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Hybrid marine propulsion system technology for smart ships

Marine propulsion is the mechanism or system used to generate thrust to move a ship or boat across water. At present, 90% of the sea-going naval ship are diesel-powered. To achieve the safe upper limit for global warming of 1.5 degrees Celsius, ‘all ships designed and built today must operate in a net zero emission world at the end of their service life,’ says Gavin Allwright, Secretary General of the International Windship Association.


The International Windship Association (IWSA) is spearheading a ‘Decade of Wind Propulsion’ campaign to accelerate the adoption of hybrid alternative propulsion methods for shipping, helping to steer the industry toward decarbonisation. Three key elements of the Decade of Wind Propulsion campaign include the delivery of retrofit systems and new build projects, the optimisation of existing systems alongside new concepts, and the facilitation of hybrid systems.


While the pressure to reduce fuel consumption and emissions has increased, the operating profile of ships has become increasingly diverse: offshore vessels perform numerous tasks, such as transit and critical dynamic positioning (DP) operations; heavy crane vessels, such as the Pioneering Spirit, exhibit an increased capacity and complexity for diverse offshore operations; naval ships perform traditional patrol operations in open sea, but are also deployed in littoral operations; and tugs require full bollard pull when towing and require limited power during transit or standby. Due to these diverse operating profiles, the power and propulsion plant has to perform well on many performance criteria, such as: Fuel consumption; Emissions; Radiated noise; Propulsion availability; Manoeuvrability; Comfort due to minimal noise,  vibrations and smell; Maintenance cost due to engine thermal and mechanical loading; and Purchase cost.


Moreover, the classification in parallel, series and series-parallel hybrid electric vehicles does not apply to ship’s power and propulsion architectures, as ships can have multiple propulsion engines, electric propulsion motors, diesel generators, fuel cells, and energy storage systems. Marine propulsion topology can be classified into mechanical propulsion, electrical propulsion and hybrid propulsion and the power system topology in combustion power supply, electrochemical power supply, stored power supply and hybrid power supply.


A hybrid marine propulsion system is any combination of a combustion engine and an electric motor. Electricity can be produced by one or a combination of the following: a combustion engine generator, a wind generator, a towed water generator or solar panels. A purely electric solution with solar panels is enviable due to its zero carbon footprint and low operating costs and various takes on these systems have been gaining traction on alternative energy vessels. Advances in both energy storage and solar panel technology have reduced costs and physical footprint making solar power propulsion systems more feasible for use on boats.


Marine hybrid propulsion is a combination of a battery-powered propulsion system along with alternate fuel such as diesel and liquefied natural gas (LNG) utilized during propulsion of the naval vessels. A hybrid vehicle can achieve propulsion using a fuelled power source for e.g. a diesel engine or through a stored energy source, which is a battery bank and electric motor. Hybrid propulsion systems can be differentiated between configurations, where the diesel engines and the E-motors work in parallel on the propeller.


The main applications of marine hybrid propulsion market are in offshore vessels and navy applications. Offshore patrol ships are good examples of ships that are equipped with the hybrid propulsion system. Patrol ships can be operated at low speeds by the electric motor and at a high power demand by the main engine. Moreover, Anchor Handling Tug Supply vessels are good examples of offshore ships with highly flexible power demand and per consequence, different operation modes, and sailing speeds.



Mechanical propulsion

A typical architecture for a modern ship with mechanical propulsion comprises of a prime mover, typically a diesel engine or gas turbine, drives the propulsor, typically a propeller, either directly or through a gearbox. A separate electrical AC network is required for generating and distributing electric power of auxiliary loads, such as variable speed drives, heating ventilation and air-conditioning (HVAC) and other mission-critical and auxiliary systems. Diesel, steam-turbine or gas-turbine generators feed this electrical network.

For large cargo ships, driven by low-speed diesel engines, no gearbox is required and reversing can be achieved by reversing engine rotation. On the other hand, smaller ships do require a gearbox to reduce the engine speed, as they are driven by medium- or high-speed diesel engines. This gearbox can also be used for reversing shaft rotation.


The most applied propulsor is a Fixed Pitch Propeller (FPP). It requires a reversible engine or gearbox for stopping and reversing. Alternatively, a controllable pitch propeller (CPP) can provide negative thrust for stopping and reversing. Other propulsors are water jets, surface-piercing propellers, cycloïdal propellers, paddle wheels, whale-tails, and magnetohydrodynamic propulsion. Furthermore, propulsion and steering can be combined in steerable thrusters.


