4.5 Electrical Transmission
A direct drive marine diesel engine installation will have higher energy efficiency and have lower first cost than a diesel-electric plant. However, the greater flexibility in installed units is available with a diesel-electric system. The concept of fully integrated electric propulsion has grown in popularity, particularly for military vessels and cruise ships. Current generations of warships utilise a combination of large and small prime movers to achieve efficient operation at cruise and sprint speeds. Very often there are two sprint engines and two cruise engines mechanically coupled to propulsion, and a number of diesel generator sets. This means there can be as many as ten separate prime movers of different types on board such vessels. Employing fully integrated electric propulsion, whereby a small number of common and distributed prime movers provide propulsion and auxiliary power, can reduce the number and variety of prime movers on board.
Cruise ships have extremely large lighting, air conditioning and other hotel loads, and the machinery layout is driven by the need to maximise internal space for passengers. By locating all of the prime movers in spaces that would otherwise be unusable, the revenue generating space can be increased. Using electric motors to drive the propellers allows the prime movers to be located away from the shaft line, even high up in the superstructure.
Full electric propulsion can be utilised to allow prime movers to be operated more efficiently. With traditional mechanical propulsion the propeller speed is proportional to the rotational speed of the prime mover, forcing the engines to operate outside their most efficient speed and power ranges whenever the vessel is not at full power. By removing the mechanical link between propeller and engine the propeller speed can be made independent of the engine speed, enabling the engine to be operated at its most efficient speed at all times. When operating at part load the number of prime movers in operation can be reduced, with the remaining ones still running close to full power and maximum efficiency.
For naval applications it is desirable to divide the vessel into zones, which can be isolated and operated independently in the event of serious damage to the vessel. Electric propulsion is ideally suited to this philosophy, allowing the prime movers to be located in different zones, each connected by a main supply bus. All loads can then be supplied by any combination of generators in normal operation. In the event of damage to the system, the affected zone can be isolated without affecting the rest of the system. A typical system architecture for a naval vessel with fully integrated electric propulsion is shown in Figure 6 [27].
Diesel generating sets are the most common method of electricity generation on board ships, but the increasing acceptance of gas turbines is extending into the electrical sector. Gas turbine alternators are likely to become widely accepted for both military and cruise-ship electricity generation applications, especially with the move away from 50/60Hz distribution towards either 400 Hz or DC systems. The very high rotational speed of gas turbines makes high frequency distribution more attractive.
4.6 Operability and maintainability
Environmental sustainability relies on continued efficient and safe operation of marine systems. Greater emphasis on plant design for simplicity and ease of maintenance will be necessary in the future, with remote as well as on-board engineers becoming more reliant on computer based total system 'healthcare' tools to diagnose potential problems and plan their rectification, Figure 7.
At present, the major engine manufacturers have developed a range of software to assist in condition monitoring and fault diagnosis. However, this type of software is specific to a small number of individual machines within an engine room and is not generically applicable. One of the key issues to be addressed is the healthcare of the whole powering and propulsion system, which is an integration of a large number of components. The application of methods such as neural networks and fuzzy logic can be utilised in the development of advanced condition monitoring, fault diagnosis and maintenance scheduling to provide total system healthcare.
The standardisation and modularisation of equipment will allow operators to capitalise on the skills they have available on-board and to adopt a shore-based unit replacement approach. One of the biggest problems currently facing marine automation is the ability to interface systems in a safe manner. While there are a number of proprietary and manufacturing/processes industry standards for data communication available, there is no single commonly agreed practise in the marine industry. There are a number of emerging standards, but there is not a standard that will suit every application.
As the information technology revolution gains momentum, there are increased research efforts in the field of applied telematics. This could have a huge bearing on marine operations and opens up the greater potential for remote operation, monitoring and maintenance of marine systems. However, this concept relies heavily on the capacity and reliability of the communication infrastructure to be utilised.
5. CONCLUSIONS
LCA studies will provide important information and data to optimise transport chains for environmental sustainability. Marine transportation of cargo and passengers has been shown by a number of studies to have a lower environmental 'cost' than other modes. This therefore suggests that greater use should be made of inland waterways and shipping routes in future planning.
The developments in marine powering and propulsion continue at pace, with designers continually striving for the most efficient, reliable and cost-effective means by which to power a ship. The technologies outlined within this paper are at varying stages of development. Fuel cell technology looks to have very good long term potential, however it is still in its infancy, and above all lacks the global bunkering infrastructure necessary to implement the transition to hydrogen based fuel.
The diesel engine still offers the economic advantage in its ability to burn low quality fuel, although tightening environmental legislation continues to place more stringent demands on plant, which are being met by a variety of solutions.
Recent developments in gas turbine technology have improved efficiency levels to make them increasingly competitive with diesel engine power and propulsion units.
Increased reliance on electrical technology is one way of reducing mechanical complexity and utilising generator engines at close to their design load for optimum efficiency. Developments such as electronic fuel injection and electro-hydraulic valve actuation have opened up the possibility to total system integration and controllability. This allows optimisation of plant performance to a given operating profile such as low NOx or low fuel consumption running. This must come with the assurance of reliability if systems are to be supplemented with the widespread implementation of electronic control. The implementation of remote condition monitoring technology, fault diagnosis and intelligent decision making tools will improve system continued efficiency, safety and reliability.
