Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Extremely high current generators

  1. Apr 3, 2015 #1
    I'm doing some research into capturing a large amount of energy over a short period of time. The energy will be stored in supercapacitors but I'm finding it hard to find information on how a generator would cope with this.

    My project investigates the feasibility of using a regenerative braking system on an aircraft and I'm looking for a rough idea of what the limits are of capturing a very high current with a generator. The reason a high current will be required is to capture as much energy as possible over a short period of time. The amount of available kinetic energy I have calculated after losses is around 460MJ or 129kWH.

    I have found some examples online such as a 200KW generator which is capable of dealing with 300V at 400 amps. If the time available to capture the energy is 60 seconds, would this mean that 120kW x 1/60 = 2kWh could be captured in this time? This is nowhere near enough to prove the concept of what I'm looking at if so

    Sorry if I am confusing KWh with joules here, any input pointing me in the right direction would be much appreciated
  2. jcsd
  3. Apr 3, 2015 #2


    User Avatar

    Staff: Mentor

    Electric trains use regenerative braking. Look into their capacities.
  4. Apr 3, 2015 #3


    User Avatar
    Gold Member

    I can't give you any help, but I'm confused by your terminology. I assume you mean that you are going to have the plane wheels/brakes power a generator and you then want to store the generated electricity but you say "capturing a very high current with a generator" which doesn't make sense because generators don't capture electricity, they generate it (see how the name makes sense that way ? :smile: )

    You'll need to give some thought to how you want the power output of the generator distributed. That is, you get a tradeoff between voltage and current. If you can generate 10amps at 100volts then you could presumable generate 1amp at 1000 volts, so your concern about amps is oversimplified.
  5. Apr 3, 2015 #4

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    How big an aircraft are you contemplating?
    Weight is a huge concern and generators are not light.
    I've seen in my metal recycling yard aircraft starters rated at 1,000 amps yet not much bigger than a football. They're heavy though, copper and iron mostly.
    Still they're only 40 kw or so.
    I doubt the small amount of energy recovered in landing would buy the extra fuel burnt to haul the weight.
    460MJ is only about 11kg of jet fuel, maybe 4 gallons.
    IMHO it's better to just dissipate the energy as heat in the brake linings. (Asbestos is great stuff.)

    What airlines would buy is a lightweight gizmo that'd spin up the wheels to match aircraft's ground speed just before landing.
    Ever noticed how long are those skidmarks at end of runway ? Well, those tires are expensive.

    that's my thoughts

    old jim
    Last edited: Apr 3, 2015
  6. Apr 4, 2015 #5


    User Avatar
    Science Advisor

    It only takes about one second to spin up a wheel on contact. The skid marks look long because many aircraft land, all in slightly different places. The skid marks heat the tyre and so give some initial braking. The heat also prepares the tyre temperature and pressure for the braking process.
  7. Apr 4, 2015 #6


    Staff: Mentor

    Alex282, You are being sloppy in your nomenclature as phinds said.

    You want to do dynamic braking. That means you will need one generator on each axle of the landing gear. On a 747 that means 9 generators (or is it 5? or is it 18? there are 18 tires but I'm not sure how many axles.)

    The generators will have to spin up at the same time the wheels spin up, which will reduce tire life as Jim Hardy pointed out.

    Presumably, the generators will generate DC power for the supercapacitors. As phinds said, the current magnitude can be traded off with high voltage. But don't supercapacitors come only in low voltages? Adding complexity and weight, you could generate high voltage three-phase AC (lighter generators, less current), then use a transformer and diodes to reduce the voltage and convert it to DC.

    Then the supercapcitor stored energy will have to be converted to 400 hertz AC for use on airplane loads. (Is 400 still the standard for airplanes?) The conversion and consumption of this energy will have to be fast because supercapcitors leak the energy away rapidly. I am thinking that the energy will have to be used up before the airplane leaves the taxiway. The engines will already be running at idle and spinning their own generators (except when the engines are used for reverse thrust during landing.) Excess electric energy may have to be dumped someplace and extra energy captured with dynmaic braking may be very unwelcome at that time.

    Perhaps you are thinking that the supercapacitors can store the energy long enough for the aircraft to reach the gate. Then the supercapcitors could feed a land-based inverter to help power the airport's power grid. That may or may not be possible. it depends on the leakage rate of the supercapcitors.

    Finally, the weight of the 9 generators, the transformers, the supercapcitors, and the inverters will add the weight of the airplane as Jim Hardy mentioned.

    I remember how frustrating it can be to propose a radical new idea in engineering circles. It seems that other engineers lie in wait to shoot down anything new or anything not invented by me. On the other hand, some ideas really don't deserve a lot of research. The burden is on the proposer is to think it through enough to counter that presumption. We can help on this forum if you get stuck on some technical issues, but you need to do a lot more thinking before posting your questions here.
  8. Apr 4, 2015 #7

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    They look small from an airliner window forty feet off the ground..
    Through the windshield of a little Cessna they look immense.

