24 International HISWA Symposium on Yacht Design and Yacht ... · 24th International HISWA Symposium on Yacht Design and Yacht ... the mega yacht industry has been confronted with

24th International HISWA Symposium on Yacht Design and Yacht Construction 14 and 15 November 2016, Amsterdam, The Netherlands, RAI Amsterdam

ISBN / EAN: 978-94-6186-749-0

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ELECTRIC PROPULSION OPTIMIZATION FOR FAST YACHTS R.J. Niessink, de Voogt Naval Architects (Feadship), the Netherlands

SUMMARY For years, the mega yacht industry has been confronted with the unending demand of high-power, highly-efficient, silent, lightweight and more environmentally friendly propulsion systems. More often than not, these requirements are joined by the client-driven yearning for the uniqueness of an innovation. Realistically a single one of these requirements mostly results in the choice for conventional propulsion systems. However, the culmination of these desires opens up the way for electric systems specifically tailored to the client’s wishes. For this reason, Feadship has conducted a study to determine the factors that comprise the ‘optimal’ electric power-train for large yachts in the range of 1 to 100 MW of required propulsion power. This paper shows the outcome of this study by presenting the key drivers involved in the optimization of electric propulsion, as well as exploring a few of the possible configurations. 1. INTRODUCTION As with any vessel, one of the first stages of design for a yacht would be to determine the propulsion train. The principles for this propulsion train is more often than not based on a small scope of requirements (i.e. max. speed & hotel load) to derive its capabilities and thus the specifications for its components. Realistically however, the yacht’s usage or operating profile is not focused around this small scope of requirements, but of a larger enveloping scope that takes into account all of the yacht’s ‘modes’ of operation ( Figure 1).

Figure 1 Power train usage This realization has caused many marine industries in recent years to change their design philosophy regarding propulsion trains and look into more diverse and complex configurations for the entire power train [2]. Among these new configurations many (partially) electrical propulsion solutions have been suggested. Praised for their potential regarding operating flexibility, electrical power trains would have a major benefit over conventional alternatives for propulsion (Figure 2) with more operating flexibility due to not requiring a complex mechanical transmission for each driver.


ISBN / EAN: 978-94-6186-749-0

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Figure 2 Mechanical propulsion alternatives [1]

However, apart from operating flexibility, many more factors play a role in determining an optimized system which goes beyond the inherent properties of electric or mechanical propulsion principles and is dependent on the actual units used: A gas turbine or diesel engine could be used for both electric and mechanical propulsion methods but would still maintain different properties. Therefore, a truly optimized and custom propulsion system requires exploring the boundaries of what is possible within electric power trains in order to find the best possible configurations. 2. DETERMINING FACTORS Just as operating flexibility, the factors that play a role in an optimized system are the same as for any propulsion train used regardless of whether this is concluded for electrical propulsion methods or mechanical. Next to the governing cost of the systems, there are far more defining factors that allow for a system tailored exactly to the client’s wishes. One of the main limiting factors to a propulsion train is space, whether a propulsion train is used in large or smaller vessels; the required volume for all components that play in a role in the propulsion should be taken into account and as such, extend beyond the engine room compartment. This includes required casings with regards to allowable location, shaft line and tank capacities. Next to the volume, weight plays an important role in the propulsion train, as this can be translated into an impact to the stability and range/speed. Furthermore, efficiency of the entire propulsion train plays a role as the fuel consumption can also be translated into the range, speed and operating costs of the yacht. For every unit used, the efficiency varies under different loads, which is not only true for power generation such as engines, but also for drivers such as electric motors (Figure 3).

Figure 3 Performance of typical brushless AC motor

Another factor that has become increasingly more dominant over the past years is emissions. Dictated by global necessity, these emissions are limited and a potentially constraining factor for the propulsion train.


