To meet the power demand requirements of autonomous underwater vehicles (AUVs), the power supply is generally composed of a large number of high-energy lithium battery groups. The lithium battery heat dissipation properties not only affect the underwater vehicle performance but also bring some security risks. Based on the widespread application of lithium batteries, lithium batteries in an AUV are taken as an example to investigate the heat dissipation characteristics of the lithium battery spatial layout in an AUV. With the aim of increasing the safety of lithium batteries, a model is developed for the heat transfer process based on the energy conservation equation, and the battery heat dissipation characteristics of the spatial layout are analyzed. The results indicate that the most suitable distance between the cells and the cross arrangement is better than the sequence arrangement in terms of cooling characteristics. The temperature gradient and the temperature change inside the cabin with time are primarily affected by the navigation speed, but they have little relationship with the environmental temperature.
As autonomous underwater vehicles (AUVs) are developing toward the direction of long ranges and high speeds, increasingly more power is urgently needed to support navigation. Because electrochemical reactions occurring within lithium-ion batteries will generate heat, the battery compartment of autonomous underwater vehicles works for a long time on large-scale integrated lithium-ion battery packs in a confined space, and thus security and reliability problems will exist. In [
At present, domestic and foreign scholars have been focusing on the security problem of AUVs using a lithium battery group to conduct related studies. In [
The existing studies primarily focus on battery thermal balance control system design. Regarding investigations of the AUV battery cooling layout, analyses have been performed only for navigation within the battery compartment temperature field, but little connections exist with structural layout studies on the thermal performance of the battery pack. In addition, compared to electric power vehicles, the battery cabin of AUVs is a closed compact space, and the use of common cooling methods, such as cold wind and solvent cooling, is limited. The battery pack heat conduction can only be achieved through the battery shell body and seawater, and physical problems involved are how to realize battery cooling through air flow engendered by local temperature variations inside the battery cabin and the heat conduction structure.
The main contribution of this paper is twofold: (i) we analyze the heat exchange process of the vehicle battery pack and establish the natural convection and heat transfer model for the confined space of the battery compartment and (ii) we investigate the heat transfer characteristics of lithium batteries in different spatial distributions.
According to the internal structure of the AUV battery cabin and theoretical knowledge of heat transfer, the heat transferred from the battery to the outside seawater can be summarized as the following three aspects of heat conduction. The first part of the heat conduction includes the heat generated by the battery pack and the heat exchange process between the battery cabin and the housing wall. The second part of the heat conduction process is between the cabin housing wall and the outer wall of the housing. Finally, the third part of the heat conduction is battery heat exchange between the outer wall of the cabin housing and seawater. The procedure is shown in Figure
Schematic diagram of heat dissipation and conduction of battery cabin.
To facilitate analysis of the temperature distribution in the battery cabin under different working conditions, the heat transfer process in the battery cabin was hypothesized and simplified as follows: The ends of the battery cabin and inner battery pack are insulated. The temperature distributions inside the battery cabin and battery pack only change in the radial direction and remain essentially unchanged in the axial direction. When the battery cabin works, the thermal parameters do not change over time.
Based on the above analysis, the heat dissipation model of the confined space of the battery cabin is equivalent to the problems of constant properties, inner heat source, and three-dimensional unsteady heat transfer.
The internal heat transfer process in a lithium battery can be simplified into a regular physical, three-dimensional unsteady heat transfer process within the heat source. For this reason, the internal lithium/thionyl chloride battery energy equation can be expressed as
When the Biot number of the battery is less than 0.1 under the natural convection environment, the internal temperature of the battery can be considered to be approximately evenly distributed. According to the Bernardi hypothesis, the heat generation rate in a single battery is constant, which can be approximately expressed as follows:
The heat convection of single cells occurs primarily through air convection and radiative heat transfer according to the ideal gas equation:
As shown by the above equation, a change in temperature can cause a change in air density in the battery cabin, and natural convection forms under the effect of gravity. Ignoring the effects of volume force and viscous force, the momentum conservation equation of air in the battery cabin can be expressed as follows:
The integral equation of the conservation of energy in the form of the cabin battery operation equation can be expressed as follows:
Heat transfers between the battery and vehicle wall primarily through natural air convection, and the heat generated by the battery part dissipates into the environment via the shell. The other part of the heat is absorbed by the shell of the vehicle, which causes the battery cabin temperature to increase. The purpose of this study is aimed at increasing the proportion of
Heat transfer from the inner wall to the outer wall of the battery cabin can be considered as heat conduction of a cylindrical wall, which can be expressed as follows:
During underwater movement, forced convection heat transfer occurs between the outer wall of the battery cabin and seawater, which can be expressed as follows:
The forced convection heat transfer coefficient between the outer wall and seawater is related to the sailing speed, which can be determined by the Reynolds number and Nusselt number of convective heat transfer between the outer wall and seawater:
Based on the lithium-ion battery pack for underwater space external thermal model and because the AUV battery compartment is a closed and compact space, the distance between batteries and combining types for the distribution of the temperature gradient inside the battery compartment have a great impact. This paper selects a winding-type lithium/thionyl chloride battery named 18650 as an example to analyze cells with different spaces and different permutations. Numerical parameters related to a single 18650 battery are shown in Table
Parameters of 18650-type battery.
