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Hydrogen storage is a crucial technology that supports the progress of hydrogen and fuel cell systems in various sectors.While hydrogen boasts the highest energy content by mass compared to other fuels, its low density at room temperature limits its energy per unit volume.

This makes the development of advanced storage techniques essential to increase energy density and make hydrogen storage more efficient. Emerging storage solutions aim to maximize both gravimetric and volumetric hydrogen densities. As these technologies evolve, they hold the potential to revolutionize energy systems and reduce dependence on conventional fossil fuels.


Storage & Transportation
Key Challenges

Hydrogen storage presents several challenges, impacting efficiency, safety, and cost. Key challenges include:

Hydrogen Embrittlement

A significant issue in hydrogen storage and transportation. Hydrogen atoms can diffuse into metals, causing them to become brittle and prone to cracking. This affects materials like steel used in pipelines, tanks, and valves, potentially leading to failures.

Low Energy Density

Hydrogen has low energy density by volume, requiring either high-pressure storage (350-700 bar) or liquefaction at -253°C. Both approaches demand specialized, costly equipment and energy.

Hydrogen Leakage

Hydrogen's small molecular size allows it to easily escape through many materials, causing efficiency losses and posing safety hazards, such as explosions.

Boil-off and Energy Loss

In liquid hydrogen storage, boil-off occurs as hydrogen evaporates due to heat ingress. This leads to continuous energy loss and complicates long-term storage.

Energy for Compression

Compressing hydrogen can consume up to 15% of its energy, and liquefaction can use about 30%, making both processes energy-intensive.

Safety Risks

Hydrogen is highly flammable and can form explosive mixtures when it accumulates in confined spaces, making leak detection and prevention crucial.

Heavy Hydride Materials

Metal hydrides can store hydrogen, but they are heavy and require heat to release the gas, limiting their practicality in mobile or large-scale applications.

Infrastructure Costs

The need for specialized, high-strength materials and safety systems makes building large-scale hydrogen storage and transportation infrastructure expensive.

Economic Challenges

High costs associated with storage systems, material degradation, and maintenance impact the overall economic feasibility of hydrogen storage solutions.

Storage Duration

Current hydrogen storage methods face challenges with long-duration storage, such as energy loss (boil-off) and material degradation over time.

Combating Challenges with Physical- and Material-Based Storage
Combating Challenges with Physical- and Material-Based Storage

Physical-Based Storage

Compressed Gas

Hydrogen is stored as a gas in high-pressure tanks. While hydrogen gas has a low density at ambient conditions, compressing it to high pressures (typically 350 to 700 bar) can store more hydrogen in a smaller volume. However, this method requires robust materials to withstand high pressures and efficient compression technology to minimize energy loss.

Specifications:
  • 🌡️ Temperature: Ambient
  • ⚖️ Density: 0.017kg/L
  • 📊 TRL: 9

Cold/Cryo-Compressed

Hydrogen gas is cooled to cryogenic temperatures to reduce its volume while being stored at high pressure. This method enhances the volumetric storage capacity compared to standard compressed gas, but it requires maintaining very low temperatures, which can be energy-intensive.

Specifications:
  • 🌡️ Temperature: -150°C
  • ⚖️ Density: 0.08 kg/L
  • 📊 TRL: 9

Liquid Hydrogen

In this method, hydrogen is stored in a liquid state at extremely low temperatures (around -253°C or 20 K). Liquid hydrogen storage offers a much higher energy density by volume compared to gaseous hydrogen, but the process of liquefying hydrogen requires significant energy. In addition, heat management and boil-off losses are concerns with liquid storage.

