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.
Coefficient of Thermal Expansion and their Importance.pptx
Heat Transfer in Metallic Hydrides
1. HEAT TRANSFER IN METAL HYDRIDES
Hare Rama Hare Rama Rama Rama Hare Hare
Hare Krishna Hare Krishna Krishna Krishna Hare Hare
Presented by
Mohamed Ali Jahar
M7, 12402034
Guided by
Dr. Mohan. G
Associate Professor
Dept. of Mechanical Engineering,
Sree Chitra Thirunal College of Engineering
Thiruvananthapuram
2. Introduction
• Hydrogen - alternative to fossil fuels
• Characteristics
a. High energy density
b. Abundant & lightest
c. Environment friendly
• Greatest challenge:
Safe, reliable compact and cost effective hydrogen
storage methods.
No hydrogen economy without storage methods
3. Hydrogen storage
• Essential requirements:
a. cost d. durability
b. weight & volume e. refueling time
c. efficiency
H2 storage
onboard
Compressed
hydrogen
Liquid
hydrogen
Solid state
hydrogen
storage
stationary
Power to gas
Underground
hydrogen
storage
4. Solid state hydrogen storage
• Hydrogen stored as chemical compounds (Metal
Hydrides) from which it is readily recovered by
heating.
• Advantages:
a. lower volume requirement for a given mass of
hydrogen
b. greater energy efficiency
c. higher purity hydrogen output
d. large number of charge- discharge cycles
e. safety and ease of use
5. Cylindrical container with axial filter and outer cooling
jacket
Container
Cooling Jacket
Coolant
Filter tube
6. Hydrogen sorption
• The H2 molecule is first weakly physisorbed on the
surface and then dissociatively chemisorbed as strongly
bound, individual H-atoms.
• diffuse quickly from the surface into the periodic sites in
the metal crystal lattice
7. Heat transfer issues in metal hydrides
• Hydriding process release large amount of heat
• Effective thermal conductivity of hydride bed is 0.1 W/m-K
• High bed temperatures increase the corresponding
equilibrium pressure
• High equilibrium pressure reduces reaction rate, adversely
affecting hydrogen storage characteristics of the device.
• Process will stall, metal hydrides may sinter at high
internal temperature
8. Heat transfer enhancement methods
• Extended areas like fins, foams, or meshes
a. Foams:
• Normally used Al, support metal hydride, enhance heat
transfer
• Vessel divided into compartments & each compartment
is filled with a metal form
9. • Increasing thermal conductivity of hydride bed
improves the rate of the hydriding process.
• Only marginal improvement for thermal conductivity
beyond 5 W/m-K
10. b. Compacting
• Binder – expanded graphite
• Mixed homogeneously with metal hydride.
• The mixture pressed into small blocks to make
compacts.
• Effective thermal conductivity reaches above 3 W/m-K
• particles pressed too tightly against each other suffer
reduced hydrogen storage capacity.
11. c. Fins
• Heat transfer to surrounding air stream enhanced by
fins.
• High finned tubes -lower equilibrium pressures within
the bed
12. d. Multi tubular and spiral heat exchanger
• HX tubes and filters arranged in specific stalked
orientation
• Net heat exchange rate increases
13. Performance Study Of Metal Hydride
Storage Device
• Annular cylindrical metal hydride reaction bed is taken. It’s of 27mm
internal diameter, 3mm wall thickness and 450mm length containing
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.
14. Salient points on the configuration and
hydriding
• 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
• Bed thickness b refers to the thickness of hydride bed
b = r – rf rf = radius of filter tube, 6 mm
• Charging of hydride bed – supply hydrogen at pressure
higher than the equilibrium pressure
• Heat transfer fluid circulated through cooling jacket carries
away heat of sorption
• During hydrogen sorption, the granules swell and cause the
bed volume to increase.
16. • Reaction Kinetics
The equilibrium pressure is given by the Vant Hoff equation
where A and B are the Vant Hoff constants.
- Material constant - Activation energy - Saturation density of the hydride
- Material constant - Activation energy - Density of alloy without
hydrogen sorption
17. Thermo physical Properties of materials used in the
storage device
Density of metal 8400 kg/m3
Specific heat of metal (Cps ) 419 J/kg-K
Effective thermal conductivity of metal (ks ) 1·6 W/m-K
Porosity (ε) 0·5
Effective density of the hydride at
saturation (ρss )
4259 kg/m3
Effective density of hydride (ρs ) 4200 kg/m3
Activation energy (Ea) 21170 J/mol H2
Permeability (λ) 10−8
Thermal conductivity of hydrogen (kg) 0·1272 W/m-K
Specific heat hydrogen (Cpg) 14283 J/kg-K
Density of hydrogen (ρg) 0·0838 kg/m3
18. Universal gas constant (R) 8·314 J/mol-K
Reaction constant (σ ) 75 s−1
Slope factor (ϕs ) 0·35
Constant (ϕo) 0·15
Hystersis factor (ϕ) 0·2
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
S.No. Operating parameter Range of parameters
20. • for first few seconds of the reaction, the amount of
hydrogen absorbed close to the porous filter is more.
• Later, the rate of reaction drops significantly (due to fall
in pressure difference (Ps−Peq) and becomes negligible.
• 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.
• the rise in bed temperature close to the convection
boundary region is lower, resulting in larger driving
potential (Ps−Peq) for hydrogen absorption.
22. • 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.
• The excess heat is stored in the bed itself, resulting
in sudden rise in bed temperature.
• the bed temperature decreases due to fall in the
reaction rate and increase in heat transfer from the
bed to the cooling fluid.
24. • 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.
• Rate of absorption reaches peak at beginning and
decreases gradually toward zero at the end of
process
25. 4. Effect of cooling fluid/absorption temperature
26. • at lower cooling fluid temperatures, the hydrogen
absorption proceeds at a faster rate.
• At low absorption temperature, the equilibrium
pressure (Peq) which is the function of bed
temperature is lower.
• at lower absorption temperatures, the temperature
difference (T − Tf ) is also higher, leading to a faster
heat removal during the hydriding reaction.
• at lower absorption temperatures the hydride
absorbs more hydrogen with shorter reaction time.
• For a given supply pressure,hydrogen storage
capacity is found to increase significantly at lower
absorption temperature due to prevailing lesser
plateau slope.
28. • 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.
• For better heat and mass transfer characteristics,
hydride bed thickness should be kept as minimum
(below 10 mm).
29. Conclusions
Hydrogen economy stands shoulder with the same fossil fuel economy,
but due to the enhanced efficiency hydrogen economy makes it stand for
the future.
Among the storage techniques used, 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. The effects of these techniques to enhance heat transfer
have been discussed.
Supply pressure, coolant temperature and bed thickness are important
operating & geometric parameters controlling the sorption rate.
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