While weight is very important in electric aircraft, I was hoping to see an efficiency advantage as well (batteries weigh more than motors).
The top advantage of electric motors for aircraft is that they can fly at much higher altitude, therefore much lower air resistance, reducing energy requirements. Fuel-burning jet engines require a certain oxygen density to operate[1]. Electric motors don't need oxygen, and propellers/fans can operate even with very low air pressure.
"The limit on maximum altitude for engines is set by flammability- at very high altitudes the air becomes too thin to burn, or after compression, too hot. For turbojet engines altitudes of about 40 km appear to be possible, whereas for ramjet engines 55 km may be achievable. Scramjets may theoretically manage 75 km."
> and propellers can operate even with very low air pressure
How does that work exactly? Of course, my intuition is that lower air pressure means less to "push off of" but also less drag. And you've got certain constant overhead, like internal friction. Is there some sweet spot of air pressure where you'll get the best efficiency?
There is far more benefit to be had by reducing the weight of the motor than increasing the conversion efficiency, given that modern motors are already generally over 95% efficient, so there is just not that much left to gain there.
Unfortunately one of the pilots disadvantages is that they don't work at high altitudes. Turbo prop and turbo jet planes get around this with PACKs and bleed air. To make a high altitude electric small plane you're going to have to make a mini Dreamliner PACK or carry tanks of liquid/gaseous oxygen.
A candidate plane for this engine would be the Cesna Turbo Stationair.
-It is a six passenger plane with a base operating weight of about a ton.
-It's engine delivers 231 kW, which is slightly less than Siemens electric engine. It's mass is approx. 200 kg, so the Siemens engine frees up 150 kg of weight.
-It's fuel capacity is 237 kg of 100LL avgas. With an energy density of 44.0 MJ/kg, this gives the plane a stored energy of 10.4 GJ. The plane's max range is 1300 km.
-Tesla's best battery (the Smart) has an energy density of 132 WH/kg, which is 475.2 kJ/kg. That means you'd need 21.9 thousand kilograms of battery to store the same energy that's in the Cesna's tank when full.
-If we assume the plane's range is linearly related to it's stored fuel (an approximation not really accounting for take-offs), 237 kg plus the 150 kg saved in engine weight gives 0.051 GJ of stored energy, which would reduce the plane's range to 5% of normal, or about 63 km.
Based on this, don't expect an EV passenger plane anytime soon. Battery technology just isn't good enough yet. A hybrid with a gas-turbine generator is much more likely in the near future. This engine may also have applications where range is not important, such as training, stunt and racing planes.
Well, to begin with, you are off by a factor of 3 on useable energy, and perhaps by a factor of 2 on the energy density of best batteries of today (Wikipedia stated 0.95 MJ/KG for best Li batteries), so achievable range should be ~380 km to begin with. I'll also note that the fuel tank itself has weight (though I do not know how much it is).
Next, the Stationair is a really old airplane, the basic design is over 50 years old. If we take the 2004-ish Cessna 400 instead, which has a 60% greater maximum range than the Stationair... perhaps we can claim that 500km+ is achievable, without calling upon radical new designs or further technology improvements.
And an airplane with ~500 km range is pretty sensible. (frankly, long trips in small planes are a bit of a trial, though probably less so in a smooth and quiet electric plane).
OTOH, you do need to consider reserve range, that does not work in favour of electrically powered aircraft. But the effect might be limited to having to add some battery capacity, sacrificing some performance along the way.
In conclusion... It does not even matter if the first commercially available electric airplane matches the petrol airplane on range. It would be nice to have a useable range, but ~500 km is enough for many applications. The electric airplane does not need to dominate the market from the start... just enter it.
Did you forget to take the engines efficiency into account? I think the electric engine is going to be in the 90 percentiles. The fuel engine in the 30s.
You also failed to read the word "hybrid". But not to worry, battery tech will continue to improve too. Fossil fuel efficiency? Not so much.
Piston engines are only about 30% efficient in converting gasoline to mechanical energy. While battery/electric can be around 90%. So you may be off by a factor of 3.
My intention was just to get ballpark close. 15% of normal range is still not very good. Battery's may get there eventually, but not yet. As stated in the article, they're going after hybrid applications and this is probably why.
Ah, but it is if you're running on batteries! Unlike fuel tanks, batteries don't get lighter (at least, not measurably; e=mc^2 tells us that there will be a little change) as you drain them.
It's irrelevantly small. You can build solar-powered aircraft, but they have to be extremely light, huge, extremely slow, and carry almost no payload.
Insolation is approximately 1hp per square meter. The Cessna in question has about 16m^2 of wing, so that's 16hp of incoming sunlight. Solar cells are maybe 20% efficient, so that's about 3hp of actual electricity generated. That's not even counting cosine error, which will hurt you badly any time you're not flying at high noon in the tropics.
converting to OP units: 1 hp is about .75 kW, so 3 hp is 2.25 kW, compared with the 231 kW used by the engine. 1/100.
