The performance of electric buses depends much on the performance of power batteries; so, the operating temperature of power batteries is highly correlated with their performance and influences also their life and safety. Power batteries' temperature management system is thus also rather crucial.
Power batteries' thermal management calls for heating them at low temperatures and cooling them at high ones.
Types of battery thermal management system
1.1 Battery cooling methods
The cooling methods of power batteries mainly include natural air cooling, forced air cooling, liquid cooling and refrigerant direct cooling.
-Forced air cooling: By introducing air conditioning wind, natural wind or convection wind outside the vehicle into the battery compartment, the battery temperature is reduced.
-Liquid cooling: Rely on the air outlet of the air conditioning system or the refrigerant of the independent refrigeration equipment to cool the coolant, and then transport the coolant to the heat exchanger in the battery pack to cool the battery cell.
-Refrigerant direct cooling: Directly introduce the refrigerant of the refrigeration equipment into the heat exchanger in the battery pack to cool the battery cell by heat conduction.
The comparison of different cooling methods is as follows:
◆Natural air cooling: The cooling performance is affected by the external environment, the system does not require additional control, no energy consumption, small space occupation, low cost, reliable process, and low risk of water wading.
◆ Forced air cooling: The cooling performance is average, the system volume is large, but the weight is light, the energy consumption is low, it is easy to control, the system cost is low, the process is reliable, and the risk of water wading is high.
◆ Liquid cooling: The cooling effect is good, the system volume is moderate, the control principle is mature, and the implementation difficulty is moderate. However, this method has a heavy system, high energy consumption, the highest cost, average process reliability, and a high risk of water wading. Currently, liquid cooling is the most widely used cooling solution in the industry.
◆ Refrigerant direct cooling: The cooling effect is the best, the system volume is small, the weight is light, the energy consumption is low, the system cost is moderate, but the control is complex, the technical difficulty is high, the process reliability is high, and the risk of water wading is high. At present, refrigerant direct cooling is still in the research stage and has not yet been widely used.
Although the heat exchange efficiency of refrigerant direct cooling is high, when the refrigerant evaporates in the battery pack, the temperature difference at various places is large, which affects the temperature uniformity of the battery and thus reduces the charging and discharging performance. In addition, the power demand of buses is large, the number of battery packs is large, and the pipeline layout of the refrigerant direct cooling system is relatively complex, and there is a risk of leakage.
1.2 Battery heating method
At present, the common power battery heating methods mainly include:
◆ Electric heating film heating: Integrate the electric heating film inside the battery pack to directly heat the battery cell.
- When the ambient temperature is higher than 0℃, this method has a good heating effect, no additional control is required, low energy consumption, low cost, small space occupation, and easy to implement.
- When the ambient temperature is lower than 0℃, the heating effect is greatly reduced and is usually not used.
◆Electric liquid heating: Connect an electric liquid heater in series in the water circulation loop of the battery pack thermal management to heat the antifreeze to increase the battery temperature.
- This method has a good heating effect, a moderate system size, less space occupation, mature control principle, high process reliability, and low difficulty in implementation.
- Although the cost is relatively high, it is currently widely used in battery heating systems due to its stability and efficiency.
Whether it is winter or summer, the normal operating temperature of the battery is 25 ℃±5 ℃. In winter, it needs to be heated by the battery thermal management equipment, and the target water temperature for heating is 25 ℃±5 ℃; in summer, it needs to be cooled by the BTMS system, and the target water temperature for cooling is also 25 ℃±5 ℃. The following introduces three types of liquid-cooled (heated) battery thermal management systems commonly used in buses. These three types of battery thermal management are all integrated heating and cooling systems. According to the use and ambient temperature requirements of pure electric buses, when the system needs to be cooled, the antifreeze is directly cooled through the heat exchanger of the BTMS; when the system needs to be heated, the PTC electric liquid heater connected in series in the BTMS water circulation system heats the antifreeze.
Simple unit form
System composition
The simple unit is mainly composed of a plate heat exchanger, a water pump, a fan, and a PTC electric liquid heater. Its principle is shown in Figure 1, and its composition is shown in Figure 2.
Figure 1: Simplified BTMS System Diagram
Figure 2: Simplified BTMS Structure Diagram
1 - Plate Heat Exchanger; 2、5 - Solenoid Valves; 3、4 - Water Temperature Sensors; 6 - PTC; 7 - Fan; 8 - Controller; 9 - Water Pump
Working principle
When the system receives a cooling signal, solenoid valve 2 opens and solenoid valve 5 closes.
The fan and water pump start running, extracting cold air from the air conditioning duct, and cooling the antifreeze through the plate heat exchanger inside the unit.
The cooled antifreeze is transported to the heat exchanger inside the battery by the water pump, and heat is exchanged with the battery cell to reduce the battery temperature.
When the system receives a heating signal, solenoid valve 2 closes and solenoid valve 5 opens.
The PTC electric liquid heater and water pump inside the unit start running to heat the antifreeze in the system.
The heated antifreeze circulates into the heat exchanger inside the battery pack, and increases the battery temperature through heat conduction.
In addition to the heating and cooling functions, the water-cooled unit also has a self-circulation function to reduce the temperature difference inside the battery.
When the BTMS sends a self-circulation command, the PTC heater and fan stop running, but the water pump continues to work, and the antifreeze circulates in the water circuit to ensure uniform battery temperature.