Mechanical propulsion is particularly efficient at design speed, between 80 and 100% of top speed. In this range the diesel engine operates in its most efficient working point. Moreover, mechanical propulsion consists of only three power conversion stages, the main engine, the gearbox and the propeller, which leads to low conversion losses.


Mechanical propulsion has a poor fuel efficiency and high emissions when sailing at speeds below 70% of top speed, because engine fuel consumption significantly increases below 50% of rated power. Mechanical propulsion exhibits poor availability, because failure of any of the components in the drive train directly leads to loss of propulsion. The manoeuvrability is limited by the engine’s operating envelope.


Electrical propulsion

A typical architecture of an electric propulsion system is depicted in Fig. Multiple diesel generator sets (1) feed a fixed frequency high voltage electrical bus (2). This bus feeds the electrical propulsion motor drive (5) and the hotel load (6), in most cases through a transformer (3). The electric propulsion motor drive consists of a power electronic converter (4) used to control shaft line speed and thus ship speed.


Benefits and challenges of electrical propulsion

In the first place, electric propulsion is a fuel-efficient propulsion solution when the hotel load is a significant fraction of the propulsion power requirement and the operating profile is diverse, because the generator power can be used for both propulsion, through the electric motors, and auxiliary systems. To achieve this, a power management system (PMS) matches the amount of running engines with the required combined propulsion and hotel load power. This control strategy ensures engines do not run inefficiently in part load and is often referred to as the power station concept.


Secondly, the NOx emissions of electric propulsion are likely to be less than those of mechanical propulsion, because the propulsion power at full ship speed is, in most cases, split over more engines, which due to their lower individual power run at a higher speed. The third advantage of electrical propulsion is the reduced maintenance load, as engines are shared between propulsion and auxiliary load and are switched off when they are not required. Fourthly, electric propulsion can achieve reduced radiated noise due to the absence of a mechanical transmission path from the engine to the propeller. To this aim, the design of motor and power converter has to be optimised for minimal torque fluctuation.


On the contrary, electrical propulsion faces the following challenges: Due to the additional conversion stages in power converters and electric motors, electrical propulsion incurs increased losses. These losses lead to an increase in SFC, particularly near top speed of the ship. Most ships with electric propulsion use FPP, because electric motors with variable speed drives can provide maximum torque at every speed and run in reverse. Cavitation potentially increases under operational conditions, particularly for electric propulsion with fixed pitch propellers and speed control, as well as for mechanical propulsion with FPP.


Even though the fuel savings attributed to the power station concept are mostly offset by the increased electrical losses, electric propulsion has been very successful in the cruise industry. This is mainly attributed to the robustness of the power station concept; failure of a diesel generator has hardly any impact on the operation of the vessel. Additionally, electrical propulsion allows flexibility in positioning machinery spaces, due to the absence of the shaft-line, which traditionally determines the engine room layout. Finally, the absence of the shaft-line also allows isolation of noise from the diesel engines, by installing diesel-generator sets on flexible, noise-isolating mountings.


The success of electrical propulsion in commercial ships and the drive to reduce running cost has prompted significant development programmes to enable electric propulsion for naval destroyers in the UK and US. These development programmes were targeted to increase the power density with advanced technologies, consisting of new permanent magnet and high temperature super conducting motor technologies in order to fit electric propulsion in frigates and meet military requirements.


Major development in naval ship propulsion has been the adoption of integrated electric propulsion (IEP) by the UK Royal Navy (RN) for its Type 45 anti-air warfare destroyers and the forthcoming Queen Elizabeth-class aircraft carriers, and by the US Navy (USN) for the DDG 1000 Zumwalt-class destroyers, the first of which currently is in its pre-commissioning phase.


These development programmes have led to the application of electric propulsion in Royal Navy’s Type 45 destroyer,  and Queen Elizabeth aircraft carriers, and in US Navy’s DDG-1000 destroyer. In spite of development programmes for new motor technologies, these naval applications are still all based on the Advanced Induction Motor (AIM) with Pulse Width Modulation (PWM) frequency converter drives. This AIM drive is an advanced development of asynchronous motor technology. These naval applications consist of traditional fixed frequency high voltage AC generator sets with conventional control strategy, despite programmes to develop DC architectures.



Hybrid marine propulsion system technology

When the auxiliary load is only a fraction of the required propulsive power, the losses associated with the electrical conversion lead to increased fuel consumption for electric propulsion systems [65]. The extra electrical equipment also leads to increased weight, size and cost. Therefore, ships that frequently operate at low speed can benefit from a hybrid propulsion system.


