A SEMINAR REPORT ON

HEAT AND MASS TRANSFER IN METAL

HYDRIDE

Submitted in partial fulfilment of the requirements for the

Award of the degree

BACHELOR OF TECHNOLOGY

In

MECHANICAL ENGINEERING

Submitted by

MOHAMED ALI JAHAR: 12402034

DEPARTMENT OF MECHANICAL ENGINEERING

SREE CHITRA THIRUNAL COLLEGE OF ENGINEERING

THIRUVANANTHAPURAM-695018

OCTOBER 2015

SREE CHITRA THIRUNAL COLLEGE OF ENGINEERING

THIRUVANANTHAPURAM-695018

DEPARTMENT OF MECHANICAL ENGINEERING

CERTIFICATE

Certified that seminar work entitled “HEAT AND MASS TRANSFER IN METAL

HYDRIDE” is a bonafide work carried out in the seventh semester by

“MOHAMED ALI JAHAR” in partial fulfillment for the award of Bachelor of

Technology in “MECHANICAL ENGINEERING” from University of Kerala during

the academic year 2015-2016 who carried out the seminar work under the guidance and

no part of this work has been submitted earlier for the award of any degree

SEMINAR COORDINATOR SEMINAR GUIDE

E.JAYAKUMAR Dr. MOHAN.G Assistant professor Professor

Department of Mechanical Engg. Department of Mechanical Engg.

SCT College of Engineering SCT College of Engineering

Thiruvananthapuram-18 Thiruvananthapuram-18

HEAD OF THE DEPARTMENT

Dr. S. H. ANILKUMAR Professor,

Department of Mechanical Engg.

SCT College of Engineering,

Thiruvananthapuram-18

i

ACKNOWLEDGEMENTS

I express ardent and earnest of gratitude to my seminar guide Dr. Mohan G, Professor,

Department of Mechanical Engineering, Sree Chitra College of Engineering,

Pappanamcode, Thiruvananthapuram, for his cooperation and guidance for preparing

and presenting the seminar.

I also take this opportunity to express my heartfelt thanks to seminar coordinator

Sri. E.Jayakumar, Assistant Professor, Department of Mechanical Engineering for the

coordination and support provided to make this seminar a success.

I am extremely happy to mention a great word of gratitude to Prof. S.H. Anilkumar,

Head of the Department of Mechanical Engineering, Sree Chitra Thirunal College of

Engineering, Trivandrum for providing me with all facilities for the presentation of this

paper.

I would also extend my gratefulness to all the staff members in the department. I also

thank all my friends and well-wishers who greatly helped me in my endeavor.

Mohamed Ali Jahar

ii

ABSTRACT

The hydrogen economy has been under rapid growth and development in recent years.

Metal hydride based hydrogen storage systems deserve attention as they offer higher

storage densities compared to high-pressure gas storage. It is the most compatible and

economic method to store hydrogen. In these metal hydride storage devices, low heat

transfer has been a key issue. The heat transfer rate can be enhanced by using various

techniques.

A two-dimensional numerical analysis of coupled heat and mass transfer processes in a

cylindrical metal hydride reactor containing MmNi4·6Al0·4 is presented. Performance

studies on MmNi4·6Al0·4 based hydrogen storage device are carried out by varying the

hydrogen supply pressure, absorption (cooling fluid) temperature and hydride bed

thickness.