Specifications:
  • 🌡️ Temperature: -243°C
  • ⚖️ Density: 0.071 kg/L
  • 📊 TRL: 9

Material-Based Storage

Material-based storage involves storing hydrogen in solid-state materials, which can offer higher hydrogen densities under safer and more moderate conditions. The subcategories include:


Adsorbent Materials

Hydrogen can be stored in the porous structure of materials like Metal-Organic Frameworks (MOFs). These materials can adsorb hydrogen molecules at their surfaces due to their high surface area and tunable pore sizes. MOF-5, for example, is known for its potential in hydrogen storage, offering reversible adsorption under certain conditions.

reaction reaction> <div class=
Salient Feature:
  • High Surface Area
  • Tunable Pore Sizes
  • Reversible Storage
  • Low Operating Temperatures
  • Lower Pressure Requirements
Additional Benefits
  • Lightweight and Compact
  • Safe and Stable
  • Scalability and Flexibility
  • Emerging Technology
MOF Structure

LOHC (Liquid Organic Hydrogen Carriers)

TRL 5

In this method, hydrogen is chemically bonded to organic liquids, such as BN-methylcyclopentane, for storage and transportation. The hydrogen can be released when needed by applying heat or a catalytic process. LOHC systems allow for easy handling and transport of hydrogen in liquid form.


Process:

Hydrogenation (Storing Hydrogen):  C 7H 8 + 3H 2 → C 7H 14

Dehydrogenation (Releasing Hydrogen):   C 7H 14→ C 7H 8 + 3H 2

Salient Features:
  • Reversible Hydrogen Storage
  • Safe and Stable Storage
  • Existing Infrastructure Compatibility
  • High Energy Density
  • Easy Handling and Transport
  • Non-toxic and Non-flammable in Carrier Form
  • Multiple Carrier Options
  • Energy-Intensive Hydrogen Release
  • Long-Term Storage without Loss
  • Reusable Carrier
LOHC Structure

Interstitial Hydrides

TRL 7

These are metallic materials, such as LaNi₅H₆, that absorb hydrogen atoms into their crystal lattice. The hydrogen atoms are stored in the interstitial spaces between metal atoms. These materials offer moderate storage capacities and are being researched for their potential to store and release hydrogen under manageable conditions.


Salient Features:
  • Hydrogen Stored in Metal Lattices
  • Reversible Hydrogen Storage
  • Moderate Temperature and Pressure Requirements
  • Good Volumetric Storage Capacity
  • Metallic Bonding with Hydrogen
  • Fast Kinetics for Absorption/Desorption
  • High Gravimetric Density
  • Long-Term Stability
  • Controlled Release of Hydrogen
  • Non-toxic and Safe Handling
  • Suitable for Stationary and Portable Applications
Interstitial Hydrides

Complex Hydrides

TRL 6

Complex metal hydrides, such as sodium alanate (NaAlH₄), store hydrogen in the form of chemical bonds with metal atoms. These hydrides can store large amounts of hydrogen by weight, but they often require heat to release the hydrogen, which can complicate the system's efficiency.


Salient Features:
  • High Hydrogen Storage Capacity
  • Lightweight Materials
  • Strong Chemical Bonding
  • Temperature-Dependent Release
  • Reversible Hydrogen Storage
  • Thermodynamic Stability
  • Suitable for Various Applications
  • Slow Kinetics
  • Synthesis and Cost Challenges
  • Safe and Stable
Complex Hydrides

Chemical Hydrogen Storage

TRL 4

Chemical compounds like ammonia borane (NH₃BH₃) can store hydrogen within their chemical structure and release it through decomposition. These materials offer high hydrogen content by mass, but the process of releasing hydrogen can be slow or require high temperatures.


Salient Features:
  • High Hydrogen Storage Capacity
  • Lightweight Materials
  • Strong Chemical Bonding
  • Temperature-Dependent Release
  • Reversible Hydrogen Storage
  • Thermodynamic Stability
  • Suitable for Various Applications
  • Slow Kinetics
  • Synthesis and Cost Challenges
Complex Hydrides

Ethanol and Methanol as Hydrogen Carriers

Both ethanol and methanol are simple alcohols that can serve as liquid hydrogen carriers. They are widely available, easy to transport, and can be processed to generate hydrogen for use in fuel cells and other applications.

Methanol is often produced from natural gas or biomass. It is used as a chemical feedstock and a fuel in various industrial processes. Methanol is also a candidate for clean energy applications because it can be made from renewable resources, including carbon dioxide, water, and renewable energy (electro-methanol).

Ethanol is a renewable alcohol typically produced through the fermentation of biomass, such as corn, sugarcane, or other plant materials. It is commonly used as a biofuel (bioethanol) in transportation and energy systems.


