pressure at 5-times atmospheric pressure (5-ATM) would deliver 8,000-horsepower at comparable thermal efficiency with mean system pressure reduced to 1-ATM. During operation, some exhaust heat will be reintroduced into the engine downstream of the high-pressure compressor. The combination of heat rejected from the low-pressure compressor inter-cooler and residual exhaust heat would partially sustain the operation of a bottom-cycle steam engine. Future development of high-temperature heat exchangers made from compounds such as highly purified boron arsenide with high thermal conductivity promise to assure engine efficiency.
Stored Thermal Energy Solar thermal power stations use stored thermal energy to generate several hours of electric power after sunset. The thermal storage medium is low-cost salt compounds of sodium and potassium that melt at sufficiently high temperature to generate steam. Advances in high- temperature ceramic chemistry have produced corrosion-resistant compounds that maintain constant mechanical properties up to 1400-degrees C (2550 F). Open-cycle gas turbine engines require heated air or gas to enter the power turbine at temperatures to 1200-degrees C, while single-shaft, closed-cycle turbines with single compressor and single turbine can operate with air heated to 900-degrees C.
Sodium fluoride (NaF) that historically was a waste byproduct of the aluminium smelting industry, melts at just under 1,000-degrees C (1800 F) with heat-of-fusion of 300- BTU/pound. The aluminium industry mixes the mineral cryolite (Na3AlF6) from Iceland and Greenland with bauxite (Al2O3) to produce aluminium. A mixture of aluminium oxide (Al2O3) and cryolite can melt at under 1200-degrees C (2,200 F) with heat-of-fusion at some 400-BTU/pound, with usable life expectancy of 1-million deep- drain cycles. Future research offers the possibility of closed-cycle gas turbine engines operating from stored thermal energy in both stationary land-based applications and in future maritime propulsion.
Thermal Recharge
Much advancement has occurred over the past decade in the development of micro and mini- nuclear power conversion. One future possibility would involve a micro or mini reactor being brought to quayside on a railway carriage, then transferred on board a ship for the purpose of replenishing the onboard thermal storage supply that would sustain ship propulsion for several hundred nautical miles. A fleet of ships assigned to short-sea shipping service could operate between ports located within the operating range of each ship, allowing a small number of micro or mini reactors to sustain the operation of a fleet of ships.
Conclusions
A 3-shaft open-cycle gas turbine engine using modern electronic engine management technology installed in duplicate or even triplicate, would operate efficiently and reliably over a wide range of power output, even for extended duration cycles. Likewise, a single- shaft closed-cycle gas turbine engine using modern and evolving ceramic- based, annular configuration heat- exchange technology could operate at competitive levels of efficiency using a wide range of fuels, including stored thermal energy. The exhaust heat of both types of engines would sustain the operation of a bottom-cycle steam engine, allowing the combined-cycle engine to deliver very competitive levels of thermal efficiency.
While reciprocating marine engines require massive volumes of lubricating oil, turbine engines require a fraction of the amount of lubricant to assure proper operation of engine bearings. Turbine engines also avoid the problem of internal friction and engine wear caused by piston rings sliding on cylinder walls, thereby extending usable service life. While turbine engines are compact, the heat exchangers attached to them will occupy a substantial amount of volume, which would likely be available inside the hull of a ship. The combination of gas turbine engine with steam bottom-cycle engine represents a future propulsion option for large ships.
THE REPORT | DEC 2024 | ISSUE 110 | 69
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