iii

CONTENTS

ACKNOWLEDGMENTS

i

ABSTRACT ii

LIST OF FIGURES iv

LIST OF TABLES v

NOMENCLATURE vi

Chapter 1 INTRODUCTION

1.1 ‘Hydrogen economy’ 1

1.2 Storage of hydrogen 3

1.3 Types of metal hydrides 4

1.4 Thermodynamics of hydrogen sorption 7

Chapter 2 LITERATURE REVIEW 9

Chapter 3 HEAT AND MASS TRANSFER IN METAL HYDRIDES

3.1 Introduction 11

3.2 Heat transfer enhancement methods 11

Chapter 4 SORPTION PERFORMANCE OF METAL HYDRIDE

STORAGE DEVICE

4.1 Physical model 14

4.2 Problem formulation 15

4.3 Results and discussions 17

Chapter 5 CONCLUSIONS 22

APPENDIX - 1 23

REFERENCES 32

iv

LIST OF FIGURES

Sl. No. Title Page No

1.1

Ideal PCT diagram and the corresponding Van’t Hoff of

metal hydrides

7

1.2 Comparison of actual and ideal cases (a) Plateau slope (b)

Hysteresis

8

3.1 Schematic of a metal-hydride hydrogen storage bed 12

3.2 Effect of thermal conductivity of hydride bed on rate of

hydriding

13

3.3 Schematic of a multi tubular heat exchanger 13

3.4 Schematic of a spiral heat exchanger 13

4.1 Physical model of hydrogen storage device 14

4.2 Variation of hydride concentration with time for first 40 s 17

4.3 Variation of hydride concentration with time. 18

4.4 Variation of bed temperature with time 18

4.5 Effect of supply pressure on hydrogen storage capacity 19

4.6 Effect of cooling fluid temperature on average bed

temperature

20

4.7 Effect of cooling fluid temperature on hydrogen storage

capacity

20

4.8 Effect of bed thickness on average bed temperature 21

4.9 Effect of bed thickness on hydrogen storage capacity

21

v

LIST OF TABLES

Sl. No Title Page no

4.1

Thermo physical Properties of materials used in the

storage device

16

4.2 List of parameters and their operating range 17

vi

NOMENCLATURE

b bed thickness, mm

Cp specific heat, J kg-1 K-1

d diameter, mm

Ea activation energy, J mol-1

h heat transfer coefficient, W m-2 K-1

Ca reaction rate

M molecular weight kg kmol-1

�̇� rate of hydrogen absorbed,

kg m-3 s-1

p pressure, Pa

r radial coordinate, mm

R Univ. gas constant

(8.314 J mol-1 K-1 )

T time, s

T temperature, K

Z axial coordinate, mm

Greek letters

Δ H0 heat of formation, J kg-1

α deformed factor

ϵ Porosity

κ permeability, m2

λ thermal conductivity, W m-2 K-1

ϕ expansion ratio

θ polar coordinate, radian

ρ density, kg m-3

Subscripts

ab Absorption

e Effective

eq Equilibrium

f Fluid

H Hydrogen

p Particle

s Solid

sat Saturated

0 Initial

1

Chapter 1

INTRODUCTION

1.1 Hydrogen Economy

Most of the energy needs of today's world are fulfilled by fossil fuels in the form of coal,

natural gas and oil. However, these sources of energy are neither inexhaustible nor

sustainable. It would not be far off the mark to say that global economy depends heavily

on the vicissitudes of oil prices and this in turn leads to a thoroughly disparate

distribution of power as well as wealth among the nations of the world. Countries with

little or no oil reserves are dependent on oil producing nations for energy - and

consequently development - since on the whole; there can be no development without

expending energy. Burning of fossil fuels produce green house gases which are directly

responsible for global warming - a phenomenon which threatens the existence of many

varieties of plants, animals, agricultural practices and even humans! In addition

chemicals released into the atmosphere due to the combustion of these fuels are often

pollutants capable of causing serious health hazards. The fast depleting oil reserves are

non-renewable and cannot be expected to last for more than a few decades at the most.

Thus, it is in the interest of the greater good that clean, abundant, safe, affordable and

reliable sources of energy must be identified.

The term 'hydrogen economy' was coined by John Bockris as early as 1971 as a proposed

system of delivering energy using hydrogen as energy carrier. It is expected to evolve

into an alternative for the fossil fuel-based economy.

Hydrogen is the most abundant element on earth. The chemical energy per unit mass of

hydrogen (142 MJ/kg) is higher than other fuels. However, hydrogen is not a primary

energy source, but a secondary energy carrier. In hydrogen economy, it is expected to be

the major secondary energy carrier along with electricity. Hydrogen has several attractive

characteristics to be the ideal energy carrier of the future. It can be produced from various

primary energy sources and can be converted to other energy forms at high efficiency.

2

Hydrogen is normally obtained by either reforming of hydrocarbons such as methane or

by electrolysis of water. Electrolysis can be widely used for in situ production of

hydrogen. However significant amount of electricity used for this process must ideally be

obtained from renewable sources such as solar or wind energy in order to realize the ideal

of 'hydrogen economy'. The greatest challenges in widespread use of hydrogen energy is

thus two fold - production of hydrogen in the required quantity and storage &

transportation of hydrogen in a convenient and safe manner.

In nature, less than 1 percent of hydrogen is available in the elemental form. Convenient

natural reservoirs of molecular hydrogen are not available in nature. Hydrogen is

available abundantly in the bonded form as water, hydrocarbons etc. Production of high

purity hydrogen from these sources forms one of the greatest challenges faced by the

hydrogen economy. Hydrogen is highly combustible and stringent safety measures must

be followed in its handling. Also, for onboard hydrogen storage, the storage system must

be compact and low in weight.

The establishment of a 'hydrogen infrastructure' with hydrogen pipelines and filling

stations is necessary for making fuel available to the end user. Another option is the use

of efficient decentralized hydrogen production. The combination of a hydrogen fuel cell

and electric motor is more efficient than an internal-combustion engine. However, the

capital cost associated with fuel cells is quite high making it economically inefficient in

comparison. Overall reduction of cost is thus another issue which requires much

attention.

