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1. Overview of heat pipe-based battery cooling
Heat pipe (HP) technology, which has developed rapidly in recent years, has been widely used in many fields. The heat pipe is a high-efficiency heat exchange element that uses the phase change of the medium in the pipe to absorb heat and release heat. The application of heat pipes in battery thermal management is mainly for heat dissipation. It was first used for battery cooling in space equipment such as satellites and spacecraft. Heat pipes are used in electric vehicle power battery systems, and they have also begun to attract attention as the battery thermal problem becomes increasingly prominent. Wu et al. have successively studied the heat dissipation effect of heat pipes in Ni-MH batteries and Li-ion batteries, and in the heat dissipation design of a cylindrical Li-ion battery with a capacity of 12A·h (diameter 40mm, length 110mm), a method of adding aluminum fins and fans to the condensation end of the heat pipe to enhance heat transfer was proposed. Zhang Guoqing and others conducted heat dissipation experiments on a module composed of 6 SC-type nickel-metal hydride batteries (capacity 2200A·h, diameter 22mm, length 42.5mm). Swanepoel analyzed the heat transfer performance of different wall materials and media in pulsating heat pipes (PHPs), and designed the thermal management of PHPs in HEV. The lead-acid battery pack (optima spirocell, 12V, 65A·h) used is placed at the rear of the car, and combined with the flowing air during the driving process to enhance heat dissipation. Jang and Rhi designed a loop thermosyphon for thermal management of Li-ion batteries, and experiments showed that the battery temperature could be kept below 50°C. They believe that heat pipes are suitable for battery thermal management in future EVs and HEVs.
2. The basic principle of heat pipe cooling
A heat pipe is an artificial component with good heat transfer performance. The commonly used heat pipe consists of three parts: the main body is a closed metal tube shell, there is a small amount of working medium and capillary structure in the inner cavity of the heat pipe, and the air and other debris in the pipe must be discharged to keep the heat pipe in a vacuum state. The heat pipe mainly uses the following three laws of physics when it works:
(1) When the liquid inside the heat pipe is in a vacuum state, the boiling point is relatively low.
(2) The porous capillary structure can generate suction force on the liquid and promote the flow of the liquid.
(3) When the same substance vaporizes, the latent heat is much higher than the sensible heat.
The heat pipe can be generally divided into three parts according to the heat transfer conditions: the evaporation end, the adiabatic end and the condensation end. The basic working principle of the heat pipe is shown in Figure 1. Generally, the heat pipe consists of three parts: the tube shell, the wick and the end cover. Pump the inside of the heat pipe to a negative pressure of 1.3×(10-1~10-4) Pa and then fill it with an appropriate amount of working liquid, so that the capillary porous material of the liquid-absorbing wick close to the inner wall of the pipe is filled with liquid and then sealed. The heat-absorbing end of the heat pipe is the evaporation end, and the heat-dissipating end is the condensing end. According to the needs of the application, an adiabatic section can be arranged in the middle of the heat pipe. The working principle of the heat pipe is as follows: when the heating end of the heat pipe is heated, the working medium is heated and evaporated and flows to the condensation end under the action of a weak pressure difference, and then the vapor dissipates heat at the condensing end and turns into liquid again, and the liquid at the condensing end flows back to the evaporating end by the capillary force or gravity of the porous material. According to the heat dissipation principle of the heat pipe, the evaporating end stores the heat generated by the battery in the working medium in the form of phase change heat, transfers the heat to the condensing end with the help of the working medium transport capacity, and the condensing end transfers the heat to the outside to achieve the effect of heat dissipation. The working medium in the heat pipe can be continuously circulated, and the heat generated by the battery can be continuously transferred to the ambient air, so as to realize the transmission of large heat flow under a small temperature difference and reduce the temperature of the battery rapidly.
Figure 1 - Working principle of heat pipe
3. Selection of fluid working medium in heat pipe
The heat pipe mainly relies on the change of phase of the working medium to transfer heat, so various physical properties of the working liquid have a very important influence on the working characteristics of the heat pipe. Generally, the following principles should be considered when choosing a working fluid:
(1) The working liquid should have good comprehensive thermophysical properties.
(2) The working fluid should be compatible with the material of the shell and the wick.
(3) The working liquid should meet the requirements of economical rationality, non-toxic and non-polluting.
(4) The working fluid should have good thermal stability.
(5) The working liquid needs to adapt to the working temperature area of the heat pipe and has an appropriate saturated vapor pressure.
In general, the working fluid can be divided into the following categories according to the working temperature range of the heat pipe.
(1) Cryogenic heat pipe working fluid. Cryogenic heat pipe refers to a heat pipe with a temperature working range of 0~200K. The working medium of the heat pipe operating in this temperature range can choose pure chemical substances (such as oxygen, hydrogen, hydrogen, krypton, ammonia) and compounds (such as ethane and freon) in the form of single elements.
(2) Low temperature heat pipe working fluid. The low temperature heat pipe refers to the heat pipe with the operating temperature range of 200~550K. Here, the working fluids inside the heat pipe operating in the temperature range include acetone, water, ammonia, alcohol, freon and some organic compounds. Among these working fluids, water and ammonia are the most widely used. They have good thermophysical properties and can meet the requirements of most low temperature occasions.
(3) Medium temperature heat pipe working fluid. The medium temperature heat pipe refers to the heat pipe operating in the temperature range of 500~750K. This type of heat pipe usually selects sulfur, mercury, alkali metals (such as cyan, rubidium) or some compounds (such as thermal conductivity) heat exchangers according to its working temperature environment.
(4) High temperature heat pipe working fluid. The high temperature heat pipe refers to the heat pipe whose working temperature is above 750K. Such heat pipes usually use high melting point metals as their working medium, such as potassium, lead, lithium, sodium, steel, etc. This kind of medium, especially metal lithium, not only has a very high melting point but also has a high axial heat transfer density, and is an ideal material as a working medium for high-temperature heat pipes.
(5) High temperature heat pipe working fluid of high melting point material. This type of heat pipe belongs to the heat pipe whose working temperature range is above 1300K. This type of heat pipe is generally used in a special high temperature environment and needs to work in a specific environment, such as a vacuum or inert gas environment. This type of heat pipe needs to consider two factors when selecting the working medium: on the one hand, it meets the needs of the working environment, and on the other hand, the working life of the heat pipe is extended as much as possible.
At present, the main heat transfer media used for thermal management of electric vehicle batteries are: air, coolant and phase change materials. Different types and sizes of battery modules have different acceptable temperature difference ranges. The maximum acceptable temperature difference for small module battery packs is 2~3°C, and the maximum acceptable temperature difference for large module battery packs is 6~7°C. More precise control requires higher heat transfer efficiency, and more compact mechanical design requires more flexible layout methods. The loop heat pipe not only has excellent heat transfer ability, but also can be flexibly arranged in the battery pack, which can well meet the needs of precise thermal management.