Climate explained: why don't we have electric aircraft?

Discussion in 'Electric Propulsion' started by aviast, Sep 24, 2019.

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  1. Sep 24, 2019 #1

    aviast

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    Climate explained: why don't we have electric aircraft?

    Dries Verstraete, University of Sydney

    CC BY-ND

    Climate Explained is a collaboration between The Conversation, Stuff and the New Zealand Science Media Centre to answer your questions about climate change.

    If you have a question you’d like an expert to answer, please send it to climate.change@stuff.co.nz

    Why do we have electric cars and trains, but few electric planes? The main reason is that it’s much simpler to radically modify a car or train, even if they look very similar to traditional fossil-fuel vehicles on the outside.

    Land vehicles can easily cope with the extra mass from electricity storage or electrical propulsion systems, but aircraft are much more sensitive.

    For instance, increasing the mass of a car by 35% leads to an increase in energy use of 13-20%. But for a plane, energy use is directly proportional to mass: increasing its mass by 35% means it needs 35% more energy (all other things being equal).

    But that is only part of the story. Aircraft also travel much further than ground vehicles, which means a flight requires far more energy than an average road trip. Aircraft must store onboard all the energy needed to move its mass for each flight (unlike a train connected to an electrical grid). Using a heavy energy source thus means more energy is needed for a flight, which leads to extra mass, and so on and on.

    For an aircraft, mass is crucial, which is why airlines fastidiously weigh luggage. Electric planes need batteries with enough energy per kilogram of battery, or the mass penalty means they simply can’t fly long distances.

    Short-range planes

    Despite this, electric aircraft are on the horizon – but you won’t be seeing electric 747s any time soon.

    Today’s best available lithium ion battery packs provide around 200 watt-hours (Wh) per kilogram, about 60 times less than current aircraft fuel. This type of battery can power small electric air taxis with up to four passengers over a distance of around 100km. For longer trips, more energy-dense cells are needed.

    Short-range electric commuter aircraft that carry up to 30 people for less than 800km, for instance, specifically require between 750 and 2,000Wh/kg, which is some 6-17% of kerosene-based jet fuel’s energy content. Even larger aircraft require increasingly lighter batteries. For example, a plane carrying 140 passengers for 1,500km consumes about 30kg of kerosene per passenger. With current battery technology, almost 1,000kg of batteries is needed per passenger.

    To make regional commuter aircraft fully electric requires a four- to tenfold reduction in battery weight. The long-term historical rate of improvement in battery energy has been around 3-4% per year, doubling roughly every two decades. Based on a continuation of this historical trend, the fourfold improvement needed for a fully electric commuter aircraft could potentially be reached around mid-century.

    While this may seem an incredibly long wait, this is consistent with the timescale of change in the aviation industry for both the infrastructure and aircraft design lifecycles. A new aircraft takes around 5-10 years to design, and will then remain in service for two to three decades. Some aircraft are still flying 50 years after their first flight.

    Here come the hybrids

    Does this mean long-distance flying will always rely on fossil fuels? Not necessarily.

    While fully electric large aircraft require a major, yet-to-be-invented shift in energy storage, there are other ways to reduce the environmental impact of flying.

    Hybrid-electric aircraft combine fuels with electric propulsion. This class of aircraft includes design without batteries, where the electric propulsion system serves to improve the thrust efficiency, reducing the amount of fuel needed.

    Hybrid-electric aircraft with batteries are also in development, where the batteries may provide extra power in specific circumstances. Batteries can then, for instance, provide clean take-off and landing to reduce emissions near airports.

    Electric planes are also not the only way to reduce the direct carbon footprint of flying. Alternative fuels, such as biofuels and hydrogen, are also being investigated.

    Biofuels, which are fuels derived from plants or algae, were first used on a commercial flight in 2008 and several airlines have performed trials with them. While not widely adopted, significant research is currently investigating sustainable biofuels that do not impact freshwater sources or food production.

    While biofuels do still produce CO₂, they don’t require significant changes to existing aircraft or airport infrastructure. Hydrogen, on the other hand, requires a complete redesign of the fuelling infrastructure of the airport and also has a significant impact on the design of the aircraft itself.