How Ethanol and Methanol Act as Hydrogen Carriers:

Both ethanol and methanol can be used to produce hydrogen through reforming processes, in which the alcohol is broken down into hydrogen and other by-products, like carbon dioxide.


1. Steam Reforming of Ethanol and Methanol:
Methanol (CH₃OH)
TRL 7

Steam reforming is the most common method for extracting hydrogen from both ethanol and methanol. In this process, the alcohol reacts with steam at high temperatures in the presence of a catalyst, producing hydrogen, carbon dioxide, and other by-products.

Steam Reforming Reaction:

CH₃OH + H₂O → CO₂ + 3H₂

This reaction shows that methanol and water (steam) react to produce hydrogen and carbon dioxide

Ethanol (C₂H₅OH)
TRL 6

Ethanol produces more hydrogen per mole of alcohol compared to methanol due to its larger molecular size.

Steam Reforming Reaction:

C₂H₅OH + 3H₂O → 2CO₂ + 6H₂

Advantages:
  • Steam reforming of ethanol and methanol operates at lower temperatures compared to traditional hydrocarbon reforming (such as natural gas), making it more energy-efficient.
  • Both ethanol and methanol are liquids, making them easier to transport and store compared to gaseous hydrogen, especially in remote or decentralized locations.
2. Partial Oxidation:

Partial oxidation is another process where methanol or ethanol reacts with oxygen (instead of steam) to produce hydrogen. This process can occur at lower temperatures and is faster than steam reforming, but it produces less hydrogen per unit of fuel.


Benefits of Using Ethanol and Methanol as Hydrogen Carriers:
  • Easy to Store and Transport:

    Since ethanol and methanol are liquids at room temperature, they are easy to handle, store, and transport using existing fuel infrastructure. This makes them practical alternatives to compressed hydrogen gas, which requires high-pressure tanks, or liquid hydrogen, which requires cryogenic temperatures.

  • Higher Volumetric Energy Density:

    Ethanol and methanol have a higher volumetric energy density than hydrogen gas, meaning more energy can be stored in a given volume. This is particularly important for transportation applications where space is limited.

  • Renewable Sources:

    Ethanol can be produced from biomass (e.g., bioethanol), making it a renewable and potentially carbon-neutral fuel. Methanol can also be synthesized from renewable resources like captured CO₂ and hydrogen from renewable energy, known as "green methanol.

Underground Hydrogen Storage

TRL 6

Underground Hydrogen Storage is an innovative solution for large-scale, long-term hydrogen storage. It involves storing hydrogen in underground formations such as salt caverns, depleted gas fields, or aquifers. This method provides a cost-effective and safe way to store vast amounts of hydrogen, making it ideal for balancing energy supply and demand, particularly in conjunction with renewable energy sources. With proven technology and existing infrastructure, underground storage offers a sustainable approach to meeting future energy needs.

Hydrogen Storage
Advantages of Underground Hydrogen Storage:
  • Large Storage Capacity:

    Underground storage can accommodate massive quantities of hydrogen, making it a suitable option for seasonal energy storage or balancing intermittent renewable energy sources like wind and solar.

  • Cost-Effective for Large-Scale Storage:

    Once the infrastructure is in place, underground storage is relatively low-cost compared to surface storage methods like pressurized tanks or cryogenic systems, especially for long-term storage.

  • Safety:

    Storing hydrogen underground reduces the risk of accidents

Transportation of Hydrogen

Hydrogen is a key player in the green energy revolution due to its potential to reduce carbon emissions across various industries. However, hydrogen transportation plays a crucial role in the entire hydrogen supply chain. As the demand for hydrogen varies from region to region, the primary consideration for hydrogen transportation is cost. Furthermore, due to the complex physicochemical properties of hydrogen, any process involving hydrogen requires careful handling. Hydrogen transport takes place in less stable environments than storage, and safety concerns also need to be considered. Transporting hydrogen efficiently and safely presents several significant challenges, given its unique properties.

