Aeroderivative turbines in ship's engine room (Photo credit: GE Power)
Re-evaluating gas turbine engines for future maritime propulsion
By Harry Valentine
The combination of advances in
engine management technology, in the design of heat
exchange technology and the chemistry of high-temperature ceramic materials and compounds provide the basis of enhancing the efficiency and market competitiveness of open-cycle and closed- cycle gas turbine engines, including in ship propulsion.
Introduction Traditional open-cycle gas turbine engines flow atmospheric air through a compressor, combustion chamber and power turbine. Peak fuel efficiency occurs when operating at maximum power output while the turbine spins at maximum RPM and maximum combustion temperature. Engine efficiency decreases as power output decreases, with half the efficiency available at 25% of output for a single- shaft engine. Externally heated closed- cycle engines continuously recirculate the same gas through compressors and turbine can deliver high efficiency at 25% of power output. However, the materials of which heat exchangers were made incurred temperature restrictions that restricted both peak efficiency and peak power output.
The complex-cycle gas turbine engine was an open-cycle, triple-shaft engine that combined 2-compressors with 3-turbines, 2-combustion chambers and 2-heat exchangers. Both high-pressure turbine and power turbine spun on separate shafts and had their own combustion chamber. During an earlier era, it
was difficult to accurately control air-fuel ratios manually for each combustion chamber that operated in series, upstream and downstream of each other. As a result, in real world operation the complex-cycle gas turbine rarely delivered peak efficiency over a range of power output. However, modern technology offers possibility to both the complex-cycle and closed-cycle gas turbine engines.
Modern advances
Modern reciprocating internal combustion engines operate with such technology as mass-flow rate sensors, air-fuel ratio sensors, air temperature sensors and computer managed fuel injection that have greatly improved fuel efficiency. There is theoretical potential to adapt such technology to the old classical complex-cycle gas turbine engine, along with the possibility of introducing new generation heat exchanger technology that operates at much higher effectiveness that earlier generation technology. Modern annular counter-flow heat exchangers developed by
THE REPORT | DEC 2024 | ISSUE 110 | 67
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