This invention relates to a vessel energy management system and method, in particular power distribution on a marine, or seagoing, vessel. Ships of all types try to reduce operating costs as much as possible in order to be competitive. The main engine operates most efficiently if it has a sufficient and substantially constant power demand, but this is difficult to achieve, other than in calm conditions. In increased sea states, the energy required to move the vessel varies between the peaks and troughs of the waves, or swell, often in a cyclical manner, similar to a sine wave. The ship needs to have sufficient power to cope with the maximum expected power requirement, but at other parts of the cycle, that power is not used.

In accordance with a first aspect of the present invention a vessel power distribution system for controlling power supply to and from a propulsion system of a vessel, the propulsion system comprising at least two drive shafts, each having a propeller; each propulsion system comprising a motor/generator unit adapted to receive power from a prime mover of the vessel and supply it to the drive shaft, or to receive power from an alternative energy source and supply power to the shaft; the power distribution system further comprising a converter unit comprising, for the first drive shaft, two shaft generator converters and a first energy storage unit coupled to a first DC bus; and for the second drive shaft, two shaft generator converters and a second energy storage unit coupled to a second DC bus; each of the DC buses being coupled together by a switching device, the switching device comprising two pairs of controllable semiconductor devices in parallel with diodes arranged such that current flow between the first and second DC buses is inhibited when at least one of the semiconductor devices is non-conducting; whereby power from one prime mover may be switched from a first drive shaft to a second drive shaft, or vice versa; or power from an alternative energy source may be directed to one of the first and second drive shafts.

Connecting the two propulsion systems via the DC bus enables load sharing and more efficient running of main engines and generators, whilst enhancing reliability by protecting against the spread of faults from one side to the other. The main engine may then be operated more efficiently, with reduced the wear and tear. The semiconductor device may be adapted to cease to conduct in the event of a fault being detected on one terminal of the device.

The motor/generator unit may comprise a permanent magnet motor.

The system may further comprise galvanic isolation between the first bus and the switching device and between the switching device and the second DC bus.

The system may further comprise one or more auxiliary generators coupled to an AC bus, or AC switchboard.

The alternative energy source may comprise one of the first or second energy storage units, or one or more of the auxiliary generators.

The switchboard or converter unit may comprise a control unit to control allocation of surplus energy to consumers on the AC bus or on the DC bus, or to control power reductions for consumers in favour of the propulsion system.

In accordance with a second aspect of the present invention, a method of method of power distribution on a vessel for controlling power supply to and from a propulsion system of a vessel, the propulsion system comprising at least two drive shafts, each having a propeller and a motor/generator unit; the method comprising receiving power from a prime mover of the vessel and supplying it to the drive shaft, or receiving power from an alternative energy source and supplying power to the shaft; wherein the power distribution system further comprises a converter unit comprising, for the first drive shaft, two shaft generator converters and a first energy storage unit coupled to a first DC bus; and for the second drive shaft, two shaft generator converters and a second energy storage unit coupled to a second DC bus; each of the DC buses being coupled together by a switching device, the switching device comprising two pairs of controllable semiconductor devices in parallel with diodes; and wherein the method further comprises controlling current flow between the first and second DC buses by switching at least one of the semiconductor devices, wherein when the or each semiconductor device is conducting, allowing power from one prime mover or alternative energy source to be directed away from a first drive shaft to a second drive shaft, or vice versa, or allowing power from an alternative energy source on the one DC bus to be directed to one of the first and second drive shafts though the other DC bus.

The method may further comprise directing power from an alternative energy source on an AC bus to one of the first and second drive shafts through the first DC bus and the second DC bus, or through the second DC bus and the first DC bus when the semiconductor devices are conducting.

An example of a vessel energy management system in accordance with the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 illustrates a typical energy management system for a marine vessel using shaft take off power;

Figure 2 illustrates an energy management system for a vessel having an energy management system according to the invention;

Figure 3 illustrates an example of a switching device for use in the system of

Fig-2;

Figure 4 is a flow diagram illustrating a method according to the present invention for controlling power on a vessel.

Electric ferries are known, having onboard batteries for storing energy when connected to a shore supply when docked, so that the ferry has a clean and quiet energy source in operation. Typically, an onboard generator is also provided, as a back-up if there is a greater than expected power requirement, or for assisting in charging the batteries when the shore supply is inadequate. The battery is discharged at a reasonably constant rate, over the course of the journey, then charged up again when the ferry docks. However, for seagoing vessels, responding to wave cycle power demand which may vary by several megawatts in the space of less than one minute, using batteries to smooth out the demand is not sufficient, as batteries do not respond well to receiving large amounts of charge and discharging by a similar amount shortly afterwards, as is required in this application. Thus, shaft power systems are used, to allow the prime mover to continue to run at a constant speed, whilst adjusting for peaks and troughs in energy demand by integrating the propulsion system with the shipboard auxiliary power system.

