Thermal Glider Research Articles

Numerical study on phase change propulsion mechanism of thermal underwater glider based on spectral method

The thermal underwater glider’s propulsion power is provided by the phase change energy storage tube. To improve the propulsion performance of the glider, it is essential to study the phase change heat transfer characteristics. This work proposes a collocation spectral method to handle the spatial derivative term and introduces a fourth-order Runge-Kutta method to handle the time derivative term. The temperature distribution and liquid volume fraction of the phase change material in the energy storage tube were numerically simulated under specific boundary conditions. The simulation results were then compared to the experimental data, and a good agreement was observed.Based on these findings, the impact of incoming free stream velocity and temperature difference intensity on phase change within the vessel was examined. The results underscored the dominance of convection in solid–liquid phase change and identified key factors influencing phase change in scenarios with Stefan numbers below 0.1342. Additionally, the study highlighted the limitations of fixed-value approximations and the nonlinear nature of phase change over time. These insights provide valuable guidance for developing phase change devices that harness volumetric potential energy aboard underwater thermal gliders.

Numerical and experimental investigations on thermal-to-work conversion process in thermal engine for thermal underwater gliders

Thermal engines are the most important component in underwater gliders (TUGs) for converting ocean thermal energy for propulsion. Although experimental achievements were made to establish diverse thermal engine, the insight thermal-to-work conversion process lacks detailed investigation. This paper unveils the fundamental principles of this thermal engine by establishing a numerical model describing the PCM expansion induced pressurized oil transfer process between two flexible oil bladders. The results of the numerical simulations well match the experimental test. Further, the effects of the outer wall temperature and the diameter of the thermal engine on the performance of the engine, as well as the velocities and streamlines evolution in the liquid phase PCM and pressurized oil are studied. This study finds that increasing the outer wall temperature of the thermal engine significantly reduces the melting duration of the PCM by changing the Stefan number (Ste) and Grashof number (Gr). Additionally, the velocity distribution in the liquid PCM resulted from natural convection affects the melting process and the performance of the thermal engine. In summary, this study provides important insights into the thermal-to-work conversion and is helpful in optimizing the performance of TUG.

A composite phase change material with large volume change rate for thermal underwater glider

The thermal underwater glider (TUG) is an important device currently used for exploring ocean resources and monitoring ocean temperature. The TUG harvests ocean thermal energy (OTE) through working medium and converts into net buoyancy variation for propulsion. The phase change material (PCM) filled in the thermal engine is the key working fluid for the TUG to harvest OTE. A large volume change rate of PCM at ocean temperature difference is important for improving the TUG's efficiency and performance. Previous studies focus on N-hexadecane for OTE conversion with limited performance. This paper proposes N-hexadecane (C16) based composite PCM for an improved thermal-to-mechanical conversion process. The experimental test results show that the composite PCM (C16:C24 = 9:1 wt%) exhibits a similar phase transition temperature (17.8–18.4℃) with C16 (18.2–18.6℃). As compared to pure C16, the volume shrinkage of this composite PCM was 26.67% higher than that of C16 and the volume expansion rate was 30.74% higher than that of C16, indicating that the composite PCM based TUG could output more net buoyancy variation and propulsion power. Following experimental test, molecular dynamics simulation of the composite PCM is carried out. The reason for this volume expansion/shrinkage improvement was revealed as the larger C–C bond length and H–C–C bond angle variations. The composite materials provided in this paper could potentially improve the propulsion performance of TUG.

