There remains the air conditioner system with a motor compressor cooler and an electric heater. However, the heat pump cycle system has recently taken root, which utilizes the Mollier diagram shown in Fig. 5.149 for saving power when heating. This is due to the fact that the efficiency, output calorie per input power, of the heat pump (COP: coefficient of performance) is usually two to three but one is an electric heater.
 The heat pump system is basically the same as a home air conditioner shown in Fig. 5.150. It controls and drives an electric compressor via an inverter, and cools, heats and dehumidifies vehicle inside.

Heat pump system for Temperate Zone

 A normal type heat pump is available for a region where the outside temperature is over -5 degree.�@An example of the heat pump system using 4-way valves is shown in Fig. 5.151. When cooling, high temperature, high-pressure refrigerant from the compressor is drawn to the external heat exchanger through 4-way valves and heat is released.

 Then, the refrigerant is decompressed through the orifice and endothermic reaction occurs in the evaporator inside the HVAC unit. When heating, refrigerant is drawn to the condenser inside the HVAC unit by switching the 4-way valves and heat is released. During this process, the external heat exchanger is operated as an evaporator.
 When dehumidifying, refrigerant flows through the condenser in the HVAC unit, the orifice for heating, the external heat exchanger, and the orifice for cooling, then to the evaporator.

 This is known as a re-heat system, in which cooled and dehumidified air will be heated. There are two heat exchangers, the condenser and evaporator, in the HVAC unit. The purpose is to get the dehumidifying effect as described above, but also there is another objective.
 Usually, the heat pump type air conditioner for home use has a heat exchanger in a room. The heat exchanger acts as an evaporator for cooling and condenser for heating. If this type is used for a vehicle air conditioner, the windshield may fog up instantly, when switched to heating after cooling, due to the evaporation of condensed water in the heat exchanger. Therefore, the condenser and evaporator are both equipped separately to ensure safety in this type air conditioner.