Note that the COP of a heat pump depends on its duty. A ground source heat pump exchanges heat with the ground. Air-source heat pumps are inexpensive relative to other heat pump systems. There is also some amount of geothermal heating and sea heat pump systems. The heating capacity of a heat pump decreases as outdoor temperatures fall. Electrical heaters are often used as backup or supplemental heat for heat pump systems. A Stirling engine can function in reverse as a heat pump for heating or cooling. Two primary classes of thermodynamic cycles are power cycles and heat pump cycles. The design of a heat pump has a considerable impact on its efficiency. A closed-cycle heat pump clothes dryer uses a heat pump to dehumidify the processing air. Heat was extracted from the river water using a heat pump. The above example applies only for an air-source heat pump. Therefore the colder the surroundings, the lower the COP of any heat pump or refrigerator. A domestic refrigerator is an example of a heat pump&colon; a heat engine in reverse. A refrigerator consists of a heat pump and a thermally insulated compartment.