Research on sailing range of thermal-electric hybrid propulsion underwater glider and comparative sea trial based on energy consumption

Underwater glider is becoming an increasingly important tool for exploring the ocean. As the underwater tasks become more challenging, it puts forward higher requirements for the endurance of electric underwater gliders. This paper proposes an operation principle for the underwater glider jointly driven by the ocean thermal energy and the electrical energy, which has the advantages of both electric underwater glider and thermal underwater glider. Based on the proposed operation principle, a universal energy consumption model and sailing range equation suitable for the electric, thermal and hybrid propulsion underwater gliders are established. Taking the sailing range, sailing range difference and sailing range ratio as the evaluation indexes, the effects of configuration parameters and navigation parameters on the endurance of three underwater gliders are analyzed. The results show that the endurance and endurance advantage of hybrid propulsion underwater glider can be improved with greater volume change rate of phase change material (PCM), heavier energy carrier and smaller average power of the task sensor, and its high performance can also be maintained with a weight ratio of PCM close to the critical value. Meanwhile, it is beneficial to obtaining stronger endurance for the hybrid propulsion glider during its operation by setting larger diving depth as well as smaller gliding angle and volume change of oil within the design range. Finally, the hybrid propulsion prototype and Petrel-II electric underwater glider are deployed to carry out the comparative sea trial, which verifies the correctness of sailing range equation and the availability of prototype. The results show that the hybrid propulsion underwater glider has a better energy consumption economy than the electric underwater glider.

Adaptive Digital Disturbance Rejection Controller Design for Underwater Thermal Vehicles

Underwater thermal vehicles, as ocean observation tools, are frequently affected by environment disturbances such as waves and currents, which may cause degradation of the observation accuracy of the vehicles. Consequently, it is important to design a controller for a vehicle that can resist ocean disturbance. In this study, an underwater thermal vehicle principle is introduced, and the mathematical model is established in the vertical plane motion. On this basis, an adaptive digital disturbance suppression control method is proposed. For known disturbance parameters, this controller could compensate for external disturbances by pre-setting control parameters using the internal model principle and parameterizations method. For the case where the disturbance parameters are unknown, disturbance parameter estimation method based on forgetting factor least-squares method is proposed to transform the unknown parameter disturbance into a disturbance with known parameters, which is then suppressed by the adaptive digital disturbance rejection control approach. This solution could effectively solve the challenges caused by parameter uncertainty and unknown time-varying ocean external disturbances. Finally, simulations are carried out for the Petrel underwater thermal glider as an example. The simulation results show the proposed control method’s superiority and inherent robustness.

Model of thermal underwater gliders with PEMFC

In all kinds of underwater gliders, the thermal underwater glider which is propelled by a thermal engine has better performance. But the properties of thermal material and ocean conditions limit the thermal glider’s behaviour. A cogeneration power system is used to improve the performance of the thermal engine. This glider uses the heat from a Proton Exchange Membrane Fuel Cell (PEMFC) stack for the Phase Change Material (PCM) to drive the thermal engine for navigation while the PEMFC stack provides electricity for the sensors, control system and control actuators of the glider. The multi-functionality assures the underwater glider to navigate in all seas. Thus, this work presents a mathematical model and a thermodynamic model of the PEMFC-thermal glider on the basis of several assumptions. A Newtonian approach has been used to model the glider under the influence of water current.

A novel power system for thermal underwater gliders with PEMFC

Underwater gliders are effective buoyancy-driven vehicles for sea sampling tasks because they are inexpensive, reusable and achieve a high duration. In all kinds of them, the thermal underwater glider which is propelled by a thermal engine has better performance. But the properties of thermal material and ocean conditions limit the thermal glider’s behaviour. A cogeneration power system for the thermal underwater glider was developed in this work. A PEMFC-thermal engine cogeneration power system was used to improve the performance of the thermal engine. Experiments of the prototype indicate that the power system proposed can promote the flexibility and adaptability of the thermal glider. When the PEMFC operates at 400 mA cm−2 current density, the heat from the PEMFC is able to make thermal material have a 12.64% volume change. Meanwhile the heat can provide up to 2.05 MPa pressure in the accumulator of the thermal engine, which assures the underwater glider to navigate in all seas.

