DOI: 10.3303/CET24114060 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Paper Received: 20 May 2024; Revised: 26 October 2024; Accepted: 10 November 2024 
Please cite this article as: Lesmana L.A., Aziz M., 2024, Metamaterial Reinforced Reactor Design with Internal Heat Transfer Channels for 
Enhanced Metal Hydride as Hydrogen Storage Solution , Chemical Engineering Transactions, 114, 355-360  DOI:10.3303/CET24114060 
  

 CHEMICAL ENGINEERING TRANSACTIONS  
 

VOL. 114, 2024 

A publication of 

 

The Italian Association 
of Chemical Engineering 
Online at www.cetjournal.it 

Guest Editors: Petar S. Varbanov, Min Zeng, Yee Van Fan, Xuechao Wang 

Copyright © 2024, AIDIC Servizi S.r.l. 

ISBN 979-12-81206-12-0; ISSN 2283-9216 

Metamaterial Reinforced Reactor Design with Internal Heat 

Transfer Channels for Enhanced Metal Hydride as Hydrogen 

Storage Solution 

Luthfan Adhy Lesmana*, Muhammad Aziz 

Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan 

luthfanadhy@g.ecc.u-tokyo.ac.jp 

Effective operation of metal hydride (MH) hydrogen storage systems hinges on reactor configuration, which is 

crucial for large-capacity and stationary applications. However, when utilised for mobility usage, MH gravimetric 

density limits the range of efficient scenarios. One way to overcome this limitation is by designing a reactor that 

can also be suitable as a frame, pursuing to create a system where the MH weight becomes negligible. 

Employing a metamaterial of lattice octet design with inner channels for heat transfer fluid flow creates a design 

that is capable of withstanding load while maintaining MH hydrogen storing performance. This study focuses on 

developing a lightweight, compact, and efficient MH reactor capable of accommodating substantial hydrogen 

inventory. Numerical analysis evaluates three reactor configurations to visualise the impact of various design 

elements during storing operation. Compared to baseline reactors, the lattice octet design demonstrates 

significantly improved performance, with a 28 % reduction in hydrogen storage time and 48 % more storage 

capacity. 

1. Introduction 

Hydrogen has emerged as a promising energy alternative to replace fossil fuels, addressing both the need for 

sustainable energy to sustain human living standards and mitigate the environmental damage caused by non-

renewable resources. Recent studies highlight hydrogen's potential to transition from fossil fuel-based energy 

production to more environmentally friendly methods (Juangsa et al., 2021), particularly in systems producing 

zero carbon emissions. 

Various methods exist for hydrogen production, including biomass gasification (Widyawati et al., 2011), methane 

decomposition, electrolysis, and chemical looping (Holladay et al., 2009). Despite its advantages, the storage 

of hydrogen poses challenges. While liquefaction and compressed gas forms are commonly used, they suffer 

from high volume per energy storage ratios, safety concerns, and efficiency issues. Pressurized storage requires 

chambers designed to withstand pressures above 700 bar, raising safety considerations. Additionally, 

maintaining hydrogen in a cryogenic state for liquefaction incurs high energy costs. Promising solutions include 

catalytic or non-catalytic NH3 synthesis and metal hydride (MH) based storage, known for its safety properties 

and volumetric energy density (Niaz et al., 2015). 

The absorption and desorption of hydrogen are influenced by heat exchanger (HE) layouts, such as pipes 

(Elhamshri and Kayfeci, 2019), finned spiral pipes (Dhaou et al., 2010), and finned multi-tubular tanks(Ma et al., 

2014). Research indicates that a higher surface area leading to uniform temperature distribution enhances 

sorption kinetics efficiency. Recent studies emphasize the importance of uniform temperature distribution across 

the metal hydride bed for improved HE performance. 

