1. Conventional air heating
In a cold environment, the battery is heated by convection heat exchange between the air with a higher temperature and the battery, which is called conventional air heating. The hot air can be obtained by electrical heating, and in the case of HEVs, the air can also be heated by providing energy from the engine.
Zhang et al. used hot air to heat the battery in a low temperature environment. The hot air is provided by the air conditioning system, which heats the cab first, and then sends it to the battery box. Zhang et al. found through the analysis of the first law and the second law of thermodynamics that this direct air heating system has a large load on the air conditioning system and is not economical. The air convection heating system established by Ji and Wang, the heat generated by the current passing through the resistor heats the air around the resistor, and the air is transported around the battery by a fan, and the battery is heated by the convective heat transfer between the air and the battery. The current of the heating resistor and the fan is provided by the on-board battery itself. Therefore, in addition to convection heat transfer, the battery itself will also generate heat due to the existence of the internal resistance of the battery, which will speed up the heating rate of the battery. Figure 1 shows the change curve of air and heater temperature with time when the air convection heating system adopts different heating resistances when the ambient temperature is -20℃. As can be seen from the figure, as the heating resistance increases, the temperature difference between the air and the heater gradually decreases, and the heating rate also decreases. When the heating resistance is 0.4Ω (the battery discharge voltage is 2.66V), the time required to heat the battery to 20℃ is 54s, and the temperature of the heater reaches 50℃ at this time; when the heater resistances were 0.6Ω and 0.8Ω, respectively, the time required to heat the battery to 20℃ was 110s and 165s, respectively. Figure 2 is a curve diagram of the heating efficiency of the air convection heating system with time. It can be seen that the smaller the resistance of the heater, the higher the heating efficiency of the air convection heating system. After heating for 50s, when the heater resistance is 0.4n, 0.62, and 0.8Ω, the system efficiency is 0.79, 0.70, and 0.65, respectively.
Figure 2 - Heating efficiency curve of air convection heating system over time
Wang Facheng et al. designed a heating wire air heating box for power battery packs. The heating wire in the heating box is used to heat the air, and then transport it to the battery box to heat the battery and increase the temperature of the battery. Wang Facheng and others first conducted an air heating experiment on the heating wire heating box. When the inlet air temperature was 22℃, a 40V DC power supply was used to provide current, and the outlet air temperature was as high as 90℃ after 380s. Using an infrared thermal imager to detect the temperature of the heating wire inside the heating box, it was found that in the air heating experiment, the heating wire could be heated from 22℃ to 141℃ in 1 minute. On the basis of this experiment, Wang Facheng et al. connected the heating box and the battery in series, and used the hot air from the outlet of the heating box to heat the battery. In order to ensure the correctness of the experiment, the battery was placed in a constant temperature box at -15℃ for 24 hours before the experiment. During the experiment, the 40V DC power supply was still used to provide current. The results showed that the battery could be heated from -15℃ to 0℃ after 1500s, and the rising rate was about 1℃ every 87s.
2. Phase change material heating
The phase change material will absorb or release a lot of heat during the phase change process. The phase change material is used to store energy (the phase change material changes from solid to liquid), and the battery can be heated in a low temperature environment (the phase change material changes from liquid to solid). Duan and Naterer studied the melting and solidification process of phase change materials through experiments. The experimental results show that the temperature of phase change materials does not change much during the phase change process. Using this feature can effectively prevent the battery from being too low in temperature in a low temperature environment.
Zhang et al. designed a cyclic Li-ion battery cooling/heating system based on phase change slurry (PCS), which are analyzed using the first and second laws of thermodynamics, respectively, and compared with direct air flow systems and refrigerant circulation systems. In the phase change suspension Li-on battery heating system, the phase change suspension absorbs heat in the cab, flows to the battery box in the circulation loop, releases the heat through the phase change, and heats the air around the battery pack. The battery pack capacity studied by Zhang et al. is 27.7kW·h-1, and the total power in the actual operation of the vehicle is assumed to be 5kW, of which 3kW is used for car driving, and 1.5kW and 0.5kW are used for the air conditioning system and auxiliary system, respectively. The phase change material used in the battery pack heating system is pentadecane (C15H31, the latent heat of phase change is 207kJ·kg-1, and the phase change temperature is 9.9℃), and the ambient temperature is below 0℃. The analysis results of the first law of thermodynamics show that when the heat loss is 0.2kW and the relative air humidity is 0.4, the heat load of the phase change suspension heating system is independent of the ambient temperature and remains at 0.1kW; when the heat loss increases, the heat load of the phase change suspension heating system increases linearly. When the heat loss is less than 2.4kW, the heat load of the direct air heating is lower than that of the phase change suspension heating.