Fig.l is a block diagram illustrating main power generation and consumption elements for a marine vessel having power control using shaft and auxiliary power. Vessels may be single screw, or twin screw. The example shown is for a twin screw vessel. Each screw is part of a stand-alone system 10, 20, to provide redundancy in the case of a fault in one side or other, but otherwise the components of the two stand-alone systems are substantially the same, but typically one is located on the port side and the other on the starboard side of the vessel. The main engine 1 turns a shaft 2 connected to a propeller 3. A motor/generator unit 4 is able to act as either motor or generator, according to the requirement, to control the supply of power to the propeller 3 and the supply of power to consumers (not shown). The motor/generator unit 4 is connected to starpoint connection 7 and an AC line 8, for example by motor operated circuit breakers 5, 6. The AC line 8 is connected to a converter unit 9. The converter unit is coupled to main switchgear 11 of the vessel through a transformer 12. The main switchboard supplies AC loads (not shown), such as motors and other loads onboard the vessel, or subsidiary AC buses at lower voltages (not shown).

The converter unit 9 typically comprises one or more AC to DC converters 13, which may include a breaking chopper. These receive power from the main switchboard AC bus 11 through transformer unit 12 and supply DC to a DC bus 14 to which one or more consumers (not shown) may be connected. The main switchboard receives its supply from auxiliary generators (not shown) and typically operates at 6.6kV, 60Hz, in most cases. In this example, the transformer transforms down from 6.6 kV to 630V AC (with an output power of 2250 kVA) and the AC voltage is then converted to 930 V in the converter unit. In the event of there being surplus power from the main engine 1, then the motor/generator unit diverts that power to the converter unit through AC to DC converters 15 and supplies power to the AC consumers through the transformer. Auxiliary generators can be stopped and started more easily than the main engine, so varying the load on the auxiliary generators is preferable to varying the load on the main engine. In the event of the propeller 3 requiring more power than is available directly from the main engine 1, the motor/generator unit supplies additional power from the auxiliary generators connected to the main switchboard. This may result in consumers on the converter unit losing power temporarily, as they have a lower priority than the propulsion system.

These stand-alone systems are good for ensuring that the vessel is still able to manoeuvre if a fault puts one of the propulsion systems out of action, but at the cost of always having to run both main engines and auxiliary generators on both the port and starboard systems. There has been some consideration of sharing power between the port and starboard sides systems by connecting the AC buses 11 together. However, this is rather complex because the two AC switchboards then need to be synchronised. An alternative solution to reduce energy usage is desirable.

An example of an energy management system according to the present invention, for a twin-screw vessel, is illustrated in Fig.2. For each propeller 33 , the main engines 31 provide power to turn the shaft 32 and a motor/generator 34, which may be a permanent magnet motor shaft generator, mounted to the shaft 32, allows excess power to be taken off and used to supply a converter unit 39, or additional power to be applied, as required, from the converter unit, or from the main switchboard 41 which is coupled to the converter unit by a transformer 40. Alternatively, the motor/generator unit 34 may be connected to the shaft 32 via gearing, or directly connected, so that energy can be given to or taken from the propulsion system. A motor operated circuit breaker 35, 36 on each side of the motor/generator 34 connects the motor/generator to the winding star point 37 and AC line 38 respectively. In the event of a fault on the motor, all the windings of the motor must be isolated from one another, as the propeller may continue to turn and generate power otherwise. The power from each main engine may be as much as 10 MW and it is desirable to run the engines with substantially constant load. The shaft rotates at a relatively slow speed, typically 70 to 80 rpm.

Within the converter unit 39, there are effectively two sub-units, each of which include a DC bus 44, 45. Each DC bus is provided with AC to DC converters between the DC bus and the AC line 38 and DC to AC converters 43, 46, typically with a breaking chopper. The number of converters depends upon the actual load. Loads, such as AC motors 55, 51 are coupled to the DC bus 44, 45 through fuses and galvanic isolation 49. An energy storage system 53a, 53b is provided in each sub-unit for the port and starboard side propulsion units. The energy storage is coupled to the DC bus via fuses and galvanic isolation 49 and a DC to DC converter 52, in this example with a choke inductor for a chopper of the converter. The chopper controls the energy being fed back and forth to the energy storage devices 53a, 53b. A switch 54 provides galvanic isolation between the converter and the energy storage devices 53a, 53bs.

Examples of energy storage devices that may be used include chemical energy storage, such as batteries, flywheels, or capacitors, including supercapacitors or ultracapacitors. The energy storage will have the capacity of the shaft generator for a short time depending on the actual project need. The use of capacitors means that excess energy is stored without a chemical reaction, which would be the case with conventional battery storage, thus allowing faster charging and discharging to cope with the variation in power demand as the vessel moves through the water. The use of batteries allows significant cost reduction. Flywheels have a good lifetime and relatively low maintenance requirement.

