To understand the basic principle of the heat pump, one must realize that heat is a form of energy, the quantity of which is quite independent of the temperature which happens to exist at the time. In air, soil and water, in air extracted from buildings, and in waste water of any kind, there are enormous quantities of heat which are useless only because the temperature is too low. From all these sources, heat can be extracted, and with a small expenditure of additional, high-grade energy a heat pump can upgrade the waste heat to a temperature suitable for room heating.
The primary heat sources include air, water and soil. In practice, air is the most common source for heat pumps while water- and soil-source systems are less commonly applicable. In general, air, soil and ground water are considered practicable as heat sources for small heat pump systems while surface water, sea water and geothermal are more suited to larger heat pump systems. As far as low-temperature sources is concerned, ground or surface water, air and soil are most commonly used.
The technical and economic performance of a heat pump is closely related to the characteristics of the heat source. An ideal heat source for heat pumps in buildings has a high and stable temperature during the heating season, is abundantly available, is not corrosive or polluted, has favorable thermophysical properties, and requires low investment and operational costs. In most cases, however, the availability of the heat source is the key factor determining its use. Table 4.4 presents commonly used heat sources. Ambient and exhaust air, soil and ground water are practical heat sources for small heat pump systems, while sea/lake/river water, rock (geothermal) and waste water are used for large heat pump systems.
Several heat pump configurations can be visualized utilizing a seemingly inexhaustible number of energy sources. Some of these energy sources are outside air, sensible heat from stream or well water, latent heat diffusion from water (ice formation), warm discharge effluents from industry, fireplace waste heat, and heat generation in sewage. Most of these energy sources are not widely available to the general public. Four types of heat pump systems are in common use in practice:
• single-package heat pumps using an air source,
• split-system heat pumps using an air source,
• single-package heat pumps using a water source, and
• split-system heat pumps using a water source.
Single-package heat pumps have all the essential components contained within a single unit while split-system heat pumps house the essential components in two separate units (i.e. one unit outdoors and one unit indoors).