . . . . . . . . . . . . . . . . . . . .. 26. Solid state hydrogen storage . . . . . . . . . . . . . . . . . . . . . . .. 27. 2.3.1. Metal hydrides. . . . . . . . . . . . . . . . . . . . . . . . . . .. 28. 2.3.2. The Lennard-Jones picture . . . . . . . . . . . . . . . . . . . .. 28. 2.3.3. Reaction mechanism. . . . . . . . . . . . . . . . . . . . . . . .. 30. 2.3.4. Thermodynamics. . . . . . . . . . . . . . . . . . . . . . . . . .. 32. 2.3.4.1. PressureComposition-Temperature (PCT) relationship. 2.3.5. . . . . . . . . . . . . . . . . . . . . . . . . . . .. 32. 2.3.4.2. Mechanism of activation . . . . . . . . . . . . . . . .. 34. Kinetics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 35. 3.

(5) 4. CONTENTS. 3 Thermal Analysis. 36. 3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 36. 3.2. Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 37. 3.3. Experimental input . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 45. 3.3.1. Sample preparation . . . . . . . . . . . . . . . . . . . . . . . .. 46. 3.3.2. Hydrogen absorption desorption experiment. . . . . . . . . . .. 47. 3.4. Results and discussion. . . . . . . . . . . . . . . . . . . . . . . . . . .. 49. 3.5. Conclusion of thermal analysis . . . . . . . . . . . . . . . . . . . . . .. 51. 4 Neutron Imaging. 53. 4.1. Introduction. 4.2. Theory of Neutron Imaging. 53. . . . . . . . . . . . . . . . . . . . . . . .. 60. 4.2.1. Neutron source. . . . . . . . . . . . . . . . . . . . . . . . . . .. 60. 4.2.2. Neutron interaction with matter . . . . . . . . . . . . . . . . .. 65. 4.2.3. Reconstruction of images for neutron radiography and tomography. 4.2.4 4.3. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Image processing. Experimental. 67. . . . . . . . . . . . . . . . . . . . . . . . . .. 71. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 73. 4.3.1. CONRAD conguration. . . . . . . . . . . . . . . . . . . . . .. 73. 4.3.2. Hydrogen storage tank and supplying set up . . . . . . . . . .. 75. Neutron radiography experiments . . . . . . . . . . . . . . . . . . . .. 77. 4.4.1. Experiments at 5 bar . . . . . . . . . . . . . . . . . . . . . . .. 78. 4.4.2. Experiments at 10 bar. . . . . . . . . . . . . . . . . . . . . . .. 82. Kinetic studies of tubular-shaped container . . . . . . . . . . . . . . .. 86. 4.5.1. Experimental setup. . . . . . . . . . . . . . . . . . . . . . . .. 86. 4.5.2. Kinetics at 4 bar. . . . . . . . . . . . . . . . . . . . . . . . . .. 86. 4.5.3. Hydrogen pressure inuence on kinetics . . . . . . . . . . . . .. 89. 4.6. Neutron tomography experiments . . . . . . . . . . . . . . . . . . . .. 93. 4.7. Conclusions of neutron imaging. 97. 4.4. 4.5. . . . . . . . . . . . . . . . . . . . . ..

(6) 5. CONTENTS. 5 Conclusions and summary 5.1. Conclusions. 5.2. Summary. Bibliography. 101. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102. 103.

(7) Acknowledgements First of all, I would like to extend a great deal of thanks to my advisor, Prof. Henryk Figiel, for giving me the opportunity to be part of his research group and for his invaluable guidance, stimulating discussions, support and encouragement during this work. I would like to thank my advisor Prof. Dominique Perreux for accepting me as a part of his research team, his supervision and guidance was very helpfull throughout this thesis. Special thanks to Dr. Lukasz Gondek for his valuable advices and helpful discussions during this study.. I would like to express my gratitude to Prof.. David. Chapelle for his help to learn the problem of heat transfer. Thanks to Prof. A. Paja for his kind advices and discussions during the whole thesis. Thanks are extended to Dr. N. Kardjilov, for his help to complete the analysis of neutron imaging studies. Thanks to Dr. J. Niewolski for his help in x-ray characterization of the metal hydride powders. I would like to thank organizers of European Union funded research training network HyTRAIN for taking me as a part of it and the extensive training in research and project management. I would like to thank all my colleagues in AGH university and LMA, France for their kind co-operation and help. I would also like to thank my friends Antony Zywczak, Kamil Kozlak, Christof Avril and Emmanuel Pillet, for their help in my life and research. I take this opportunity to thank my mentors Prof. A Sreekumar and Dr. R. Ratheesh without their constant encouragement and help, it is impossible for me to have this opportunity to study in this great university..

(8) 7. CONTENTS. Streszczenie pracy doktorskiej Znaczenie wodorków metali jako materiaªów do magazynowania wodoru jest dobrze znane. W niniejszej pracy analizowana jest koncepcja systemów magazynowania opartych na wodorkach metali. Optymalizacja ich sprawno±ci jest tu gªównym problemem. Jednym z czynników utrudniaj¡cych zastosowanie materiaªu magazynuj¡cego z wodorku metali jest jego sªabe przewodnictwo cieplne. Na skutek sªabego przewodnictwa cieplnego lokalny wzrost ciepªa w wodorku metalu jest tak du»y, »e blokuje dalsze rozprzestrzenianie si¦ reakcji wodorowania.. Aby uniemo»liwi¢. ten lokalny wzrost ciepªa nale»y projektowa¢ zbiorniki do magazynowania z wodorkiem metalu umo»liwiaj¡ce efektywn¡ wymian¡ ciepªa. Dlatego w pierwszej cz¦±ciej pracy przeprowadzono analiz¦ zjawisk termodynamicznych zwi¡zanych z transportem ciepªa.. Stosuj¡c metod¦ elementów sko«czonych stworzono model prolu temper-. aturowego zbiornika do magazynowania wraz z materiaªem absorbuj¡cym wodór. Te wyniki wskazuj¡ kierunki dalszej optymalizacji wewn¦trznej struktury takiego zbiornika.. Podkre±lono wa»no±¢ obszarów styku ±cian z aktywnym materiaªem i. ich rol¦ w procesach wymiany ciepªa i wodorowania. Ogólnie, badania materiaªów i urz¡dze« do magazynowania wodoru s¡ realizowane u»ywaj¡c przy u»yciu metod po±rednich (np. przez pomiary ci±nienia). Przyczyn¡ jest, »e wodór wewn¡trz materiaªu aktywnego (absorbuj¡cego lub oddaj¡cego wodór) jest niewidoczny przy u»yciu tradycyjnych technik charakteryzuj¡cych materiaª, takich jak dyfrakcja promieni X, a jego obecno±¢ mo»na dopiero stwierdzi¢ rejestruj¡c w otoczeniu niebezpo±rednie efekty jego istnienia.. Je±li u»yje si¦ metody dzi¦ki którejmetody, dzi¦ki której. mo»na by zycznie widzie¢ charakter reakcjimo»liwe jest uzyskanie prolu czasowoprzestrzennego reakcji wodorowania w pracuj¡cym zbiorniku, byªoby b¦dzie mo»liwe lepsze zrozumienie czynników wpªywaj¡cych na t¦ reakcj¦.. Poszukiwanie takiej. techniki doprowadziªo do obrazowania neutronowego radiograi i tomograi neutronowej, której podstawy teoretyczne zostaªy w pracy szczegóªowo omówione i przedyskutowane..

