Regan Brown
The idea of a flying car has been a staple of science fiction for decades, and while the concept has been in development since the early 20th century, it has yet to become a practical reality. Interest and investment in electric vertical takeoff and landing aircraft (VTOLs), or flying cars, have grown significantly in recent years. As the world moves towards more sustainable transportation options, the question arises: does the new flying car use fossil fuels? The answer appears to be a mix of yes and no. While some flying car prototypes may still rely on fossil fuels, the focus is now shifting towards making aviation fossil-free using alternatives such as hydrogen, biofuels, electrofuels, and batteries.
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What You'll Learn
Electric vertical takeoff and landing aircraft (VTOLs)
The concept of eVTOL aircraft first emerged in 2009 when a video of the NASA Puffin eVTOL went viral, showcasing a single-person concept rendering of the technology. The first manned eVTOL aircraft flight took place under the USAF Agility Prime programme in 2020. Since then, several companies have announced plans to develop eVTOLs, with pre-orders and investments being made.
VTOLs are designed to take off and land vertically, hover, and fly forwards, backwards, and laterally. This allows them to be used in congested or isolated areas where fixed-wing aircraft cannot operate. They are also capable of shorter commutes at lower speeds and altitudes than conventional aircraft, making them useful for urban transportation.
In terms of sustainability, the benefits of electric VTOLs over conventional fossil-fuel-powered road transportation are expected to increase in the future as more renewable energy sources are adopted. VTOLs that are fully loaded with passengers could result in fewer GHG emissions than average ground-based cars over long distances. However, VTOLs are more energy-intensive during takeoff and climb, so their overall efficiency depends on the trip distance.
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Energy intensity of flying cars
The energy intensity of flying cars, or Vertical Takeoff and Landing aircraft (VTOLs), is a topic of ongoing research and debate. VTOLs are a type of aircraft that can take off and land vertically, eliminating the need for a runway. This technology has the potential to revolutionize urban air transportation by minimizing space usage, which is scarce in cities.
One of the key considerations regarding the energy intensity of flying cars is the efficiency of batteries. Jet fuel contains approximately 12,000 Watt-hours per kilogram, while modern Li-ion batteries, the best of which offer around 200 Wh/kg, are still far behind in terms of energy content. However, significant progress has been made in battery performance and cost over the last decade, largely driven by the electric vehicle revolution.
The energy efficiency of flying cars also depends on various factors such as trip distance, occupancy, and cruise segments. For shorter distances, VTOLs enter an energy-intensive hover mode, resulting in higher energy consumption. However, for longer trips, VTOLs can leverage efficient cruise performance, leading to improved energy efficiency. Additionally, fully loaded VTOLs with higher occupancy rates can further enhance their energy efficiency relative to ground-based vehicles.
Research by Michael Sivak of the University of Michigan's Transportation Research Institute suggests that the average energy intensity of driving is about twice that of flying. This conclusion takes into account the current average on-road fuel economy of various vehicle types. However, it is important to note that carpooling can significantly impact this comparison, as having multiple occupants in a vehicle reduces the energy intensity per person.
The carbon intensity of the electricity grid also plays a crucial role in the overall energy intensity of electric VTOLs. As more renewable generation comes online, the carbon intensity of electric grids is expected to decrease, enhancing the sustainability of electric VTOLs over conventional fossil-fuel-powered road transportation.
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GHG emissions of VTOLs
Electric vertical takeoff and landing aircraft (VTOLs), commonly known as flying cars, have gained significant interest and investment. However, their sustainability implications are still uncertain. VTOLs are efficient when cruising but consume a lot of energy during takeoff and climb, so their environmental impact depends heavily on the trip distance.
A study published in Nature Communications compared the GHG emissions of a VTOL travelling 100 km with one pilot on board to that of a one-occupant internal combustion engine vehicle (ICEV) and a battery electric vehicle (BEV). The VTOL produced 35% lower emissions than the ICEV but 28% higher emissions than the BEV. When comparing a fully loaded VTOL (three passengers) to ground-based cars with an average occupancy of 1.54, the VTOL's GHG emissions per passenger-kilometer were 52% lower than ICEVs and 6% lower than BEVs.
