The U.S. Navy has called for increased stamina in unmanned undersea vehicles to enable missions that can last for weeks, not just one or two days; this exceeds the energy capability of traditional battery technologies. Raytheon engineers are addressing the need for an alternative power source through the use of external combustion engines and monopropellant fuels. The team investigated a number of engine types. Particularly promising technologies included a modified Rankine cycle engine developed by Cyclone Power Technologies, Inc.
External vs. Internal Combustion Engines
More than 150 years ago, the first practical steam engine (using external combustion), built by James Watt, started the industrial revolution. The use of low pressure and temperature was the sign of the times; a new technology and lack of materials set the pace. At the turn of the 20th century, power generation brought in turbine high pressure boilers and steam-powered automobiles, and in 1906, the Stanley Steamer set a land speed record of 127 mph.
Throughout the 20th century, steam remained the dominant means of electric power generation, increasing its efficiency through the use of high temperatures, high pressures and heat regeneration. These supercritical steam power plants are now able to deliver efficiencies greater than 45 percent, competing with the best diesel internal combustion engines, and with fewer and less toxic by-products.
The Cyclone engine uses external combustion, which is very insensitive to fuel formulations or the degree of refining required to meet its performance specifications. If it can burn, the Cyclone engine can harvest the energy content. That characteristic opens up a new realm of promising possibilities.
External Combustion for Undersea Power and Propulsion
The Rankine cycle combustion process is external to the cylinder containing the working gas. The Rankine cycle is characterized by the working gas undergoing a phase change (from liquid to gas), which can be utilized to achieve high-power densities. The most familiar Rankine engine is the steam engine, where water boiled by an external heat source, expands and exerts pressure on a piston or turbine rotor, and hence, does useful work. Until now, oil was used to lubricate the moving components.
Cyclone's Schoell cycle steam engine with heat regeneration (Figure 1) is a modified Rankine cycle engine where deionized water operates in a closed cycle within the radial piston engine as both the working and lubricating fluid (blue lines). Using today's high-temperature, water-lubricated bearings and radial pistons, with meticulous attention to material compatibility, Cyclone has developed a higher efficiency, smaller, environmentally friendly external combustion engine. The combustion is external and the fuel and exhaust (red lines) are the only elements exposed to external pressure.
Three heat exchange stages (cylinder, condenser, combustion) are used to recover waste heat from the cylinders and combustion exhaust to improve overall engine efficiency. The cylinder exhaust is used to pre-heat the input air (green lines) and the working fluid prior to the main heat exchanger. The input air is also heated by the combustion exhaust.
Raytheon is employing a monopropellant developed by James R. Moden Inc. to fuel this engine, and is developing the surrounding system components to change the air-breathing, external combustion engine into an undersea vehicle propulsion system providing electrical energy for electronic control, vehicle/payload power and battery charging. Figure 2 illustrates the operation of this system.
Cyclone Power Technology has a family of external combustion engines as shown in Figure 3. Cyclone's Waste Heat Engine (WHE) recaptures heat from external sources to create steam, which powers the engine. The WHE models are designed to provide 5 to 10 kilowatts to run a grid-tied or primary electric power generator while producing zero emissions. The Solar One provides one to three kilowatts of electricity from solar steam generators. These engines can also run a grid-tied or primary electric power generator while producing zero emissions. The MK 5 produces 100 hp and is designed for automotive, marine propulsion, power generation, off-road equipment, industrial co-generation and specialty applications. Raytheon customers often have needs for remote or high-dynamic range power. Environmentally friendly, high-efficiency external combustion engines yield an intriguing alternative.
The U.S. Navy employs a number of large-diameter, large-payload undersea vehicles, and has plans to expand that fleet in the next decade. The Navy requires that these next-generation undersea vehicles have high speed and long endurance (as long as 120 days). Game-changing technologies like this are needed to address these undersea vehicle requirements.
Kevin P. Bowen