Solar Impulse recently completed the first ever circumnavigation using a solar powered aircraft. So it might be an appropriate time to review the options as regards alternative fuelled aircraft.
A long standing assumption of many has been that given the very high rates of energy consumption by aircraft and the heavy dependency of aviation on fossil fuels, once oil supplies peaked (or we were forced to curb consumption to fight climate change) this would mean the end of commercial aviation as we know it. No more cheap flights, no more holidays in the sun, the world would suddenly become a much larger place. However its possible that this may not be the case. The technology of Solar Impulse 2 hints at a range of possible solutions that are either in the works or already exist.
The eternal plane
Firstly the concept behind Solar Impulse is not new. Electric powered aircraft have been around since the 1970’s and as early as 1979 the first ever solar powered aircraft had taken flight, the Gossamer Penguin, developed by pioneering aerospace engineer Paul MacCready.
In the 1990’s NASA developed first the Pathfinder and later the Helios aircraft. These were large solar powered UAV’s designed to stay aloft more or less indefinitely, a so called “eternal plane”, which could loiter for extended periods (potentially days at a time), performing various tasks (such as reconnaissance communications or weather/earth observation), a sort of poor man’s satellite. Due to the limitations on battery technology at the time they used fuel cells and hydrogen tanks to store energy overnight. While the project was successful at a technical level (save one crash, not an unusual occurrence with any prototype aircraft), it became a victim of funding cuts being brought in by then President Bush (lets face it anything “solar” was pretty much doomed with him in charge).
Even so, the concepts developed by the Helios/Pathfinder program have found their way into other projects, Solar Impulse is merely one of them. Airbus and Qinteq for example are trying to develop a solar powered UAV similar to Helios, while Google are trying to develop their own “eternal aircraft” in the form of a balloon. And much as various challenges and competitions within the automotive industry have driven onwards development of electric cars, so too is this true for alternatively powered aircraft.
Certainly as far as day to day aviation goes, a aircraft that crawls along at 60 knots and has a wing span larger than a football field, its not exactly a practical plane. However, there are proposals to take the technology behind these aircraft forward. For example, there are proposals for a whole series of electric aircraft that would be at least partially powered (or could be recharged while sitting on the apron) by solar energy.
Any solar aircraft is of course an electric aircraft, the question is merely do we charge its batteries in the air off the sun or on the ground off the grid.
In some respects an electric powertrain offers several advantages over a fossil fuel based powertrain for aircraft, much as it does with vehicles. Electric motors are much more energy efficient, deliver high levels of torque and they generally don’t need a complex cooling system (certain battery or fuel cell systems can require cooling thought). They are also less prone to many of the technical issues that can be devil conventional aircraft, e.g. engines cutting out due to aircraft inversion, compressor stall, or high g manoeuvres. Also as an electric motor doesn’t need oxygen to operate, the service ceiling can be higher (of course there are many other factors that limit the service ceiling of an aircraft).
Noise issues can also potentially be reduced, an important factor, given how aircraft noise is a major issue at many airports. Another consideration is that by relocating the power plant of the aircraft (i.e. we are no longer limited to a set of big turbines or prop’s, we could have several sets of smaller motors instead), we can do things with our electric powertrain that you can’t do with a conventional aircraft. Things like VTOL can potentially become simpler to implement (that said, all VTOL aircraft pay a “premium” in that provisions for VTOL tend to come at the expense of increased fuel consumption in level flight).
The main disadvantage is of course where do we get the power from? And generally that means batteries (we’ll talk about fuel cells and hydrogen a bit later). As with electric cars, we face the dilemma that to give the aircraft a decent range, we need to add more and more batteries. But this makes the aircraft heavier and thus its energy consumption increases, meaning it needs yet more batteries. In other words, there might be range issues.
However, it should be noted that in much the same way that the bulk of car journey’s are relatively short and well within the range of electric cars, the same is true for aviation. The vast bulk of flights are either short haul or are the sort of trans-continental flights that could be broken down into short sections. So while it is probably true to argue that electric planes cannot offer a like for like replacement with aviation fuel, there’s still a large number of roles that they can perform.
Already a number of electric planes are entering the market. Most of these early entrants are short range light aircraft intended for use as trainers, or for general aviation use. For light aircraft the benefits of an electric power train offer a significant advantage, while the main downside (range) is less of an issue, as trainers rarely stray particularly far from their home airfield. There is something of a price premium to be paid, as they are slightly more expensive than a similar piston powered aircraft. But the fuel efficiency savings (the major cost of ownership of any plane tends to be fuel) would presumably reduce the burden of those costs.
And some companies see these light aircraft as a natural stepping stone towards larger aircraft flying perhaps commercial flights with fare paying passengers and cargo.
One concept for example, is the Ce-Liner. This will use ducted fan electric motors powered by Li-Ion batteries. Should you be wondering how long does it take to recharge all those batteries (and how many coffee’s does the pilot need to drink while he waits), well the batteries are mounted on standard cargo pallets, the same ones used to carry air-freight and luggage in. This means the amount of batteries the aircraft carries can be lowered as necessary (freeing up space for more cargo), improving fuel economy on shorter flights.
