IEEE Electrification - December 2021 - 71

at which it remains competitive with planes (fewer than
3 h). What if the cities in the table were linked through a
single hyperloop main line? The issue is whether it is preferable
to take a 3-h, 30-min flight from Spain to Sweden or
a 4-h hyperloop ride, assuming a sustained 650 km/h. This
example does not factor in time spent in stations and airports,
which has become a drawback to air travel, and it
does not consider the fact that hyperloop stations could be
placed in city centers. Safety will be discussed in more
detail in the following, but a quick note is that hyperloops
require no takeoffs and landings.
Electric Propulsion for Hyperloops
As explained, hyperloops are based on vehicles on closed
tracks in low-pressure tubes that minimize energy consumption
while operating safely and reliably. The purpose
of the propulsion system is to achieve this and overcome
the natural drag that the fluid inside the track exerts on
the vehicles and other resistance, such as magnetic drag.
The vehicles will transmit kinetic energy to the surrounding
fluid, dragging a wake behind them. If the pod velocity
is high enough, shock waves will occur, similar to those
that take place in a high-speed train tunnel. However,
shock waves in hyperloops cannot be released into the
atmosphere, as they are for trains. They will bounce back
and forth, hitting the vehicles and potentially jeopardizing
stability.
Several hyperloop concepts use linear motors all along a
route in a complete vacuum tube to avoid this effect. That
level of vacuum may require a more complex and delicate
two-stage pump. The high vacuum is required to create
conditions where there is no aerodynamic drag. Nevertheless,
the adverse effects of atmospheric pressure inside the
tube can be overcome with a potentially better solution, an
electric turbofan-like propulsion system. Turbofan engines,
which are the most common for short-, medium-, and
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750 1,000 1,250 1,500 1,750 2,000 2,250 2,500 2,750
Distance From Barcelona (km)
Figure 4. The true speed of flights from Barcelona, Spain, operated by Vueling Airlines on 17 November 2019, using an Airbus 320 aircraft.
(Source: Flight Aware.)
IEEE Electrification Magazine / DECEMBER 2021
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long-haul aircraft, are following the road map of other traditional
propulsion systems, such as cars. Car engines, where
mechanical power was traditionally generated by burning a
hydrocarbon fuel, are being replaced by electric power units
fed from batteries. Turbofan engines with electromechanical
power units have the advantage of producing no emissions.
This enables their use in confined facilities without
incurring recirculating gas problems.
The use of an electric turbofan-like system is mandatory
when the pressure inside the tube is at nonspace vacuum
levels, which is basically due to vehicle aerodynamic
drag. This system has two main effects: first, there is a
way to cope with the piston effect. Air is transferred after
a compression-decompression cycle from the front to the
tail of the vehicle. Second, a propulsion system based on
compressed air is obtained, compensating for the higher
drag. A value of 10 kPa is enough to reduce the aerodynamic
drag by more than one order of magnitude and
enables the compressor to properly operate.
Electric turbofan systems have a disadvantage in that
they are fit to operate only at their design point in an
enclosed environment. However, at the design point,
hyperloops are extremely competitive from an energy
efficiency point of view, compared to aircraft and piston
engine vehicles, which operate outside their design conditions
for significant amounts of time. For short-haul
routes, meaning fewer than 1,500 km, an aircraft is at its
cruising speed for only about 70% of the distance (Figure
5). The efficiency of a system with such a characteristic
is quite poor. Considering energy, a significant portion
is wasted, especially during taxiing, takeoffs, and climbing
(Figure 6). Hyperloops have the advantage that for
routes in this range, they can reach their travel speed and
maximum efficiency without a phase equivalent to
climbing, which increases their cruising phase to approximately
95% of a trip.
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591
Speed (km/h)

IEEE Electrification - December 2021

Table of Contents for the Digital Edition of IEEE Electrification - December 2021