The GHG emissions of electric VTOLs are influenced by the carbon intensity of the electricity grid. As more renewable energy sources are integrated, the benefits of electric VTOLs over conventional fossil-fuel-powered road transportation are expected to increase. Additionally, VTOL emissions can be reduced through technologies like DEP, which enable the use of smaller, electric-driven propulsors.
Another study by the University of Michigan Transportation Research Institute's Michael Sivak found that the average energy intensity of driving is about twice that of flying. However, this conclusion has been disputed, with critics arguing that aviation fuel efficiency improvements have plateaued in recent years.
VTOLs with different propulsion systems, such as ICE, BE, and FC, have varying environmental and economic impacts depending on the range. BE-VTOLs are highly competitive for short-distance transportation, and advancements in battery technology will make them viable for longer distances in the future.
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Fossil-free hydrogen
The successful development of fossil-free hydrogen has been demonstrated by the HYBRIT initiative, a joint venture between Vattenfall, LKAB, and SSAB. HYBRIT's pilot project for hydrogen gas storage has proven that it is technically possible to store fossil-free hydrogen gas for industrial-scale production of fossil-free iron and steel. This technology increases the flexibility of the electricity system and can be used to electrify industrial processes, making them more dependent on the electricity system.
The benefits of fossil-free hydrogen extend to its ability to serve as a carrier of energy, replacing fossil fuels like gas or oil, and acting as a catalyst in industrial processes by substituting fossil materials like coking coal. This makes it a viable solution for decarbonizing various sectors, including heavy industries such as steel, refineries, chemicals, agriculture, and long-distance transport, where reducing emissions has proven challenging.
Additionally, fossil-free hydrogen can support the integration of variable renewables in the electricity system, offering a rare option for storing energy over extended periods. This aspect is particularly advantageous for addressing energy challenges and enhancing energy security. The potential of fossil-free hydrogen is recognized by 60 governments, including the European Union, which have adopted hydrogen strategies.
While the momentum for fossil-free hydrogen is strong, faster action is needed to stimulate demand and unlock investments that can expedite production scale-up and cost reduction. Nevertheless, with continued efforts, the future of hydrogen as a key component of a decarbonized economy looks promising.
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Sustainable aviation fuels
Electric vertical takeoff and landing vehicles (VTOLs), or "flying cars", are a type of vehicle that can function as both a road vehicle and an aircraft. VTOLs can shorten commute time and play a role in sustainable mobility. However, their sustainability implications are unclear.
VTOLs are efficient when cruising but consume a lot of energy during takeoff and climb, so their benefits depend on the trip distance. When travelling 100 km with one pilot, a VTOL's GHG emissions are 35% lower than those of a car with an internal combustion engine but 28% higher than those of a battery electric vehicle. On the other hand, a fully loaded VTOL with three passengers has 52% lower GHG emissions per passenger-kilometre than internal combustion engine vehicles and 6% lower than battery electric vehicles.
The GHG emissions of electric VTOLs are linked to the carbon intensity of the electricity grid, which is expected to decrease in the future as more renewable generation is brought online. This means that the benefits of electric VTOLs over conventional fossil-fuel-powered road transportation are projected to increase over time.
The U.S. Department of Energy's Sustainable Aviation Fuel Review of Technical Pathways provides details on various SAF production pathways, and the Department is working with partners to develop new SAF pathways and fuel formulations to enable testing and certification. The Sustainable Aviation Fuel Grand Challenge, announced in 2021, aims to expand domestic consumption of SAFs to 3 billion gallons in 2030 and 35 billion gallons in 2050 while reducing lifecycle greenhouse gas emissions by at least 50%.
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Frequently asked questions
A flying car or roadable aircraft can function both as a road vehicle and as an aircraft.
The new flying car does not use fossil fuels. It uses electricity, batteries, biofuels, or hydrogen.
The energy efficiency of flying cars is lower than that of conventional aircraft as they travel at lower speeds and altitudes. However, they can shorten commute time and play a role in sustainable mobility.
The sustainability implications of flying cars are unclear. While they consume substantial energy for takeoff and climb, they can be more efficient than ground-based cars over long distances with more passengers.
Alternatives to fossil fuels for vehicles include hydrogen, biofuels, electrofuels, and batteries.