ISBN / EAN: 978-94-6186-749-0

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Typical hazardous emissions consist of carbon dioxide (CO2), particulate matter and nitrous oxide (N2O). For reciprocating engines and turbines, CO2 and N2O are most common forms of Greenhouse pollution, where it should be noted that N2O accounts for 250 to 290 times the significance of CO2 emissions. In order to minimize these emissions, a choice could be made to either reduce power or use more environmentally friendly machinery. Additionally IMO regulations restrict NOx emissions to 3.4-2.0 g/kWh, depending on engine speed. This should not be confused with N2O emissions, as NO and NO2 are not directly greenhouse gasses, but are hazardous to the directly surrounding environment [3]. Often forgotten, but nonetheless a crucial parameter in the operation of any vessel, is its reliability and required maintenance. Nowadays, power dense and volumetrically sound systems hold proven reliable components that are certain to last for a long time. In general, the amount of moving parts in a system is relative to the amount of maintenance that is required. Should a machine exist of only a single moving part, it is likely prone to less maintenance than a machine that holds multiple moving parts. The practice of maintenance, reliability and overall longevity of a system is usually also linked to operational flexibility, where optimal usage of components plays a critical role. Low noise is an important measure of comfort on the yacht and is therefore also a defining factor. Although not solely originating from the propulsion train, difficult to quantify and related to comfort, the characteristics of reciprocating engines and turbines do offer some insight to the required countermeasures such as flexible mounting to reduce vibrations and the amount of noise insulation that is needed. Despite the individual incentives for each of these governing factors, there is often a relation between them. For example, dampening sound levels is possible, thus reducing the noise, but resulting in a higher overall volume of the system. Next to this relation, the client’s wishes cannot be seen as a static input, resulting in a varying significance of the aforementioned factors. For example, a current trend could be to travel ‘Green’, which would implicate the use of less power in order to reduce one’s carbon footprint. However, as this would impact the maximum speed or size of the vessel, reducing the emissions for a set amount of power becomes paramount. This, in turn, would mean having to use additional filter equipment such as SCR units that increases the system’s overall volume, causing a distinctive relation between all significant factors, which cannot be regarded independently (Figure 4).

Figure 4 Relation between determining factors

3. ELECTRICAL POWER TRAINS


ISBN / EAN: 978-94-6186-749-0

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As with any power train, the propulsion system requires both power generation and conversion in order to function. Therefore the system can be divided into four subsystems that are required: Power generation, transmission & conversion, drivers and the actual propulsion mechanism. 3.1 Power generation Both mechanical and electrical propulsion trains derive their power from the same means of power generation and can basically be narrowed down to either turbines or some form of reciprocating engine for diesel fuels in marine settings. Primarily this would come down to a comparison between the two with little difference compared to mechanical propulsion. Regarding the determining factors, gas turbines are around five to six times as power dense as conventional diesel engines in terms of weight (and to lesser extent, in size) due to their far higher compression ratios. Nevertheless, gas turbines require larger air inlets and exhaust systems due to being less fuel efficient than diesel engines in general as diesel engines offer a wider operating range than gas turbines at higher efficiencies (Figure 5) [4].

Figure 5 Fuel efficiency of conventional marine gas turbine versus diesel generator set

Furthermore, in general turbines of all sizes are less prone to vibration when compared to reciprocating engines due to the required high velocities at any stage within the turbine, although induced sounds are higher pitched than reciprocating engines due to higher frequencies. Regarding maintenance and reliability, optimal usage would require less maintenance for gas turbines than diesel engines due to less moving parts (i.e. no piston and cranks) being required, although diesel engines require less start-up time [5]. Next to these secondary factors, an increasingly larger advantage of gas turbines over reciprocating engines is that most gas turbines are inherently compliant with IMO tier III regarding NOx emissions. Whereas most diesel engines require filtering of almost 2-2.5 kg/kWh of NOx, giving an even larger advantage to the volumetric and weight power-densities of gas turbines. Yet, even despite these advantages, the operating ranges of most gas turbines (even when combined with a generator to provide maximum torque at any load) offer no uniform solution with gas turbines: Gas turbines are a favorable choice for idle or constant loads of power, although fluctuating loads are better handled by diesel engines due to their efficiency and operating ranges. As is shown in most 20/21st century propulsion configurations, a combination of both is often used, with gas turbines providing ‘boost’ options to the system [6].


