Hybrid Propulsion for Urban Air Mobility: A Key to Sustainable Flight

Urban Air Mobility (UAM) is rapidly emerging as the next frontier in transportation, promising to alleviate road congestion and offer faster, more sustainable transport solutions within and between urban areas. As the world continues to urbanize and the need for efficient, clean transportation intensifies, electric Vertical Take-Off and Landing (eVTOL) aircraft are gaining traction as a revolutionary mobility option. Yet, while battery-electric propulsion has garnered significant attention, limitations in range and endurance pose critical challenges for widespread adoption. Hybrid propulsion systems—particularly those combining hydrogen fuel cells with batteries—are increasingly viewed as a compelling solution for overcoming these barriers.

The Case for Urban Air Mobility (UAM)

The concept of UAM involves the use of small, autonomous or piloted aircraft to transport passengers or cargo within urban settings. These aircraft, often in the form of eVTOLs, are designed to operate above congested streets, providing rapid point-to-point travel. UAM promises not only faster commutes but also a reduction in carbon emissions by shifting away from traditional, fossil-fuel-powered vehicles.

In this context, the propulsion system becomes critical. While battery-electric aircraft offer zero emissions and minimal noise, their performance is hindered by the relatively low energy density of current battery technologies. This limitation directly impacts the range and operational efficiency of battery-electric aircraft, making them less suitable for longer or more demanding missions.

Hybrid Propulsion Technologies

1. Fuel Cell/Battery Hybrid

Hybrid propulsion, which pairs battery systems with hydrogen fuel cells, offers a potential solution to the limitations of fully electric UAM vehicles. The primary advantage of hybrid systems lies in their ability to combine the high-power density of batteries with the extended endurance provided by hydrogen fuel cells. This allows for a more versatile and capable aircraft, particularly in applications requiring longer flight durations or greater payload capacity.

In this configuration, hydrogen fuel cells generate electricity that powers the electric motors, while batteries provide additional energy during peak demand. This setup allows for longer range and quick bursts of power, ideal for urban air mobility scenarios.

• Performance Benefits: Fuel cells can operate efficiently for longer durations, while batteries can handle rapid acceleration and deceleration during urban flights.

In a hybrid configuration, batteries provide the high-power output needed for energy-intensive flight phases, such as take-off and vertical landing. Hydrogen fuel cells, on the other hand, offer sustained energy for cruising and lower-power segments of the flight. This synergy between the two power sources optimizes both the range and efficiency of the aircraft, while also addressing the limitations of each system when used in isolation.

2. Internal Combustion Engine/Battery Hybrid

In this hybrid system, an IC engine and batteries work together to drive the aircraft. The IC engine can either directly power the propellers or serve as a generator to recharge the batteries.

• Performance Benefits: This configuration offers flexibility and enhanced range, making it suitable for a variety of missions, from short urban hops to longer suburban routes.

Series Hybrid

In a series hybrid configuration, the IC engine or fuel cell generates electricity to power the electric motors. This setup allows the aircraft to operate solely on electric power while maintaining the flexibility to extend range as needed.

Parallel Hybrid

In a parallel hybrid configuration, both the IC engine and electric motors can directly drive the propellers. This design provides the ability to draw power from both sources simultaneously, optimizing performance during demanding flight segments.

3. Range Extender

A range extender hybrid uses a small IC engine or fuel cell to generate electricity for the electric motors, further extending the range of the aircraft while maintaining its electric operational characteristics.

Advantages of Hybrid Systems:

1. Extended Range: One of the most significant advantages of hybrid propulsion is the ability to extend the aircraft’s range beyond what is possible with batteries alone.  By combining batteries with an IC engine, the aircraft can operate on electric power for shorter urban hops and use the IC engine for longer flights or when additional power is needed. This opens up the possibility for longer missions, including regional or intercity transportation, while maintaining the benefits of electric flight.
2. Higher Payload Capacity: Hybrid systems can support larger payloads compared to fully electric configurations, which is crucial for commercial applications such as cargo transport and air taxi services.
3. Increased Efficiency: The hybrid configuration allows the aircraft to switch between or simultaneously use both power sources. For instance, the electric motors can handle takeoff and landing, while the IC engine provides additional range during cruising. Alternately by using fuel cells for sustained cruising and batteries for high-power segments, hybrid systems can operate more efficiently, resulting in better overall performance and reduced energy consumption.
4. Rapid Refueling: Hydrogen fuel cells offer faster refueling times compared to recharging batteries. This increases the operability and turnaround time for UAM aircraft, making them more suitable for high-frequency operations, such as air taxi services in urban environments.
5. Reduced Fuel Consumption and Emissions: While the IC engine does burn fuel, it operates more efficiently in a hybrid setup. The IC engine can either be smaller or used less frequently, leading to reduced fuel consumption and emissions compared to traditional aircraft, making it a more environmentally friendly solution.