(9) 8. CONTENTS. U»ywaj¡c neutronów jako sondy mo»na ªatwo zlokalizowa¢ wodór w wodorkach metali poniewa»metali, poniewa» przekrój czynny na oddziaªywanie wodoru z neutronami jest bardzo wysoki w porównaniu do takich przekrojów dla prostych metali. Ta metoda dobrze si¦ nadaje zarówno do bada« kinetyki jak i stanów równowagi wodoru w metalach. Dlatego t¦ technik¦, zastosowano w niniejszej pracy W pracy badano zjawiska absorpcji i desorpcji wodoru w typowych wodorkach absorbuj¡cych wodór (LaN i4.78 Sn0.22 i. LaN i4.6 Al0.4 ).. Dla celów prowadzonych bada« skon-. struowano pªaski, aluminiowy, niskoci±nieniowy zbiornik do magazynowania wodoru oraz maªy cylindryczny zbiornik te» wykonany tak»e z aluminium. Aluminium byªo wykorzystane do budowy zbiorników z uwagi na jego bardzo maªy przekrój czynny na oddziaªywanie z wodoremneutronami, co czyni je niemal prze¹roczystym. Badania radiograi i tomograi neutronowej dla skonstruowanych zbiorników wypeªnionych wybranym materiaªem przeprowadzono w Instytucie BENSC, Berlin Neutron Scattering Center (BENSC) w dziaªaj¡cego w ramach Instytutu Helmholtza w Berlinie posiadaj¡cym reaktor do±wiadczalny do wytwarzania neutronów dla potrzeb bada« naukowych. Wedªug posiadanej wiedzy s¡ to jedne z pierwszyche tego typu badania bada« absorpcji i desorpcji wodoru w zamkni¦tych zbiornikach przy pomocy neutronów.. W przeprowadzonych eksperymentach neutronowych badano wpªyw tem-. peratury i ci±nienia na procesy absorpcji i desorpcji wodoru. Na podstawie dynamicznych bada« in situ pokazano, »e post¦p reakcji wodorowania silnie zale»y od zastosowanego ci±nienia. Przy ci±nieniach wy»szych ni» 10 bar w badanym maªym pojemniku cylindrycznym proces wodorowania odbywaª si¦ w czasie rz¦du sekund. Z drugiej strony dla niskich ci±nie«, takich jak np.. 4 bar potrzebne byªo okoªo. godziny aby materiaª w peªni nawodorowa¢. Dane uzyskane dla wi¦kszego pojemnika pokazaªy, »e reakcja jest bardzo czuªa na wewn¦trzn¡ struktur¦ pojemnika. Wodorowanie próbki byªo szybkieSzybkie nasycenie materiaªu aktywnego wodorem zaobserwowano wzdªu» ±cianek pojemnika, podczas gdy w obj¦to±ci próbki szybko±¢ reakcji byªa okoªo rz¡d wielko±ci wolniejsza..

(10) CONTENTS. 9. Wi¡»¡c ze sob¡ obliczenia teoretyczne i obrazowanie in situ pracuj¡cego zbiornika do magazynowania wodoru potwierdzono, »e transport ciepªa w czasie reakcji jest kluczowym czynnikiem który musi by¢ wzi¦ty pod uwag¦ gdy mówi si¦ o poprawianiu wydajno±ci magazynowania wodoru. To mo»e by¢ osi¡gni¦te gªównie przez rozbudow¦ powierzchni kontaktu wodorku ze ±ciankami. Z drugiej strony udoskonalanie przewodnictwa cieplnego materiaªu aktywnego jest inna drog¡ dalszego rozwoju. Na podstawie uzyskanych wyników mo»na byªo sformuªowa¢ nast¦puj¡ce konkluzje i wnioski:. >. Zastosowanie modelu równowagi termicznej do opisu rozwoju temperatury w zbiorniku z wodorowanym materiaªem daªo wyniki zgodne z eksperymentem.. >. Po raz pierwszy wykonano eksperyment wysokorozdzielczej radiograi neutronowej in situ dla metalowego zbiornika z wodorkiem z najwy»sz¡, jak. mm), a grubo±¢ plastrów rekonstruowanych obrazów tomogracznych wyniosªa 50 mm. dot¡d rozdzielczo±ci¡ (75. >. Pokazano, »e technika radiograi neutronowej doskonale nadaje si¦ do szybkiego ±ledzenia procesów wodorowania.. >. Stwierdzono, ze technika tomograi neutronowej mo»e by¢ u»yta do precyzyjnego, w peªni trójwymiarowego, okre±lenia rozkªadu wodoru w obj¦to±ci materiaªumateriale w stanie równowagi.. >. Analiza uzyskanych danych obrazowania neutronowego doprowadziªy doprowadziªa do konkluzji, »e twierdzenie, »e i» wodór penetruje równomiernie caª¡ obj¦to±¢ materiaªu nie jest uzasadnione. Absorpcja wodoru jest bowiem silniejsza wzdªu» ±cianek zbiornika kontaktuj¡cych si¦ z aktywnym materiaªem. Ta wªa±ciwo±¢ jest bezpo±rednio zwi¡zana z lepszym przewodnictwem cieplnym w tym regionie.. >. Zaobserwowano wyra¹ny wpªyw ci±nienia wodoru na kinetyk¦ reakcji wodorowania. W zakresie ni»szych ci±nie« kinetyka reakcji wodorowania silnie zale»y.

(11) CONTENTS. 10. od ci±nienia, podczas gdy przy wy»szych ci±nieniach wzrost ci±nienia ma du»o mniejszy wpªyw na kinetyk¦ reakcji.. >. Na podstawie uzyskanych wyników mo»na jasno wskaza¢ drogi rozwoju w zakresie konstrukcji zbiorników. Optymalizacja wewn¦trznej struktury zbiornika mo»e przy±pieszy¢ kinetyk¦ procesu wodorowania, co mo»e by¢ zrealizowane poprzez uwzgl¦dnienie ilo±ci materiaªu wypeªniaj¡cego zbiornik i maksymalny mo»liwy jego kontakt ze ±cianami i wewn¦trznymi rurkami (wykonanymi z metali o wysokim przewodnictwie cieplnym).. Wprowadzenie matrycy transpar-. entnej dla wodoru, o wysokim wspóªczynniku przewodno±ci cieplnej, w której byªby umieszczony aktywny materiaª mo»e tak»e w sposób znacz¡cy poprawi¢ kinetyk¦ wodorowania..

(12) Chapter 1 Introduction 1.1. Need for hydrogen driven applications. After the invention of re and its use for daily life, clean and readily available energy source has been a concern for human race for past hundreads of thousand years. It become more important in recent time, there has been intense international interest discussion and agreements made, such as the Kyoto protocol, which have been directed at developing cleaner sources of energy to meet our ever-increasing demands without sacricing our environment. Driven by rising standards of living and a growing population worldwide, global energy consumption increased dramatically, the use of petroleum increased from 25 million barrels per day in 1960 to 90 million barrels per day in 2008 [49]. If the rate of growth of fuel demand in developing countries are considered, the global energy demand will be increased to threefold in 2050 than today. So the scientists around world are forced to look for an alternative to replace fossil fuels. Research activities around the world demonstrate that among the various alternative energy concepts, using hydrogen as the primary energy carrier (which connects a host of energy sources to diverse end user's) may enable a secure and clean energy future. Hydrogen energy systems represent such a means with which it is possible to achieve these goals. When hydrogen is combusted or electrochemically oxidized. 11.

(13) CHAPTER 1.. 12. INTRODUCTION. to create heat or electricity, respectively, the only product is water. No pollutants or greenhouse gases are generated or emitted, allowing the potential of zero emission vehicles to become a reality. This clean energy solution is known as the hydrogen economy where energy is transported and stored in the form of hydrogen. Hydrogen, the most common chemical element on the planet, exists rarely in nature in its elemental form. The majority being combined with oxygen in the form of water. Fossil fuels are the main source of industrially produced hydrogen which makes the cost of hydrogen production dependent on price of fossil fuels. In the long term, hydrogen oers a potential route for gaining energy independence from fossil fuels and we know that the present energy resources based on fossil fuels are limited. The traditional fuels like petroleum, natural gas and coal cause moreover environmental pollution mainly through producing carbon dioxide.. Therefore, renewable. and sustainable sources and energy carriers are being sought as alternatives and replacements for fossil fuels. The chemical energy per mass of hydrogen (39.4 kWh/kg) is three times larger than that of other chemical fuels (e.g.13.1 kWh/kg for liquid hydrocarbons) [1]. The combustion of hydrogen with water as the nal product of it is a very attractive exothermic chemical reaction. No. CO2 is produced, and NO. for-. mation is also prevented if clean hydrogen-air mixtures are used. Hydrogen can be produced from water using electricity, or ideally from regenerative sources like sun or wind. More over hydrogen is available/or can be produced any where in the world which makes it attractive over fossil fuels which is available only on few locations on the globe and hydrogen based systems could address the major energy challenges of the 21st century . The dream of an ideal hydrogen economy is that the energy produced from sun owing through hydrogen as a carrier performing electrical work and produces water as its only by-product.To realize hydrogen energy systems in the near future, suitable energy storage and transportation technologies need to be researched and developed. One of the key technologies in this regard, is the development of high performance hydrogen storage systems..

