Thermal Energy Source for an Unmanned Underwater Vehicle

Abstract

Mission critical requirements of a low-cost recoverable unmanned-underwater vehicle (UUV) include range, stealth and maneuverability. In response to this shortlist of requirements, a thermal energy source can be used to power a UUV for medium length missions and with low acoustic emissions. A number of these systems have been under study at Honeywell. Showing promise is a system consisting of an electrically heated ceramic matrix block which powers an argon charged closed Brayton cycle (CBC) engine. BACKGROUND Torpedoes and other unmanned underwater vehicles have been powered by a number of means, the more popular choices being monopropellant fueled Otto cycle engines and battery powered electric motors. Crucial to the control and stealth of these vehicles are issues of range, speed, exhaust gases, operating noise, mass and buoyancy, center-of-gravity migration, depth sensitivity, logistics, handling safety, and reusability. In order to deal with these issues, a new technology is examined which offers superior stealth when compared to Otto engines (conventional torpedo), and has superior range when compared to a battery system. For a point design, the 21 inch diameter reusable LMRS vehicle was chosen along with its low mission speed of 8 knots. For the engine, a closed-cycle turbocompressor was chosen. Heat for the engine is provided by a heated thermal block. Heat is rejected through the skin of the UUV to the ambient seawater. The turbine drives an alternator, which in turn drives an electric motor driven propulsor. A schematic of the UUV is shown below. This system deals well with the above issues in that it offers constant mass and buoyancy, is depth insensitive, its only physical emission is heated seawater, it has no harmful chemicals, and it has limited moving parts (minimized vibration sources and reliability risks). • Senior Principal Engineer, Honeywell Aerospace, ESS, Tempe, AZ Heat Storage Engine Figure 1. UUV with Thermal Engine Before narrowing the cycle selection down to a specific form, general features of a closed-cycle turbine engine fitted into a UUV are described. The fluid, whether singleor two-phase, is sealed in. The only external connections for the engine are electrical. Power from the turbine is routed through an alternator, an electrical converter, a brushless dc motor, and finally to the propulsor. The alternator is cooled by the working fluid. The thermal mass is heated by “induction furnace” style heater coils prior to launch. The field from these coils remotely heats the conductive thermal mass (via eddy-currents). These coils are cooled by seawater passing through convection ports on the top and bottom of the UUV skin. The thermal mass is well insulated in order to provide extended standby time, but must be heated in advance of the UUV mission. The engine has minimal moving parts. These consist of the turbo-compressor rotating group, a mixing valve, a shutoff valve, and a check valve. Power is enabled and modulated via the shutoff valve, or a fluid inventory control. A passive mixing valve is required to temper the extremely hot fluid leaving the thermal mass, down to a moderate temperature – for use by the turbine, by mixing in unheated fluid. DISCUSSION In order to compare and contrast the value of the argon charged Brayton cycle, an H2O Rankine cycle is first examined. The Rankine cycle involves gaseous and liquid H2O and utilizes a positive displacement pump and a gas turbine. The cycle components also include the heater and a skin cooled condenser. Thermal Mass Heater Pressure Compensated Pump accumulator