MH forms when a metal compound absorbs hydrogen through sorption, which is illustrated by the pressure-

composition-temperature (PCT) connection curve. Initially, a metal solution (α phase) forms just before an 

increase in hydrogen pressure and concentration. MH formation (β phase) begins under these conditions, 

reaching a plateau once α and β phases reach equilibrium. This process is represented by the equation: 

𝐻2 +  𝑀𝑒𝑡𝑎𝑙 ⇌ 𝑀𝐻 +  𝛥𝐻𝑅;  𝛥𝐻𝑅 >  0                                                                                            (1) 

355



Due to its safe operation and high hydrogen density per unit volume, MH technology holds promise for hydrogen 

storage. However, its low hydrogen density per unit mass and slow charging and discharging rates limit its 

application in mobile contexts. One potential solution is integrating MH reactors into structural elements of 

mobile applications, similar to incorporating lithium batteries into electric cars, to make them smaller and lighter 

(Lesmana and Aziz, 2023). Previous research has explored utilizing metamaterial-based reactors to enhance 

charging efficiency and hydrogen capacity within vehicle structures. The metamaterial structural qualities, such 

as high compression capability and surface area density, enable this feature. 

Cellular materials exhibit opposing mechanical characteristics, combining low density with high stiffness (Sinha 

and Mukhopadhyay, 2023). However, finding naturally occurring materials that possess all the necessary 

qualities in a single cohesive unit is challenging. There are limitations to extending the qualities of natural 

materials. The mechanical and physical properties of natural materials, like mass density, are interconnected. 

Typically, traditional materials with high strength and stiffness have high mass density and vice versa. 

Overcoming these connections to invent materials that are simultaneously stiff, strong, and lightweight is crucial 

in fields like mechanical engineering, robotics, and aerospace. Such materials are often classified as mechanical 

metamaterials (Contreras et al., 2024). 

 

Figure 1: Visualisation of (a) single cell lattice octet and (b) 2x2x2 matrix of connected octet cells 

Previous studies have explored the relationship between topology and increasing nodal connectivity, leading to 

enhanced macroscopic rigidity (Panesar et al., 2018). The octet lattice demonstrates unique characteristics, 

including high compressive strength and linear scaling strength with consistent and reliable density under 

various conditions. As a lattice metamaterial, the octet lattice is recognised for its ability to produce combinations 

of high stiffness and strength. Figure 1 illustrates the octet lattice and its visualisation from the strut junction to 

the matrix of interconnected cells. 

The above study indicates a strong correlation between MH bed design and HE implementation to further 

achieve more effective sorption kinetics of hydrogen, and other studies show cellular materials characteristic of 

high compression strength capability (Kaur and Singh, 2021). However, a numerical study of hydrogen storage 

utilising metamaterial in a lattice structure was found to be limited. The object of this study is to analyse the 

opportunity for octet structure to be implemented as a solid hydrogen storage option that could replace structural 

parts for high mobility usage scenarios like hydrogen cars as car chassis. Previous studies utilising the gyroid 

structure show great usability of the same concept (Lesmana and Aziz, 2022). In this study, the effect of 

hydrogen charging rate by using the octet structure is investigated using CFD analysis. 

2. Method 

2.1 Reactor Design Preparation 

This study uses a simple case of a canister supported by an octet structure. The octet structure, inherently self-

supporting, is feasible for manufacturing through 3D printing. However, if certain portions of the strut are absent, 

it results in overhangs, making the structure challenging to handle during processing. All the octet struts are 

required to exist. This study uses a tube canister model with hollowed walls and piping channels inside the bed 

following the octet strut shapes. The size of the model is 60 mm in height and 40 mm in diameter. Octet struts 

with 3 mm thickness were then designed to have 1 mm channels inside them for the heat transfer fluid to flow. 

MH bed and the internal octet are visualised in Figure 2(a), while the octet structure itself is depicted in Figure 

2(b). The fluid chamber can be viewed in Figure 2(c), which contains both the inner channel inside the octet 

strut and the chamber that envelops the MH bed. The MH bed 3D model was then used in CFD analysis for 

evaluating the performance of hydrogen absorption and compared with a simple pipe-in-pipe design that uses 

the same heat transfer fluid domain volume as depicted in Figure 2(d).  

