Boiling refers to the process of generating bubbles inside the liquid for vaporization. It is a very effective method for removing heat. It has a high heat transfer rate and requires a small heat transfer temperature difference. Boiling cooling is used in thermal management of high-power electronic components, and also in internal combustion engine cooling. The boiling heat transfer battery thermal management method is expected to solve the thermal runaway problem of the lithium-ion battery module in the auxiliary power compartment of the Boeing 787 aircraft. Pool boiling is the main physical mechanism used in boiling heat transfer devices, and it is very important to control the boiling within a set boiling range. Controlling the boiling interval is quite challenging due to fluctuating and uncertain heat transfer loads, so in battery thermal management, the boiling process must be regulated in real time. Boiling is affected by factors such as how the fluid and heat-generating body are combined, surface roughness, system pressure, etc. Boiling heat transfer is divided into pool boiling and in-tube boiling. With the increase of heat transfer power and temperature difference, pool boiling is further divided into four zones: natural convection zone, nucleate boiling zone, transition boiling zone, and film boiling zone. Figure 1 is a typical relationship curve between the surface heat flux and surface superheat degree of pool boiling, and the approximate distribution area of the four intervals is given in the figure.
The application of boiling cooling in the thermal management of single cells is still in the preliminary experimental stage, and there are very few people in the research. The Netherlands van Gils et al. used an experimental method to study the ability of pool boiling to control the thermal properties of electric vehicle batteries. The non-conductive Novec7000 was selected as the boiling working medium (The chemical composition is C3F7OCH3 with a purity of 99.5%, 1-methyl 7-fluoropropane, and the boiling point is 34℃ under standard conditions).
Their experiment is divided into the following four parts: the selection of boiling medium, the test of cooling ability, the test of temperature uniformity, and the control of boiling process. In the first step, the battery is completely immersed in a working medium of 1 atm (1 atm=1.01325×105Pa). The working medium is contained in a double-walled container, and constant temperature water is passed between the two walls to preheat the boiling working medium. At the same time, a temperature sensor was used to test the temperature of the top, bottom and middle of the battery, as well as the ambient temperature and the temperature of the working fluid, and a 5C constant current discharge experiment was carried out to test the cooling ability of Novec7000 liquid, and compared with air cooling. The results are shown in Figure 2(a). At the end of discharge, the temperature rise of the battery is less than 5℃, the liquid does not start to boil, and the heat transfer method is natural convection. In the second step, the volume/pressure control piston on the device is used to control the pressure in the container, and the temperature sensor is used to measure the temperature of the top and bottom of the battery and the temperature of the liquid. First, a long-term pulsating charge-discharge experiment with a current size of 5A was carried out, and the period was 180s. In this case the heat generation rate of the battery reaches the maximum value, which is used to test the heat soaking ability of the boiling cooling. When the wall temperature of the container is 33℃ and boiling cooling is performed, the temperature at the top and bottom of the battery is almost equal, as shown in Figure 2(b), indicating that the boiling cooling has a strong heat soaking ability. The boiling process is affected by the pressure in the container, reducing the pressure can increase the intensity of the boiling, which shows that, in principle, adjusting the pressure can effectively and timely control the boiling. Under the condition of different heat production loads of the battery, it is necessary to adjust the appropriate pressure for cooling control.
For the liquid boiling cooling of the battery pack, no one outside China has carried out relevant research at present. The problem of temperature consistency is more prominent in large battery packs. Overheating of a certain part in the middle will not only gradually increase the temperature difference of the entire battery pack, and greatly reduce the cycle life, but also further increase the heat production of the battery and cause thermal runaway. Boiling cooling has great advantages in the ability to control temperature and uniform heat, and the control is convenient and flexible, and the successful application of boiling cooling in thermal management of large battery packs has great prospects.