Oct 312013
 
A futuristic hybrid plane design from Airbus.  What are the chances we'll see these types of planes in our lifetimes?

A futuristic hybrid plane design from Airbus. What are the chances we’ll see these types of planes in our lifetimes?

A reader wrote in, pointing to several recent articles about electrically powered airplanes of the future, and wondered why planes don’t work the same way as a Toyota Prius or Chevy Volt.

If it works for a car, he figured, why not for an airplane.  That’s a fair and sensible question, and definitely worth a careful considered reply.

He writes

Here is an interesting concept for you.  Hybrid power works really well on cars and in theory could adapt to planes (as long as Boeing doesn’t make them!).

The secret is in energy management.  My Volt is officially supposed to return 39 miles per full charge.  Driving with a lead foot gives less than that.  Driving with a feather foot easily yields 60+ miles on a single charge finishing at the same spot the drive started (ie a fair comparison rather than a one way journey with a big downhill to charge the battery).

Surely there is no reason that by managing the power usage, significant energy efficiencies can’t be obtained by planes.

When there is an economic reason to change it will happen.  Hybrid technology is more efficient but costs more.  When the price differential is removed the changeover will be rapid.  The world will soon have to pay the real cost of energy (which includes the cost of pollution) and only that economic stimulus will clean things up.

This is not feel-good greeny-liberal stuff, it is simple economics.  It is obvious that change will occur once something truly shocking happens.  Sadly we wont have to wait long.

So, might we see a hybrid airplane?  In addition to the article he linked to, here’s a very-light-on-details feel-good piece from Boeing about future airplane development too, but as you can see if you watch it, they hint at things but actually focus more on design than a hybrid energy recovery/reuse system.

I’ll go on record as saying we’ll never see hybrid technology in an airplane.  The reason is actually fairly easy to appreciate.

First of all, think about hybrid cars.  As you may know, they work best around town, and actually give lower economy on long journeys.  The reason for this is the underlying reason why hybrids would never work in the air.

How and Why Hybrids Work in Cars

A car’s power requirements, when driving around town, comprise high power demand (when accelerating), low power demand (when cruising or coasting), almost no power demand (when stopped at a light) and power return (when braking).  The hybrid has a ‘reservoir’ of power (ie its battery) which stores power that is recaptured when the car brakes.  The car uses that reservoir to keep the systems working with the engine stopped during periods of almost no power demand or low power demand.

Due to the frequent cycling between recovering power from braking and using the power while driving, and significant periods of low/no power demand, even a small reservoir/buffer does a great job of recapturing otherwise wasted energy and also limiting the petrol engine to work only in an efficient medium/high power mode, not in an inefficient very low/idle power mode.

But on the open road, with freeway driving, you might go an hour at a time without the need to do any braking, and the car, at freeway speeds, almost immediately depletes any stored power in its ‘reservoir’ battery, and so is running almost entirely on the petrol engine.  The good news is the petrol engine is working at peak efficiency and no power is being wasted, but the bad news is that the reservoir is doing nothing.  For this reason, the vehicle’s fuel economy actually goes down – quite the opposite of a normal non-hybrid car.

How and Why Hybrids Won’t Work in Passenger Planes

Now think about an airplane’s power requirements.  They are very different to a car driving around town.

To look at a typical flight profile, and assuming the plane is using ‘shore power’ while at the gate, when the plane first starts its engines and taxis prior to take-off, it is using a very low or low power setting.  But then, when it takes off it is using maximum power, when it climbs it is using not much less than maximum power, and when it settles into the cruise, it is using medium/low power with the engines tuned to optimum efficiency at this setting.

At the end of the cruise, when it goes into descent, the plane almost never ‘brakes’ but descends merely by using less power (but still some power).  Only rarely will it actually ‘brake’ (you see those rectangular panels on the wings lift up when it is braking), and then once it lands, it brakes at close to maximum braking for a very brief period, before transitioning to low and very low power as it taxis to the gate and then turns off its engines and switches over to shore power again.

