June 13, 2021

Flying cars will only be sustainable if they are full | Science

Flying cars will only be sustainable if they are full | Science

With the flying cars it will pass like with the drones: they will seem something of the future until one day be everywhere. There are already at least 60 prototypes of these hybrid vehicles of a helicopter, an airplane and a drone and everything indicates that the first commercial versions will be ready in the next five years. Now, a study that compares its consumption and emissions with those of the usual cars and with the electric ones shows that they are losing. Only in long journeys and going full length can be a viable transportation alternative.

Giants of aviation such as Airbus and Boeing have their designs. Both the European and American space agencies are already working with prototypes to scale. Car manufacturers such as Toyota or Daimler are investing in various projects and there are dozens of technologists with their own idea of ​​what a vertical take-off and landing vehicle (or VTOL) should be. Most seem more like a mini-plane or a maxidrón, but their personal transport mission brings them closer to the idea of ​​flying cars. The success of drones and the advantages of distributed electric propulsion and the depletion of the transport model based on cars with internal combustion engines have helped this fever.

Now, a group of researchers from the Ford R & D center and the University of Michigan (USA) have developed a model of one of the key aspects for the success or failure of the VTOL, its sustainability. Summing up what the prototypes can already do and what they promise will do little to advance the technology, the engineers estimated the consumption and the emissions in different routes, between 5 and 250 kilometers, and in different load scenarios, with one to four occupants. The results, published in Nature CommunicationsThey are bittersweet.

More than 60 prototypes of these vertical take-off and landing electric vehicles have already been tested

"In our base case, we compared a VTOL with a single occupant with land vehicles with only one passenger, in which case, traveling 100 kilometers (point to point) with a pilot in a VTOL generates greenhouse gas (GHG) emissions. that are 35% lower than those of a vehicle with an internal combustion engine, but 28% higher than those of a battery-powered electric vehicle, and both with only one occupant ", summarizes the senior author of the study and director of the Center for Sustainable Systems of University of Michigan, Gregory Keoleian.

In 100 kilometers, a VTOL emits 15.7 kilograms of COtwo compared to 24.3 for a normal car and 12.3 for an electric car. But the first consume proportionally more in the phases of takeoff, climb, descent and landing than in the cruise, hence one of the keys to its viability is in the distance. While in a five-kilometer journey the VTOL emits six times more than an electric, the emissions are equal to around 250 kilometers. For their calculation, they took into account the complete cycle of the energy used, from its generation (or extraction in the case of oil) to its consumption.

The other big key is occupation. In the different simulations, flying cars always lose when the distance is less than 35 kilometers. But they start to win the combustion engines from that distance and the terrestrial electric ones if they are loaded with people. In energy consumption, expressed in megajoules, the VTOL multiply between five and six times the consumption of land vehicles until, from those 35 kilometers, consumption is equalized. From 120 kilometers, the flyer spends less than the combustion engine, but always more than the electric.

Idealized aspect of how the DeLorean DR7 will be. The current prototype is one tenth of its final size.
Idealized aspect of how the DeLorean DR7 will be. The current prototype is one tenth of its final size.

Only if the comparison is unequal, the flying car wins. The study starts from a reality: current cars have an average occupancy per trip of 1.54 occupants, according to official data (in Spanish cities such as Madrid it drops to 1.35, according to a study by the Municipal Transport Company of 2017). It is a percentage that makes road transport very inefficient, without including the added problem of traffic congestion. So if a VTOL goes with its pilot and three passengers it emits half of GHGs that a combustion car and up to 6% less than an electric one that take an average of 1.5 occupants in a 100 kilometers journey.

"A high occupation is not only better for emissions, it also favors the profitability of flying cars, and users could be encouraged to share the journey, given the great time savings if we compare the return with driving", says the researcher at the Research and Innovation Center of the Ford Motor Company and co-author of the study, Akshat Kasliwal. Although it is less sustainable, the shortest distance between one point and another is always the straight line. And a VTOL can do 100 kilometers in 27 minutes compared to the 154 minutes it can take a conventional car traveling several roads at rush hour.

But reality today is very different. According to the previous survey of mobility in the US, the average route of the American driver (similar to Spanish) is 17 kilometers, half of what is necessary for a VTOL to emit less gas than a conventional car. In fact, only 15% of journeys exceed that distance.

If the electricity from the batteries came from renewable sources, emissions would be reduced by half

For the aeronautical engineer David Ortiz, from the technical point of view, the study is impeccable, but part of optimistic assumptions, "like the willingness of several people to share a journey because it is shorter". In addition, Ortiz stresses that the work does not include the environmental cost of manufacturing the vehicles. "Making a car contaminates a lot and probably doing a VTOL will contaminate it even more and the issue of batteries remains," adds the director of engineering at UAV Works, one of the first companies that have manufactured a VTOL.

A final aspect to consider is the origin of electricity. Although an electric car (flying or not) is seen as zero emissions, it has been emitted by the generation of the electricity that moves it. Depending on where it comes from (a coal-fired power plant, a gas station, a solar-panel garden), that's how your real emissions will be. The authors of the study start from the reality of the USA, where the burning of coal still accounts for more than a third of the mix energetic. But a reduction of its contribution to half, up to figures of 15%, close to those that Spain has today, would reduce the emissions of the VTOL also by half. So the more green your energy, the more green the flying cars will be.

In what coincides Ortiz with the authors of the study is that the flying cars will not be for everyone. Among its possible gaps will be the services of air taxi or companies dedicated to the sharing of transport type Uber, which already has its own platform, Uber Elevate. Only the wealthy can buy one of these Ferrari air for personal use.


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