Towards the optimal operation of a thermal-recharging float in the ocean

A large number of profiling floats are currently in use around the world oceans to monitor oceanographic conditions. Generally, floats are in motion by the buoyancy force, and missions of data collection, storage, and communication are carried out relying on Lithium-ion batteries. Chao et al. developed the TREC float which is operated on batteries rechargeable from the ocean thermocline. It uses a patented TREC (Thermal RECharging) engine which is a thermo-mechano-electric converter utilizing a Phase Change Material (PCM) system for energy harvesting. Data are desired from a target area, but the floats have lacked a way to stay on target, carried away from it by ocean currents. An alternative solution is proposed in the present study combining a profile float (the TREC) with small thrusters to allow for location correction. The main purpose of this study is to solve an optimal energy allocation problem through the numerical analysis of the nonlinear correlation between energy harvesting and consumption. A powered TREC is designed by sizing a feasible thruster to translate back to the target area. A parameter study on energy allocation is carried out with various types of translation motion (vertical, horizontal, and rotational for Options V, H, and R), and the geometry of the float (spherical and cylindrical end shapes for Options S, and C). Option R is the most promising candidate under the design requirements, though it must be noted that the location correction phase takes much longer due to the rotation time.