    While hydrogen is very light – hydrogen contains three times more energy per kilogram than kerosene – its density is very low, even when stored as a liquid at -250℃. This means that fuel can no longer be stored in the wing but needs to be moved to relatively heavy and bulky tanks inside the fuselage. Despite these drawbacks, hydrogen-fuelled long-distance flights can consume up to 12% less energy than kerosene.

    This article is part of The Covering Climate Now series

    This is a concerted effort among news organisations to put the climate crisis at the forefront of our coverage. This article is published under a Creative Commons licence and can be reproduced for free – just hit the “Republish this article” button on the page to copy the full HTML coding. The Conversation also runs Imagine, a newsletter in which academics explore how the world can rise to the challenge of climate change. Sign up here.[​IMG]

    Dries Verstraete, Senior Lecturer in Aerospace Design and Propulsion, University of Sydney

    This article is republished from The Conversation under a Creative Commons license. Read the original article.
     
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  2. Sep 25, 2019 #2

    Dan Thomas

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    Have I missed something? Are there electric air taxis carrying four people 100 km already?

    And the 50 times less energy density thing: That would mean that this air taxi, using fossil aircraft fuel instead of lithium-ion batteries, could cover 6000 km (3700 statute miles) with four passengers on the same weight of fuel. I don't know of any airplane like that, either.

    Maybe I'm just sleepy...
     
  3. Sep 25, 2019 #3

    BBerson

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    60 times less energy, but electric motors are three times as efficient. So 20 times worse than fuel. 100 km range is likely possible, but with a heavier takeoff weight. Batteries are not ideal for aviation in most ways, mentioned in the article.
     
  4. Sep 25, 2019 #4

    radfordc

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  5. Sep 25, 2019 #5

    Dan Thomas

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    I read about that more than a year ago. Anxious to see it proven or otherwise. Those are large airplanes to drive with electric. The article says that Harbour Air will convert their entire fleet to electric, which I think is journalistic overconfidence. Harbour still has to fly one yet to prove it, physically, economically, and safety-wise.
     
  6. Sep 25, 2019 #6

    Hephaestus

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    A beaver that does two flights a day - one to, one from on the Vancouver to Victoria route... Serviced already by bc ferries, doesn't make it viable.

    It's a short hop, for it's purposes it makes sense... But it's no game changer.
     
  7. Sep 25, 2019 #7

    12notes

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    "This type of battery can power small electric air taxis with up to four passengers over a distance of around 100km." - This statement, while somewhat misleading, may be technically correct, currently in use batteries may contain enough energy to do that. An aircraft does not currently exists that can do that, but the battery type is in the ballpark to have sufficient energy density.

    EDIT: I found the correct battery weight (2 @ 53kg each) for the Electro in Pipistrel's literature, original speculative numbers are in red, updated number based on correct battery weights in blue.

    Fudging some numbers for the Pipistrel Alpha Electro vs Pipistrel Alpha Trainer for a sniff test:
    Electro empty weight: 810 lbs
    Trainer empty weight: 615 lbs
    Electro motor weight: 40 lbs (wild guess, should be correct ballpark)
    Trainer motor weight: 140 lbs (Rotax 912 ULS)
    Electro Estimated battery weight: 295 lbs (based on fudged math and a wild guess, rough approximation at best) 233 lbs (from Pipistrel literature)
    Trainer Equivalent fuel volume : 49 gallons 39 gallons
    Electro Range/mpg based on equivalent fuel: 86 miles / 1.7 mpg 2.2 mpg
    Trainer Range/ fuel / mpg : 324 miles / 10.9 gallons (12.7 - 1/2 hour reserve) / 29.7 mpg
    Energy density ratio Trainer:Electro : 17.5 : 1 (based on fudged math and a wild guess, rough approximation at best) 13.5:1
    This is quite a bit less than the 20:1 ratio that BBerson discussed.

    At the same battery equivalent fuel weight as the Alpha Electro, a piston aircraft would need to get 25.4 mpg 31.8 mpg to go 1240 miles (20*100km). At the same percentage of battery equivalent fuel weight vs. gross weight, a plane twice the gross weight would need to get 12.4mpg 15.9 mpg , a Cessna 172 is roughly twice the gross weight, has 4 seats, and gets 16.4 mpg.