Hydrogen economy consists of different technologies for production, transportation and

storage of hydrogen. Hydrogen can be produced from water using electrolysis,

thermolysis, thermochemical cycles, ferrosilicon method, photobiological water splitting

etc. Infrastructure for establishment of hydrogen economy would consists of hydrogen

production centres and hydrogen pipelines connecting these to filling stations. Other

methods of transporting hydrogen can include hydrogen tanks, compressed hydrogen

tube trailers, liquid hydrogen trailers etc. Major bottleneck in the deployment of hydrogen

is storage.

3

Hydrogen can be the energy source for both stationary and on-board applications.

Onboard hydrogen storage in automobiles requires a light, safe, compact and affordable

containment for the storage of hydrogen.

1.2 Storage of Hydrogen

Hydrogen can be the energy source for both stationary and on-board applications.

Onboard hydrogen storage in automobiles requires a light, safe, compact and affordable

containment for the storage of hydrogen.

Main storage technologies used are:

1. As compressed gas

2. As cryogenic liquid

3. In solid state devices

Compression might be the simplest way to store hydrogen in a cylinder of pressure upto

20 Mpa, but the energy density is too low to satisfy the fuel demand of driving practice.

About four times higher pressure is needed to meet the driving purpose, however, such

industrial cylinders have not been commercially available.

Liquefaction method faces two challenges: the efficiency of the liquefaction process and

the boil-off of the liquid. Gasification of liquid hydrogen inside the cryogenic (21.2 K)

vessel is an inevitable loss even with a perfect insulation technique. The critical

temperature of hydrogen is very low (33.2 K), above which liquid state cannot exist.

Therefore, liquid hydrogen can only be stored in an open system otherwise the pressure

in a closed system can be high as 1000 MPa at room temperature. The boil-off of liquid

means the emission of H2 into the atmosphere. The relatively large amount of energy

necessary for liquefaction and the continuous boil-off of liquid limit this storage system

to utilizations where the cost of hydrogen is not an important issue and the hydrogen is

consumed in a rather short time, e.g. air and space applications.

Among the technologies used solid state storage device is considered as a promising

field. Hydrogen storage in solid form involves either physisorption or chemisorption of

hydrogen in suitable storage materials which include metal hydrides, complex hydrides,

4

carbon based materials, zeolites, silica etc. Gravimetric hydrogen density of metal and

complex metal hydrides is higher than other storage forms.

In solid state hydrogen storage a material that can be reversibly absorb or absorb

hydrogen in atomic (H) or molecular (H2) form is used to compress hydrogen to high

storage densities. These materials store hydrogen in molecular form at low temperatures.

Within the micropores the hydrogen is effectively compressed to high density, resulting

in an increase in the volumetric density of the hydrogen in comparison with free gas at

the same temperature and pressure. Metal hydrides store atomic hydrogen in the bulk of

the material. That is, this method uses an alloy that can absorb and hold large amounts of

hydrogen by chemical bonding and formation of hydrides. A hydrogen storage inter-

metallic alloy is capable of absorbing and releasing hydrogen without compromising its

own structure. Hence, metal hydride based hydrogen storage systems deserve attention as

they possess higher storage density than that of the compressed gas storage. Besides

larger hydrogen capacity, metal hydrides offer several other practical advantages. Their

formation does not require any special processes / machinery as they are formed simply

by coming in close contact with the hydrogen gas.

Unlike compressed hydrogen and liquid hydrogen, metal hydrides can be kept for a long

period in simple metal containers at atmospheric conditions. They are quite stable below

their dissociation temperatures and since the dissociation of metal hydride is an

endothermic reaction, the self cooling effect limits the evolution of hydrogen in the event

of a leak developing accidentally in the storage tank. Storage of hydrogen in metal

hydrides does not require thick walled containers or special thermal insulation and,

therefore the possibility of explosion in hydride containers is minimal.

1.3 Types of Metal Hydrides

Metal hydride compounds may broadly be divided into three categories on the basis of

the bonding between the alloy and the hydrogen atom, namely ionic, covalent and

metallic. They differ in their chemical and physical properties.

5

Four important categories of metal hydrides have been developed over the past three

decades for hydrogen storage and other engineering applications. They are:

1. AB-type alloys (e.g. with titanium as the major hydride forming

constituent)

2. A2B-type alloys (e.g. with magnesium as the major hydride forming

constituent) -

3. AB2-type alloys (e.g. with zirconium as the major hydride forming

constituent)

4. AB5-type alloys (e.g. with rare earth metals such as lanthanum as the major

hydride forming constituent)

A and B are different metals or group of metals with the former being the major hydride

forming constituent and the latter being the constituent that influences the kinetics,

stability and the storage capacity of the alloy.

a) AB-Type Metal Hydrides

Titanium forms two hydrides, a monohydride (TiH) and dihydride (TiH2), upon exposure

to hydrogen. But, these are very stable and hence cannot be used for engineering

applications. Therefore titanium is usually combined with metals like Fe, Co, Ni, Cr, Zr

and V to reduce the stability of the hydride to the acceptable levels. In this type, Ti is the

major hydride forming constituent. TiFe is a suitable alloy for hydrogen storage

applications as it is of low cost and has agreeable storage capacity with convenient

pressure-temperature variation for charging and discharging of hydrogen.

