The success of HEVs in automotive applications has led certain other vehicular areas to consider this technology as well. These include ships and aircraft, which are non‐ground vehicles. Diesel‐electric locomotives have already been using this technology in a slightly different form, and we will discuss this as well.
6.4.1 Ships
Obviously, the need in a ship involves very large sizes of everything. It can ultimately be considered as an industrial power system, with both utility types of systems and a pro- pulsion system. The overall power need in a ship could be anywhere from 1 MW to almost 100 MW. The focus of this book, however, will be on propulsion and not on the overall electric power system of a ship.
Historically, ships evolved in various stages, from steam or diesel propulsion to diesel‐
electric propulsion. Initially, the motors used in ship propulsion were DC motors. With the advent of power electronics, DC motors can now be replaced by robust induction motors with very good control, based on power electronics. The motors could also be field‐excited synchronous motors. The benefits of using these motors are reliability and efficiency. In addition, recently, the technology of pod propulsion has become popular [9]. In this scheme, the propulsion motor is located separately in a pod, which is physi- cally secured at a distance from the main body of the ship. The power electronics system is located within the main body of the ship and electrical wiring is run to the pod, which houses the motor. The size of the pod can be very large – something like 10–12 ft (3–4 m) in diameter. The propulsion motor shaft is connected to the propellers. Also, the pod is capable of being turned through a full 360° if needed. This eliminates the need to have a rudder in the ship. Let us now look at the architecture of the ship propulsion system [10] shown in Figure 6.19.
Figure 6.19 aims to give a complete overview of the possibilities in terms of generation, distribution, and propulsion in a ship. The choice of the particular architecture is depend- ent on the size, and this can significantly affect the cost. Not all of the components mentioned above will be suitable under all circumstances. If the ship generation system uses a diesel or gas turbine, then the overall ship system could be considered to be a hybrid system, whereas if there is no mechanical power system – for example, if a fuel cell is the only source of power – then the ship will be a completely electric ship. Even if the power is generated by nuclear energy – ultimately to get electricity – then it will need some kind of electric generator, which in turn will need to be turned by non‐electrical means, and in this case the ship can be considered in the hybrid vehicle category. Whether it will be hybrid in terms of propulsion will depend on the exact propulsion means used, and what kind of devices are directly contributing toward propulsion.
So, a simpler subset of the above system could consist of a gas turbine as the prime mover, which drives a field‐excited synchronous generator (or it could be a permanent magnet generator). The advantage of a field‐excited generator is the ability to control the voltage by controlling the magnetic field, which can be done using semiconductors of relatively lower current rating. If a permanent magnet generator is used, the voltage control has to be at full power level at the stator terminal with much higher‐rated semi- conductors. The advantages of permanent magnet generators are higher efficiency and
Special Hybrid Vehicles 165
simplicity, with no need to use slip rings for the field. On the propulsion side, we can use synchronous or induction motors. Synchronous motors have to be doubly fed, whereas induction motors are fed only at the stators. The architecture of a possible system is shown in Figure 6.20.
The propulsion motor voltage may be rated at several hundred to several thousand volts, depending on the size and needs.
With the above architecture in mind, it will now be instructive to look at the pod propulsion we referred to earlier. As its name suggests, it merely involves the physical location of the propulsion motor. In other words, it is the items on the right of Figure 6.20 – the propulsion motor and propeller – which are housed inside the pod.
The pod itself is outside the ship’s main body structure (below the stern), but of course is secured to it through mechanical structures. The electric wiring runs from the main ship to the pod. So the items shown inside the dot‐dashed lines in Figure 6.20 are in the pod. Figure 6.21 shows what the pod looks like.
The figure is reasonably self‐explanatory, showing the various components. The slip ring unit has to be connected to the power system, which is located inside the main body of the ship. The man in the picture gives an idea of the size of the pod and its components.