    Airliner tire life ranges from ten to 300 landings.

    From an interview with an aircraft tire supplier:

    see also http://ask.metafilter.com/70629/Spinning-airplane-wheels-before-landing
    and https://www.gcaa.ae/en/ePublication...oeing 737-800NG - VT-AXP - Report 05 2009.pdf

    now that's interesting,
    also gives pilot tactile feedback that he's on the ground.
    I do recall one very gentle landing on a rain slicked runway where there was absolutely no sensation as plane touched down, first clue was engines reversing.
    Captain switched on the intercom announced for all to hear : "Ladies and gentlemen that was our copilot's very first landing with passengers. Please give him a round of applause."

    old jim
  9. Apr 4, 2015 #8
    This is a University group project and I am looking at the generation section.The plane we're looking at is an Airbus A380 as a base model . The idea would be to use 3 phase AC induction generator(s) and then convert this to DC for the supercapacitors to charge and then use all of this energy to move the plane by using the generators as motors. So this system is independent of the current power systems on the plane.

    Apologies, I know I haven't listed every detail of the project here as there is a lot, but so far everyone has been helpful in giving me food for thought, which is what I'm looking for in order to write up some theory to scale the system up to a real size from the small scale prototype which we have built.

    The prototype basically uses a 150W DC motor coupled with a 150W DC generator to charge supercapacitors. The speed of the motor over the test simulates the landing of an aircraft e.g. a variable power supply decreases the speed automatically to match the real braking profile of a plane. The results are that after the simulation, the supercapacitors are able to run the same 150W motor for a few minutes

    The part I'm struggling with the most is theoretically scaling the system up for use on an aircraft to investigate the feasability taking weight etc into consideration. How much energy can be generated during the short time period of the landing and what is the best way to do this fast, high voltage, high current? How much torque can be extracted from this energy and how long can this torque be used for from the generated energy? Can the batteries already on the plane be used as backup power to the electric motors?

    I know that there is already a working system similar to this (Honeywell and Saffran EGTS) except it uses the auxiliary power unit of the plane to generate the energy instead of regenerative braking. My project is basically just to document why a regenerative braking system will or will not work
  10. Apr 4, 2015 #9

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Well it sure sounds interesting.

    I always thought disc brake rotors looked like candidates for an axial flux electrical machine rotor.

    If you can embed windings or magnets in the brake rotors you might get an electrical machine without a lot of rotating inertia.
    And it could spin up the wheels before touchdown.

    It was not my intent to curb enthusiasm. I do think there's an opportunity to save a lot of tire changes and a little bit of jetfuel. It is wasteful to use the main engines for taxi.

    I grew up in a town where the major employer was airlines. Several of my friends' dads were airplane mechanics. They changed LOTS of tires . They told us boys if we'd find a way to stop that skid on touchdown we'd get rich. So i was imprinted early....

    I do hope you come up with something.

    http://apexdrivelabs.com/pdf/Apex_Motor_Offers_Superior_Torque_Density.pdf [Broken]

    Good luck.

    old jim
    Last edited by a moderator: May 7, 2017
  11. Apr 4, 2015 #10
    That is an interesting point about 460MJ being equal to about 11kg of fuel, but I did read that an aeroplane uses around 1 ton of fuel taxiing per day which could mean moving on the ground wastes a lot of energy by using the jet engines
  12. Apr 5, 2015 #11

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

  13. Apr 5, 2015 #12


    Staff: Mentor

    OK, now we can get somewhere. Your original question about the maximum amount of energy you can capture is a difficult question. It might take detailed calculations or experimetation to answer it. I suggest starting with simpler quesitons. You may be able to complete the project while avoiding having to answer the difficult quesiton.

    First, what is the definition of success for your project? How far must the plane taxi, at what speed to succeed? Are stop/starts required during taxi? Are there size or weight constraints?

    Airbus, or the people who make the tractors that tow aircraft at airports, should be able to tell you how much power is needed to tow a 380 at normal taxi speeds. Don't be afraid to reach out to them. I remember seeing a video of a stunt where a strong man pulled a 747 50 feet using his teeth. That suggests that the pulling power may be fairly small. The smaller the energy requirements, the easier everything elese becomes. For example, if it is small enough, you may be able to do dynamic braking and taxi power with only one axle, instead of all the axles.

    Next, find out from Airbus how much energy the braking system is designed to dissipate during a landing. It is not the total K.E. of the plane. Thrust reversers, I believe get rid of more K.E. than the bakes do. Ask them if the energy per wheel is the same. If not, then ask for the energy specs for each wheel separately.

    Now, with the total energy needed to taxi, and the total energy available from braking, you can calculate the minimum required efficiency of your brake-store-taxi system. If the answer is more than 100%, then you can't succeed; look for another way.

