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.

Concepts NREC will use a Brayton cycle-based S-CO2 system to recoverwaste heat from a gas turbine. This graph shows a typical supercriticalCO2 heat engine process using the Brayton cycle.
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.

Rolls-Royce MT-30 Gas Turbine
The TEG systems use this temperature difference to directly generate electric power and thus, effectively increase the cycle efficiency closer to the Carnot Efficiency for the S-CO2 heat engine cycle. An advantage of the TEG is that there are no moving parts, so operation and maintenance costs are negligible while providing reliable power, even if only from the alternative-fueled combustion system, and if the S-CO2 system is not available.

The Phase 1 feasibility analysis revealed a power improvement over the MT-30 gas turbine engine of 20%, with only the baseline, simple S-CO2 waste heat recovery system, to as high as 27% when as many as four TEG systems are included in various places within the S-CO2 cycle.

CN’s efforts in Phase 1 will focus on the thermodynamic modeling of the S-CO2 cycle and conducting a feasibility analysis of an advanced design for a TEG-heat exchanger that integrates TEG modules with sufficient surface area for the necessary heat transfer required to benefit the S-CO2 cycle. An assessment of benefits to using TEG systems with the S-CO2 cycle and an auxiliary-fueled combustion system to maintain thermal stability and provide auxiliary (boost) power will be included. Also detailed will be the conceptualized design of an S-CO2 compressor, turbine, and generator using a single drive shaft in a compact, modular package that reduces bearings, seals, and gearbox drives, reduces operation and maintenance costs, and facilitates installation in the confined spaces of the engine room. An auxiliary combustion system adds heat energy input to the cycle if the prime mover is either unavailable or at a part-load demand, and enables the supercritical heat exchanger to maintain relatively constant pressure and material temperatures, thus minimizing thermal or mechanical fatigue. Bioderived fuels for the auxiliary combustion system will also be considered to further increase economic viability.

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This article first appeared in the October, 2011 issue of Defense Tech Briefs Magazine.

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