However, its reaction kinetics is severely affected by the presence of impurities such as

oxygen, water vapour and carbon monoxide found in the hydrogen supply. Partial

substitution of Fe by Mn improves the hydriding properties of the FeTi alloy. Like

titanium, zirconium can also form stable AB type binary compounds with Ni and Co.

These types of alloys are usually characterized by the presence of two plateaus. The

thermal conductivity is better and kinetics is faster for these types of alloys.

6

b) A2B-Type Metal Hydrides

Magnesium is the major hydride forming constituent in A2B category. Magnesium based

alloys are particularly suitable for thermal energy storage applications at temperature

ranges of above 250°C as hydrogen desorption takes place only above 300°C and the heat

of desorption as high as 70 kJ/mol H2. Among all types of metal hydrides, magnesium

based alloys possess the highest hydrogen storage capacity of about 7 wt%, while most of

the other alloys are below 2 wt%. Other advantages include lightweight and availability

at low cost. Addition of Ni, Al and Cu improves the reaction kinetics of Mg based alloys.

c) AB2-Type Metal Hydrides

Many metal hydrides with Zirconium as the hydride-forming constituent belong to AB2

category. Zirconium based alloys are characterized by low plateau pressures. AB2 alloys

usually consist of Mn, V, Cr, Mo, Fe and Co in combination with Zr. Addition of

manganese, chromium and vanadium increase the hydrogen storage capacity. The

advantages include high hydrogen capacity, ease of activation, rapid absorption and

desorption rates, moderate thermal stability, long cyclic life and low cost. AB2 type

titanium–manganese alloys also have fast kinetics and large hydrogen storage capacities.

The major disadvantages of these hydrides are sloping plateaus and large hysteresis,

which reduce their useful storage capacity.

c) AB5 – Type Metal Hydrides

Rare earth metals like lanthanum and misch-metal combined with nickel and cobalt

constitute AB5 type of metal hydrides. AB5 metal hydrides generally have a hexagonal

crystal structure. Different elemental combinations are possible which gives a good

amount of versatility in forming metal hydrides with suitable properties. Most

commercial combinations of this type use misch-metal for element A, owing to its low

cost and favourable thermodynamic properties. However the dissociation pressure and

hysteresis increase substantially. Broad range of PCT versatility, low hysteresis and easy

initial activation are some noted advantages of the above alloys. Hydrogen storage

capacity is low at 1.3 wt% approx. It is slightly expensive. They decrepitate during the

initial cycle and are pyrophoric. They are not prone to easy formation of oxide layers

which is an added advantage. Intrinsic kinetics of AB5 alloys is very good.

7

1.4 Thermodynamics of Hydrogen Sorption

Metal hydrides are formed when the hydride alloys are exposed to hydrogen at certain

pressures and temperatures. The hydrogen molecules initially dissolve randomly into the

metal lattice forming a solid solution of hydrogen in the metal (α-phase) and after

reaching a saturation level, hydrogen atoms arrange themselves in a specific

configuration with the metal atoms (β-phase) by forming a metallic bond between the

metal and hydrogen atom. The two processes which constitute the total process of

hydrogen uptake by the metal, may be represented by the following equations:

𝑀 + 𝑥

2 𝐻2 ↔ 𝑀𝐻𝑥 + 𝐻𝑒𝑎𝑡

(1.1)

𝑀𝐻𝑦 +

(𝑥 − 𝑦)

2𝐻2 ↔ 𝑀𝐻𝑥

(1.2)

Where x and y are number of moles of hydrogen. Metal hydride formation is

accompanied by heat generation. When heat is supplied to the metal hydride, the bonds

are broken thereby releasing the hydrogen gas. The absorption and desorption processes

of intermetallic hydrogen system is represented in the ideal PCT (pressure concentration

temperature) diagram shown in Fig 1.1. Three distinct regions namely α, α+β and β are

observed.

Fig.1.1: Ideal PCT diagram and the corresponding Van’t Hoff of metal hydrides

8

The initial steep slope corresponds to the hydrogen forming into a solid solution (α-

phase). As the α-phase reaches saturation, a second distinct solid phase β begins to form.

This region (α+β) in which the pressure remains nearly constant as the concentration

increases is termed as plateau region. The flat of (α+β) region is the most important part

of the PCT diagram as most of the hydrogen is absorbed in this region at near constant

pressure. Hence, metal hydrides with wide plateau regions are preferred as they absorb

large quantities of hydrogen. It can be observed that the width of the plateau reduces as

the temperature increases and tends to vanish at the critical temperature, Tcwhere the

α−phase is converted directly into β-phase. At a given temperature, the value of plateau

pressure is a direct indication of the thermal stability of the metal hydrides.