Comparing Hybrid Configurations for UAM

To determine the most suitable propulsion system for different UAM missions, it is essential to evaluate the performance of various hybrid configurations. For instance, comparing a battery-electric design with hybrid systems that incorporate different fuel cell sizes reveals key insights into the advantages and trade-offs associated with each setup.

The article explores the potential of hybrid fuel cell/battery powertrains for lightweight helicopters in Urban Air Mobility (UAM) scenarios. It compares three hybrid configurations using hydrogen fuel cells against a purely battery-electric powertrain. The study is based on a helicopter designed for two passengers and a 175 kg payload, tested across three mission types: regional transport, suburban routes, and urban routes.

Configuration 1: 70 kW Fuel Cell Hybrid

This configuration pairs a 70 kW fuel cell with a battery pack. It excels in longer regional missions, tripling the number of regional transports compared to a battery-electric counterpart. However, its performance on short urban routes is less impressive, as the added weight and complexity of the fuel cell system limit its agility and efficiency in stop-and-go urban flight patterns.

Configuration 2: 90 kW Fuel Cell Hybrid

With a larger 90 kW fuel cell, this hybrid design is optimized for high-power duty cycles, making it ideal for urban missions that require frequent take-offs and landings. It offers enhanced performance on short, energy-intensive routes while still benefiting from the extended range capabilities of the fuel cell system.

Battery-Electric Design

Fully electric configurations are lightweight and efficient for short, low-power urban missions. However, their limited range restricts them to shorter flights and smaller payloads. This makes them less viable for regional transportation or applications requiring sustained energy output over longer distances.

Mission-Based Performance Study

To further refine the analysis, performance studies of the different propulsion systems are conducted over three specific mission types: short urban routes, regional transport, and high-power demand missions (such as emergency response or cargo delivery). The results indicate that:

• Urban Routes: A battery-electric design may outperform hybrid systems for short urban missions where low weight and agility are key. The simplicity and efficiency of the all-electric system offer significant advantages in these scenarios.
• Regional Transport: The 70 kW hybrid configuration demonstrates a clear advantage for regional missions, significantly extending the operational range and allowing for more trips compared to a battery-only design.
• High-Power Demand Missions: The 90 kW hybrid system excels in missions requiring high sustained power, such as emergency response or rapid cargo delivery, where both power density and range are crucial.

The study concludes that hybrid fuel cell/battery powertrains enhance helicopter performance in UAM by balancing power and energy density. The optimal configuration depends on the mission type: the 70 kW fuel cell is ideal for regional flights, while the 90 kW system excels in power-demanding suburban and urban environments. Hybrid powertrains offer a flexible, sustainable propulsion solution for UAM applications.

Challenges and Opportunities

1. Weight and Complexity: Hybrid propulsion systems can be heavier and more complex than traditional systems, impacting aircraft design and performance. Advances in lightweight materials and efficient design practices are essential to overcoming these challenges.
2. Fuel Cell Technology: Continued advancements in  fuel cell efficiency are crucial for improving the energy density and operational capabilities of hybrid electric aircraft.
3. Battery Technology: The effectiveness of hybrid propulsion is closely tied to advances in battery technology. Improving energy density (the amount of energy stored per unit of weight) is crucial for extending the electric range and reducing reliance on combustion engines. Current efforts in solid-state batteries and lithium-sulfur technology promise significant gains in the near future.
4. Infrastructure Development: Building the necessary infrastructure, such as hydrogen refueling stations and charging facilities, is essential to support hybrid UAM operations and facilitate widespread adoption.

Real-World Examples

Several aerospace companies and startups are already exploring hybrid-electric powertrains for UAM:

• Embraer: Their UAM division, Eve Air Mobility, is considering hybrid-electric systems to extend the range and reduce emissions of their eVTOL aircraft.
• Rolls-Royce: Rolls-Royce has been developing hybrid-electric propulsion systems that combine IC engines with batteries, targeting UAM applications where both range and environmental performance are critical.
• Joby Aviation: While focused on all-electric aircraft, Joby and other companies are also investigating hybrid options as a way to address the battery limitations for longer routes in UAM networks.

Conclusion:

Hybrid propulsion offers a promising solution for achieving sustainable and efficient urban air mobility. By combining the advantages of electric motors with either fuel cells or internal combustion engines, hybrid systems can address the challenges of range, emissions, and noise.

While fully electric aircraft may dominate short-range urban transport, hybrid systems will likely play a crucial role in extending the operational capabilities of UAM vehicles, particularly for regional or high-power-demand missions. By balancing the high power density of batteries with the endurance of hydrogen fuel cells, hybrid systems offer a flexible and sustainable solution for the diverse needs of urban air mobility.

As battery and fuel cell technologies continue to improve and infrastructure development progresses, hybrid propulsion is poised to play a key role in the future of urban transportation, paving the way for cleaner, more efficient air travel in our cities.