Gas Turbine Exhaust Recovery Design Improves Propulsion Efficiency

Power Recovery System
Concepts NREC
White River Junction, VT
802-296-2321
www.conceptsnrec.com

In January, Concepts NREC (CN) was awarded a Phase I Small Business Innovative Research (SBIR) grant from the Navy to improve the power efficiency of its gas turbine prime movers used for ship propulsion. The eight-month analytical study is in collaboration with the Maine Maritime Academy and its principal consultant, Travis Wallace, President, Thermoelectric Power Systems, LLC. The Navy’s RFP required that the power recovery system improve the power output of the prime mover by at least 20%. However, considerations included the effects that transient power demand from the prime mover has on the waste heat flow rate and temperature, which may consequently affect the fatigue integrity of the heat exchangers and stability of the turbomachinery subsystems.

The power improvement system must comply with space constraints inherent with onboard marine vessel power plants, as well as the interest in being economical. The complexity of using steam heat recovery systems precluded their consideration as a solution for this project.

CN’s winning proposal suggested the use of a Brayton cyclebased, supercritical carbon dioxide (S-CO₂) system to recover waste heat from a Rolls-Royce MT-30 gas turbine, a prime mover used in marine applications. CN also suggested the viability of integrating one or more thermoelectric generator (TEG) system(s) within the S-CO₂ cycle to further increase the power recovery, and using an auxiliary combustion system, perhaps powered by onboard, combustible bio-refuse, and waste oils. This could provide thermal stability within the power recovery system during periods of transient power demands. The S-CO₂ cycle has been promoted in several U.S. Department of Energy (DOE) project studies as an efficient prime mover system using nuclear energy as the heat source. A CN study, commissioned by Knolls Atomic Power Laboratories (KAPL), demonstrated the viability an S-CO₂ compressor and turbine design.

The Navy proposal was strengthened by the detailed description of a compressor and turbine-generator module design developed in the KAPL study.

Thermoelectric generators (TEGs) are solid state, direct energy conversion devices that generate direct current electric power via the temperature difference between hot and cold surfaces using the Seebeck Effect. TEGs are getting increased visibility as an alternative power generation system when there is waste heat available from the prime mover and a need/desire to increase the efficiency of the prime mover. The use of TEG systems within the proposed S-CO₂ bottoming cycle takes advantage of temperature differences between the cycle components.