(14) CHAPTER 1.. 1.2. 13. INTRODUCTION. Challenges of hydrogen-based energy systems. One of the challenges facing the widespread adoption of hydrogen as an energy vector is the lack of an ecient, economical, and sustainable method of hydrogen storage. Hydrogen as a gas is about 14 times gravimetrically lighter than air. Compressed gaseous hydrogen transport is only possible in heavy, expensive vessels that can withstand pressures up to. 80. MPa, or a system of pipelines that must either be. constructed from the bottom up or retrotted from existing natural gas pipelines. Cryogenic hydrogen can be transported more easily than gaseous hydrogen, but the conversion from gaseous to liquid hydrogen is energy consuming, inecient, and considerably expensive. At this time, pipelines are considered the most likely transport method for a hydrogen economy. Major concerns surrounding hydrogen distribution include high cost and a phenomenon known as hydrogen embrittlement that causes pipelines and storage vessels to crack and fail over time. Decentralized production of hydrogen eliminates losses associated with long distance transport but increases the demand for eective hydrogen storage on-site. As a crucial step from production to use, hydrogen storage is considered to be one of the most technically challenging aspects for the future hydrogen economy. The advantage that hydrogen posses over other energy carriers such as electricity is a higher capacity to be stored for use at a later time. Storage research is primarily focused on compressed gas, cryogenic hydrogen, and metal hydrides, but a growing number of alternative methods including carbon novel materials, chemical hydrides, and glass microspheres are also being tested. Compressed gas is the most mature storage technology, but compression adds ineciencies to the hydrogen life-cycle and requires stronger, costlier materials for tank construction. Extensive materials research is being conducted to improve compressed gas storage technology; advancements have already been made in carbon-ber wrapped tanks, which are lighter and safer than traditional steel tanks.. Cryogenic hydrogen is denser than compressed. gaseous hydrogen, therefore requiring less storage volume.. Energy and economic.

(15) CHAPTER 1.. 14. INTRODUCTION. costs associated with cryogenic hydrogen storage are higher than compressed gas storage costs. Between 10 and 30 percent of the energetic fuel value of hydrogen is required for liquefaction, and tanks must be super-insulated to maintain cryogenic. 0 temperatures near -250 C . Solid storage in metal hydrides is the next available method on going research in that eld suggests that metal hydrides will be prominent in the future hydrogen economy. Solid-state storage in materials like metals or complex hydrides has potential advantages in terms of volume or weight storage capacity and safety in comparison with gaseous and liquid hydrogen storage [13].. But research using traditional. materials discovery and development techniques has not found a viable candidate for on-board storage of hydrogen for mobile applications, in which about 10 wt.%. °. hydrogen should be stored/released at 0 100. C and 0.1 1 MPa [1]. The main. bottlenecks are high release temperature, slow reaction kinetics, insucient storage capacity and poor thermal conductivity.. 1.3. Hydrogen basics. Hydrogen is the rst element in the periodic table of the elements having the atomic number 1 and the electron conguration. 1s1 .. Hydrogen was prepared many years. before it was recognized as a distinct substance by Cavendish in 1766 and it was named by Lavoisier.. The atomic radius of the free hydrogen atom in the ground. state , the Bohr radius is. a0 = 52.9. in crystal structures is between. 30. pm and the covalent atomic radius of hydrogen and. 35. pm. The phase diagram of hydrogen is. shown in Fig.1.1, hydrogen is the lightest element in the world with a density of. 0.08988. 3 kg/m under ambient conditions, where it is in gas phase. The liquefaction. and solidication temperatures are. 20. K and. 14. K respectively at atmospheric pres-. sure. The maximum boiling point can not exceed. 33. K under a pressure of. 1.3. MPa.. Higher pressures stay ineective in increasing this temperature. Hydrogen(H ) is one of the most abundant elements on earth and is able of re-.

(16) CHAPTER 1.. 15. INTRODUCTION. leasing energy through chemical reactions with. O2. in heat engines and fuel cells. producing only water as byproduct. As the smallest element, hydrogen has the highest energy density which is to overall particles.. 120. MJ/kg since it has the highest ratio of electrons. This energy density is approximately three times the energy. density of gasoline (44.5 MJ/kg) and diesel (42.5 MJ/kg). The comparison of other properties of hydrogen, methane and gasoline is summarized in table 1.1.. Figure 1.1: Phase diagram of hydrogen. Also compared to other forms of energy as mechanical energy, chemical energy (gasoline), electric or magnetic elds and nuclear fuels [4], hydrogen has the advantage being most environmental friendly, having unlimited resources and making it possible, the fuel cell technology to achieve high eciencies. a range of. 640km in a fuel cell driven vehicle [12].. 5kg of hydrogen provides. Also hydrogen remains nontoxic if. it reacts with oxygen (the reaction with air results in toxic nitrogen oxides). Owing to these features, it is being considered as a promising alternative to replace the fossil fuels, which are polluting the environment. However, hydrogen is not found in.

(17) CHAPTER 1.. 16. INTRODUCTION. nature forming a pure gas, rather, it is combined with other elements as in water and organic molecules. A hydrogen storage system suitable for mobile applications must meet simultaneously the following requirements set up based on economical and environmental considerations:. >. development of ecient hydrogen production means [3]. >. safe, ecient and cheap storage of hydrogen. i.e. High gravimetric (>9 wt%). and volumetric (> 81g H2 /L) densities,. >. good thermodynamics. >. fast kinetics. >. eective heat transfer (The operation temperature approximately in the range. °. 60-120 C). >. long lifetime cycles in absorption and desorption. >. safety under normal use and acceptable risk under extreme conditions [2].. The fast kinetics and eective heat transfer among these can be considered as the crucial factors which inuence the commercialization of fuel cell technology. A hydrogen storage system suitable for mobile applications must meet simultaneously the above mentioned requirements set up based on economical and environmental considerations. A promising prospective approach to achieving this goal is the solid-state storage where H atoms are stored in the lattice of a host material.. This method is moti-. vated by the fact that hydrogen storage in gaseous form under high pressure or in liquid state are not suciently ecient.. Besides the concerns on safety and costs. in compressing and liquefying the hydrogen gas, the energy density of hydrogen gas under. 700. bar (70 MPa) (4.4 MJ/L) and liquid hydrogen at. 20. K (10.1 MJ/L) are. smaller than that in gasoline (34.8 MJ/L). The hydrogenation/dehydrogenation processes in some transition metals like Nb and V take place at ambient conditions but.

(18) CHAPTER 1.. 17. INTRODUCTION. Properties. · °. Lower heating value [kWh kg. −1. Hydrogen (H2). Methane (CH4). Gasoline (-CH2-). 33.33. 13.9. 12.4. 585. 540. 228-501. 2045. 1875. 2200. 4 - 75. 5.3 - 15. 1.0 - 7.6. 0.02. 0.29. 0.24. 2.65. 0.4. 0.4. 13 - 65. 6.3 - 13.5. 1.1 - 3.3. 1.48 - 2.15. 1.39 - 1.64. 1.4 - 1.7. 2.02. 7.03. 44.22. 0.61. 0.16. 0.05. ]. Self ignition temperature [ C]. °. Flame temperature [ C] Ignition limits in air [Vol%] Minimal ignition energy [mWs]. ·. −1 Flame propagation in air [m s ] Detonation limits [Vol%]. ·. −1 Detonation velocity [km s ]. ·. Explosion energy [kg TNT m. −3. ]. ·. 2 −1 Diusion coecient in air [cm s ]. Table 1.1: Properties of fuels [1]. the hydrogen gravimetric densities in these transition metals seldom exceed 3 wt% so that the gravimetric requirement is not fullled. In fact, for a material to store more than 6 wt% of hydrogen there is high chance that it could be made of light elements. Therefore, a great deal of research has turned to investigate light metal hydrides (LM H ) like alanates (KAlH4 ,. N aBH4. and. LiBH4 ,. etc.), amide-imide. N aAlH4 , LiAlH4 ,. etc.), borates (KBH4 ,. (LiN H2 -Li2 N H ). [5] and saline hydrides. (M gH2 ) [6]. These materials display high storage capacity reaching up to. LiBH4 .. However, in this compounds. H. 18wt%. in. atoms are held by strong ionic and covalent. bonds resulting in unfavorable thermodynamics and slow kinetics for the hydrogen sorption reactions. Nevertheless, they are still being considered as good candidates thanks to the development of new methods to destabilize them. In fact, to improve hydrogen storage properties of. LM H. the following approaches are currently being.