Features and scope of application
✅ Advantages
Simple structure and low cost.
Possess basic cooling, heating and self-circulation functions.
❌ Limitations
Rely on the cabin air conditioner to provide cold air, which is greatly affected by the operating status of the air conditioning system.
The cooling capacity is limited. Due to the lack of an independent refrigeration system, the cooling power is usually less than 2kW, which is difficult to meet the needs of high-power batteries.
It is suitable for slow-charging batteries with low-rate charge and discharge, and is often used in hybrid buses, but not for pure electric buses with stricter temperature control requirements.
Non-independent unit form
System composition and working principle
The non-independent unit uses the refrigerant of the air conditioning system to control the temperature of the battery, without the need for a separate refrigeration system. Its composition is shown in the figure.
- Evaporator 1: The cabin air conditioning evaporator provides cold air for the passenger area.
- Evaporator 2: The water-cooled unit evaporator is responsible for cooling the antifreeze and delivering the low-temperature antifreeze to the battery heat exchanger to reduce the battery temperature.
- Refrigeration circuit: Two evaporators are connected in parallel, sharing key components such as the compressor, condenser, and drying bottle. The refrigerant flow is controlled by solenoid valves 1 and 2, and expansion valves 1 and 2 are responsible for regulating the flow of the two refrigerants.
- Solenoid valve 2 is opened, and the refrigerant enters evaporator 2 to exchange heat with the antifreeze to cool the antifreeze.
- The low-temperature antifreeze circulates into the battery pack heat exchanger to reduce the battery temperature.
- Solenoid valve 2 is closed, the water pump and PTC electric liquid heater are started, and the antifreeze enters the battery heat exchanger after being heated to warm the battery.
- Solenoid valve 2 is closed, PTC stops working, only the water pump runs, and the antifreeze circulates in the water circuit to balance the battery temperature and prevent excessive temperature difference.
Features and scope of application
✅ Advantages
- No additional refrigeration system is required, reducing equipment costs.
- Strong refrigeration capacity (≥6kW), suitable for pure electric buses with high-rate fast-charging batteries.
❌ Limitations
- Affects the cooling of the passenger area, and the diversion of refrigerant will weaken the air conditioning effect and increase the system load.
- The system matching is complex, and the air conditioning manufacturers and models of different models are not fixed, making it difficult to match the water cooling unit with the air conditioning.
- The installation is limited, the high and low pressure pipelines are long, especially when the battery is placed at the bottom, the connection between the air conditioning and the water cooling unit is more difficult.
- The control logic is complex, and the cooling demand of the battery and the vehicle air conditioning system needs to be coordinated, and the control strategy requirements are high.
Applicable scenarios:
- Applicable to fast-charging batteries with high charge and discharge rates.
- Suitable for pure electric buses with high requirements for cooling capacity (≥6kW).
Independent unit form
System principle and structure
The independent unit is equivalent to a small pure electric air conditioning system with an independent and complete refrigeration system. The main difference lies in the structure of the evaporator:
- The evaporator of the ordinary air conditioning is used for heat exchange between the air conditioning refrigerant and the air;
- The evaporator of the independent unit is used for heat exchange between the air conditioning refrigerant and the antifreeze.
- This unique heat exchanger generally adopts a shell-and-tube structure, in which:
- The inner pipe transmits the refrigerant;
- The outer pipe transmits the cooling water;
- Fins are distributed between the two layers of pipes to increase the heat exchange area and improve the heat exchange efficiency.
- There are two design forms for independent units:
- Integrated design
- Split design
Working principle
- After the system receives the refrigeration signal, it starts the fan and water pump.
- The refrigerant exchanges heat with the antifreeze through the plate heat exchanger to reduce the temperature of the antifreeze.
- The antifreeze enters the battery heat exchanger through the water pump circulation to achieve battery cooling.
- After the system receives the heating signal, it starts the PTC electric liquid heater and water pump.
- After the antifreeze is heated, it flows into the heat exchanger inside the battery to increase the battery temperature through heat exchange.
Features and scope of application
✅ Advantages
- Independent system, not affected by the air conditioning refrigeration performance, can quickly respond to changes in battery temperature.
- Flexible layout, power can be adjusted as needed to meet different cooling needs.
- The control logic is simple, because there is an independent refrigeration system, there is no need to consider the cooling needs of the passenger area air conditioning.
- The cooling capacity is adjustable, usually above 2 kW, suitable for hybrid or pure electric buses with high-rate fast charging batteries.
❌ Limitations
- Compared with the non-independent unit system, the independent unit has an additional set of compressors and condensers for independent refrigeration, and the cost is slightly higher. But because it is an independent system, the control logic is simpler than that of the non-independent unit.
Applicable scenarios:
- Applicable to fast-charging batteries with high battery charge and discharge rates, such as hybrid buses and pure electric buses.
Conclusion
As a key component of electric buses, the BTMS directly affects the performance, safety and life of the power battery. Therefore, in the design and development of electric buses, it is very important to have a deep understanding and mastery of the structure and working principle of battery thermal management equipment. This not only helps to improve the overall efficiency of the vehicle, but also ensures the long-term reliability of the battery, thereby providing stronger support for the operation of the vehicle.Learn More about the Guchen Battery Thermal Management System
