Propulsion system and method for a cable ferry

Abstract

The invention provides a flywheel-based propulsion system for cable ferries and method of operating such a propulsion system. A flywheel is energized from an on-shore source of electrical power during unloading and loading of the ferry and the flywheel is then used to generate electrical power for an electrical motor to power the ferry during the crossings.

1 . A propulsion system for a cable ferry comprising:
 i) a variable frequency electric drive and motor configured to propel said ferry along a cable;   ii) a flywheel;   iii) an electric charging motor connected to said flywheel for selectively driving said flywheel;   iv) an electric generator connected to said flywheel for selectively being driven by said flywheel to generate electrical power;   v) means for connecting said charging motor to an external source of electric power;   vi) an electrical bus for connecting said generator to said electric drive;   vii) switch means for selectively connecting and disconnecting the output of said generator to said drive motor and the external power source to said charging motor; and   viii) control means.   
   
   
       2 . The propulsion system of  claim 1  wherein said charging motor and generator are combined in a motor/generator which selectively functions as either a motor or generator. 
   
   
       3 . The propulsion system of  claim 2  wherein said motor/generator is AC. 
   
   
       4 . The propulsion system of  claim 3  further comprising a DC/AC converter electrically connected to said motor/generator. 
   
   
       5 . The propulsion system of  claim 2  comprising a plurality of flywheel-based energy storage units, each comprising said flywheel, motor/generator and DC/AC converter. 
   
   
       6 . The propulsion system of  claim 1  wherein said electrical bus is DC. 
   
   
       7 . The propulsion system of  claim 1  wherein said electric drive motor is an AC motor. 
   
   
       8 . The propulsion system of  claim 1  further comprising a variable frequency drive electrically connected to said drive motor to convert DC power from said generator to AC power. 
   
   
       9 . The propulsion system of  claim 1  further comprising an AC to DC converter to deliver power at a rate adequate to re-energize the ferry in the short time it is at the ferry terminal. 
   
   
       10 . The propulsion system of  claim 1  further comprising a back-up diesel motor and generator configured to deliver electric power to said charging motor. 
   
   
       11 . A method of operating a cable ferry, wherein said cable ferry is propelled by an electric motor and comprises a flywheel-based energy storage unit, said method comprising:
 i) charging said flywheel-based energy storage unit by connection to a source of electric power;   ii) powering the propulsion of the ferry during a ferry crossing with electric power from said charged flywheel-based energy storage unit;   iii) re-charging said flywheel-based energy storage unit by connection to a source of electric power while said car ferry is docked after said crossing;   iv) repeating steps ii) and iii).   
   
   
       12 . The method of  claim 10  further comprising the steps of providing a diesel motor-powered generator and initiating said generator to re-charge said flywheel-based energy storage unit if required during one of said crossings. 
   
   
       13 . The method of  claim 11  wherein said flywheel-based energy storage unit comprises a flywheel, an AC motor/generator and DC/AC converter.