Parameter | Value |
---|---|
Diameter [m] | 0.018 |
Length [m] | 0.065 |
Weight [kg] | 0.048 |
Internal resistance [ |
0.03–0.06 |
Density [kg·m−3] | 2900 |
Specific heat capacity [J·kg−1·K−1] | 1000 |
Equivalent heat conductivity [W·m−1·K−1] | 3 |
Nominal voltage [V] | 3.6 |
Rated capacity [Ah] | 2.5 |
The domain is described using triangular elements, with a total number of approximately 20,000. Grids closest to the profiles of the batteries were refined with triangular boundary elements to describe the boundary flow with sufficient precision. The distance between two adjacent cells is the same, and the distance between the boundary and batteries remains constant. The distances between the batteries change continually under the premise of shape and constant number of batteries.
With 5 single 18650 batteries as the objects, the space between the batteries (
Temperature distribution inside the battery compartment under different spacings between cells.
Table
Changes in temperature differences with varying cell spacings.
Cell spacing (m) |
|
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Temperature difference value (°C) | 0.68 | 0.63 | 0.59 | 0.56 | 0.53 | 0.51 | 0.50 | 0.49 | 0.48 | 0.47 |
Curve of the temperature difference variations with the distance between the batteries.
Table
Under the premise of the most suitable distance (
Simulation results of sequential arrangement and cross arrangement.
Sequential arrangement
Cross arrangement
Figure
On the basis of the previous discussion, take the batteries that are in the cross arrangement. Additionally, select the distance between batteries as
Certain structural layout AUV.
Certain underwater vehicle powertrain design specifications are shown in Table
Certain underwater vehicle dynamic system indicators.
Parameter | Value |
---|---|
Speed | 4 kn |
Flight | 70 km |
Power | 180 W |
Operating voltage | 21 V~30 V |
The formula for calculating the number of batteries is as follows:
The battery pack sets 189 batteries into 7 series groups, with each group including 27 parallel batteries. The battery pack was installed in the battery cabin with a diameter of 200 mm to provide an operating voltage of 21 to 30 V. See Figure
Schematic layout of the battery pack.
According to the analysis assumptions, the heat dissipation model of the confined space of the battery cabin is equivalent to the problems of constant properties, inner heat source, and two-dimensional unsteady heat transfer. A cross section of the battery cabin was taken as the calculation area, and the preprocessing program of ANSYS was used to construct the finite element analysis model through cell type selection, material parameters decision, geometric modeling, and cell generation.
The domain is described with triangular elements, with a total number of approximately 72,000. The total number of nodes is approximately 7300, and some grids closest to the profiles of the batteries and navigation shell were refined with triangular boundary elements to describe the boundary flow with sufficient precision. The grid of the model was divided as shown in Figure
Underwater vehicle battery compartment mesh.
Batteries for underwater cabins have different heats per unit of time when sailing at different speeds. After analysis, the direction in which the vehicle experiences resistance (the direction of the center of gravity speed) is opposite to the direction of navigation. Namely, the speed line is in the opposite direction of the
Therefore, the current through a single battery is
It can be seen that the heat production of a single battery is associated with the vehicle speed. Select vehicle speeds of 4 kn, 5 kn, and 6 kn. The performance parameters of batteries at different speeds are shown in Table
Battery performance parameters at different speeds.
Speed/kn | Single battery current/A | Surface heat transfer coefficient (W/(m2·K)) | Heat rate |
---|---|---|---|
4 | 0.27 | 1564 | 172 |
5 | 0.52 | 2854 | 655 |
6 | 0.90 | 4094 | 1958 |
After the batteries discharge for 10 hours, the simulation analysis shows that the temperature distribution in the battery cabin is as shown in Figure
Battery compartment internal temperature profile at different speeds.
