Electric vehicle air conditioners have a certain foundation in heat pump air conditioners, and Honda pure electric vehicles in Japan have adopted electric drive heat pump air conditioners. In addition, when used in particularly cold areas, some customers can choose to install a fuel heater heating system.
Denso Corporation (DENSO) has developed an electric vehicle heat pump air conditioning system using R134 refrigerant, which uses the structure of an in-vehicle condenser and evaporator in the air duct of the heat pump system. In recent years, Denso has also developed a CO2 heat pump air-conditioning system. The system also adopts the scheme of installing two heat exchangers, an evaporator and a condenser in the air duct. The difference from the R134 system is that when the system is in cooling mode When the refrigerant flows through the internal condenser and the external condenser at the same time.
When only one heat exchanger is used in the air duct, it is an evaporator in the cooling mode and a condenser in the heating mode. The heat pump air conditioning system with this structure not only needs to develop an expansion valve that allows two-way flow, but also under heat pump conditions, when the system is defrosting, the condensed water on the heat exchanger in the air duct will quickly evaporate and form on the windshield. Frost is not conducive to safe driving. Therefore, it is necessary to adopt a structure with an internal condenser and an evaporator in the air duct of the heat pump system, and the condenser and evaporator outside the vehicle share a heat exchanger.
In order to reduce the power consumption of the battery by the air conditioner, the American company Amerigon has developed an air-conditioned seat. This air-conditioned seat is equipped with an electric heat pump. The function of the electric heat pump is to transfer heat to the water tank outside the space where the temperature needs to be adjusted, so as to realize the demand. The temperature-regulated space is cooled or heated. In addition to energy saving, this kind of air-conditioned seat can also improve driving and riding comfort, and is more suitable for supporting use in electric vehicles.
Pure electric vehicles do not have engine waste heat for heating, and cannot be used as a heat source for vehicle air conditioning in winter. The air conditioning system of pure electric vehicles must have the function of heating by itself, that is, a heat pump type air conditioning system is required. At the same time, the compressor can only be directly driven by an electric motor, and the structure is not exactly the same as the existing compressor type. Since the battery (power battery) used to power the heat pump air-conditioning system is mainly used to drive the car, the energy consumption of the air-conditioning system has a great impact on the journey of the car every time it is charged. If the electric vehicle still uses the existing low-efficiency air-conditioning system, it will consume more than 10% of the electric power, which means that it is necessary to choose between increasing the manufacturing cost of the battery and reducing the driving performance index of the electric vehicle. Compared with fuel vehicles, higher requirements are placed on the energy-saving and high-efficiency air-conditioning systems of electric vehicles. At the same time, electric vehicle air conditioners must solve the two major problems of cooling and heating. According to the unique properties of electric vehicles, the current heating methods of electric vehicle air conditioning include semiconductor (thermocouple), electric heat pump, PTC heating, fuel heating, etc. Among them, fuel heating heating is generally used in hybrid electric vehicles , Electric heat pump air conditioners are the most promising.
(1) Semiconductor type
Semiconductor refrigeration, also known as electronic refrigeration, or thermoelectric refrigeration, has been developed from the 1950s, which is between compression refrigeration and absorption refrigeration and is called the world’s three major refrigeration methods. The basic device of the semi-conductor is a thermocouple pair, that is, an N-type semiconductor and a P-type semiconductor are connected to form a thermocouple, as shown in Figure 1. After the direct current is connected, the temperature difference and heat will be generated at the interface Transfer. Several pairs of semiconductor thermocouples are connected in series on the circuit, and the heat transfer is connected in parallel, which constitutes a common refrigeration thermopile. With the help of various heat transfer methods such as heat exchangers, the hot end of the thermopile is continuously dissipated and maintained at a certain temperature, and the cold end of the thermopile is placed in the working environment to absorb heat and cool down. This is the principle of semiconductor refrigeration.
As a special cold source, semiconductor refrigeration has the following characteristics in technical application: no refrigerant is needed; continuous work; no pollution source; no rotating parts, no rotation effect: no sliding parts, no vibration, noise during work, and longevity Long; easy to install.
Semiconductor refrigeration fins can both refrigerate and heat. The refrigeration efficiency is generally not high, but the heating efficiency is very high, always greater than 1. Therefore, the use of a single piece can replace separate heating and cooling systems. Semiconductor refrigeration chip is a current-transducer type chip. Through the control of input current, high-precision temperature control can be realized. Coupled with temperature detection and control means, it is easy to realize remote control, program control, and computer control, which is convenient to form an automatic control system. The thermal inertia of the semiconductor refrigeration fin is very small and the heating time is very fast. When the heat dissipation of the cooked end is good and the cold end is unloaded, the refrigeration fin can reach the maximum temperature difference in less than 1 min. The reverse use of semiconductor refrigeration fins is temperature difference power generation. Semiconductor refrigeration fins are generally suitable for power generation in the middle and low temperature areas. The power of a single refrigeration element pair of a semiconductor refrigeration chip is very small, but it is combined into a stack and combined into a refrigeration system with the same type of stacks in series and parallel. The power can be very large, so the cooling power can be several milliseconds. The range of watts to tens of thousands of watts. The temperature difference range of the semiconductor refrigeration film is wide, and it can be realized at -130~90℃.