(19) CHAPTER 1.. 18. INTRODUCTION. pursued: addition of catalysts, synthesis of nano-particles and mixture of dierent hydride phases. Another alternative approach to store hydrogen in solid state systems consist on adsorbing hydrogen molecules on the surface of materials in a non dissociative manner. One important advantage of this approach is that the kinetics of H-loading and releasing is fast, which is due to the fact that no chemical bond reconstruction takes place. However, since the hydrogen molecules are held by weak van der Waals forces, cryogenic temperatures are required in order to keep the tank loaded. There are many materials with high surface areas that are able of storing signicant amount of hydrogen in this way, e.g. carbon nanotubes [14], molecular clathrates [9] and metal-organic frameworks [10].. 1.4. Topic of the thesis. Storing hydrogen safely and economically is the major problem we face in transfer from fossil fuels to hydrogen driven world. In this thesis currently available hydrogen storage methods are reviewed briey. discussed in chapter 2.. The dierent hydrogen storing methods are. The compressed hydrogen storing methods using dierent. types of tank such as type 1 (haves only metal liner), type 2 (metal liner + composite wrap on the mid part of tank), type 3 (metal liner+ composite wrap all over tank) and type 4 (plastic liner + composite wrapping all over it) are discussed. The advantages and disadvantage with the technology of compressed hydrogen storage are discussed in the rst section. Storing hydrogen in liquid form is discussed in the next section. Hydrogen storage in materials in details is discussed in next section which includes metal hydrides and the hydrogen storing mechanism. One of the major problem arising in metal hydrides are due to their low thermal conductivity, local heat development increases so high that it stops further proceeding of the reaction. In chapter 3 will be discussed the thermal problems which arises in MH storage tank, in order to understand the temperature prole during operation of tank, nite dierence (FD) method is used to study the evolution of temperature.

(20) CHAPTER 1.. INTRODUCTION. 19. inside the tank. Results from this calculations are presented and discussed in this chapter. Studying the properties of hydrogen storing elements using X-rays, sieverts apparatus or similar experimental set up provides us only indirect information's, using which we have to deduce the reality. If we have a characterization method by which we could directly study the hydrogen absorption desorption behaviors of metal hydrides that will be very interesting. In this aspect neutron imaging techniques has advantage over other characterization techniques, since neutrons heavily interacts with hydrogen.. This property of high interaction cross section for neutrons with. hydrogen helps to distinguish easily between the metal part and hydrided part of the system. Using this information the hydrogen absorption/desorption properties of metal hydrides could be understood well. Neutron imaging techniques are very powerful tools to study the hydrogen sorption experiments since we could see insitu experiment results in real time. In order to study the eect of hydrogen pressure on the kinetics of reactions two dierent shapes were designed one with at cylindrical type and the second tubular shaped tank.. In chapter 4 the experimental results. obtained from the neutron imaging studies using designed tanks are presented and discussed..

(21) Chapter 2 Hydrogen Storage Methods Signicant amount of research has been done on the storage of hydrogen. Though all these researches resulted in improvements or opened up new areas of study in storage techniques, they could not reach the targets to establish a hydrogen based economy yet. There are various ways to store hydrogen and researches continue on all of these areas. Three main techniques to store hydrogen may be distinguished: 1- Compressed Gas Storage 2- Liquid Hydrogen Storage 3- Storage in Hydrides These are discussed in this chapter. 2.1. Hydrogen in compressed gaseous form. Traditionally, compressed hydrogen gas is stored in metallic(steel) cylinders (Type1 cylinder). The storage pressure for this type of cylinder is low (usually less than 25 MPa) and the capacity of hydrogen gas storage is limited. Also, hydrogen embrittlement can result in micro-cracks and reduce the strength of the cylinder. The Type2 cylinder is developed by wrapping ber-reinforced composite materials in the hoop direction on the metallic (usually steel and alloys of it) cylinder [4]. It reduces the weight of the cylinder signicantly. However, this still does not meet the requirement. 20.

(22) CHAPTER 2.. HYDROGEN STORAGE METHODS. 21. of hydrogen storage capacity for auto motives. To further reduce the weight and increase the storage capacity, fully wrapped composite cylinders (Type3 and Type4 ) have been introduced (Fig.2.1).. Figure 2.1: Schematic representation of dierent types of compressed hydrogen storage tanks. The cylinders are manufactured by the lament winding technique. In this process, a liner is used as a mandrel and the high strength, low density composite (ber strength. 5−6. GPa, composite density. 1.6. g/cm3) is fully wrapped in both the heli-. cal and the hoop directions. The helical winding produces helical layers to bear the axial loading, while the hoop winding generates hoop layers to carry circumferential loading. The liner serves as a gas barrier after the cylinder is manufactured. The Type III dierentiates the Type IV by the material used for the liner (aluminum for Type III and polymer for Type IV). To obtain satisfactory storage capacity, the cylinders are designed for pressures as high as. 80. MPa , hence, the failure of the.

(23) CHAPTER 2.. 22. HYDROGEN STORAGE METHODS. cylinder would lead to a catastrophic accident.. 2.1.1 Recent developments New light weight composite cylinders have been developed which are able to withstand a pressure up to 80 MPa and so the hydrogen can reach a volumetric density of 36 kg. ·m. −3. , approximately half as much as in its liquid form at the normal boiling. point. The gravimetric hydrogen density decreases with increasing pressure due to the increasing thickness of the walls of the pressure cylinder. High pressure tanks for hydrogen storage are already available in the market, which can be pressurized up to 30 MPa [4]. These are usually made of steel and their capacities are not big enough for fuel cell applications. of. 320. l is needed to store. 5. kg of hydrogen at around. It is reported that a tank. 25. MPa [16]. To make them. applicable to vehicular storage, their pressures have to be increased greatly which results in increases in both the tank weight and the material cost.. Even the use. of lightweight materials such as carbon-ber reinforced compounds or stainless steel can not reduce the thickness to desired values [2]. Wall materials usually used are steel alloys (Yield Strength. σ. alloys (σ. = 414. ρ = 2800. ρ = 4430. 3 kg/m ) and carbon composite (σ. MPa and. =. 703. MPa and density. ρ = 7860. kg/m3), aluminum. 3 kg/m ), titanium alloys (σ. = 2070. MPa and. = 924. ρ = 1900. MPa and. 3 kg/m ) [16].. Among these, stainless steel has been used mostly for pressure vessels. High tensile strength, low density and non reactivity with hydrogen along with low diusivity are the main, desired properties for hydrogen storage tanks. Tensile strength presents a limitation to the maximum allowable pressure in the tank. The consequence of this situation is the limitation of the volumetric density.. Here, the volumetric density. is dened as the mass amount of hydrogen to the cylinder volume. Given a certain volume of a tank, the pressure puts a limit to the mass of hydrogen. volumetric density can not be bigger than a certain value.. Hence, the. On the other hand,. the inverse relation is present between the pressure and the gravimetric density..

(24) CHAPTER 2.. HYDROGEN STORAGE METHODS. 23. Gravimetric density is the ratio of hydrogen mass to the tank mass and expressed as hydrogen mass percentage. Therefore, it is high at low pressures and has a maximum at vacuum [13] since low pressures would not necessitate thick and heavy walls. This presents an optimization problem where both densities have to be adjusted for maximum hydrogen storage performances. It should be noted that yield strength and density are not the only important properties in selecting a wall material.. There are also eects arising from cycling. loadings. Aging and fatigue are some of these problems. These problems are arising from hydrogen's capability of being able to escape through many elements and compounds. Hydrogen as the smallest element has a very high permeability rate through many materials. For example, carbon composites have high yield stresses and low densities, but they do not oer any solution to hydrogen leakage which ends up in capacity reduction during operation.. Therefore hydrogen barrier coatings such as. liners are required for carbon composites to stop the hydrogen escape to keep the usable hydrogen capacity at hand. Other main tasks of the liners are to have low permeability of hydrogen, close stiness to other wall elements to prevent cracking and low costs and weights.. Liners are usually compounds such as aluminum and. copper alloys or polymers like cross-linked polyethylene covered with graphite ber epoxy layer [13].. 2.2. Liquid Hydrogen Storage. At normal conditions, hydrogen is in gaseous form. hydrogen can get into the liquid state under. 20.4. At the atmospheric pressure. K, which is below the critical point. temperature (33 K, 1.29 MPa). This temperature is in the region of cryogenic temperatures which is dened as the range below. 123. K.. To store hydrogen in liquid. form has the advantage that the volumetric density is much higher than in gas form to give better storage capacities and the tank pressure does not need to be high, so the total mass is much lower. Also this technique can benet from improvements in.