Fig.1.2: Comparison of actual and ideal cases (a) Plateau slope (b) Hysteresis

In reality most of the metal hydrides exhibit two irreversibilities, namely, the sloped

plateau and the hysteresis effect as shown in Figs 1.2 (a) and (b) respectively. The

hysteresis effect is not desirable in practical applications as at the same temperature,

hydrogen is desorbed at a pressure lower than that at during absorption. Because of

plateau slope, the equilibrium pressure in the (α+β) region increases with hydride

concentration during absorption and decreases with concentration during desorption,

which results in the reduction in the usable hydrogen capacity of the material

9

Chapter 2

LITERATURE SURVEY

It is necessary to review some researches done in the past that are related to this study.

Some of the reviewed journals, review paper etc. are:

Abdulkadir Dogan, Yuksel Kaplan and T. Nejat Veziroglu published a paper on

“Numerical investigation of heat and mass transfer in a metal hydride bed”. This paper

presents a mathematical model for hydrogen storage in a metal hydride bed. For this

purpose, a two dimensional mathematical model which considers complex heat and mass

transfer during the hydriding process is developed. The coupled differential equations are

solved with a numerical method based on integrations of governing equation over finite

control volumes. The driving force is considered to be pressure difference because of

temperature distribution in the system. The numerical results showed that the hydriding

performance depends on the temperature distribution in the hydride bed. Fluid flow

enhances hydriding rate in the system by driving the hot fluid to colder regions.

Abdelmajid Jemni, Sassi Ben Nasrallah and Jilani Lamloumi published a paper on

“Experimental and theoretical study of a metal-hydrogen reactor”. In this paper the

effective thermal conductivity, the conductance between the hydride bed and the fluid

around the reactor, the equilibrium pressure and the expression of the reaction kinetics

are determined experimentally taking into account initial conditions, the temperature and

the applied hydrogen pressure temporal evolution. Also validity of theoretical model is

tested by comparison between theoretical and experimental results.

Jinsong Zhang and Timothy S Fisher had done review on on “heat transfer issues

in hydrogen storage technologies. Heat transfer issues could become the determining

factor in the feasibility of given storage methods. Enhanced cooling during compression

process is not only important for compressed hydrogen systems, but also essential for

hydrogen storage approaches involving liquid hydrogen and metal hydrides. Enhanced

heat transfer for metal hydride systems is essential in allowing rapid hydrogen uptake and

sufficiently fast kinetics for hydrogen release.

10

Manvendra M Umekar, P Muthukumar published a paper on "Study of coupled heat and

mass transfer during absorption of hydrogen in MmNi4·6Al0·4 based hydrogen storage

device.” A two-dimensional numerical analysis of coupled heat and mass transfer

processes in a cylindrical metal hydride reactor containing MmNi4·6Al0·4 is presented. To

understand the hydrogen absorption mechanism the governing equations for energy, mass

conservation and reaction kinetic equations are solved simultaneously using the finite

volume method (FVM). Performance studies on MmNi4·6Al0·4 based hydrogen storage

device are carried out by varying the hydrogen supply pressure, absorption (cooling fluid)

temperature and hydride bed thickness. The results obtained from the computer

simulation showed good agreement with the available experimental data. At the supply

conditions of 30 bar and 298 K, MmNi4·6Al0·4 stores about 1·28 wt%, which is very close

to the experimental value of 1·3 wt%. Overall high heat transfer coefficients are found to

reduce the absorption time significantly.

G. Mohan, M.Prakash Maiya, and S. Srinivasa Murthy published a paper on

“Performance of air cooled hydrogen storage device with external fins. In this paper, a

parametric study of hydrogen sorption in an air cooled annular cylindrical hydrogen

storage device with external fins is reported. LaNi5, which has excellent hydrogen storage

properties is used as the hydriding material. The influence of different geometric

parameters such as hydride bed thickness and fin height, and operational parameters such

as hydrogen supply pressure and cooling air temperature are studied.

11

Chapter 3

HEAT AND MASS TRANSFER IN METAL HYDRIDES

3.1. Introduction

Hydriding processes release large amounts of heat, as dictated by the heat of reaction,

while the hydride expands in volume. The effective thermal conductivity of hydride bed

is low. High bed temperatures increase the corresponding equilibrium pressure and

therefore reduce the reaction rate, adversely affecting hydrogen storage characteristics of

the device. If the heat is not removed efficiently, the temperature rise can be so large that

the process will stall.

