1. A brine* circulates in a collector loop and absorbs heat energy from the rock, ground, air or water.
2. The heat pump contains a heat exchanger called an evaporator. The energy is transferred to a refrigerant** with a low boiling point which evaporates and circulates as gas in a closed system.
3. The pressure of the refrigerant is raised in the compressor, which also raises the temperature to a useable level.
4. In the condenser, the refrigerant releases the heat to the heating system for the house.
5. The separate cooler functions as an extra heat exchanger and squeezes out the remaining heat energy at which point the refrigerant returns to liquid form.
6. The pressure falls in the expansion valve.
7. The refrigerant returns to the evaporator and the process starts again.

* Brine is a mixture which cannot freeze, for example alcohol or glycol.
** These days environmentally friendly refrigerants such as carbon dioxide and hydrocarbons are used.