1. Cooling and Heating Functions of the Heat Pump Air Conditioner
A heat pump air conditioning system is an energy-efficient device capable of both cooling and heating. During heating, it operates in reverse cycle, transferring heat from a low-temperature environment to a high-temperature one. This process consumes a small amount of reverse cycle power while providing a large amount of heat, thus achieving energy savings. As shown in Figure 1, the heat pump air conditioning system mainly includes an
electric compressor, three heat exchangers (outdoor condenser, indoor condenser, and indoor evaporator), two solenoid valves (cooling solenoid valve and heating solenoid valve), two electronic expansion valves (cooling and heating electronic expansion valves), as well as refrigerant pressure and temperature sensors.
Figure 1: Schematic Diagram of the Heat Pump Air Conditioning Cooling Principle.
The
air conditioning compressor is driven by AC high-voltage electricity and is typically a fixed displacement, scroll-type compressor. By varying the motor speed, the compressor delivers the required refrigerant flow to the air conditioning system. Solenoid valves act as switches, enabling the flow of refrigerant when energized. The electronic expansion valve operates based on commands that rotate a stepper motor, which moves the needle valve along its axis to adjust the refrigerant flow. This regulates the refrigerant flow rate to match the thermal load.
1.1 Cooling Principle
During cooling, the cooling solenoid valve and cooling electronic expansion valve work, as shown in Figure 1. The high-temperature, high-pressure refrigerant from the compressor passes through the cooling solenoid valve and enters the outdoor condenser, where it exchanges heat with the outdoor air and turns into a high-pressure, medium-temperature liquid. After being throttled by the cooling electronic expansion valve, the refrigerant enters the indoor evaporator. Here, it absorbs heat from the cabin, and the liquid refrigerant turns into a low-pressure, low-temperature gas, which returns to the compressor, completing the cooling cycle.
1.2 Heating Principle
During heating, the heating electronic expansion valve and heating solenoid valve work, as shown in Figure 2. The high-temperature, high-pressure refrigerant from the compressor enters the indoor cooler, releasing heat, and after cooling, it turns into a high-pressure, medium-temperature liquid. After throttling through the heating electronic expansion valve, the refrigerant enters the outdoor condenser, where it absorbs heat from the outside environment, turning the refrigerant into a low-pressure, low-temperature gas. The gas then passes through the heating solenoid valve back to the compressor, completing the heating cycle.
Figure 2: Schematic Diagram of the Heat Pump Air Conditioning Heating Principle.
2. Dolphin Car Heat Pump Air Conditioning System
In September 2021, BYD launched the first model of the Ocean series, the Dolphin Car, which was the first to feature the heat pump air conditioning system, greatly improving the vehicle's thermal management efficiency.
2.1 Dolphin Car Heat Pump Air Conditioning System Composition
As shown in Figure 3, the Dolphin Car heat pump air conditioning system primarily consists of an
electric air conditioning compressor (maximum power 6kW), electronic fan, motor cooling radiator, outdoor condenser, indoor condenser, and evaporator, direct cold-heat plate for the power battery, gas-liquid separator, thermal management integrated module, and plate heat exchanger (located below the thermal management integrated module). The refrigerant used is R134a (BYD’s other electric vehicle models use R410a).
Figure 3: Composition of the Dolphin Car Heat Pump Air Conditioning System.
The thermal management integrated module includes six solenoid valves, three electronic expansion valves (Figure 4), and nine refrigerant tube connections (Figure 5).
Figure 4: Thermal Management Integrated Module;
Figure 5: Thermal Management Integrated Module Tube Connections.
2.2 Dolphin Car Heat Pump Air Conditioning System Working Principle
The working principle of the Dolphin Car heat pump air conditioning system is shown in Figure 6. In the figure, PT-1 and PT-2 represent refrigerant pressure and temperature sensors, P-1 represents the refrigerant pressure sensor, and T-1 and T-2 represent the refrigerant temperature sensors.
Figure 6: Working Principle of the Dolphin Car Heat Pump Air Conditioning System.
The Dolphin Car heat pump system eliminates the traditional high-voltage PTC heater used in electric vehicles, replacing it with a low-voltage fan-assisted PTC heater (1kW), which provides auxiliary heating in extremely low-temperature environments. In addition to providing cooling and heating for the cabin, the Dolphin Car heat pump system is the world’s first to use refrigerant to directly cool and heat the power battery, and utilize heat from the drive motor and motor controller, among other electric drive units, achieving five major functions and integrated intelligent thermal management.