356



 

Figure 2: Visualisation of (a) MH bed, (b) octet reinforcement structure ,(c) heat transfer fluid domain and (d) 

simple baseline reactor design of pipe inside pipe with same fluid domain volume 

2.2 Numerical Model 

The governing equations for the sorption kinetics of hydrogen in LaNi5 bed include volume averaged energy 

balance equation, volume averaged mass balance equation, and reaction kinetics of MH. To simplify the 

numerical model, the following assumptions are made: (1) hydrogen in the gas phase is treated as an ideal gas, 

(2) local thermal equilibrium is assumed between hydrogen and the metal hydride (MH), (3) the MH bed is 

regarded as a homogeneous, porous, and isotropic medium, (4) the volumetric expansion of the MH bed is 

ignored, and its properties, such as porosity, permeability, and thermal conductivity, are considered constant, 

and (5) the MH bed is assumed to be perfectly in contact and uniformly packed. The energy balance equation 

is expressed as,  

(𝜌cp)eff

𝜕T

𝜕t
=

𝜕

𝜕x
(keff

𝜕𝑇

𝜕x
) +

𝜕

𝜕y
(keff

𝜕𝑇

𝜕y
) +

𝜕

𝜕z
(keff

𝜕𝑇

𝜕z
) − (1 − 𝜀)ṁ ∆H (2) 

(𝜌cp)eff
= 𝜀(𝜌cp)gas

+ (1 − 𝜀)(𝜌cp)mh
 (3) 

keff = 𝜀 kgas + (1 − 𝜀) kmh (4) 

The mass balance equation is expressed as, 

(𝜀)
𝜕𝜌

𝜕t
= ṁ + (1 − 𝜀)

𝜕

𝜕x
(𝐷

𝜕𝜌

𝜕x
) + (1 − 𝜀)

𝜕

𝜕y
(𝐷

𝜕𝜌

𝜕y
) + (1 − 𝜀)

𝜕

𝜕z
(𝐷

𝜕𝜌

𝜕z
) (5) 

𝐷 = 𝐷𝑜exp (
−Ha

kB𝑇
) (6) 

The rate at which hydrogen is absorbed per unit volume during absorption can be expressed as a function of 

several parameters, including density (ρ), mass diffusion coefficient (D), porosity of the material (ε), and the 

mass of hydrogen absorbed over time (ṁ). Additionally, there are other factors involved such as the pre-

exponential factor (Do), the Boltzmann constant (kB), the activation enthalpy (Ha), and the temperature (T). 

ṁa = 𝐶aexp (
−𝐸a

RT
) ln (

𝑃

𝑃eq
) (𝜌sat − 𝜌t,a) (7) 

In this context, ρsat and ρt,a denote the saturated density and the density of LaNi5 MH when it's empty. Peq stands 

for pressure equilibrium, which can be mathematically described using the Van 't Hoff equation. 

ln(𝑃eq) = 𝐴 −
𝐵

𝑇
 (8) 

where A and B denote Van ‘t Hoff constants. 

With HE is integrated into the system, the flow of the coolant at a constant rate is controlled by an energy 

balance equation. 

(𝜌cp)f

𝜕Tf

𝜕t
+ (𝜌cp)f

 u⃗  gradTf = div(kf gradTf) (9) 

357



Initially, the MH pressure, density, and temperature are considered constant. 

𝑇 = 𝑇o,  𝜌 = 𝜌o,  𝑃 = 𝑃o   𝑢𝑥 = 𝑢𝑦 = 0, 𝑢𝑧 = 𝑢𝑖𝑛, 𝑇 = 𝑇𝑖𝑛 (10) 

𝑢𝑥 = 𝑢𝑦 = 0, 𝑢𝑧 = 𝑢𝑖𝑛, 𝑇 = 𝑇𝑖𝑛 (11) 

A case study was used to validate the numerical model created in COMSOL. By using material properties from 

Table 1, a case study canister was designed and used as geometry for MH bed analysis. A fine mesh setting 

was used for this study. Hydrogen inlet pressure is set to 15 bar. The comparison between the numerical model 

and experiment is presented in Figure 3. The comparison shows that the mathematical model represents the 

experiment result with the deviation that is assumed caused by the assumption that is being made.  

