# An External Ocean Thermal Energy Power Generation Modular Device for Powering Smart Float

^{*}

## Abstract

**:**

## 1. Introduction

^{9}km

^{2}that contains rich water and natural resources. Currently, the development and utilization of the ocean have become an emerging field of strategic significance [1,2,3]. Unmanned underwater vehicles are important platforms for ocean observation and detection, which are generally divided into autonomous underwater vehicle, autonomous underwater glider, remotely operated vehicle, or Argo float. An autonomous underwater glider is an autonomous observation platform powered by buoyancy and sails in a zigzag trajectory underwater, which can be used for continuous observation of a wide range. Argo float mainly operates below the sea surface to realize long-term, fixed-point, continuous, multi-level, and synchronous observation of the marine environment, which has good concealability.

## 2. Working Principle and Modular Design

## 3. Modular Integration

#### 3.1. Counterweight Characteristic

_{i}, and the center of mass in the reference coordinate system is denoted by (x

_{i}, y

_{i}, z

_{i}). Then, the center of mass (X

_{G}, Y

_{G}, Z

_{G}) of the integrated vehicle in the reference coordinate system can be calculated by Equation (1).

_{iB}, y

_{iB}, z

_{iB}). Then, the center of buoyancy (X

_{B}, Y

_{B}, Z

_{B}) of the integrated vehicle in the reference coordinate system can be calculated by Equation (2).

#### 3.2. Hydrodynamic Characteristic

_{m}is set to 0.1~0.4 m/s, and the angle of attack α is 0°, 2°, 4°, 6°, 8°, 10°, and 12° for CFD modeling and simulation.

_{m}= 0.3 m/s and α = 8°, as shown in Figure 5. The three models share the same surface pressure distribution, and the high-pressure area of the thermal power generation module appears at the head of the thermal energy conversion device and the heat exchange tubes. Among the three models, the third model shows the most uniform surface pressure distribution, with no obvious pressure concentration, and the second model shows a slightly worse performance. For the first model, there is a relatively obvious pressure concentration at the head of the thermal energy conversion device and the heat exchange tubes. Under the same conditions, a more uniform surface pressure distribution means a greater lift and resistance of the underwater vehicle. Therefore, the inclined conical fairing of the thermal energy power generation module is more conducive to reducing the resistance of the Smart Float.

_{m}= 0.3 m/s and α = 8°, as shown in Figure 6.

_{m}= 0.3 m/s and α = 8°, as shown in Figure 6.

- According to Figure 6a,c,e, a relatively large wake is produced at the tail when the water flow is separated from the thermal power generation device. The thermal energy conversion device with a flat fairing shows the largest wake and flow velocity loss, which has a great interference with the flow field at the tail. In comparison, the flow velocity loss of the device with an inclined conical fairing is slightly less. The device with a conical fairing has the smallest wake and flow velocity loss, showing the smallest interference to the flow field at the tail;
- Based on Figure 6b,d,f, the heat exchange tube with a flat fairing shows the most serious turbulent dissipation at the head, the tube with a conical fairing shows slightly less turbulent dissipation, while the tube with an inclined conical fairing has the least turbulence dissipation, almost negligible;
- The wake and flow velocity loss of heat exchange tubes with different fairings can also be found in Figure 6b,d,f. The tube with a flat fairing has the largest wake and flow velocity loss at the tail when the water flow is separated from the tube at the tail, causing an obvious flow velocity loss on the upper surface of the wing. By contrast, the tube with a conical fairing has a smaller flow velocity loss at the tail, with a weaker influence on the flow velocity of the upper surface of the wing. The tube with an inclined conical fairing shows optimal performance in the wake and flow velocity loss at the tail, bringing the least influence on the flow velocity of the upper surface of the wing.

- The lift-drag ratio of Smart Float has a small correlation with the speed v
_{m}, experiencing smaller variations under different speed conditions; - At a small angle of attack (0° ≤ α ≤ 8°), the increase in the lift flow surface of the vehicle outweighs that in the resistance flow surface, and therefore the lift-drag ratio increases with the angle of attack. At a large angle of attack (α > 8°), the increase in the lift flow surface is less than that of the resistance flow surface, and thus the lift-drag ratio decreases with the increase of the angle of attack. This changing trend of the lift-drag ratio is related to the flow surface of the model;
- The wake and flow velocity loss of heat exchange tubes with different fairings can also be found in Figure 6b,d,f. The tube with a flat fairing has the largest wake and flow velocity loss at the tail when the water flow is separated from the tube at the tail, causing an obvious flow velocity loss on the upper surface of the wing. By contrast, the tube with a conical fairing has a smaller flow velocity loss at the tail, with a weaker influence on the flow velocity of the upper surface of the wing. The tube with an inclined conical fairing shows optimal performance in the wake and flow velocity loss at the tail, bringing the least influence on the flow velocity of the upper surface of the wing.

