How heat pump air conditioners work
The air conditioning system of electric vehicles mainly uses PTC (Positive Temperature Coefficient, positive temperature coefficient) for heating in winter. The main disadvantage of PTC heating is that it is not efficient, which will reduce the driving range of electric vehicles in winter by about 30%. The best solution for this problem The solution is to use heat pump air conditioning for heating. At present, most of the working fluid in electric vehicle heat pump air conditioning systems is R134a. The biggest problem is that the GWP (Global Warming Potential) is too high, causing great environmental pollution and serious greenhouse effect, while R290 The GWP of the working fluid is low and it is a better choice to replace R134a working fluid.
A heat pump is an energy utilization device that transfers heat energy from low temperature to high temperature by consuming part of the energy. The heat pump air conditioning system mainly consists of a compressor, outdoor heat exchanger, indoor heat exchanger, throttling device, liquid storage dryer, and four-way valve It is composed of components such as temperature and pressure sensors. During the heating cycle of the heat pump air conditioner, the compressor changes the low-temperature and low-pressure working fluid into high-temperature and high-pressure gas through work. Some of the gas re-enters the compressor through the cycle, and the other part of the gas passes through the four-way valve. It enters the indoor heat exchanger, where it cools down and condenses to release heat, which increases the temperature inside the vehicle and changes the high-temperature and high-pressure gaseous working fluid into a medium-temperature and high-pressure supercooled liquid. Then it enters the throttling device to greatly reduce the pressure, and further changes into a low-temperature and low-pressure fluid. The gas-liquid two-phase working fluid enters the outdoor heat exchanger to absorb heat, and transfers the heat from the external environment to the heat pump system. The working fluid changes back to a low-temperature and low-pressure superheated gas state, and is again passed through the compressor to complete the cycle. Electric vehicle heat pump The principle of the air conditioning system is shown in Figure 1.
As can be seen from Figure 1, the role of the working fluid in the entire heating cycle can be simplified into four basic processes: isentropic compression, isobaric condensation, adiabatic expansion, isobaric evaporation. Thermodynamic cycle pressure-enthalpy diagram of electric vehicle heat pump air conditioning system as shown in picture 2.
In Figure 2, 0 is the form of the saturated low-temperature and low-pressure working fluid, 1 is the form of the working fluid at the compressor inlet and the outdoor heat exchanger outlet, 2 is the form of the working fluid at the compressor outlet and the indoor heat exchanger inlet, and 3 is the saturated The form of the working fluid at high temperature and high pressure, 4 is the form of the working fluid at the outlet of the indoor heat exchanger and the inlet of the throttling device, 5 is the form of the working fluid at the outlet of the throttling device and the inlet of the outdoor heat exchanger, a is the enthalpy value of the throttling device, b is the enthalpy value of the compressor outlet. Process 1→2 is the work process of the compressor, which is isentropic compression under ideal conditions; Process 2→4 is the circulation of the working fluid in the indoor heat exchanger, which is an isobaric condensation heat release process; process Process 4→5 is the adiabatic expansion of the working fluid in the throttling device; process 5→1 is the equal pressure evaporation of the working fluid in the outdoor heat exchanger, which is an endothermic process.