So, here’s the question.  At which points in this flight can the airplane ‘recycle’ power, and at which points can it use that recycled power?

Ignoring weight issues for a minute, there would possibly be a very few opportunities to use some sort of as yet not fully invented (but not hard to invent) device to recapture power when the airbrakes are otherwise used.  The braking on the ground would still have to be mainly mechanical braking, but perhaps that could be augmented by electrical braking too and some more power could be recaptured for the – what – 15 seconds or so that the plane slows after touching down.

The good news is that recaptured power on the descent could perhaps be used to then propel the plane on the ground towards the gate, and subsequently to move it from the gate towards the take-off runway.  But it could not be used during take-off, and in the unlikely event there was any available when the plane settled into its lower powered cruise, it would be used up very quickly indeed.

Do you see what this means?  The plane is having to carry a discharged battery for almost the entire flight.  Unlike a car around town, which is using its battery continuously, the plane charges the battery on descent/landing and uses it on the ground. It is deadweight most of the way.  And you know how airlines feel about flying unnecessary deadweight.

Now think about the cost/weight and fuel implications of this.  The airplane would have to have additional devices added to act as an airbrake/generator (some planes have something a bit like that already – a RAT – a ‘Ram Air Turbine’ that is designed to produce emergency power if the plane’s engines all fail.

In addition, the plane would need generator/motors on its wheels (although there are proposals to add motors to a plane’s wheels at present, in the hope that electrical propulsion would be more economical than the inefficient use of the jets to drive the plane on the ground).  If it was going to also use stored power to spin the jet fans during cruise, it would need additional machinery on the jet engines, too.  And, most of all, it would need a bank of batteries to store the power.

Let’s ignore the important question of ‘how to you generate/recapture the energy’ and also the important question of ‘how do you reuse the energy in the air’ and focus just on the batteries alone.  As you’re about to see, the problem with the batteries is so stark as to spare us the need to consider about all the other problems associated with trying to make a hybrid plane.

The Battery Weight Costs More Fuel than it Saves

A Chevy Volt Li-ion battery weighs 435 lbs and stores 16 kWhr of energy, of which 10.3 kWhr is usable.  Now think about this – in a plane, that 435 lbs could also translate to at least two more passengers.  There’s your first hint that there is an unavoidable trade-off associated with adding batteries to a plane.  We’ll come back to that in a minute.

What does 10.3 kWhr of energy actually mean?  What can you do with that?  Perhaps the easiest way to appreciate how much energy that is would be to equate it to how much jet fuel it saves.  This amount of energy can be provided by burning one single gallon of diesel (which weighs about 7.2 lbs) in an efficient generator.  I don’t know, but let’s say that a jet engine is a less efficient way of converting fuel to energy and say it might burn two gallons of jet fuel to create 10.3 kWhr of work done.

On a five-hour flight, the 435lb battery therefore promises to save two, at the most three gallons of jet fuel (and possibly only one gallon – but let’s stick with the number two).  But it will also require another nine gallons of jetfuel to be burned to carry its weight on the flight.

So – and without considering the additional weight of all the generating equipment, control equipment, wiring, and so on, the bottom line is that on the plane, the airline has to burn four and a half times more jetfuel to ‘save’ the jetfuel that the battery might possibly save.  Not a very positive trade-off, at all!

The Problem is Not Energy Cost Dependent

Two more points on this.  First, this calculation stays the same, whether jet fuel is a penny a gallon or $1000 a gallon.  You’re burning 4.5 gallons of jetfuel for each gallon you ‘save’.  That will never make sense, and it is so far from making sense, that it just isn’t going to happen in anyone’s lifetime.

Lithium ion batteries, which are currently the ‘best’ form of energy storage possible, and which weigh the least per kWhr of energy stored, are a ‘mature’ technology, and there’s little reason to hope for a five fold improvement in their energy storage per weight capacity any time soon.