(25) CHAPTER 2.. 24. HYDROGEN STORAGE METHODS. compressed gas storage tanks in the way that it can adopt the novel wall materials and pressure control advances, since these two aspects are excessively as important for liquid hydrogen storage. Liquid hydrogen storage tanks are usually thin wall pressure insulation vessels . Some designs have been proposed where hydrogen pressure vessels with a few adjustments can be adapted to liquid hydrogen storage technique [20]. Like in the pressure vessels, maximum useful volume is reached in tanks with cylindrical shape. The only concern here is the tting of the tank into the available space inside the vehicle. Operational pressures for liquid storage of hydrogen range from 0.1 MPa to 0.35 MPa [20]. Even at these pressures the energy density is much higher than compressed gas tanks. Compared to the 4.4 MJ/l capacity of compressed gas, liquid hydrogen is able to contain 8.4 MJ/l energy. On the other hand, they have to go through heating processes, since hydrogen has to be fed at high temperatures into the fuel cells in vehicular applications. Still high capacities make it desired to gaseous storage. One proof on higher densities is the hydrogen transporting trailers. It is illustrated that trailers equipped with liquid hydrogen storage tanks are capable of carrying six times more hydrogen than those with compressed gas tanks [20, 21].. 2.2.1 Challenges in liquid hydrogen storage The main problem in liquid hydrogen storage is the hydrogen boil-o, which can lead to hydrogen consumption without any engine operation. Boil-o refers to the phenomena that some portion of the liquid boils under heat exchange and becomes gaseous, which can escape by permeating.. It is a function of thermal insulation,. tank size, tank shape and molecular states ratio of hydrogen with thermal insulation being the most eective parameter among these [13].. In currently used insulated. pressure vessels, evaporation rates of hydrogen can reach 2 to 3 vol.percent per day.. The rst evaporation's are usually observed after 3-4 days in passenger cars. under parking [22].. Tanks have to include ecient boil-o minimizing systems in. order to reduce it. Otherwise evaporating losses will occur at unacceptable levels for.

(26) CHAPTER 2.. HYDROGEN STORAGE METHODS. 25. vehicular applications, even the operation is paused for three days. In this sense the walls have to be very good insulated. Highest rates of hydrogen escape are observed in low pressure tanks fueled with liquid hydrogen.. Vacuumed and multi layered. insulation shows again small amounts of losses compared to micro sphere insulated pressure vessels [12]. As a result it can be said that insulated pressure vessel is a better option than low pressure liquid hydrogen tanks in the sense that they oer less hydrogen losses in operation and during parking periods. To obtain the same performances low pressure liquid hydrogen tanks have to have 6.5 to 10 times thicker walls than those of insulated pressure vessels.. Also low pressure liquid hydrogen. tanks lose 8 percent of their energy in fueling. The main advantage of these tanks is their low weight and compactness . Other than that, the inevitable boil-o losses of hydrogen will force to vent additional fuel after a long period of parking or to put more fuel until a pressure is obtained which is more than the design pressure. Commercial pressure vessels can be applied on the hydrogen storage applications with a few necessary adjustments. Commercial pressure vessels lack some properties to operate at cryogenic temperatures (20 K). Insulation has to be taken care of in the design. Also for gaseous hydrogen charge, the walls must be stronger to withstand the high pressure. Heat insulation together with high pressure capacity in insulated pressure vessels can improve hydrogen capacity in vehicles. Low eciency in liquefaction of hydrogen presents another subject to necessary improvements in the liquid storage technique. The work needed to liquefy hydrogen can reach 30 % of the total energy content of the tank compared to the 18 % of total energy to compress the gas in high pressure vessels this is a signicantly bigger consumption. However the liquid storage tanks are reported to be much more capable of storing large amounts of hydrogen, so that the total carried energy is still higher though there are these liquefaction losses [20]. Like in the compressed gas storage, hydrogen embrittlement has some eects in liquid storage as well. Inevitable boilo and/or gap formation in the tank due to hydrogen consumption brings on the.

(27) CHAPTER 2.. 26. HYDROGEN STORAGE METHODS. gas permeation issue through the walls. The wide temperature range of the system (from. 20. K to fuel cell temperature) sets the requirement for system materials with. low thermal expansion and contraction coecients.. This creates the tendency to. use one type of tank material because of few material choices with low expansioncontraction coecients and other properties being similar. But unlike compressed gas tanks, the liquid hydrogen tanks are thin-wall vessels made of lightweight and strong walls like carbon composites. Such materials require a second layer of liner to prevent hydrogen permeation meaning a second type of material. Also insulation problems may necessitate multi layers with dierent types of materials. Hence this contradiction presents a dicult design problem for liquid hydrogen storage tanks.. 2.2.2 Recent developments It has been reported that current helium transport technology can be adapted to hydrogen transportation with a few adjustments and also with some improvements [22]. Helium is carried in vessels in liquid form at cryogenic temperatures which is even lower than liquid hydrogen tank temperature. It is protected from surrounding heat by a covering super insulation and vacuum.. There exists also a barrier. of thin metal which is in contact with liquid nitrogen from the outer side.. Hence. the nitrogen acts as a shield by absorbing and evaporating the heat rst. However the total spending of liquid nitrogen leaves the insulation system vulnerable to heat losses, hence nitrogen recharging is mandatory. The same system with liqueed air barrier instead of nitrogen can make a promising liquid hydrogen storage tank. The advantage to use air is that it can be collected easily from the environment while in operation, so the eort is simpler and actually it can be further improved. Hydrogen needs to be at higher temperatures before provided to fuel cells. Hence by putting it in contact with the air shield during operation, the air can be liqueed. The pressure drop followed by this liquefaction can also result in additional air extraction from the atmosphere through a tube. This will seal the system due to the solidication of.

(28) CHAPTER 2.. CO2. 27. HYDROGEN STORAGE METHODS. and water in the air and will complete the heat barrier. Like in compressed gas. applications two cylinders of dierent sizes can be merged to use maximum available space in the vehicle. Also, the tank operation is assisted with a compressor where evaporated hydrogen can be collected and pressurized for small applications like energy providing to start the vehicle. The idea of liqueed hydrogen can be extended to a state called gelled hydrogen .. It is reported that the introduction [12, 11] of. a gellant into the liquid hydrogen can increase capacities by. 10%.. Other benets. of the gelled hydrogen are 200-300 % reduced boil o rates and higher safety due to smaller spill radii.. As gellant materials, solid ethane, solid methane and silica. particles are suggested. Liquid hydrogen storage can be considered as a promising candidate to solve the on-board storage problem as it has been already developed and used in industrial application with high densities (7 wt % gravimetric,. 58. g/l. volumetric [20, 21]). However it still needs to be improved further to meet all the requirements of the future targets. First of all, liquid hydrogen at cryogenic temperatures necessitates a completely new fueling infrastructure. Apart from economical problems, there are also some technical challenges. With ongoing improvements, the operating temperature can increase in the future which may require new technologies to overcome pressure increase problems. Boil-o hydrogen losses along with the high liquefaction energy required are standing as primary shortcomings of liquid hydrogen storage which is being eliminated with developments in the insulation media.. 2.3. Solid state hydrogen storage. Compressed gas and liquid hydrogen storage resemble conventional storage techniques like gasoline tanks in cars, where the substance is in pure element form. Though the fueling ease, performances are not quite satisfactory. An alternative way is to dissolve hydrogen in other materials where the resulting compound is called a hydride.. This can be considered as a solid state storage since hydrides are at-. tained by putting hydrogen into solids which exhibit crystalline, quasi crystalline or.

