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Views: 3 Author: Site Editor Publish Time: 2023-05-16 Origin: Site
The operation of pure electric vehicles uses electricity as energy, and the vehicle is driven by an electric drive system. The thermal management system of new energy vehicles is very different from that of traditional fuel vehicles. In new energy vehicles, batteries and motors generate heat during work, while in traditional fuel vehicles, the engine generates heat. The thermal management system of new energy vehicles requires precise temperature control, mainly the cabin temperature, battery temperature and powertrain temperature. Therefore, the thermal management system of new energy vehicles will be more complicated. For new energy vehicles, solving the contradiction between the cruising range and ride comfort of new energy vehicles is an urgent problem that new energy vehicles need to solve.
The air conditioning system is the key to the thermal management of the car. Both the driver and the passengers want to pursue the comfort of the car. The important function of the car air conditioner is to make the passenger compartment achieve a comfortable driving and riding environment by adjusting the temperature, humidity and wind speed in the car's passenger compartment. The principle of the mainstream car air conditioner is to cool or heat the temperature inside the car through the thermophysical principle of evaporative heat absorption and condensation heat release. When the outside temperature is low, heated air can be delivered to the cabin so that the driver and passengers do not feel cold. The ability to deliver low-temperature air into the cabin when the outside temperature is high keeps the driver and passengers feeling cooler. Therefore, the car air conditioner plays a very important role in the air conditioning in the car and the comfort of the occupants.
Traditional fuel vehicle air conditioners are mainly composed of four components: evaporator, condenser, compressor, and expansion valve.
A compressor is a power device that can compress a low-temperature and low-pressure gaseous refrigerant into a high-temperature and high-pressure gaseous refrigerant. Compressors are generally installed on the engine in fuel vehicles, and the compressor is driven by the engine to work.
The evaporator is a heat exchange device installed in the cockpit. The working principle of the evaporator is to use evaporation to absorb heat to cool down. When the low-temperature and low-pressure liquid refrigerant passes through the evaporator, the liquid refrigerant vaporizes and absorbs the heat in the compartment, thereby rapidly cooling the compartment.
The condenser is also a heat exchange device and is installed outside the compartment. The working principle of the condenser is to heat up by condensing and absorbing heat. When the high-temperature and high-pressure gaseous refrigerant passes through the condenser, the heat is released to the outside air through forced cooling by the fan, so that the high-temperature and high-pressure gaseous refrigerant is transformed into a medium-temperature and high-pressure liquid refrigerant.
An expansion valve is a device that expands medium temperature and high pressure liquid into low temperature and low pressure liquid. The expansion valve is generally installed at the entrance of the evaporator, and expands the medium-temperature and high-pressure liquid refrigerant into a low-temperature and low-pressure liquid refrigerant, so that the refrigerant enters the evaporator to absorb the heat in the vehicle compartment.
Automobile air conditioning is composed of refrigeration system, heating system and ventilation system. These three systems make up the assembly of the car air conditioner. The principle of traditional fuel vehicle refrigeration is the four steps of compression, condensation, expansion, and evaporation, as shown in Figure 1. By repeating these four steps, the operation of the refrigeration system can be guaranteed. The evaporator then continues to cool the cabin.
The principle of traditional fuel vehicle heating is to use the waste heat of the vehicle engine to heat the vehicle cabin. Firstly, the cooling water with higher temperature coming out of the engine cooling water jacket enters the warm air core. Cool air is blown through the heater core by a fan. The heated air can then be blown into the cabin for cabin heating or window defrosting. The cooling water returns to the engine after leaving the heater, completing a cycle.
The driving devices of new energy vehicles and traditional fuel vehicles are different. The air-conditioning compressor of a fuel vehicle is driven by the engine, while the air-conditioning compressor of a new energy vehicle is driven by a motor. Therefore, the air-conditioning compressor on the new energy vehicle cannot be driven by the engine, but an electric compressor is used to compress the refrigerant. The basic principle of new energy vehicles is the same as that of traditional fuel vehicles, using condensation to release heat and evaporation to absorb heat to cool the passenger compartment. It's just that the compressor is changed to an electric compressor. At present, the scroll compressor is mainly used to compress the refrigerant.
The heating mode of new energy vehicles is quite different from that of traditional fuel vehicles. The heating mode of traditional fuel vehicles is to transfer the waste heat of the engine to the compartment through the coolant to heat up the compartment. New energy vehicles do not have an engine, so there is no process for the engine to heat the cabin. It uses other heating modes to heat the cabin. Several new energy vehicle air conditioning heating methods are given below.
The semiconductor heater is composed of semiconductor elements and terminals to perform cooling and heating functions. The thermocouple in this system is the basic component for cooling and heating, and its structure is shown in Figure 2. Connect two semiconductor devices to form a thermocouple. After the direct current is connected, heat and temperature difference will be generated at the interface to heat the interior of the cabin. The main advantage of semiconductor heating is that it can heat the cabin quickly. Its main disadvantage is that semiconductor heating consumes a lot of electricity. For new energy vehicles that need to pursue mileage, its shortcomings are fatal. Therefore, it cannot meet the requirements of new energy vehicles for energy saving of air conditioners. It is also more necessary for people to conduct research on semiconductor heating methods and design an efficient and energy-saving semiconductor heating method.
