With the development of electric vehicle technology, some customers have higher and higher requirements for power performance such as acceleration and climbing of electric vehicles, and high power is bound to bring high heat production. When ordinary air cooling cannot meet cooling needs, liquid cooling cannot meet safety needs, and phase change material cooling cannot meet long-term thermal management, the rational use of thermoelectric cooling for thermal management of the battery pack is undoubtedly an ideal solution.
Thermoelectric cooling, also known as semiconductor cooling, is mainly an electrical cooling method based on the Peltier effect. Its advantages are small size, no noise and no vibration, compact structure, no moving parts, convenient operation and maintenance, no need for refrigerant, and the cooling capacity and cooling speed can be adjusted by changing the current size. Its working principle is shown in Figure 1. When current flows from a p-type semiconductor material to an n-type semiconductor material, the carriers (holes) in the p-type semiconductor and the carriers (free electrons) in the type semiconductor move toward the junction. The free electrons recombine with the holes in the p-type semiconductor immediately after entering the p-type semiconductor to generate heat, and the holes immediately recombine with the free electrons in the n-type semiconductor to generate heat. Since these two parts of energy greatly exceed the energy absorbed by them in order to overcome the contact potential difference, they still exhibit exothermic state after offsetting, and the final result is that the temperature at the joint increases and becomes a hot end, which releases heat to the outside world; when the current direction is opposite, the temperature at the joint decreases to become the cold end and absorbs heat to the outside. At the same time, when there is a temperature difference between the connection terminals, a Seebeck voltage will be generated, and the current will pass through the thermoelectric element with a temperature gradient. Energy exchange occurs between the element and the environment due to the Thomson heating effect, which is proportional to the magnitude of the current and temperature gradient. If several pairs of semiconductor thermocouples are connected in series on the circuit according to Figure 2, and in parallel in terms of heat transfer, a common refrigeration thermopile is formed.
The cooling capacity of the thermoelectric refrigerator is determined by the dimensionless figure of merit ZT of the material, and its expression is shown in formula (1-1), where Z is the figure of merit whose dimension is 1/K. The power factor α2σT of semiconductor materials with narrow channels is a function of the carrier concentration, and the power factor is usually optimized to obtain the maximum ZT value. Ignoring the influence of unavoidable factors (such as contact resistance, radiation) that can reduce the performance of the equipment, for the thermoelectric cooling unit with two semiconductor materials, the calculation method of the figure of merit ZT is shown in formula (1-2). The cooling coefficient COP is the ratio of the absorbed heat to the consumed electric energy, and the calculation method is shown in formula (1-3). Figure 3 is related to formula (1-3), which reflects the relationship between the cooling coefficient COP of the thermoelectric cooling unit and ZT and the temperature of the cold end when the temperature of the hot end is a fixed value of 300K.
where α is the Seebeck coefficient of the material; σ is the electrical conductivity of the material; ρ is the resistivity of the material; k is the thermal conductivity of the material.
Among them, TH is the temperature of the hot end; TC is the temperature of the cold end; ZTM is the figure of merit of the material at the average temperature of the cold and hot ends.
Figure 3 - Relationship between COP of thermoelectric cooling unit and cold end temperature and ZT
Thermoelectric cooling is restricted by low COP, and its application range is relatively narrow, but with the development of technology, more and more applications will appear. At present, thermoelectric cooling technology is mainly used in the civilian market, such as household refrigerators, beverage cooling, etc.; medical equipment for cooling laser diodes or integrated chips; high-power electronic device cooling and industrial temperature control; automotive industry, such as car mini refrigerators, car air conditioners, and car seat cooling/heating, etc.
There are many commercial thermoelectric coolers on the market today. Circular and arc-shaped stand-alone thermoelectric coolers, the shape of which can be changed according to actual needs. Figure 4 is an application of the annular thermoelectric cooler in the cooling of aerospace electronic devices. There are 7 thermoelectric cooling plates in the picture to form a closed ring structure. The inner cold ring is the cold end part, which is used to absorb the heat generated by the electronic device. Air is introduced into the hot side for forced convection cooling and heat dissipation to maintain a suitable temperature difference between the hot and cold ends of the thermoelectric cooler. For the cooling of cylindrical single cells, this structure can be used for reference. When the heat dissipation system at the hot end of the thermoelectric cooler cannot effectively take away the generated heat, thermal runaway is prone to occur. Therefore, the heat dissipation design is one of the main factors affecting the thermoelectric cooling technology. Thermoelectric coolers are usually combined with one or more of air cooling, liquid cooling, heat pipe cooling or phase change material cooling to improve cooling efficiency. For large cylindrical or square battery packs, the thermoelectric cooler can be coupled with other cooling methods after reasonable structural design, and directly serve as part of the thermal management cooling component; thermoelectric refrigeration technology can also be perfectly combined with solar photovoltaic technology and automotive air conditioning technology, providing cooling air for the battery cooling system through the air conditioning refrigeration system, and indirectly participating in battery thermal management.
Figure 4 - Application of annular thermoelectric cooler in aerospace
The solar thermoelectric cooling system works by installing solar photovoltaic panels on the car body, using the solar energy irradiated on the battery panel to generate electricity by the photovoltaic effect, and transport it to the battery in the car through the charging wire, the battery provides stable DC power and transmits it out to provide energy for the thermoelectric refrigeration and air conditioner in the car box. At present, the application of thermoelectric refrigeration technology in battery thermal management has yet to be developed. With the further maturity of thermoelectric refrigeration technology, it is believed that battery thermal management technology and thermoelectric cooling technology will have a moment of perfect combination.