Air
Outdoor air is omnipresent (exist everywhere) and contains useful heat even at air temperatures as low as -20 °C. This heat can be used for heating if the right heat pump technology is used. Air-source heat pumps have a relatively low first cost and are often used for both heating and cooling. Such installations are particularly cost-effective.
Air can be used as heat source by means of several types of heat pumps such as air-to-air, air-to-water, air-to-liquid, for which the heat sink is either air or heating water (or liquid) and/or domestic hot water. It can be used in buildings or industrial applications.
Another widely used air-based heat source is ventilation air in residential, commercial and industrial buildings. Exhaust air has a relatively high temperature, and its flow rate is often directly related to the demand for space heating in the building. This is a common source for heat recovery which can provide space, water heating and process heating, either alternately or in combination.
Ground
Some meters below the ground surface, geothermal heat from the inner parts of the earth contributes to the heat supply. The ground heat is abundantly available and can be extracted and used for heating in both residential,commercial and industrial applications.
The ground also provides an eminent, low cost heat storage facility. The storage properties result in low temperature swings and large time delays compared to the temperature variation at the surface. In fact, at 5-6 m below surface it is winter when it is summer above ground. This makes the ground not only an excellent heat source in winter but also a good heat sink for cooling in summer. Even better is the combination of heat extraction in winter with heat rejection in summer. The storage function means that excess heat rejected during summer can be reused for heating in winter.
There are two main types of ground-source installations, horizontal shallow-grid systems and vertical borehole systems. The first type uses the surface layer of soil and hence relies mainly on stored solar heat and the second type uses part solar and part geothermal heat. Vertical boreholes are preferably drilled in solid bedrock but can be used in more or less any type of ground. The vertical systems are slightly more costly than the horizontal ones but require little surface area, are well-protected against external damage, have more favourable and stable temperature conditions and extremely good storage possibilities.
Water or Liquid
For water-based systems, there are a number of available natural heat sources such as ground, surface and sea water. Other sources are raw or cleaned sewage water, industrial wastewater etc.
Ground water or water close to the bottom of seas and lakes are easily available and offer a stable temperature range and the possibility of continuous heat extraction in many regions.
Wastewater usually has a higher temperature than the natural water sources. It is an eminent heat source for district heating heat pumps as there is a direct relation between the number of inhabitants of a municipality, the amount of wastewater and the demand for heating.
In industrial processes there are also possibilities to extract heat from other types of liquid flows than water flows and use different process streams as heat sinks.