The two sub-units of the converter unit 39 are coupled together by a switching device, or breaker, which is also provided with motor-controlled isolators 57 at each terminal of the switching device. More detail of the switching device can be seen in Fig.3. The breaker comprises a semiconductor device 61, such as an insulated gate bipolar transistor, and a diode 62. Back to back pairs of semiconductor device 61 and diode 62 allow for current flow in either direction when the transistors are conducting. Current flow is prevented if either of the transistors is non-conducting. The motor- controlled isolators 57 provide galvanic isolation after the current flow has been stopped by one or other transistor 61 becoming non-conducting, which may occur if there is a fault in the sub-unit on the side of the converter unit 39 closest to that transistor. There may be additional inductance (not shown) added to slow current rise, or the natural inductance in the components of the switching device may perform that function.

The energy management system may be provided with a controller (not shown), for example in the main switchgear 41 to manage ship service power and the auxiliary generators (not shown) which are connected to the AC buses. The AC power on the AC switchgear 41 is coupled through the transformer unit 40 to the converter unit 39. Galvanic isolation of the AC bus 41 and transformer 40 is provided by switches 42.

In operation, the main engines usually keep running and the auxiliary supply on the AC bus, as well ss the consumers in the converter unit allow peaks and troughs of propulsion demand to be smoothed out. This reduces fuel consumption on the vessel, as compared with having to adjust the power output of the main engine. By connecting the two propulsion systems via the DC bus of the converter units, the need for synchronisation of the AC buses is avoided. In addition, there is a reduction in the losses from transforming between DC and AC which the option of connecting at the AC bus requires in both directions.

The energy management system operates in a number of different modes. The energy storage devices 53a, 53b may be used to boost energy to the main shafts 32, or to offload energy from the main shafts, to ensure stable operation for the main engines 31 when moving through the sea. When more propulsion power is needed than that which is available from the main engine, the motor/generator units 34 act as a motor with the required electrical power being generated by the auxiliary engines supplying the main switchboards 41. When there is more energy available from the main propulsion then is needed for propulsion, the energy management system takes off power to feed one or more auxiliary switchboards (not shown), either in the converter unit, or coupled to the main switchboard.

If the main engine 31 on one side is out of service for some reason, the energy management system may use power from the other side by virtue of the connection and switching device 56 in the converter unit 39. For example, the switching device 56 allows power to the port propeller 33 to be obtained from the starboard main engine 31, or the starboard energy storage unit 53a, as well as from the port energy storage unit 53b, thereby ensuring the vessel still has some manoeuvring, or propulsion capability. The energy management system may use any of these energy sources (i.e. the other main engine, the auxiliary generators on the main switchboard, or the energy storage on either side of the vessel) to start one of the main engines 31 in case of failure of the primary start system, or in case of a crash stop having put an engine out of use. Further fuel efficiencies can be found in better use of the auxiliary generators, i.e. only running those when needed and shutting them down or starting them up as the demand changes, rather than leaving them running on low load. The energy storage devices 53a, 53b may also assist in a transient startup sequence for heavy start consumers.

Fig.4 is a flow diagram of a method of operating a power control system of the present invention. At intervals, the power requirement of the propulsion system is determined 70. Within the converter unit 39, the state of charge of the energy storage 53a, 53b may be monitored 71. Additionally, the power requirements of consumers on the DC bus and AC bus may be monitored 72. If power required for propulsion is less than power generated by operating the main engines, then any additional power available may be diverted 73 to an energy storage unit, or consumer. A determination 74 is made of which system or consumer type that has an unmet power requirement takes priority, then that system or consumer receives available power. To supply the energy storage or consumers, AC voltage generated from the motor/generator unit, whether directly connected to the prime mover shaft, mounted on the propeller shaft, or connected via a gearbox, is converted to DC voltage, or if the excess is from the auxiliary generators on the AC bus, this is converted to DC for consumers or energy storage on the DC bus, or supplied directly from the AC bus, for consumers on the AC bus.

If the power required for propulsion exceeds 75 the power generated by the main engines, then additional power may be supplied from one or more alternative energy sources. Availability of these sources is determined 76, which may include determining which consumers can be allocated a reduced power or shut down temporarily. The alternative power sources may be power extracted from the energy storage 53a, 53b, or supplied from the auxiliary generators via the main switchboard 41, or if one engine has failed, then the other engine can share its power through the DC bus connection to enable both propellers to operate to manoeuvre the vessel, or allow it to make progress at a lower speed. A combination of power sources may be used in the event of one main engine failing. Monitoring of the status of all supply and demand continues 77.

In normal use, the operation of the main engine is more efficient if it can be kept generating at a substantially constant rate, typically a little below full power. The main engines 31 generate a steady output, typically chosen to be sufficient to provide the actual average power required and the peaks and trough in demand are handled by the energy storage 53a, 53b storing energy generated by the prime mover which exceeds the instantaneous required power and supplying energy to the shaft 32 when the instantaneous power demand exceeds the power generated by the prime mover. Operating the system of the present invention may reduce fuel consumption and emissions; reduce wear and tear on the equipment and so reduce maintenance and down-time, increasing up-time and performance. A vessel set up to operate in this way may need less installed engine power, giving direct and indirect cost savings, as well as reducing weight and space requirements. The interconnections of energy storage and auxiliary generators and the main engines improve safety by maintaining propulsion, for at least a short time, in case of the loss of the main engine.