Furthermore, metal hydrides may sinter at high internal temperatures and lose

hydrogen storage capacity. The dehydriding process requires heating at a specific

temperature, dictated by chemical thermodynamics, to proceed. Without sufficient heat

supply, the release of hydrogen will cease because of the reduced temperature in the

hydride bed. Therefore, enhanced internal heat transfer is essential to improve system

performance and to maintain system reliability. Metal-hydride powder has a typical grain

size of 50–100 um. After approximately 10 to 100 hydriding/dehydriding cycles, the

powder decomposes to an equilibrium grain size of approximately 1um. Such fine metal-

hydride powders typically have low effective thermal conductivities of the order of 0.1

W/m-K.

3.2 Heat Transfer Enhancement Methods

Heat and mass transfer enhancement methods suggested for metal-hydride beds can be

broadly classified into two categories: extended areas in the forms of fins, foams, or

meshes, and binding metal hydrides into a solid matrix formed by high-conductivity

materials such as copper, aluminum, or nickel

a) Foams

The design of a developmental metal-hydride hydrogen storage vessel using metal foams

is effective in increasing the effective thermal conductivity.

12

Fig 3.1: Schematic of a metal-hydride hydrogen storage bed

The vessel is divided into compartments by metal plates, and each compartment is filled

with metal foam and fitted with a porous metal filter and U-shape coolant tube. The foam

material, normally aluminum, provides support for the metal-hydride powders and

enhances internal heat transfer. The foam generally occupies approximately 6% of the

volume. Metal-hydride powders occupy approximately 80% of the open space, while the

remainder is left for expansion.

b) Compacting

Expanded natural graphite/metal-hydride compacts have recently been proposed to

enhance heat transfer and have exhibited promising results. To make such compacts,

commercially available expandable graphite is heated in order to expand in volume, and

the expanded graphite is then homogeneously mixed with metal-hydride particles. The

mixture is then pressed into small cylindrical blocks to make compacts. Compacts with

2.1% mass fraction of expanded graphite can increase the effective thermal conductivity

of metal-hydride powders from 0.1 W/m-K to above 3 W/m-K.

c) Fins

The device with fins shows a visible improvement in heat transfer over the one with no

fin, leading to higher rates of absorption. High finned tubes leads to lower equilibrium

pressures within the bed, which causes higher pressure differential and sorption rates. It

may be noted further that the performance improvement of the high-fin (13 mm) tube

over the low-fin (3 mm) tube is small. Moreover maintenance of low finned tubes is

easier in applications where soot deposition is problematic. So low fin tubes may be

preferred for applications where reduction in system weight is important and flue gases

are used as the heat transfer media.

13

.

Fig 3.2: Effect of thermal conductivity of hydride bed on rate of hydriding

d) Multi tubular heat exchanger& spiral heat exchanger

The multi tubular heat exchanger involves arrangement of an array of heat exchanger

tubes and filters in a specified stalked orientation. The multi tubular increases the net heat

exchange rate by increasing heat transfer area for a specific volume.

Fig 3.3: Schematic diagram of multi tubular Fig 3.4: Schematic of

heat exchanger spiral heat exchanger

In spiral heat exchanger secondary flows induced by the centrifugal force have significant

ability to enhance the heat transfer rate. Helical and spiral coils are the known types of

curved tubes which have been used in a wide variety of applications.

14

Chapter 4

SORPTION PERFORMANCE OF METAL HYDRIDE

STORAGE DEVICE

4.1. Physical Model

The study has been performed by considering an annular cylindrical metal hydride

reaction bed as shown in figure 4.1. It’s of 27 mm internal diameter, 3 mm wall thickness

and 450 mm length containing MmNi4·6Al0·4 is chosen for the analysis. Metal hydride

alloy fills the space between the filter (inner wall of hydride bed) and the inner concentric

tube of the reactor. Hydrogen is supplied into the bed radially through a porous filter. The

heat transfer fluid flows spirally through the space between inner and outer concentric

tubes of the reactor.

Fig 4.1: Physical model of hydrogen storage device.

The following assumptions are made in the heat and mass transfer analysis.

(i) The medium is in local thermal equilibrium (Ts= Tg= T). It means that the conduction

in solid and gas phases takes place in parallel and there is no net heat transfer from one

phase to another.

(ii) Effect of radiative heat transfer is negligible. This assumption is valid for all Mm, La,

Zr and Ti based alloys whose minimum absorption temperatures are well below 30◦C.

(iii) The gas phase is ideal from thermodynamic point of view.

15

(iv) Only mass transfer and no heat transfer takes place through the porous filter.

(v) The thermal conductivity and specific heat of the hydride bed are assumed to be

constant. This assumption underestimates the bed performance slightly, because in actual

case the effective thermal conductivity varies with bed pressure and hydrogen

concentration.

(vi) The other thermo-chemical properties such as enthalpy of formation, entropy of

formation and activation energy of the metal hydride are independent of temperature and

pressure.

