## 1 Introduction  
The South China Sea is vast and located south of the strong solar radiation zone of the Tropic of Cancer, with abundant temperature-difference energy reserves [1, 2]. Advanced scientific instruments and monitoring devices are indispensable for exploring the ocean and obtaining on-site observation data and information [3, 4, 5]. Therefore, the research and design of effective ocean monitoring devices are of great significance [6, 7, 8]. Among them, the vertical-lifting underwater monitoring device is one of the important tools for studying and exploring the ocean. Currently, the energy forms required by vertical-lifting underwater monitoring devices mainly fall into two categories: one is that all the energy consumption is provided by battery packs; the other is that part of the energy is provided by battery packs and the other part comes from ocean temperature-difference energy [9, 10, 11]. Usually, the energy consumption of the buoyancy drive system comes from ocean temperature-difference energy, but the power for the control system, communication, and observation equipment is still provided by the battery packs carried by the underwater monitoring device [12; 13; 14]. Due to the limitation of battery pack capacity, the operation time of traditional vertical-lifting underwater monitoring devices is restricted, making it difficult to perform long-term remote operations [15; 16]. This paper explores a new type of underwater monitoring device that is driven by ocean thermal energy and operates vertically. The key difference from traditional vertical underwater monitoring devices is that the energy required by this device comes entirely from ocean thermal energy. Ocean thermal energy is inexhaustible and unlimited, and it no longer depends on the capacity of batteries. Therefore, this device can significantly increase its operational cycle and working area [17; 18]. The device absorbs ocean temperature-difference energy through a solid-liquid-phase-change heat exchanger and converts part of the absorbed energy into mechanical energy to meet the mechanical energy requirements of the buoyancy drive system [19; 20]. Meanwhile, part of the pressure energy is converted into electrical energy by driving a small-displacement hydraulic motor to rotate a micro-generator, providing power for the observation, communication, and control instruments of the underwater monitoring device [21]. References [22; 23; 24] indicate that the marine autonomous monitoring device based on thermal energy utilization absorbs thermal energy from the surface warm water layer of the ocean through reciprocating motion between the thermal boundary layers of the ocean, and releases thermal energy to the deeper cold water layer. The vertical ascending and descending underwater monitoring device based on thermal energy-driven proposed in this paper operates by reciprocating motion at different depths. During this process of heat storage and release, the volume changes due to the solid liquid phase change of the phase change material in the phase change heater, thereby altering the buoyancy of the underwater monitoring device and enabling its upward and downward movement. In this paper, the Fluent software is used to simulate the melting/solidification process of the phase change material by the enthalpy-porosity method [25; 26; 27; 28; 29]. This method introduces a quantity called the liquid fraction to represent the proportion of liquid material in the entire control volume [30; 31; 32; 33; 34]. Additionally, an effective thermal conductivity is introduced in this paper to represent the influence of natural convection in the liquid phase during the melting process on the phase change process [35; 36; 37]. Under the premise of ensuring a certain accuracy, this greatly simplifies the calculation and analysis process.