In the refrigerant alternative, CO has attracted widespread attention and overall optimism from academia and industry with its excellent environmental protection effects, wide service adaptability and compatibility with vehicle systems.
With the development of new energy vehicles, the waste heat utilization of heating without engines, and the transcritical CO cycle has received more and more attention because of its strong heating characteristics.
01
Why are CO heat pumps optimistic?
In the context of carbon neutrality, the fourth generation of refrigerants (natural or HFO low GWP refrigerants) has become a green alternative to the third generation of HFC refrigerants because of its excellent performance and environmental protection, but these alternatives have their own advantages and disadvantages (see "The Next Generation of New Energy Vehicle Refrigerants, Who is the Protagonist?"). "), at this stage, there is still controversy about the selection of the next generation of refrigerants.
Among them, CO, with its excellent environmental protection effect, wide working condition adaptability and compatibility with the vehicle system, has attracted widespread attention and overall optimism from the academic and industry circles.
CO is a natural refrigerant, chemically stable, as a refrigerant safety class of A1, its ODP is zero, GWP is only 1, the environmental impact is minimal. In fact, as early as 1993, the International Society of Refrigeration proposed to apply CO to automotive air conditioning systems, but the development was hindered due to poor refrigeration effect. In recent years, with the development of new energy vehicles, the waste heat utilization of its heating without engines, and the transcritical CO cycle has received more and more attention because of its strong heating characteristics.
In the field of household use, foreign CO heat pumps have been applied in Japan, the United States, Denmark and other countries. In the automotive sector, in 2013 the Volkswagen Group announced that all its models will use CO as a refrigerant in the future, and in 2021, many models in the pure electric MEB platform will be equipped with CO heat pump air conditioning, kicking off the prelude to large-scale in-vehicle applications.
The image comes from the Internet
With the optional CO heat pump for Volkswagen MEB models, more car companies will also join the team of CO heat pumps in the future. CITIC Securities expects that in 2025, the new energy vehicle field will be equipped with about 4.6 million sets of CO heat pumps, corresponding to a market space of about 36 billion yuan.
02
Transcritical CO heat pumps and their optimization
CO is very different from the thermal physical properties of traditional refrigeration working fluids, with a critical temperature of only 31.1°C and a critical pressure of 7.37MPa. For the automotive CO heat pump system, the exothermic process on the high-pressure side is in the supercritical area above the critical point, and the evaporative heat absorption process on the low-pressure side is in the subcritical region, so the automotive CO heat pump system can only use transcritical circulation.
In the transcritical CO cycle, the high-pressure side CO has been in the supercritical region during the exothermic process, only the heat exchange does not occur, so the condenser is called a gas cooler in the transcritical CO cycle. The transcritical CO circulation system is not much different from the traditional working fluid heat pump system in terms of device composition, which is mainly composed of four parts: compressor, gas cooler, evaporator and throttling device, in addition, auxiliary devices such as gas-liquid separators, control systems and automatic control devices are sometimes provided.
Transcritical CO circulation system diagram / Image source: CNKI
The pure transcritical CO cycle has no advantage over other refrigerant cycles, so the researchers optimized the transcritical CO cycle system. At this stage, the solution suitable for improving the performance of the ELECTRIC vehicle CO heat pump air conditioning system is mainly to add a reheater to the system and introduce the supplementary gas enthalpy technology.
First, the introduction of the recuperator may improve the performance of the circulatory system, but also may reduce the performance of the circulatory system, and whether it is increased or reduced will depend on the thermal properties of the selected working medium. A large number of scholars have studied the effect of the recuperator on the transcritical CO system under different working conditions, and the results of the study point out that if the system obtains the maximum COP as the design goal, the system recuperation rate is about 15%. Based on enthalpy difference, the researchers derive a method to effectively express the efficiency of the recuperator, and the theoretical calculation results show that the efficient heat collector in the transcritical CO system is an important factor in obtaining high system performance.
