The heat generated by battery cells during operation is mainly composed of four parts: polarization heat, reaction heat, side reaction heat, and Joule heat. For lithium-ion batteries, the heat generated by side reactions is extremely small, so it can be ignored. The internal reaction heat of the battery only needs to consider the remaining three parts of heat:
The reaction heat Q1 during battery charging and discharging can be expressed as: Q1=0.0104Q * I
In the formula, Q is the algebraic sum of the heat generated by the positive and negative electrodes of a chemical reaction process, measured in kJ/mol; I is the discharge current, measured in A.
Joule heating generated by the internal ohmic resistance Re in the battery cell Q2: Q2=I ^ 2 * Re
Polarization heat generated by internal polarization resistance Rp in the battery cell Q3: Q3=I ^ 2 * Rp
When the operating temperature of the battery is between 70-80 ℃, the heat of reaction accounts for the vast majority. At the normal operating temperature of lithium-ion, Joule heat and polarization heat account for the vast majority.
The heating component is mainly composed of a battery heating core and an expanding foam between the cores (the foam itself does not generate heat and mainly serves as a buffer).
The heating element is divided into ten battery module groups, with eight groups arranged horizontally and two groups arranged vertically. The heat transfer components mainly include heat transfer aluminum plates between the cores, cooling plates, connecting adhesives (connecting heat transfer aluminum plates and cooling plates), and inlet and outlet cooling water pipes.
The cooling plate is embedded with a cooling water jacket inside, and the coolant inside the cooling water jacket carries out the heat transferred to the cooling plate through convective heat exchange.
The overall diagram of the power battery and the structural diagram of the cooling plate (dark gray indicates the internal water jacket) are shown in the following figure.
1. Pre simulation processing
Due to the complex internal structure of lithium-ion batteries, direct modeling is difficult and facilitates CFD calculation and analysis, requiring simplified processing of the batteries. Here, the battery cell is simplified into a rectangular model with uniform heating.
The heating of the battery cells in this scheme is mainly carried out by the flow of coolant through thermal conduction and convective heat transfer.
Therefore, the components that need to be considered for the cooling of the entire power battery include: the heat-conducting aluminum plate between the battery cells, the cooling plate, the connecting adhesive between the heat-conducting aluminum plate and the cooling plate, and the expansion foam between the cores. The connecting adhesive and expansion foam are simplified into a homogeneous rectangular model.
The heat emitted from the outer frame of the heating core is relatively small and can be ignored here. The simplified model of the entire cooling system is shown in the following figure.
The flow is incompressible turbulence, and the standard K-Epslion model is used to simulate turbulent flow, with the wall treated using standard wall functions.
The battery cell is treated as an anisotropic uniform thermal conductivity, and the battery is simplified as a uniform heating element. The mass of a single battery cell is 0.85kg, the specific heat capacity of the battery cell is 1020J/kg •℃, and the initial temperature for cooling the battery is 30 ℃.
The thermal conductivity coefficient of the battery cell is 1W/m • k along the thickness direction and 25W/m • k perpendicular to the thickness direction.
The coolant is a mixture of 50% ethylene glycol and 50% water by volume, with an inlet flow rate of 6.5L/min (estimated based on the total heat generated by the battery) and an inlet water temperature of 25 ℃.
The battery heating evaluation condition adopts the US06 operating condition. The battery heating situation within one US06 cycle condition (600s) is shown in the following figure. The instantaneous heating power of the battery cell is converted to the average equivalent heating power of 3.367W in steady state.
2. Analysis of simulation results
The overall temperature distribution of the battery cells is not uniform, and the two vertically placed battery packs have a low overall temperature due to being cooled first; On the contrary, the other eight horizontally arranged battery packs were finally cooled, resulting in a high overall temperature. In addition, due to the use of single-sided cooling in the battery pack, each group of cells also has a temperature difference of about 5 ℃. After steady-state calculation equilibrium, the maximum temperature of the battery cell as a whole reached 40.17 ℃ (allowable value of 45 ℃), and the maximum temperature difference of the battery cell as a whole was as high as 10.44 ℃ (exceeding the allowable value of 5 ℃), indicating uneven cooling of the battery cell as a whole.
