Global electric car sales are surging based off the public concern for greenhouse gas emissions—understandable, considering that transport makes up 14% of global emissions. However, this isn’t confined solely to road vehicles. International aviation and shipping each account for more than 2% of this figure. So the big question is, can we also electrify aviation and shipping? What materials and technologies exist that could even make this possible?
The big difference in electrical motors for aircraft and shipping compared to those for electric vehicles is that they have much more extreme requirements in terms of weight and power output. So conventional electric motors composed of copper, iron, and permanent magnets, which suffice for electric cars, just won’t do if we want to get an airliner off the ground.
High-temperature superconductors (HTS) are making high power, low-weight motors possible. HTS materials lose their electrical resistance below a superconducting transition temperature.
For conventional superconductors these transition temperatures are so low that they need mostly to be cooled using liquid helium (−269°C). HTS, on the other hand, operate at comparatively high temperatures and can be cooled using liquid nitrogen (−196°C), a cheap and abundant coolant. Multiple manufacturers have developed methods for producing HTS wire, the price of which is even approaching that of copper.
A visitor touches a superconducting magnet at the Elemento Science Museum in Minsk on September 19, 2015. The superconducting element is cooled by liquid nitrogen, resulting in the magnet levitating in the strong magnetic field produced. (Photo Credit: AFP)
Electric Cargo Ships
So how do HTS fit into the world of transport? One of the main constraints in constructing ever-larger cargo ships is the size and complexity of the propulsion system. Electric ship propulsion has been around since the 19th Century, though it has mostly been limited to small vessels.
HTS wire can conduct the same current as a copper cable in about one-tenth the cross section. So, when used to replace copper windings and permanent magnets, HTS wire provided a huge volume reduction and can create much higher magnetic fields. This allows for much more compact, higher-power electrical motors.
Another major advantage of replacing copper with HTS in motors is the absence of resistive heating during operation, meaning that only a very small cooling power is required once the superconductor is below its transition temperature. Of course, one of the ever-present major challenges is figuring out how to implement the cryogenic system required to cool rotating HTS coils. This, however, is a challenge engineers have risen to.
Over the past few decades, several manufacturers have been constructing and testing powerful HTS motors with the high torque required in marine propulsion. Siemens, for one, has demonstrated a motor with a power of 4 MW and AMSC, a 36.5-MW system.
Getting HTS Off the Ground
When it comes to aviation, electric aircraft seem even farther-fetched than electric ships. The work put into HTS ship motors over the years, however, has demonstrated that the advantages HTS bring to motor technology are even more applicable to aviation.
Aircraft have very strict weight requirements, which is evident in the industry’s interest in additive manufactured components (as well as some less-technological ideas, such as one Japanese airline even asking their passengers to relieve themselves before boarding). Reducing fuel consumption is therefore not only essential for reducing emissions but is also a massive financial driver. Add to that the benefits of reduced noise and air pollution, and electrification of aviation becomes very attractive to the industry.
Developments for passenger electric aircraft are already in full swing, with multiple companies working on prototypes including Airbus, Wright Electric, and Zunum Aero. These are mostly hybrid concepts which will demonstrate electrical machines work in tandem with turbine engines for propulsion. In such a configuration that HTS motors are likely to make significant contributions.
Looking beyond this, NASA have laid out plans to develop the N3-X aircraft. This should provide a 70 % reduction in fuel consumption by using two gas-driven HTS generators to power the distributed HTS motor-driven fans.
Developments for passenger electric aircraft prototypes with superconductor capacities are being built at multiple companies, including Airbus, Wright Electric, and Zunum Aero.
With Great Power Comes Superconductivity
Despite the advantages, in reality the adoption of HTS in propulsion has been slow. Most probably this is down to the complexities of the technology and the associated developmental costs.
Nevertheless, the advancements made in exploiting HTS material properties since their discovery in the ’80s have been immense. Effort is still required to implement HTS motors on a large scale; and yet, especially in the case of aviation, ambitious development goals have never stood in the way of progress.
As pressure increases to reduce emissions in transport, HTS won’t just offer improvements on conventional devices but will become a key enabling technology.
Ben Stafford is materials science expert at online materials search engine Matmatch.