With the heat pump air conditioning system, the Dolphin Car’s winter driving range is improved by over 10%, covering a wide temperature range from -30°C to 40°C, with the minimum energy consumption reaching 10.3 kWh per 100 km.
(1) Air Conditioner Heating
When the vehicle is driving or stopped in low temperatures, the air conditioning system provides heating. The
heat pump system activates the electric compressor, and the heating electronic expansion valve operates. The water source heat exchange solenoid valve and air conditioning heating solenoid valve both open. The refrigerant passes through the indoor condenser to release heat, and the plate heat exchanger absorbs heat from the electric drive units, such as the drive motor and motor controller. In extremely low temperatures, the PTC heater can be turned on for auxiliary heating to extend the temperature range of the heat pump system.
The flow path of the refrigerant during heating is as follows: compressor → indoor condenser → heating electronic expansion valve → water source heat exchange solenoid valve → plate heat exchanger → air conditioning heating solenoid valve → gas-liquid separator → compressor.
Figure 7: Air Conditioner Heating.
(2) Power Battery Heating
In low-temperature environments, the heat pump air conditioning system heats the power battery to shorten charging time or improve vehicle performance during low-temperature driving. The battery electronic expansion valve, battery heating solenoid valve, water source heat exchange solenoid valve, and air conditioning heating solenoid valve all open. The refrigerant absorbs waste heat from the electric drive unit via the plate heat exchanger and heats the direct cold-heat plate of the power battery.
The refrigerant flow path during battery heating is as follows: compressor → battery heating solenoid valve → power battery direct cold-heat plate → battery electronic expansion valve → one-way valve 1 → water source heat exchange solenoid valve → plate heat exchanger → air conditioning heating solenoid valve → gas-liquid separator → compressor.
Figure 8: Power Battery Heating.
(3) Simultaneous Heating of Cabin and Power Battery
When the vehicle is operating in low temperatures or charging in cold environments, the system can simultaneously heat the cabin and the power battery. The heat pump system activates the electric compressor, and both the heating electronic expansion valve and battery electronic expansion valve open. The water source heat exchange solenoid valve, battery heating solenoid valve, and air conditioning heating solenoid valve all open to absorb heat from the electric drive units. The indoor condenser and power battery direct cold-heat plate release heat, and the PTC heater can be used as needed.
The refrigerant flow paths are similar to those in Figures 7 and 8.
(4) Air Conditioner Cooling
When the vehicle is driving or stopped in high temperatures, the air conditioning system provides cooling. The heat pump system activates the electric compressor, and the cooling electronic expansion valve operates. Both the air conditioning cooling solenoid valve and the air exchange heat solenoid valve open. The refrigerant passes through the outdoor condenser to release heat, and the indoor evaporator absorbs heat from the cabin.
The refrigerant flow path during cooling is as follows: compressor → indoor condenser → air conditioning cooling solenoid valve → air exchange heat solenoid valve → one-way valve 5 → cooling electronic expansion valve → indoor evaporator → one-way valve 4 → gas-liquid separator → compressor.
Figure 9: Air Conditioner Cooling.
(5) Power Battery Cooling
During charging, especially with high-power charging, to prevent the power battery from overheating, the
heat pump system operates to cool the battery. When the battery temperature exceeds a set value during vehicle operation, the system starts working. The battery electronic expansion valve opens, and the air conditioning cooling solenoid valve, air exchange heat solenoid valve, and battery cooling solenoid valve all open. The refrigerant passes through the outdoor heat exchanger to release heat, and absorbs heat through the direct cold-heat plate of the power battery.
The refrigerant flow path during battery cooling is as follows: compressor → indoor condenser → air conditioning cooling solenoid valve → air exchange heat solenoid valve → one-way valve 5 → one-way valve 2 → battery electronic expansion valve → power battery direct cold-heat plate → battery cooling solenoid valve → one-way valve 3 → gas-liquid separator → compressor.
Figure 10: Power Battery Cooling.
(6) Simultaneous Cooling of Cabin and Power Battery
When the vehicle is charging or driving, if both the cabin and the power battery need cooling simultaneously, the heat pump system works by activating both the battery electronic expansion valve and the cooling electronic expansion valve. The air conditioning cooling solenoid valve, air exchange heat solenoid valve, and battery cooling solenoid valve all open. The refrigerant flows according to the paths described in Figures 9 and 10.
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