Hermetic 40-kW-Class Steam Turbine System for the Bottoming Cycle of Internal Combustion Engines

An entirely new 40-kW-class steam turbine prototype has been constructed and successfully tested for more than 6,500 hours. The two-stage Curtis wheel with a nominal pressure ratio of about 130 has been designed for applications in bottoming water/steam cycles. The axial turbine runs at a rated rotational speed of 36,000 rpm, utilizing magnetic bearings and a permanent magnet synchronous generator which is coupled to the grid by frequency inverters. The integral turbine-generator set has been designed as a hermetically sealed assembly group. The turbine design allows both the turbine’s oil supply system and the bottoming cycle’s feed-water treatment system to be eliminated. The turbine has been designed to allow unmanned operation of the entire cycle with minimum maintenance requirements and reduced costs. Extensive turbine testing, including rated power, overload and load rejection tests was carried out to verify functionality. Long-term operational capability was also demonstrated, giving particular attention to generator performance. Exemplarily a bottoming cycle, utilizing the turbine, matching the exhaust conditions of internal combustion engines was designed, increasing electrical efficiency from 40.4% to 43.4%, which represents a relative net improvement of 7.5%. In this application a turbine isentropic efficiency of more than 55% is expected, based on the congruence of measurement and calculation.