The main component of the PTC is the thermistor. Heating by electric heating wire is a device that directly converts electrical energy into heat energy. The PTC air heating system is to turn the warm air core of a traditional fuel vehicle into a PTC air heater. A fan is used to drive the outside air through the PTC heater for heating. The heated air is sent into the interior of the cabin to heat the cabin. Due to its direct power consumption, the energy consumption of new energy vehicles is also relatively large when the heater is turned on.
PTC plumbing, like PTC air heating, generates heat by using electricity consumption. But the plumbing system works by first heating the coolant with the PTC. After the coolant is heated to a certain temperature, the coolant is pumped into the heater core to exchange heat with the surrounding air. A fan sends heated air into the cabin to heat the cabin. Then the cooling water is heated by PTC and reciprocated. This heating system is more reliable and safer than PTC air cooling.
The principle of the heat pump air conditioning system is the same as that of the traditional automobile air conditioning system. The heat pump air conditioner can realize the conversion of cabin heating and cooling, and its principle is shown in Figure 3 and Figure 4. A four-way reversing valve is used to change the flow direction of the refrigerant in the system, so as to achieve the process of refrigeration and heat exchange. Since the heat pump air conditioner does not directly consume electric energy to generate heat, the energy saving degree of the heat pump air conditioner is higher than that of the PTC heater. At present, heat pump air conditioners have been mass-produced in some vehicles.
The thermal management of the automotive power system is divided into the thermal management of the traditional fuel vehicle power system and the thermal management of the new energy vehicle power system. Now the thermal management of the power system of traditional fuel vehicles is very mature. Traditional fuel vehicles are powered by engines, so engine thermal management is the focus of traditional vehicle thermal management. The thermal management of the engine mainly includes the cooling system of the engine. More than 30% of the heat in a car's system needs to be dissipated by the engine cooling circuit to keep the engine from overheating under high loads. The engine's coolant is used to heat the cabin.
The power plant of a traditional fuel vehicle consists of the engine and transmission of a traditional fuel vehicle, while a new energy vehicle consists of a battery, motor, and electronic control. The thermal management methods of the two have changed a lot. The normal operating temperature range of the power battery of new energy vehicles is 25-40 ℃. Therefore, thermal management of a battery requires both keeping it warm and dissipating it. At the same time, the temperature of the motor should not be too high. If the temperature of the motor is too high, it will affect the service life of the motor. Therefore, the motor also needs to carry out necessary heat dissipation measures during use. The following is an introduction to the thermal management system of the battery and the thermal management system of the motor electronic control and other components.
The thermal management system of the power battery is mainly divided into air cooling, liquid cooling, phase change material cooling and heat pipe cooling based on different cooling media. The principles and system structures of different cooling methods are quite different.
Through the flow of air, the battery pack conducts convective heat exchange with the outside air. Air cooling is generally divided into natural cooling and forced cooling. Natural cooling is when the outside air cools the battery pack when the car is running. Forced air cooling is to install a fan for forced cooling against the battery pack. The advantages of air cooling are low cost and easy commercial application. The disadvantages are low heat dissipation efficiency, large space occupation ratio, and serious noise problems.
Heat is removed from the battery pack by the flow of liquid. Since the specific heat capacity of liquid is larger than that of air, the cooling effect of liquid cooling is better than that of air cooling, and the cooling speed is also faster than that of air cooling. The temperature distribution after cooling the battery pack is also relatively uniform. Therefore, liquid cooling is also widely used commercially. But liquid cooling also has disadvantages. The disadvantage is that there is a risk of leakage, the complexity is relatively large, and the maintenance cost is high.
Phase change materials (PCM) include paraffin, hydrated salts, fatty acids, etc. When a phase change occurs, it can absorb or release a large amount of latent heat while its own temperature remains unchanged. PCM has a large thermal energy storage capacity without additional energy consumption, and is widely used in battery cooling of electronic products such as mobile phones. However, the application of automotive power batteries is still in the research state. Phase-change materials suffer from low thermal conductivity, causing the PCM to melt on the side that contacts the battery. However, other parts are not melted, which reduces the heat transfer performance of the system and is not suitable for large-sized power batteries. If these problems can be solved, PCM cooling will become the most potential development solution for thermal management of new energy vehicles.
A heat pipe is a device based on phase change heat transfer. A heat pipe is a sealed container or sealed pipe filled with a working medium/liquid (water, ethylene glycol or acetone, etc.) in a saturated state. One section of the heat pipe is the evaporation end, and the other end is the condensation end. It can not only absorb the heat of the battery pack but also heat the battery pack. It is currently the most ideal power battery thermal management system.
Direct cooling is a way to install the evaporator of the air conditioning system in the battery box to cool the battery box quickly by using the principle of evaporation and heat absorption of refrigerants such as R134a refrigerant. The direct cooling system has high cooling efficiency and large cooling capacity.
The above are the mainstream thermal management methods of the five power batteries, and their respective advantages and disadvantages are shown in Table 1.
