# Investigation of Self-Driven Profiler with Buoyancy Adjusting System towards Ocean Thermal Energy
## 1 Introduction  
Oceans account for 70% of the earth's surface area with abundant biological resources and mineral resources, so the exploration of the ocean is very important. The underwater thermal profiler, which can be used to detect the ocean, is a kind of autonomous vehicle that moves up and down between the sea surface and deep sea. It can harvest OTE through the utilization of phase change materials (PCMs) and convert OTE into mechanical energy for a buoyancy-driven system. When phase change material melts, it expands in volume and does work; therefore, the study of phase change rate is essential. A mathematical model aimed at analyzing the characteristics of the volumetric change rate as well as the freezing and melting times of the mixed PCMs is proposed, and calculation methods of phase change time are given [1]. Similarly, an analytical solution is developed to study volumetric heat generation effects during the melting and solidification of nano-enhanced PCMs that are encapsulated in a horizontal cylindrical container [2]. [PERSON] et al. proposed an improved Nusselt boundary layer model to describe the contact melting process of PCM in a rectangular cavity. The melting process is recorded by a camera, and results showed that melting is fastest when the container is tilted at 60 degrees [3]. Research proves that adding materials with high thermal conductivity and specific surface area could increase the rate of thermal charging and discharging. [PERSON] et al. studied two enhancement materials, namely, aluminum and graphite foams, saturated with PCM, and results showed that the thermal charging enhancement of graphite foams is superior to that of aluminum foams [4]. [PERSON] et al. prepared NPCM using paraffin as the PCM and high conductive MWCNT as thenanomaterial. Lessening in melting and solidification time of 30% and 43% was attained in the case of NPCM with 0.3% and 0.9% in experiments, respectively [5]. Based on the principle of work done by melting of PCM, some underwater observation platforms have been developed. The first to use this principle is the American Webb Research Corporation (WRC), which first developed the thermally driven glider Slocum. The first-generation Slocum thermal glider platform is equipped with three temperature difference energy capture heat engines, using PCM as the energy capture working medium. The experiment was carried out in the Seneca lake area of the United States in 1998. The temperature difference between the lake surface and the bottom of the lake was 13 \({}^{\circ}\)C, the maximum diving depth was 125 m, and the heat exchange efficiency was about 3%. This test verified that the underwater mobile observation platform can capture the temperature difference energy, and the feasibility of doing work [6]. After that, WRC successively produced the second- and third-generation thermal glider platforms, which only use the captured temperature difference energy to drive the buoyancy of the underwater mobile observation platform. In 2013, WRC developed the fourth-generation Slocum thermal glider, which not only uses ocean thermal energy for buoyancy driving, but also converts it into electrical energy. Therefore, its observation range and frequency are higher than those of the previous three generations, and more frequent profile observations can be carried out [7]. The Scripps Institute of Oceanography in the United States developed an underwater mobile observation platform entitled SOLO-TREC, which is equipped with 10 thermal energy capture heat engines. The captured ocean thermal energy is stored in the accumulator in the form of high-voltage potential energy. In addition to being used for buoyancy drive, it can also be converted into electrical energy by a hydraulic motor [8; 9]. The American Seatreac company designed a set of temperature difference energy capture and power generation system, SL1, to supplement the energy required for the buoyancy adjustment of the Navis series profiler.