(29) CHAPTER 2.. HYDROGEN STORAGE METHODS. 28. amorphous nature . Researches in that area have ended up in volumetric densities of hydrogen more than those of compressed gas and liquid hydrogen thus receiving the most attention. applications.. However, these are still below the future targets in practical. Also the infrastructure for a hydride based storage economy is infe-. rior compared to other. There are dierent types of hydrides as possible hydrogen storage candidates. These are classied mainly in two groups as metal hydrides and chemical hydrides. Some researches address more complex chemical compounds and alloys, in which case the structure is called a complex hydride. In chemical hydrides hydrogen forms a covalent bond in the compound, whereas metal hydrides are ionic compounds of hydrogen and the metal.. 2.3.1 Metal hydrides Metal Hydrogen reaction The reaction between hydrogen and a metal can be expressed by the following reaction M(s)+xH2 (g) MHx (s)+Q. where Q is the released heat during reaction. are exothermic.. (2.1). Metal hydrides of practical interest. While the above reaction can be sucient for thermodynamical. considerations, it is very misleading, from a kinetic point of view, to think of the formation of a metal hydride as a single reaction. In the following it will be illustrated, starting with a picture of the adsorption process and then turning to the overall processes involved in the formation of a bulk hydride, that the mechanism of hydride formation has a high degree of complexity.. 2.3.2 The Lennard-Jones picture The reaction between gas phase H2 and a metal surface is schematically illustrated in gure2.2 where the one-dimensional Lennard-Jones potential of atomic H (orange.

(30) CHAPTER 2.. 29. HYDROGEN STORAGE METHODS. Figure 2.2: Chemisorption of hydrogen in metals is schematically represented using Lennard-Jones potential energy diagram [4, 13, 56, 57].. line) and molecular H2 is shown (maroon line). Far from the surface the two lines are separated by the hydrogen dissociation energy which is. 218 kJ/mol H. A H2. molecule. moving towards the surface will at some point feel a weak attractive force expressed by the potential energy in the range of approx.. 0 − 20. kJ/mol H (van der Waals. forces) corresponding to molecular physisorption (point 1 in Fig.2.2). If the molecule is moved closer to the surface the potential energy will increase due to repulsion. At some point the potential energy of the H2 molecule will intersect with the potential energy of the H atom. After this point, it is energetically more favorable for the two H atoms to be separated and bonded to the metal surface rather than bonded to each other. Hence dissociation will occur. If this intersection is at a potential energy larger than zero relative to gas phase H2 (point 2) dissociation is said to be activated and the height of point 2 determines the activation barrier.. If the intersection is.

(31) CHAPTER 2.. located at approx. (point 3).. 30. HYDROGEN STORAGE METHODS. zero potential energy dissociation is said to be non-activated. In the former case only the fraction of. H2. molecules with an energy. larger than the activation barrier will be able to dissociate. After dissociation the H atoms nd a potential energy minimum shown as point 4 (chemisorption) which corresponds to the H atoms being bonded to the metal surface.. If the H-M bond. is stronger than the H-H bond, chemisorption is said to be exothermic.. Likewise. if theH-H bond is the strongest, chemisorption is said to be endothermic. Beyond the point of chemisorption the H-atoms can penetrate the rst metal atomic layer into the subsurface through an activated process from which it can diuse into the bulk (as a solid solution) of the metal. If the potential energy of bulk H-atoms is below zero relative to gas phase H2 , hydrogen solid solution is said to exothermic, likewise if the potential energy of bulk H-atoms is above zero hydrogen solid solution is said to be endothermic. The explanation of Lennard-Jones picture is referred from [4, 13, 56, 57].. 2.3.3 Reaction mechanism The reaction mechanisms involved in the formation of metal hydride including bulk processes is schematically represented in the Fig2.3. The various steps in the reactions are.. >. Dissociation/adsorption: The rst step is the dissociative adsorption of hydrogen on the metal/hydride surface. This is shown as point 1 in gure 2.3.. >. Surface penetration: From the surface the hydrogen atoms can penetrate into the sub-surface (point 3).. >. Bulk diusion: From the sub-surface, the hydrogen atoms can diuse into the bulk or from the bulk and further hydrogen diuses between interstitial sites in the metal lattice (point 4)..

(32) CHAPTER 2.. HYDROGEN STORAGE METHODS. Figure 2.3: Mechanisms involved in the formation of a metal hydride.. 31. Hydrogen. atoms shown in red, the metal host lattice in yellow and the metal hydride in green [57].. >. Hydride formation:. Hydrogen atoms in the bulk (corresponding to a solid. solution) can create a hydride nuclei which can grow to larger hydride grains by trapping of additional hydrogen atoms (point 6). The formation of a hydride phase complicates the picture somewhat since hydrogen diusion can also take place through the hydride (point 5).. For dehydrogenation the process is the reverse i.e.. the hydride phase decomposes. and hydrogen atoms diuse to the sub-surface and subsequently to the surface, where the hydrogen atoms recombine and desorb as H2 (point 2). A reaction mechanism can be proposed on the basis of the above reversible elementary reactions. For illustrative purposes only the rst processes including dissociation, surface penetration.

(33) CHAPTER 2.. HYDROGEN STORAGE METHODS. 32. and diusion are written out in a simplied manner:. H2 (g)+2*surf 2H*surf. H*surf + ∗subsurf H*subsurf + ∗surf H*subsurf + ∗bulk H*bulk + ∗subsurf. where * denotes a free interstitial position where one H-atom can occupy. Each of the involved elementary reactions can be assigned a net rate.. 2.3.4 Thermodynamics 2.3.4.1. PressureComposition-Temperature (PCT) relationship. Figure 2.4: Ideal PCT Curve of a hydride adapted from [4]. One of the basic properties of hydrogen storage materials is the PCT behavior. One common method to describe the hydrogen concentration in metal is the atom ratio of hydrogen to metal H/M, and other methods are also used, such as mass percent (weight percent) and volume percent.. PCT curve shows the relationship. between the hydrogen pressure and hydrogen concentration in the alloy at a temperature. Figure2.3.4.1 shows a generalized form of PCT curve and the mathematical.

(34) CHAPTER 2.. 33. HYDROGEN STORAGE METHODS. denitions of the hysteresis, plateau slope and H-capacities.. The whole curve can. a phase region with low hydrogen concentration, b phase region with high hydrogen concentration and a+b region. In the a phase region, the. be divided into 3 regions:. host metal dissolves some hydrogen as a solid solution,and the hydrogen concentration in the solid follows Sieverts' law. H/M=kP. 0.5. (2.2). As the hydrogen pressure increases, the concentration of dissolved hydrogen is increased, the H-H interaction becomes locally important and nucleation and growth of the hydride phase (. b phase) start.. The. b phase usually represents a discontinuous. change in crystal structure or at least in lattice parameters. While the two phases coexist, the isotherm shows a plateau. In the plateau region, generally the hydrogen concentration is linear vs.. lnP ,. and the plateau slope is dened as. Slope. Binary intermetallics, such as. = [d(ln P )]/[d(H/M )]. LaN i5 ,. (2.3). TiFe and MgNi, when prepared carefully, gen-. erally show very little plateau slope, while ternary intermetallics, due to the segregation formed during solidication, usually exhibit plateau slope. Careful preparation techniques, especially annealing before use, can minimize plateau slope [4]. For most applications, a near zero plateau slope is highly desirable. When the whole. α. phase. changes toβ phase, the H2 pressure rises steeply with the H concentration. Generally, in the. β. phase region, the hydrogen pressure and concentration does not follow. Sieverts' law 2.2. Practically, three kinds of PCT curves may be found. In the rst one, only one hydride phase is formed. In the second one, multi-hydride phases are formed. In the third one, there is no apparent plateau. This happens when hydrogen forms continuous solid solution in the metal and no crystallographically distinct hydride phase is formed.. Due to the fact that thermodynamic paths associated.

(35) CHAPTER 2.. 34. HYDROGEN STORAGE METHODS. with the process of phase growth, hydride formation and hydride decomposition are thermodynamically dierent which leads to pressures and phase boundaries changes. The pressures corresponding to the same H/M usually are dierent in the hydriding process and dehydriding process. The hysteresis is dened as :. Hysterisis. = ln(Pa /Pd ). Where Pa and Pd are absorption and desorption pressures at some value of. (2.4). H/M. in. the plateau range. The hysteresis is dependent on the samples' history and on the test procedure used. With increasing temperature, the desorption plateau pressure increases, as shown in Fig.2.4.. When the temperature is above a critical Tc, the. dierence between the two phases ends, and the plateau disappears. The relationship of the mid-desorption plateau pressure, Pd , and temperature can be described by van't Ho equation:. lnPd = ∇H/RT − ∇S/R where. ∇H. (2.5). enthalpy of the hydriding reaction T is absolute temperature R universal. gas constant and. ∇S. entropy of the hydriding reaction. Usually hydriding reaction. is exothermic, and the de-hydriding reaction is endothermic, so both∇H and. ∇S. are. negative.. 2.3.4.2. Mechanism of activation. Activation is the procedure needed to hydride a hydrogen storage material for the rst time and bring it to its maximum hydrogen storage capacity and hydriding/dehydriding rates. When hydrogen is absorbed by a metal, the metal-hydrogen reaction is composed of several partial steps which are represented in Fig.2.3. The components of hydrogen storage alloys are very active elements and when they are exposed to air oxides and hydroxides are formed on the surface. The oxides/hydroxides can not dissociate molecular hydrogen into hydrogen atoms, thus.