Secondly, even if we ignore the nine gallons of fuel needed to ‘save’ us two gallons, the other issue is that the airline has to think ‘Would I rather fly two more passengers, and possibly make an extra $500 on their fares, or would I rather save two gallons of fuel?’.  Until such time as jetfuel exceeds $250 a gallon, the answer to that is easy.

Now, let’s also throw in the extra weight and complexity and cost of all the associated systems to generate the energy, to charge and control the batteries, and extra weight to put the batteries in special fireproof boxes so they don’t explode into flames and cause the plane to crash and burn, and let’s also look at the very trivial amount of energy that would actually be recaptured during the brief air-braking and ground-braking portions of the flight, and the situation becomes more and more impractical.

Hybrid technology works well in a car driving around town, because its reservoir is all the time being emptied and refilled.  It only needs a small reservoir to store a few minutes of power, and it will cycle through emptying and filling its reservoir, potentially several times per hour.  But a plane would not be all the time emptying and filling its reservoir.  It would be filling it only during the descent and landing stage of a flight, and emptying it only during the time on the ground.  The rest of the time, for the entire flight, it is just deadweight and requiring more jet fuel to be burned to carry it.

I’ll concede I’ve made some oversimplifications in this explanation.  You’re welcome to rework the figures anyway you like, but whatever you do and however you do it, you’ll find yourself confronted with a clear impossibility.

Hybrid technology is totally impractical for airplanes.

If Hybrid Technology is Impractical/Impossible, Why do the Airlines Talk it Up?

Lastly, if hybrid technology truly is totally impractical, why does it seem that both Airbus and Boeing are working on developing the technology.

That’s a very good question, with several answers.  The first answer is that neither company is primarily in the business of power plant development.  They develop airframes, not engines.  So their concepts are little more than pie-in-the-sky exercise, with not a shred of corporate commitment to making any such starry-eyed dreaming occur.

This leads to the second answer, which we’ll start off addressing with another question.  Why do airlines make big deals out of ‘experimenting with’ and ‘trialing’ bio-fuels in their planes?

That’s all a total nonsense designed merely to win them some favorable press.  No airline would be experimenting or trialing any sort of fuel on a commercial flight full of passengers that wasn’t already 500% trialed and tested by the fuel and engine manufacturers.  You’ll note that none of these high visibility ‘trials’ end up with an airline committing to buy bio-fuel, for the simple reason it is much more expensive than regular fuel.

Hybrid powered planes and bio-fueled planes are both exercises in public relations rather than practical mechanics.

As long as airlines give highest priority to operating at the lowest cost, neither are in our future.

  2 Responses to “Will We Ever See Hybrid Powered Planes?”

  1. Good morning, David,

    In your article about using hybrid power for commercial aircraft, I think that you are overlooking one very important – and crucial – point.

    Commercial aircraft today are powered primarily by turbine engines. How would you make a hybrid powered turbine engine? The turbine engine relies on the combustion of fuel to expand gas (air) to drive the turbine. I don’t think it would be possible to make a turbine powered by a battery that would or could produce the necessary energy to propel a large aircraft with enough speed to fly for any length of time at high altitudes.

    Bob E
    Windsor, CA.

    • Hi, Bob

      Thanks for adding another one of the challenges associated with dual-powered airplanes. I didn’t intend my article to be a definitive look at every problem associated with this concept, because I felt that after the overwhelming problem with the weight ‘cost’ of batteries, there was little need to consider all the other associated challenges too.

      You are correct, however. Although a ‘high bypass’ turbo-fan jet could have an electrical motor added to either spin the fans or act as a generator when the fans were being spun by the jet engine, this would not be an efficient way of converting electrical energy to thrust.

      The question of how to efficiently recover energy and then how to efficiently use that energy subsequently is of course an essential issue in any type of energy recycling environment. Hybrid cars have a fairly effective solution to this; I’m not sure there’s yet an obvious way of doing this for planes.

      In other words, even if the battery weight wasn’t a deal breaker, the inefficiencies in the process would further detract from the break even point.

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