# Investigation of Self-Driven Profiler with Buoyancy Adjusting System towards Ocean Thermal Energy
## 5 Experimental Research of Profiler Driven by Ocean Thermal Energy  
Figure 6 shows the shape and internal structure of the profiler. The profiler carries 8 L PCM, and the capacity of the buoyancy adjustment system is 8 N. Buoyancy materials are installed on the profiler to make buoyancy equal to gravity, and two deflectors are installed at the end of the profiler to reduce flow resistance. The upper-end cap is equipped with a depth sensor and a temperature sensor. The lower-end cap is equipped with the external cylinder and external bladder. Ten thermal engines are evenly arranged on the outside of the main cavity for thermal energy capture. The thermal engines are connected to the lower-end cap through titanium alloy tubes, and then connected to the hydraulic manifold block, which is composed of pressure sensors, solenoid valves, check valves and other components. By installing the pull wire displacement sensor on the inner cylinder and magnetostrictive displacement sensor on the external cylinder, accurate measurements of the oil volume are achieved.  
As shown in Figure 7, the experiment was carried out in Qiandao Lake, where the surface water temperature is 21 degrees, and the bottom water temperature is 11.5 degrees. The ocean thermal profiler prototype was deployed with a float-rope-profiler structure so that the profiler can float freely. Compared with other deployment methods, this deployment layout can make the profiler less affected by the current and the tension of the rope, and the fixed-length rope could prevent the profiler from touching the bottom. During the lake test, setting the profiler to neutral buoyancy first, and then adding a 3.5 N counterweight on the profiler, the profiler starts to dive. After the PCM is solidified to the set value, the profiler will automatically float up. The profiler relies on the temperature difference energy to automatically float up and down, and was recovered after three cycles.  
Figure 6: The profiler prototype.  
The profiler state of the lake test is shown in Figure 8. Key data, such as outer cylinder piston position, water depth, water temperature, accumulator pressure, and inner cylinder piston position, are expressed in the diagram. At the beginning of the experiment, the atmospheric temperature was 24 degrees, which is higher than the phase change temperature of PCM (18 \({}^{\circ}\)C), so the PCM had already melted before the profiler was placed in the water. The accumulator was under high pressure, the outer cylinder piston position was in low position and the profiler was in a negative buoyancy state. After putting the profiler into the water, it started to sink. Because the water temperature at the bottom of the lake was 12.5 degrees, which is lower than the phase change temperature of PCM, the PCM started to freeze and shrink. The oil flowed from the internal cylinder to the inner oil sacs, which were installed in the PCM containers, and the internal cylinder piston started to move. When the piston reached the 550 mm position, solenoid valve 1 (shown in Figure 1) opened and the oil entered the external cylinder from the accumulator, and the external cylinder piston moved from the 18 mm to the 104 mm position, which led to an increase of buoyancy and floats up of the profiler. As the surface water temperature was higher (22 \({}^{\circ}\)C), the PCM gradually melted and the accumulator pressure increased. Solenoid valve 2 opened when the accumulator pressure raised to 5.2 MPa, and the oil flowed from the external cylinder into the internal cylinder, which caused the inner and outer cylinder pistons to move, and the profiler converted to a negative buoyancy state. The profiler started to sink and the second cycle began. In the experiment, the energy that drives the motion of the profiler came from the accumulator, which originated from the thermal energy of water, so the profiler achieved self-driven movement relying on environmental energy.  
Figure 7: Photos of lake test.  
Figure 8: _Cont_.