A self-searching optimal ADRC for the pitch angle control of an underwater thermal glider in the vertical plane motion

An underwater thermal glider is efficient when it takes a stable zigzag-gliding in the vertical plane. To increase its endurance, improving the control and energy efficiency over the pitch angle adjustment is necessary. Firstly, the working and control principle of the thermal glider is given, and the mathematical motion model is built on this principle for a performance test. Secondly, based on the Tracking Differentiator and Active Disturbance Rejection Control (ADRC) theory, a self-searching optimal control method is proposed. With this method, the external disturbance can be compensated by adjusting the reference in advance, which makes the required control signal optimized and small. Finally, this method is used for the pitch-keeping control of the glider during the descent and ascent, or the posture transition control between the descent and ascent. The results show that the control system has a good steady and dynamic performance under a simulated ocean current and noise disturbance, as well as an adaptive effect. The control signal to keep and track the trajectory is much smaller. Thus, by assistantly adjusting the reference in advance, this method takes much less energy to resist the disturbance than by only changing the control output and adjusting the actuator with the ADRC or PID. Since the thermal glider carries a limited power, saving energy means an increased endurance. Theory is a guide before practice. This will have a great significance in the marine survey or military usage. Besides, the control algorithm is also simple. All these make it a better control strategy.

Model and simulation of the energy retrieved by thermoelectric generators in an underwater glider

The need of a more persistent human presence on sea is driving the development of increasingly more efficient Autonomous Underwater Vehicles. A particularly promising solution to power these vehicles is the retrieval of energy from the ocean temperature gradient. This paper investigates the use of thermoelectric generators (TEGs) for that purpose, by presenting a complete model of thermoelectric generators embedded in the hull of an underwater glider. The model includes not only the thermoelectric generators but also the heat transfer process and the behaviour of different heat storage materials. Several simulations are performed, for each storage material, to determine the adequate design parameters leading to sufficient energy generation for a thermal glider. It is shown that the use of Phase Change Materials (PCM) as an energy storage material is highly advantageous over the use of sea water or stainless steel. Furthermore, it is shown that this approach, although leading to a high number of TEGs and mass of PCM, is compatible with current existing glider dimensions.

Ocean thermal energy harvesting with phase change material for underwater glider

Ocean thermal energy is formed by temperature difference between sea surface and deep sea. Underwater glider is a new kind of autonomous underwater vehicle. It ascends and descends between the sea surface and deep sea periodically by adjusting its net buoyancy. Thermal underwater glider can harvest ocean thermal energy through the utilization of phase change material (PCM) and convert it into mechanical energy for buoyancy-driven. In this paper, the fundamental principle of this thermal engine is unveiled. A nonlinear model for relationship between system pressure and phase change rate is established. Based on the established nonlinear model, influencing factors for system pressure and stored energy are analyzed comprehensively. Selected PCM is Hexadecane. Value range for air solubility in liquidus PCM is 0.06–0.1, and for residual air in the system is −0.02 to 0.05. The influence of these two factors on relative energy storage is similar. For typical values of air solubility 0.08 and residual air 0.005, utilization ratio of the thermal engine is less than 50%. Through experiment in the lake, the nonlinear model was verified, maximum system pressure was 12.5MPa, and average stored energy of each cycle was 2.48kJ. A prototype of thermal underwater glider has also been tested in the South China Sea. It had worked continuously for 29days without any failure. Total number of working profile was 121 and total cruising range was 677km, total stored energy was 300kJ. High reliability and performance was validated by the sea trial.

Model of a thermal driven volumetric pump for energy harvesting in an underwater glider

Underwater gliders are one of the most promising approaches to achieve an increase of human presence in the oceans. Among existing solutions, thermal driven gliders present long range and endurance capabilities, offering the possibility of remaining years beneath water collecting and transmitting data to shore. A key component in thermal gliders lies in the process used to collect ocean's thermal energy. In this paper a new quasi-static model of a thermal driven volumetric pump, for use in underwater gliders, is presented. The study also encompasses an analysis of the influence different hydraulic system parameters have on the thermodynamic cycle efficiency. Finally, the paper proposes a simple dynamic model of a heat exchanger that uses commercially available materials for the Phase Change Material (PCM) container. Simulation results validate the models developed.