    So it's still a reasonable claim, but phrased poorly. They should have explicitly stated that such a plane is not yet available. But the battery has the density needed to power a standard configuration, well designed GA 4 seat GA aircraft for 100km.
     
    Last edited: Sep 25, 2019
  8. Sep 25, 2019 #8

    BJC

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    When considering range, one needs also to consider reserve energy for enough power for a go-around at the end of the flight. Full power is available with a combustion engine until all the available fuel has been used. I don’t know what power is available from a battery and controller when the battery's energy is almost spent. I would appreciate comments from those of you knowledgable about the batteries being postulated for, or used, in aircraft.

    Thanks.


    BJC
     
  9. Sep 25, 2019 #9

    12notes

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    Lithium Cobalt batteries (commonly known as lithium ion) have a fairly linear power output until they get to about 20% energy remaining, and still have over 90% rated power until 10% remaining, but it declines pretty fast after that. However, their life is significantly reduced if you discharge this far repeatedly, so electric aircraft treat 10% (possibly a little higher) as empty, and add the reserve above that. So it's like a combustion engine that will run for a while, increasingly weakly, after the fuel has run out, but it'll ruin your gas tank.

    [​IMG]
    This is for a cell rated at 3.7V, it makes over 100% of rated power above ~95% charge.
     
  10. Sep 25, 2019 #10

    BJC

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    Are the electric ranges in post #7 absolute range, or range with 10% charge plus enough power and energy for a go-around still in the battery?

    The Electro spokesman at Oshkosh indicated that they don't like to run the batters below 20% (IIRC) because of the detriment to battery life, which already is short / expensive.


    BJC
     
  11. Sep 25, 2019 #11

    12notes

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    20% might be correct or really conservative, I don't have an exact number. It may be about 15%, and we both heard different people rounding different ways.

    On the Pipistrel, it's range without using the reserve, and, although it isn't explicitly stated, this is typically over the 20% not used for battery longevity. It's a 21kWh battery, and maximum distance cruise is 45 minutes at 18kW, which is 13.5kWh or 64%, a 20 minute reserve (LSA) at the 18kW cruise power leaves about 10% battery life, but I don't know what power level is considered reserve so the energy burn could be less.

    Also, researching this I found that it actually has two 53kg battery packs, I've updated the original post with the corrected numbers, the conclusion did not change.
     
    Last edited: Sep 25, 2019
  12. Sep 25, 2019 #12

    BBerson

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    There are plenty of variable details. The battery output efficiency depends on the discharge rate, the motor efficiency depends on power load. Exact comparisons would be rare with an optimal fuel engine at same speed.
    Electric can be great at short range, if that is the mission. Overall cost including battery replacement cost is still largely unknown.
     
  13. Sep 26, 2019 #13

    pictsidhe

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    With 21700 Tesla cells 75% drained, they struggle to provide 1C. You probably want to try your go-around with more charge in them... Other cells show similar behavior. If you want high power, you won't get it if they are significantly drained. Not draining cells too far significantly improves life, as well as giving some emergency reserve. Not fully charging cells also helps life. DeWalt only charge their powertool batteries with 4V per cell for this reason.
     
  14. Sep 26, 2019 #14

    Dan Thomas

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    Those Beavers and Otters are flying back and forth all day, coming and going out of Vancouver's and Victoria's harbours. They're going to have to address turnaround time.

    I grew up in BC, spent all but 20 of my 66 years there. Been on the ferries plenty.
     
  15. Sep 26, 2019 #15

    Hephaestus

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    One of the harbour air interviews when they first announced this they said it would be one to and one from daily. 2 flights a day.

    All I know for sure, is it'd have to be a super nice day before I climbed on board :) I get nervous every time I head to the island, I seem to be a fog magnet. o_O
     
  16. Sep 26, 2019 #16

    Dan Thomas

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    Probably due to the recharge time.

    They have 22 Otters, 3 Twin Otters, 14 Beavers and 1 Cessna Grand Caravan. On their schedule for this week, they show 19 flights daily Vancouver to Victoria and 18 flights daily back to Vancouver. That's in addition to all the flights they make between a bunch of other places. They're going to have to address recharge times.
     
  17. Sep 26, 2019 #17

    radfordc

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    First steps seldom are.
     
  18. Sep 26, 2019 #18

    BBerson

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    They will probably need a new electric grid to charge them.
     

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