Towards an all-electric society?

Electric drives for ships and planes

Trains are increasingly electrified, electric motorbikes are booming in Asia, and the all-electric car might be close to mass production. Some people are openly suggesting that water and air transport could become electrically powered as well, completing the move to an 'all electric society'. What are the chances of this vision becoming a reality? Ships with electric propulsion systems already existed at the beginning of the 20th century and are currently ready for a (limited) revival. However, development in powering large aircraft by electric propulsion is much less in evidence.

The new electrically powered ships are coming our way

For ship propulsion, diesel engines and gas turbines are currently considered the conventional power systems. They drive the ship propeller via a large gearbox. It has not always been like that, however. At the beginning of the 20th century, turbo-electric systems were competing with mechanical drives. The American navy even had a class of electrically powered battleships that served in the Second World War. After World War II, the idea of electric ship propulsion was completely abandoned as being too large and too heavy.

Recently, following the development of power electronics and smaller, more powerful electric motors, the idea of turbo-electric ship propulsion has been brought to the attention of ship designers and builders again.

In turbo-electric ship propulsion, the power is not supplied by battery packs as in electric vehicles, but by a gas turbine or diesel engine. Compared to conventional ship propulsion, it is basically an electric generator and motor replacing the gearbox. This is introducing an extra energy conversion step, decreasing inherent efficiency. (This extra conversion step is also present in electric cars, but there it allows the small and relatively inefficient fuel engine to be replaced by an electric motor running on power generated in highly efficient or renewable energy power stations.) Consequently, it does not come as a surprise that at maximum load, a turbo-electric ship propulsion system has a lower efficiency than a conventional system of Diesel engine plus gearbox.

However, the efficiency of a Diesel engine or a gas turbine drops steeply when working at less than maximum output. An electric drive, on the contrary, could be composed of a series of electric motors which can be employed individually as needed and each run at their most economical setting. For ships that run most of the time at less than full power, such a modular electric drive becomes more efficient than any other propulsion system. Spare electrical power could be stored in battery packs or used for other appliances on board. Ships need electricity onboard anyway for radar, navigation, lighting, and several kinds of appliances. By using an electric drive, all the power can be delivered by a single system, making power infrastructure simpler and easier to maintain.

Considering these advantages, it is not surprising that electric propulsion is mainly being reconsidered for naval military vessels and cruise ships. These types of ships typically run most of the time at less than full power and require a great deal of electricity for other uses. A class of electrically driven warships is currently being developed for the British Navy. It will be the first series of vessels to be powered entirely by an electrical system since World War II.

Electricity in airplanes but not for propulsion

While the first commercial electrically driven ships are on their way, electric propulsion for large airplanes is still very much in the future. Electrically driven planes do exist already today, namely gliders with small electric motors for auxiliary propulsion. A few small experimental aircraft with electric propellers have also been built, utilising high discharge lithium-ion batteries as those being used in some electric cars, or even photovoltaic cells. However, large airplanes with electric motors are not a viable option at present. This is because the size and power-to-weight ratio of a battery or fuel cell powered electric system cannot compete with that of a kerosene powered jet engine.

Still, aeronautical engineers are working on constructing designs such as a 'blended wing', a flat, tail-less structure resembling a giant wing. This shape would allow much more space for electric batteries or generators. Such planes could be driven by 'superconducting motors', an electric engine that can generate three times the torque of a conventional motor of the same weight and power input. Then again, if those would be powered by hydrogen fuel cells, an extra energy conversion step is introduced reducing the intrinsic efficiency. Moreover, having hydrogen on board creates an additional safety issue. On the other hand, if aircraft motors are to be powered by battery packs, a significant breakthrough in battery technology development would have to take place.

The 'blended wing' aircraft still seem to be in the realm of science fiction at present, but this does not mean that electricity has no major role to play in air transport systems. An increasing number of electrically powered devices such as landing gears and flaps are being added to aircraft. Those electric systems are lighter and can be made more reliable than their hydraulic and mechanical counterparts.

Increasingly electric, but not all electric

The cases above demonstrate that in many fields, electricity is gaining ground compared to other energy carriers. In most cases this has positive effect on efficiency, emission reduction, and flexibility. However, the 'all-electric-economy' may well remain the eternal science-fiction dream. So what? After all, the real goal is efficiency, not mere consistency.

References

• Article 'Making waves / Maritime engineers are already embracing electric propulsion for ships — and electric planes could be next' in the Economist
• Article 'Electric Drives for Battle ships' by Nicolas Tesla, New York Herald, February 25, 1917
• Article 'Sonex Aircraft and AeroConversions Show Electric Propulsion System for Sport Aircraft' on Green Car Congress