In recent years, the market of novel energy vehicles, including electric vehicles and hybrid electric vehicles, has grown fast [1
]. Power batteries, as one of the most important parts in electric vehicles (EVs) and hybrid electric vehicles (HEVs) [3
], have attracted the majority of attention from researchers and engineers. A whole battery pack in a vehicle commonly contains several single cells connected serially or in parallel [4
]. A large quantity of battery cells will generate a large amount of heat and cause temperatures to rise during the charge and discharge process [5
]. A cell’s voltage, energy, efficiency, and life cycle are deeply influenced by its operating temperature [7
]. Further thermal runaway and safety issues can occur if the heat is not able to be dissipated effectively [9
]. In addition, the temperature difference between each cell can lead to a variable capacity descending rate, which also reduces the life cycle of a battery pack. At temperatures exceeding 50 °C, charging efficiency and battery life deteriorate the most rapidly due to heat [10
]. Therefore, it is necessary to pay more attention to the research of battery thermal management system (BTMS), which is very important for battery performance, life and safety. Pesaran et al. [12
] noted that Ni-MH and Li-ion batteries’ ideal operating temperatures range from 25 to 40 °C, and the temperature differences between the cells within a battery pack should be below 5 °C.
1.1. Literature Review
Plenty of works have been done in the field of battery thermal management system for commercially sold EVs and HEVs in the market, such as direct air cooling, liquid cooling, phase change material, heat pipes, hybrid cooling system and other emerging cooling technologies.
The air-cooling system uses air as the heat exchange medium to let the air sweep across the battery surface to take away the heat generated by the battery. The air-cooling system is the most widely used cooling mode with simple structure, light weight, low cost and convenient maintenance. Chen YF et al. [13
] showed that the thermal conductivity of Li-ion battery was low, and air cooling alone could not meet the requirements of thermal management. Harmel et al. [14
] and Chen et al. [15
] analyzed the thermal balance of li-ion batteries and found that when the wind speed reached a certain degree, increasing the wind speed had little effect on the heat dissipation effect of the batteries. Nelson et al. [16
] have shown that the cooling effect of an air-cooling system is poor when the ambient temperature is high. In addition, the air-cooling system has a large volume and poor protection level, which limits its application in battery packs with a large scale.
The liquid-cooling system uses the circulating flow of the coolant to take away the heat generated in the battery. The coolant has higher specific heat capacity and higher heat exchange efficiency, which makes it easier to meet the cooling and heating requirements of the battery comparing with air. The research of Pesaran et al. [7
] and Nelson et al. [16
] confirmed this point of view. The liquid-cooling system has been widely studied and applied in recent years because of its good cooling effect and reasonable cost, which has allowed the technology to develop rapidly. Huo et al. [17
] showed that the number of channels, flow rate and liquid flow direction have an important impact on the cooling effect of the straight cooling channel, and gave suggestions on the number of channels and the requirement flow rate. Jarrett et al. [18
] studied the influence of different serpentine structures on the temperature uniformity and average temperature of the cooling plate. However, their research only focuses on a single cooling plate corresponding to several cells, not on the real, large-scale battery thermal management system, which comprehensively considered the flow distribution, pressure drop and temperature difference. Moreover, the effect of heat generation and heat dissipation under different current ratio is worthy of further study.
The heat pipe is a technology of thermal management based on the principle of phase change heat transfer. Flexible geometry, low maintenance requirements and good thermal conductivity make it attractive as a BTMS option [19
], however, the complex manufacturing process and high cost limit the large-scale use of heat pipe [22
]. The PCM cooling system uses phase change materials to absorb the heat generated by the battery in the form of latent heat, which has the advantages of excellent uniform temperature effect and large heat storage capacity [23
], but with the disadvantages of low thermal conductivity and volume change affecting the application and promotion of PCM [25
]. The independent passive phase change material system and heat pipe system are only conducive to heat transfer and storage. For large-scale or high-power battery systems, it is difficult to transfer the heat to the outside of the battery pack by itself in the natural state. It is necessary to combine independent cooling technology into a hybrid heat management system [26
], for example, PCM and air-cooling combination [27
], PCM and liquid cooling combination [28
], PCM, HP and air-cooling combination [29
], etc. The hybrid thermal management system combines the advantages of various cooling technologies, but the design cost and material cost are too expensive, and the implementation of the system is very complex.
Several other emerging cooling technologies have been studied recently, such as thermoelectric coolers (TECs) [30
], thermo–acoustic refrigerators (TARs) [31
], active magnetic regenerators (AMRs) [32
], and internal cooling in battery cells [33
]. These techniques offer many advantages, but they require much more works before they can be commercially applied in vehicles. Technology readiness level (TRL), designed by NASA in the late 1990s is a useful evaluation towards the market (Table 1
). The TRL index can be used as a rough estimation of the required costs, time to market, risks taken, and so on [34
]. With the increase of TRL, the technical is more mature and close to mass production.
In conclusion, although different thermal management systems have their own advantages and disadvantages, liquid cooling is generally the best solution of thermal management systems. Qu [35
] evaluated the battery thermal management system by the analytic hierarchy process (AHP) proposed by Saaty [36
], a famous American operational research scientist. The results also prove the advantages of a liquid-cooling system comprehensively considering technology, maturity, consumption, cost and other factors. The global market sales data shows the absolute advantages of air-cooling and liquid- cooling systems. The battery pack of HEVs is small and arranged inside the vehicle, and air cooling is basically adopted, such as in the Toyota Prius; the battery pack of EVs and PHEVs is large and arranged outside the vehicle, and liquid-cooling system is basically adopted, such as in the Tesla Model 3.
The research object of this paper is a Ni-MH battery pack applied to a hybrid sport utility vehicle (SUV), which has a higher current ratio than Prius and is mounted on the bottom of the chassis to obtain a flat trunk space. In order to achieve better thermal management performance, a liquid-cooling system is selected and applied.
1.2. Contributions and Organization
Pesaran et al. [37
] systematically put forward the design process of BTMS: (1) determine the design objective and layout boundary; (2) determine the heat generation rate and thermal characteristic parameters of the battery; (3) determine the channel structure and preliminary formulation scheme; (4) determine the feasibility scheme through numerical simulation; and (5) experiment verification and design optimization.
In this paper, a performance-optimized and engineering-oriented properly liquid cooled plate was designed to meet the requirements of a battery pack in a certain hybrid SUV. In addition to the five steps before, this paper particularly emphasizes the role of theoretical calculation, which, together with numerical simulation, effectively reduces development costs, shortens development cycle, and improves development success probability and maturity. The organization of this study is introduced in Figure 1
The internal parameters of this cooling plate, including the cross-sectional area ratio of the main pipe to the branch pipes and the number of internal supporting walls in the branch pipe, were analyzed and studied. In addition to these inner parameters, the boundary conditions for the flow uniformity, pressure loss, flow rate, differential currents, differential inlet temperatures, and temperature performance of the pipe-based mixed parallel-series liquid-cooling system were also analyzed and studied. Three-dimensional finite element models of the packs’ cooling systems and batteries were set up and numerically simulated. Then, a temperature test platform with the battery pack, battery tester, and cooling tank was established to verify the theoretical design and simulation.