Testing of Locally Available Jordanian Materials for Hydrogen Storage

Several locally available materials were tested for hydrogen storage. This study was conducted theoretically using suitable software. The materials tested were Sweileh |San, Zeolite, Nickel, Magnesium and Lanthanium. It was found that using the locally available Sweileh sand alone did not give promising results. Pure Magnesium showed the best results followed with Nickle then lanthanium. On mixture basis, adding Magnesium to local Sweileh sand improved its storage ability more than adding other materials like Ni or La.


Introduction
The increase in energy consumption particularly in the past several decades has raised fears of exhausting the globe's reserves of natural resources in the future.Due to industrialization and population growth as well as influx of refugees from neighboring conflict areas, today, Jordan largely depend upon natural resources, which are not replaceable.Approximately 90% of Jordan's energy consumption comes from fossil fuels.
The primary energy consumption consists of Natural gas, crude oil and oil products, imported electricity and renewable energy.The bulk of the consumption in 2011 being Crude oil and related products (Khammash, 2012).
Natural gas is utilized by the Kingdom to generate electricity, with some 80% of the electricity produced in Jordan depending on the imported gas from Egypt.This is shown below in Figure (1 and 2).The disruption in the supply of natural gas to the Kingdom, which led to a decline in the amounts imported of around 65% in 2011 compared to 2010, coupled with the increasing burden of the high oil prices, have highlighted the urgency of persevering with the energy master plan to diversify the sources of energy going forward.In 2011, the total cost of imported crude oil, oil products and natural gas reached a whopping JOD3.84 billion, up by 51% compared to 2010.
The main objective of this paper is to investigate a potential improvement to hydrogen storage materials for fuel cells using locally available sources in Jordan.It also aims to improve the mathematical model used worldwide with the aim of adding to its accuracy.There are many available materials in Jordan that we can use in Hydrogen storage such as:

Silica Sand
Silica sand is defined as a high purity industrial mineral in which the sand grains are made entirely of quartz.Impurities are very minor.The term silica sand is applied to quartz sand that conforms to the specifications of which the main composition is SiO 2 > 99%, with very little contaminant oxide contents (i.e., Al 2 O 3 , TiO 2 , CaO, and Fe 2 O 3 ) and heavy minerals of < 0.1%.
The granules of silica absorbed hydrogen readily and withstood many hydrogen absorption cycles without producing fines.The porosity of the silica matrices was optimized so that the pores were large enough to allow hydrogen to easily move within the matrix and react with the metal hydride particles, but small enough to act as a filter to prevent passage of larger molecule including harmful impurities.
Silica sand are located at Ras En Naqb Area, Petra / Ein El BiadaArea , Al Jayoshia Area and Qa'a Ed Disi Area which have the highest reserve (Madanat, Kailani & Mehyar, 1998).

Zeolite
Zeolites from a large group of hydrous silicates that show close similarities in composition, association, and mode of occurrence.They are framework aluminasilicates with Na and Ca, and highly variable amounts of H 2 O in the voids of the framework.
Porous materials have shown that their high surface areas are useful in the desorption of hydrogen.Zeolite absorbs a small amount at room temperature and ambient pressures, meaning it would be an incurred energy cost from maintaining low temperatures and elevated pressures in use.It could be used to enhance pressurized storage systems, and clearly low weight frameworks.The chemical tuning available in zeolites could allow for improved binding of hydrogen.