4.2 PROBLEM FORMULATION

a) Energy balance

Heat conduction through the alloy bed is represented by the following equation.

(𝜌𝐶𝑝)

𝑒

𝜕𝑇

𝜕𝑡= 𝜆𝑒

1

𝑟

𝜕

𝜕𝑟(𝑟

𝜕𝑇

𝜕𝑟) + 𝜆𝑒

𝜕

𝜕𝑧(

𝜕𝑇

𝜕𝑧) − �̇�Δ𝐻0

(4.1)

Sorption heat during absorption/desorption represents the source term. As solid and

hydrogen is in local thermal equilibrium, heat transfer between the two phases is

negligible. Heat transfer to the ambient due to radiation is also neglected.

Effective volumetric heat capacity

(𝜌𝐶𝑝)𝑒

= 𝜖𝜌𝑔𝐶𝑝𝑔 + (1 − 𝜖)𝜌𝑠𝐶𝑝𝑠

(4.2)

b) Mass balance

During hydriding and dehydriding, the metal hydride alloy cracks into small particles and

becomes fine powder. The mass conservation equation of solid phase is given by

(1 − 𝜖)

𝜕𝜌𝑠

𝜕𝑡= 𝑚 ̇

(4.3)

16

c) Reaction Kinetics

The hydrogen mass absorbed per unit time and unit volume m, is given by

Where Ca is reaction rate constant (s−1) and Ea is activation energy (J/mol H2).

𝑚𝑎̇ = −𝐶𝑎 exp (−

𝐸𝑎

𝑅𝑇) ln (

𝑝

𝑝𝑒𝑞) (𝜌𝑠𝑎𝑡 − 𝜌𝑠)

(4.4)

The equilibrium pressure is given by the Vant Hoff equation

ln 𝑝𝑒𝑞 = 𝐴 −

𝐵

𝑇

(4.5)

where A and B are the Vant Hoff constants.

TABLE 4.1 Properties of MmNi4·6Al0·4

Density of metal (ρ) 8400 kg/m3

Specific heat of metal (Cps) 419 J/kg-K

Effective thermal conductivity of metal

(including copper additive) (λs) 1·6 W/m-K

Porosity (ϵ) 0·5

Effective density of the hydride at saturation (ρsat) 4259 kg/m3

Effective density of hydride (ρs) 4200 kg/m3

Activation energy (Ea) 21170 J/mol H2

Permeability (κ) 10−8

Properties of hydrogen

Thermal conductivity of hydrogen (λg) 0·1272 W/m-K

Specific heat hydrogen (Cpg) 14283 J/kg-K

Density of hydrogen (ρg) 0·0838 kg/m3

Constants used

Universal gas constant (R) 8·314 J/mol-K

17

TABLE 4.2 List of parameters and their operating range

S. No. Operating parameter Range of parameters

1. Supply pressure (Ps), bar 10 20 30

2. Cooling fluid temperature (Tf),◦C 15 20 25

3. Overall heat transfer coefficient (U), W/m2-K 750 1000 1250

4. Bed thickness (ro− ri), mm 7·5 12·5 17·5

Note: 1. Effective thermal conductivity of the bed is fixed at 1·6 W/m-K

2. Numbers in italic bold show the values around which other parameters are

varied.

4.3 Results and Discussions

At the supply condition of 20 bar and 25◦C absorption temperature, it is observed from

figure 4.2 that for first few seconds of the reaction, the amount of hydrogen absorbed

close to the porous filter (0·5mm from filter) is more due to high reaction rate

accompanied by larger pressure difference between supply pressure and hydride

equilibrium pressure (Ps –Peq).

Fig 4.2: Variation of hydride concentration with time for first 40 s.

18

Later, due to rapid heat generation close to the porous filter, the rate of reaction drops

significantly due to fall in pressure difference (Ps −Peq) and becomes negligible till the

hydride equilibrium pressure falls below a certain value. However, the region close to the

convection boundary starts to absorb hydrogen at relatively faster rate and reaches the

saturation state much before the net absorption comes to an end. Due to the effective heat

transfer from the hydride bed to the cooling fluid, the rise in bed temperature close to the

convection boundary region is lower, resulting in larger driving potential (Ps − Peq) for

hydrogen absorption.

Fig 4.3: Variation of hydride Fig 4.4: Variation of bed

concentration with time. temperature with time

Figures 4.3 and 4.4 together reveal that the low temperature regions in the hydride bed

ensure high rate of absorption and vice-versa. It is observed from figure 4.4 that in the

beginning of the absorption process the bed temperature increases sharply, reaches its

maximum and then decreases gradually, and becomes equal to the cooling fluid

temperature at the end of the absorption process. Due to poor thermal conductivity of the

hydride bed, the bed is not able to transfer the complete heat of absorption during the

initial rapid reaction. Hence, the excess heat is stored in the bed itself, resulting in sudden

rise in bed temperature. Later, the bed temperature decreases due to fall in the reaction

rate and increase in heat transfer from the bed to the cooling fluid.

