Research and design on frosting of outdoor heat exchanger in heat pump type electric vehicle air conditioning system

Aiming at the problem that the outdoor heat exchanger of the electric vehicle heat pump air conditioning system is prone to frost during winter operation, which affects the heating performance of the system, this paper designs and builds an experimental system and conducts research, and analyzes the frosting characteristics of the multi-flow microchannel heat exchanger. The experimental results show that there is a linkage relationship between the growth of the frost layer on the surface of the outdoor heat exchanger and the decrease in the surface temperature. The distribution of the frost layer in each flow channel is uneven. The density of the two-phase refrigerant in the heat exchanger is different and affected by gravity, resulting in gas-liquid phase separation and uneven distribution in the flow channel. In addition, the increase in the frost coverage rate on the surface of the outdoor heat exchanger will lead to a decrease in the temperature of the refrigerant side of the outdoor heat exchanger, the suction and exhaust pressure of the compressor, and the heating capacity, resulting in an increase in the unit power consumption of the compressor. Taking the data of the ambient temperature of 0 ℃ as an example, when the frost coverage rate reaches 77.4%, the decreases in the suction pressure, exhaust pressure and heating capacity are 33.4%, 12.1% and 25.8% respectively, and the unit power consumption increases by 32.0%, resulting in a decrease in the stability of the system operation, so that the heating capacity cannot meet the heating requirements of the passenger compartment.

Energy crisis and climate warming have become global problems that need to be solved urgently, making electric vehicles replacing fuel vehicles a mainstream trend, which puts higher requirements on automotive air conditioners. The waste heat of electric vehicle motors cannot meet the heating needs in winter. The use of positive temperature coefficient thermistors (PTC) for heating has become a common solution, but the mileage is greatly reduced due to the consumption of more battery power, and the reduction is even more than 50% in low temperature environments. The development of more efficient automotive air conditioning systems has become a common direction for academia and industry. Heat pump systems have become a feasible alternative because their COP is greater than 1. S. Bellocchi et al.'s research shows that replacing PTC with heat pumps can save 17% to 52% of energy consumption at different ambient temperatures. However, there are also some problems in the application of heat pump systems. When operating in heating mode, the outdoor heat exchanger acts as an evaporator, and the refrigerant temperature in the flat tube is lower than 0 °C. Because the thermal resistance of aluminum is small, after heat conduction, the surface temperature of the flat tube drops below the dew point and freezing point of water vapor and frost forms.

Many scholars at home and abroad have studied the frosting problem of heat pump electric vehicle air conditioning systems. In terms of frosting mechanism, Li Jingshan et al. conducted an experimental study on the growth characteristics of the frost layer on the surface of the outdoor heat exchanger of the air source heat pump and the dynamic performance of the system. The results show that the deterioration of system performance is the result of the cyclic action between the three factors of the decrease in surface temperature of the outdoor heat exchanger, the increase in frost layer thickness and the increase in air flow resistance leading to a decrease in air volume, and the headwind speed has an important influence on the average frost density. Other scholars have studied the effect of heat exchanger type on frosting and the effect of refrigerant flow distribution in the heat exchanger on system performance. Xu Bo et al. studied the performance and mechanism of cyclic frosting and defrosting of two types of microchannel heat exchangers, horizontal and vertical flat tubes, and found that for the horizontal flat tube microchannel heat exchanger, the water accumulated between the fins during the cyclic frosting and defrosting process will accelerate the frosting rate and trigger the defrosting action in advance, while the vertical flat tube microchannel heat exchanger has better drainage capacity and better parameter stability.

L. Feng et al. established a heat pump system model, including theoretical models of components such as compressors, liquid storage tanks, and microchannel heat exchangers. The simulation results show that the model is consistent with the experimental results under other working conditions, but there are problems in heating mode where the power consumption and heating capacity are higher than the experimental values. This is mainly caused by the uneven distribution of refrigerant flow in the outdoor heat exchanger during actual measurement. In order to alleviate the frosting problem, J. S. Byun et al. designed a heat pump system that leads a branch from the compressor exhaust pipe to the inlet of the outdoor heat exchanger to slow down the growth and diffusion of the frost layer on the surface of the outdoor heat exchanger. It was found that the performance was best when the branch refrigerant flow accounted for 20% of the total flow, but in this process, the increase in the refrigerant flow in the outdoor heat exchanger would cause transient fluctuations in the operation of the heat pump. The growth distribution characteristics of the frost layer on the surface of the outdoor heat exchanger during heating mode operation and the influence of frosting on the main parameters of the system.