    I am suspicious about the leakage of supercapacitors. You should check that to see how long they retain the energy.

    Yes, the airplane's other batteries might be able to contribute some energy. Specs on how much energy they store should be available from Airbus. Compare that with the energy needed to meet your taxi requirements. There may also be some energy available from the jet engine driven generators at idle speed. I assume that the jet engines will not be totally shut off during taxi, and they have a minimum idle speed. (If the jets are shut off, then you must supply energy for air conditioning and other hotel loads in additiono to taxi propulsion.)

    So, as a first step, I suggest an analysis based soley on energy/power resources and requirements. Leave energy conversion and storage complexities for step two. Also, don't let anyone pressure you to start designing until the requirements (the definition of success) are very thouroughly nailed down. You may even be able to negotiate the requirements with the professor. For example, taxi 1000m at 5 knots, on level pavement with zero wind, is success. I would not accept a vague requirement like "as far as possible."

    For step two, study the torque-slip characteristics of induction machines. Slip is the difference between applied AC voltage and the speed of the rotor. During taxi, you might be able to assume constant speed (unless stop/start taxi is part of the requirements). That simplifies the design of the inverter to feed the induction motor. During braking, you need a variable frequency AC excitation source to control the slip and maximize the torque.

    Good luck.

    It was my own words that I thought sounded discouraging Jim. Not yours.
  14. Apr 5, 2015 #13

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    not a scientific source,
    more than anybody could want to know about tires at ...
    http://www.goodyearaviation.com/resources/pdf/aircraftmanual.pdf [Broken]

    main engine idle fuel flow is a few kg per second.
    I can remember in early 80's incoming airliners stopped at the taxiway to get towed rest of the way to the gate , to save the fuel. Waiting for the tow tractor was unpopular with both pilots and passengers.
    Last edited by a moderator: May 7, 2017
  15. Apr 5, 2015 #14


    Staff: Mentor

    If your project requirements are to just save fuel and money during taxiing, regardless of method, I have a completely different approach to suggest. One that is very mainstream 2015 technology. Jim Hardy hinted at it in #13.

    You could have a fleet of driverless towing robots (call them taxi drones if you like). They could be controlled by the pilot's tablet using Bluetooth, or controlled by the air traffic control ground controller using 3G communications. Using an umbilical cord, they could supply the plane's hotel power during towing so that the jet engines could be completely shut down as soon as the plane leaves the runway. Look up the numbers; I believe that gas turbines need lots of fuel to run at idle, so this scheme might save more fuel than regenerative braking.

    Taxi drones would have numberous economic, safety and legal side effects.

    Since Airbus Industrie seems to prefer to rely on machines and automation rather than humans, turning over control and responsibility from pilots to automated ground control upon leaving the runway would be very consistent with Airbus policy. Fewer humans, fewer mistakes. (Even though many engineers vigorously disagree.)

    If you had a fleet of taxi drones, such that each drone was used only once per day, then it may be practical to make the drones 100% solar powered.

    Do you have flexibility in your project?
  16. Apr 5, 2015 #15


    User Avatar
    Gold Member

    There are three major competing systems for the taxiing of airplanes to go GREEN. The airlines have been given the task of reducing the carbon footprint of operation as an industry, and political pressure seems to have inspired several methods ( which previously were probably thought of as not feasible due to the technology not being there, or from the mindset of the industry ) to gain traction for the reduction of CO2 emmissions during the taxiing phase of airline travel.

    One is the EGTS as mentioned previously, where the power to taxi is delivered by a system permanently attached to the main wheels, and supplied by the APU.

    Another is WheelTug, where the propulsion motor is placed at the nose wheel.

    In both instances complexity is added to the airplane.

    Another is the Taxibot, an external semi-autonomous tug system that can move the plane from terminal to take-off holding area.

    In all three systems. main engines can be powered down while taxiing.

    The OP situation is from a completely different perspective, ie regenerative breaking, and the capture and usage of what would appear to be wasted energy upon landing.
    Compexity would be an issue, with generators, motors, and energy storage. Also in question is whether the amount of energy capture is sufficient enough to carry the plane from location A, end of runway, to B, terminal, with engines off.
  17. Apr 7, 2015 #16
    Alex - Didn't this nearly exact thread come up before? In your October post?

    In short - the advantage of having a motor / gen connected to the wheels is to save fuel while taxiing. You can not effective make a Generator big enough to perform this breaking and not add too much weight to the aircraft. However once the Motor system is in place utilizing it to assist in the breaking is practical, even if just to capture some of the energy - or charge the caps that then are used for wheel drive? The only feasible way I can think to really capture the energy is with a tailhook type - ground based breaking assist system - that way you are not "paying" for the weight of the generator - but then you have de-coupled the Motoring from the generating
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook