Design and simulation of liquid cooled system for power battery of PHEV

Various battery chemistries have different responses to failure, but the most common failure mode of a cell under abusive conditions is the generation of heat and gas. To prevent battery thermal abuse, a battery thermal management system is essential. An excellent design of battery thermal management system can ensure that the battery is working at a suitable temperature and keeps the battery temperature diffenence at 2-3 °C. This paper presents a thermal-elcetric coupling model for a 37Ah lithium battery using AMESim. A liquid cooled system of hybrid electric vehicle power battery is designed to control the battery temperature.A liquid cooled model of thermal management system is built using AMESim, the simulation results showed that the temperature difference within 3°C of cell in the pack.


Introduction
To meet the needs of the market, electric vehicles must have performance especially considering safety 、range and battery reliability comparable to that of a modern combustion engine vehicle [1]. Research shows that the battery components are completely stable below 80°C, but once the temperature reaches 120-130°C, the passivation SEI (Solid Electrolyte Interface) layer starts dissolving progressively in the electrolyte, which causes electrolyte to react with the least protected surface of graphite generating heat. The battery temperature impacts on life, safety and performance of lithium-ion batteries and suggested a range of 15-35°C as desired working temperature. One of the key is in the thermal control of the battery: operation at low temperature reduces the power output due to suppressed electro-chemical reactions, while elevated temperature accelerates corrosion leading to reduced battery life [2]. It is also important that the temperature within a battery cell is uniform: variations will cause the electrochemical reactions to proceed at different rates in different regions of the cell, thereby leading to incomplete energy utilization, and inefficient management of the battery life [2]. Any power consumed for the regulation of temperature reduces the power available to the primary vehicle functions, and so the efficient operation of a thermal management system is also desirable [3].
An air-cooling system worked very well in HEVs during standard drive cycles that could control the maximum temperature below the limit of 55 °C and the temperature difference was no more than 5°C. Excessive ambient temperature and higher energy density batteries, an active liquid cooled system gives the most effective and efficient thermal management [4]. 11.4kWh The total pack nominal voltage is 310.8 V. The total energy is 11.4 kWh. Figure 1 shows three dimensional representation of the lithium-ion battery pack layouts considered in this study.There are 84 battey cells total which are packed in seven modules.  3 Thermo-electric model Bernardi derived an expression for battery heat using a thermodynamic energy balance on a complete cell [5].Its simplified form [6] : = − + ( − ) (1) The first term is the reversible reaction heat and the second term is irreversible reaction heat. The reversible heat flow rate can be calculated by the following relationship:  = is the specific heat m is the battery mass h is the convective heat transfer coefficient A is the convective surface or radiation surface is the emissivity is the boltzmann constant is ambient air temperature l is the condution surface K is the condution heat transfer coefficient is the battery contact solid temperature  Fig. 4 shows the battery cooling system contains a water pump, liquid cooled plate, chiller, expansion valve, condenser, compressor, evaporator and chiller. The heat generated from the battery is passed through the cold plate to the coolant.The coolant was driven into the chiller by the water pump ,where happen heat exchange with the refrigerant . Figure 5 Liquid cold plate modeling principles In order to model in AMESim,the cold plate was discretized. Figure 5 shows the liquid cold plate is divided into battery discrete mass, cold plate discrete mass and flow channel discrete region. Fig. 6 show that the entair AMESim model for a liquid cooled system of a PHEV. Figure 7 Liquid cold plate modeling principles in AMESim Cold plate modeling needs to consider the simulation accuracy requirements and the complexity of the cold plate structure.Abse on lumped therml capacity ,a battery cell can be regarded as a thermal mass ,cold plate can be discreted into many mass units and flow channel inside the plate can be discreted into many regions, as shown in Figure 5. Figure 7 show that the cold plate AMESim model ,the heat convection coefficient is setted to 5.

Simulation
Wate pump model need to enter the pump head, flow and speed test data,as shown in Figure 8. If only limited data or the pump operating status is out of existing data range , the data will be interpolated based on the principle of similarity.  Fig. 9 shows the air conditioning system amesim model, including the compressor, condenser, chiller, expansion valve model,more detail can be found in [5]. Figure 9 Air conditioning system amesim model

Thermal management Strategies
When battery maximum temperature in the pack rise to 35℃, the water pump and air conditioning open at the same time; when the battery minimum temperature down to 30 ℃, the watre pump and air conditioning close at the same time.

Results and discussion
Figure 10 Battery current and heat generation rate Fig.10 shows that current and heat generation rate of battery.The battery is charged-discharged at C -rates,the heat generation rate was calculated from the battery model in AMESim.Next the battery temperature and maximum temperature difference at 8L/min,4L/mim and 2L/min flow rate will be simulated based on the same working condition in Fig.10.   Fig.12 shows the coolant flow rate of 8L / min, the battery pressure drop includes four cold plate pressure loss and pipe pressure loss ,it is 28.9kPa in total. Figure 13.The maximum battery temperature difference in the pack Compared with air cooling system, the advantage of liquid-cooling system is that the better uniformity of battery temperature. The simulation results show that the maximum battery temperature difference in pack is 2 ℃ at 8L/min flow rate. The maximum temperature difference occurs when the air conditioner is turned off.   The simulation results show that the maximum battery temperature difference in pack is 5 ℃ at 2L/min flow rate.

Conclusions
In this paper, A novel Liquid cooled system is designed for a battery pack of PHEV. A liquidcoolling system model in AMESim was developed to research the battery thermal concern,it is very important especially at the initial stage of the design.
1. The maximum temperature difference occurs when the air conditioner is turned off; 2. As the coolant flow decreases, the battery maximum temperaturec in pack decreases increases; 3. The simulation results show that the power battery cooling system to meet the design requirements, the liquid cooling system can control the battery temperature difference within 3 ℃.