Jul 26, 2024
This article has been re-shared from the World Economic Forum. You can read the original article here.
Ewan Gribbin, Research Assistant, University of Cambridge
Dr Deepanshu Singh, Research Associate, University of Cambridge
- The aviation industry needs to cut its carbon emissions but this will require multiple solutions.
- One option, hybrid-electric aircraft, could improve the range limitations of battery-electric planes to help significantly reduce emissions in short-haul aviation.
- But the carbon-cutting benefits of a hybrid solution that involves battery power can only be achieved if the electricity used for manufacturing and charging is green.
Aviation accounts for around 3% of global annual CO2 emissions, but it is often regarded as one of the most challenging industries to decarbonize. This is because the number of people travelling by air is increasing every year and the technologies needed for decarbonization are far from ready for mass deployment.
As a result, a single technology or change in aircraft operations will not be enough to achieve net-zero emissions. A combination of different strategies, encouraged by appropriate incentives and policies, will be required. This could include upgrading airport infrastructure, producing sustainable fuels or adopting new propulsion technologies.
Electrification has been used to decarbonize the automotive industry and now it’s also being explored as a possible pathway for aviation. A battery-powered flight uses the energy stored in batteries to power electric fans for propulsion. Since it doesn’t involve burning fossil fuels, electrification could eliminate in-flight emissions of carbon, as well as other environmentally unfriendly gases.
But greenhouse gases could still be emitted while producing the battery and generating the electricity needed to charge it. So, sustainable sources of electricity must be used for charging, alongside sustainable battery manufacturing practices, to significantly reduce overall emissions compared to using fossil jet fuel.
Battery-powered planes present challenges, however. Today’s batteries weigh nearly 50 times more than a comparable amount of jet fuel. Aircraft must also carry more fuel – or battery power – than they need for a journey so they can continue to fly and land safely if diverted to a different airport. For short missions this is a significant percentage of the total fuel carried and the extra weight adds to the fuel the plane uses, reducing the distance it can travel.
Helping hybrid planes take off
This is why hybrid aircraft could form part of the solution to aviation’s decarbonization challenge. Hybrid planes use an additional source of energy as well as battery power.
There are multiple options for the secondary fuel source and the way in which the propulsion systems are integrated: the secondary fuel could be burnt in its own separate engine or in a generator to power the same motors as the battery would, for example. Alternatively, a fuel cell could generate electricity from chemical reactions using hydrogen. The battery power and secondary fuel can be used to power the aircraft simultaneously or alternately.
One promising hybrid option is to use fossil fuels and a turbogenerator to provide the energy for the required reserves. Including an additional fuel source increases the complexity and weight of the aircraft, but an AIA analysis conducted for this article* showed that the increase could be smaller than the weight of the batteries that would otherwise be needed for reserves. And while the additional weight could also raise energy consumption, it would increase the range of the aircraft.
The performance of hybrid- and battery-powered aircraft could be improved even further by using electric motors. The jet engines typically used on passenger aircraft are more efficient if they are larger, but this is not the case for electric propellers. Positioning multiple small propellers along the length of the wing or using wingtip propellers could significantly improve aerodynamic efficiency.
Future hybrid plane designs could exploit these opportunities to lower greenhouse gases and extend their range.
Airports can also benefit from hybrid planes
Hybrid planes could also reduce an airport’s carbon footprint and improve local air quality. Aircraft can account for up to 95% of airport emissions. This could be reduced by using battery-electric planes to eliminate carbon emissions and other harmful pollutants.
A fully electric flight would require charging infrastructure at both the arrival and departure airports. On the other hand, hybrid-electric aircraft present a readily deployable solution while this charging infrastructure is built.
Hybrid aircraft batteries could be charged in-flight or on the ground by an on-board jet-fuelled powerplant if a destination airport has no charging infrastructure. This would reduce the environmental benefits of hybrid-electric operation, but it would expand the number of airports to which hybrid planes could fly while charging infrastructure is being rolled out.
Limitations of hybrid-electric aircraft
The emissions associated with battery charging depend heavily on the source of electricity used. Of course, grid emissions are expected to fall as renewable energy sources grow, but the AIA analysis* shows that using electricity from today’s power grid to power an aircraft could would, in certain locations and under certain circumstances, produce around 25% more carbon dioxide than using a jet-fuelled aircraft.
Producing batteries can also be more polluting than processing jet fuel as the mining and manufacturing processes involved are both energy intensive. These methods are likely to improve, but uncertainty remains and is dependent on the type of battery chosen for an aircraft.
This is why it’s important that a clear path to decarbonization is created for all industries – the benefits of building more renewable generation, for example, will extend beyond the energy sector.
Making short-haul flights hybrid
At present, most hybrid-electric planes in development have a target range of below 500km – a market segment that’s responsible for less than 6% of global passenger emissions. As batteries become more powerful, hybrid-electric aircraft could take over the short-haul aviation market (that is, journeys of up to 1,500km), which accounts for 33% of global passenger emissions.
Alternative approaches will be needed to decarbonize medium- and long-haul aviation, for example developing sustainable jet fuel substitutes or switching to completely new fuels such as hydrogen. Increasing efficiency through fleet changes and operational improvements will also form an important part of the industry-wide net-zero solution, of course.