In hybrid propulsion, a direct mechanical drive (1) provides propulsion for high speeds with high efficiency. Additionally, an electric motor (2), which is coupled to the same shaft through a gearbox (3) or directly to the shaft driving the propeller, provides propulsion for low speeds, thus avoiding running the main engine inefficiently in part load. This motor could also be used as a generator for electrical loads on the ship’s services electrical network



When the mechanical drive engine is running, this system allows generating capacity either from the electric generator or from the generating sets. Typically, rule-based control or the operator determines the generating capacity.


Because hybrid propulsion is a combination of electrical and mechanical propulsion, it can benefit from the advantages of both . However, in order to achieve these benefits, a proper design (of the hybrid propulsion) is required and often a trade-off between these requirements has to be made. The control strategy allows an optimal trade-off and can use the extra degree of control by transferring electrical power from the mechanical drive to the electrical network and vice versa. The main challenge for the hybrid propulsion design is to balance the trade-off between all requirements and design a control strategy to achieve this balance.


Typical applications of hybrid power and propulsion systems are naval frigates and destroyers, towing vessels and offshore vessels. Castles and Bendre  describe the economic benefits of a hybrid propulsion system for US Navy DDG-51 class assuming rule-based control. The US Navy uses gas turbines as its prime movers, also for its ship services’ generators. The part load specific fuel consumption of gas turbines is very poor, much worse than that of diesel engines. With gas turbines, hybrid propulsion thus can lead to significant fuel savings. Sulligoi et al.  discuss the Italian Navy FREMM frigate configuration with diesel generators and a sprint gas turbine main engine.


Electrical propulsion with hybrid power supply

In electrical propulsion with hybrid power supply, a combination of two or more types of power source can provide electrical power. We propose to classify power sources into: Combustion power supply, from diesel engines, gas turbines or steam turbines; Electrochemical power supply from fuel cells; or Stored power supply from energy storage systems (2) such as batteries, flywheels or super capacitors.



A typical architecture of an electrical propulsion plant with hybrid power supply is shown in Fig. In this case, energy storage (2) is connected to the main distribution bus. However, energy storage can be connected at various locations of the electrical system: At the main high voltage bus bar through an AC/DC converter; At the LV bus bar through an AC/DC converter; Directly or through a DC/DC converter to the DC link of the propulsion converter.


The benefits of applying stored and hybrid power supply in ship power and propulsion plants can be diverse: The energy storage can provide the required electrical power and enable switching off one or more engines when they would be running inefficiently at part load. The energy storage can then be recharged when the engine is running in an operating point with lower SFC and CO2 and NOx emissions. This can save fuel, reduce emissions, reduce noise, increase comfort and enable temporarily sailing without emissions, noise and vibrations from the engines.


The battery can provide back-up power during a failure of combustion power supplies (diesel generators). This can omit the need for running extra diesel engines as spinning reserve and can potentially reduce the installed power on vessels with a requirement for a high availability of propulsion, for example DP vessels.


Batteries have only recently been applied in maritime applications, but their popularity is growing very quickly. For tugs and ferries, for example, the potential reduction of fuel consumption and emissions has led to investigation and application of electrical propulsion with hybrid power supply


Gearboxes with power take in and take out motors can reduce requirements for gensets and even cut engine loads

Demand is growing for gearboxes designed with hybrid propulsion in mind, as tug owners seek to cut fuel consumption and emissions, while reducing the number of generator sets on board. A new generation of gearboxes are being developed for four-stroke engines to meet the requirements of hybrid propulsion systems on harbour, escort and coastal towage tugs. These units can be installed on new vessels or retrofitted to existing tugs, helping reduce fuel consumption, engine load and running time and emissions.


During Riviera Maritime Media’s recent Gearboxes: gearing up for hybrid propulsion applications webinar, executives from two gearbox manufacturers outlined the latest developments in this area. Katsa gearbox business sales manager Mikko Happonen outlined the latest gearbox technology for four-stroke engines and how these can reduce requirements on gensets.


During the discussion, Mr Happonen outlined developments in the Katsa L350 and L490 gearbox series. These are compact, clutched power PTI/PTO gearboxes designed for marine hybrid applications. “These gearboxes combine two power inputs to one main output with flexible clutch options,” said Mr Happonen. The two main applications are for driving thrusters on workboats, offshore support vessels and tugs, and for propulsion drivetrains. The L350 has a power range of 200-1,000 kW and the L490 range spans 1,000-2,500 kW. These gearboxes have an integrated wet running, multi-plate clutch.