Second, the technology of replenishing gas and increasing enthalpy can significantly improve the adaptability of heat pump air conditioning in cold areas. The system application form of replenishment gas enthalpy technology is mainly divided into two types: belt economy and flasher. It works by spraying part of the refrigerant from the outlet gas cooler into the compressor after flashing or economizer heat transfer.
03
Application of CO heat pumps in electric vehicles
Direct heat pump system
Household heat pump air conditioner is the use of indoor and outdoor heat exchangers, the use of four-way reversing valve to switch the direction of refrigerant flow. The difference in electric vehicles is that when the air conditioning system switches from defrost mode to heating mode in winter, the condensate of the heat exchanger in the car will evaporate and fog up on the windshield of the car with the fresh wind blowing, bringing safety hazards to the driving. The researchers developed an electric vehicle heat pump system that uses three heat exchangers, two of which are arranged in the electric vehicle air duct. This enables free switching between cooling, heating and defrosting and dehumidification.
Heat pump air conditioning system for electric vehicles with four-way reversing valve
Image source: CNKI
The existing four-way reversing valve is a copper product, mainly supplied to the household heat pump air conditioner, and the aluminum weldability on the car air conditioner is poor, resulting in its easy corrosion, poor vibration resistance, so that there is a lack of leakage between high pressure and low pressure frequent switching. Denso designed a bypass valve system to replace a four-way reversing valve to switch the refrigerant direction of the automotive heat pump air conditioning system.
The 2017 Prius Prime is equipped with a direct heat pump air conditioning system for electric vehicles with replenishment and enthalpy. The system improves the heating performance of the passenger compartment in low temperatures and realizes dehumidification without electric heating assistance. Compared with the heat pump system without replenishment enthalpy, the heat of the heat pump air conditioning system with replenishment enthalpy increase will increase by 26% at the same speed of the compressor. In addition, compared with the heat pump air conditioning system with PTC, the energy consumption ratio of the heat pump air conditioning system with supplemental gas enthalpy increase in the case of the same heat is 63%, which can increase the driving range of electric vehicles by 21%.
The image comes from the Internet
Secondary circuit heat pump system
Due to the large number of valves for electric vehicle heat pump air conditioners, and the CO heat pump system needs to run across critical cycles, the operating pressure of the system is higher, and the sealing and material requirements of the valve body are higher. In a secondary circuit electric vehicle heat pump air conditioning system, the refrigerant does not enter the crew compartment directly for refrigeration or heating. The system will be divided into refrigerant circuit and refrigerant circuit, by the condenser, evaporator, compressor, gas-liquid separator and electronic expansion valve to form a refrigerant circuit, of which the refrigerant circuit has only one expansion valve, which can greatly reduce the number of high-pressure valves. When there is a cooling or heating demand in the crew compartment, the carrier refrigerant will flow into the condenser or evaporator for heat exchange and heat or cool the air in the crew compartment to meet the needs of cooling or heating.
The electric vehicle heat pump air conditioning system of the secondary circuit can reduce the number of high-pressure valves, but the same secondary circuit will increase the loss of energy, and the COP of the system will be slightly lower than that of the direct heat pump. At this stage, there is a controversy about whether the CO heat pump air conditioning system uses direct heat pump or secondary circuit, and there are obvious advantages and disadvantages of both schemes, and further research and demonstration will be required for the final choice.
Analysis of the CO heat pump system of volkswagen ID.4
Image source: Future Think Tank
03
Thermal management of electric vehicles based on CO heat pumps
Electric vehicles are different from fuel vehicles, not only the crew compartment needs thermal management, the safety and efficiency of the battery and the motor are closely related to its temperature, the heat inside the battery pack can not be dissipated in time will lead to a rise in battery temperature, battery temperature becomes larger, and even cause thermal runaway, so the temperature control of the battery and motor is particularly important for electric vehicles. Therefore, for electric vehicles, it is necessary to use the heat and cold volume generated by the heat pump air conditioner to control the temperature and humidity of the crew cabin and the temperature control of the power battery and the motor, so as to realize the integrated management of the heat of the electric vehicle and improve the overall energy utilization rate.