#### 3.3. Heat Transfer Characteristic

_{1}, P

_{2,}and P

_{3}is fitted with the following equations respectively.

_{2}is shorter than that at P

_{1}and P

_{3}. In particular, the volume reduction rate of the PCM at P

_{2}is about twice that at P

_{1}and P

_{3}within the diving depth range from 0 m to 200 m, when the volume variation curves of P

_{1}and P

_{3}are almost coincident. At the maximum diving depth (400 m), the volume of the remaining PCM is almost equal at the three positions. In the ascending stage, the volume growth rate of the PCM at P

_{1}and P

_{3}is higher than that at P

_{2}. When Smart Float returns to the sea surface and starts to float, the volume growth rate at P

_{1}is the highest, and that at P

_{3}is the lowest. The volume expansion of a single tube at three positions during this cycle is 0.555 L, 0.526 L, and 0.527 L respectively.

_{2}in the depth range of 33 m to 250 m (thermocline) is 1.86 times and 2.16 times that at P

_{1}and P

_{3}. In the thermocline, the temperature at P

_{2}decreases fastest with the increase of depth. The volume shrinkage rate of the PCM in the diving stage increases with the seawater temperature gradient. Among the three points, P

_{1}has the lowest latitude and the highest sea surface temperature, while P

_{3}has the highest latitude and the lowest sea surface temperature. When Smart Float is on the sea surface, the volume expansion rate of the PCM is related to the surface seawater temperature, and the volume expansion rate of the PCM at P

_{1}is the highest in this stage. The complete phase change of the PCM can be realized during a power generation process at all three positions. The volume change rate of the material will slightly attenuate because the innermost layer of the material cannot reach the surrounding water temperature.

_{1}and P

_{3}is slightly higher than that at P

_{2}. The reason is that although the PCM at P

_{1}and P

_{3}can be completely solidified, the innermost layer still has a relatively high temperature when entering the melting stage. At the diving depth of 400 m, the volume change rate of materials decreases obviously at all three positions, since the solidification time of PCM is shortened in a smaller diving depth, and the complete phase of the PCM cannot be realized. The innermost layer still keeps a liquid phase when the melting stage is started. Because of the large temperature gradient, the PCM P

_{2}can undergo relatively sufficient solidification in a limited time compared with the materials at P

_{1}and P

_{3}, thus showing the smallest remaining volume of the PCM and the largest volume change rate. In addition, the ratio of the time for the PCM to shrink to the minimum volume value and the time for the complete profile gradually increases with the decrease of diving depth. For example, with the diving depth of 1000 m, 700 m, and 400 m, the ratios at P

_{1}are 37%, 54%, and 64% respectively.

## 4. Sea Trial and Model Test

## 5. Conclusions

- The external integrated ocean thermal energy power generation module does not influence the attitude adjustment ability of the Smart Float by analyzing the counterweight characteristic and selecting the optimal installation angle of 75°;
- According to the analysis of the hydrodynamic characteristic, it is more conducive to choose an inclined conical fairing of the ocean thermal energy power generation module for improving the lift-drag ratio of the vehicle, and the underwater navigation economy of the thermal energy power generation device;
- Based on the simulation analysis of the heat transfer characteristic, the complete phase change cycle can be realized at a depth equal to or larger than 700 m;
- The sea trial results show that energy generation of 1.368 Wh can be generated in a single profile, with the hydraulic-to-electric efficiency of about 60% in the power generation stage, which verifies the performance of thermal energy power generation module is integrated into Smart Float under real ocean conditions.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Abbreviations

d | diving depth |

P_{buoyancy} (x, y, z) | center of buoyancy position |

P_{mass} (x, y, z) | center of mass position |

P_{batteryl} (x, y, z) | the limit center of mass position when the battery pack is on the left |

P_{batteryr} (x, y, z) | the limit center of mass position when the battery pack is on the right |

T_{seawater} | seawater temperature |

Δh | height difference between center of mass and buoyancy |

θ_{Inst} | installation angle |

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**Figure 1.**Structure and main parameters of the thermal energy power generation module. The module mainly includes a thermal energy conversion device and PCM heat exchange tube. Schematics of the thermal conversion device: (