1 Experimental setup and methods

1.1 Experimental setup

The above studies mainly focus on the frosting mechanism of air source heat pump systems, the influence of heat exchanger type or system design on frosting rate, while the influence of outdoor heat exchanger flow channel design of automobile air conditioning system on frosting and the influence of frosting on the system are less studied. Therefore, this paper designs and builds an experimental bench for a heat pump air conditioning system of an electric vehicle. The research system experiment is carried out in the automobile air conditioning enthalpy difference chamber, and the refrigerant filled in the heat pump system is R134a. The experimental device and test system are shown in Figure 1. The enthalpy difference chamber consists of two environmental chambers, the inner and outer chambers, and the temperature, humidity and air volume of the air in the environmental chamber are controlled by an independently installed air handling device to maintain the set value.

Due to the limitation of the enthalpy difference chamber capacity, the relative humidity of the external environment chamber cannot reach the value required for the experiment, so an independent humidifier is used in this experiment to assist humidification. The heat pump system consists of a compressor, an outdoor heat exchanger, an indoor condenser, an indoor evaporator, an electronic expansion valve, a thermal expansion valve, a liquid storage tank, etc., in which the electronic expansion valve and the thermal expansion valve both have a cut-off function. The system switches the heating and cooling modes through the solenoid valve. The relevant component parameters of the heating mode used in this experiment are shown in Table 1. The black thin line in Figure 1 indicates that there is no refrigerant flow. In the heating mode, the indoor evaporator does not work, the indoor evaporator side mode damper of the internal environment room air duct is closed, the indoor condenser side mode damper is open, solenoid valve 1 is closed, and solenoid valve 2 is open.

Fig.1 Experimental device and test system(in heating mode)
Pt100 platinum resistance and pressure sensor are arranged at the refrigerant inlet and outlet of the compressor, outdoor heat exchanger and indoor condenser to measure the temperature and pressure of the refrigerant. Coriolis mass flowmeter is used to measure the mass flow of refrigerant, and power meter is used to measure the power consumption of compressor. In order to measure the temperature change and distribution of each area on the surface of outdoor heat exchanger during heating operation, 12 K-type thermocouples are arranged 2 mm away from the end face of flat tube on the windward side. The specific position is shown in Figure 2. From left to right, they are marked as the first to fourth processes. Three temperature measurement points are arranged at the upper, middle and lower positions of each process close to the central axis. The measurement accuracy is shown in Table 2. All data in the experiment are read and stored by self-compiled software, and the sampling time interval is set to 6 s. Table 1 Specifications
of heat pump system

Fig.2 Structure of outdoor heat exchanger and layout of measuring points
Tab.2 Main measured parameters and the precision

In addition, after testing with a handheld temperature and humidity measuring instrument, the relative humidity of the windward surface of the outdoor heat exchanger can be maintained at 80% to 85% in the experiment. The sources of relative humidity error are: 5% error caused by the independent humidifier and ±3% instrument error of the handheld temperature and humidity meter. The error transfer method proposed by R. J. Moffat was used for analysis. The above two error components are independent of each other, so considering the uniform distribution, the overall error of relative humidity is 5.8%.

1.2 Experimental methods

The refrigerant charge of this heat pump system is 650 g. The research of Qu Xiaohua et al. shows that the evaporator is prone to frost under high humidity, non-extremely low ambient temperature, high heat exchange and low wind speed. Therefore, the experimental conditions shown in Table 3 are selected. Since an independent humidifier is used to assist humidification on the outdoor side, although it is a fresh air condition, the relative humidity on the indoor side is different from that on the outdoor side, which is different from the actual vehicle operation. During the experiment, photos were taken every 5 minutes to record the frosting phenomenon. Table 3 Test
conditions