Hydrogen Storage

The mass density of hydrogen gas at normal atmospheric pressure, temperature (NTP) is 0.0898 kg/m³. Comparing with the density of air which is approximately 1.225 kg/m³, hydrogen is approximately 15 times lighter than air at NTP.

Gasoline vapor density / Hydrogen density = 3.5 kg/m³ / 0.0898 kg/m³ ≈ 39. Therefore, hydrogen is approximately 39 times lighter than gasoline vapor.

A cubic meter of water weighs approx. 1,000 kilograms, where as a cubic meter of hydrogen gas weighs only 0.0898 kilograms. Therefore, hydrogen is approximately 11,940 times lighter than water at NTP.

Under normal pressure and temperature conditions, 1 kg of hydrogen occupies approximately 12,000 Litre, where as 1 kg of gasoline = 1.34 Litre.

Now you can easily imagine the density of the hydrogen!

Due to its extremely low density, hydrogen in gaseous form takes up huge volume at normal atmospheric pressure. As a result, large, giant containers are required to store useful quantities of hydrogen in gaseous form in normal conditions. Thus hydrogen is virtually not stored or transported in gaseous form at atmospheric pressure because it is simply highly inefficient. Efficient and safe storage, transport of hydrogen is not a straightforward unlike fossil fuels gasoline, diesel, natural gas, coal, etc.,

Also hydrogen has very poor volumetric energy density, the amount of energy carried per unit volume.

Imagine a container holding one gallon of liquid hydrogen. That same amount of hydrogen, if it were a gas at standard temperature and pressure, would fill up a whopping 800 gallon containers approximately! This is because liquid hydrogen is incredibly dense compared to its gaseous form. When it changes state (from liquid to gas), it expands dramatically, undergoing a rapid phase change.

Hence for storage, transportation and for all practical purposes of hydrogen use, it has to be compressed. This is the key limitation of using hydrogen as a fuel in mobility (Cars, ships, Trucks, etc.,). As hydrogen is produced at low production pressure (20-30 bar), hydrogen requires compression before transportation.

Two Main Approaches to Hydrogen Storage:

A. Physical Storage: focuses on changing the physical state of hydrogen to increase its density for storage. Here there are two key traditional strategies: high-pressure compression and cryogenic liquefaction.

Compressed Gaseous Hydrogen: (CGH2) By compressing hydrogen gas to pressures ranging from 350 to 700 bar within specialized containment vessels, its density is significantly increased. This is the most mature hydrogen storage technology, low cost. This energy intensive process needs approximately ranges 4.2 – 15% of the energy content (LHV) of the resulting compressed hydrogen.

Liquid Hydrogen (LH2): Hydrogen can be liquefied at cryogenic temperatures (around -253°C). This dramatic phase change results in a roughly 800 times increase in its density compared to the gaseous state. This means that 800 times more hydrogen can be stored in the same tank or container. The tank needs specialized thermal insulation. Storing liquefied hydrogen in cryogenic tanks is a very complex, energy intensive process.

However, this cryogenic liquid approach presents significant challenges:

• Hydrogen has a low ignition limit, high diffusion rate in air, and a low boiling point (-252.8 C). These properties necessitate specialized infrastructure for safe handling, significantly increasing the overall cost of using hydrogen as a fuel. While these methods offer a path to increased density, the safety considerations and infrastructure costs associated with them limit their widespread adoption.
• Storing liquid hydrogen costs 4–5 times more than using compressed gas technology.
• Environmental heat (heat from outside air, environment) or boil off phenomenon can cause up to 2-3% of hydrogen vaporizes per day from the liquid cryogenic tank through the safety valve. This release of hydrogen could potentially pose a significant safety risk.
• This cryogenic process is also highly energy-intensive. In practice, the energy input for liquefaction is typically around 20- 35% of the energy content (LHV) of the resulting liquid hydrogen.

Combination of the above two:

Cryo-Compressed Hydrogen storage (CcH2): This method combines elements of both compressed and cryogenic storage. Here, hydrogen is kept at a cold temperature (around -20°C) while simultaneously being stored under high pressure. This approach aims to achieve higher storage densities than compressed hydrogen alone, while reducing the complexity of maintaining extremely low temperatures needed for liquefaction. However, this method is still under development and requires further research to optimize performance and address potential challenges.

B. Material Based Storage:

This approach utilizes special materials both solids and liquids that act like “carriers” for hydrogen. These materials can bond with hydrogen molecules or atoms, either physically or chemically. This method offers several advantages over physical storage:

• Higher Storage Density: By binding with hydrogen, these materials can pack it in more tightly, increasing storage capacity.
• Improved Safety: The bonding with the carrier material can make hydrogen less volatile and easier to handle safely.

Liquid Organic Hydrogen Carriers (LOHCs): This promising approach is at the forefront of innovation. LOHCs store hydrogen by chemically binding it to readily available liquid molecules at ambient conditions. The hydrogen can then be released through a controlled dehydrogenation process. LOHCs offer a trifecta of benefits:

• Safe and Simple: No high pressure or extreme temperatures are needed, reducing infrastructure costs and safety concerns.
• Tailored Options: Different LOHC molecules can be chosen based on desired properties. Some options even utilize atmospheric derivatives like CO2, potentially capturing greenhouse gases during hydrogen production.
• Easy Separation: Hydrogen separation from LOHCs is achieved through simple condensation due to the use of high-boiling-point liquid molecules.

Toluene / Methylcyclohexane is a well-studied LOHC system.

Metal-Organic Frameworks (MOFs): MOFs are porous crystal materials made of metal ions, where large pores within the crystals can store hydrogen gas1. MOFs have high surface areas and hydrogen adsorption capacities, where hydrogen molecules can cling to the surface of the MOF cavities. They have a simple charge/discharge mechanism, allowing the stored hydrogen to be released immediately upon discharge without the use of chemical reactions, which typically require high temperatures. MOFs have the potential to store hydrogen through adsorption at moderate pressures and temperatures.

Metal Hydrides: These materials store hydrogen through chemical bonding. This involves the formation of metal hydrides using elements like palladium, which has the capacity to soak up hydrogen up to 900 times its own volume, in addition to other elements such as magnesium, aluminum, and specific alloys. While some hydrides offer high storage capacities, challenges remain in terms of reversibility and energy efficiency of the hydrogen release process.

Nanomaterials: Nanomaterials are being explored for their potential in hydrogen storage.

Power Fuels: Power fuels are another form of material-based storage where hydrogen is stored in the form of power fuels.

Ongoing research is focused on developing new materials and techniques for even more efficient and safe hydrogen storage.

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