There are only a handful of pod manufacturers in the world. The main ones are ABB, Rolls‐Royce, and Schottel. ABB makes the Azipod [11] and Rolls‐Royce makes the Mermaid pod. Schottel has a low‐power pod known as Schottel Electric Propulsion (SEP) and there is a high‐power version called Siemens Schottel Propulsion (SSP) in a
Power sources
Transformers Power conditioning, power electronics, and distribution
Gear or direct drive Fuel
Gas turbine
Fuel cell Nuclear power plant Diesel engine
Heat engine
Generator Power generation
Power electronics converters
Optional energy storage
Motor drives Loads
Induction, synchronous, reluctance, and other motors
Auxiliary loads
Podded or non- podded propulsion
Hydrogen
Figure 6.19 A generic architecture of a ship’s electrical system. Paths shown by arrowheads entering a particular block merely imply multiple possibilities and do not necessarily indicate concurrent paths.
Hybrid Electric Vehicles 166
Diesel engine
or gas turbine Generator Power conditioning
Loads Loads Loads
Ship internal or auxiliary load at various voltage and frequency levels
Propulsion motor
Propeller
Figure 6.20 System architecture of a hybrid electric ship.
Slip ring unit Hydraulic motor
Slewing bearing
Seal
Internal seal
Thrust bearing Exciter
Stator Rotor
Drainage compartments Shaft earthing
device Radial bearing Internal seal
External seal Propeller
Brake &
locking device Propeller shaft
Electric motor
Air flow
Figure 6.21 A pod propulsion system used in ships. Source: Courtesy of Rolls‐Royce plc.
Special Hybrid Vehicles 167
joint venture with Siemens. The Azipod and the Mermaid are shown in Figure 6.22 to give an idea of their appearances.
6.4.2 Aircraft
Since this book is about electric and hybrid vehicles from a propulsion point of view, it might be instructive to consider an aircraft propulsion system and whether hybrid pro- pulsion can be considered for this purpose or not. First, with the energy technology currently available to us, electric and hybrid propulsion systems cannot be considered feasible but we will give some explanation.
There are several fundamental issues that are different for aircraft compared to land or water‐borne vehicles. First, why do we consider hybrid or electric technology? From our previous discussions in this book, the predominant considerations are: (1) fuel economy, (2) environmental friendliness, (3) size, (4) cost, (5) reliability, (6) weight of the mechanical transmission system in certain off‐road applications, including locomotives, and (7) the drive cycle of the vehicle. The choice of hybrid or electric vehicle is dependent on the trade- off between these seven items. The drive cycle of a ground vehicle can fluctuate according to city or highway driving. Water vehicles, particularly ocean‐going ships (including some smaller watersport and similar vehicles), and aircraft do not have a fluctuating drive cycle.
Hence many of the considerations that are important in ground vehicles do not apply.
Another consideration is available space. Ships have a lot of space available, whereas air- craft do not. Hence it is possible to place a high‐power motor in a ship for propulsion, along with a large battery, which may be unrealistic in an aircraft.
Consider a Boeing 747 type of commercial aircraft, which can need around 90 MW of power during takeoff and about half that during cruising, depending on the speed.
Regardless of the technology used, accommodating a motor with that kind of power or having a means (battery or energy source) to drive it is unrealistic – at least with the current technology of motors and energy sources. If that technology existed, we would certainly have quieter aircraft, and they would, generally, also be safer in the event of a crash, due to the absence of any combustible fuel that could result in fire. Just to give an idea of the specific energy and power required, it may be instructive to refer to Table 6.1, from the “Battery University” web site, with some modifications.
Figure 6.22 External view of actual pod propulsion systems in a ship: left, Azipod® by ABB Oy; right, Mermaid pod by Rolls‐Royce. Source: Courtesy of Rolls-Royce plc.