Table 1: MH LaNi5 properties 

Properties Value   

Density, 𝜌 (kg m−3) 8,200 

Specific heat, 𝑐𝑝  (J kg
−1 K−1) 419 

Thermal conductivity, k (W m−1 K−1) 2.4 

Porosity, ε (−) 0.5 

Van 't Hoff constants used in Eq. (2.7), A (−), B (K) 12.99; 3,704.59 

Plateau slope, α (−) 0.038 

Hysteresis factor, β (−) 0.137 

Initial concentration of MH bed, c0 (mol m−3) 18,981.6 

Activation energy ‒ Absorption, Ea (J mol−1) 21,170 

Initial pressure 𝑃o (MPa) 2 

Material constant used in Eq. (2.6), Ca (s
−1) 59.187 

 

 

Figure 3: Comparison between past experiment and numerical model 

3. Result and Discussion 

The simulation used 300
𝑊

𝑚2𝐾
  heat transfer coefficient to simulate the heat transfer fluid flowing trough the 

channel inside the octet struts. Figure 4a show the hydrogen fraction over time alongside the concentration map. 

During the first charging state under 1,000 s, the high absorption state is only concentrated around the cooled 

area. The result shows direct corelation with heat transfer performance to improve the reaction rate of hydration. 

The trend also shows that the growing hydrogen fraction is increasing from the area where octet strut are located.  

Compared with baseline design, the octet insert shows faster charging rate on average (see Figure 5). 

0

0.5

1

1.5

0 500 1000 1500 2000 2500A
ve

ra
ge

 B
ed

 C
o

n
ce

n
tr

at
io

n
 

(w
t%

)

Time (s)

Singh et al

358



 

Figure 4: Concentration of hydrogen fraction over time of (a) octet insert structure canister and (b)simple pipe 

in pipe canister for baseline design 

 

Figure 5.  Average hydrogen fraction over time of (a) octet insert structure canister and (b)simple pipe in pipe 

canister for baseline design 

During 1,000 s, the charging rate slope for octet design is at the highest rate, and the effect of cooling of the 

octet strut can be observed through the initial MH that reaches the full fraction located on the parameter of the 

strut. Full charge state of this MH bed reached at 2,000 s, however 90 % of charging state only requires 1,200 s. 

This shows a favourable outcome considering charging and discharging rate of MH bed can be scaled to a 

bigger volume by applying same heat transfer solution. Compared with octet-based reactor, the Baseline reactor 

shows a slower charging rate, requiring 2,000 s for 90 % charged and almost 3,500 s to be fully charged.  

The temperature map during the same period also provided in Figure 6. The heat transfer effect from the fluid 

chamber to the MH bed also can be observed and the temperature returns back to room temperature once the 

hydrogen fraction becomes 1, resulted from the exothermic reaction is no longer occurred. Enhancing the 

performance of this canister could involve addressing the blue spotted region noticeable on the state of charge 

map depicted in Figure 5. This could be achieved by either incorporating additional struts or increasing the cell 

count. This adjustment would lead to more uniform structures that function as effective heat transfer surfaces, 

augmenting the reaction rate of the metal hydride (MH). 

 

Figure 5: MH temperature distribution during charging 

359



4. Conclusion 

The effect of octet insert as canister support and heat transfer enhancement is investigated in this study. This 

study investigated the use of an octet lattice structure in MH reactor for hydrogen storage, targeting high mobility 

applications like hydrogen-powered vehicles. The octet structure, designed for 3D printing, integrated fluid 

channels to enhance heat transfer, crucial for efficient hydrogen absorption. Numerical model analysis showed 

a strong link between efficient heat transfer and increased hydrogen absorption rates, with full hydrogenation 

achieved in about 2,000 s and 90 % capacity in 1,200 s. Improved heat transfer was observed around the cooled 

octet struts compared to baseline reactor design with only pipe in pipe, indicating faster hydrogen absorption 

that connected with more surface area provided by octet insert when compared with simple pipe in pipe deseign. 