(36) CHAPTER 2.. 35. HYDROGEN STORAGE METHODS. they inhibit the absorption of hydrogen by the metals and alloys, and the metals or alloys lose the capability of absorbing hydrogen. In order for the alloy to absorb hydrogen, the alloy must be activated. The activation process usually comprises a slow and complicated annealing process at high temperature and at high pressure in hydrogen environment to get rid of oxides and hydroxides break the surface barrier and increase the surface area. Generally the pressure needed to activate the alloy is much higher than the plateau pressure. Other process involved in activation mechanism of some hydride is annealing under high vacuum at high temperatures to desorb spurious phases from the surface of the active material.. 2.3.5 Kinetics The designing of hydrogen storage devices can be improved by better knowledge of kinetics of alloy hydriding and dehydriding reactions.. Selection of operating tem-. peratures, pressures and cycle times of hydrogen storing devises heavily depend on the reaction kinetics of the alloy. Insight into reaction mechanisms gained from kinetic analysis can be valuable in assessing poisoning eects which may alter the rate controlling processes. For these reasons the kinetics of alloy hydriding have been of considerable and growing interest. The hydriding/dehydriding rates of the hydrogen storage material is also very important in the practical application. The material must possess fast enough reaction rates, and slow rates will limit the application of the hydrogen storage material..

(37) Chapter 3 Thermal Analysis 3.1. Introduction. Metal-hydride systems have become a secure and eective way to store hydrogen for fuel cells and to produce thermodynamic or electrochemical work for stationary and mobile applications[61, 60]. Indeed, they reach the highest volumetric capacity of all hydrogen storage techniques and new hydride compounds are currently being developed to enhance their gravimetric storage ability. The heat energy exchanged during the absorption and desorption processes is quite large (near of hydrogen gas for LaNi5 hydride [27] and. 30.8kJ. per mole. 75.0 for MgH2 [28]) and the metal hydride. reactor should manage this heat to prevent inhibiting reaction. Several experimental studies on hydride powder beds or compacted mixtures have been carried out to determine the limit reaction rate that corresponds to a nearly isothermal process [30]. They show that, in AB5 hydrides, the intrinsic reaction kinetics is very fast but the heat transfer controls the overall reaction rate. In fact, because metal hydrides have rather low thermal conductivity (around 0.1 W/mK for AB5 powders [32]), it is often necessary to include an additional heat transfer material inside the hydride bed, such as metal foam. Marty etal. [33] presented a numerical approach for predicting the heat and mass transfer characteristics in a hydride tank during the absorption of hydrogen. Jemni and Ben Nasrallah [35] presented experimental and theoretical. 36.

(38) CHAPTER 3.. 37. THERMAL ANALYSIS. studies of a metal hydrogen storage reactor. They have shown that the theoretical results agreed satisfactorily with the experimental data. For many practical heat transfer problems it is not possible to obtain a solution by means of analytical techniques. Instead, solving them requires the use of numerical methods, which in many cases allow such problems to be solved quickly.. Often,. we can easily see the eect of changes in parameters when modeling a problem numerically. To work in this way is much faster and tends to be more inexpensive, than assembling and working with the actual experimental apparatus. The aim of this calculation has been to develop a model for transient and steady-state heat conduction in MH storage tank.. The focus is set on modeling the evolution of. temperature at the surface of the MH tank during operation (i.e during hydrogen absorption/desorption)[37].. 3.2. Theory. The amount of thermal energy crossing a unit area per unit time while owing in the direction of decreasing temperature is the heat ux vector. q = −k∇θ. 2 Here, q is the heat ux J/m s,. θ. (3.1). is the temperature (K), and k is the thermal. conductivity of the medium (W/mK). The term∇θ is thus the temperature gradient vector. The minus sign on the right hand side indicates that thermal energy ows from hot regions to cold regions. The thermal conductivity is the rate of thermal energy transfer per unit area and per unit temperature gradient. Thermal energy is transported within a solid by the electrons and the phonon's (lattice vibrations) inside the material.. The transport. of energy is hindered by the presence of imperfections or by any kind of scattering sites..

(39) CHAPTER 3.. THERMAL ANALYSIS. 38. general form of the dierential thermal energy balance equation is. ∂H = ∇.k∇θ + g(r, t) ∂t Were. g(r, t). is the generation/absorption of heat within the material. We have. ∂H ∂θ = ρCp ∂t ∂t where. ρ. (3.2). and. Cp. (3.3). 3 are, respectively the density (in kg/m ) and specic heat (in J/kg. K) of the material. ρCp. Substituting. α=. For steady state. ∂θ = ∇.k∇θ + g(r, t) ∂t. (3.4). ∂θ k 1 = ∇. ∇θ + g(r, t) ∂t ρCp ρCp. (3.5). ∂θ 1 = ∇.α∇θ + g(r, t) ∂t ρCp. (3.6). ∂θ = α∇2 θ ∂t. (3.7). k ρCp. g(r, t) = 0. For one dimensional problem the above equation reduces to. ∂θ ∂ 2T −α 2θ =0 ∂t ∂x. (3.8). In cylindrical coordinates the above equation can be written as. ∂ 2 θ 1 ∂θ g ∂θ + ( )+ −λ =0 2 ∂r r ∂r k ∂t. (3.9). Physical model The system to be simulated is schematically shown in Fig.3.1. The reactor consists of a cylindrical tube of radius. r2. lled by hydride particles (LaNi4.78 Sn0.22 ) inside the.

(40) CHAPTER 3.. 39. THERMAL ANALYSIS. radius r1 . Hydrogen is introduced in the reactor, at ambient pressure, and diuses radially. The aim is to get rst hand information about the temperature developed at the surface of the cylinder. In order to derive the mathematical model, the following main assumptions have been done:. >. A one-dimensional treatment is assumed: the axial variations of parameters are neglected.. >. Local thermal equilibrium between gas and solid is considered.. >. Hydrogen is supplied at the inlet to the bed at a known constant pressure.. >. Solid phase is considered isotropic and with uniform porosity.. >. Eect of porosity variation on the temperature evolution is negligible; and the volumetric expansion of the alloy on hydriding is neglected.. >. Gaseous phase is considered to be ideal.. >. Van't Ho 's relation Eq.2.5 is assumed for equilibrium gas pressure. Slope and hysteresis on the plateau of the real pressure/concentration/ isotherms (PCT) are neglected.. >. Thermo-physical properties are assumed to be constant.. Finite dierence method was used to solve the problem.. The model equations. which describe the studied system involve energy balance for hydrogen in gaseous and solid phase, as follows. Consider the cylinder made up of two type of materials, from the center to to. R = Re. R = Ri. is material A(in our case Metal Hydride) and from. is material B (Aluminum).. Let us divide the cylinder radius into center to. R = Ri. Ri. there is. n1. parts and. Ri. to. n. Re. parts each of is. n2. parts (n. ∆r. thickness, so that from. = n1 + n2 ).. Let say1/λ1 is. the thermal diusivity of material A, and 1/λ2 is that of material B.. ∂θ θi,j+1 − θi,j = ∂t ∆t. (3.10).

(41) CHAPTER 3.. THERMAL ANALYSIS. 40. Figure 3.1: Schematic representation of the system. ∂θ θi+1,j − θi,j = ∂r ∆r 2 ∂ θ θi+1,j + θi−1,j − 2θi,j = 2 ∂r ∆r2. (3.11). (3.12). In cylindrical co-ordinates the equation for heat ux one dimension is. ∂ 2 θ 1 ∂θ g ∂θ + ( ) + − λ =0 ∂r2 r ∂r k ∂t. (3.13). here the term g is representing heat produced during absorption of hydrogen and is given by following expression. g=. Q − bt t exp b2. (3.14).

