Growing world trade, stiffening emissions requirements, and rising fuel prices are driving the marine industry to improve fuel utilization in large vessels. Exhaust steam boilers are a popular means of capturing energy from ship engine exhaust. The steam is used in various systems around the ship from laundry services to the fresh water generator. However, in most cases the supply outstrips the demand, and energy from un-used steam is dumped into the ocean.
Calnetix Technologies is developing a marine based Organic Rankine Cycle (ORC) designed to produce 350 kW gross power from un-used ship steam. The ORC will operate with sat urated steam at 8 Bar G and 175 °C and cooling sea water at 25 °C. The additional power will be used to offset the fuel demand from onboard diesel generators. This results in a significant fuel savings as well as lowered emissions.
The ORC is comprised of a closed loop system with R245fa refrigerant as the working fluid. A pump is used to move the working fluid around the loop and increase its pressure into the evaporator. The high pressure refrigerant is vaporized using heat from the ship’s steam. The refrigerant then passes through a radial turbine where work is extracted and converted into electricity. Low pressure refrigerant is cooled and condensed by sea water, and then pumped back around the loop.
This paper will focus on the design of the ORC’s integrated power module which is comprised of a high speed radial turbine, variable speed motor/generator, active magnetic bearings, and backup bearings. A high frequency, bidirectional inverter is used to operate the radial turbine at high speed (25,000 RPM.) This allows the turbine to reach an isentropic efficiency of 0.88. The PM bias active magnetic bearing system allows the turbine/rotor combination to operate without the need for lubricating oil and with minimal friction. This reduces the required maintenance and drastically improves the life span of the unit compared to using conventional bearings.
An over-hung turbine design was chosen due to the high operating temperatures of the cycle. Cooling of the stator will be accomplished using a cooling water jacket. Cooling of the rotor will be accomplished by a cooled gas stream generated from a throttling process of the IPM inlet fluid and heat rejection via a heat exchanger.
Various analyses will be presented including calculation of the turbine isentropic efficiency, rotor/stator thermal analysis, component stress analysis, and linear system dynamics for the rotor and magnetic bearing system.