VEHICLE AIR CONDITIONING SYSTEM AND VEHICLE AIR CONDITIONING METHOD

Improved are a vehicle air conditioning system including a refrigerant circuit and a heat medium circuit and including a plurality of vehicle-exterior heat exchangers that exchange heat between outside air and a heat medium and a vehicle air conditioning method. The vehicle air conditioning system includes a refrigerant circuit, a heat medium circuit, a vehicle-interior heat exchanger, a vehicle-exterior heat exchanger including a first heat exchanger and a second heat exchanger disposed in series with respect to a flow of air generated by an air blower, a switching unit, a detection unit configured to detect a degree of frost formation, and a control device. The operation mode includes a defrosting mode in which the heat medium in the high-temperature medium circuit is supplied to a defrosting target to be selected as either the first heat exchanger or the second heat exchanger connected in parallel in the heat medium circuit by the operation of the switching unit. The control device selects, as the defrosting target, the first heat exchanger located on an upwind side in preference to the second heat exchanger located on a downwind side when it is determined that both the first heat exchanger and the second heat exchanger need to be defrosted.