What role does the expansion valve play in a heat pump system?

The expansion valve is a critical component in a heat pump system, primarily responsible for reducing the pressure of the refrigerant. When the refrigerant exits the expansion valve, it experiences a significant drop in pressure, which allows it to expand. This expansion results in a decrease in temperature, enabling the refrigerant to absorb heat from the surrounding environment.

This cooling effect is essential for the overall function of the heat pump, particularly in the evaporator coil, where the refrigerant is tasked with extracting heat. By lowering the pressure and temperature of the refrigerant, the expansion valve creates the necessary conditions for efficient heat transfer, thus facilitating the heat pump's ability to provide heating or cooling.

The other choices describe functions that are not characteristic of the expansion valve. The increase of refrigerant pressure, maintenance of constant temperature, or filtration of impurities are roles more suited to other components in the heat pump system, such as the compressor or filters.