“PTO or PTI electric motors can be connected for running either as a generator or as a driving motor,” said Mr Happonen. “Our standalone hybrid PTO/PTI clutch gearboxes have compact designs enabling the selection of main engine or propulsion gearbox as standard,” he said, adding, “They are easy to handle, assemble and maintain.” These gearboxes enable hybrid propulsion to be integrated with thruster-based propulsion, reducing emissions from tugboats. They can be controlled by a vessel master from the bridge.


Katsa’s hybrid PTO gearbox has a hydraulic clutch dynamic torque of up to 16,000 nautical miles. It has independent oil circulation, with pump and integrated oil sump, hydraulic clutch control system with proportional valves and a clutch protection system with remote bridge control. These PTO gearboxes also have flexible gear ratios for optimal engine and pump revolutions.


Mr Happonen anticipates higher demand for these hybrid gearboxes as more owners select them for different vessel types to reduce emissions and fuel consumption, and increase periods between engine and generator set maintenance. “Hybrid is the main technical solution for the future,” he said. “We are focusing our research on developing smart hybrid gearboxes and ensuring the electric power fits well with the mechanical side.”


Mr Happonen said most of the applications for Katsa hybrid gearboxes were on newbuild vessels with thruster-based propulsion, although these products are available for retrofits if required. “Retrofit projects would be challenging because of the limited space in enginerooms,” he said. “This is why we make [gearboxes] as compact as possible.” Katsa is also developing new winch gears and winch electric powerpacks, incorporating permanent magnet motor technology. For the marine and offshore sector, these could be used for demanding applications such as escort winches on tugs. Still, there has been some reticence among shipowners as regards investing in hybrid propulsion gearboxes for newbuildings.


Rival transmission manufacturer ZF has expanded its range to include electric power and hybrid drivelines. ZF also said it can supplement its existing scope of supply with electric motors, power electronics and controls. ZF Marine’s parallel hybrid systems enable installation of electric drives with power levels of 150-750 kW. It can be a complete hybrid driveline from a single source, which some shipyards prefer for newbuildings, said ZF head of sales of commercial and fast craft Wolfram Frei. “We provide standardised overall solutions,” he said. ZF hybrid transmission is part of a propulsion solutions portfolio. It enables full-electric operation, or supplements the engine load, reducing operating hours and maintenance costs. Electric motors have minimal noise and can provide a boost function augmenting power from engines when tugging heavy cargo.


Marine Hybrid Propulsion Market

As per a report by Fortune Business Insights™, titled, “Marine Hybrid Propulsion Market, 2021-2028,” the market size was USD 3,053.2 million in 2020. It is projected to grow from USD 3,370.9 million in 2021 to USD 8,122.2 million in 2028 at a CAGR of 13.39% in the forecast period.


Drivers & Restraints:

The emergence of Electric Propulsion Technology to Accelerate Growth

The rising usage of electric propulsion technology in naval ships, merchant ships, and recreational boats is expected to propel the marine hybrid propulsion market growth in the near future. This technology helps to reduce the need for clutches and gears, as well as lowers the noise created by marine engines. Besides, the technology has the ability to provide improved maneuverability and enhanced performance amid harsh climatic conditions. However, the COVID-19 pandemic has led to a major drop in the global trade, thereby hindering the demand for these hybrid propulsion systems.


Based on the type, this market is categorized into diesel-electric, gas-electric, and others. Out of these, the diesel-electric segment was in the dominant position in 2020 backed by its ability to produce more energy in a naval vessel. By operation, it is segregated into a parallel hybrid propulsion system and serial hybrid propulsion system.


Market Segments

Based on the deadweight, it is classified into 5K DWT, 5K-10K DWT, and more than 10K DWT. By the ship type, it is divided into anchor handling tug supply vessels, platform supply vessels, yachts, motor ferry, cruise liner, small cargo ships, naval ships, submarines, ROVs, UUVs, and AUVs. Lastly, by installation, it is bifurcated into line fit and retrofit.


Top companies  in marine hybrid propulsion market are ABB Ltd. (Switzerland), BAE Systems (The U.K.), Caterpillar Inc. (The U.S.), General Electric Company (The U.S.), MAN Diesel & Turbo SE (Germany), Mitsubishi Heavy Industries, Ltd. (Japan), Rolls-Royce plc (The U.K.), Schottel GmbH (Germany), Siemens AG (Germany), Steyr Motors GmbH (Austria), Torqeedo GmbH (Germany), Wartsila Corporation (Finland),


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