From the perspective of the maturity of air-conditioning technology and the comparison of energy utilization efficiency, electric vehicle air-conditioning systems that use semiconductor refrigeration technology currently have problems such as low figure of merit and unsatisfactory refrigeration performance of thermoelectric materials, and the output of thermopile is affected by the composition of thermoelectric materials. The limitation of the output of component elements does not meet the energy-saving and high-efficiency requirements of electric vehicle air conditioners.
(2) Electric heat pump type
In theory, reversal of the refrigeration cycle can be used for heating. However, when the ambient temperature is low, the heating performance will decrease, which cannot meet the requirements for high heating performance in low temperature areas. When driving, the electric compressor is used to compress the refrigerant and the circulating medium is cooled by the wind in the condenser, and in winter, when the condenser (change to the evaporator when heating) is frosted, the heating performance is also difficult to exert. This requires consideration of adding a system for heating and defrosting the condenser (the evaporator when heating).
Heating originally requires higher performance than cooling in some cases. For example, when heating and driving in winter, air from outside the car is generally introduced to prevent fogging of the car windows. Since the car must discharge heated air to the outside of the car while driving, heating requires higher performance than cooling.
The heat pump air conditioning system is improved on the basis of the original fuel vehicle air conditioning system. The compressor is directly driven by a permanent magnet DC brushless electric motor. The working principle of the system is shown in Figure 2. There is no essential difference between this system and ordinary heat pump air-conditioning systems. Because it is used in electric vehicles, the main components such as compressors have their particularities. The biggest advantage of this technology is its high cooling and heating efficiency. The fully enclosed electric scroll compressor is driven by a DC brushless motor and cooled by the return air of the refrigerant. It has the advantages of low noise, low vibration, compact structure and light weight. Under the test conditions of an ambient temperature of 40°C, an interior temperature of 27°C, and a relative humidity of 50%, when the system is stable, a cooling capacity of 29kW can be obtained with 1kW of energy consumption; when the ambient temperature is -10°C, the interior temperature of the vehicle At 25°C, when the system is stable, 2.3kW of heating capacity can be obtained with kW of energy consumption. Under the ambient temperature of -10~40℃, it can provide a comfortable driving environment for pure electric vehicles with higher efficiency. If the parts technology can be improved, the corresponding efficiency can also be improved.
At present, the biggest bottleneck of the heat pump-type pure electric vehicle air conditioner is the problem of low-temperature heating, which is also one of the future research problems in the industry. In order to make the heat pump type pure electric vehicle air conditioner more energy-saving and efficient, generally focus on the solution from the following perspectives: develop a more efficient DC scroll compressor; develop a more precise and energy-saving silicon electronic expansion valve; use an efficient subcooling type Parallel flow condenser; improve the structure of the evaporator, so that the refrigerant evaporates more uniformly. In addition, pure electric vehicles are affected by the number of door openings, speed, light and other factors during driving, and the air conditioner has a heavy heat and humidity load. The compressor and even the entire air-conditioning system must adapt to this multi-factor changing working conditions. Therefore, the design of the heat-pump-type pure electric vehicle air-conditioning system is particularly important.
The relationship between the air volume of the evaporator fan and the cabin temperature, set temperature, ambient temperature, solar radiation intensity, and evaporator outlet temperature is non-linear.
The automobile air conditioner heat pump system is similar to the ordinary household air conditioner, and is an extension of the common household air conditioner. In order to prevent the comfort of the car from being reduced due to the removal of the box during heating, a hot air bypass method for uninterrupted heating and removal of the box is adopted. When defrosting, the operating principle is basically the same as that of heating, except that the defrosting solenoid valve is opened, so that part of the high-temperature and high-pressure superheated gas from the compressor is diverted to the inlet of the outdoor heat exchanger, and the outdoor heat exchanger is quickly turned off. The humidity is increased to above 0°C to melt the frost layer on the outdoor heat exchanger, so that the heat exchanger maintains a good heat exchange efficiency.
(3) PTC heating type
When a pure electric vehicle adopts the electric heating method of a heater, the heater is generally arranged under the floor between the driver’s seat and the passenger’s seat. The heater consists of a PTC heater element that can generate electricity, a cooling fan that transfers the heat of the heater element to the radiator (cooling water), a radiator flow path, and a control board. Because the heater is required to have a higher heating performance, the power source uses a high-voltage power battery. If it is a special product for pure electric vehicles, it is not necessary to use coolant. The heating unit made directly with a blower needs to use the high voltage of the power battery, and a small amount of heat dissipation element generates a lot of heat, so the warm air heated by the PTC heater is heated. .
Since the manufactured heating unit needs to use the high voltage of the power battery, and a small amount of exothermic element is used to generate a large amount of heat, the heater needs rich design and manufacturing technology experience. There is a plate-shaped heater element inside the heater body, and heat dissipation is improved by passing a heat sink (cooling water) on both sides of the element. The heater element adopts the ordinary PTC element, and the PTC element sandwiched between the electrodes has the property that the resistance changes with the temperature of the element. In the low temperature area, the resistance is low, and the current flows to generate heat. As the temperature increases, the resistance gradually increases, and the current is difficult to flow, and the heat generation decreases. The characteristics of PTC components meet the heating performance requirements of automobiles, that is, they have high heating performance in low temperature areas.