**1**) end cover, (

**2**) leather bag, (

**3**) speed increaser, (

**4**) solenoid valve, (

**5**) stiffening ring, (

**6**) accumulator, (

**7**) battery pack, (

**8**) generator, (

**9**) hydraulic motor, and (

**10**) circuit board.

**Figure 4.**Three types of fairings for the thermal energy power generation module (

**a**) flat fairing; (

**b**) conical fairing; (

**c**) inclined conical fairing.

**Figure 5.**The pressure distribution on the surface and the symmetry planes of the three models are (

**a**) flat fairing; (

**b**) conical fairing; (

**c**) inclined conical fairing.

**Figure 6.**The cross-sectional flow velocity distribution of the thermal energy conversion device and the heat exchange tubes (

**a**) The flow velocity of the thermal energy conversion device with a flat fairing; (

**b**) The flow velocity of heat exchange tubes with a flat fairing; (

**c**) The flow velocity of the thermal energy conversion device with a conical fairing; (

**d**) The flow velocity of the central section of the heat exchange tubes with a conical fairing; (

**e**) The flow velocity of the thermal energy conversion device with an inclined conical fairing; (

**f**) The flow velocity of the central section of the heat exchange tubes with an inclined conical fairing.

**Figure 7.**The lift-resistance ratio and angle of attack of the vehicle with different fairings of the thermal energy power generation module under different speed conditions (

**a**) v

_{m}= 0.1 m/s; (

**b**) v

_{m}= 0.2 m/s; (

**c**) v

_{m}= 0.3 m/s; (

**d**) v

_{m}= 0.4 m/s.

**Figure 9.**The variation of PCM volume with diving depth at different positions. (

**a**) At the diving depth of 1000 m; (

**b**) At the diving depth of 700 m; (

**c**) At the diving depth of 400 m.

**Figure 13.**Power generation performance of thermal energy power generation module during the sea trial.

**Table 1.**Between center of mass and buoyancy of the integrated vehicle at different installation angles.

θ_{Inst}/° | P_{buoyancy} (x, y, z) | P_{mass} (x, y, z) | Δh/mm | P_{batteryl} (x, y, z) | P_{batteryr} (x, y, z) |
---|---|---|---|---|---|

Not integrated | x = 1180 | x = 1179.945 | 3.000 | x = 1168.068 | x = 1192.742 |

y = 0 | y = 0 | y = 0 | y = 0 | ||

z = 0 | z = −3 | z = −3 | z = −3 | ||

45 | x = 1172.420 | x = 1180.047 | 5.088 | x = 1170.568 | x = 1190.255 |

y = 0.012 | y = 0.203 | y = 0.203 | y = 0.203 | ||

z = −36.569 | z = −41.657 | z = −41.657 | z = −41.657 | ||

60 | x = 1172.420 | x = 1180.047 | 3.865 | x = 1170.568 | x = 1190.255 |

y = 0.011 | y = 0.201 | y = 0.201 | y = 0.201 | ||

z = −34.342 | z = −38.207 | z = −38.207 | z = −38.207 | ||

75 | x = 1172.420 | x = 1180.047 | 2.442 | x = 1170.567 | x = 1190.255 |

y = 0.010 | y = 0.200 | y = 0.200 | y = 0.200 | ||

z = −31.748 | z = −34.190 | z = −34.190 | z = −34.190 | ||

90 | x = 1172.420 | x = 1180.047 | 0.914 | x = 1170.568 | x = 1190.255 |

y = 0.010 | y = 0.199 | y = 0.199 | y = 0.199 | ||

z = −28.965 | z = −29.879 | z = −29.879 | z = −29.879 | ||

135 | x = 1172.420 | x = 1180.047 | −3.261 | x = 1170.568 | x = 1190.255 |

y = 0.007 | y = 0.195 | y = 0.195 | y = 0.195 | ||

z = −21.362 | z = −18.101 | z = −18.101 | z = −18.101 | ||

150 | x = 1172.420 | x = 1180.047 | −4.199 | x = 1170.568 | x = 1190.255 |

y = 0.006 | y = 0.194 | y = 0.194 | y = 0.194 | ||

z = −18.653 | z = −15.454 | z = −15.454 | z = −15.454 |

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**MDPI and ACS Style**

Zhang, H.; Ma, X.; Yang, Y. An External Ocean Thermal Energy Power Generation Modular Device for Powering Smart Float. *Energies* **2022**, *15*, 3747.
https://doi.org/10.3390/en15103747

**AMA Style**

Zhang H, Ma X, Yang Y. An External Ocean Thermal Energy Power Generation Modular Device for Powering Smart Float. *Energies*. 2022; 15(10):3747.
https://doi.org/10.3390/en15103747

**Chicago/Turabian Style**

Zhang, Hongwei, Xinghai Ma, and Yanan Yang. 2022. "An External Ocean Thermal Energy Power Generation Modular Device for Powering Smart Float" *Energies* 15, no. 10: 3747.
https://doi.org/10.3390/en15103747