19

a) Effect of supply pressure

An attempt was made for validating the present numerical results with the experimental

data of similar reactor geometry under same operating conditions. Figure 4.5 shows that

Fig 4.5: Effect of supply

pressure on hydrogen storage capacity

the numerical results are in good agreement with the experimental data reported by

Muthukumar et al (2005). It is also observed from figure 4.5 that the maximum storage

capacity of about 1·3 wt% is obtained at the supply conditions of 30 bar and 298 K. The

effect of supply pressure on the hydrogen storage capacity is more predominant for the

supply pressures of above 10 bar. This is due to the large slope of the PCT characteristic

of the alloy; higher supply pressures increase the storage capacity significantly.

b) Effect of cooling fluid/absorption temperature

Figures 4.6 and 4.7 together reveal that at lower cooling fluid temperatures, the hydrogen

absorption proceeds at a faster rate due to the availability of larger driving potential for

mass transfer. At low absorption temperature, the equilibrium pressure (Peq) which is the

function of bed temperature is lower, resulting in larger pressure difference (Ps − Peq).

20

In addition, at lower absorption temperatures, the temperature difference (T − Tf) is also

higher, leading to a faster heat removal during the hydriding reaction. Hence, at lower

absorption temperatures the hydride absorbs more hydrogen with shorter reaction time.

For a given supply pressure, hydrogen storage capacity is also found to increase

significantly at lower absorption temperature due to prevailing lesser plateau slope.

Fig 4.6: Effect of cooling fluid Fig 4.7: Effect of cooling fluid

temperature on average bed temperature. temperature on hydrogen storage

capacity

c) Effect of hydride bed thickness

Effect of hydride bed thicknesses on average bed temperature and hydrogen storage

capacity are illustrated in figures 4.8 and 4.9.

Different bed thicknesses are obtained by keeping the filter radius and volume of the

reactor as constant and by varying the outer radius ro. It is observed that the higher bed

thicknesses offer larger resistance to heat transfer resulting in slower reaction and large

cycle time. While lower bed thicknesses offer smaller resistance to heat transfer resulting

in higher reaction and small cycle time. For better heat and mass transfer characteristics,

hydride bed thickness should be kept as minimum (below 10 mm).

21

Fig 4.8: Effect of bed thickness Fig 4.9: Effect of bed thickness on

on average bed temperature hydrogen storage capacity

22

Chapter 5

CONCLUSIONS

Metal hydride hydrogen storage demands low storage energy and provides high

gravimetric efficiency. In the metal hydride hydrogen storage technique, heat transfer rate

has been the key issue. The heat transfer rate can be enhanced using various techniques as

discussed.

Two-dimensional numerical model dealing with the coupled heat and mass

transfer processes in a cylindrical hydride bed is developed. At the start of the absorption

process, rapid hydrogen absorption close to the filter is observed. The analysis reveals

that the hydrogen absorption is a function of heat transfer rate and driving force (Ps −

Peq). A maximum storage capacity of about 1·3 wt% is achieved at supply pressure of 30

bar and 298 K. Thin beds are found to be better both in terms of higher absorption rate

and good heat transfer characteristics.

23

Appendix - 1

SEMINAR PRESENTATION SLIDES

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REFERENCES

[1] Abdelmajid Jemni, Ben Nasrallah, Jilani Lamloumi. “Experimental and

theoretical study of a metal-hydrogen reactor.” International Journal of Hydrogen

Energy 24 (1999): 631-644.

[2] A. Isselhorst. “Heat and mass transfer in coupled hydride beds.” Journal of

Alloys and Compounds 231 (1995): 871-879.

[3] Abdulkadir Dogan, T. Nejat, Yuksel Kaplan. “Numerical investigation of heat and

mass transfer in a metal hydride bed." Journal of Applied Mathematics and

Computation 150 (2004): 169-180.

[4] A. Jemni, S. Ben Nasrallah. “Heat and mass transfer models in metal-hydrogen

reactor." International Journal of Hydrogen Energy 22 (1997): 67-76.

[5] G. Mohan, Lekshmi Dinachandran. "Numerical simulation of the parametric

influence on the wall strain distribution of vertically placed metal hydride based

hydrogen storage container." International Journal of hydrogen energy 40 (2015):

5689-5700.

[6] J Zhang, T S Fisher. "A review of heat transfer issues in hydrogen storage

technologies." Journal of Heat Transfer (2005): 127-1391

[7] Manvendra M Umekar, P Muthukumar. "Study of coupled heat and mass transfer

during absorption of hydrogen in MmNi4·6Al0·4 based hydrogen storage device.”

Sadhana 34 (2009): 255–270.