But apart from drones, perhaps a specifically designed aircraft to target this could help: lighter, smaller, larger surface area, slower cruising speed would reduce air-friction so increasing efficiency. Mightn't be practical to get down to self-fueling, but at least make a significant contribution to range. NB: longer flights collect more fuel.
EDIT2 Solar Impulse 2 is currently on a circumnavigation of the globe (they store power to continue to fly at night)
http://wikipedia.org/wiki/Solar_Impulse At the moment waiting for ideal weather in the China leg
Solar aircraft can definitely be done (what with the existence proofs you provided) but they're not very practical. Power consumption is roughly proportional to the cube of speed, so to get the Solar Impulse 2's cruise speed up to, say, 150kts instead of the 49kts it actually does would require 27x more solar collection surface area. If you want to carry two people instead of one, it'll need to be about twice again as big. You'll hit annoying scaling laws since larger structures are proportionally weaker so it'll get even worse.
The whole point of hybrid cars are the fact that you're only on the throttle periodically. You start, stop, speed up, slow down all the time (especially in S. Florida!). Normally when you're slowing down, you're burning off the energy as heat (friction from the brakes), etc.
Adding an electric motor/generator to this allows you to recapture energy that is normally wasted slowing back down, so it can be reused to re-accelerate back up to speed. This is why hybrids (unlike non-hybrid cars) get better fuel mileage in town than on the freeway.
Aircraft, on the other hand, are at full-throttle on takeoff, and close to 3/4 throttle during the vast majority of the flight. There is no slowdown to recoup the energy until coming in for a landing. @beloch has already done the math showing the electric range would be downright miniscule, so there's very, very little to be gained here.
Until we have some utterly astounding advances in battery (or more likely, supercapacitor) tech (probably via graphene), this won't really be a cost-effective solution.
However, this would be an absolutely incredible motor for electric car conversions.
Imagine a pair of these (each powering an axle)... it would be considerably more powerful than even the Tesla Model S P85D.
Regenerative braking is only one advantage of the hybrid approach. The other significant advantages are:
1. The engine can be sized for average output, not peak output, which makes it more efficient.
2. The engine can stay closer to its optimally efficient RPM.
3. Because RPM variation is smaller, and because power requirements are smaller, you can use more efficient engine cycles like the Atkinson cycle.
I'm doubtful that the advantages will be worth the extra weight. #1 is not going to be as big of a deal for an airplane, since as you note, power output is a decent fraction of the maximum during cruise. (In contrast with a car, which might have a 200hp engine but only use 30hp of that in cruise.) But it should count for something. #2 might make a nice difference paired with a variable-pitch propellor. #3 gives a decent efficiency boost. Put it all together and it doesn't seem likely to be worth the extra weight, since that counts for so much in an airplane, but it's not completely absurd.
Note for the Tesla comparison that the limiting factor in the P85D's power output is the battery, not the motors. Lighter motors would certainly be good, but it'll be a small effect. If you really want more power, you either need a bigger battery, or a battery chemistry that can discharge faster.
Electric aircraft can recharge in flight from the sun. This will mostly be long endurance, high altitude utility aircraft, like Facebook/Titan Aerospace's wifi UAV or surveillance drones that can stay airborne indefinitely. These aircraft can be pseudo satellites without requiring a rocket launch. They must be extremely low in drag and weight efficient, with powerplant weight being a major issue because previously large electric motors have not had application in aerospace and have not been weight optimized as gas turbines have. Siemens has seen the growing market (QinetiQ Zephyr for another example) and is responding to it. We definitely won't be seeing any passenger aircraft electrically propelled anytime soon.
The article specifically mentions using this motor for hybrid passenger aircraft (which is what my post addressed). I mentioned nothing about UAVs or solar power (which is only possible with an enormous wing/fuselage ratio).
FTA:
"This innovation will make it possible to build series hybrid-electric aircraft with four or more seats," said Frank Anton, Head of eAircraft at Siemens Corporate Technology
These could dramatically improve the throttle command response of the aircraft and make the aircraft far quieter on departure and approach. Civil aviation is one of the most annoying hobbies that anyone has ever conceived, from the perspective of people who live near airfields. If this tech opens up the possibility of living near and airfield but not having to put up with the sound of private pilots coming and going all day long in Korean-War-era airplanes, that would be nice for those people.
> from the perspective of people who live near airfields.
Things more annoying than airplane noise:
* People who voluntarily choose to live near airports (which predictably have associated airplane noise), then complain about it.
* The ubiquitous, intentionally loud motorcycles (at closest approach, much louder than any small airplane, due to power law d^-2), which all get a free pass for some bizarre cultural reason, and are largely inescapable.
Some of the airplane noise perception problem is jealousy, and is easy to fix: go down to the airport, chat someone up, and they'll give you a ride.
Elon Musk has joked that one day he will merge the work of Tesla and SpaceX into an electric jet. I'm hoping we'll end up with electric-powered Star Trek shuttlpods with Falcon-derrived takeoff-landing thrusters.