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An interesting thing to notice is that the power/passenger is lowest for a bike and highest in a 747 jumbo jet. However, the energy/passenger per kilometer is somewhat different – it is lowest in a bike, but highest in a ship. Interestingly, an SUV needs more energy/person/kilometer than the jet. The fourth row is the power at cruising, leading to the sixth row upon dividing by the number of passengers (i.e., the fifth row). This shows that power/passenger is very high in a jet and a ship. The number of passengers indicated above for the 747 and Queen Mary 2 are approximate, hence the sixth row is slightly different for those columns compared to those derived from fourth and fifth rows. But a ship has a lot more space available, hence an electric ship is a viable possibility, whereas a jet equivalent is not a viable option with the present technology.
Thus, the only electric aircraft that we see today are some unmanned drones or very tiny propeller‐driven planes.
With the above in mind, we can discuss the work done on electric aircraft and where things stand at this time. While the equivalent of a jet engine using electrical means is not currently possible, to alleviate the problem of energy requirement, small solar air- craft have been designed using solar panels. Some of these will now be discussed.
A very recent example of a solar panel aircraft is shown in Figure 6.23. This aircraft, called Helios Prototype, has been developed by NASA in the United States. It weighs 1600 lb (725 kg), has a wingspan of 247 ft (75.3 m), and a wing area of 1976 sq ft (184 m2).
The upper side of the wing carries the solar panels, which are very thin, like a sheet of paper. The solar power is fed into backup lithium–sulfur batteries so that the aircraft can fly in the absence of daylight.
Another aircraft is a hybrid, made by Falx Air Vehicles in the UK (Figure 6.24). It has a tilt rotor, uses a 100 hp combustion engine, a solar array, and an electric motor rated
Table 6.1 Comparison of energy and power demands in different systems.
Specifications/
power and energy
demand Boeing 747
(jumbo jet)
Queen Mary 2 or large ocean‐going liner
Sports utility
vehicle Bicycle Person on foot
Weight 369 t
(fully loaded)
81,000 t 2.5 t 100 kg with
person 80 kg (180 lb) Cruising speed 900 km/h
(560 mph)
52 km/h (32 mph)
100 km/h (62 mph)
20 km/h (12.5 mph)
5 km/h (3 mph) Maximum power 77,000 kW
(100,000 hp)
120,000 kW (160,000 hp)
200 kW (275 hp)
2000 W (professional)
2000 W Power at cruising 65,000 kW
(87–000 hp)
90,000 kW (120–000 hp)
130 kW (174 hp)
80 W (0.1 hp) 280 W (0.38 hp) Number of
passengers 450 3000 4 1 1
Power/passenger 140 kW 40 kW 50 kW 80 W 280 W
Energy/passenger
per kilometer 580 kJ 2800 kJ 1800 kJ 14.4 kJ 200 kJ
Courtesy Battery University web site: http://batteryuniversity.com/parttwo‐53.htm.
Special Hybrid Vehicles 169
Figure 6.23 NASA’s Helios Prototype solar aircraft. Source: Wikimedia.
Figure 6.24 A hybrid electric solar aircraft by Falx Air. Source: Courtesy of NewsUSA.com.
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at 240 hp peak power. The fuel consumption is claimed to be 10 l/h of flight. This is substantially lower than a regular helicopter, which consumes about 17 times more fuel.
It is obvious from the above that electric and hybrid aircraft are limited in size. The issue is in essence due to limitations of the energy storage mechanism and the extremely high power needed by large commercial aircraft during takeoff.
