A systematic classification of the different types of heat pumps is difficult because the classification can be made from numerous points of view, e.g. purpose of application, output, type of heat source, type of heat pump process, etc. If the heat is distributed via a mass flow, e.g. warm air or warm water, this mass flow is called the heat carrier.
Customarily, in the USA heat pumps are classified for the heating of buildings according to the type of heat source (first place) and type of heat carrier (second place). A distinction can be made between the terms:
• heat pump, covering only the refrigeration machine aspect, and
• heat pump plant which besides the heat pump itself also contains the heat source.
This differentiation is due to heat from the heat source being transferred to the cold side of the heat pump by an intermediate circuit, the cold carrier.
Another usual classification differentiates between
• primary heat pumps which utilize a natural heat source present in the environment, such as external air, soil, ground water, and surface water,
• secondary heat pumps which reuse waste heat as heat source, i.e. already used heat, such as extract air, waste water, waste heat from rooms to be cooled, and
• tertiary heat pumps which are in series with a primary or secondary heat pump in order to raise the achieved, but still relatively low temperature further, e.g. for hot water preparation.
Furthermore, heat pumps are generally classified by their respective heat sources and sinks. Depending on cooling requirements, various heat source and heat sink arrangements are possible in practical applications. The six basic types of heat pump are as follows:
• water-to-water,
• water-to-air,
• air-to-air,
• air-to-water,
• ground-to-water and
• ground-to-air.
In each of these types the first term represents a heat source for heating or a heat sink for cooling applications. Schematics of the common types of heat pumps are also shown in Figure 4.2.