What are the benefits of having an electric motor? I imagine that fewer moving parts means less maintenance and lower odds of failure, but what are the other advantages? Quieter perhaps? Are these aircraft going to be loaded up with battery packs? How is that going to work?
this article does a fair amount of discussion on advantages. Basically it is quiet, has no on-site emissions, and is theoretically more efficient. They also go on to mention that it loosens some design constraints. http://www.technologyreview.com/news/516576/once-a-joke-batt...
Those wouldbe advantages of an all-electric aircraft, however this article is not about that. This article is about an electric motor that could be used in a series-hybrid aircraft, that is, one with a fixed-speed combustion engine and small battery or capacitor.
The original article mentions hybrid but does not say the motors are exclusive to hybrid drives, merely that the motor will be flight tested in a series-hybrid DA36. Besides, the MIT article I linked enumerates the benefits of the hybrid drive. The word 'hybrid' is in the subtitle. I suspect Siemens is focusing on hybrid tech at the moment because battery tech is such a moving target and they want to use an existing airframe. Ultimately, I think all-electric aircraft will find their market in short-flight urban transport applications where noise, pollution, and reliability are huge factors.
The quiet aspect probably has a lot of value in military applications. And it'd be useful in the solar powered unmanned planes we see, but maybe the energy requirements for this are too high.
Even if the motor is quiet you still have the propeller making a lot of noise. I (honestly) don't know if you can really get it quiet enough for the motor's noise to be important.
The largest source of propeller noise is transonic flow at the tips, so turning the propeller slowly helps significantly. The tradeoff is a larger propeller and more expensive engine installation due to the required gearbox.
A solar-powered sea-plane would be pretty awesome. Probably not enough range to cross an ocean in one go, but you just land, do some fishing, then take off again the next day.
Whatever reduces our reliance on fossil fuels and leads to reduced emissions I am down with. I see some people getting confused, this is not about a solely electric plane. The applications here would be for a hybrid plane that utilises both existing jet engine technology and electric tech. Like a hybrid vehicle, which hopefully leads to reduce fuel costs, greater distances possible to travel in a plane.
Imagine if for take off and landing the pilot can fire up the existing jet engines, then whilst cruising fire up the electric engines and wind back the output from the jet engines (leading to a slower burn rate). You would still be using both, but you would have some additional boost from the electric engines. Put some lightweight solar panels on the roof of the plane, charging up the batteries while the plane sits on the tarmac and readies for taxi. Take the output of the existing engines and use it to charge the batteries in flight (like an alternator in a vehicle, but on steroids).
Of course, weight is the limiting factor of all of this. I am more thinking out aloud here, when it comes to reducing our reliance on fossil fuels, I think anything (even a marginally small improvement) is a great step forward.
Would driving one of these engines concurrently with the power generated by a turbine engine make sense?
Does the thrust generated by a turbine engine hit some point of diminishing returns that converting the engine energy to electricity and powering this engine be more efficient?
I know Boeing have made advancements in the efficiency of drawing electric power from a turbine engine with the 787, the generators are now directly connected to the transmission of the engine.
Normal helicopters can autorotate[1] to a landing if the motor stops. A quad is unflyable with one motor out[2]. An octo can self correct for a single motor failure, and is still controllable with two motors out if you get lucky about which of the motors fail second.
[2] it's probably theoretically possible that a quad with reversible motors (or reverse pitchable blades) could be built to survive a single motor failure, but so far as I can tell nobody, at least in the hobby size quadcopter world, has done so (thoughI'd be surprised if the KMel Robotics and Pennsylvania University research teams don't know exactly how to do it).
>> but so far as I can tell nobody, at least in the hobby size quadcopter world, has done so
> It's not the motors that are the problem. They can spin freely any direction. It's programming the ESCs to be reversible.
True - poor wording on my part. And even the ESC programming is a "solved problem" - all the brushless RC cars run ESCs which understand how to run brushless motors in both directions.
So actually reversing the thrust is, as you say, already happening. What I've not seen (yet) is a controller board with software designed to use that ability to stabilise and safely land a quad with one motor out.
There are quadcopters with collective pitch and mechanical linkages to a central motor. They should be able to autorotate in theory (although there might not actually be enough rotor momentum), and not to mention fly upside down. Of course, you don't get the mechanical simplicity of 4-motor quadcopters, and there's still no redundancy for propeller loss.
Ahhh yeah - ETA Zurich. They're the other team besides KMel and PennU that Id expect to have worked (successfully) on this. Thanks. That's _really_ impressive. (I wonder how far away from showing up in the 3DR or DJI gear, or even the NAZE32 or KK stuff?)
The top advantage of electric motors for aircraft is that they can fly at much higher altitude, therefore much lower air resistance, reducing energy requirements. Fuel-burning jet engines require a certain oxygen density to operate[1]. Electric motors don't need oxygen, and propellers/fans can operate even with very low air pressure.
[1] http://en.wikipedia.org/wiki/Jet_engine:
"The limit on maximum altitude for engines is set by flammability- at very high altitudes the air becomes too thin to burn, or after compression, too hot. For turbojet engines altitudes of about 40 km appear to be possible, whereas for ramjet engines 55 km may be achievable. Scramjets may theoretically manage 75 km."