Energy storage system with ultracapacitors for thermal underwater glider

A power system with proton exchange membrane fuel cells (PEMFC) was designed for thermal underwater glider. Heat generated by PEMFC is used as the propulsion power of the glider, and the electricity is used in the control and sensor system. An electric energy storage system (ESS) is required which possesses high power density with good cycle life. Ultracapacitors which exhibit high power density and cycle life are considered as energy storage devices. Simulations based on a specific voyage condition indicate that ESS with ultracapacitors has positive effects on reducing the output power demand of PEMFC and lightening the power system. Experimental results show that the state of charge (SOC) is related to the capacitance and resistance in ultracapacitor ESS.

Optimization of displacement and gliding path and improvement of performance for an underwater thermal glider

The underwater thermal glider utilizes ocean thermal energy to change its buoyancy, which enables it to ascend and descend. A Phase Change Material (PCM) as the working fluid inside the thermal engine tubes is sensitive to the surrounding seawater temperature, whose effects are different with the various displacements and gliding angles of the glider. In this paper, the effects of the displacement and the gliding angle on the performance of the thermal engine were studied numerically and experimentally. On this basis, the ways to eliminate the negative effect of a thermocline on the performance of the thermal engine were obtained. The results show that the displacement and gliding angle affect the transition time of the PCM. There exist the threshold values of the displacement and gliding angle for the normal work of thermal engine. There are two means of eliminating the negative effect of a thermocline on the performance of the thermal engine and improving glider performance: one is to increase the displacement, and the other is to decrease the absolute value of the gliding angle. There is also another better way to improve glider performance.

Numerical and experimental study of the phase change process for underwater glider propelled by ocean thermal energy

Ocean thermal energy is a type of renewable energy. The phase change process of the thermal glider propelled by ocean thermal energy was numerically and experimentally investigated in this paper. The gliding process was analyzed and optimized while the glider was operating in the ocean thermocline near the equator. The results showed that the time of a cycle decreases by 30% and the gliding depth decreases by 40% after optimization. The gliding process was also analyzed when the PCMs were partly melted with the volumetric expansion rate at 50%, 60%, 70%, 80% and 90% of the total value. With the decreasing of the volumetric expansion rate, the depth and the duration of a cycle decrease significantly, but the output work does not decrease obviously. It is helpful to reduce the manufacturing cost and prolong the duration of the glider. In addition, it can improve the energy harvest efficiency from the ocean thermocline. The vehicle's gliding process can be further investigated in different ocean thermoclines based on the numerical model presented in this paper, which can provide helpful information for optimal design of the thermal glider.

Effects of thermocline on performance of underwater glider’s power system propelled by ocean thermal energy

The thermal glider’s changeable volume produces propelling force to power the glider’s descending and ascending through the thermocline. The different depth, thickness, and intensity of the thermocline at different seasons and locations affect the working processes of the glider’s power system. Based on the enthalpy method, a mathematical model of the underwater glider’s power system was established and the time efficiency of operation was introduced, so that the effects of different thermoclines on the underwater glider’s power system were analyzed theoretically. The simulation result shows that the thermocline affects the transition time of the phase change processes of working fluids within the thermal engine tubes. There exist the threshold values of the thermocline’s depth and upper thickness for the power system’s operation. A depth or upper thickness of the thermocline less than the corresponding threshold leads the power system to work abnormally. To keep the power system working efficiently, a glider must be kept in warm surface water for a certain period before it moves through cold water, so that the time efficiency of operation is reduced. A less time efficiency of operation is unfavorable to the thermal glider to penetrate through the ocean currents.