Further enhancements could be made by increasing the number of struts or reducing cell sizes for better heat 

distribution. The octet lattice structure shows great potential for mobile hydrogen storage due to its structural 

strength and efficient heat transfer. Integrating MH beds into structural components such as chassis, cases, and 

bodies can offset the low gravimetric density of MH. With advancements in hydrogen utilization efficiency, 

hydrogen-based vehan MH reactor for hydrogen storage, targeting high-mobilityhat electric cars currently 

incorporate lithium batteries for frame integrity, utilizing lattice insert with MH bed similarly could expand 

hydrogen use, providing a safe and clean energy solution without the hazardous waste issues associated with 

lithium batteries. Future research should optimize this design and explore larger-scale applications to fully 

leverage its advantages. 

Acknowledgments 

This work was supported by JST SPRING, Grant Number JPMJSP2108. 

References 

Contreras N., Zhang X., Hao H., Hernández F., 2024, Application of elastic metamaterials/meta-structures in 

civil engineering: A review. Composite Structures, 327, 117663. 

Dhaou H., Souahlia A., Mellouli S., Askri F., Jemni A., Ben Nasrallah S., 2010, Experimental study of a metal 

hydride vessel based on a finned spiral heat exchanger. International Journal of Hydrogen Energy, 35, 1674–

1680. 

Elhamshri F.A.M., Kayfeci M., 2019, Enhancement of hydrogen charging in metal hydride-based storage 

systems using heat pipe. International Journal of Hydrogen Energy, 44, 18927–18938. 

Holladay J.D., Hu J., King D.L., Wang Y., 2009, An overview of hydrogen production technologies. Catalysis 

Today, 139, 244–260. 

Juangsa F.B., Irhamna A.R., Aziz M., 2021, Production of ammonia as potential hydrogen carrier: Review on 

thermochemical and electrochemical processes. International Journal of Hydrogen Energy, 27, 14455–1447. 

Kaur I., Singh P., 2021, Critical evaluation of additively manufactured metal lattices for viability in advanced heat 

exchangers. International Journal of Heat and Mass Transfer, 168, 120858. 

Lesmana L.A., Aziz M, 2022. Mechanical Behaviour and Fluid Dynamics Analysis of Metal Hydride for Hydrogen 

Storage Based on Triply Periodic Minimal Surface Structure. Chemical Engineering Transactions, 94, 901–

906. 

Lesmana L.A., Aziz M., 2023, Structural Topology Optimization of Metal Hydride Gyroid Container for Hydrogen 

Storage. Chemical Engineering Transactions, 103, 469–474. 

Ma J., Wang Y., Shi S., Yang F., Bao Z., Zhang Z., 2014. Optimization of heat transfer device and analysis of 

heat and mass transfer on the finned multi-tubular metal hydride tank. International Journal of Hydrogen 

Energy, 39(25), 13583–13595. 

Niaz S., Manzoor T., Pandith A.H., 2015, Hydrogen storage: Materials, methods and perspectives. Renewable 

and Sustainable Energy Reviews, 50, 457–469. 

Panesar A., Abdi M., Hickman D., Ashcroft I., 2018, Strategies for functionally graded lattice structures derived 

using topology optimisation for Additive Manufacturing. Additive Manufacturing, 19, 81–94. 

Sinha P., Mukhopadhyay T., 2023, Programmable multi-physical mechanics of mechanical metamaterials. 

Materials Science and Engineering R: Reports, 155, 100745. 

Widyawati M., Church T.L., Florin N.H., Harris A.T., 2011, Hydrogen synthesis from biomass pyrolysis with in 

situ carbon dioxide capture using calcium oxide. International Journal of Hydrogen Energy, 36, 4800–

4813.Click or tap here to enter text. 

360