Final answer:
The expansion valve is the component in a heat pump that controls the direction of the refrigerant flow, allowing the heat pump to switch between heating and cooling modes.
Step-by-step explanation:
The component used to change the direction of refrigerant flow in a heat pump is the expansion valve. When a heat pump operates in heating mode, it reverses the refrigeration cycle allowing it to transfer heat from the colder outdoor environment into the heated space inside. Four primary components make up a simple heat pump system: an evaporator, a compressor, a condenser, and the expansion valve. Specifically, the expansion valve regulates the flow of the refrigerant, which enables it to alternately evaporate and absorb heat from the outside air or to condense and release heat into the indoor space, depending on the mode of operation.