For example, the vehicle thermal management system of Apple Electric Vehicle adopts the secondary circuit scheme to manage the heat of the whole vehicle for the crew compartment, power battery and motor. Its primary circuit uses CO as a refrigerant independently designed, the secondary circuit uses water as the carrier refrigerant, including heating circuit, cooling circuit, battery circuit and motor circuit, which can realize the functions of crew compartment heating and cooling, motor heat dissipation and heating, battery cooling and heating. The system exchanges heat and cooling capacity with the refrigerant circuit via a Liquid Cooled Gas Cooler (LCGC) and a refrigerant-water heat exchanger (Chiller), respectively.
The heat obtained by the secondary circuit in the gas-liquid heat exchanger can be selected to be utilized by a three-way valve or flowed to the heat exchanger outside the vehicle. When the crew compartment needs heating, pump 1 opens the use of refrigerant to carry heat to the warm air core in the air conditioning box to heat the air in the crew compartment; in the case of extremely low temperature, the heating battery is also needed while heating the crew compartment, at this time the use of heating PTC material auxiliary heating, at this time the carrier refrigerant will release heat in the warm air core body at the same time through the heat exchanger 1 heat exchanger, pump 3 open the heating power battery.
The cold volume is transmitted to the secondary circuit through the Chiller, at this time the pump 2 is opened, when the crew compartment needs to be cooled, it can be carried to the cooling core to cool the crew compartment air, and at the same time, in the case of pump 3 open, the cold volume can be transmitted to the battery through the heat exchanger 2, and the cooling amount can be achieved through the heat exchanger 3 through the four-way valve to achieve the role of cooling the motor, and when the vehicle has no refrigeration needs, it can be discharged through the external heat exchanger.
Apple electric vehicle vehicle thermal management system
Image source: CNKI
03
Conclusions and Outlook
CO will play a huge role in the refrigerant change process due to its environmental friendliness, wide range of sources and good safety factor. In addition, the characteristics of high CO heat transfer coefficient and large cooling capacity per unit volume are conducive to reducing the volume of the heat exchanger and the displacement of the compressor, which can effectively reduce the volume of the system, which is very suitable for automotive air conditioning. Secondly, the CO heat pump air conditioning system has a good low temperature start heating function, which can still provide larger heat and maintain a high COP in the case of extremely low outdoor temperatures. Therefore, the CO heat pump air conditioning system will be the development direction of electric vehicle heat pump air conditioning in the future.
However, CO heat pump systems need to be promoted on a large scale in the field of electric vehicles, and future work also needs to solve the following problems:
(1) The transcritical CO system has a high calorific system at low temperatures, but in the refrigeration mode of high temperature environment, the cooling effect of the air cooler is limited, the dryness after throttling is high, and the refrigeration performance is poor. Therefore, it is necessary to solve the problem of insufficient cooling capacity in high temperature environment COP reduction;
(2) Since the critical pressure of CO is 7.3MPa, the operating pressure of the transcritical CO system is high, the low pressure side is 3~5MPa, and the high-pressure side reaches 8~14MPa. Therefore, the entire system components need to solve the problem of high pressure resistance and the life cycle problem under high pressure, in addition to considering the sealing problem of the whole system to ensure the safe and reliable operation of the system;
(3) The exothermic process of CO in the gas cooler of the transcritical cycle is in a supercritical state, no phase transition occurs, and the temperature after the exothermic heat is independent of the high pressure pressure, and has a large temperature slip, which increases the difficulty of system control.
Electric vehicle heat pump air conditioning not only needs to provide cooling, heating, defrosting and dehumidification functions for the crew compartment, but also needs to consider the temperature control of the motor and battery pack, so it is necessary to design an efficient and concise integrated thermal management system for the whole vehicle. In addition, the thermal management of pure electric vehicles has many valve bodies and complex pipelines, which not only increases the cost but also increases the space required for the system. Therefore, the integration of multi-valve bodies and pipelines will become the focus of the next work.