Mathematical Model
Before describing the mathematical model and its modifications, the following assumptions are used : 1. Storage is symmetric along the vertical (Z) axis.
2. Thermal equilibrium between solid and gas volumes, 3. Pressure variation in the tank is small, hence, neglected.
4. The effect of the hydrogen concentration on the equilibrium pressure variation is negligible (flat desorption and absorption plateaus on the P-C diagram).
6.The advection terms in the energy equation can be neglected.7. Hydrogen behaves as an ideal gas.
The first assumption adequately approximates the physical scenario being modeled here since there is likely to be little variation in the convection coefficient or temperature values along the perimeter of the exterior tank walls.
The second through fifth assumptions were validated by two through five are validated by Jemni and Ben Nasrallah (Kikkinides, Georgiadis & Stubos, 2006).The sixth assumption is a consequence of the third assumption.
As can be concluded from the literature review, 2D models have been found to adequately represent hydrogen adsorption/desorption processes without extra computational complexity.Therefore, present work considers two-dimensional axisymmetric cylindrical geometry.A metal hydride reactor comprises both an expansion volume of length Lg and abed filled with a LaNi5 metal hydride alloy of length L.
The bed and expansion parts are surrounded by a jacket with the cooling fluid flowing in it.The hydrogen gas enters the storage tank from the cylindrical inlet of the specified radius Rf.In the study of Kikkinides et al. (2006) the heat exchange configuration consisting of additional concentric annular ring and tube demonstrated significant improvement for the hydrogen storage time optimization.Thus, for the current research the same heat exchange system within the bed has been used as shown in Fig. (3): HE1-inner tube heat exchanger filled with cooling fluid and placed in the center of the reactor from r=0 to r=r0; HE2-inner annular ring heat exchanger for which thickness is specified as δr=r 2 -r 1 (filled with cooling medium); HE3-outer surface heat exchanger of radius r3 that surrounds both the expansion volume and porous media.There are many mathematical models in the literature on metal hydrides.All these models describe the same processes that take place during adsorption and desorption in a metal hydrogen reactor.However, significant differences occur in the models presented by different authors due to the scope of model consideration, the presence of additional assumptions, model dimensions, etc.Since 2D models are of particular interest for this research project, some basic modeling equations are discussed below.

Mass balance
General mass balances for hydrogen (Eq. 1) and solid (Eq.2) depending on the rand z axes ε + .r. ρ .u + .ρ u = −m (1) Energy balance Energy balances for metal hydride beds (Eq. 3) can be expressed as a single equation if the assumption of local thermal equilibrium is valid and used for model development. Where; (ρ .c ) = (ℰ.ρ .c + (1 − ℰ). ρ .c ) λe = ελg + (1 − ε)λs In case if the temperatures of solid and gas phases are not equal, a single energy equation is not sufficient to determine the temperature field in the reactor so that energy balances are derived separately.

Momentum balance
The gas velocity is calculated using either Darcy's law (Eq.4), i.e. the axial and radial pressure drops depend linearly on the superficial velocity, or more detailed z-and momentum balances.In most of studies reviewed, the Blake-Cozeny equation is used for the model development:

Mass balance
The differences between expansion and porous volume mass balances can be easily determined.In case of expansion compartment of the metal hydride reactor, there is no hydrogen adsorbed or desorbed, thus m is equal to 0, and there is no need to account for the volume porosity since it comprises the gas phase only.This is shown in (Eq.5) Energy Balance: Energy balance can also be written as follows : ρ .c .+ .ρ .c .u .(r. T) + ρ .C .u .= .r. λ .+ .λ .+ (6)

Reaction kinetics
Based on previous studies (Kikkinides, Georgiadis & Stubos, 2006;Kaplan & Veziroglu, 2003), the following reaction kinetics is used for the modeling of the LaNi5-H2 system: -for hydriding case Where ρsat is the density of the saturated metal hydride bed, i.e. after the reaction is completed.
The equilibrium pressure was calculated using the following equation:

Modification of Model
The modifications applied to the above model include : 1. Specific heat for Hydrogen gas was taken as function of temperature.
2. Viscosity variation as a function of hydride bed temperature.
3. Equivalent thermal conductivity as a function of diameter of hydride material.
4. Pressure of gas according to Darcy law is combined with momentum equations.