6.4.3 Locomotives
Locomotives have evolved over more than 200 years, the steam locomotive being around in the early 1800s. The power demand in these vehicles could be 3000–6000 hp on aver- age, depending on the application. There are some exceptions where the size could be even bigger. Diesel locomotives began replacing steam ones starting about a decade before the mid‐twentieth century. They are easier to maintain than steam locomotives and are more efficient (www.railway‐technical.com/st‐vs‐de.shtml). Purely electric loco- motives were introduced in 1894 (www.itdh.com/resource.aspx?ResourceID=GREAT21) by Kálmán Kandó, using a three‐phase induction motor. However, this needed electrifi- cation of the railway track, to be successful in the long run. Then there was the gas tur- bine locomotive, where the gas turbine engine was used to run an electric generator, and the electricity was used to drive a propulsion motor. A gas turbine has the benefit of high specific power density. But the efficiency of gas turbines drops after a certain engine speed, so they become uneconomic in terms of fuel consumption. This is unlike a diesel engine, whose efficiency is flatter at a higher speed.
For these reasons, most locomotives now use diesel engines, but the propulsion sys- tem is implemented through an electric motor, as there are a couple of main advantages of the diesel‐electric system. First, if the propulsion were purely mechanical, a rather large transmission system with a gearbox and other components, would be needed to create the necessary torque in the wheels. Second, particularly in short‐haul trains, with frequent speed fluctuations, a diesel engine with a finite and large number of gears would need to be operated at speeds other than the most optimal in terms of efficiency.
By using a diesel‐electric system, we can remove both of these issues. In this system, the diesel engine can be run at the optimal speed, and a generator is run to produce electric- ity, which can be used to run a traction motor to drive the wheels. With the advent of power electronics, this system is very easy to realize, using reliable and efficient traction motors, which can be an induction motor or a synchronous motor – either permanent magnet or field excited. With this background then, let us look at the basic architecture of the diesel‐electric system (Figure 6.25).
In general, the traction motors are placed on each axle of the locomotive to drive the wheel pairs. A couple of diesel‐electric locomotives [12] are shown in Figure 6.26.
One particular locomotive example is indicated in Figure 6.27 [13], with some tech- nical specifications. Its diesel engine is rated at 4000 hp, while the power at the wheels is 3350 hp. Six motors are used, one per axle – three axles at the front and three at the back. The motors are four‐pole, squirrel cage, three‐phase induction motors, maxi- mum voltage 2030 V, with a 433 kW continuous power rating. So the total motor power is 2598 kW, or about 3500 hp. The maximum speed of the motor is 3220 rpm, and there is a gear ratio of 85:16 between the motor and the wheel. Note that the squirrel cage induction motor is a very reliable device for such applications. Although the specific power density can be somewhat smaller than in a permanent magnet motor, in a loco- motive application where space may not be a premium, say in a small passenger car,
Main inverter
Auxiliary inverter
HEP inverter
Coach loads
Traction motors
Auxiliary
rectifier Battery
Motor blowers
Air compressor
Locomotive cooling fans Main
rectifier Diesel
generator
Figure 6.25 Electrical and propulsion system architecture for a diesel‐electric locomotive.
Figure 6.26 Pictures of two diesel‐electric locomotives by Siemens. Courtesy Siemens AG.
Hybrid Electric Vehicles 172
this may be the ideal choice. In addition, induction motors are very resilient to tem- perature conditions, unlike permanent magnet motors.
It should be mentioned that in many applications there is no traction system battery in the locomotive (of course, the diesel engine needs a starting mechanism, which can be electric, hydraulic, or pneumatic; if it is electric, a small starter battery is needed).
However, for capturing regenerative energy, it is necessary to have a s torage battery, ultracapacitor, or a combination of the two, or even a flywheel storage unit, which can be used during regeneration. The regeneration can help improve fuel economy and is of more importance for short‐haul trains but not so important for a long‐haul train. The architecture of a locomotive system capable of regeneration is shown in Figure 6.28.
The system shown in this figure is essentially the same as Figure 6.25, except that it now has a propulsion battery and/or ultracapacitor, which can feed the propulsion motor. All the other principles remain the same. It is worth to mention that, nowadays, electrified rail transportation, especially high-speed trains, is more and more popular, which removes the need of a on-board diesel generator on a train. In these applications, the electric machines get power from its overhead cable.