Heat-pump-type vehicular air-conditioning system

A heat-pump-type vehicular air-conditioning system, comprising:

• a cooling refrigerant circuit in which an electric compressor, a vehicle-mounted external heat exchanger, a receiver, a first decompression unit, and an onboard evaporator provided within an HVAC unit are connected in an above stated order;
• an onboard condenser disposed on a downstream side of the onboard evaporator within the HVAC unit, including a refrigerant inlet connected to a discharge circuit of the electric compressor via a switching unit, and a refrigerant outlet connected to the receiver;
• a first circuit including a second decompression unit connected between an outlet side of the receiver and one end side of the vehicle-mounted external heat exchanger;
• and
• a second circuit including a solenoid valve connected between another end side of the vehicle-mounted external heat exchanger and an intake circuit of the electric compressor, the solenoid valve being open during heating,
• a heating refrigerant circuit being configurable in such a manner that the electric compressor, the switching unit, the onboard condenser, the receiver, the first circuit including the second decompression unit, the vehicle-mounted external heat exchanger, and the second circuit including the solenoid valve are connected in an above stated order, and
• in a heating mode, upon frost formation on the vehicle-mounted external heat exchanger, defrosting being enabled by switching the heating refrigerant circuit to the cooling refrigerant circuit and directly guiding a hot gas from the electric compressor to the vehicle-mounted external heat exchanger,
• wherein the first decompression unit and the second decompression unit are each decompression units equipped with an on-off valve function, and the vehicle-mounted external heat exchanger and the onboard evaporator are jointly usable as evaporators while in the heating mode and while in a dehumidifying heating mode by using the on-off valve function of the first decompression unit and the second decompression unit, and
• while in the dehumidifying heating mode, on-off control of the on-off valve function of the first decompression unit is performed in accordance with a temperature of one of air blown form the onboard evaporator and a fin thereof, so as to cause one of flow and obstruction of refrigerant to the onboard evaporator.