ISBN / EAN: 978-94-6186-749-0

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Figure 6 Example power density of common power generators

Batteries are another form of power generation next to engines or turbines in combination with fuel tanks. However, due to the currently extremely low power density of batteries when compared to conventional engines or turbines (Figure 6), the feasibility of these solutions for higher power ratings is limited to peak shaving in the form of compensating peaks in fluctuating loads such as hotel loads or auxiliary thrusters. 3.2 Transmission and conversion Next to mechanical transmission, configurations of electrical transmission require additional units: The large amount of power generated by the gas turbine or diesel engine will have to be converted to electrical power. Once the electrical power has reached the propulsion jets or boosters, it will once again have to be converted into mechanical energy. Depending on the speed, power and torque of the system, using this conversion necessitates the use of units that would otherwise not directly impact the system (e.g. higher torques or speeds would require a differently sized motor or generator).

3.2 (a) Alternator/generator To convert the mechanical power coming from the turbine and/or diesel engine, an alternator changes mechanical energy into a multiphase alternating current (AC). Direct current (DC) generators exist, but are less applicable due to their inability to convert large amounts of power efficiently, as changing the voltage or current of DC relies on carbon brushes to maintain energy gain from the rotating shaft [7]. Multiple AC generator types exist, but can usually be narrowed down to synchronous and asynchronous generators. With asynchronous generators virtually being the same as an inversed induction motor. The benefits of either machines can be narrowed down to cost and efficiency. Whereas synchronous machines are more complex and thus more expensive, they offer higher efficiencies and can more easily accommodate to large fluctuations in required loads [8]. Depending on the type of stator/rotor excitement, synchronous generators can also be more power dense, due to less heat losses within the generator. Furthermore, synchronous generators are capable of providing different loads at constant speeds and thus allowing optimal loads for the turbine or engine. For most propulsion trains, a synchronous generator is therefore more beneficial regarding the above mentioned determining factors. Although they are more expensive, especially when permanent magnets are used in place of commutator rings in order to boost efficiency even more.


ISBN / EAN: 978-94-6186-749-0

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Common combinations of engines with generators are available on the market to date, usually combined with an asynchronous generator, the generator sets are up to 220% heavier and 180% larger than their conventional engine counterpart (Figure 7).

Figure 7 Generator sets compared to their engines

The difference becomes even larger when compared to turbines, as these are already far more power-dense than reciprocating engines with almost a linear trend, as the amount of copper required increases relatively to the increase of power output.

3.2 (b) Transmission After the power has been converted to electrical current, it has to be transferred through the ship to an electric motor in order to be converted back into mechanical power for propulsion. This transfer of power is usually done through conducting wires with insulation to avoid interruption or disruption of the flowing current. The amount of current that can travel through a wire is dependent on parameters such as electrical conductivity/resistivity and the dimensions of the wire. Naturally, as copper contains nearly the highest electrical conductivity of all materials (next to silver), it is often chosen as a conductor. As the amount of current passing through the wire has a direct relation to its size, a choice could be made to increase the voltage (and thus decreasing the current) of the system. However, increasing the voltage also increases the required insulation, as higher voltages allow the current to more easily create shocks and arc flashes which could be hazardous to the system’s safety requirements and reliability. As this risk is increased for direct current systems, voltages above 1 kV DC are impractical as the components that would have to withstand these fault current are not readily available due to technical challenges [9]. Therefore, for most power ranges, the transmission principle is usually confined to AC systems in order to avoid large and heavy cabling that would otherwise be required for DC systems that operate below 1 kV. Nevertheless, DC systems provide slightly higher efficiencies and are capable of using smaller components due to not directly requiring AC-DC-AC converters (Figure 8).


