A heat pump is a device that transfers heat energy from a source of heat to what is called a thermal reservoir. Heat pumps move thermal energy in the opposite direction of spontaneous heat transfer, by absorbing heat from a cold space and releasing it to a warmer one. A heat pump uses external power to accomplish the work of transferring energy from the heat source to the heat sink. The most common design of a heat pump involves four main components – a condenser, an expansion valve, an evaporator and a compressor.
Heat pumps are machines that can pump heat in both directions—from the inside to the outside (cooling) and from the outside to the inside (heating).
In many ways, heat pumps are functionally the same as conventional air conditioners. The only real difference is that a heat pump can reverse itself so it can provide heating when needed. So basically, it’s an air conditioner that can reverse itself.