Gas Turbine Engine Exhaust Waste Heat Recovery Navy Shipboard Module Development

by Francis A. Di Bella
Concepts ETI, Inc., d.b.a. Concepts NREC

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CConcepts NREC (Wilder, VT, and Woburn, MA) is collaborating with the Maine Maritime Academy (Castine, ME) and their principal consultant, Thermoelectric Power Systems LLC, to address a need as expressed in a recent Small Business Research Innovation (SBIR) request for proposal offered by the Navy for an increase in the power output from the prime mover propulsion systems aboard naval vessels. A power recovery system must be able to improve the power output of the prime mover by 20% while also considering the effects that transient power demand from the prime mover have 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 complexity of using steam heat recovery systems, as well as the lower expected cycle efficiencies, temperature limitations, toxicity, material compatibilities, and/or costs of organic fluids in Rankine cycle power systems, precludes their consideration as a solution to power improvement for this application in naval vessels. The power improvement system must also comply with the space constraints inherent with onboard marine vessel power plants, as well as the interest to be economical with respect to the cost of the power recovery system compared to the fuel that can be saved per naval exercise.

Concepts NREC (CN) will perform a feasibility analysis on a Brayton cycle-based, supercritical carbon dioxide (S-CO2) system to recover waste heat from a Rolls-Royce MT-30 gas turbine used in marine applications. The analysis will also consider the integration of one or more thermoelectric generator (TEG) systems within the S-CO2 cycle in order to further increase the power recovery. The use of an auxiliary combustion system to provide thermal stability within the power recovery system during the transient power demands, required of the vessel’s prime mover propulsion system, will also be considered. The use of TEG systems within the heat engine bottoming cycle takes advantage of the temperature differences between the cycle components that are a consequence of the second law of thermodynamics. The TEG systems use this temperature difference to generate electric power directly and without “moving parts” and effectively increase the cycle efficiency to almost the limit of the Carnot Efficiency for the S-CO2 heat engine cycle. CN’s preliminary feasibility analysis has indicated a power improvement over the MT-30 gas turbine engine of from 20%, for the simple S-CO2 waste heat recovery system, to as high as 24% when the TEG systems are included. The supercritical CO2 cycle has been promoted in several Department of Energy (DOE) project studies as an efficient prime mover system using high temperature heat sources as may be available, for example, from nuclear energy as the heat source. A waste heat recovery application of the CO2 supercritical cycle that will be prepared by CN integrates the TEG work done by Maine Maritime Academy (MMA) and will culminate in the sizing of the major components, concluding with an engineering cost analysis to determine the Simple Payback for the system and ultimate cost per kWe ($/kWe).