Question

Describe the three ways in which hydrogen can be stored in vehicles for use as a fuel, and discuss briefly the disadvantages of each method.

Short answer

Hydrogen can be stored as compressed gas, liquid hydrogen, or in metal hydrides, each with disadvantages such as weight, energy requirements, and storage efficiency.

Step-by-step solution

Storage as Compressed Gas

Hydrogen can be stored as a compressed gas in high-pressure tanks. Typically, these tanks operate at pressures around 350 to 700 bar. This method is quite straightforward and allows relatively easy refueling. However, the main disadvantage is the requirement for very robust and heavy tanks to withstand high pressures, making the vehicle heavier and consuming more space, which could affect vehicle performance and design.

Storage as Liquid Hydrogen

Hydrogen can also be stored in its liquid form, but this requires maintaining extremely low temperatures (around -253°C or -423°F) to keep it liquefied. Liquid hydrogen storage offers higher energy density than compressed gas, allowing longer vehicle range. However, the cold temperatures require significant energy for refrigeration and specialized insulation, which can increase overall energy consumption and cost. Additionally, some hydrogen may be lost through evaporation, also known as boil-off.

Storage in Metal Hydrides

Metal hydrides can store hydrogen through chemical reactions, absorbing hydrogen into a metal lattice. This method can be quite efficient and allows for safer storage at lower pressures. However, metal hydrides tend to be heavy, which increases vehicle weight. Furthermore, the process of absorbing and releasing hydrogen can be slow, affecting the refueling speed and overall convenience.

Key concepts

Compressed Gas Storage

Storing hydrogen as a compressed gas is one of the most common methods used in hydrogen-powered vehicles. In this method, hydrogen gas is stored in high-pressure tanks usually operating between 350 and 700 bar. These tanks need to be extremely sturdy to contain the pressurized hydrogen safely.
This method has several advantages, such as:

• Simple and straightforward design
• Quick and easy refueling

However, there are notable disadvantages:

• Heavy and robust tanks are required, adding weight to the vehicle
• Large space is needed for the tanks, affecting vehicle design and performance

The weight and space impacts can lead to reduced vehicle efficiency and limit the design flexibility for other components.

Liquid Hydrogen Storage

Liquid hydrogen storage involves cooling hydrogen gas to extremely low temperatures—around -253°C—or -423°F. This process converts the gas into a liquid form. One of the main advantages of liquid hydrogen is its higher energy density compared to compressed gas, which can extend the vehicle's range.
Despite its benefits, liquid hydrogen storage faces several challenges:

• Maintaining low temperatures requires significant refrigeration energy
• Specialized insulation is needed, impacting costs
• The risk of hydrogen loss through evaporation, known as boil-off

Evaporation can lead to a gradual decrease in the stored fuel over time, which negatively impacts the total efficiency and economy of the vehicle's fuel system.

Metal Hydride Storage

Hydrogen storage in metal hydrides utilizes the chemical absorption of hydrogen into metal matrices. This makes the hydrogen safer to store at much lower pressures, which can be advantageous for vehicle safety and design.
The benefits of this method include:

• Lower pressure storage increases safety
• Potentially higher storage efficiency due to the chemical bond

Despite these benefits, the disadvantages must also be considered:

• Metal hydrides are typically heavy, burdening the vehicle with additional weight
• The release and absorption processes can be slow, adversely affecting refueling speed and convenience

Addressing these issues is crucial for making metal hydride storage a more viable option for wider adoption in hydrogen fuel systems.

Vehicle Fuel Systems

Vehicle fuel systems for hydrogen are designed to utilize one of these storage methods to deliver power efficiently and safely. Each system requires specific components and safety measures tailored to its storage type.
Key components include:

• High-pressure tanks or specialized cryogenic containers for gas and liquid storage
• Catalyst beds and heating elements for metal hydride systems

Safety is a major concern in designing these systems due to hydrogen's flammable nature. Advanced sensors and safety valves are also integral to prevent leaks and manage pressure.
It's important that these systems are lightweight and cost-effective, balancing performance with the economics of production and operation.

Hydrogen Fuel Challenges

The widespread adoption of hydrogen as a fuel faces several challenges related to storage, infrastructure, and economics.
Key challenges include:

• High production and storage costs due to technology and material requirements
• Lack of established refueling infrastructure comparable to fossil fuels
• Hydrogen's physical characteristics, such as flammability and small molecule size, which can lead to leaks

To overcome these challenges, innovative technologies and policy interventions are needed, alongside industry investment in infrastructure development. Progress in these areas holds the promise of cleaner, sustainable energy through hydrogen fuel vehicles.