ISBN / EAN: 978-94-6186-749-0

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Figure 8 AC vs. DC system

If multiple means of power generation or drivers are used, an additional unit is required to combine the different inputs and outputs in the form of a switchboard that is able to distribute and regulate the flow of power through the system. Although a switchboard is not necessary when independent means of power generation drive independent propulsors. Switchboard sizes vary depending on the amount of inputs and outputs used; in order to minimize the impact of the switchboard on the determining factors, a minimum amount of inputs and outputs is favourable, although this goes against availability of operational flexibility. Furthermore, in order to connect auxiliary systems to the switchboard, such as hotel loads, bow/stern thrusters or shore connections running on different voltages, the voltage difference will have to be bridged by making use of conventional transformers, which require large and heavy copper windings the larger the voltage difference becomes. 3.3 Drivers With power distributed over the different propulsors, electrical power will have to be converted back into mechanical propulsion in order to supply a propeller or waterjet with its required energy. In theory, this is done by inverted generators that now function as motors. The advantages and disadvantages remain the same; synchronous motors (inverted synchronous generators) offer higher efficiency characteristics at the cost of more complex components when compared to induction motors (inverted asynchronous generators). One added disadvantage of synchronous motors, however, is that they are inherently not self-starting and would have to be mechanically powered for a short time before being able to completely run according to the AC input from the switchboard. Synchronous motors can be altered to adapt some properties of induction motors (i.e. self-starting), but this requires a more complex motor or more complex motor control. Electric motors are driven by a variable frequency drive in order to control speed. This way, the motor is capable of regulating torque and speed within limits by using a constant voltage from the input, whereas it would otherwise be operating at the exact same speed as the frequency of the grid. As directly controlling this AC input is not possible due to the fixed frequency of the grid, a variable frequency drive (VFD) converts the AC current into DC in order to store energy in capacitors and then convert it back to AC current via a controlled sinusoidal output. This common method of controlling AC motors from an AC grid is flexible and reliable, but does include two additional conversions of energy with included losses. Furthermore, the additional steps within the frequency drive require large and heavy units, which also have an impact on the volume and weight. Driven by variable frequency drives, multiple variations of electric motors exist within asynchronous and synchronous types. Recent innovations in electric motor technology have opened up the way for designs such as superconducting motors to be a realistic option. Popular variants in recent marine propulsion systems are advanced induction motors (AIM) which are of the asynchronous type [10]. These are more efficient when compared to conventional induction motors, but still


ISBN / EAN: 978-94-6186-749-0

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fall behind synchronous motors of the permanent magnet (PM) type. With higher demands of power, superconducting motors (SC) offer even larger power densities, despite their constant cooling requirement and even higher efficiencies (Figure 9).

Figure 9 Conventional induction motors vs. superconducting & permanent magnet

With superconducting motors being able to almost completely negate the impact on efficiency from electrical to mechanical conversion, as well as reducing the size and weight required by a third. Naturally, this technology can also be used for generators and other components, but even when unpowered the superconducting environment within the component would still have to be maintained, requiring constant power (although marginal). Although a transmission component in reality, reduction gears could also be used in an electrical propulsion train when used in combination with an electric motor. As with reciprocating engines and turbines, electric motors also benefit from higher speeds due to having to deliver less torque whilst maintaining the same power output; meaning the size of the motors can be decreased by increasing the nominal speeds and reducing torque. However, as propulsors require different nominal speeds, a reduction gear is often required. Combining both the motor and the reduction gear with regards to the overall volume and weight required, an optimal gear ratio for a set propulsor speed can be derived for a given scope (Figure 10).

Figure 10 Example of optimal gear ratio for permanent magnet synchronous motor for a given scope

From which the propulsor speed becomes a determining factor for its powertrain. In order to reduce the required volume/weight, a choice could be made to require propulsors with higher speeds. However, this is where hydrodynamic efficiency should be taken into account: As, in general, conventional fixed pitch propellers favor slower ship speeds (up to ~20 kn.), whereas (linear) jets require higher speeds for optimal efficiency depending on the ship’s lines [11].


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4. CONFIGURATIONS Based on the above obtained information, a plethora of electric powertrain configurations can be derived depending on its determining factors. These could include any setup of diesel/turbine generator sets driving a unique electric powertrain, or a partly electric powertrain in conjunction with a mechanical drive. Regarding fully electric powertrains however, integrated fully electric propulsion (IFEP) is commonly applied today. A wide array of vessels currently use IFEP configurations, ranging from large cruise ships to navy aircraft carriers, albeit for different incentives [12].

Figure 11 Today’s and tomorrow’s use of integrated fully electric propulsion systems

Due to the capabilities of present day technology, most electrical propulsion configurations are powered by a variety of power generators such as a combination of diesel reciprocating engines or turbines with varying power ratings. This is done to obtain near optimal loading conditions for the diesels/turbines at any operating mode (Figure 12).

Figure 12 Specific fuel consumption for 50 MW powertrain

A typical IFEP system would allow for maximum redundancy and flexibility by choosing any power consumer to be supplied by any of the units of power generation through a switchboard connecting all the inputs and outputs of the system (Figure 13). By choosing various power ranges for the units that generate the power, the system is able to provide optimal usage of subsystems based on the consumer’s power requirements. In terms of operating flexibility as stated in chapter 1, this is increasingly becoming a necessity for yachts.


