Prospects for Marine Reciprocating Engines to Operate on Stored Heat
By Harry Valentine
An earlier article focused on the prospects of using stored heat to energize a closed air/gas turbine engine driving electrical generating equipment. Several sectors of the commercial marine transport industry prefer to use direct-drive between the engine and the propeller. Research undertaken by Quasiturbine Engines of Montreal revolved around a Brayton-cycle engine using a positive-displacement air compressor system and a positive-displacement rotary engine, from Quasiturbine. While the research focused on an internal combustion concept, there may be potential to adapt the concept to operate on an external heat source.
The heat would be produced in a reaction chamber onboard ship, where 2-compounds such as a metallic oxide and carbon dioxide would combine to form a metallic carbonate and release much heat in the process. That heat would provide ship propulsion, via a Brayton-cycle engine. The controlled thermal decomposition of the carbonate would occur off the ship and possibly away from port, at a specialized facility that would be part of a high-temperature nuclear fission or radiation-free nuclear fusion thermal power facility. Such an energy system may become competitive in the future as a result of increasing oil prices.
Much of the hardware needed to build an externally heated, closed-cycle, positive-displacement Brayton-cycle power system already exists in various sectors of the energy and marine propulsive industries. Closed-cycle operation will allow for higher inlet pressure into the low-pressure compressor and subsequently allow for a more compact engine system to deliver higher specific output. There may be scope to adapt compressors from natural gas industry to operate in a Brayton-cycle system, using modified valve systems that may adjust to changing demands of pressure and flow-rate. Compressed air leaving the low-pressure compressor would pass through a separator to swirl out small amounts of lubricating oil that may enter the air stream. That oil would return to the compressor.
The heated air would then be cooled by seawater in a heat exchanger that would serve as an intercooler, before entering the high-pressure compressor. Heated high-pressure air would pass through a separator to swirl out lubricating oil, then through a one-way valve before entering a gas storage tank or accumulator from the natural gas industry. The engine control system would regulate pressure in the tank and adjust the valves of the low and high-pressure compressors accordingly. Compressed air leaving the storage tank would be heated by exhaust heat in a recuperative heat exchanger, then further heated in a reaction chamber before entering the multistage engine system. The heat exchangers in the reaction chamber may be adapted from the high-temperate nuclear power industry.
There may be scope to adapt existing marine piston engines to operate on externally heated compressed air, by modified the engine inlet valve systems to operate in a manner similar to the inlet valves on compressed air powered mining locomotives and steam locomotives of an earlier era. These inlet valves could adjust according to the angle of the crankshaft, admitting compressed air or high-pressure steam over the range of between 20% and 80% of cylinder volume. Adjusting the cut-off ratio of the inlet valves allowed operators to adjust engine power output. The same system can be adapted to a modern engine.
Air leaving the high-pressure engine would swirl through a separator before being reheated and expanded in the low-pressure engine. To save space, the high and low-pressure compressors may be built into a V-configuration, as could the high and low-pressure piston engines. Air leaving the low-pressure engine would swirl through a separator before being cooled by incoming compressed air in the recuperative heat exchanger. Final cooling of the air would occur in a seawater-cooled heat exchanger, from where the pressurized air would re-enter the system, into the low-pressure compressor. There would be the option of passing the exhaust air through a turbo-charger, then through a cooling system before the turbo-compressor pushes the air through an intercooler and into the low-pressure piston compressor.
The externally heated engines would drive both the compressors as well as the propeller. A starter driven by compressed air would start a stationery engine and the valve systems on both the compressors and engines may be modified to operate bi-directionally, that is offer both clockwise and anti-clockwise rotational capability on the crankshafts and propeller. Compared to the externally heated air turbine engine that drives electrical generating equipment to power electrically driven propellers, a direct drive system could bypass the electrical transmission losses that reduce efficiency to between 85% and 90%. A direct-drive system that may offer extended operating range.
While Quasiturbine of Montreal undertook original research into adapting their positive-displacement rotary engine to Brayton-cycle operation, Quasiturbine may need to adapt their engine to bi-directional rotation operation that is required in some marine propulsive applications. Such capability would give the compact Quasiturbine engine with its high specific power output, application in commercial marine transportation. It may be able to operate using 3-stage expansion with inter-stage reheat, to raise overall thermal efficiency. There may be scope to include variable adjustment to the radial distance of the inlet ports of the high-pressure engine to provide the equivalent of a cut-off ratio. The engine could operate at higher inlet pressure and higher efficiency.
Advances have occurred in high-temperature nuclear power conversion that provides opportunity to develop chemical thermal storage technology for a variety of applications, including short-distance marine propulsion. Ships powered by such technology may become viable on short routes in regions where future oil prices may be high, or oil resources restrained. There are several suitable routes around the Baltic and Mediterranean Seas as well as in the region around and near the South China.
About the Author
Mr. Valentine holds a degree in mechanical engineering from Carleton University, Ottawa, Canada, with specialization in thermodynamics (energy conversion) and transportation technology.
He served as a research assistant to Dr Ata Khan, professor of transportation engineering who is still on staff at Carleton University. Mr. Valentine has a background in free-market economics and has worked as a technical journalist for the past 10-years in the energy and transportation industries.
Over a period of 20 years he has undertaken extensive research, authored and published numerous technical articles in the field of transportation energy. His economics commentaries have included several articles on issues that pertain to electric power generation.
Mr. Valentine has technical journalistic experience covering low-grade and high-grade geothermal energy, steam generators (with continuous blow down to keep the boiler water clean), engine exhausts, solar thermal (low-grade and low-grade thermal), nuclear and coal-fired thermal steam-power stations. He can be reached at [email protected].
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The opinions expressed herein are the author's and not necessarily those of The Maritime Executive.