(42) CHAPTER 3.. Here. Q. 41. THERMAL ANALYSIS. is calculated assuming after innite time the heat produced is temperature. independent and equal to the heat produced by the chemical reaction (Q. 109 J/m3 ). = 3.4 ×. and is taken from the experimental value, b is the time constant that is. the time at which heat creation is maximum and t is the time.. We assume. b. is. temperature independent. Substituting the values above equation becomes. θi+1,j + θi−1,j − 2θi,j 1 θi+1,j − θi,j g θi,j+1 − θi,j ( ) + − λ =0 + ∆r2 r ∆r k1 ∆t where. 0 ≤ i ≤ n1 we. (3.15). can re arrange that equation as follows. ∆r g θi,j+1 − θi,j 1 (θ + θ − 2θ + (θ − θ )) + = i+1,j i−1,j i,j i+1,j i,j λ∆r2 r k1 ∆t. (3.16). to get the temperature dierence's in consecutive parts we take the temperature variables in one side as follows. ∆r g ∆t (θi+1,j + θi−1,j − 2θi,j + (θi+1,j − θi,j )) + = θi,j+1 − θi,j 2 λ∆r r k1. (3.17). the temperature at the next reference point can be calculated as. θi,j+1 =. ∆t ∆r g (θi+1,j + θi−1,j − 2θi,j + (θi+1,j − θi,j ) + ∆r2 ) + θi,j 2 λ∆r r k1. hereθi,j+1 is the temperature at point i and at time (j + 1). Let us assume and. α2 =. ∆t , λ2 ∆r2. α1 =. (3.18). ∆t λ1 ∆r2. ri =i × ∆r. by applying this we will get the energy balance equation for the Metal Hydride material as. g ∆r (θi+1,j − θi,j ) + ∆r2 ) + θi,j r1 k1 ∆r g = α1 (θ3,1 + θ1,1 − 2θ2,1 + (θ3,1 − θ2,1 ) + ∆r2 ) + θ2,1 r2 k1 ∆r g = α1 (θ3,2 + θ1,2 − 2θ2,2 + (θ3,2 − θ2,2 ) + ∆r2 ) + θ2,2 r2 k1. θi,j+1 = α1 (θi+1,j + θi−1,j − 2θi,j + θ2,2 θ2,3. (3.19). (3.20). (3.21).

(43) CHAPTER 3.. 42. THERMAL ANALYSIS. θ2,4 = α1 (θ3,3 + θ1,3 − 2θ2,3 +. g ∆r (θ3,3 − θ2,3 ) + ∆r2 ) + θ2,3 r2 k1. (3.22). θn1 ,j = α1 (θn1 +1,j−1 + θn1 −1,j−1 − 2θn1 ,j−1 ) + α1 (. ∆r g (θn1 +1,j−1 − θn1 ,j−1 ) + ∆r2 ) + θn1 ,j−1 rn1 k1. (3.23). Here the boundary condition is. k1. (θn ,j − θn1 ,j ) (θn1 ,j − θn1 −1,j ) = k2 1+1 ∆r ∆r. The boundary condition at the surface of the cylinder (at. (3.24). n2 ). is. θn2 ,j − θn2 −1,j = h(Text − Tmat ) ∆r. (3.25). or. θn2 ,j = here. Text. h∆rText + θn2 −1,j 1 + h∆r. is the external temperature.. θn1 +1,j =. k1 (θn ,j − θn1 −1,j ) + θn1 ,j k2 1. (3.26). similarly the energy balance equation for aluminum can be written as. θn1 +l,j = α1 (θn1 +(l+1),j−1 +θn1 +(l−2),j−1 −2θn1 +l,j−1 +. ∆r (θn +(l+1),j−1 −θn1 +l,j−1 ))+θn1 +l,j−1 rn1 +l 1.

(44) CHAPTER 3.. where. 43. THERMAL ANALYSIS. 0 ≤ l ≤ n2. We can write this in a matrix form as follows Equation for material. A is.                . .  θ1,j     θ2,j   1     α1 (1 + θ3,j   =     0 θ4,j      . .  . . . .   θn1 ,j  0    α ∆r2 g  1 k1    α1 ∆r2 kg1 +    α1 ∆r2 g k1   .  . .   α1 ∆r2 kg1. 0 ∆r r2. − 2α1 ). α1. (1 + (1 +. . . .. 0. 0. 0 0 . .. ∆r )α1 r2. 0. 0 0 . .. ∆r )α1 r3 . . .. 0 0 . .. ∆r − r3 . . .. 2α1 ) (1 +. . . .. .. . .. θ1,j−1    θ 2,j−1    θ3,j−1    θ4,j−1    .  . .   θn1 ,j−1.                . This equation can be written as.                . . . θ1,j   θ1,j−1    θ θ2,j   2,j−1       θ θ3,j   = A  3,j−1    θ4,j−1 θ4,j      . .   . . . .     θn1 ,j θn1 ,j−1   1   α1 (1 +  A =   0   . . .. . . . 0       α ∆r2 g   1 k1       α1 ∆r2 kg1 +     α1 ∆r2 g k1     .   . .     α1 ∆r2 kg1.               . 0 ∆r r2. − 2α1 ). α1 . . .. (1 + (1 +. (3.27). 0. 0. ∆r )α1 r2. 0. ∆r − r3 . . .. 2α1 ) (1 +. And the equation for material B can be written as. ∆r )α1 r3 . . .. . 0 . . . 0 ..   0 . . . 0 ..    0 . . . 0 ..    . . . . . . ... ...                .

(45) CHAPTER 3..                . 44. THERMAL ANALYSIS.  θn1 +1,j     θn1 +2,j  0 0 0 0  1     α2 (1 + ∆r − 2α2 ) (1 + ∆r )α2 0 0 θn1 +3,j  r r  =     0 α2 (1 + ∆r − 2α2 ) (1 + ∆r )α2 0 θn1 +4,j   r r    . . . . . .  . . . . . . . . . . . .   θn2 ,j    θm,j−1     θ   2,j−1       θ3,j−1       θ4,j−1      .   . .     θn2 ,j−1 . . . .               . θn1 +1,j      θn1 +2,j         θn1 +3,j   = B    θn1 +4,j      .   . .     θn2 ,j. θm,j−1   θ2,j−1     θ3,j−1    θ4,j−1    .  . .   θn2 ,j−1. 0 0 0 0 ...  1   α2 (1 + ∆r − 2α2 ) (1 + ∆r )α2 0 0 ...  r r Where B =   0 α2 (1 + ∆r − 2α2 ) (1 + ∆r )α2 0 . . .  r r  . . .. . . .. 0 0 h∆rText +θn2 −1,j 1+h∆r. ···   ···    ···    ···. (3.28). (3.29). . . . .. 0. . . . .. The combined equation for whole cylinder can be written as. . . .. . . .. 0 0 0 h∆rText +θn2 −1,j 1+h∆r. . ..   ..    ..    ...

(46) CHAPTER 3.. 45. THERMAL ANALYSIS. . . .  θ1,j   θ 2,j     θ3,j    θ4,j   .  . .     θn1 ,j   . .  .    θ  n2 −1,j  θn2 ,j.   θ1,j−1     θ 2,j−1         θ3,j−1        θ4,j−1     A 0  . = .  .     0 B     θn1 ,j−1     . .   .       θ   n2 −1,j−1   θn2 ,j−1. . .  0.                         +                        . α1 ∆r2 kg1 α1 ∆r2 kg1 α1 ∆r2 kg1 . . .. α1 ∆r2 kg1 0 0 . . ..                         . (3.30). This model was used to calculate the temperature at the surface of the container. The mathematical treatment of the problem is completed by the following additional equations. K1 = ε (ρCp )g + (1 − ε) (ρCp )m K2 = (ρCp )Al λ1 = ελg + (1 − ε) λm. where the sux m, Al and g stands for metal hydride, Aluminum and gas.. 3.3. Experimental input. The initial information's to model the evolution of temperature prole was taken from the hydrogen absorption/desorption experiments done using LaNi4.78 Sn0.22 metal hydride. The experimental procedures are discussed below. LaNi5 is one of the most studied compound as a hydrogen storage material. Several investigations have been devoted to the crystal structure, thermodynamic and. =. =. electrochemical properties. So far, the multi-component La1 x REx Ni5 x Mx system has been extensively studied (M = Mn, Cr, Fe, Co, Cu, Al, Sn, Ge, Si and RE.