If we are to consider other forms of energy storage we should also explore how it could be put to use. Developments with superconductors are laying the way for very high power to weight electric motors that could be used for aviation with the potential to be lighter than a turbofan based propulsion system, allowing the use of an energy store that does not need to be ejected to create thrust. With the potential development of room-temperature superconductors (currently at 254K or -19 degrees C), this becomes even more appealing by making refrigeration of the motors redundant (another sizeable weight saving).
The turbofan propulsion system used in a Boeing 747 is powered on jet fuel (Kerosene) with an energy density of about 45MJ/kg. Currently only being able to hypothesise on energy conversion efficiencies and weight reductions obtainable above, it would only be realistic to power an aircraft with an energy store with a comparable energy density. EEstor is a battery company claiming to have a device (a form of an ultracapacitor) capable of 1.47 MJ/kg. This claim is not without its sceptics, after all this is three times the energy density of today’s lithium-ion batteries (0.58 MJ/kg). While well suited to automotive applications, this is currently far too low to be used for aviation but progress is progress. Lithium-air batteries offer hope with theoretical energy densities in excess of 5,000 Wh/kg (18 MJ/kg). While battery energy densities may not reach the required levels to allow for all-electric propulsion (or no hydrocarbon fuels), a hybrid generator powering superconductor engines could be the path of the future.
A Boeing 747 has an average power consumption of 140MW. Power delivery is as crucial as energy density for an aircraft. Numbers of this scale are normally associated with power stations. If we look far enough into the future it may be reasonable to put a power station on an aircraft, after all we do have nuclear submarines. Granted, there is no shortage of obstacles when it comes to considering a flying nuclear power station. However, there is a lot of progress being made on nuclear fusion. There is a program being funded by DARPA for naval power generation with the aim of developing a 100MW-1GW fusion reactor for ship propulsion.
IEC Fusion, if successful, would be able to provide a source of nuclear energy generation within a relatively tiny space, producing no nuclear waste and no risk of a runaway nuclear reaction via a proton-Boron fuel (PB11). Updates available on their blog. The ignition and waste products of this reaction are not radioactive, in fact far safer than current aviation fuel. On the other hand, if nuclear fusion would be able to be harnessed on a commercial aircraft I think we would have found a solution to a much larger problem. The final alternative to flight dependent energy storage is none (well, excluding reserve systems and batteries at least). Strictly speaking I am referring to remote energy generation and transmission.
The electrical genius Nikola Tesla had a vision of global wireless power at the turn of the 20th century. This has eventually led to technologies such as electromagnetic resonance and microwave transmission.Witricity is focusing on using short-range energy transmission at home, providing a means to charge devices without wires within several meters. While electromagnetic induction is well suited for domestic applications, microwave appears the only current feasible contender for wireless energy transmission to aircraft. Whilst microwave power transmission has been proved to be very efficient obstacles remain with distance (currently effective up to 1km) and public image. No one wants to be boiled from the inside out on their way to a summer retreat.
Air travel may be seen as indispensable in modern times; however there are possibilities of replacing long-haul flights with a greener alternative. Why do we fly? – To get across vast distances on earth in short periods of time. For shorter distances a dedicated maglev train with speeds reaching over 350 mph would be a possible substitute. If green air travel can be achieved, it will start with shorter distances. The real issue is transatlantic / long-haul flights. A variation of the maglev offers an alternative with an exceptional engineering challenge. The Discovery Channel recently aired an episode of Extreme Engineering called Transatlantic Tunnel which explored this option. Known as a Vactrain, it is merely a maglev train placed within a vacuum tube. The reduced drag from wind resistance and friction offers speeds in excess of 4,000 mph, shortening a trip from London to New York to around one hour. The costs of submerging a transatlantic vacuum-pumped tunnel 300m below sea level for nearly 3,500 miles are staggering (estimated at $1trillion). Of course this would require a clean energy source to be considered green. The good news is that it wouldn’t have to be airborne.