Initial Conditions
Initially the pressure, the temperature, and the metal hydride density in the tank are assumed to be constant, the tank is fully charged, and is in a state of equilibrium: T(r, z, 0) =Ti, Pg(r, z, 0) =Pi and Ps(r, z, 0) =Pi

Boundary Conditions
Top The following table (1) shows the properties of the materials were used in the simulation models in all of its stages.

Results and Discussion
Figures (2) show the model output for general LaNi5 material.The aim was to test the validity of the model.The model was found to predict the behavior of the tested hydride material as shown in [5].
This part discusses the initial screening test done to choose the best suitable material.In this part, several materials were selected and modeled for the hydrogen storage under the same conditions to try to find the best performing materials.
The main criterion for the selection used was the ability to store at moderate temperatures and pressures as well as less time.Other criteria which may be used for the selection can be the volume of canister needed to store certain amount of hydrogen.
In this project the first two were selected.The materials used in this study were selected after thorough search in the available literature as well as the available materials locally in Jordan.Modeling was done with the help of specialized software designed for fuel cell testing e.g.PDESol.The results of the initial screening is presented below in Figures (3).The hotness of the materials refer to the absorption reaction between the material and hydrogen.As can be noticed, Sweileh sand and Magnesium showed the best performance (though at the cost of high temperature) and the best storage capacity compared with other materials like Nickel and Lanthanum.
Though Lanthanum showed relatively cooler behavior, however the storage capacity was not as showed by other materials.
Therefore, the three best performing materials selected for the next screening test were Magnesium, Lanthanum and Sweileh sand.The next step taken in this project was the attempt to find the best combination between the two selected materials.
As an initial test, mixtures of 50-50% was selected and their respective properties were calculated by linearly interpolating the properties of each one.The last step in this initial screening was to find out which is more effective in hydrogen storage, is it the Sand or other materials.For this purpose, 25-75% of the materials mixtures were selected and again their respective properties were calculated by linear interpolation.
As clearly shown in Figures 4 the addition of Magnesium was more effective to the adsorption reaction than that of Sweileh sand.This is in total agreement with the available literature.
According to Temperature distribution, the amount of absorption of hydrogen is better when we use the higher percentage of Magnesium.
The original model was modified with the use of variable specific heat and viscosity equation for hydrogen to accommodate for the temperature and pressure change during the process.Figures (5,a,b,c,d and e) show the temperature variation throughout the absorption process.It is clearly noticed that Magnesium was the most uniform with maximum temperature rise during the adsorption process.This is clear indication that the amount of reaction that took place with Mg was higher than others.This means that the amount of hydrogen stored was higher than other materials.This is a clear advantage of the material as it will help in easily release the stored hydrogen during the desorption process.This is clearly shown in This means that the amount of hydrogen stored was low compared with Mg.And hence, less hydrogen will be evolved during the desorption process.This is expected since the properties of sand are not as superior as that for Mg.This is shown in Table (1).

Conclusion
After studying the different materials available in Jordan it is concluded that no single material shall be used for hydrogen storage; rather a mixture of several materials may be better for good hydride performance.It is also concluded that Mg is recommended to be used with other materials for better hydrogen storage capacity.

Figure 1 .
Figure 1.Composition of Primary Consumption in 2011

Figure 3 .
Figure 3. Metal hydride reactor with heat exchangers (in black) inside and outside of the bed surface boundary conditions: Pg = Pt and T = Tt or Kt = qt" Centre axis boundary conditions: Pg = Pc and T = Tc or Kc = qc" Side surface boundary conditions: Pg = Ps and T = Ts or Ks = qs" Bottom surface boundary conditions: Pg = Pb and T = Tb or Kb = qb" Figure 4. Model output data

Figure 5 .
Figure 5.The amount of absorption for different pure materials Figure (5-a).Sand, on the other hand, shown in Figure (5-b), showed relatively lower temperature rise compared with Mg.

Figure 6 .
Figure 6.The amount of absorption for different percentage of sand and Mg mixture

Table 1 .
Properties of Materials Use in Modeling