Thermal characteristics analysis and waste heat utilization of the buoyancy engine in hadal class unmanned underwater vehicles

• The waste heat of the buoyancy engine is verified to have effective utilization value. • An efficiency model of the buoyancy engine is established considering the temperature and pressure. • Thermal models of the buoyancy engine are established by analyzing the heat transfer path. • A thermal energy recovery scheme is proposed and optimized. • The accuracy of the established model and the effectiveness of the scheme are verified by experiments. During the underwater operation of unmanned underwater vehicles (UUVs), the discharge efficiency of the onboard lithium batteries is significantly reduced by the low-temperature environment, which further decreases the energy utilization rate, especially for hadal class UUVs. This study aimed to analyze the thermal characteristics of the buoyancy engine, the most significant energy-consuming unit in UUVs, and explore the potential for waste heat recovery. Based on the buoyancy engine of the Petrel-X hadal-class underwater glider, we established models for component power loss, system efficiency, and multi-node lumped parameters, which were validated through experimental data. The analysis revealed that the total efficiency of the piston system was 46.16 %, with over half of the energy consumed dissipating as heat. The motor of the piston pump exhibited the highest temperature rise, accounting for approximately 34.5 % of the total power for heat generation. Building on these findings, a waste heat recovery scheme suitable for hadal class UUVs was proposed for the first time, which utilized heat pipes as the phase change heat transfer element. In the verification experiment, the surface temperature of the piston pump motor decreased by approximately 9.63 °C, and the air temperature around the lithium batteries rose by 3.18 °C, corresponding to a 7 % improvement in discharge efficiency, confirming the effective utilization of waste heat from the buoyancy engine. This study provides a novel approach to improving the energy efficiency of hadal class UUVs.