ISBN / EAN: 978-94-6186-749-0

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Figure 13 Example of 50 MW IFEP system

The downside of such a fully integrated system is that the entire electrical transmission section is required to distribute the full power range of the system, as all components are connected to a single switchboard. In the example of Figure 13 the system should be able to handle the combined power of 50 MW in order to preserve the system’s flexibility. This leads to oversizing due to the larger current/voltages required to run through cables, components and rails. An option would therefore be to physically split the system and connecting the system’s components only to the consumers that require their power input. This marginally sacrifices the system’s operating flexibility, but drastically reduces the flow of power that the transmission components need to handle. In the example of Figure 13, splitting the two turbine alternators used for boost speeds from the main system, removes the ability to power for example the hotel load and auxiliary thruster with the turbines, but greatly reduces the size of the main switchboard and other components. By looking closely at the intended function for each of the components, the system can be greatly optimized according to the design intent (Figure 14).

Figure 14 Example of system optimized for function


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Although reducing the size of various components due to lesser power requirements, separating the system’s components results in greater system complexity by requiring additional means of synchronization and providing more challenges for the system’s power management. It should be noted that by completely disconnecting the subsystems e.g. using a separate power train for boost speeds, the systems complexity can greatly be reduced. By doing so, the propulsor(s) used in specific operation modes (e.g. boost) are completely unpowered in all other modes, creating trailing propellers and/or jets. Combining this with the frequency of these operating modes and the impact on hydrodynamic efficiency depending on the ship’s lines, the consequences of separating the power trains completely negates the determining factors that the systems would be optimized for. This would lead to larger systems due to higher power requirements in the other operating modes, as well as higher operating costs and tank volume and is therefore only rarely part of an optimized system. 5. CONCLUSION 21st century demands for propulsionystems allow for optimization based on not only a single static factor such as maximum speeds, but rather looking at the actual operating profile of the yacht while combining the yacht’s function with overall determining factors such as volume, weight and efficiency. This allows for more innovative propulsion systems such as fully electric power trains. It was found that a large amount of interconnected and variable parameters form the scope of an optimized propulsion train. Next to volume and weight, a present day powertrain is also dictated by emissions and noise, causing a relation between the determining factors based on their significance. Despite being generally larger, heavier and less efficient overall than their conventional mechanical or hybrid counterparts, electric power trains offer superior operating flexibility as well as large amounts of options in terms the system’s configuration allowing to specifically tune the propulsion system to the client’s wishes. It was found that for the above mentioned parameters, solutions in the form of electrical configurations can be found that tailor to multiple significant factors whereas conventional mechanical solutions would require compromises in the system. Often chosen for maximum efficiency at multiple power ranges, integrated fully electric propulsion systems (IFEP) are being widely implemented in vessels around the globe. Ranging from cruise ships to naval vessels, the incentives vary but are comparable with yachts due to the greatly varying power demands from different consumers whilst aiming for optimal usage of the power generating units. By looking at the specific functions for each of the different power ranges, IFEP systems can be further optimized in line with its determining factors by physically splitting subsystems from the powertrain. This slightly decreases the operating flexibility and increases system complexity, but allows for more optimized components. Further optimization for these systems is possible in the form of changing the system’s distribution principles from AC to DC and using battery technology to compensate fluctuating loads within the system with the use of peak shaving. However these options are currently limited due to technical constraints in the form of component reliability and safety margins by regulatory bodies. Overall with upcoming and current innovations regarding electric propulsion such as permanent magnet machines and superconducting generators/motors, the gap between electric and conventional mechanical propulsion is rapidly decreasing. With permanent magnet machines offering higher power densities and greater stability at fluctuating loads and superconducting components having close to zero thermal losses and requiring far smaller and lighter components. The distinctive choice between operating flexibility and the


ISBN / EAN: 978-94-6186-749-0

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simplicity and size/weight of conventional mechanical systems should become far less paramount in the coming years. REFERENCES

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[7] Portela, P., Sepúlveda J., Esteves, J. E., “Alternating Current and Direct Current Generator”, in The

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24 International HISWA Symposium on Yacht Design and Yacht ... · 24th International HISWA Symposium on Yacht Design and Yacht ... the mega yacht industry has been confronted with

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