Hydrogen Fuel Cell Buses: Powering The Future Of Transport

As the world shifts towards cleaner energy sources, hydrogen fuel cell buses are becoming an increasingly popular alternative to traditional fossil fuel-powered vehicles. These buses use hydrogen as fuel to generate electricity and power an electric motor, which drives the wheels. With no harmful emissions, high energy efficiency, and a quiet engine, hydrogen fuel cell buses are an attractive option for public transport, particularly in urban areas. So, how does a hydrogen fuel cell bus work?

Hydrogen storage tanks

The hydrogen storage tanks are an essential part of the hydrogen fuel cell system, providing all the energy required for the bus to operate. The number of tanks can vary depending on the size and capacity of the system, with some buses equipped with seven tanks. The tanks are designed to store hydrogen as a liquid or compressed gas, with a typical bus requiring 20 to 30 kilograms of hydrogen per day.

Refuelling of the hydrogen storage tanks typically takes around seven minutes, which is comparable to the time it takes to refuel a traditional fossil fuel-powered bus. The refuelling process is similar to that of a gasoline or diesel vehicle, with the hydrogen being compressed and stored in high-pressure tanks. The safety of hydrogen storage tanks is a key consideration, and they are designed and built to strict safety standards to prevent leaks or accidents. Collision sensors are also incorporated, which can activate a "safe shutdown" sequence that locks the high-pressure hydrogen in the tank in the event of a collision.

The size of the hydrogen storage tanks and their capacity play a crucial role in determining the range of a hydrogen fuel cell bus. With advancements in technology and infrastructure, the range of these buses is expected to improve, making them an increasingly viable option for clean and sustainable public transportation.

Fuel cell stack

The fuel cell stack is a crucial component of a hydrogen fuel cell bus, serving as the heart of the fuel cell system and generating the electrical energy required to power the vehicle. This stack is formed by arranging hundreds of individual fuel cells in series, each undergoing a "cold combustion" process that converts chemical energy into electricity through the combination of hydrogen and oxygen.

The individual fuel cells in the stack play a vital role in this process. Each cell facilitates a chemical reaction where hydrogen molecules are separated into positively charged hydrogen ions and negatively charged electrons. This occurs at the anode side of the fuel cell. The hydrogen ions then pass through a proton exchange membrane, specifically designed to accommodate their movement. Meanwhile, the electrons are forced to travel through an external circuit, generating electricity in the process.

The protons, now diffusing through the polymer membrane, move towards the cathode side of the fuel cell, where they reunite with the electrons and combine with oxygen molecules from the air. This reaction produces water vapour as a byproduct, completing the "cold combustion" process. The electricity generated in this process is then supplied to the electric motor, powering the wheels of the bus.

The power output of a fuel cell stack is influenced by its size. By increasing the number of cells in the stack, the voltage can be raised, while expanding the surface area of the cells boosts the current. This flexibility in design allows for customisation to meet specific power requirements.

Electric motor

A hydrogen fuel cell bus uses electricity to power an electric motor, which in turn drives the bus's wheels. The electricity is generated onboard through a chemical reaction that occurs within the fuel cell stack.

The electric motor is connected to the fuel cell stack and uses the electricity produced to power the bus. The fuel cell stack is made up of individual fuel cells that combine hydrogen and oxygen to produce electricity, heat, and water vapour as byproducts. The hydrogen molecules enter the anode side of the fuel cell, where they are separated into positively charged hydrogen ions and negatively charged electrons. The electrons generate electricity as they are forced to travel through an external circuit. The hydrogen ions and electrons then recombine on the cathode side of the fuel cell with oxygen molecules from the air. The electricity generated by the fuel cell stack is then supplied to the electric motor, which powers the wheels of the bus.

The electric motor's power is defined by the vehicle manufacturer during the design process. The size of the electric motor is determined by the amount of electric power it receives from the appropriately sized fuel cell and battery combination. The electric motor can also be powered by the battery pack, which stores energy generated from regenerative braking and provides supplemental power to the electric motor.

The electric motor drives the vehicle's wheels, with some vehicles using motor generators that perform both the drive and regeneration functions. The transmission transfers mechanical power from the electric motor to drive the wheels.

Hydrogen refuelling stations

Hydrogen Refuelling Station Technology

The technology and processes used in hydrogen refuelling stations can vary. One common method is the liquefaction of hydrogen, which involves cooling it to a temperature of -253 °C and storing it in cryogenic containers. However, liquefaction is energy-intensive, consuming up to 35% of the energy in the stored hydrogen. Another approach is to locate the refuelling station near or on top of a hydrogen pipeline, simplifying the process by only requiring compression, cooling, and dispensing of hydrogen. Additionally, hydrogen can be produced through electrolysis of water, resulting in zero emissions if renewable electricity is used.

Hydrogen Supply and Storage

Compressed gaseous hydrogen is typically delivered in containers suitable for the quantity of hydrogen required. The refuelling process involves delivering hydrogen to the vehicle's tank at the correct pressure through a flexible hose and nozzle, similar to refilling with petrol or diesel. Hydrogen can also be stored on-site through methods like electrolysis or by utilising by-product hydrogen from industrial processes, such as the production of chlorine.

Refuelling Infrastructure

Different refuelling infrastructure concepts are available to meet varying levels of hydrogen demand. This can range from refuelling a single car to large refilling stations capable of serving multiple cars and buses daily. Hydrogen refuelling stations can be designed to refuel vehicles quickly, with some stations boasting refuelling times of around 7 minutes, comparable to refuelling fossil fuel-powered vehicles.

Integration with Bus Depots

Real-World Examples

The establishment of hydrogen refuelling stations is already underway in various parts of the world. For instance, Hyundai Motor Company partnered with Incheon International Airport Corporation and other entities to set up a hydrogen refuelling station at Incheon Airport's Terminal 2. This station supports the operation of fuel cell electric buses, contributing to the airport's transition to eco-friendly transportation.

Hydrogen supply and storage

Refuelling stations play a vital role in the hydrogen supply chain, and their strategic placement is essential for efficient operations.

Hydrogen refuelling stations are essential for the operation of hydrogen fuel cell buses. These stations can be centralised locations, such as a bus depot, or strategically placed along bus routes to facilitate quick refuelling. The refuelling process for hydrogen fuel cell buses is similar to that of traditional gasoline or diesel vehicles, with hydrogen being compressed and stored in high-pressure tanks. The time required for refuelling is significantly shorter than that of battery-powered electric vehicles, taking approximately 7 minutes for a typical fill-up.

The hydrogen supply for these refuelling stations can come from various sources. One method is to generate hydrogen on-site using an electrolyser or natural gas reformer. Alternatively, hydrogen can be delivered to the refuelling station, typically produced at large-scale production facilities. This delivered hydrogen can be stored as a liquid or compressed gas, depending on the infrastructure available at the station.

The hydrogen storage tanks on the buses themselves are designed to strict safety standards to prevent leaks or accidents. In the event of any malfunction or leak, the fuel cell system is designed to shut down automatically, ensuring the safety of passengers, drivers, and pedestrians.

The development of hydrogen refuelling infrastructure is crucial to the widespread adoption of hydrogen fuel cell buses. As technology advances and investments in infrastructure increase, refuelling times are expected to decrease further, making hydrogen fuel cell buses an increasingly attractive option for clean and sustainable public transportation.

Frequently asked questions