Design and Simulation of a Shape Memory Alloy Buoyancy Heat Engine in the Low Latitude Thermocline

摘要

Recent developments in Autonomous Underwater Vehicles (AUVs) have highlighted, among others, interest in long range and endurance. One of the technologies of interest for propulsion of long range, high endurance AUVs has been buoyancy engines. The buoyancy engines on contemporary underwater "gliders" require an active energy input to do useful mechanical work, typically using battery powered pumps to change the vehicle buoyancy. The gliders contain a wet and dry volume and change overall buoyancy by transferring low density oil into and out of a bladder in the wet volume. This battery powered design has been implemented on configurations such as Slocum and Spray. However, increase in glider weight, complexity, and control requirement associated with the implementation of such systems places a limit on their benefits. An innovative concept for a passive buoyancy engine was presented by the authors in previous work. This concept exploited oceanic thermoclines (the difference between the higher ocean surface temperatures and the lower temperatures at depth) to drive a Shape Memory Alloy (SMA) based Buoyancy Heat Engine (BHE). The unique ability of SMAs to recover strain when heated was leveraged to produce a buoyancy change related to temperature. Prior work determined that the early SMA-BHE concept could not resist the hydrostatic pressure in the oceanic thermocline. The dynamic study showed that significant changes must be made to the design specifications to make the SMA-BHE feasible for ocean operation. This paper focuses on designing an environmental engine based on shape memory alloys to operate within the oceanic thermocline that satisfies the constraints placed on an SMA-BHE in the thermocline. The design was optimized with respect to its buoyancy change per unit engine mass by way of a parametric study. A dynamic analysis demonstrated the oceanic operation of the SMA-BHE within the low latitude thermocline.