HEAT TRANSFER IN METAL HYDRIDES

Hare Rama Hare Rama Rama Rama Hare HareHare 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 EngineeringThiruvananthapuram

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

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 gasUnderground

hydrogen storage

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

Cylindrical container with axial filter and outer cooling jacket

Container

Cooling Jacket

Coolant

Filter tube

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

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

Heat transfer enhancement methods

• Extended areas like fins, foams, or meshesa. Foams:

• Normally used Al, support metal hydride, enhance heat transfer

• Vessel divided into compartments & each compartment is filled with a metal form

• Increasing thermal conductivity of hydride bed improves the rate of the hydriding process.

• Only marginal improvement for thermal conductivity beyond 5 W/m-K

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.

c. Fins• Heat transfer to surrounding air stream enhanced by

fins.• High finned tubes -lower equilibrium pressures within

the bed

d. Multi tubular and spiral heat exchanger• HX tubes and filters arranged in specific stalked

orientation• Net heat exchange rate increases

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.

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.

Governing equations• Energy balance

• Mass balance

Effective volumetric heat capacity

• 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

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

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

Results & Discussions

1.

• 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.

2.

• 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.

3. Effect of supply pressure

• 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

4. Effect of cooling fluid/absorption temperature

• 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.

• Effect of hydride bed thickness

• 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).

ConclusionsHydrogen 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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