But, how long would it take to load all of those extra cargo pallets? Well a study conducted by Schmit et al (2016) showed that it would only take marginally longer (or around about the same time), to load up and service an electric plane like the Ce-liner compared to a conventional aircraft. However, crucially in this situation loading of passengers ceases to be part of the critical path. Nine times out of ten, a flight delay is due to a “technical issue” with “self loading cargo”. We’ve probably all had that experience, some guy starts queuing to board twenty minutes before the gate is called, then stands in the middle of the aisle with everyone behind him, forced too wait while he sorts through his luggage, takes off his coat, flosses his teeth, etc. while the stewardess starts to turn blue with rage.
I think most airlines would take an extra few minutes on the stand as a small price to pay for a lower fuel bill and a greater certainty of the aircraft leaving on time.
Others look to a slightly less radical approach of hybrid engines. Much like how hybrid cars operate, a hybrid plane would use a mix of electric motors (powering the aircraft along) with a fossil fuel powered APU and batteries powering everything. Alternatively, Boeing has proposed the Sugar Volt concept, which will use hybrid engines that will use fuel during take off and then switch to electrical power during cruise. While these planes will still be dependant on fossil fuels, the concept does offer significant fuel savings and the range issues mentioned earlier are resolved. And again recall that the one thing above all else airlines are usually trying to do is cut fuel costs.
Also longer term, we could see these hybrid aircraft, switch from aviation fuels to either biofuels or hydrogen. And its worth noting that biofuelled planes are not a new thing. Virgin Atlantic has already flown commercial passenger flights using biofuels to power a conventional 747 (back in 2008). So the only real limiting factor her relates to fuel supplies, rather than any technical barrier within the plane.
Hydrogen powered aircraft
Hydrogen planes fall into two flavours, the first time use fuel cells in place of batteries, but are otherwise electric planes. The second replace jet fuel with hydrogen fuel, but run on “conventional” engines (anything from gas turbines to rocket motors, basically you burn stuff and it goes out the back of the plane very fast!).
As noted earlier, fuel cell aircraft have been developed and tested. Boeing’s Phantom Works for example has developed a small light aircraft (trainer type) powered by a fuel cell. This was used as a development step towards a much larger aircraft, the Phantom Eye, a long range hydrogen powered drone with the capability for several days endurance, intended for various covert and military applications.
Hydrogen as a combustion fuel in aircraft is nothing new. Back in the 50’s, Locheed’s Skunk works developed the CL-400 Suntan aircraft as a high speed, high altitude reconnaissance aircraft. The Russians experimented with a hydrogen powered airliner, the TU-155, back in the late 1980’s. And the idea has not died a death, Reaction Engines, a group of researchers working on a variety of hydrogen powered aircraft (including an SSTO concept called Skylon) have proposed the LAPCAT A2, a Mach 5 airliner powered by hydrogen.
Hydrogen offers several advantages over conventional aviation fuel, its got a much higher calorific value and delivers twice the specific impulse, allowing aircraft to fly higher and faster. So why don’t we use it? Well there’s the small matter of getting the hydrogen onto the plane. Up until now the focus has been on using cryogenically cooled liquid hydrogen, which comes with a series of problems and practicalities (boil off being a problem as well as tankage).
A number of solutions are proposed to solve this. Developments related to the automotive FCEV means that lightweight carbon fibre reinforced tanks, capable of operating pressures as high as 700 bar, are now available. The Wh/kg ratio for hydrogen at these pressures is higher that it is for gasoline, even when we account for the weight of the tank itself. That said, there’s still something of a weight premium to be paid (even when empty they still add weight), which would impact on range and cargo in the case of shorter flights. Given that such tanks would have to be mounted in the fuselage (rather than wing tanks), this would take away from the aircraft’s internal cargo volume.
Another concept is to use hydrogen stored in solid form. A company called Cella Energy is working on the idea of hydrogen stored in the form of solid oxide pellets that absorb hydrogen like a sponge. When heated, they release the hydrogen, with the pellets then returned for “recharging”. The system is being developed for use both in aviation and in vehicles, with energy densities three times better than Li-ion batteries being claimed. Test flights of small drones powered by hydrogen pellets are already underway and tests with a fuel cell powered car are to follow.
Many options available – but not much time
In summary, there are many ways by which the aviation industry’s fossil fuel addiction can be broken. Many of these could indeed radically transform aviation, allowing for aircraft that will be able to fly faster, with greater fuel economy (some drones capable of staying aloft almost indefinitely) and less noise. Climate change deniers often try to claim that going green means giving up what we have, retreating into the woods and becoming hippies, but this is simply not true. There are alternatives.
But as is so often the case with green technology, its unlikely we’ll be able to develop a like for like replacement for fossil fuels. We’ll likely end up with a range of different aircraft powered by various different technologies. Also, there’s the ticking clock. In short, will we develop the technology quickly enough to allow aviation to adopt a smooth transition away from fossil fuels? If not, then its possible some disruption to air travel becomes a risk. But it is those who stand in the way of change who would be the cause of this disruption, not a lack of ideas.