Transportation of Hydrogen

Why Transporting Hydrogen is Difficult

Low Energy Density

Hydrogen is the lightest element and has a much lower energy density compared to traditional fuels like gasoline. Even when compressed or liquefied, hydrogen requires a much larger volume to deliver the same energy output, which complicates its transport. For example, liquid hydrogen still only offers around 25% of the energy density of gasoline, making bulk transport less efficient.

Storage Requirements

To be transported, hydrogen is either compressed to very high pressures (up to 700 bar) or cooled to extremely low temperatures (-253°C) to be stored as a liquid. Both of these processes require significant energy and specialized equipment. The high-pressure containers are heavy and expensive, and maintaining hydrogen in a liquid state involves energy-intensive cooling systems.

Material Compatibility
Material Compatibility

Hydrogen can cause embrittlement in certain metals, which makes it difficult to use traditional pipelines or containers for transportation. This requires the use of specialized materials that can safely store and transport hydrogen without degradation over time.

Transportation Methods

Repurposing Natural Gas Infrastructure

Repurposing existing natural gas infrastructure for hydrogen transport introduces several technical challenges due to the differing chemical properties of hydrogen compared to natural gas. Key considerations include evaluating the technical condition and chemical composition of infrastructure materials to determine if they can handle 100% hydrogen.

Gas for Climate suggests that most existing infrastructure materials are already suited for hydrogen, implying that extensive modifications may not be necessary. However, a case-by-case assessment remains essential to ensure that the pipelines' technical and chemical properties can accommodate 100% hydrogen safely and efficiently.

Main Elements of Conversion Process:
Technical Conditions of Gas Pipeline
Cleaning
Integrity Management of Steel Pipes and Fittings

Regular inspections are essential to detect potential cracks. Hydrogen transport can hasten crack propagation due to embrittlement, typically occurring with pre-existing fractures and dynamic stresses. Continuous monitoring of crack development is vital.

Compression of Hydrogen

Requires new turbines, motors, and compressors. Operating below maximum capacity can reduce transport costs per MWh, minimizing the need for costly, high-capacity compressor stations.

System Tightness

As hydrogen is a much smaller molecule than methane, internal and external tightness needs certification, with appropriate sealing materials.

Measuring Equipment Replacement

Requires specific equipment modifications including gas chromatographs, pressure transducers, and calibrated gas meters.

Physical-based methods rely on compressing or liquefying hydrogen to move it from one location to another. These methods are widely used for hydrogen transport over long distances or for industrial applications.

Compressed Hydrogen Gas

Hydrogen is compressed at high pressures (usually 350-700 bar) and stored in high-pressure tanks. Transport can occur via pipeline, cylinders, or trailers.

Liquid Hydrogen Transport

Hydrogen is cooled to extremely low temperatures (-253°C) to reach a liquid state, which increases its energy density. It is then transported in cryogenic tanks. This method is energy-intensive due to the cooling requirements but enables transporting larger quantities in a smaller volume.

Pipeline Transport

Hydrogen pipelines transport compressed hydrogen gas over long distances. Dedicated pipelines are feasible for large-scale hydrogen infrastructure, while natural gas pipelines may require retrofitting.

Metal Hydrides for Storage and Transport

Some physical-based methods combine with material-based approaches. Metal hydrides absorb and release hydrogen and allow transport in solid form under moderate conditions.

Material-based methods involve hydrogen storage within specific materials, which allows for safer, denser storage and transport at lower pressures. These approaches are still developing and are aimed at making hydrogen transport more energy-efficient and secure.

Chemical Carriers

Hydrogen is bound in liquid organic hydrogen carriers (LOHCs) like toluene or methylcyclohexane, which act as reversible hydrogen storage solutions. The hydrogen is released through catalytic processes upon reaching its destination.

Metal Hydrides

Metal hydrides, such as magnesium hydride, absorb hydrogen, forming a solid compound that releases hydrogen upon heating. This method is especially useful for portable applications or smaller transport volumes.

Ammonia (NH₃)

Ammonia is considered an indirect hydrogen carrier. It can be transported as a liquid at relatively moderate pressures and decomposed to produce hydrogen on demand. However, handling ammonia requires additional safety measures.

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