Vehicle speed is 4 kn
Vehicle speed is 5 kn
Vehicle speed is 6 kn
After continuous discharging for 10 h, the curve of the maximum temperature of the battery compartment over time is as shown in Figure
The maximum temperature changes over time.
As shown in Figures
Therefore, as the vehicle speed increases, the maximum temperature inside the battery compartment increases and temperature differences increase accordingly. The reasons for this phenomenon are summarized as follows: as the underwater vehicle speed increases, the heat production rate is greater, and more heat is generated per unit time. Because the surface heat transfer coefficient is small, the battery heat generation per unit of time is less than the shell heat distribution per unit of time, resulting in heat concentration and the maximum temperature increasing. In addition, the minimum temperature is always the environmental temperature, which remains unchanged, and the battery compartment temperature increases as the vehicle speed increases.
For accuracy, select navigation speeds of 4 kn, 5 kn, and 6 kn to study the effects of seawater temperature on the temperature distribution inside the battery cabin when the water temperatures are 10°C, 15°C, and 20°C, respectively.
After the batteries continuously discharge for 10 hours, the simulation analysis shows that when the vehicle cruising speed is 4 kn, the seawater temperatures are 10°C, 15°C, and 20°C, and the battery compartment temperature distribution is shown in Figure
The battery compartment temperature distribution when the speed is 4 kn.
Seawater temperature is 10°C
Seawater temperature is 15°C
Seawater temperature is 20°C
After continuously discharging for 10 h, the battery compartment maximum temperature curve over time is as shown in Figure
The battery compartment maximum temperatures over time when the speed is 4 kn.
After the batteries continuously discharge for 10 hours, the simulation analysis shows that when the vehicle cruising speed is 5 kn, the seawater temperatures are 10°C, 15°C, and 20°C, and the battery compartment temperature distribution is shown in Figure
The battery compartment temperature distribution when the speed is 5 kn.
Seawater temperature is 10°C
Seawater temperature is 15°C
Seawater temperature is 20°C
After continuously discharging for 10 h, the battery compartment maximum temperature curve over time is as shown in Figure
The battery compartment maximum temperatures over time when the speed is 5 kn.
After the batteries continuously discharge for 10 hours, the simulation analysis shows that when the vehicle cruising speed is 6 kn, the seawater temperatures are 10°C, 15°C, and 20°C, and the battery compartment temperature distribution is shown in Figure
The battery compartment temperature distribution when the speed is 6 kn.
Seawater temperature is 10°C
Seawater temperature is 15°C
Seawater temperature is 20°C
After continuously discharging for 10 h, the maximum temperature inside the battery compartment over time is as shown in Figure
The battery compartment maximum temperatures over time when the speed is 6 kn.
To summarize, the water temperature has essentially no effect on the temperature difference inside the battery compartments. The reasons for this phenomenon are summarized as follows: the heat generation rate of batteries within unit time and the forced convection heat transfer coefficient between the outer wall of the vehicle and seawater are unchanged and almost equivalent when the sailing speed is constant. When the seawater temperature increases, the overall temperature in the battery cabin increases, but the temperature difference is basically unchanged.
In this paper, using theoretical analysis combined with the actual situation and the finite element software ANSYS, we establish the space of lithium batteries for an underwater thermal model. Then, we study the spatial layout of thermal characteristics, and the conclusions are as follows: A correlation exists between the underwater battery compartment temperature and the distance between the batteries. As the distance between the batteries increases, the temperature gradient gradually changes, and when it reaches a certain threshold, the gradient gradually stabilizes. Different permutations and combinations between the batteries have effects on the temperature gradient. The cross arrangement is better than the sequential arrangement in terms of the temperature gradient. The sailing speed affects the change in the temperature gradient and the maximum temperature inside the battery cabin over time. The greater the speed and the battery discharge current, the more the heat that the batteries generate. Then, the temperature increases faster, and less time is required to achieve the steady state. The seawater temperature rise may cause an overall temperature increase in the battery cabin. However, it has almost no impact on the temperature gradient and uniformity.
The authors declare that they have no competing interests.
This work was supported by the National Natural Science Foundation of China (NSFC) under Grant 51509205 and by the Shaanxi Provincial Natural Science Foundation of China 2015JQ5136.