Vapor injection heat pump system and operation method thereof

A vapor injection heat pump system comprising:

• a compressor;
• an indoor heat exchanger;
• a vapor injection module;
• an outdoor heat exchanger;
• an evaporator;
• and
• a controller,
• wherein the vapor injection module includes:
• a first expansion unit which blocks a flow of a condensed refrigerant or expands and transfers the condensed refrigerant according to a first heating mode which is a non-vapor injection heating mode in which the refrigerant introduced into the vapor injection module is introduced into the outdoor heat exchanger or a second heating mode which is a vapor injection heating mode in which some of the refrigerant introduced into the vapor module is introduced into the compressor and the remaining refrigerant is introduced into the outdoor exchanger;
• a gas-liquid separator which separates the refrigerant received from the first expansion unit into a gas phase refrigerant and a liquid phase refrigerant;
• and
• a second expansion unit which allows the condensed refrigerant to pass therethrough or expand or allows the liquid phase refrigerant separated in the gas-liquid separator to expand according to the first heating mode or the second heating mode, and
• the controller controls the first expansion unit and the second expansion unit according to the first heating mode or the second heating mode and controls the flow of the refrigerant to be maintained in the vapor injection module when the first heating mode is switched to the second heating mode or the second heating mode is switched to the first heating mode.